# A Random Walk with Heavy Flavours

Authors: Surasree Mazumder; Trambak Bhattacharyya; Santosh K. Das
Journal: Advances in High Energy Physics
 (2013)

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## Abstract

We focus on evaluating transport coefficients
like drag and diffusion of heavy quarks (HQ) passing
through quark-gluon plasma using perturbative QCD (pQCD).
Experimental-observable-like nuclear suppression factor
(RAA) of HQ is evaluated for
both zero and nonzero baryonic chemical potential (μB) scenarios
using Fokker-Planck equation. Theoretical estimates of RAA
are contrasted with experiments.


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## Body

## 1. Introduction

When nuclear matter is subjected to an ambience of very high density, the individual quarks and gluons would no longer be confined within the hadrons but melt into a deconfined state of quarks and gluons. Just after the discovery of asymptotic freedom [1–3], Collins and Perry [4] also suggested that at very high density the degrees of freedom of the strongly interacting matters are not hadrons but quarks and gluons. The same is true when QCD vacuum is excited to high temperatures, too [5]. With increasing temperature, new and new hadrons are produced thereby increasing the corresponding number density, and at a certain temperature, there is an overlap of hadrons. Such a phase of matter is called quark-gluon plasma (QGP) and its study needs QCD, the theory of strong interaction which is extremely successful in vacuum, to be applied in a thermal medium. So, the deconfined state of quarks and gluons gives an opportunity to peruse “condensed matter physics” of elementary particles in the new domain of nonabelian gauge theory.Lattice-QCD-based calculations predict that the typical value of the temperature for the quark-hadron transition,Tc, is ~170 MeV [6, 7] (latest lattice QCD results show that Tc~160 MeV [8–10]). According to the cosmological big bang model, the universe has undergone several phase transitions (GUT, electroweak, quark to hadron, etc.) at different stages of its evolution. The quark-hadron transition occurred when the universe was few microseconds (μs) old and this is the only transition which can be accessed in the laboratory currently. The study of quark-hadron transition demands special importance in understanding the evolution of the μs old early universe. The issue is very crucial for astrophysics too, as the core of the compact astrophysical objects like neutron stars may contain quark matter at high baryon density and low temperature. So there is a multitude of reasons behind creating QGP in laboratories.Temperature and energy density required to produce QGP in the laboratory can be achieved by colliding heavy ions at relativistic energies, under controlled laboratory environment. The nuclear collisions at Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) energies (200 GeV/A and 2.76 TeV/A, resp.) are aimed at creating QGP. Once the QGP medium is created, we must try to understand the transport properties of the medium, that is, whether it is liquid or gas. The study of the transport coefficients of strongly correlated system is a field of high contemporary interest both theoretically and experimentally. In one hand, the calculation of the lower bound on the shear viscosity (η) to entropy density (s) ratio (η/s) within the frame work of AdS/CFT model [11] has ignited enormous interests among the theorists. On the other hand, the experimental study of the η/s for cold atomic systems and QGP and their similarities has generated huge interest across various branches of physics (see [12] for a review).In general, the interaction of probes with a medium brings out useful information about the nature of the medium. As the magnitude of the transport coefficients is sensitive to the coupling strength, so these quantities qualify as useful quantities to characterize a medium.In context of probing QGP, the heavy quarks (HQs), mainly, charm and bottom quarks, play a vital role. The reasons are as follows.(i)
HQ mass is significantly larger than the typically attained temperatures and other nonperturbative scales,M≫Tc, ΛQCD (intrinsic energy scale for the strong interaction); that is, the production of HQs is essentially constrained to the early, primordial stage of a heavy-ion collision and they do not dictate the bulk properties of the matter. Therefore, the heavy flavours are the witness to the entire space-time evolution of the system.(ii)
Their thermalization time scale is larger by a factor ofm/T, where m is the mass of heavy quarks and T is the temperature, than that of the light quarks and gluons and hence heavy quarks can retain the interaction history very effectively.From experimental point of view, however, the issue of HQ thermalization in QGP can be addressed by measuring the elliptic flow (v2) of leptons from the decays of HQs. Moreover, the observed transverse momentum suppression (RAA) of leptons originating from the decays of D and B mesons produced in nuclear collisions as compared to those produced in proton + proton (pp) collisions at the same colliding energy [13–15] offers us an opportunity to estimate the drag and diffusion coefficients of QGP. (It is now possible to detect directly D mesons at LHC detectors like ALICE, see [16]). Hence, no wonder that in the recent past a large number of attempts have been made to study both heavy flavour suppression [13, 14] and elliptic flow [17] within the framework of perturbative QCD (pQCD) [18–34].


## 2. Motion of Heavy Quarks (HQs) in QGP

In the introduction, we have already discussed that HQs act as effective probes to look into the properties of QGP. As HQs are much heavier than the particles constituting the QGP thermal bath, one expects that they will execute Brownian motion in QGP medium [35, 36]. The system under study, then, would have two components: (i) the QGP formed at an initial temperature Ti and initial thermalization time τi consisting of light quarks and gluons and (ii) heavy quark, the Brownian particle formed due to hard collisions at very early stage of heavy ion collision. The momentum distribution of HQ is governed by a nonlinear integrodifferential equation which is the Boltzman transport equation (BTE) as
(1)[∂∂t+pE.∂∂x+F.∂∂p]f(x,p,t)=[∂f∂t]collisions.Fis the force exerted on the HQ by the surrounding colour field. p and E denote the three momentums and the energy of the HQ, respectively. The right hand side of (1), which is called the collision integral, C[f], is attributed to the QCD interactions of HQ with light quarks (q), antiquarks (q-), and gluons (g). One should, in principle, solve this differential equation under the influence of potential involving interaction of HQs with light quarks/anti-quarks and the background colour field in the force term. But, here, we will set F=0 and will treat QGP to be uniform. Therefore, the second and the third term of the left hand side of (1) vanish under these approximations. Again defining
(2)f(p,t)=1V∫‍d3xf(x,p,t)
which is the normalized probability distribution in the momentum space, we have
(3)∂f(p,t)∂t=[∂f∂t]collisions.
Equation (3) signifies that all variation of the distribution function of HQ with time is due to the collisions only.


## 3. Formalism

Our main aim is to determine the collision integral of the transport (3). Once, we determine certain form of C[f], we can proceed towards solving the differential equation. There are lots of approximations through which the integrodifferential equation can be solved. Of course, under certain conditions, (3) can be reduced to the simple form
(4)∂f(p,t)∂t=-f-f0τ
which is a useful first approximation. Here, f0 is the equilibrium distribution function and τ is the relaxation time that determines the rate at which the fluctuations in the system drive it to a state of equilibrium again. In this form, the equation is very easy to solve. But, our case is not so simple. We will deal with a more sophisticated approximation that leads to the Fokker-Planck equation [37].To start with, we apply the Landau approximation which allows only soft scattering in the collision integral. If we definew(p,k) to be the rate of collisions which change the momentum of the HQ from p to p-k, we have
(5)[∂f∂t]collisions=∫‍d3k[w(p+k,k)f(p+k)-w(p,k)f(p)].
The second part of the integral corresponds to all those transitions that remove HQ from momentum p to p-k and therefore represents a net loss to the distribution function. Likewise, the first part of the integral represents a net gain to the distribution function of HQ. With these, (3) becomes
(6)∂f(p,t)∂t=∫‍d3k[w(p+k,k)f(p+k)-w(p,k)f(p)].
Equation (6) is a linear equation in f. We can simplify it by assuming the previously discussed Landau approximation. Mathematically, this approximation amounts to assuming w(p,k) to fall off rapidly to zero with |k|; that is, transition probability function, w(p,k), is sharply peaked around |k|=0. Therefore, if we expand the integrand in the right hand side of (6) in powers of k, we have
(7)w(p+k,k)f(p+k)≈w(p,k)f(p)+k·∂∂p(wf)+12kikj∂2∂pi∂pj(wf).
Retaining terms up to the second order only, we obtain Fokker-Planck equation [37]
(8)∂f∂t=∂∂pi[Ai(p)f+∂∂pj[Bij(p)f]],
where the kernels are defined as follows:
(9)Ai=∫‍d3kw(p,k)ki,Bij=12∫‍d3kw(p,k)kikj.
In the present formalism, we considered the elastic scattering of the HQ with the gluon, light quark, and the corresponding anti-quarks. All these processes contribute to determine w(p,k) [37] and, in turn, the above defined kernels. Now, to explore the physical significance of the A and B coefficients, let us consider Ai=piγ(p) and Bij=D(p)δij, which assume very low p; that is, the medium of QGP is isotropic to the HQ. If, in this limit, we neglect all the derivatives of A and B coefficients with momentum of HQ, (8) reduces to
(10)∂f∂t=γ∂∂p·(pf)+D[∂∂p]2f.
The method of solution of this equation is elaborately discussed in [37]. Now, if we want to extend this formalism to the regime where the momentum of the HQ is no longer small, that is, the particular kinematic domain where HQ becomes relativistic, obviously, we would like to know how the transport coefficients drag (γ) and diffusion (D) behave at high p region. In order to do so, we extrapolate the concept of isotropy to the higher momentum of HQ in such a way that only the first derivatives of the drag and diffusion coefficients are considered and the momentum dependence of A and B coefficients is encoded inside γ and D. Therefore, the Fokker-Planck equation, under this approximation, in Cartesian coordinate system becomes [28]
(11)∂f∂t=C1(px,py,t)∂2f∂px2+C2(px,py,t)∂2f∂py2+C3(px,py,t)∂f∂px+C4(px,py,t)∂f∂py+C5(px,py,t)f+C6(px,py,t),
where,
(12)C1=D,C2=D,C3=γpx+2∂D∂pTpxpT,C4=γpy+2∂D∂pTpypT,C5=2γ+∂γ∂pTpx2pT+∂γ∂pTpy2pT,C6=0,
where the momentum p=(pT,pz)=(px,py,pz). We numerically solve (11) [38] with the boundary conditions f(px,py,t)→0 for px,py→∞, and the initial (at time t=τi) momentum distribution of charm and bottom quarks is taken from MNR code [39]. It is evident from (11) that with the momentum-dependent transport coefficients the FP equation becomes complicated.It is possible to write down the solution of the FP equation in closed analytical form [40] in the special case of momentum-independent drag and diffusion coefficients. To find out the solution f for momentum independent drag and diffusion coefficients, we can consider the one-dimensional version of (10) as
(13)∂f∂t=γ∂∂(pf)+D∂2f∂p2,
which is also called the Rayleigh’s equation. For the initial condition f(p,t=0)=δ(p-p0), the solution of f is
(14)f(p,t)=(γ2πD(1-e-2γt))-1/2×exp(-γ2D(p-p0e-γt)21-e-2γt).However, (14) is solution for a very simplified scenario and we will study the momentum dependence of drag and diffusion coefficients and their effects on RAA in the sections to come.


## 4. Drag Coefficients

### 4.1.  Elastic Processes

We need to evaluate the drag coefficient as a function of temperature and momentum of HQ. The expression to evaluate collisional drag can be written as [37]
(15)γcoll=12Ep∫‍d3q(2π)32Eq∫‍d3q′(2π)32Eq′×∫‍d3p′(2π)32Ep′1γQ∑‍|M|2×(2π)4δ4(p+q-p′-q′)f′(q)[1-p·p′p2],
where p′=p-k and q′=q+k. The scattering matrix elements are given explicitly in [41] where the t channel divergence occurring due to very soft gluon exchange has been shielded by replacing t in the denominator of matrix elements by t-mD2 in an ad hoc manner, where t is the Mandelstam variable and mD is the thermal mass of gluon. However, the same problem can be approached from hard thermal loop (HTL) perturbation theory. The gluon propagator for t channel diagram is then replaced by HTL propagator. But calculation of even the elastic matrix elements for HQs scattering with light quarks and gluons in this approach is very lengthy and radiative matrix elements are even clumsier. However, an outline of calculating collisional drag and diffusion coefficients in HTL perturbation theory approach is given for the interested readers in the Appendix. Here we proceed with the conventional process of shielding t channel divergence with the gluon thermal mass, mD.The integrations in (15) have been performed using the standard techniques [23–25, 37]. Results of the present calculation of drag coefficients are plotted with respect to momenta of charm and bottom (Figure 1). We can observe that the momentum dependence of γ is nonnegligible for the shown momentum range. The value of γ for charm due to collisional processes at p=5 GeV is about 0.036 fm−1 which reduces to a value of 0.018 fm−1 at p=10 GeV (can be compared with, e.g., [42–45]). In the inset, the drag coefficients of HQs due to elastic collision are plotted in the lower momentum region, where the drag remains more or less constant with p. Therefore, it is clear that had we taken the value of drag at low momentum and extrapolated that value to higher momentum, the final result would have been overestimated. The diffusion coefficient of HQ can be evaluated from Einstein relation D=γMT. Later, it will be seen that the momentum dependence of transport coefficients of a high energy HQ will have considerable effects on the nuclear suppression factor, RAA of HQ.Figure 1
Drag coefficients of charm and bottom with their momentum atT=300 MeV assuming running strong coupling, αs(T), and temperature-dependent Debye screening mass, mD(T) for gluon, quark, and antiquark scattering.However, there are also radiative processes taking place in the medium. The radiative drag can be obtained in the present formalism by computing the radiative energy loss of a HQ passing through QGP. In the next section, we will discuss the methods and intricacies of calculating the transport coefficients in radiative domain.


### 4.2. Radiative Processes

It is already known that the two mechanisms of energy loss of heavy quarks are collisional and radiative energy loss. Though at low transverse momentum (pT) region the collisional and radiative losses of HQs are comparable (see Figure 5), the radiative one tends to dominate with increasing momentum. So it is worthwhile, after discussing about evaluation of collisional transport coefficients (drag, diffusion) within the ambit of T=0 perturbative QCD (pQCD) [37] in the previous section, to contemplate on radiative processes and to inspect how pQCD approach can be utilized in finding out radiative transport coefficients.It is well known from QED that for high energies radiative energy loss becomes dominant [46]. Also the hard thermal loop calculations in context of QCD show that the radiative energy loss contributes to the same order of strong coupling as that of collisional loss [47]. The above discussions tempt one to infer that the observed large suppression of heavy quarks at RHIC is predominantly due to bremsstrahlung processes [25, 28], at least at high pT, but, as already said, the comparable values of energy losses in low momentum region leave an ample room for ambiguity in this statement particularly at “not-so-high” pT   (~2 GeV) region.Theoretical estimate of nuclear suppression factor (RAA) for charm quarks in [28] shows ~4 times more suppression due to inclusion of radiation (Figure 2) with the same initial conditions. Inclusion of radiative processes leads to a good description of charm RAA at RHIC energies. At LHC energy (2.76 TeV/A), the collisional energy loss due to hard (momentum transfer > 2 GeV) collisions could be about one-third of the total [29]. The rest may be attributed to radiative loss.Figure 2
Suppression of transverse momentum of charm quarks in QGP as a functionpT.From the discussion on collisional transport coefficients we know that (collisional) drag is given by [37]
(16)γcoll=-1p(dEdx)coll,
where p is the momentum of the probe. We employ a similar argument and relate radiative energy loss ((dE/dx)rad) to inelastic drag (γrad) in the same way. The effective drag obtained is a summation of collisional and radiative parts. As energy loss is related with the transport properties like drag offered by the medium, we must concentrate on more and more accurate determination of radiative energy loss which will enable us to understand the properties of QGP.The radiative energy loss in general has been studied in [48–53] incorporating Landau-Pomeranchuk-Migdal (LPM) effect [54, 55], to be discussed later, due to multiple scattering. The authors of [56] treat the problem of radiative energy loss of HQs by building a model in scalar QCD approach. Attempts have been taken [25, 28, 30] to incorporate the formalism of [48], the Gyulassy-Wang potential model (GWPM), to compute the radiative energy loss of heavy quarks traveling through QGP. In stead of going to the energy loss in GWPM approach directly, we may try to acquire some familiarity with the model and the notations used for the description of it.


#### 4.2.1. Gyulassy Wang Potential Model (GWPM) and Radiation Spectrum

To analyze the multiple scattering and the induced gluon radiation in GWPM, certain simplifications have to be made. For example, [49] assumes static interaction between propagating parton and bath particles. This interaction is modelled by a static Debye screening potential. Now, it is possible to approximate the effective average random colour field produced by the bath particles by a potential provided the distance between two successive scatterers is large compared to colour screening length (μ-1). The screened potential is given by
(17)VAA′a(q→)=AAA′a(q→)e-iq→·x→=gTAA′ae-iq→·x→q→2+μ2,
where μ is the colour screening mass, Ta are the generators corresponding to the representation of target partons at x→ transferring (three) momentum q→, and g is the coupling. The Feynman diagrams contributing to induced gluon radiation from a single quark-quark scattering are given in Figure 3.Figure 3
Feynman diagrams for gluon radiation from a single quark-quark (qq′) scattering.The calculation of Feynman diagrams is done in light-cone coordinates [57, 58] where light-cone representations of four-vectors are done in the following way:
(18)p=(p+,p-,p⊥)=(p0+p3,p0-p3,p⊥).
In the potential model, one can neglect radiation from the target lines (external lines appearing at the bottom of gluon propagator in Figure 3) provided one decides to work in light cone gauge, A+=0, for emitted gluon fields (For details see [59]). For radiation in the midrapidity region, the celebrated Gunion-Bertsch distribution formula (GB formula) for soft gluon radiation [60] can be reproduced from this approach.We now discuss the GB formula and recent attempts to generalize. (Interested readers are referred to the Appendix for other such very recent endeavors.) We will see that this formula and/or its generalizations will play a significant role in finding out the energy loss due to gluon emission of quarks. The early attempts of generalizing GB formula [61–63] consider the general gg→ggg matrix element elegantly written by [64] and factoring out the elastic gg→gg scattering amplitude to obtain the distribution of emitted gluon (~|Mgg→ggg|2/|Mgg→gg|2). We can write the following form of gluon distribution [63], (for details, see Appendix of [63])
(19)dngd2k⊥dη=[dngd2k⊥dη]GB[(1+t2s+5t22s2-t3s3)-(32s+4tss-3t22s2s)k⊥+(52s+t2s2+5t2s3)k⊥2],
where η is the rapidity of the radiated gluon, and the subscript GB has been used to indicate the gluon spectrum obtained using the approximation considered in [60] (see also [65]) which is generally given by
(20)[dngd2k⊥dη]GB=CAαsπ2q⊥2k⊥2[(k→⊥-q→⊥)2+mD2],
where mD=(2π/3)αs(T)(CA+NF/2)T is the thermal mass of the gluon [66, 67], NF is the number of flavours contributing in the gluon self-energy loop, CA=3 is the Casimir invariant for the SU(3) adjoint representation, αs is the temperature-dependent strong coupling [68], k⊥ is the transverse momentum of the emitted gluon, and q⊥ is the transverse momentum transfer. In general, one introduces the thermal mass in the denominator of (20) to shield the divergence arising from collinearity (i.e., when k→⊥=q→⊥) of emitted gluon. But, for the present case (19) is written under the assumption that q→⊥≫k→⊥, and hence there is no need to write the mD2 factor in GB spectrum.Now, we must consider multiple scattering encountered by incoming particles, too. The many-body effect due to presence of medium results in interference of scattering amplitudes. This interference effect is called LPM effect [54, 55]. LPM effect is discussed in QED domain in [69]. LPM suppression can be understood in a qualitative manner as an interplay between two time scales, the formation time (τf) and the scattering time (τsc). τF is the time needed for the emission of the induced gluon. Actually, τf determines the time span after which a radiation can be separately identified from the parent parton from which the radiation is being given off. Now, a collision just before formation of the gluon results in suppression of the radiation. This destructive interference is called LPM effect and in the next section we will see that it puts a constraint on the phase space of the emitted gluon. If k0 is the energy of the emitted (soft) gluon and k⊥ is its transverse momentum, then formation time τf~2k0/k⊥2 [49]. When this τf is much less than collision time, that is, separation between two scattering centres, L, then the intensity of radiation is additive and Bethe-Heitler (BH) limit is reached. In the factorization limit, L≪τf, the interference in radiation amplitude takes place and LPM effect dominates.After this brief discussion on possible effects of multiple scattering, emitted gluon distribution due to multiple scattering in terms of that due to single scattering can be written as [49]
(21)dng(m)d2k⊥dη=Cm(k)dng(1)d2k⊥dη,
where “m” stands for multiple scattering and “1” stands for single scattering. Cm is called radiation formation factor characterizing the interference pattern due to multiple scattering. Naturally, in the BH limit, Cm≈m, which implies that scattering amplitudes are just additive and the resultant intensity shows no interference pattern. On the other hand, the factorization limit gives [49, 50]
(22)Cm(k)≈89[1-(-18)m]forquarks≈2(1-12m)forgluons.
Equation (22) shows that the interference effect due to many multiple scatterings for quarks leaves corresponding radiation spectrum a factor of ~8/9 of that due to single scattering. It can also be checked that the gluon intensity radiated by gluon jet is 9/4 times higher than that radiated by quark jets in multiple scattering. Thus, the LPM effect in QCD depends on colour representation due to nonabelian nature of the problem under discussion.


#### 4.2.2. Energy Loss of Light Particles in GWPM

The energy loss in potential model can be evaluated by integrating over the transverse momentum and rapidity of emitted gluon. The phase space is, of course, constrained by LPM effect due to multiple scattering. The multiple scattering is implemented through the differential change of the factorCm with number of scattering m, dCm/dm, which can be approximated as a θ-function [49]. Also, the emitted gluon must have energy less than that of the parent parton. So the energy loss formula implementing the energy constraint gives
(23)ΔErad=Em+1-Emm=∫‍d2k⊥dηdngd2k⊥dηk0Cm+1-Cmmθ(E-k⊥coshη)=∫‍d2k⊥dηdngd2k⊥dηk0dCmdmθ(E-k⊥coshη)~∫d2k⊥dηdngd2k⊥dηk0θ(τsc-τF)θ(E-k⊥coshη),
where E is the energy of the parent parton and k0=k⊥coshη, Em+1 denotes energy loss in (m+1)th, collision and Em is the corresponding value in the mth collision. The first θ-function involving scattering time τsc [70] gives a lower limit of k⊥, that is, k⊥>Λcoshη. The second θ-function yields k⊥<E/coshη. Utilizing (23), [63] compares the energy loss (Figure 4) of a gluon jet passing through gluonic plasma obtained by generalized GB formulae given in [61–63]. We will see how (23) can be employed to find out HQ energy loss in the next section.Figure 4
Temperature variation ofΔER, that is, ΔE normalized by the corresponding value obtained from GB approximation, of a 15 GeV gluon due to traversal of 4 fm in a gluonic heat bath. Solid (dashed) line indicates result for the gluon spectrum obtained (Color online) in [61, 63]. The dot-dashed line stands for the results for the gluon spectrum of [62].Figure 5
Drag coefficients of charm assuming running strong coupling,αs(T) and temperature dependent Debye screening mass, mD(T) due to its interaction with thermal gluons, quarks, and antiquarks.


#### 4.2.3.   Energy Loss of HQs and Radiative Drag

While calculating the radiative energy loss of HQs [56] takes the GB distribution for small and moderate x and for mass m≠0. The distribution, so obtained, can be shown to yield GB distribution when x≪1 and when m=0. However the authors of [25, 28] use the original GB distribution formula weighted by a radiative suppression factor, called “dead-cone” factor, originating due to mass of HQs. Since dead-cone factor plays an important role in radiative energy loss of heavy quarks, we will pause here to spend some words on dead-cone effect in QCD [71] and its generalizations.The dead-cone suppression obtained in [71] actually has an analogy with radiated power distribution of a nonrelativistic, accelerating charge particle. The average power radiated per unit solid angle is given by [72]
(24)dPdΩ∝|β→˙|2sin2θ,
where θ is the angle between acceleration β→˙ of the particle and the direction of propagation of radiation, n→. This is a simple sin2θ behaviour showing no radiation at θ=0(ornπ,n∈Z). It can be shown [73] that the behaviour of (24) is, indeed, similar to what one gets for conventional dead cone [71]. Qualitatively speaking, it is hard to cause a deceleration of a high energy heavy quarks along their directions of motion, and this is why the bremsstrahlung radiation is suppressed along this direction (see Figure 24). Indeed, the distribution of radiated gluons emanating from HQs is shown to be related to those from light quarks (LQs) by the following formula [71] (for small radiation angle θ):
(25)dPHQ=(1+θ02θ2)-2dPLQ,
where θ0=m/E, m is the mass, and E is the energy of heavy quark. It is worth noting that when E→∞, the radiation from HQs converges with that of LQs. However, there are some very recent developments in generalizing the dead-cone effect either from Qq→Qqg matrix element or assuming effects of off-shellness of quarks which, after being produced, take some time to become on-shell. The details may be seen in the Appendix.The radiative transport coefficients like drag and diffusion have been evaluated in [25, 28] employing potential approach. The radiative drag coefficient can be obtained by finding out the radiative energy loss of HQs passing through medium. The GB spectrum for emitted gluon, weighted by the dead-cone factor and the energy of gluon k0  (=k⊥coshη), is integrated over the transverse momentum (k⊥) and rapidity (η) of the emitted gluon. As already stated, the lower and upper limits of k⊥ have been obtained from the θ-functions of (23). The average energy loss per collision, ΔErad, can be written with the help of (23) as
(26)ΔErad=〈ngk0〉=∫‍dηd2k⊥dngdηd2k⊥×k0Θ(τsc-τF)Θ(E-k⊥coshη)FDC2,where the formation time of the emitted gluon [49], τF=(CA/2C2)2coshη/k⊥, and CA/2C2=N2/(N2-1) for quarks with C2=CF=4/3. Dead-cone factor of (25) (dPHQ/dPLQ) can be written in the following way, provided one replaces k⊥/k0~sinθ~θ for small θ:
(27)FDC2=(k⊥2k02θ02+k⊥2)2,
where dE/dx can be obtained if one multiplies ΔErad with Λ, which can be obtained from [70]. Drag (γ) and diffusion (D=γmT) coefficients in [28] are functions of momentum as well as temperature. The effective drag can be obtained by adding collisional and radiative drags, γeff=γcoll+γrad. The momentum dependence of the drag coefficient of the charm quark propagating through the QGP is displayed in Figure 5 for T=300MeV. For pT~2 GeV of charm quarks the collisional and radiative contributions tend to merge with each other. However, it is interesting to note the dominance of radiative drag in Figure 5 over its collisional counterpart for higher momentum.


## 4.1.  Elastic Processes

We need to evaluate the drag coefficient as a function of temperature and momentum of HQ. The expression to evaluate collisional drag can be written as [37]
(15)γcoll=12Ep∫‍d3q(2π)32Eq∫‍d3q′(2π)32Eq′×∫‍d3p′(2π)32Ep′1γQ∑‍|M|2×(2π)4δ4(p+q-p′-q′)f′(q)[1-p·p′p2],
where p′=p-k and q′=q+k. The scattering matrix elements are given explicitly in [41] where the t channel divergence occurring due to very soft gluon exchange has been shielded by replacing t in the denominator of matrix elements by t-mD2 in an ad hoc manner, where t is the Mandelstam variable and mD is the thermal mass of gluon. However, the same problem can be approached from hard thermal loop (HTL) perturbation theory. The gluon propagator for t channel diagram is then replaced by HTL propagator. But calculation of even the elastic matrix elements for HQs scattering with light quarks and gluons in this approach is very lengthy and radiative matrix elements are even clumsier. However, an outline of calculating collisional drag and diffusion coefficients in HTL perturbation theory approach is given for the interested readers in the Appendix. Here we proceed with the conventional process of shielding t channel divergence with the gluon thermal mass, mD.The integrations in (15) have been performed using the standard techniques [23–25, 37]. Results of the present calculation of drag coefficients are plotted with respect to momenta of charm and bottom (Figure 1). We can observe that the momentum dependence of γ is nonnegligible for the shown momentum range. The value of γ for charm due to collisional processes at p=5 GeV is about 0.036 fm−1 which reduces to a value of 0.018 fm−1 at p=10 GeV (can be compared with, e.g., [42–45]). In the inset, the drag coefficients of HQs due to elastic collision are plotted in the lower momentum region, where the drag remains more or less constant with p. Therefore, it is clear that had we taken the value of drag at low momentum and extrapolated that value to higher momentum, the final result would have been overestimated. The diffusion coefficient of HQ can be evaluated from Einstein relation D=γMT. Later, it will be seen that the momentum dependence of transport coefficients of a high energy HQ will have considerable effects on the nuclear suppression factor, RAA of HQ.Figure 1
Drag coefficients of charm and bottom with their momentum atT=300 MeV assuming running strong coupling, αs(T), and temperature-dependent Debye screening mass, mD(T) for gluon, quark, and antiquark scattering.However, there are also radiative processes taking place in the medium. The radiative drag can be obtained in the present formalism by computing the radiative energy loss of a HQ passing through QGP. In the next section, we will discuss the methods and intricacies of calculating the transport coefficients in radiative domain.


## 4.2. Radiative Processes

It is already known that the two mechanisms of energy loss of heavy quarks are collisional and radiative energy loss. Though at low transverse momentum (pT) region the collisional and radiative losses of HQs are comparable (see Figure 5), the radiative one tends to dominate with increasing momentum. So it is worthwhile, after discussing about evaluation of collisional transport coefficients (drag, diffusion) within the ambit of T=0 perturbative QCD (pQCD) [37] in the previous section, to contemplate on radiative processes and to inspect how pQCD approach can be utilized in finding out radiative transport coefficients.It is well known from QED that for high energies radiative energy loss becomes dominant [46]. Also the hard thermal loop calculations in context of QCD show that the radiative energy loss contributes to the same order of strong coupling as that of collisional loss [47]. The above discussions tempt one to infer that the observed large suppression of heavy quarks at RHIC is predominantly due to bremsstrahlung processes [25, 28], at least at high pT, but, as already said, the comparable values of energy losses in low momentum region leave an ample room for ambiguity in this statement particularly at “not-so-high” pT   (~2 GeV) region.Theoretical estimate of nuclear suppression factor (RAA) for charm quarks in [28] shows ~4 times more suppression due to inclusion of radiation (Figure 2) with the same initial conditions. Inclusion of radiative processes leads to a good description of charm RAA at RHIC energies. At LHC energy (2.76 TeV/A), the collisional energy loss due to hard (momentum transfer > 2 GeV) collisions could be about one-third of the total [29]. The rest may be attributed to radiative loss.Figure 2
Suppression of transverse momentum of charm quarks in QGP as a functionpT.From the discussion on collisional transport coefficients we know that (collisional) drag is given by [37]
(16)γcoll=-1p(dEdx)coll,
where p is the momentum of the probe. We employ a similar argument and relate radiative energy loss ((dE/dx)rad) to inelastic drag (γrad) in the same way. The effective drag obtained is a summation of collisional and radiative parts. As energy loss is related with the transport properties like drag offered by the medium, we must concentrate on more and more accurate determination of radiative energy loss which will enable us to understand the properties of QGP.The radiative energy loss in general has been studied in [48–53] incorporating Landau-Pomeranchuk-Migdal (LPM) effect [54, 55], to be discussed later, due to multiple scattering. The authors of [56] treat the problem of radiative energy loss of HQs by building a model in scalar QCD approach. Attempts have been taken [25, 28, 30] to incorporate the formalism of [48], the Gyulassy-Wang potential model (GWPM), to compute the radiative energy loss of heavy quarks traveling through QGP. In stead of going to the energy loss in GWPM approach directly, we may try to acquire some familiarity with the model and the notations used for the description of it.


### 4.2.1. Gyulassy Wang Potential Model (GWPM) and Radiation Spectrum

To analyze the multiple scattering and the induced gluon radiation in GWPM, certain simplifications have to be made. For example, [49] assumes static interaction between propagating parton and bath particles. This interaction is modelled by a static Debye screening potential. Now, it is possible to approximate the effective average random colour field produced by the bath particles by a potential provided the distance between two successive scatterers is large compared to colour screening length (μ-1). The screened potential is given by
(17)VAA′a(q→)=AAA′a(q→)e-iq→·x→=gTAA′ae-iq→·x→q→2+μ2,
where μ is the colour screening mass, Ta are the generators corresponding to the representation of target partons at x→ transferring (three) momentum q→, and g is the coupling. The Feynman diagrams contributing to induced gluon radiation from a single quark-quark scattering are given in Figure 3.Figure 3
Feynman diagrams for gluon radiation from a single quark-quark (qq′) scattering.The calculation of Feynman diagrams is done in light-cone coordinates [57, 58] where light-cone representations of four-vectors are done in the following way:
(18)p=(p+,p-,p⊥)=(p0+p3,p0-p3,p⊥).
In the potential model, one can neglect radiation from the target lines (external lines appearing at the bottom of gluon propagator in Figure 3) provided one decides to work in light cone gauge, A+=0, for emitted gluon fields (For details see [59]). For radiation in the midrapidity region, the celebrated Gunion-Bertsch distribution formula (GB formula) for soft gluon radiation [60] can be reproduced from this approach.We now discuss the GB formula and recent attempts to generalize. (Interested readers are referred to the Appendix for other such very recent endeavors.) We will see that this formula and/or its generalizations will play a significant role in finding out the energy loss due to gluon emission of quarks. The early attempts of generalizing GB formula [61–63] consider the general gg→ggg matrix element elegantly written by [64] and factoring out the elastic gg→gg scattering amplitude to obtain the distribution of emitted gluon (~|Mgg→ggg|2/|Mgg→gg|2). We can write the following form of gluon distribution [63], (for details, see Appendix of [63])
(19)dngd2k⊥dη=[dngd2k⊥dη]GB[(1+t2s+5t22s2-t3s3)-(32s+4tss-3t22s2s)k⊥+(52s+t2s2+5t2s3)k⊥2],
where η is the rapidity of the radiated gluon, and the subscript GB has been used to indicate the gluon spectrum obtained using the approximation considered in [60] (see also [65]) which is generally given by
(20)[dngd2k⊥dη]GB=CAαsπ2q⊥2k⊥2[(k→⊥-q→⊥)2+mD2],
where mD=(2π/3)αs(T)(CA+NF/2)T is the thermal mass of the gluon [66, 67], NF is the number of flavours contributing in the gluon self-energy loop, CA=3 is the Casimir invariant for the SU(3) adjoint representation, αs is the temperature-dependent strong coupling [68], k⊥ is the transverse momentum of the emitted gluon, and q⊥ is the transverse momentum transfer. In general, one introduces the thermal mass in the denominator of (20) to shield the divergence arising from collinearity (i.e., when k→⊥=q→⊥) of emitted gluon. But, for the present case (19) is written under the assumption that q→⊥≫k→⊥, and hence there is no need to write the mD2 factor in GB spectrum.Now, we must consider multiple scattering encountered by incoming particles, too. The many-body effect due to presence of medium results in interference of scattering amplitudes. This interference effect is called LPM effect [54, 55]. LPM effect is discussed in QED domain in [69]. LPM suppression can be understood in a qualitative manner as an interplay between two time scales, the formation time (τf) and the scattering time (τsc). τF is the time needed for the emission of the induced gluon. Actually, τf determines the time span after which a radiation can be separately identified from the parent parton from which the radiation is being given off. Now, a collision just before formation of the gluon results in suppression of the radiation. This destructive interference is called LPM effect and in the next section we will see that it puts a constraint on the phase space of the emitted gluon. If k0 is the energy of the emitted (soft) gluon and k⊥ is its transverse momentum, then formation time τf~2k0/k⊥2 [49]. When this τf is much less than collision time, that is, separation between two scattering centres, L, then the intensity of radiation is additive and Bethe-Heitler (BH) limit is reached. In the factorization limit, L≪τf, the interference in radiation amplitude takes place and LPM effect dominates.After this brief discussion on possible effects of multiple scattering, emitted gluon distribution due to multiple scattering in terms of that due to single scattering can be written as [49]
(21)dng(m)d2k⊥dη=Cm(k)dng(1)d2k⊥dη,
where “m” stands for multiple scattering and “1” stands for single scattering. Cm is called radiation formation factor characterizing the interference pattern due to multiple scattering. Naturally, in the BH limit, Cm≈m, which implies that scattering amplitudes are just additive and the resultant intensity shows no interference pattern. On the other hand, the factorization limit gives [49, 50]
(22)Cm(k)≈89[1-(-18)m]forquarks≈2(1-12m)forgluons.
Equation (22) shows that the interference effect due to many multiple scatterings for quarks leaves corresponding radiation spectrum a factor of ~8/9 of that due to single scattering. It can also be checked that the gluon intensity radiated by gluon jet is 9/4 times higher than that radiated by quark jets in multiple scattering. Thus, the LPM effect in QCD depends on colour representation due to nonabelian nature of the problem under discussion.


### 4.2.2. Energy Loss of Light Particles in GWPM

The energy loss in potential model can be evaluated by integrating over the transverse momentum and rapidity of emitted gluon. The phase space is, of course, constrained by LPM effect due to multiple scattering. The multiple scattering is implemented through the differential change of the factorCm with number of scattering m, dCm/dm, which can be approximated as a θ-function [49]. Also, the emitted gluon must have energy less than that of the parent parton. So the energy loss formula implementing the energy constraint gives
(23)ΔErad=Em+1-Emm=∫‍d2k⊥dηdngd2k⊥dηk0Cm+1-Cmmθ(E-k⊥coshη)=∫‍d2k⊥dηdngd2k⊥dηk0dCmdmθ(E-k⊥coshη)~∫d2k⊥dηdngd2k⊥dηk0θ(τsc-τF)θ(E-k⊥coshη),
where E is the energy of the parent parton and k0=k⊥coshη, Em+1 denotes energy loss in (m+1)th, collision and Em is the corresponding value in the mth collision. The first θ-function involving scattering time τsc [70] gives a lower limit of k⊥, that is, k⊥>Λcoshη. The second θ-function yields k⊥<E/coshη. Utilizing (23), [63] compares the energy loss (Figure 4) of a gluon jet passing through gluonic plasma obtained by generalized GB formulae given in [61–63]. We will see how (23) can be employed to find out HQ energy loss in the next section.Figure 4
Temperature variation ofΔER, that is, ΔE normalized by the corresponding value obtained from GB approximation, of a 15 GeV gluon due to traversal of 4 fm in a gluonic heat bath. Solid (dashed) line indicates result for the gluon spectrum obtained (Color online) in [61, 63]. The dot-dashed line stands for the results for the gluon spectrum of [62].Figure 5
Drag coefficients of charm assuming running strong coupling,αs(T) and temperature dependent Debye screening mass, mD(T) due to its interaction with thermal gluons, quarks, and antiquarks.


### 4.2.3.   Energy Loss of HQs and Radiative Drag

While calculating the radiative energy loss of HQs [56] takes the GB distribution for small and moderate x and for mass m≠0. The distribution, so obtained, can be shown to yield GB distribution when x≪1 and when m=0. However the authors of [25, 28] use the original GB distribution formula weighted by a radiative suppression factor, called “dead-cone” factor, originating due to mass of HQs. Since dead-cone factor plays an important role in radiative energy loss of heavy quarks, we will pause here to spend some words on dead-cone effect in QCD [71] and its generalizations.The dead-cone suppression obtained in [71] actually has an analogy with radiated power distribution of a nonrelativistic, accelerating charge particle. The average power radiated per unit solid angle is given by [72]
(24)dPdΩ∝|β→˙|2sin2θ,
where θ is the angle between acceleration β→˙ of the particle and the direction of propagation of radiation, n→. This is a simple sin2θ behaviour showing no radiation at θ=0(ornπ,n∈Z). It can be shown [73] that the behaviour of (24) is, indeed, similar to what one gets for conventional dead cone [71]. Qualitatively speaking, it is hard to cause a deceleration of a high energy heavy quarks along their directions of motion, and this is why the bremsstrahlung radiation is suppressed along this direction (see Figure 24). Indeed, the distribution of radiated gluons emanating from HQs is shown to be related to those from light quarks (LQs) by the following formula [71] (for small radiation angle θ):
(25)dPHQ=(1+θ02θ2)-2dPLQ,
where θ0=m/E, m is the mass, and E is the energy of heavy quark. It is worth noting that when E→∞, the radiation from HQs converges with that of LQs. However, there are some very recent developments in generalizing the dead-cone effect either from Qq→Qqg matrix element or assuming effects of off-shellness of quarks which, after being produced, take some time to become on-shell. The details may be seen in the Appendix.The radiative transport coefficients like drag and diffusion have been evaluated in [25, 28] employing potential approach. The radiative drag coefficient can be obtained by finding out the radiative energy loss of HQs passing through medium. The GB spectrum for emitted gluon, weighted by the dead-cone factor and the energy of gluon k0  (=k⊥coshη), is integrated over the transverse momentum (k⊥) and rapidity (η) of the emitted gluon. As already stated, the lower and upper limits of k⊥ have been obtained from the θ-functions of (23). The average energy loss per collision, ΔErad, can be written with the help of (23) as
(26)ΔErad=〈ngk0〉=∫‍dηd2k⊥dngdηd2k⊥×k0Θ(τsc-τF)Θ(E-k⊥coshη)FDC2,where the formation time of the emitted gluon [49], τF=(CA/2C2)2coshη/k⊥, and CA/2C2=N2/(N2-1) for quarks with C2=CF=4/3. Dead-cone factor of (25) (dPHQ/dPLQ) can be written in the following way, provided one replaces k⊥/k0~sinθ~θ for small θ:
(27)FDC2=(k⊥2k02θ02+k⊥2)2,
where dE/dx can be obtained if one multiplies ΔErad with Λ, which can be obtained from [70]. Drag (γ) and diffusion (D=γmT) coefficients in [28] are functions of momentum as well as temperature. The effective drag can be obtained by adding collisional and radiative drags, γeff=γcoll+γrad. The momentum dependence of the drag coefficient of the charm quark propagating through the QGP is displayed in Figure 5 for T=300MeV. For pT~2 GeV of charm quarks the collisional and radiative contributions tend to merge with each other. However, it is interesting to note the dominance of radiative drag in Figure 5 over its collisional counterpart for higher momentum.


## 4.2.1. Gyulassy Wang Potential Model (GWPM) and Radiation Spectrum

To analyze the multiple scattering and the induced gluon radiation in GWPM, certain simplifications have to be made. For example, [49] assumes static interaction between propagating parton and bath particles. This interaction is modelled by a static Debye screening potential. Now, it is possible to approximate the effective average random colour field produced by the bath particles by a potential provided the distance between two successive scatterers is large compared to colour screening length (μ-1). The screened potential is given by
(17)VAA′a(q→)=AAA′a(q→)e-iq→·x→=gTAA′ae-iq→·x→q→2+μ2,
where μ is the colour screening mass, Ta are the generators corresponding to the representation of target partons at x→ transferring (three) momentum q→, and g is the coupling. The Feynman diagrams contributing to induced gluon radiation from a single quark-quark scattering are given in Figure 3.Figure 3
Feynman diagrams for gluon radiation from a single quark-quark (qq′) scattering.The calculation of Feynman diagrams is done in light-cone coordinates [57, 58] where light-cone representations of four-vectors are done in the following way:
(18)p=(p+,p-,p⊥)=(p0+p3,p0-p3,p⊥).
In the potential model, one can neglect radiation from the target lines (external lines appearing at the bottom of gluon propagator in Figure 3) provided one decides to work in light cone gauge, A+=0, for emitted gluon fields (For details see [59]). For radiation in the midrapidity region, the celebrated Gunion-Bertsch distribution formula (GB formula) for soft gluon radiation [60] can be reproduced from this approach.We now discuss the GB formula and recent attempts to generalize. (Interested readers are referred to the Appendix for other such very recent endeavors.) We will see that this formula and/or its generalizations will play a significant role in finding out the energy loss due to gluon emission of quarks. The early attempts of generalizing GB formula [61–63] consider the general gg→ggg matrix element elegantly written by [64] and factoring out the elastic gg→gg scattering amplitude to obtain the distribution of emitted gluon (~|Mgg→ggg|2/|Mgg→gg|2). We can write the following form of gluon distribution [63], (for details, see Appendix of [63])
(19)dngd2k⊥dη=[dngd2k⊥dη]GB[(1+t2s+5t22s2-t3s3)-(32s+4tss-3t22s2s)k⊥+(52s+t2s2+5t2s3)k⊥2],
where η is the rapidity of the radiated gluon, and the subscript GB has been used to indicate the gluon spectrum obtained using the approximation considered in [60] (see also [65]) which is generally given by
(20)[dngd2k⊥dη]GB=CAαsπ2q⊥2k⊥2[(k→⊥-q→⊥)2+mD2],
where mD=(2π/3)αs(T)(CA+NF/2)T is the thermal mass of the gluon [66, 67], NF is the number of flavours contributing in the gluon self-energy loop, CA=3 is the Casimir invariant for the SU(3) adjoint representation, αs is the temperature-dependent strong coupling [68], k⊥ is the transverse momentum of the emitted gluon, and q⊥ is the transverse momentum transfer. In general, one introduces the thermal mass in the denominator of (20) to shield the divergence arising from collinearity (i.e., when k→⊥=q→⊥) of emitted gluon. But, for the present case (19) is written under the assumption that q→⊥≫k→⊥, and hence there is no need to write the mD2 factor in GB spectrum.Now, we must consider multiple scattering encountered by incoming particles, too. The many-body effect due to presence of medium results in interference of scattering amplitudes. This interference effect is called LPM effect [54, 55]. LPM effect is discussed in QED domain in [69]. LPM suppression can be understood in a qualitative manner as an interplay between two time scales, the formation time (τf) and the scattering time (τsc). τF is the time needed for the emission of the induced gluon. Actually, τf determines the time span after which a radiation can be separately identified from the parent parton from which the radiation is being given off. Now, a collision just before formation of the gluon results in suppression of the radiation. This destructive interference is called LPM effect and in the next section we will see that it puts a constraint on the phase space of the emitted gluon. If k0 is the energy of the emitted (soft) gluon and k⊥ is its transverse momentum, then formation time τf~2k0/k⊥2 [49]. When this τf is much less than collision time, that is, separation between two scattering centres, L, then the intensity of radiation is additive and Bethe-Heitler (BH) limit is reached. In the factorization limit, L≪τf, the interference in radiation amplitude takes place and LPM effect dominates.After this brief discussion on possible effects of multiple scattering, emitted gluon distribution due to multiple scattering in terms of that due to single scattering can be written as [49]
(21)dng(m)d2k⊥dη=Cm(k)dng(1)d2k⊥dη,
where “m” stands for multiple scattering and “1” stands for single scattering. Cm is called radiation formation factor characterizing the interference pattern due to multiple scattering. Naturally, in the BH limit, Cm≈m, which implies that scattering amplitudes are just additive and the resultant intensity shows no interference pattern. On the other hand, the factorization limit gives [49, 50]
(22)Cm(k)≈89[1-(-18)m]forquarks≈2(1-12m)forgluons.
Equation (22) shows that the interference effect due to many multiple scatterings for quarks leaves corresponding radiation spectrum a factor of ~8/9 of that due to single scattering. It can also be checked that the gluon intensity radiated by gluon jet is 9/4 times higher than that radiated by quark jets in multiple scattering. Thus, the LPM effect in QCD depends on colour representation due to nonabelian nature of the problem under discussion.


## 4.2.2. Energy Loss of Light Particles in GWPM

The energy loss in potential model can be evaluated by integrating over the transverse momentum and rapidity of emitted gluon. The phase space is, of course, constrained by LPM effect due to multiple scattering. The multiple scattering is implemented through the differential change of the factorCm with number of scattering m, dCm/dm, which can be approximated as a θ-function [49]. Also, the emitted gluon must have energy less than that of the parent parton. So the energy loss formula implementing the energy constraint gives
(23)ΔErad=Em+1-Emm=∫‍d2k⊥dηdngd2k⊥dηk0Cm+1-Cmmθ(E-k⊥coshη)=∫‍d2k⊥dηdngd2k⊥dηk0dCmdmθ(E-k⊥coshη)~∫d2k⊥dηdngd2k⊥dηk0θ(τsc-τF)θ(E-k⊥coshη),
where E is the energy of the parent parton and k0=k⊥coshη, Em+1 denotes energy loss in (m+1)th, collision and Em is the corresponding value in the mth collision. The first θ-function involving scattering time τsc [70] gives a lower limit of k⊥, that is, k⊥>Λcoshη. The second θ-function yields k⊥<E/coshη. Utilizing (23), [63] compares the energy loss (Figure 4) of a gluon jet passing through gluonic plasma obtained by generalized GB formulae given in [61–63]. We will see how (23) can be employed to find out HQ energy loss in the next section.Figure 4
Temperature variation ofΔER, that is, ΔE normalized by the corresponding value obtained from GB approximation, of a 15 GeV gluon due to traversal of 4 fm in a gluonic heat bath. Solid (dashed) line indicates result for the gluon spectrum obtained (Color online) in [61, 63]. The dot-dashed line stands for the results for the gluon spectrum of [62].Figure 5
Drag coefficients of charm assuming running strong coupling,αs(T) and temperature dependent Debye screening mass, mD(T) due to its interaction with thermal gluons, quarks, and antiquarks.


## 4.2.3.   Energy Loss of HQs and Radiative Drag

While calculating the radiative energy loss of HQs [56] takes the GB distribution for small and moderate x and for mass m≠0. The distribution, so obtained, can be shown to yield GB distribution when x≪1 and when m=0. However the authors of [25, 28] use the original GB distribution formula weighted by a radiative suppression factor, called “dead-cone” factor, originating due to mass of HQs. Since dead-cone factor plays an important role in radiative energy loss of heavy quarks, we will pause here to spend some words on dead-cone effect in QCD [71] and its generalizations.The dead-cone suppression obtained in [71] actually has an analogy with radiated power distribution of a nonrelativistic, accelerating charge particle. The average power radiated per unit solid angle is given by [72]
(24)dPdΩ∝|β→˙|2sin2θ,
where θ is the angle between acceleration β→˙ of the particle and the direction of propagation of radiation, n→. This is a simple sin2θ behaviour showing no radiation at θ=0(ornπ,n∈Z). It can be shown [73] that the behaviour of (24) is, indeed, similar to what one gets for conventional dead cone [71]. Qualitatively speaking, it is hard to cause a deceleration of a high energy heavy quarks along their directions of motion, and this is why the bremsstrahlung radiation is suppressed along this direction (see Figure 24). Indeed, the distribution of radiated gluons emanating from HQs is shown to be related to those from light quarks (LQs) by the following formula [71] (for small radiation angle θ):
(25)dPHQ=(1+θ02θ2)-2dPLQ,
where θ0=m/E, m is the mass, and E is the energy of heavy quark. It is worth noting that when E→∞, the radiation from HQs converges with that of LQs. However, there are some very recent developments in generalizing the dead-cone effect either from Qq→Qqg matrix element or assuming effects of off-shellness of quarks which, after being produced, take some time to become on-shell. The details may be seen in the Appendix.The radiative transport coefficients like drag and diffusion have been evaluated in [25, 28] employing potential approach. The radiative drag coefficient can be obtained by finding out the radiative energy loss of HQs passing through medium. The GB spectrum for emitted gluon, weighted by the dead-cone factor and the energy of gluon k0  (=k⊥coshη), is integrated over the transverse momentum (k⊥) and rapidity (η) of the emitted gluon. As already stated, the lower and upper limits of k⊥ have been obtained from the θ-functions of (23). The average energy loss per collision, ΔErad, can be written with the help of (23) as
(26)ΔErad=〈ngk0〉=∫‍dηd2k⊥dngdηd2k⊥×k0Θ(τsc-τF)Θ(E-k⊥coshη)FDC2,where the formation time of the emitted gluon [49], τF=(CA/2C2)2coshη/k⊥, and CA/2C2=N2/(N2-1) for quarks with C2=CF=4/3. Dead-cone factor of (25) (dPHQ/dPLQ) can be written in the following way, provided one replaces k⊥/k0~sinθ~θ for small θ:
(27)FDC2=(k⊥2k02θ02+k⊥2)2,
where dE/dx can be obtained if one multiplies ΔErad with Λ, which can be obtained from [70]. Drag (γ) and diffusion (D=γmT) coefficients in [28] are functions of momentum as well as temperature. The effective drag can be obtained by adding collisional and radiative drags, γeff=γcoll+γrad. The momentum dependence of the drag coefficient of the charm quark propagating through the QGP is displayed in Figure 5 for T=300MeV. For pT~2 GeV of charm quarks the collisional and radiative contributions tend to merge with each other. However, it is interesting to note the dominance of radiative drag in Figure 5 over its collisional counterpart for higher momentum.


## 5. Space-Time Evolution

Once we know the total drag (γeff) and diffusion (Deff) coefficients, we need the initial conditions for both the HQ and the QGP background which is also evolving with time as follows.
(28)∂μTμν=0,
where Tμν=(ϵ+P)uμuν-gμνP is the energy momentum tensor for ideal fluid, ϵ is the energy density, P is the pressure, uμ is the hydrodynamic four velocity, and gμν is the metric tensor. We will solve this equation for longitudinal expansion assuming boost invariance along the z direction [74]. It is expected that the central rapidity region of the system formed after nuclear collisions at RHIC and LHC energy is almost net baryon free. Therefore, the equation governing the conservation of net baryon number need not be considered here. Under this circumstance, (28) reduces to:
(29)∂ϵ∂τ+ϵ+Pτ=0.
To solve (29), we use the relation P=cs2ϵ, where cs is the velocity of sound and τ is time. With this we arrive at the relation
(30)ϵτ1+cs2=C,
where C is a constant. For the ideal equation of state, cs2=1/3, (30) reads ϵτ4/3=C. In terms of temperature, this relation can be written as τT3=C.The total amount of energy dissipated by a heavy quark in the expanding QGP depends on the path length it traverses. Each parton traverses a different path length which depends on the geometry of the system and on the point where it is created. The probability that a parton is produced at a point (r,ϕ) in the plasma depends on the number of binary collisions at that point which can be taken as [75]
(31)P(r,ϕ)=2πR2(1-r2R2)θ(R-r),
where R is the nuclear radius. It should be mentioned here that the expression in (31) is an approximation for the collisions with zero impact parameter. A very high energy parton created at (r,ϕ) in the transverse plane propagates a distance L=R2-r2sin2ϕ-rcosϕ in the medium provided its direction remains unaltered. In the present work, we use the following equation for the geometric average of the integral involving drag coefficient
(32)Γ=∫‍rdrdϕP(r,ϕ)∫L/vdτγ(τ)∫‍rdrdϕP(r,ϕ),
where v is the velocity of the propagating partons. Similar averaging has been performed for the diffusion coefficient, too. For a static system, the temperature dependence of the drag and diffusion coefficients of the heavy quarks enters via the thermal distributions of light quarks and gluons through which it is propagating. However, in the present scenario, the variation of temperature with time is governed by the equation of state or velocity of sound of the thermalized system undergoing hydrodynamic expansion. In such a scenario the quantities like Γ (32) and hence the HQ suppression become sensitive to cs.


## 6. Initial Conditions and Nuclear Modification Factor

In order to solve (11), the initial distribution functions, fin(pT,t) for charm and bottom quarks have been supplied from the well-known MNR code [39]. The ratio between the solution of Fokker-Planck equation at the transition temperature, Tc=175MeV, and the initial distribution function of HQ is the required nuclear modification factor, RAA of open HQ. But in order to compare results from the present formalism with experimental data from RHIC and LHC, we need to have RAA of nonphotonic single electron originating from the decays of D and B mesons.Therefore, the hadronization of charm and bottom quarks toD and B mesons, respectively, are done by using Peterson fragmentation function [76] as
(33)f(z)∝1[z[z-1/z-ϵQ/(1-z)]2],
where z is the fraction of momentum carried by the hadrons and ϵQ is 0.05 for charm and (mc/mb)2ϵQ for bottom where mc(mb) is the mass of charm (bottom). One may use different kinds of available fragmentation functions, but the final result will not be sensitive to the choice of f(z). In point of fact, describing hadronization has always been a formidable task because the hadron bound states are nonperturbative in nature. The fragmentation scheme is a popular method to deal with hadronization. For more relevant information, interested readers are referred to [77, 78], which discusses coalescence model, a hadronization formulation based on recombination of heavy flavours.Both the final solution of FP Equation and the initial distribution of HQ are convoluted with the above fragmentation function (33) and their ratio will give RAA of D and B mesons. The final and initial distribution functions are obtained for the single electrons originated from the decays of D and B mesons and the final nuclear modification factor is
(34)RAAD(B)→e=fD(B)→e(pT,Tc)fD(B)→e(pT,Ti).
Now, once we know how to determine RAA, we will compare our results with the RHIC and LHC data. In the process of doing so, we will study the effects of the equation of state on the nuclear suppression of heavy flavours in quark gluon plasma and estimate the initial entropy density of the QGP formed at the RHIC. For this purpose, the experimental data on the charged particle multiplicity and the nuclear suppression of single electron spectra originating from the semileptonic decays of D and B mesons have been employed. We have used inputs from lattice QCD (LQCD) to minimize the model dependence of the results.The initial entropy density and the thermalization time (τi) for the QGP can be constrained to the measured (final) multiplicity by the following relation [79] which is boost invariant:
(35)siτi=κ1A⊥dNdy,
where A⊥ is the transverse area of the system which can be determined from the collision geometry and κ is a known constant (=3.7 for massless Bosons).Equation of state (EoS), which is taken asP=cs2ϵ for almost baryon free QGP expected at RHIC energy, also has its own role to play in the space-time evolution of QGP. This sound velocity squared appearing in the EoS shows a significant variation with temperature in LQCD calculations (Figure 6).Figure 6
Velocity of sound squared as a function temperature [8].In Figure10, we show the variation of Ti with cs2 obtained by constraints imposed by the experimental data on RAA and dN/dy. The value of Ti varies from 210 to 300 MeV depending on the value of cs. It is interesting to note that the lowest value of Ti obtained from the present analysis is well above the quark-hadron phase transition temperature, indicating the fact that the system formed in Au+Au collisions at sNN=200 GeV might be formed in the partonic phase.The data forRAA of D0 from ALICE are also contrasted with our results (see Figure 11) when cs2=1/3 [12]. The EoS sets the expansion time scale for the system as τexp~[(1/ϵ)dϵ/dτ]-1~τ/(1+cs2) indicating the fact that lower value of cs makes the expansion time scale longer, that is, the rate of expansion is slower. This issue is further discussed in details in [80].The result ofRAA, where the EoS contains temperature-dependent cs2, is displayed in Figure 7. Here, the experimentally measured suppression [13, 14] has been reproduced reasonably well at Ti=250MeV and τi=0.83 fm/c. The Ti value, however, may increase if we take into account the transverse expansion because the inflation dilutes the medium. Now, we know that larger cs makes the QGP life time smaller leading to lesser suppression of HQ propagating through QGP for a shorter time. Therefore, when the value of cs2 is taken to be 1/3, that is, the highest possible value, the maximum value of Ti, which is in this analysis 300 MeV, will be reached.  The value of si at cs2=1/3 is ~59/fm3. These values of Ti and si are considered as the highest values of them admitted by the data. The result for the highest value of cs2 is illustrated in Figure 8. Likewise, the result of RAA in the case when cs2=1/5 is depicted in Figure 9. In this case, the data is well reproduced at Ti=210MeV and si~19.66/fm3.Figure 7
Variation ofRAA with pT for the space time evolution with initial condition Ti=250MeV and τi=0.84 fm/c and the EoS which includes the variation of cs with T (colour online).Figure 8
Variation ofRAA with pT for for cs2=1/3 and Ti=300MeV (colour online).Figure 9
Variation ofRAA with pT for cs2=1/5 and Ti=210MeV (colour online).Figure 10
The variation ofTi with cs2 for fixed dN/dy.Figure 11
R
A
A as a function of pT for D and B mesons at LHC. Experimental data taken from [16].Now, it is expected that the central rapidity region of the system formed after nuclear collisions at high energy RHIC and LHC run is almost net baryon free. Therefore, the equation governing the conservation of net baryon number need not be considered here and all our calculations are valid for zero baryonic chemical potential (μB=0) cases. One may be interested in calculating transport coefficients in μB≠0 case which may be of importance in low energy RHIC run [81, 82] and GSI-FAIR [83]. This aspect is discussed in the next section.


## 7. Drag and Diffusion at Finite Baryonic Chemical Potential

The nuclear collisions at low energy RHIC run [81, 82] and GSI-FAIR [83] are expected to create a thermal medium with large baryonic chemical potential (μB) and moderate temperature (T). So the effect of baryonic chemical potential (μB) on the transport coefficients of HQ should also be taken into account. Both the temperature (T) and quark chemical potential, μ (=μB/3), dependence of drag enter through the thermal distribution.The variation of the drag coefficients of charm quarks (due to its interactions with quarks and antiquarks) with the baryonic chemical potential for differentT is displayed in Figure 12. The drag coefficient for the process: Qg→Qg is ~8.42×10-3 fm−1 (1.86×10-2 fm−1) for T=140MeV (190 MeV) (not displayed in Figure 12). The T and μ dependence of the drag and diffusion coefficients may be understood as follows. As discussed earlier, the drag may be defined as the thermal average of the momentum transfer weighted by the square of the invariant transition amplitude for the reactions qQ→qQ,  Qq-→Qq-, and gQ→gQ.Figure 12
Variation of the drag coefficient of charm quark due to its interactions with light quarks and anti-quarks as a function ofμ for different temperatures.The average momentum of the quarks of the thermal bath increases with bothT and μ. The increase in average momenta enables the thermal quarks to transfer larger momentum and thus, in turn, enhances the drag coefficient. This trend is clearly observed in the results displayed in Figure 12 for charm quark. The drag due to the process Qq→Qq is larger than the Qq-→Qq- interaction because of non-zero chemical potential, the Q propagating through which the medium encounters more q than q- at a given μ. For vanishing chemical potential, the contributions from quarks and anti-quarks are same.In the same way, it may be argued that the diffusion coefficient involves the square of the momentum transfer, which should also increase withT and μ as observed in Figure 13. The diffusion coefficient for charm quarks due to its interaction with gluons is given by ~1.42×10-3 GeV2/fm (4.31×10-3 GeV2/fm) for T=140MeV (190 MeV). The drag and diffusion coefficients for bottom quarks are displayed in Figures 14 and 15, respectively, showing qualitatively similar behaviour as charm quarks. The drag coefficients for bottom quarks due to the process Qg→Qg are given by ~3.15×10-3 fm−1 and 6.93×10-3 fm−1 at T=140 MeV and 190 MeV respectively. The corresponding diffusion coefficients are ~1.79×10-3 GeV2/fm and 5.38×10-3 GeV2/fm at T=140 MeV and 190 MeV, respectively. The μ dependent drag and diffusion coefficients will be used later to evaluate the nuclear suppression of the heavy flavours for low energy RHIC experiments.Figure 13
Variation of the diffusion coefficient of charm quark due to its interactions with light quarks and anti-quarks as a function ofμ for different temperatures.Figure 14
The same as Figure12 for bottom quark.Figure 15
The same as Figure13 for bottom quark.


## 8. Nuclear Suppression in Baryon Rich QGP

For low energy collisions, the radiative energy loss of heavy quarks will be much smaller than the loss caused by elastic processes as they are produced with very low momentum. Moreover, the thermal production of charm and bottom quarks can be ignored in the range of temperature and baryonic chemical potential under study. Therefore, we can apply the FP equation for the description of HQ evolution in the baryon rich QGP. Here we need to solve the FP equation for non-zeroμB. The drag and diffusion coefficients are functions of both the thermodynamical variables: μB and T.The energy (sNN) dependence of the chemical potential can be obtained from the parametrization of the experimental data on hadronic ratios as [84] (see also [85])
(36)μB(sNN)=a(1+sNNb)-1,
where a=0.967±0.032 GeV and b=6.138±0.399 GeV. The parametrization in (36) gives the values of μB. At midrapidity, the chemical potential of the system decreases with respect to the colliding energy as observed in Figure 16. So the composition of matter produced at LHC and RHIC is different from the matter produced at low energy collision. At LHC and RHIC, the matter produced at the midrapidity is almost baryon free, whereas the matter produced at the low colliding energy is dominated by baryons.Figure 16
Variation ofμB with respect to the sNN.To take care of this extra baryons, we need to solve the baryon-number conservation equation along with the energy-momentum conservation equation; that is, we simultaneously solve(37)∂μTμν=0,∂μnBμ=0
for (1+1) dimension with boost invariance along the longitudinal direction [74]. In the above equation, nBμ=nBuμ is the baryonic flux and uμ is the hydrodynamic 4-velocity. The initial baryonic chemical potential carried by the quarks μ(=μB/3) is shown in Table 1 for various sNN under consideration.Table 1
The values center of mass energysNN, dN/dy, initial temperature (Ti), and quark chemical potential used in the present calculations.
(
s
N
N
) (GeV)
d
N
/
d
y
T
i (MeV)
μ (MeV)
39
617
240
62
27
592
199
70
17.3
574
198
100
7.7
561
197
165The value of the multiplicities for varioussNN has been calculated from the equation below [86] as
(38)dNdy=dnppdy[(1-x)〈Npart〉2+x〈Ncoll〉]Ncoll is the number of collisions and contributes x fraction to the multiplicity dnpp/dy measured in pp collision. The number of participants, Npart, contributes a fraction (1-x) to dnpp/dy, which is given by
(39)dnppdy=2.5-0.25ln(s)+0.023ln2(s).
The values of Npart and Ncoll are estimated for (0%–5%) centrality by using Glauber model [87]. The value of x depends very weakly on sNN [88], and in the present work we have taken x=0.1 for all the energies.We need the initial heavy quark momentum distributions for solving the FP equation. For low collision energy, rigorous QCD-based calculations for heavy flavour production are not available. In the present work, the initial HQ distribution is obtained from pQCD calculation [41, 89] for the processes gg→QQ- and qq-→QQ-. To demonstrate the effect of non-zero baryonic chemical potential, we evaluate RAA for μ=200MeV and μ=0 for a given Ti=200MeV. The results are displayed in Figure 17 representing the combined effects of temperature and baryon density on the drag and diffusion coefficients. The drag of the heavy quarks due to its interaction with quarks is larger than that due to its interactions with the anti-quarks (Figure 12), resulting in larger suppression in the former case than the latter. The net suppression of the electron spectra from the Au+Au collisions compared to p+p collisions is affected by quarks, anti-quarks, and gluons. The results for net suppressions are displayed for μ=200MeV (dashed line) and μ=0 (with asterisk). The experimental detection of the non-zero baryonic effects will shed light on the net baryon density (and hence baryon stopping) in the central rapidity region. However, whether the effects of non-zero baryonic chemical potential are detectable or not will depend on the overall experimental performance.Figure 17
The nuclear suppression factorRAA as a function of pT due to the interaction of the charm quark (solid line) and anti-quark (dashed-dot line) for μ=200MeV. The net suppressions including the interaction of quarks, anti-quarks, and gluons for μ=200MeV (dashed line) and μ=0 (with asterisk) are also shown.The results forRAA are shown in Figure 18 for various sNN with inputs from Table 1. We observe that at large pT the suppression is similar for all energies under consideration. This is because the collisions at high sNN are associated with large temperature but small baryon density at midrapidity, which is compensated by large baryon density and small temperature at low sNN collisions. Low pT particles predominantly originate from low temperature and low density part of the evolution where drag is less and so is the nuclear suppression.Figure 18
Nuclear suppression factor,RAA, as function of pT for various sNN.In our earlier work [23], we have evaluated the RAA for nonphotonic single electron spectra resulting from the semileptonic decays of hadrons containing heavy flavours and observed that the data from RHIC collisions at sNN=200 GeV are well reproduced by enhancing the pQCD cross sections by a factor 2 and with an equation of state P=ϵ/4 for collisional loss. In the same spirit, we evaluate RAA with twice enhanced pQCD cross section and keeping all other quantities unaltered (Figure 19). The results in Figure 19 show stronger suppression as compared to the results displayed in Figure 18, but it is similar in all the energies under consideration. When we have enhanced the pQCD cross section for the interaction of the heavy quarks with the thermal system by a factor of two—the resulting suppressions in RAA are between 20% and 30% for sNN=39-7.7 GeV.Figure 19
The same as Figure18 with enhancement of cross section by a factor of 2.


## 9. Glimpses of Elliptic Flow

However, apart from the nuclear suppression factorRAA, another experimental observable of heavy flavour, elliptic flow (v2), can be studied within the framework of Fokker-Planck equation. The elliptic flow (v2) of the produced particles has been considered as one of the most promising signals for the early thermalization of the matter formed in heavy ion collision. If we consider a thermalized ellipsoidal spatial domain of QGP, originated due to noncentral nucleus nucleus collisions, with major and minor axes of lengths ly and lx (determined by the collision geometry), respectively, then the pressure gradient is larger along the minor axis compared to that along the major axis because ly>lx. Now, pressure gradient is force and hence force along the minor axis is larger than that along the minor axis. Consequently, the HQ moves faster in this direction. Therefore, the momentum distribution of electrons originating from the decays of charmed hadrons (D mesons) produced from the charm quark fragmentation will be anisotropic, and since v2 is the second Fourier’s coefficient of the momentum distribution, the spatial anisotropy is thus reflected in the momentum space anisotropy.v
2 is sensitive to the initial conditions and the equation of state (EoS) of the evolving matter formed in heavy ion collision [90–92]. Several theoretical attempts have been made in this direction to study both the nuclear suppression factor and elliptic flow of the heavy flavour within a single framework. Although several authors have reproduced the large suppression of high momentum heavy quarks reasonably well yet it is still difficult to explain HQ elliptic flow simultaneously within the same set of initial parameters. It is also imperative to mention that no calculation with radiative energy loss is still able to reproduce heavy flavour elliptic flow. After this brief discussion, we refer the interested readers to [18, 27, 32, 33, 56, 93–96] for more interesting aspects of elliptic flow.


## 10. Summary and Discussions

In summary, we have tried to give a systematic account of evaluating collisional and radiative transport coefficients of heavy quarks passing through quark-gluon plasma. The nuclear suppression factor (RAA) of heavy quarks has been evaluated using Fokker-Planck equation. The present paper confines the discussions within perturbative QCD. Recently, the authors of [97] find out the drag force and RAA of charm quarks propagating through thermalized QGP within the framework of both AdS/CFT and AdS/non-CFT. Interested readers are referred to [98, 99] for similar discussions.We have studied the momentum dependence of transport coefficients and observe that the momentum dependence is crucial in reproducing the trend in thepT dependence of the experimental data. Also, the dead-cone factor (FDC) weighted by the Gunion-Bertsch spectrum for radiated gluon from heavy quarks is implemented. It is worth mentioning that though the present paper speaks at length about elastic as well as bremsstrahlung energy loss of HQs and tells about dominance of the latter in high pT region, yet there are a plethora of articles which show the importance of elastic energy loss in context of experimental observables in entire momentum range attainable by HQs [18, 93, 100]. This issue, as a matter of fact, still awaits an unambiguous settlement.Momentum independent transport coefficients in non-zero baryonic chemical potential region have also been studied and theoretical estimates of the suppression factor are provided. In the Appendix, we have discussed the calculation of elastic transport coefficients within the ambit of QCD hard thermal loop perturbation theory. Also, radiative suppression due to off-shellness of produced partons, which we may call the “dead cone due to virtuality,” has also been discussed. This effect of off-shellness of heavy quarks as well as HTL calculations may be extended to encompass radiative processes which, as we have already discussed, will play the most significant part in high energy regime. The calculations ofRAA including the aspects just mentioned may be contrasted with the experimental data.Though [18, 23–25, 27, 34, 56, 93, 95] attempt to study RAA and v2 of the heavy quark, yet the role of hadronic matter has been ignored. However, to make the characterization of QGP reliable, the role of the hadronic phase should be taken into consideration and its contribution must be subtracted out from the observables. Although a large amount of work has been done on the diffusion of heavy quarks in QGP, the diffusion of heavy mesons in hadronic matter has received much less attention so far. Recently, the diffusion coefficient of D meson has been evaluated using heavy meson chiral perturbation theory [101] and also by using the empirical elastic scattering amplitudes [102] of D mesons with thermal hadrons. The interactions of D meson with pions, nucleons, kaons, and eta particles have been evaluated using Born amplitudes [103] and unitarized chiral effective Dπ interactions [104, 105]. D and B-meson scattering lengths have also been used as dynamical input to study the drag and diffusion coefficients [106, 107]. All these studies observed that the magnitude of both the transport coefficients is significant, indicating substantial amount of interaction of the heavy mesons with the thermal bath. The results may have significant impact on the experimental observables like the suppression of single electron spectra [108] originating from the decays of heavy mesons produced in nuclear collisions at RHIC and LHC energies.



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