Final-state Energy Loss Research Articles

Measurements of the charm jet cross section and nuclear modification factor in pPb collisions at [formula omitted

The CMS Collaboration presents the first measurement of the differential cross section of jets from charm quarks produced in proton–lead (pPb) collisions at a nucleon–nucleon center-of-mass energy of sNN=5.02TeV, as well as results from charm quark jets in proton–proton (pp) collisions at s=2.76 and 5.02TeV. By comparing the yields of the pPb and pp collision systems at the same energy, a nuclear modification factor for charm jets from 55 to 400 GeV/c in pPb collisions at sNN=5.02TeV of RpA=0.92±0.07(stat)±0.11(syst) is obtained. This is consistent with an absence of final-state energy loss for charm quarks in pPb collisions. In addition, the fraction of jets coming from charm quarks is found to be consistent with that predicted by pythia 6 for pp collisions at s=2.76 and 5.02TeV, and is independent of the jet transverse momentum from 55 to 400 GeV/c.

Heavy-flavor measurements in heavy-ion collisions with the ALICE experiment

Open and hidden heavy-flavor measurements with the ALICE experiment at the LHC are reported. Emphasis goes to the recent results in p–Pb and Pb–Pb collisions at sNN=5.02 and 2.76 TeV respectively. Heavy-flavor measurements are presented in the form of either the ratio of the production cross sections in heavy-ion and pp collisions normalized by the average number of nucleon-nucleon collisions, or the per-event yields as a function of charged-particle multiplicity. Possible interpretations of these results in pp, p–Pb or Pb–Pb collisions in terms of multi-parton interactions, gluon saturation, initial or final state energy loss, system collective motion, color-charge screening or recombination of uncorrelated quarks are discussed.

Open heavy-flavor measurements with ALICE at the LHC

ALICE is well equipped to reconstruct heavy-flavor particles down to low transverse momentum pT at mid and forward rapidity. An overview of the ALICE results obtained with the Run 1 data in pp (√5=2.76TeV and √s=7TeV), Pb-Pb (√Snn=2.76TeV) and p-Pb (√sNN=5.02 TeV) collisions is presented. In pp collisions the measured cross sections are well described by pQCD calculations. The charged-particle multiplicity dependence of heavy-flavor yields indicates that Multi-Parton Interactions contribute to the high-multiplicity pp collisions and affect charm and beauty production in a similar way. In p-Pb and Pb-Pb collisions the measured nuclear modification factors indicate a final-state energy loss of heavy- quarks in central Pb-Pb collisions. Furthermore, the observed positive heavy-flavor elliptic flow in semi-central Pb-Pb collisions gives a hint that charm quarks participate in the collective expansion of the medium at low pT. In high-multiplicity p-Pb collisions, a double-ridge structure is observed in the heavy-flavor decay electron-hadron azimuthal correlations at low pT similar to what is measured in the light-flavor sector. Such long-range correlations in ŋ could originate from a collective expansion of the system, as well as from gluon saturation in the initial state (color-glass condensate) or other mechanisms.

Measurements of electrons from heavy-flavour hadron decays in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Heavy-flavour hadrons, i.e. hadrons carrying charm or beauty quarks, are a well-suited probe to study the Quark–Gluon Plasma (QGP) in relativistic heavy-ion collisions. For this reason, measurements of electrons from heavy-flavour hadron decays have been performed in pp, p–Pb and Pb–Pb collisions at the LHC with the ALICE detector. Results for the nuclear modification factors ( R pA and R AA ) support a final-state energy loss of heavy quarks in central Pb–Pb collisions and, in semi-central collisions a positive elliptic flow coefficient v 2 of electrons from heavy-flavour hadron decays was observed. Furthermore, a double-ridge structure was observed in the measured two-particle angular correlation distribution, triggered by heavy-flavour decay electrons, in high-multiplicity p–Pb collisions relative to low-multiplicity p–Pb collisions and to pp collisions.

Nuclear-modification factor for open-heavy-flavor production at forward rapidity in Cu+Cu collisions atsNN=200GeV

Background: Heavy-flavor production in p+p collisions tests perturbative-quantum-chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p+p results, quantified with the nuclear-modification factor (R_AA), provides information on both cold- and hot-nuclear-matter effects. Purpose: Determine transverse-momentum, pt, spectra and the corresponding R_AA for muons from heavy-flavor mesons decay in p+p and Cu+Cu collisions at sqrt(s_NN)=200 GeV and y=1.65. Method: Results are obtained using the semi-leptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the p_T spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte-Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p+p collisions at sqrt{s}=200 GeV, integrated over pt and in the rapidity range 1.4<y<1.9 is found to be dsigma_ccbar/dy = 0.139 +/- 0.029 (stat) ^{+0.051}_{-0.058} (syst) mb. This result is consistent with calculations and with expectations based on the corresponding midrapidity charm-production cross section measured earlier by PHENIX. The R_AA for heavy-flavor muons in Cu+Cu collisions is measured in three centrality intervals for 1<pt<4 GeV/c. Suppression relative to binary-collision scaling (R_AA<1) increases with centrality. Conclusions: Within experimental and theoretical uncertainties, the measured heavy-flavor yield in p+p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu+Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss.

Initial-state quark energy loss from Drell–Yan production in proton-proton and proton-nucleus collisions

Drell-Yan production cross sections were measured by the NA3 and E866 experiments, in p-Pt and pp collisions, as a function of the dimuon mass and x F . We compare these measurements to next-to-leading order Drell-Yan calculations, made with the CTEQ6M parton densities modified (or not) by nuclear effects, using the EPS09 parameterization. The analysis of the data allows us to evaluate the initial-state quark energy loss. Drell-Yan measurements are ideally suited to isolate the initial-state parton energy loss, given the absence of final-state effects on the produced dimuon. Our study shows that these data indicate negligible quark energy loss and allow us to derive rather strict upper limits. For completeness, our study has been repeated using the less accurate measurements of Drell-Yan cross section ratios between heavy and light nuclear targets, provided by the E772 and E866 experiments. Our results provide an additional constraint on the models trying to explain quarkonium production in proton-nucleus collisions, as a function of quarkonium rapidity and collision energy, where initial- and final-state energy loss has frequently been assumed to play an important role, convoluted with several other complex mechanisms, including final-state quarkonium breakup, formation time effects, etc.

Determination of the effect of final state energy loss on the acoplanarity and momentum imbalance in jets

The energy loss of partons leaving the hot media created in the heavy ion collisions has been at the center of the interest of the experimental and theoretical communities of the field. Hence an increase in the transverse momentum imbalance and acoplanarity of dijets may be a way to study the parton energy loss. In order to study the effect, we have made simulations of dijet events where the intrinsic kT of the partons, the initial and final state radiation are taken into account. The results of these simulations for p + p and Pb + Pb collisions, as a function of the center of mass energy of collision will be presented.

Cold nuclear modifications at RHIC and LHC

We use recent nuclear parton distributions, among them the Hirai — Kumano — Nagai (HKN) and Eskola — Paukkunen — Salgado (EPS08) parameterizations, in our pQCD-improved parton model to calculate the nuclear modification factor, R AA′ (p T ), at RHIC and at the LHC. At RHIC, the deuteron-gold nuclear modification factor for pions, measured at p T ≥ 10 GeV/c in central collisions, appears to deviate more from unity than the model results. The slopes of the calculated R dAu (p T ) are similar to the slopes of the PHENIX pion and photon data. At LHC, without final-state effects we see a small enhancement of R dPb (p T ) in the transverse momentum range 10 GeV/c ≥ p T ≥ 100 GeV/c for most parameterizations. The inclusion of final-state energy loss will reduce the R dPb (p T ) values.

Non-Abelian energy loss in cold nuclear matter

We use a formal recurrence relation approach to multiple parton scattering to find the complete solution to the problem of medium-induced gluon emission from partons propagating in cold nuclear matter. The differential bremsstrahlung spectrum, where Landau-Pomeranchuk-Migdal destructive interference effects are fully accounted for, is calculated for three different cases: (i) a generalization of the incoherent Bertsch-Gunion solution for asymptotic on-shell jets, (ii) initial-state energy loss of incoming jets that undergo hard scattering, and (iii) final-state energy loss of jets that emerge out of a hard scatter. Our analytic solutions are given as an infinite opacity series, which represents a cluster expansion of the sequential multiple scattering. These new solutions allow, for the first time, direct comparison between initial- and final-state energy loss in cold nuclei. We demonstrate that, contrary to the naive assumption, energy loss in cold nuclear matter can be large. Numerical results to first order in opacity show that, in the limit of large jet energies, initial- and final-state energy losses exhibit different path length dependences, linear versus quadratic, in contrast to earlier findings. In addition, in this asymptotic limit, initial-state energy loss is considerably larger than final-state energy loss. These new results have significant implications for heavy-ion phenomenology in both $p+A$ and $A+A$ reactions.