# A Review of Elliptic Flow of Light Nuclei in Heavy-Ion Collisions at RHIC and LHC Energies

Authors: Md. Rihan Haque; Chitrasen Jena; Bedangadas Mohanty
Journal: Advances in High Energy Physics
 (2017)

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## Abstract

We present a review of the measurements of elliptic flow (v2) of light nuclei (d, d¯, t, He3, and He¯3) from the RHIC and LHC experiments. Light (anti)nuclei v2 have been compared with that of (anti)proton. We observed a similar trend in light nuclei v2 to that in identified hadron v2 with respect to the general observations such as pT dependence, low pT mass ordering, and centrality dependence. We also compared the difference of nuclei and antinuclei v2 with the corresponding difference of v2 of proton and antiproton at various collision energies. Qualitatively they depict similar behavior. We also compare the data on light nuclei v2 to various theoretical models such as blast-wave and coalescence. We then present a prediction of v2 for He3 and He4 using coalescence and blast-wave models.


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## Body

## 1. Introduction

The main goals of high energy heavy-ion collision experiments have primarily been to study the properties of Quark Gluon Plasma (QGP) and the other phase structures in the QCD phase diagram [1–11]. The energy densities created in these high energy collisions are similar to that found in the universe, microseconds after the Big Bang [5–8, 12–14]. Subsequently, the universe cooled down to form nuclei. It is expected that high energy heavy-ion collisions will allow studying the production of light nuclei such as d, t, He3, and their corresponding antinuclei. There are two possible production mechanisms for light (anti)nuclei. The first mechanism is thermal production of nucleus-antinucleus pairs in elementary nucleon-nucleon or parton-parton interactions [15–21]. However, due to their small (~few MeV) binding energies, the directly produced nuclei or antinuclei are likely to break up in the medium before escaping. The second mechanism is via final state coalescence of produced (anti)nucleons or from transported nucleons [22–36]. The quark coalescence as a mechanism of hadron production at intermediate transverse momentum has been well established by studying the number of constituent quarks (NCQ) scaling for v2 of identified hadrons measured at RHIC [37–45]. Light nuclei may also be produced via coalescence of quarks similar to the hadrons. But the nuclei formed via quark coalescence are highly unlikely to survive in the high temperature environment due to their small binding energies. In case of hadron formation by quark coalescence, the momentum space distribution of quarks is not directly measurable in experiments. However, in case of nucleon coalescence, momentum space distributions of both the constituents (nucleons) and the products (nuclei) are measurable in heavy-ion collision experiments. Therefore, measurements of v2 of light nuclei provide a tool to understand the production mechanism of light nuclei and freeze-out properties at a later stage of the evolution. It also provides an excellent opportunity to understand the mechanism of coalescence at work in high energy heavy-ion collisions.The production of light (anti)nuclei has been studied extensively at lower energies in Bevelac at LBNL [24, 46–49], AGS at RHIC [50–53],  and SPS at CERN [54–58]. In the AGS experiments, it was found that the coalescence parameter (B2) is of similar magnitude for both d and d¯ indicating similar freeze-out hypersurface of nucleons and antinucleons. Furthermore, the dependence of B2 on collision energy and pT indicated that light nuclei production is strongly influenced by the source volume and transverse expansion profile of the system [58, 59]. In this paper, we review the results of elliptic flow of light nuclei measured at RHIC and LHC and discuss the possible mechanisms for the light nuclei production.The paper is organized as follows. Section2 briefly describes the definition of v2, identification of light (anti)nuclei in the experiments and measurement of v2 of light (anti)nuclei. In Section 3, we present the v2 results for minimum bias collisions from various experiments. We also discuss the centrality dependence, difference between nuclei and antinuclei v2, and the energy dependence of deuteron v2. In the same section, we present the atomic mass number scaling and also compare the experimental results with various theoretical models. Finally in Section 4, we summarize our observations and discuss the main conclusions of this review.


## 2. Experimental Method

### 2.1. Elliptic Flowv2

The azimuthal distribution of produced particles in heavy-ion collision can be expressed in terms of a Fourier series,(1)dNdϕ-Ψr∝1+∑n2vncos⁡nϕ-Ψr,where ϕ is the azimuthal angle of produced particle, Ψr is called the reaction plane angle, and the Fourier coefficients v1, v2, and so on are called flow coefficients [60]. Ψr is defined as the angle between the impact parameter vector and the x-axis of the reference detector in the laboratory frame. Since it is impossible to measure the direction of impact parameter in heavy-ion collisions, a proxy of Ψr, namely, the event plane angle Ψn, is used for the flow analysis in heavy-ion collisions [61]. v2 is measured with respect to the 2nd-order event plane angle Ψ2 [61]. Ψ2 is calculated using the azimuthal distribution of the produced particles. In an event with N particles, the event plane angle Ψ2 is defined as [61](2)Ψ2=12tan-1⁡Y2X2.X2 and Y2 are defined as(3a)X2=∑i=1Nwicos⁡2ϕi,(3b)Y2=∑i=1Nwisin⁡2ϕi,where wi are weights given to each particle to optimise the event plane resolution [61, 62]. Usually the magnitude of particle transverse momentum pT is used as weights as v2 increases with pT. Special techniques are followed while calculating the event plane angle so that it does not contain the particle of interest whose v2 is to be calculated (self-correlation) and also the nonflow effects (e.g., jets and short range correlations) are removed as much as possible [41, 42, 61, 63]. Heavy-ion experiments use the η-subevent plane method to calculate the elliptic flow of identified hadrons as well as for light nuclei. In this method, each event is divided into two subevents in two different η-windows (e.g., positive and negative η). Then two subevent plane angles are calculated with the particles in each subevent. Each particle with a particular η is then correlated with the subevent plane of the opposite η. This ensures that the particle of interest is not included in the calculation of event plane angle. A finite η gap is applied between the two subevents to reduce short range correlations which does not originate from flow.The distribution of the event plane angles should be isotropic in the laboratory frame for a azimuthally isotropic detector. If the distribution of the event plane angles is not flat in the laboratory frame (due to detector acceptance and/or detector inefficiency) then special techniques are applied to make the distribution uniform. The popular methods to make theΨ2 distribution uniform is the ϕ-weight and recentering [64, 65]. In the ϕ-weight method, one takes the actual azimuthal distribution of the produced particle, averaged over large sample of events, and then uses inverse of this distribution as weights while calculating the correlation of the particles with the event plane angle [64, 65]. In the recentering method, one subtracts Xn and Yn from the event-by-event Xn and Yn, respectively, where Xn and Yn denote the average of Xn and Yn over a large sample of similar events. The main disadvantage of applying one of these methods is that it does not remove the contribution from higher flow harmonics. Therefore, another correction method known as the shift correction is used to remove the effects coming from higher flow harmonics. In this method, one fits the Ψ2 distribution (after applying ϕ-weight and/or recentering method) averaged over all events, with a Fourier function. The Fourier coefficients from this fit (obtained as fit parameters) are used to shift Ψ2 of each event, to make the distribution uniform in the laboratory frame [64, 65].Since the number of particles produced in heavy-ion collisions are finite, the calculated event plane angleΨ2 may not coincide with Ψr. For this reason, the measured v2obs with respect to Ψ2 is corrected with the event plane resolution factor R2, where(4)R2=cos⁡2Ψ2-Ψr.In order to calculate the event plane resolution, one calculates two subevent plane anglesΨ2a and Ψ2b, where a and b correspond to two independent subevents. If the multiplicities of each subevent are approximately half of the full event plane, then the resolution of each of subevent plane can be calculated as [60, 61],(5)cos⁡2Ψ2a-Ψr=cos⁡2Ψ2a-Ψ2b.However, the full event plane resolution can be expressed as(6)cos⁡2Ψ2-Ψr=π22χ2exp⁡-χ224×I0χ224+I2χ224,where χ2 = v2/σ and I0, I2 are modified Bessel functions [60, 61]. The parameter σ is inversely proportional to the square-root of N, the number of particles used to determine the event plane [60, 61]. To calculate the resolution for full event plane (Ψ2), one has to solve (6) iteratively for the value of χ2 using the subevent plane resolution (〈cos⁡2Ψ2a-Ψr〉) which is calculated experimentally using (5). The χ2 value is then multiplied with 2 as χ2 is proportional to N and reused in (6) to calculate the resolution of the full event plane. In a case of very low magnitudes, the full event plane resolution can be approximately given as [60, 61](7)cos⁡2Ψ2-Ψr=2cos⁡2Ψ2a-Ψr=2cos⁡2Ψ2a-Ψ2b.The procedure for calculating full and subevent plane resolutions using subevent plane angles and various approximations is discussed in detail in [60, 61].


### 2.2. Data on Light Nuclei

For this review, we have collected light nucleiv2 data from the STAR [63] and PHENIX [66] experiments at RHIC and ALICE experiment at LHC [67]. Table 1 summarizes the measurement of light nuclei v2 available till date.Table 1
Available measurements of light nucleiv2.
Experiment
Nuclei
s
N
N (GeV)
Centrality
STAR [63]
d
,
d
¯
,
t,
7.7, 11.5, 19.6, 27,
0–80%, 0–30%, 30–80%
H
e
3
,
H
e
¯
3
39, 62.4, 200
(0–10%, 10–40%, 40–80% in 200 GeV)
PHENIX [66]
d
+
d
¯
200
0–20%, 20–60%
ALICE [67]
d
+
d
¯
2760
0–5%, 5–10%, 10–20%, 20–30%, 30–40%, 40–50%


### 2.3. Extraction of Light Nucleiv2

In heavy-ion collisions, light nuclei are primarily identified by comparing the mean ionization energy loss per unit length (〈dE/dx〉) in the Time Projection Chamber (TPC) with that from the theoretical predictions (dE/dx|theo) [41, 42, 63, 67–71]. Light nuclei are also identified via the time-of-flight measurement techniques using the Time-of-Flight (TOF) detector [66, 67, 71–74].In the STAR experiment, to identify light nuclei using TPC, a variableZ [63] is defined as(8)Z=log⁡dE/dxdE/dxtheo.Then the light nuclei yields are extracted from these Z-distributions in differential pT and ϕ-Ψ2 bins either for minimum bias collisions or in selected centrality classes. The ϕ-Ψ2 distribution is then fitted with a 2nd-order Fourier function; namely,(9)dNdϕ-Ψ2~1+2v2cos⁡ϕ-Ψ2.The Fourier coefficient v2 is called elliptic flow and is extracted from the fit. As we discussed in the previous subsection this measured v2 is then corrected with the event plane resolution factor (R2) [41, 42, 63].In the ALICE experiment, light nuclei in the lowpT region (<1.0 GeV/c for d, d¯) are identified by comparing the variance (σ〈dE/dx〉) of the measured dE/dx in the TPC with the corresponding theoretical estimate (dE/dxtheo) [67, 71]. Light nuclei are considered identified if the measured 〈dE/dx〉 lies within ±3σ〈dE/dx〉 of the dE/dxtheo. On the other hand, the light nuclei yield is extracted from the mass squared (mTOF2) distribution using the TOF detector. The mass of each particle is calculated using the time-of-flight (t) from the TOF detector and the momentum (p) information from the TPC [66, 67, 71]. Both the ALICE and PHENIX experiments use the TOF detector to identify light nuclei at high pT (>1.0 GeV/c). The mass of a particle can be calculated using the TOF detector as(10)mTOF2=p2c2c2t2L2-1,where the track length L and momentum p are determined with the tracking detectors placed inside magnetic field [44, 66, 67, 71]. After getting m2 for each particle, a selection cut is implemented to reject tracks which have their m2 several σ away from the true m2 value of the light nuclei, as done in the STAR experiment [63]. The ALICE experiment, on the other hand, defines a quantity Δm such that Δm=mTOF-mnucl, where mnucl is the mass of the light nuclei under study. The distribution of Δm is then fitted with a Gaussian + exponential function for signal and an exponential function for the background [67]. Then v2 of light nuclei is calculated by fitting v2(Δm) with the weighted function,(11)v2TotΔm=v2SigΔmNSigNTotΔm+v2BkgΔmNBkgNTotΔm,where the total measured v2Tot is the weighted sum of that from the signal (v2Sig) and background (v2Bkg). v2Tot of the candidate particles are calculated using the scalar product method and corrected for the event plane resolution [67].The PHENIX experiment calculates charged averagev2 of (anti) deuterons as(12)v2=cos⁡2ϕ-Ψ2R2.The quantity R2=cos⁡2Ψ2-Ψr can readily be identified as the resolution of the event plane angle [66]. The resolution of full event plane Ψ2 is calculated with subevent planes (Ψ2a, Ψ2b) estimated using two Beam-Beam Counter (BBC) detectors [44, 66]. The detailed procedure of calculating the full event plane resolution from subevents is already mentioned in the previous subsection. The large η gap between the central TOF and the BBCs (Δη>2.75) reduces the effects of nonflow significantly [44, 66]. The nuclei v2 calculated in PHENIX are also corrected for the contribution coming from backgrounds, mainly consisting of misidentification of other particles (e.g., protons) as nuclei. A pT dependent correction factor was applied on the total v2 (referred to as v2Sig+BkgpT) such that(13)v2dd¯pT=v2Sig+BkgpT-1-Rv2BkgpTR,where v2Sig+BkgpT is the measured v2 for dd¯+background at a given pT, v2d(d¯) is the corrected v2 of dd¯, and R is the ratio of signal and signal + background.


## 2.1. Elliptic Flowv2

The azimuthal distribution of produced particles in heavy-ion collision can be expressed in terms of a Fourier series,(1)dNdϕ-Ψr∝1+∑n2vncos⁡nϕ-Ψr,where ϕ is the azimuthal angle of produced particle, Ψr is called the reaction plane angle, and the Fourier coefficients v1, v2, and so on are called flow coefficients [60]. Ψr is defined as the angle between the impact parameter vector and the x-axis of the reference detector in the laboratory frame. Since it is impossible to measure the direction of impact parameter in heavy-ion collisions, a proxy of Ψr, namely, the event plane angle Ψn, is used for the flow analysis in heavy-ion collisions [61]. v2 is measured with respect to the 2nd-order event plane angle Ψ2 [61]. Ψ2 is calculated using the azimuthal distribution of the produced particles. In an event with N particles, the event plane angle Ψ2 is defined as [61](2)Ψ2=12tan-1⁡Y2X2.X2 and Y2 are defined as(3a)X2=∑i=1Nwicos⁡2ϕi,(3b)Y2=∑i=1Nwisin⁡2ϕi,where wi are weights given to each particle to optimise the event plane resolution [61, 62]. Usually the magnitude of particle transverse momentum pT is used as weights as v2 increases with pT. Special techniques are followed while calculating the event plane angle so that it does not contain the particle of interest whose v2 is to be calculated (self-correlation) and also the nonflow effects (e.g., jets and short range correlations) are removed as much as possible [41, 42, 61, 63]. Heavy-ion experiments use the η-subevent plane method to calculate the elliptic flow of identified hadrons as well as for light nuclei. In this method, each event is divided into two subevents in two different η-windows (e.g., positive and negative η). Then two subevent plane angles are calculated with the particles in each subevent. Each particle with a particular η is then correlated with the subevent plane of the opposite η. This ensures that the particle of interest is not included in the calculation of event plane angle. A finite η gap is applied between the two subevents to reduce short range correlations which does not originate from flow.The distribution of the event plane angles should be isotropic in the laboratory frame for a azimuthally isotropic detector. If the distribution of the event plane angles is not flat in the laboratory frame (due to detector acceptance and/or detector inefficiency) then special techniques are applied to make the distribution uniform. The popular methods to make theΨ2 distribution uniform is the ϕ-weight and recentering [64, 65]. In the ϕ-weight method, one takes the actual azimuthal distribution of the produced particle, averaged over large sample of events, and then uses inverse of this distribution as weights while calculating the correlation of the particles with the event plane angle [64, 65]. In the recentering method, one subtracts Xn and Yn from the event-by-event Xn and Yn, respectively, where Xn and Yn denote the average of Xn and Yn over a large sample of similar events. The main disadvantage of applying one of these methods is that it does not remove the contribution from higher flow harmonics. Therefore, another correction method known as the shift correction is used to remove the effects coming from higher flow harmonics. In this method, one fits the Ψ2 distribution (after applying ϕ-weight and/or recentering method) averaged over all events, with a Fourier function. The Fourier coefficients from this fit (obtained as fit parameters) are used to shift Ψ2 of each event, to make the distribution uniform in the laboratory frame [64, 65].Since the number of particles produced in heavy-ion collisions are finite, the calculated event plane angleΨ2 may not coincide with Ψr. For this reason, the measured v2obs with respect to Ψ2 is corrected with the event plane resolution factor R2, where(4)R2=cos⁡2Ψ2-Ψr.In order to calculate the event plane resolution, one calculates two subevent plane anglesΨ2a and Ψ2b, where a and b correspond to two independent subevents. If the multiplicities of each subevent are approximately half of the full event plane, then the resolution of each of subevent plane can be calculated as [60, 61],(5)cos⁡2Ψ2a-Ψr=cos⁡2Ψ2a-Ψ2b.However, the full event plane resolution can be expressed as(6)cos⁡2Ψ2-Ψr=π22χ2exp⁡-χ224×I0χ224+I2χ224,where χ2 = v2/σ and I0, I2 are modified Bessel functions [60, 61]. The parameter σ is inversely proportional to the square-root of N, the number of particles used to determine the event plane [60, 61]. To calculate the resolution for full event plane (Ψ2), one has to solve (6) iteratively for the value of χ2 using the subevent plane resolution (〈cos⁡2Ψ2a-Ψr〉) which is calculated experimentally using (5). The χ2 value is then multiplied with 2 as χ2 is proportional to N and reused in (6) to calculate the resolution of the full event plane. In a case of very low magnitudes, the full event plane resolution can be approximately given as [60, 61](7)cos⁡2Ψ2-Ψr=2cos⁡2Ψ2a-Ψr=2cos⁡2Ψ2a-Ψ2b.The procedure for calculating full and subevent plane resolutions using subevent plane angles and various approximations is discussed in detail in [60, 61].


## 2.2. Data on Light Nuclei

For this review, we have collected light nucleiv2 data from the STAR [63] and PHENIX [66] experiments at RHIC and ALICE experiment at LHC [67]. Table 1 summarizes the measurement of light nuclei v2 available till date.Table 1
Available measurements of light nucleiv2.
Experiment
Nuclei
s
N
N (GeV)
Centrality
STAR [63]
d
,
d
¯
,
t,
7.7, 11.5, 19.6, 27,
0–80%, 0–30%, 30–80%
H
e
3
,
H
e
¯
3
39, 62.4, 200
(0–10%, 10–40%, 40–80% in 200 GeV)
PHENIX [66]
d
+
d
¯
200
0–20%, 20–60%
ALICE [67]
d
+
d
¯
2760
0–5%, 5–10%, 10–20%, 20–30%, 30–40%, 40–50%


## 2.3. Extraction of Light Nucleiv2

In heavy-ion collisions, light nuclei are primarily identified by comparing the mean ionization energy loss per unit length (〈dE/dx〉) in the Time Projection Chamber (TPC) with that from the theoretical predictions (dE/dx|theo) [41, 42, 63, 67–71]. Light nuclei are also identified via the time-of-flight measurement techniques using the Time-of-Flight (TOF) detector [66, 67, 71–74].In the STAR experiment, to identify light nuclei using TPC, a variableZ [63] is defined as(8)Z=log⁡dE/dxdE/dxtheo.Then the light nuclei yields are extracted from these Z-distributions in differential pT and ϕ-Ψ2 bins either for minimum bias collisions or in selected centrality classes. The ϕ-Ψ2 distribution is then fitted with a 2nd-order Fourier function; namely,(9)dNdϕ-Ψ2~1+2v2cos⁡ϕ-Ψ2.The Fourier coefficient v2 is called elliptic flow and is extracted from the fit. As we discussed in the previous subsection this measured v2 is then corrected with the event plane resolution factor (R2) [41, 42, 63].In the ALICE experiment, light nuclei in the lowpT region (<1.0 GeV/c for d, d¯) are identified by comparing the variance (σ〈dE/dx〉) of the measured dE/dx in the TPC with the corresponding theoretical estimate (dE/dxtheo) [67, 71]. Light nuclei are considered identified if the measured 〈dE/dx〉 lies within ±3σ〈dE/dx〉 of the dE/dxtheo. On the other hand, the light nuclei yield is extracted from the mass squared (mTOF2) distribution using the TOF detector. The mass of each particle is calculated using the time-of-flight (t) from the TOF detector and the momentum (p) information from the TPC [66, 67, 71]. Both the ALICE and PHENIX experiments use the TOF detector to identify light nuclei at high pT (>1.0 GeV/c). The mass of a particle can be calculated using the TOF detector as(10)mTOF2=p2c2c2t2L2-1,where the track length L and momentum p are determined with the tracking detectors placed inside magnetic field [44, 66, 67, 71]. After getting m2 for each particle, a selection cut is implemented to reject tracks which have their m2 several σ away from the true m2 value of the light nuclei, as done in the STAR experiment [63]. The ALICE experiment, on the other hand, defines a quantity Δm such that Δm=mTOF-mnucl, where mnucl is the mass of the light nuclei under study. The distribution of Δm is then fitted with a Gaussian + exponential function for signal and an exponential function for the background [67]. Then v2 of light nuclei is calculated by fitting v2(Δm) with the weighted function,(11)v2TotΔm=v2SigΔmNSigNTotΔm+v2BkgΔmNBkgNTotΔm,where the total measured v2Tot is the weighted sum of that from the signal (v2Sig) and background (v2Bkg). v2Tot of the candidate particles are calculated using the scalar product method and corrected for the event plane resolution [67].The PHENIX experiment calculates charged averagev2 of (anti) deuterons as(12)v2=cos⁡2ϕ-Ψ2R2.The quantity R2=cos⁡2Ψ2-Ψr can readily be identified as the resolution of the event plane angle [66]. The resolution of full event plane Ψ2 is calculated with subevent planes (Ψ2a, Ψ2b) estimated using two Beam-Beam Counter (BBC) detectors [44, 66]. The detailed procedure of calculating the full event plane resolution from subevents is already mentioned in the previous subsection. The large η gap between the central TOF and the BBCs (Δη>2.75) reduces the effects of nonflow significantly [44, 66]. The nuclei v2 calculated in PHENIX are also corrected for the contribution coming from backgrounds, mainly consisting of misidentification of other particles (e.g., protons) as nuclei. A pT dependent correction factor was applied on the total v2 (referred to as v2Sig+BkgpT) such that(13)v2dd¯pT=v2Sig+BkgpT-1-Rv2BkgpTR,where v2Sig+BkgpT is the measured v2 for dd¯+background at a given pT, v2d(d¯) is the corrected v2 of dd¯, and R is the ratio of signal and signal + background.


## 3. Results and Discussion

### 3.1. General Aspects of Light Nucleiv2

Figure1 shows the energy dependence of light (anti)nuclei v2 for sNN=7.7, 11.5, 19.6, 27, 39, 62.4, 200, and 2760 GeV. The panels are arranged by increasing energy from left to right and top to bottom. The pT dependence of v2 of d, d¯, t, He3, and 3He¯ is shown for 0–80% centrality in STAR, 20–60% centrality in PHENIX, and 30–40% centrality in ALICE. Since PHENIX and ALICE do not have measurements in the minimum bias collisions, we only show the results for mid-central collisions. The data points of PHENIX and ALICE correspond to inclusive d+d¯  v2. The general trend of nuclei v2 of all species is the same: it increases with increasing pT. The slight difference of v2 between STAR and PHENIX is due to the difference in centrality ranges. The centrality range for PHENIX is 20–60% and that for STAR is 0–80%.Figure 1
Midrapidityv2(pT) for light nuclei (d, d¯, t, He3, and He¯3) in 0–80%, 20–60%, and 30–40% centrality from STAR, PHENIX, and ALICE, respectively. Proton v2(pT) is also shown as open circles [41–44, 75] for comparison. Lines and boxes at each marker corresponds to statistical and systematic errors, respectively.From the trend in Figure1 it seems that light nuclei v2 show mass ordering; that is, heavier particles have smaller v2 value compared to lighter ones, similar to v2 of identified particles [41, 42, 44, 75]. In order to see the mass ordering effect more clearly, we restrict the x–axis range to 2.5 GeV/c and compare v2 of d with v2 of identified particles such as π+, Ks0 (K in Pb + Pb), and p as shown in Figure 2. We see that dv2 at all collision energies is lower than v2 of the identified hadrons at a fixed value of pT. Although mass ordering is a theoretical expectation from the hydrodynamical approach to heavy-ion collisions [76], coalescence formalism for light nuclei can also give rise to this effect. Recent studies using AMPT and VISHNU hybrid model suggest that mass ordering is also expected from transport approach to heavy-ion collisions [77–79].v2 of light nuclei is negative for some collision energies as shown in Figure 1. This negative v2 is expected to be the outcome of strong radial flow in heavy-ion collisions [80].Figure 2
Midrapidityv2(pT) for π+ (squares), Ks0 (K in Pb + Pb) (triangles), p (open circles), and d (crosses) in 0–80%, 20–60%, and 30–40% centrality from STAR, PHENIX, and ALICE, respectively.In order to study the energy dependence of light nucleiv2, we compare the deuteron v2 from collision energy sNN=7.7 GeV to 2760 GeV as shown in Figure 3. The deuteron v2(pT) shows energy dependence prominently for high pT (pT>2.4 GeV/c), where v2 is highest for top collision energy (sNN=2760 GeV) and gradually decreases with decreasing collision energy. This energy dependent trend of light nuclei v2 is similar to the energy dependence of identified hadron v2 where v2(pT) also decreases with decreasing collision energy [41, 42].Figure 3
Energy dependence of midrapidityv2pT of d for minimum bias (30–40% for ALICE) collisions.The STAR experiment has measured the difference of nuclei (d) and antinuclei (d¯) v2 for collision energies sNN=19.6, 27, 39, 62.4, and 200 GeV [63]. Figure 4 shows the difference of d and d¯v2 as a function of collision energy. For comparison, the difference of proton and antiproton v2 is also shown [41, 42]. We observe that the difference of d and d¯v2 remains positive for sNN=7.7–39 GeV. However, for sNN≥62.4 GeV the difference of d and d¯v2 is almost zero. The difference of d and d¯v2 qualitatively follows the same trend as seen for difference of p and p¯v2[41, 42]. It is easy to infer from simple coalescence model that light (anti)nuclei formed via coalescence of (anti)nucleons will also reflect similar difference in v2 as the constituent nucleon and antinucleon. The difference in v2 between particles and their antiparticles has been attributed to the chiral magnetic effect in finite baryon-density matter [81], different v2 of produced and transported particles [82], different rapidity distributions for quarks and antiquarks [83], the conservation of baryon number, strangeness, and isospin [84], and different mean-field potentials acting on particles and their antiparticles [85–88].Figure 4
Difference ofd and d¯v2pT as a function of collision energy for minimum bias Au + Au collisions in STAR.The centrality dependence of light nucleiv2 measured by the STAR and ALICE is shown in Figure 5. STAR has measured d and d¯v2 in two different centrality ranges, namely, 0–30% and 30–80% for collision energies below sNN=200 GeV. In case of sNN=200 GeV, the light nuclei v2 are measured in three different centrality ranges, namely, 0–10% (central), 10–40% (mid-central), and 40–80% (peripheral) as high statistics data were available. ALICE has measured inclusive d+d¯v2 in 6 different centrality ranges, namely, 0–5%, 5–10%, 10–20%, 20–30%, 30–40%, and 40–50%. We only present the results from 0–5%, 20–30%, and 40–50% centrality from ALICE as shown in Figure 5. v2 of d shows strong centrality dependence for all collision energies studied in the STAR experiment. We observe that more central events have lower v2 compared to peripheral events. d¯ shows the same trend as d for collision energies down to sNN=27 GeV.Figure 5
Centrality dependence ofv2 of d(d¯) as a function of pT.The STAR experiment could not study centrality dependence ofd¯ below sNN=27 GeV due to limited event statistics [63]. Comparing the centrality dependence of d(d¯)v2 from STAR and ALICE we can see that both experiments show strong centrality dependence of light nuclei v2. The centrality dependence of light nuclei v2 is analogous to the centrality dependence observed for identified nucleon (p,p¯) v2 [89, 90].


### 3.2. Mass Number Scaling and Model Comparison

It is expected from the formulations of coalescence model that if light nuclei are formed via the coalescence of nucleons then the elliptic flow of light nuclei, when divided by atomic mass number (A), should scale with the elliptic flow of nucleons [91, 92]. Therefore, we expect that the light (anti)nuclei v2 divided by A should scale with p(p¯) v2. Here, we essentially assume that v2 of (anti)proton and (anti)neutron are the same as expected from the observed NCQ scaling of identified particle v2[41, 42]. Figure 6 shows the atomic mass number scaling of light nuclei v2 from STAR, PHENIX, and ALICE experiments. Since ALICE does not have results in minimum bias events, we used both p+p¯ and d+d¯v2 from 30–40% centrality range. We observe that light nuclei v2 from STAR and PHENIX show atomic mass number scaling up to pT/A~1.5 GeV/c. However, deviation of the scaling of the order of 20% is observed for d+d¯v2 from ALICE. The scaling of light (anti)nuclei v2 with (anti)proton v2 suggests that light (anti)nuclei might have formed via coalescence of (anti)nucleons at a later stage of the evolution at RHIC energies for pT/A up to 1.5 GeV/c [22–32]. However, this simple picture of coalescence may not be holding for ALICE experiment at LHC energies. On the contrary, there is another method to produce light nuclei, for example, by thermal production in which it is assumed that light nuclei are produced thermally like any other primary particles [17–21]. Various thermal model studies have successfully reproduced the different ratios of produced particles as well as light nuclei in heavy-ion collisions [17–21].Figure 6
Atomic mass number scalingv2/A of light nuclei as a function of pT/A for STAR (0–80%), PHENIX (20–60%), and ALICE (30–40%).In order to investigate the success of these models, both STAR and ALICE have compared the elliptic flow of light nuclei with the predictions from blast-wave models [63, 67]. Figure 7 shows the v2 of light nuclei predicted from blast-wave model using the parameters obtained from fits to the identified particles v2 [67, 93]. We observe that blast-wave model reproduces v2 of light nuclei from STAR with moderate success except for low pT  (<1.0 GeV/c), where v2 of d(d¯) are underpredicted for all collision energies. However, the blast-wave model seems to successfully reproduce the d+d¯v2 from ALICE. The low relative production of light nuclei compared to identified nucleons at RHIC collisions energies supports the procedure of light nuclei production via coalescence mechanism [22–32]. However, the success of blast-wave model in reproducing the nuclei v2 at LHC and moderate success at RHIC suggest that the light nuclei production is also supported by thermal process [17–21]. The light nuclei production in general might be a more complicated coalescence process, for example, coalescence of nucleons in the local rest frame of the fluid cell. This scenario might give rise to deviations from simple A scaling [63].Figure 7
Light nucleiv2 as a function of pT from blast-wave model (lines). For comparison, p+p¯  v2 is also shown. Marker for STAR corresponds to 0–80%, PHENIX corresponds to 20–60%, and ALICE corresponds to 30–40% central events.At RHIC energies the light nucleiv2 have been compared with results from a hybrid AMPT + coalescence model [63]. A Multiphase Transport (AMPT) model is an event generator with Glauber Monte Carlo initial state [94]. The AMPT model includes Zhang’s Partonic Cascade (ZPC) model for initial partonic interactions and A Relativistic Transport (ART) model for later hadronic interactions [94]. The nucleon phase-space information from the AMPT model is fed to the coalescence model to generate light nuclei [63, 95]. Figure 8 shows the light nuclei v2 from the coalescence model and compared to the data. The coalescence model prediction for d+d¯ in Pb + Pb collisions at sNN=2760 GeV is taken from [96]. The coalescence model fairly reproduces the measurement from data for all collision energies except for the lowest energy sNN=7.7 GeV. The AMPT model generates nucleon v2 from both partonic and hadronic interactions for all the collision energies presented. However, increased hadronic interactions compared to partonic, at lowest collision energies, are not implemented in the AMPT + coalescence model. This could be the reason behind the deviation of the data from the model predictions at lowest collision energy [41, 42].Figure 8
Light nucleiv2 as a function of pT from AMPT + coalescence model (solid lines). Marker for STAR experiment corresponds to 0–80%, PHENIX corresponds to 20–60%, and ALICE corresponds to 30–40% central events.We have performed simultaneous fit to thev2 and pT spectra of identified hadrons + light nuclei using the same blast-wave model as used in [67, 75]. The simultaneous fit of v2 and pT spectra for measurements from the PHENIX and the ALICE experiment is shown in Figure 9. We find that the inclusion of light nuclei results to the fit does not change the fit results compared to the blast-wave fit performed only on identified hadron v2 and pT spectra. This indicates that the light nuclei v2 and pT spectra are well described by the blast-wave model.Figure 9
(a) Blast-wave fit ofπ, K, p(p¯), d(d¯)v2, and (c) pT spectra from the PHENIX experiment. The same is shown for the ALICE experiment in panels (b) and (d). pT spectra are used from [45, 101]. Marker for PHENIX data corresponds to 20–60% and marker for ALICE data corresponds to 30–40% central events.
(a)
(b)
(c)
(d)


### 3.3. Model Prediction ofHe3 and He4v2

We have predictedv2 of He3 and He4 using the simple coalescence and blast-wave model. Since protons and neutrons have similar masses and the same number of constituent quarks, they should exhibit similar collective behavior and, hence, similar magnitude of v2. Therefore, we parametrize the elliptic flow of p+p¯v2 using the fit formula [97],(14)fv2pTn=an1+e-pT/n-b/c-dn,where a, b, c, and d are fit parameters and n is the constituent quark number of the particle [97]. The fit to p+p¯v2 (solid lines) from the PHENIX and ALICE experiment is shown in Figures 10(a) and 10(b), respectively. Assuming similar magnitude of neutron v2 as that of proton, we then predict v2 of He3 and He4 as(15a)v2pTHe3≈3v2pT3p,(15b)v2pTHe4≈4v2pT4p.Figure 10
(a) Coalescence model predictions (blue lines) ofHe3 and He4v2 for (a) sNN=200 GeV and (b) for sNN=2760 GeV. The blast-wave predictions of He3 and He4v2 are also shown in red lines.
(a)
(b)This simplified coalescence model prediction ofHe3 and He4v2 is shown in Figures 10(a) and 10(b) as blue (dotted and dashed) lines. For comparison, the blast-wave model predicted v2 of He3 and He4 from the fit parameters obtained in Figure 9 are also shown in red (dotted and dashed) lines. We observe characteristic difference in the prediction of He3 and He4v2 from the coalescence and the blast-wave model. As one expects from the mass ordering effect of blast-wave model, v2 of He3 and He4 are predicted to be almost zero in the intermediate pT range (1.0 <pT<2.5 GeV/c). On the other hand, the simple coalescence model predicts orders of magnitude higher v2 compared to blast-wave for both He3 and He4 in the same pT range. Hence, experimental measurements of 3He and 4Hev2 in future would significantly improve our knowledge on the mechanisms of light nuclei formation in heavy-ion collisions [71, 98–100].


## 3.1. General Aspects of Light Nucleiv2

Figure1 shows the energy dependence of light (anti)nuclei v2 for sNN=7.7, 11.5, 19.6, 27, 39, 62.4, 200, and 2760 GeV. The panels are arranged by increasing energy from left to right and top to bottom. The pT dependence of v2 of d, d¯, t, He3, and 3He¯ is shown for 0–80% centrality in STAR, 20–60% centrality in PHENIX, and 30–40% centrality in ALICE. Since PHENIX and ALICE do not have measurements in the minimum bias collisions, we only show the results for mid-central collisions. The data points of PHENIX and ALICE correspond to inclusive d+d¯  v2. The general trend of nuclei v2 of all species is the same: it increases with increasing pT. The slight difference of v2 between STAR and PHENIX is due to the difference in centrality ranges. The centrality range for PHENIX is 20–60% and that for STAR is 0–80%.Figure 1
Midrapidityv2(pT) for light nuclei (d, d¯, t, He3, and He¯3) in 0–80%, 20–60%, and 30–40% centrality from STAR, PHENIX, and ALICE, respectively. Proton v2(pT) is also shown as open circles [41–44, 75] for comparison. Lines and boxes at each marker corresponds to statistical and systematic errors, respectively.From the trend in Figure1 it seems that light nuclei v2 show mass ordering; that is, heavier particles have smaller v2 value compared to lighter ones, similar to v2 of identified particles [41, 42, 44, 75]. In order to see the mass ordering effect more clearly, we restrict the x–axis range to 2.5 GeV/c and compare v2 of d with v2 of identified particles such as π+, Ks0 (K in Pb + Pb), and p as shown in Figure 2. We see that dv2 at all collision energies is lower than v2 of the identified hadrons at a fixed value of pT. Although mass ordering is a theoretical expectation from the hydrodynamical approach to heavy-ion collisions [76], coalescence formalism for light nuclei can also give rise to this effect. Recent studies using AMPT and VISHNU hybrid model suggest that mass ordering is also expected from transport approach to heavy-ion collisions [77–79].v2 of light nuclei is negative for some collision energies as shown in Figure 1. This negative v2 is expected to be the outcome of strong radial flow in heavy-ion collisions [80].Figure 2
Midrapidityv2(pT) for π+ (squares), Ks0 (K in Pb + Pb) (triangles), p (open circles), and d (crosses) in 0–80%, 20–60%, and 30–40% centrality from STAR, PHENIX, and ALICE, respectively.In order to study the energy dependence of light nucleiv2, we compare the deuteron v2 from collision energy sNN=7.7 GeV to 2760 GeV as shown in Figure 3. The deuteron v2(pT) shows energy dependence prominently for high pT (pT>2.4 GeV/c), where v2 is highest for top collision energy (sNN=2760 GeV) and gradually decreases with decreasing collision energy. This energy dependent trend of light nuclei v2 is similar to the energy dependence of identified hadron v2 where v2(pT) also decreases with decreasing collision energy [41, 42].Figure 3
Energy dependence of midrapidityv2pT of d for minimum bias (30–40% for ALICE) collisions.The STAR experiment has measured the difference of nuclei (d) and antinuclei (d¯) v2 for collision energies sNN=19.6, 27, 39, 62.4, and 200 GeV [63]. Figure 4 shows the difference of d and d¯v2 as a function of collision energy. For comparison, the difference of proton and antiproton v2 is also shown [41, 42]. We observe that the difference of d and d¯v2 remains positive for sNN=7.7–39 GeV. However, for sNN≥62.4 GeV the difference of d and d¯v2 is almost zero. The difference of d and d¯v2 qualitatively follows the same trend as seen for difference of p and p¯v2[41, 42]. It is easy to infer from simple coalescence model that light (anti)nuclei formed via coalescence of (anti)nucleons will also reflect similar difference in v2 as the constituent nucleon and antinucleon. The difference in v2 between particles and their antiparticles has been attributed to the chiral magnetic effect in finite baryon-density matter [81], different v2 of produced and transported particles [82], different rapidity distributions for quarks and antiquarks [83], the conservation of baryon number, strangeness, and isospin [84], and different mean-field potentials acting on particles and their antiparticles [85–88].Figure 4
Difference ofd and d¯v2pT as a function of collision energy for minimum bias Au + Au collisions in STAR.The centrality dependence of light nucleiv2 measured by the STAR and ALICE is shown in Figure 5. STAR has measured d and d¯v2 in two different centrality ranges, namely, 0–30% and 30–80% for collision energies below sNN=200 GeV. In case of sNN=200 GeV, the light nuclei v2 are measured in three different centrality ranges, namely, 0–10% (central), 10–40% (mid-central), and 40–80% (peripheral) as high statistics data were available. ALICE has measured inclusive d+d¯v2 in 6 different centrality ranges, namely, 0–5%, 5–10%, 10–20%, 20–30%, 30–40%, and 40–50%. We only present the results from 0–5%, 20–30%, and 40–50% centrality from ALICE as shown in Figure 5. v2 of d shows strong centrality dependence for all collision energies studied in the STAR experiment. We observe that more central events have lower v2 compared to peripheral events. d¯ shows the same trend as d for collision energies down to sNN=27 GeV.Figure 5
Centrality dependence ofv2 of d(d¯) as a function of pT.The STAR experiment could not study centrality dependence ofd¯ below sNN=27 GeV due to limited event statistics [63]. Comparing the centrality dependence of d(d¯)v2 from STAR and ALICE we can see that both experiments show strong centrality dependence of light nuclei v2. The centrality dependence of light nuclei v2 is analogous to the centrality dependence observed for identified nucleon (p,p¯) v2 [89, 90].


## 3.2. Mass Number Scaling and Model Comparison

It is expected from the formulations of coalescence model that if light nuclei are formed via the coalescence of nucleons then the elliptic flow of light nuclei, when divided by atomic mass number (A), should scale with the elliptic flow of nucleons [91, 92]. Therefore, we expect that the light (anti)nuclei v2 divided by A should scale with p(p¯) v2. Here, we essentially assume that v2 of (anti)proton and (anti)neutron are the same as expected from the observed NCQ scaling of identified particle v2[41, 42]. Figure 6 shows the atomic mass number scaling of light nuclei v2 from STAR, PHENIX, and ALICE experiments. Since ALICE does not have results in minimum bias events, we used both p+p¯ and d+d¯v2 from 30–40% centrality range. We observe that light nuclei v2 from STAR and PHENIX show atomic mass number scaling up to pT/A~1.5 GeV/c. However, deviation of the scaling of the order of 20% is observed for d+d¯v2 from ALICE. The scaling of light (anti)nuclei v2 with (anti)proton v2 suggests that light (anti)nuclei might have formed via coalescence of (anti)nucleons at a later stage of the evolution at RHIC energies for pT/A up to 1.5 GeV/c [22–32]. However, this simple picture of coalescence may not be holding for ALICE experiment at LHC energies. On the contrary, there is another method to produce light nuclei, for example, by thermal production in which it is assumed that light nuclei are produced thermally like any other primary particles [17–21]. Various thermal model studies have successfully reproduced the different ratios of produced particles as well as light nuclei in heavy-ion collisions [17–21].Figure 6
Atomic mass number scalingv2/A of light nuclei as a function of pT/A for STAR (0–80%), PHENIX (20–60%), and ALICE (30–40%).In order to investigate the success of these models, both STAR and ALICE have compared the elliptic flow of light nuclei with the predictions from blast-wave models [63, 67]. Figure 7 shows the v2 of light nuclei predicted from blast-wave model using the parameters obtained from fits to the identified particles v2 [67, 93]. We observe that blast-wave model reproduces v2 of light nuclei from STAR with moderate success except for low pT  (<1.0 GeV/c), where v2 of d(d¯) are underpredicted for all collision energies. However, the blast-wave model seems to successfully reproduce the d+d¯v2 from ALICE. The low relative production of light nuclei compared to identified nucleons at RHIC collisions energies supports the procedure of light nuclei production via coalescence mechanism [22–32]. However, the success of blast-wave model in reproducing the nuclei v2 at LHC and moderate success at RHIC suggest that the light nuclei production is also supported by thermal process [17–21]. The light nuclei production in general might be a more complicated coalescence process, for example, coalescence of nucleons in the local rest frame of the fluid cell. This scenario might give rise to deviations from simple A scaling [63].Figure 7
Light nucleiv2 as a function of pT from blast-wave model (lines). For comparison, p+p¯  v2 is also shown. Marker for STAR corresponds to 0–80%, PHENIX corresponds to 20–60%, and ALICE corresponds to 30–40% central events.At RHIC energies the light nucleiv2 have been compared with results from a hybrid AMPT + coalescence model [63]. A Multiphase Transport (AMPT) model is an event generator with Glauber Monte Carlo initial state [94]. The AMPT model includes Zhang’s Partonic Cascade (ZPC) model for initial partonic interactions and A Relativistic Transport (ART) model for later hadronic interactions [94]. The nucleon phase-space information from the AMPT model is fed to the coalescence model to generate light nuclei [63, 95]. Figure 8 shows the light nuclei v2 from the coalescence model and compared to the data. The coalescence model prediction for d+d¯ in Pb + Pb collisions at sNN=2760 GeV is taken from [96]. The coalescence model fairly reproduces the measurement from data for all collision energies except for the lowest energy sNN=7.7 GeV. The AMPT model generates nucleon v2 from both partonic and hadronic interactions for all the collision energies presented. However, increased hadronic interactions compared to partonic, at lowest collision energies, are not implemented in the AMPT + coalescence model. This could be the reason behind the deviation of the data from the model predictions at lowest collision energy [41, 42].Figure 8
Light nucleiv2 as a function of pT from AMPT + coalescence model (solid lines). Marker for STAR experiment corresponds to 0–80%, PHENIX corresponds to 20–60%, and ALICE corresponds to 30–40% central events.We have performed simultaneous fit to thev2 and pT spectra of identified hadrons + light nuclei using the same blast-wave model as used in [67, 75]. The simultaneous fit of v2 and pT spectra for measurements from the PHENIX and the ALICE experiment is shown in Figure 9. We find that the inclusion of light nuclei results to the fit does not change the fit results compared to the blast-wave fit performed only on identified hadron v2 and pT spectra. This indicates that the light nuclei v2 and pT spectra are well described by the blast-wave model.Figure 9
(a) Blast-wave fit ofπ, K, p(p¯), d(d¯)v2, and (c) pT spectra from the PHENIX experiment. The same is shown for the ALICE experiment in panels (b) and (d). pT spectra are used from [45, 101]. Marker for PHENIX data corresponds to 20–60% and marker for ALICE data corresponds to 30–40% central events.
(a)
(b)
(c)
(d)


## 3.3. Model Prediction ofHe3 and He4v2

We have predictedv2 of He3 and He4 using the simple coalescence and blast-wave model. Since protons and neutrons have similar masses and the same number of constituent quarks, they should exhibit similar collective behavior and, hence, similar magnitude of v2. Therefore, we parametrize the elliptic flow of p+p¯v2 using the fit formula [97],(14)fv2pTn=an1+e-pT/n-b/c-dn,where a, b, c, and d are fit parameters and n is the constituent quark number of the particle [97]. The fit to p+p¯v2 (solid lines) from the PHENIX and ALICE experiment is shown in Figures 10(a) and 10(b), respectively. Assuming similar magnitude of neutron v2 as that of proton, we then predict v2 of He3 and He4 as(15a)v2pTHe3≈3v2pT3p,(15b)v2pTHe4≈4v2pT4p.Figure 10
(a) Coalescence model predictions (blue lines) ofHe3 and He4v2 for (a) sNN=200 GeV and (b) for sNN=2760 GeV. The blast-wave predictions of He3 and He4v2 are also shown in red lines.
(a)
(b)This simplified coalescence model prediction ofHe3 and He4v2 is shown in Figures 10(a) and 10(b) as blue (dotted and dashed) lines. For comparison, the blast-wave model predicted v2 of He3 and He4 from the fit parameters obtained in Figure 9 are also shown in red (dotted and dashed) lines. We observe characteristic difference in the prediction of He3 and He4v2 from the coalescence and the blast-wave model. As one expects from the mass ordering effect of blast-wave model, v2 of He3 and He4 are predicted to be almost zero in the intermediate pT range (1.0 <pT<2.5 GeV/c). On the other hand, the simple coalescence model predicts orders of magnitude higher v2 compared to blast-wave for both He3 and He4 in the same pT range. Hence, experimental measurements of 3He and 4Hev2 in future would significantly improve our knowledge on the mechanisms of light nuclei formation in heavy-ion collisions [71, 98–100].


## 4. Summary and Conclusions

We have presented a review of elliptic flowv2 of light nuclei (d, t, and He3) and antinuclei (d¯ and He¯3) from STAR experiment and inclusive d+d¯v2 from PHENIX at RHIC and ALICE at LHC. Similar to identified hadrons, the light nuclei v2 show a monotonic rise with increasing pT and mass ordering at low pT for all measured collision energies. The beam energy dependence of dv2 is small at intermediate pT and only prominent at high pT, which is similar to the trend as observed for the charged hadron v2. The v2 of nuclei and antinuclei are of similar magnitude for top collision energies at RHIC but at lower collision energies; the difference in v2 between nuclei and antinuclei qualitatively follows the difference in proton and antiproton v2. The centrality dependence of light (anti)nuclei v2pT is similar to that of identified hadrons v2pT.Light (anti)nucleiv2 is found to follow the atomic mass number (A) scaling for almost all collision energies at RHIC suggesting coalescence as the underlying process for the light nuclei production in heavy-ion collisions. However, a deviation from mass number scaling at the level of 20% is observed at LHC. This indicates that a simple coalescence process may not be the only underlying mechanism for light nuclei production. Furthermore, a transport-plus-coalescence model study is found to approximately reproduce the light nuclei v2 measured at RHIC and LHC. The agreement of coalescence model with the data from PHENIX and STAR is imperceptibly better than the blast-wave model. However, at the LHC energy, the light nuclei v2 are better described by blast-wave model rather than the simple coalescence model. The coalescence mechanism, intuitively, should be the prominent process of light nuclei production. However, the breaking of mass scaling at LHC energy and success of blast-wave model prevent us from drawing any definitive conclusion on the light nuclei production mechanism.We observed orders of magnitude difference inHe3 and He4v2 as predicted by blast-wave and coalescence model. The blast-wave model predicts almost zero v2 for He3 and He4 up to pT=2.5 GeV/c, whereas the coalescence model predicts significant v2 for He3 and He4 at same pT range. Hence, the precise measurements of He3 and He4v2 in the future can significantly improve the knowledge of the light nuclei production mechanism in heavy-ion collisions.



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*Source: 1248563-2017-08-15.xml*