Jet Quenching Underestimates Experimental Data [1]

Andrea Delgadoa, Rainer J. Friesbc, Ricardo Rodriguezc a Physics Department, University of Texas at El Paso b RIKEN/BNL Research Center, Brookhaven National Laboratory c Cyclotron Institute and Department of Physics and Astronomy, Texas A&M University (a) (b) Introduction Heavy ion collisions at high energies result in a phase In our study, v2 will also depend on impact parameter b for transition where quarks and gluons are no longer confined large pT > 6 GeV where hard processes dominate and LHC Energy into hadrons. This lasts about 5fm/c long. The quark-gluon dependence on p is weak. [2] T plasma (QGP) deconfined state exists above a critical energy 3 density εc ≈ 1 GeV/fm and critical temperature Tc ≈ 170 Density Dependence of Quenching MeV (1012 K). Fig. 1. Density of binary collisions for impact parameter: (a) 3.2 Previous studies have shown that quenching that grows and (b)11. with the density leads to a upper geometric limit on v2 that Jet Quenching underestimates experimental data [1]. Results If the necessary energy is achieved in a particle physics or We systematically study v2 and RAA for 6 GeV pions as heavy ion experiment, quarks and gluons can be kicked out Recently, a Layer-wise geometrical limit was proposed by functions of the quenching strength c and the suppression of a hadron. They fragment becoming jets of hadrons Shuryak and Liao[2] where quenching peaks for entropy parameter 흀. We qualitatively confirm the result by Liao and before they can be directly detected. These jets can be densities close to Tc . Shuryak that decreasing 흀 (increasing shell-like quenching) measure in particle detectors and studied in order to leads to larger v2. determine the properties of the original particle. For this model, we use a profile function that depends on a parameter λ. The profile function that gives the suppression The high-momentum quarks and gluons interact with the of quenching for large s reads [2]: Fig. 4. RAA results for Pb-Pb collisions at LHC energies using the hot, dense medium created in a ultra-relativistic heavy-ion same parameters λ and c as for RHIC. RAA 풄 풄 풄 collision. As a consequence of this interaction, the jet loses w 풔 = [ퟏ휽 풔 − 풔ퟏ 휽 풔ퟐ − 풔 + 흀휽(풔 − 풔ퟐ )] (3) (a) RHIC Energy, b = 3.2 fm (b) LHC Energy, b = 3.2 fm energy, resulting in changes in the jet fragmentation v2 functions. This process is called jet quenching. With 풔 풄 = 3/fm3 and 풔 풄 = 11/fm3 bracketing the near-T λ ퟏ ퟐ c λ region. We also introduced a parameter c which measures c High-energy nucleus-nucleus collisions allow us to change c the quenching strength per entropy density around T . We the scene of parton fragmentation from vacuum to a QCD c implement this density dependence in an LPM-inspired medium, e.g. Quark gluon plasma (QGP), and to study the energy loss formula (sLPM): Fig. 2. properties of this medium through modifications of the jet structure. Next we fit c for three different scenarios to RHIC data: (λ = 휟푬 = 풄 풅흉 흉 − 흉ퟎ 풔 풘 풔 (4) 1 [conventional], λ = 0.4 [Liao/Shuryak], λ = 0.01 [extreme shell scenario]). We find that the best values for c to fit R Observables AA (c) RHIC Energy, b = 11 fm (d) LHC Energy, b = 11 fm Parameters c and λ were used to fit the curves of Nuclear from PHENIX [3] are 0.09 GeVfm, 0.11 GeVfm and 0.135 Nuclear Modification Factor Modification Factor and Azimuthal Anisotropy to plotted GeVfm, respectively. The corresponding results for v2 are The nuclear modification factor, R , is a useful tool to data from PHENIX[3]. AA shown below. v2 data from RHIC favors the lowest values of probe jet suppression, either in the initial or final state. It is lambda. We then proceed to use the same three sets of a measure of the ratio of number of particles produced in QGP Fireball parameters to make predictions for Pb-Pb collisions at 5.5 nucleus+nucleus collisions to the number of particle We use a longitudinally expanding fireball with initial TeV at LHC. produced in p+p collisions, scaled by the average number of entropy density in the center 100(400) fm-3 for RHIC (LHC). collisions. 푨푨 The spatial distribution is modeled by the participant 풅푵 /풅풑푻 RHIC Energy Fig 푹 = (1) density 흆풑풂풓풕. The distribution of jets is given by the density 푨푨 푵 풅푵풑풑/풅풑 Fig. 5. v results for energies at RHIC and LHC. Impact 풄풐풍풍 푻 of binary collisions 흆 . These densities depend on the 2 풄풐풍풍 parameters 3.2 fm and 11 fm are shown. geometry of the interaction zone and the impact parameter Azimuthal Anisotropy (v2) b: The dense nuclear overlap at the beginning of a heavy ion Summary collision resembles an ellipsoid due to incomplete overlap 풃 풃 If we decrease λ while increasing c to fit RHIC R we 흆 = 흇 푻 (풙 + , 풚) 푻 풙 − , 풚 (5) AA of the two colliding nuclei, and thus, jets penetrating the 풄풐풍풍 풊풏풆풍 푨 ퟐ 푩 ퟐ observe less suppression at LHC. We also observe larger v2 fireball in different directions lose different amount of at LHC for decreasing λ except for the most peripheral bins. energy according to their varying paths. The azimuthal 풃 풃 흆풑풂풓풕 = 푻푨 풙 + , 풚 ퟏ − 푷 풙 − , 풚 This tells us that LHC data will be able to rule on the validity anisotropy of the spectra in the transverse plane can be ퟐ ퟐ of enhanced quenching around T 풃 풃 c. characterized in terms of the second Fourier coefficient, + 푻 풙 − , 풚 ퟏ − 푷 풙 + , 풚 (ퟔ) 푩 ퟐ ퟐ References v (p ), which dominates over the first and higher order 2 T [1]Phys. Rev. C, 66, 027902 (2002) coefficients in non-central collisions. v2 measures 흇 푻 (풙,풚) 푨 Where, 푷 풙, 풚 = ퟏ − 풊풏풆풍 푨 (7) [2]Phys. Rev. Lett., 102, 202302 (2009) eccentricity in momentum space. 푨 [3] Phys. Rev. Lett. 101, 232301 (2008) ퟐ흅 풅ퟐ푵 풅∅ 풄풐풔(ퟐ∅) Where 흇풊풏풆풍 is the inelastic cross section and 푻푨(x, y) the ퟎ 풅풑푻풅∅ Fig. 3. Fitting of R to PHENIX [3] experimental data for Au-Au 풗 풑 , 풃 ≡ (2) thickness function, the integral of the nuclear densities over AA ퟐ 푻 ퟐ흅 ퟐ Acknowledgement 풅∅ 풅 푵/풅풑푻풅∅ collisions at RHIC energy. ퟎ the longitudinal direction. Enrique Ramirez-Homs, Cyclotron REU Program. .

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