Testing the color charge and mass dependence of parton energy loss with heavy-to-light ratios at BNL RHIC and CERN LHC

Abstract

The ratio of nuclear modification factors of high-p T heavy-flavored mesons to lightflavored hadrons ("heavy-to-light ratio") in nucleus-nucleus collisions tests the partonic mechanism expected to underlie jet quenching. Heavy-to-light ratios are mainly sensitive to the mass and color-charge dependences of medium-induced parton energy loss. Here, we assess the potential for identifying these two effects in D and B meson production at RHIC and at the LHC. To this end, we supplement the perturbative QCD factorized formalism for leading hadron production with radiative parton energy loss. For D meson spectra at high but experimentally accessible transverse momentum (10 < ∼ p T < ∼ 20 GeV) in Pb-Pb collisions at the LHC, we find that charm quarks behave essentially like light quarks. However, since light-flavored hadron yields are dominated by gluon parents, the heavy-to-light ratio of D mesons is a sensitive probe of the color charge dependence of parton energy loss. In contrast, due to the larger b quark mass, the medium modification of B mesons in the same kinematical regime provides a sensitive test of the mass dependence of parton energy loss. At RHIC energies, the strategies for identifying and disentangling the color charge and mass dependence of parton energy loss are more involved because of the smaller kinematical range accessible. We argue that at RHIC, the kinematical regime best suited for such an analysis of D mesons is 7 < ∼ p T < ∼ 12 GeV, whereas the study of lower transverse momenta is further complicated due to the known dominant contribution of additional, particle species dependent, non-perturbative effects.

Negative elliptic flow of J/ψ’s: A qualitative signature for charm collectivity at RHIC

“…Major breakthroughs for the potential discovery [2,3] of this new state of matter were the observation of constituent quark number scaling of the elliptic flow v hadron 2 (p hadron T ) = n q v q 2 (p hadron T /n q ), with n q being the number of constituent quarks in the respective hadron as well as the observation of jet quenching at intermediate transverse momenta [4][5][6]. Together with the "standard" hydrodynamical interpretation this implies a rapid thermalization and a strong collective flow of the QCD matter created at RHIC.…”

Hard Probes with the STAR Experiment

“…This measurement stands in contrast to the predictions before the conference of R AA ∼0.5-0.6 in this p T range. For extreme medium densities (dN/dy gluon = 3500 [ 18] orq = 14 GeV 2 /fm [ 17]), this large level of suppression can be reproduced within the radiative framework, but only if the bottom contribution is assumed to be negligible. Resolving whether this is a viable solution will depend on direct measurement of the charm and bottom contributions separately.…”

“…The most dramatic such variation is to set the coupling to zero for the trigger particle in a dihadron measurement: this is the promise of photon-tagged correlations [ 15]. An intermediate variation is expected to be provided by charm and bottom quarks; due to their mass, it has been predicted that the radiative energy loss of such quarks is decreased in medium relative to that of light quarks [ 16,17,18]. The combination of all these different biases leads to different dependences of the suppression on density for the different measurements, and so it is hoped that the combination of the measurements will provide a precise measurement of the density of the medium.…”

CMS Physics Technical Design Report: Addendum on High Density QCD with Heavy Ions

“…From equations (1.3) and (1.4), very large initial gluon rapidity densities, dN g /dy ≈ 1100 ± 300 [96], or, equivalently, transport coefficients q ≈ 11 ± 3 GeV 2 /fm [101][102][103][104], are required in order to explain the observed amount of hadron suppression at RHIC, as quantified by the nuclear modification factor : 5) which measures the deviation of A A at impact parameter b from a simple incoherent superposition of N N collisions. In equation (1.5), T AB (b) (normalised to A · B) is the nuclear overlap function at b determined within a geometric Glauber eikonal model using the known Woods-Saxon distribution for the colliding nuclei [105].…”

“…However, the fact that the high-p T e ± spectrum from semi-leptonic D and B decays is as suppressed as the light hadrons in central AuAu [110,111] is in apparent conflict with the robust E Q < E q < E g prediction of radiative energy loss models. In order to reproduce the measured high-p T open charm/beauty suppression, jet quenching models require either initial gluon rapidity densities (dN g /dy ≈ 3000 [112]) inconsistent with the total hadron multiplicities (dN g /dy ≈ 1.8 dN /dη| η=0 [109]) or with the values (dN g /dy ≈ 1100) needed to describe the quenched light hadron spectra, or they need a smaller relative contribution of B relative to D mesons than theoretically expected in the measured p T range [103]. This discrepancy may point to an additional contribution from elastic energy loss for heavy quarks [113][114][115], so far considered negligible [94].…”