Testing pQCD and AdS/CFT energy loss at RHIC and LHC

Recent data on the high-p T pion nuclear modification factor, R AA (p T ), and its elliptic azimuthal asymmetry, v 2 (p T ), from RHIC/BNL and LHC/CERN are analyzed in terms of a wide class of jet-energy loss models coupled to different (2+1)d transverse plus Bjorken expanding hydrodynamic fields. We test the consistency of each model by demanding a simultaneous account of the azimuthal, the transverse momentum, and the centrality dependence of the data at both 0.2 and 2.76 ATeV energies. We find a rather broad class of jet-energy independent energy-loss models dE/dx = κ(T )x z T 2+z ζ q that, when coupled to bulk constrained temperature fields T (x, t), can account for the current data at the χ 2 /d.o.f. < 2 level with different temperature-dependent jet-medium couplings, κ(T ), and path-length dependence exponents 0 ≤ z ≤ 2. We extend previous studies by including a generic term, 0 < ζ q < 2 + q, to test different scenarios of energy-loss fluctuations. While a previously proposed AdS/CFT jet-energy loss model with a temperature-independent jet-medium coupling as well as a near-T c dominated, pQCD-inspired energy-loss scenario are shown to be inconsistent with the LHC data, once the parameters are constrained by fitting to RHIC results, we find several new solutions with a temperature-dependent κ(T ). We conclude that the current level of statistical and systematic uncertainties of the measured data does not allow a constraint on the path-length exponent z to a range narrower than [0 − 2].

Section: Jhep08(2014)090supporting

confidence: 77%

Constraints on the path-length dependence of jet quenching in nuclear collisions at RHIC and LHC

Betz

Gyulassy

2014

“…The interaction of heavy quarks with the medium constituents is computed considering radiative and collisional processes in the calculations indicated as Djordjevic [75], WHDG [20][21][22], CUJET3.0 [76,77], MC@sHQ+EPOS [80], BAMPS [53][54][55], and Cao, Qin, Bass [79]. Only collisional interactions are considered in the model calculations POWLANG [81,82], TAMU elastic [70] and PHSD [83].…”

Section: Comparison With Modelsmentioning

confidence: 99%

“…The dead-cone effect should reduce small-angle gluon radiation for quarks that have moderate energy-over-mass values, i.e. for c and b quarks with momenta up to about 10 and 30 GeV/c, respectively [16][17][18][19][20][21][22]. Likewise, collisional energy loss is predicted to be reduced for heavier quarks, because the spatial diffusion coefficient, which regulates the momentum transfers with the medium, scales with the inverse of the quark mass for a given quark momentum [23].…”

Section: Jhep03(2016)081mentioning

confidence: 99%

Transverse momentum dependence of D-meson production in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}}=2.76 $$ TeV

Adam¹,

Adamová²,

Aggarwal³

et al. 2016

130

The production of prompt charmed mesons D 0 , D + and D * + , and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the centre-of-mass energy per nucleon pair, √ s NN , of 2.76 TeV. The production yields for rapidity |y| < 0.5 are presented as a function of transverse momentum, p T , in the interval 1-36 GeV/c for the centrality class 0-10% and in the interval 1-16 GeV/c for the centrality class 30-50%. The nuclear modification factor R AA was computed using a proton-proton reference at √ s = 2.76 TeV, based on measurements at √ s = 7 TeV and on theoretical calculations. A maximum suppression by a factor of 5-6 with respect to binary-scaled pp yields is observed for the most central collisions at p T of about 10 GeV/c. A suppression by a factor of about 2-3 persists at the highest p T covered by the measurements. At low p T (1-3 GeV/c), the R AA has large uncertainties that span the range 0.35 (factor of about 3 suppression) to 1 (no suppression). In all p T intervals, the R AA is larger in the 30-50% centrality class compared to central collisions. The D-meson R AA is also compared with that of charged pions and, at large p T , charged hadrons, and with model calculations. The ALICE collaboration 36 IntroductionA state of strongly-interacting matter characterised by high energy density and temperature is predicted to be formed in ultra-relativistic collisions of heavy nuclei. According to calculations using Quantum Chromodynamics (QCD) on the lattice, these extreme conditions lead to the formation of a Quark-Gluon Plasma (QGP) state, in which quarks and gluons are deconfined, and chiral symmetry is partially restored (see e.g. [1][2][3][4]). Heavy quarks are produced in the hard scattering processes that occur in the early stage of the collision between partons of the incoming nuclei. Their production is characterised by a timescale ∆t < 1/(2 m c,b ), ∼ 0.1 fm/c for charm and ∼ 0.01 fm/c for beauty quarks, that is shorter than the formation time of the QGP medium, about 0.3 fm/c at Large Hadron Collider (LHC) energies [5]. They can successively interact with the constituents of the medium and lose part of their energy, via inelastic processes (gluon radiation) [6,7] or elastic scatterings (collisional processes) [8][9][10]. Energy loss can be studied using the -1 - JHEP03(2016)081nuclear modification factor R AA , which compares the transverse-momentum (p T ) differential production yields in nucleus-nucleus collisions (dN AA /dp T ) with the cross section in proton-proton collisions (dσ pp /dp T ) scaled by the average nuclear overlap function ( T AA )· dN AA /dp T dσ pp /dp T .(1.1)The average nuclear overlap function T AA over a centrality class is proportional to the number of binary nucleon-nucleon collisions per A-A collision in that class and it can be estimated via Glauber model calculations [11,12]. According to QCD calculations, quarks are expected to lose less energy than gluons because their coupling to the medium is smaller [6,7]. In the energy regime of the...

“…Likewise, collisional energy loss is expected to be reduced for heavier quarks, because the spatial diffusion coefficient that regulates the momentum exchange with the medium is expected to scale as the inverse of the quark mass [25]. In the p T interval up to about 20 GeV/c, where the masses of heavy quarks are not negligible with respect to their momenta, essentially all models predict R D AA < R B AA [17][18][19][20][26][27][28][29][30][31][32][33][34][35], which stems directly from the mass dependence of the quark-medium interaction and is only moderately affected by the different production and fragmentation kinematics of c and b quarks (see e.g. …”

mentioning

confidence: 99%

Centrality dependence of high-pT D meson suppression in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{N}\;\mathrm{N}}}=2.76 $$ TeV

Adam¹,

Milošević²,

García-Solis³

et al. 2015

The nuclear modification factor, R AA , of the prompt charmed mesons D 0 , D + and D * + , and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at a centre-of-mass energy √ s NN = 2.76 TeV in two transverse momentum intervals, 5 < p T < 8 GeV/c and 8 < p T < 16 GeV/c, and in six collision centrality classes. The R AA shows a maximum suppression of a factor of 5-6 in the 10% most central collisions. The suppression and its centrality dependence are compatible within uncertainties with those of charged pions. A comparison with the R AA of non-prompt J/ψ from B meson decays, measured by the CMS Collaboration, hints at a larger suppression of D mesons in the most central collisions.Keywords: Charm physics, Heavy Ions, Heavy-ion collision The ALICE collaboration 17 IntroductionWhen heavy nuclei collide at high energy, a state of strongly-interacting matter with high energy density is expected to form. According to Quantum Chromodynamics (QCD) calculations on the lattice, this state of matter, the so-called Quark-Gluon Plasma (QGP) is characterised by the deconfinement of the colour charge (see e.g. [1][2][3][4]). High-momentum partons, produced at the early stage of the nuclear collision, lose energy as they interact with the QGP constituents. This energy loss is expected to proceed via both inelastic (gluon radiation) [5,6] and elastic (collisional) processes [7][8][9]. The nuclear modification factor R AA is used to characterise parton energy loss by comparing particle production yields in nucleus-nucleus collisions to a scaled proton-proton (pp) reference, that corresponds to a superposition of independent nucleon-nucleon collisions. R AA is defined aswhere dσ pp /dp T and dN AA /dp T are the transverse momentum (p T ) differential cross section and yield in proton-proton and nucleus-nucleus (AA) collisions, respectively. T AA is the average nuclear overlap function, estimated within the Glauber model of the nucleusnucleus collision geometry, and proportional to the average number of nucleon-nucleon (binary) collisions [10,11]. Energy loss shifts the momentum of quarks and gluons, and thus hadrons, towards lower values, leading to a suppression of hadron yields with respect to binary scaling at p T larger than few GeV/c (R AA < 1). Energy loss is expected to be smaller for quarks than for gluons because the colour charge factor of quarks is smaller than that of gluons [5,6]. In the energy regime of the Large Hadron Collider (LHC), light-flavour hadrons with p T ranging from 5 to 20 GeV/c originate predominantly from gluon fragmentation (see e.g. As discussed in ref.[15], this should be the case also for charm and beauty quarks produced in gluon splitting processes, if their transverse momentum is lower than about 50 GeV/c. Therefore, the comparison of the heavy-flavour hadron R AA with that of pions allows the colour-charge dependence of parton energy loss to be tested. The softer fragmentation of gluons than that of charm quarks, and the observed increase of the charged hadr...