# Presentation: 901730

## Heavy-quark energy loss at RHIC and LHC

*Andrea Dainese*
*Padova – Universit**y** and INFN*
*based on work in collaboration with: *
*N.Armesto, M.Cacciari, C.Salgado, U.Wiedemann*

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Layout

- Energy loss for massive partons à la BDMPS
- Model: quenching weights + geometry
- Results for RHIC
- the baseline for “non-photonic” electrons
- RAA vs experimental data
- Testing E loss with heavy-to-light ratios at LHC
- Conclusions

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Parton E Loss in the BDMPS formalism

- Baier, Dokshitzer, Mueller, Peigne‘, Schiff, NPB 483 (1997) 291.
- Zakharov, JTEPL 63 (1996) 952.
- Salgado, Wiedemann, PRD 68(2003) 014008.

- Radiated-gluon energy distrib.:

- transport coefficient

- sets the scale of the radiated energy

- related to constraint *k*T <* ***,
- controls shape at *** << ****c*

- Casimir coupling factor: 4/3 for q, 3 for g

- (BDMPS case)

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Lower E loss for heavy quarks ?

- In vacuum, gluon radiation suppressed at ** < *m*Q/*E*Q
-                 ******   “****dead cone” effect**
*Dead cone implies lower energy loss* (Dokshitzer-Kharzeev, 2001):
- energy distribution ***d****/d*** of radiated gluons suppressed by angle-dependent factor
- suppress high-** tail

- Dokshitzer, Khoze, Troyan, JPG 17 (1991) 1602.
- Dokshitzer and Kharzeev, PLB 519 (2001) 199.

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Model vs RHIC “light” data

***Model vs RHIC “light” data***

- Model: pQCD + E loss probability (quenching weights) + Glauber collision geometry
- Density (   ) “tuned” to match RAA in central Au-Au at 200 GeV

- Dainese, Loizides, Paic, EPJC 38 (2005) 461.

- Similar results by
- Eskola, Honkanen,
- Salgado, Wiedemann

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Model for heavy quarks

- Initially only charm
- Baseline at RHIC: *PYTHIA*, with *EKS98 shadowing*, tuned to match *p*T-shape of D cross section measured in d-Au by STAR (QM04)
- Heavy-quark *E* loss using (*m/E*)-dependent QW
-      extracted from 0,*h* *R*AA (central)
*Thermalize charms that lose all energy*
-       d*N/*d*m*T  *m*T exp(-*m*T/*T*),  T = 300 MeV

- Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027.
- EKS98: Eskola, Kolhinen, Salgado, EPJC 9 (1999) 61.

- baseline d*N*/d*p*T

- baseline *R*AA

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Charm RAA at RHIC

***effect of the mass***

*thermalized component*

**Small effect of mass for charm** (~50% for D, ~30% for e) at low *p*T [large uncertainties!]
**Basically no effect in “safe” *****p*****T****-region**

- Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027.

- (extracted from light-hadron data)

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Charm + beauty + DY  electrons at RHIC(pQCD baseline revised, and its uncertainties)

- FONLL calculation: Cacciari, Nason, Vogt, hep-ph/0502203
- Drell-Yan from: Gavin et al., hep-ph/9502372
- Comparison: Armesto, Cacciari, Dainese, Salgado, Wiedemann, in preparation

- FONLL (see Ramona’s talk):
- electron spectrum may be
- ~50% c + ~50% b
- for 3 < *p*T < 8 GeV
- We investigated the DY component:
- < 10% up to 10 GeV

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## RAA of c and b electrons at RHIC

- Armesto, Cacciari, Dainese, Salgado, Wiedemann, in preparation
- Armesto @ Quark Matter 05

- Due to larger mass of b quark electron RAA increased to ~0.4
- Mass uncertainty (in E loss) and scales uncertainty (in pQCD  b/c crossing) being studied

- mass uncertainty

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## v2 at high pT from E loss

- The azimuthal asymmetry --v2 or RAA()-- of D and B mesons in *non-central* collisions tests:
- at low/moderate *p*T: recombination scenario, v2 of c/b quarks, hence degree of thermalization of medium
- at higher *p*T: path-length dependence of E loss (almond-shaped medium => v2 ~ 0.05--0.10)

**PHENIX ********0**** v****2**

- Cole @ Quark Matter 05

- v2 from E loss: Dainese, Loizides, Paic, EPJC 38 (2005) 461

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Heavy Quark Energy Loss at LHC

- ~100 cc pairs and ~5 bb pairs per central Pb-Pb collision
- Experiments will measure with good precision *R*AA for D and B, and for their decay leptons

***What can we learn from ***
***a comparative quenching study of ***
***massive and massless probes at the LHC?***

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Heavy Flavour RAA at LHC

- Baseline: PYTHIA, with EKS98 shadowing, tuned to reproduce c and b *p*T distributions from NLO pQCD (MNR)
- (*m/E*)-dep. *E* loss with

- MNR: Mangano, Nason, Ridolfi, NPB 373 (1992) 295.

- Armesto, Dainese, Salgado, Wiedemann, PRD 71 (2005) 054027.

- * EKRT Saturation model:
- Eskola, Kajantie, Ruuskanen, Tuominen,
- NPB 570 (2000) 379.

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Color-charge and mass dep. of E loss with Heavy-to-Light ratios at LHC

***Color-charge and mass dep. of E loss ******with ******Heavy-to-Light ratios****** at LHC***

- Heavy-to-light ratios:
- Compare   *g **** h* ,  *c **** D  and  b **** B*

- Armesto, Dainese, Salgado, Wiedemann, PRD71 (2005) 054027

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Conclusions

- The (open) heavy-flavour era of heavy-ion physics has begun
- Heavy flavours: testing ground for our understanding of radiative energy loss
- However, we need a solid perturbative baseline:
- c/b relative contribution has large th. uncertainty (e.g. FONLL)...
- which, still, doesn’t accomodate RHIC pp data
- DY should be included in electron comparisons at ~10 GeV
- Current *e* data tend to be overpredicted at high *p*T
- Proposed observables at the LHC:
- Heavy-to-light ratios as probes of E loss...
- ... color-charge dependence (RD/h)
- ... parton-mass dependence (RB/h)

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## EXTRA SLIDES

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## B.Muller’s QM Theory Summary

- Density of scatterings

- Range of color force

- Emergence of away-side jet will make determination of q^ easier.

**Baier’s plot**

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Medium Expansion

- The density of scattering centers is time-dependent:

- Dynamical scaling law: *same* spectrum obtained *for equivalent static trasport coefficient*
**Calculations for a static medium apply to expanding systems**

- Salgado and Wiedemann, PRL 89 (2002) 092303

**  = 1.5,  1.0,  0.5,  0

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Application: Parton Quenching Model

- Dainese, Loizides, Paic, EPJC 38 (2005) 461.

- QW + Glauber-based medium geometry and density profile + PYTHIA for parton generation and fragmentation
- The procedure in short:
- generate parton (q or g) with PYTHIA (or back-to-back pair)
- calculate its *L* and average         along the path
- use quenching weights to get energy loss
- quench parton and then hadronize it (independent fragm.)

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Extrapolation in c.m.s. energyIntermediate RHIC energy s = 62 GeV

- energy extrapolation works reasonably well

- Extrapolation in *s*: assuming      Ngluons/volume  (*s*)0.6  (EKRT saturation model)
- First test:

- EKRT: Eskola, Kajantie, Ruuskanen, Tuominen, NPB 570 (2000) 379.
- PHENIX Coll., JPG 31 (2005) S473.

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Model prediction for LHC

- Extrapolation to LHC according to saturation model gives:

**    ****indep. of *****p*****T**

- Dainese, Loizides, Paic, EPJC 38 (2005) 461.
- Vitev and Gyulassy, PRL 89 (2002) 252301.

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Why RAA is flat    Surface effect

- Most partons from inner part are totally absorbed
- Energy loss is “saturated”: many partons radiate all their energy before getting out
- Long path lengths exploited only by high energy partons
-  effectively,
* **R*AA doesn’t increase at high *p*T
-  Exercise:                      *R*AA increases with *p*T

- Prod. points in (x,y) for partons giving hadrons with *p*T > 5 GeV:

*increasing ****s*

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Lower E loss for heavy quarks ?

- Detailed massive calculation shows that:

- Armesto, Salgado, Wiedemann, PRD 69 (2004) 114003.

*Medium-induced gluon radiation fills the dead cone*

*Still, k*T*-integrated radiation is suppressed, but only for quite large m/E (i.e. large m, low p*T*)**

**Warning: large uncertainties *

*k*T

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## The heavy-to-light ratio:

- What enters the game:
- (c) quark vs gluon (Casimir factor)      *R*D/h  > 1
- harder charm *p*T distribution:               *R*D/h   up
- harder charm fragm.                            *R*D/h down
- mass effects                                        *R*D/h   up

- 1) dominates  > 12-13 GeV
- 2) and 3)  ~ compensate
- 4) dominates below 12 GeV
- Warning: significant non-pert.
- effects (e.g. reco) below 6-7 GeV

- RHIC

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## pp  QQ: pQCD calculations vs data

- Calculation of partonic cross section
- standard: Fixed Order (NLO) Massive (e.g. MNR code)
- state-of-the-art: Fixed Order Next-to-Leading Log (FONLL)

-  more accurate at high *p*T

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Calculating Parton Energy Loss

**Probe the medium**

- _gluons volume-density and _
- _interaction cross section_

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese

## Heavy-quark production at the LHC

- pp: Important test of pQCD in a new energy domain
- Remember the “15-years saga of b production at Tevatron”*
- Baseline predictions: NLO (MNR code) in pp + binary scaling (shadowing included for PDFs in the Pb)
- ALICE baseline for charm / beauty:

| system :
sNN : | Pb-Pb (0-5% centr.)
5.5 TeV | p-Pb (min. bias) 
8.8 TeV | pp
14 TeV |
| --- | --- | --- | --- |
|  | 4.3 / 0.2 | 7.2 / 0.3 | 11.2 / 0.5 |
|  | 115 / 4.6 | 0.8 / 0.03 | 0.16 / 0.007 |
|  | 0.65 / 0.80 | 0.84 / 0.90 | --  |

-                  Theoretical uncertainty of a factor 2—3 (next slide)

- * M.Mangano
- MNR code: Mangano, Nason, Ridolfi, NPB373 (1992) 295.

- PANIC 2005, Santa Fe, 24.10.2005                Andrea Dainese