Jets physics in heavy ion reactions is an important new area of active research at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) that paves the way for novel tests ...of QCD multi-parton dynamics in dense nuclear matter. At present, perturbative QCD calculations of hard probes in elementary nucleon–nucleon reactions can be consistently combined with the effects of the nuclear medium up to O(αs3). While such accuracy is desirable but not necessary for leading particle tomography, it is absolutely essential for the new jet observables. With this motivation, we present first results and predictions to O(αs3) for the recent LHC lead–lead (Pb+Pb) run at a center-of-mass energy of 2.76 TeV per nucleon–nucleon pair. Specifically, we focus on the suppression of the single and double inclusive jet cross sections. Our analysis includes not only final-state inelastic parton interactions in the QGP, but also initial-state cold nuclear matter effects and an estimate of the non-perturbative hadronization corrections. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS experiments, can be derived from these results. We show quantitatively that a fraction of this enhancement may be related to the ambiguity in the separation between the jet and the soft background medium and/or the diffusion of the parton shower energy away from the jet axis through collisional processes. We point to a suite of measurements that can help build a consistent picture of parton shower modification in heavy ion collisions at the LHC.
We present the first theoretical study of medium modifications of the global geometrical pattern, i.e., transverse sphericity (
S
⊥
) distribution of jet events with parton energy loss in ...relativistic heavy-ion collisions. In our investigation, POWHEG + PYTHIA is employed to make an accurate description of transverse sphericity in the p + p baseline, which combines the next-to-leading order (NLO) pQCD calculations with the matched parton shower (PS). The Linear Boltzmann Transport (LBT) model of the parton energy loss is implemented to simulate the in-medium evolution of jets. We calculate the event normalized transverse sphericity distribution in central Pb + Pb collisions at the LHC, and give its medium modifications. An enhancement of transverse sphericity distribution at small
S
⊥
region but a suppression at large
S
⊥
region are observed in A + A collisions as compared to their p + p references, which indicates that in overall the geometry of jet events in Pb + Pb becomes more pencil-like. We demonstrate that for events with 2 jets in the final-state of heavy-ion collisions, the jet quenching makes the geometry more sphere-like with medium-induced gluon radiation. However, for events with
≥
3
jets, parton energy loss in the QCD medium leads to the events more pencil-like due to jet number reduction, where less energetic jets may lose their energies and then fall off the jet selection kinematic cut. These two effects offset each other and in the end result in more jetty events in heavy-ion collisions relative to that in p + p.
Heavy flavor physics in high-energy heavy-ion collisions is a promising and active area to study the mass dependence of the “jet quenching” effects both at the RHIC and the LHC. In this talk, we ...present the first theoretical study on the D0 meson radial distributions relative to the jet axis both in p+p and Pb+Pb collisions at sNN=5.02TeV, where a nice agreement of our results with experimental data is observed. The in-medium parton propagations are described by a Monte Carlo transport model which uses the next-to-leading order (NLO) plus parton shower (PS) event generator SHERPA as input and includes elastic (collisional) and inelastic (radiative) in-medium interaction of heavy flavor jet. We find that, at low D0 meson pT, the radial distribution significantly shifts to larger radius indicating a strong diffusion effect, and the diffusion effects decrease quickly with pT, which is consistent with the recent CMS measurements. We demonstrate that the angular deviation of charm quarks is sensitive to Ds but not qˆ, which may provide new constrains on the collisional and radiative heavy quark energy loss.
The transverse momentum spectrum of η meson in relativistic heavy-ion collisions is studied at the Next-to-Leading Order (NLO) within the perturbative QCD, where the jet quenching effect in the QGP ...is incorporated with the effectively medium-modified η fragmentation functions using the higher-twist approach. We show that the theoretical simulations could give nice descriptions of PHENIX data on η meson in both p+p and central Au + Au collisions at the RHIC, and also provide numerical predictions of η spectra in central Pb + Pb collisions with sNN=2.76 TeV at the LHC. The ratios of η/π0 in p+p and in central Au + Au collisions at 200 GeV are found to overlap in a wide pT region, which matches well the measured ratio η/π0 by PHENIX. We demonstrate that, at the asymptotic region when pT→∞ the ratios of η/π0 in both Au + Au and p+p are almost determined only by quark jets fragmentation and thus approach to the one in e+e− scattering; in addition, the almost identical gluon (quark) contribution fractions to η and to π result in a rather moderate variation of η/π0 distribution at intermediate and high pT region in A+A relative to that in p+p; while a slightly higher η/π0 at small pT in Au + Au can be observed due to larger suppression of gluon contribution fraction to π0 as compared to the one to η. The theoretical prediction for η/π0 at the LHC has also been presented.
Reconstructed jets initiated from heavy quarks provide a powerful tool to probe the properties of the quark–gluon plasma (QGP) and to explore the mass hierarchy of jet quenching. In this article, we ...review the recent theoretical progresses on heavy-flavour jets in high-energy nuclear collisions at the RHIC and LHC. We focus on the yields and substructures of charm and bottom quark jets with jet-quenching effects, such as the nuclear modification factors, transverse momentum imbalance, angular correlation, radial profiles, fragmentation functions, the “dead-cone” effect, etc.
A
bstract
We compute the two-loop hard-thermal-loop (HTL) resummed thermodynamic potential for
N
= 4 supersymmetric Yang-Mills (SYM). Our final result is manifestly gauge-invariant and was ...renormalized using only simple vacuum energy, gluon mass, scalar mass, and quark mass counter terms. The HTL mass parameters
m
D
,
M
D
, and
m
q
are then determined self-consistently using a variational prescription which results in a set of coupled gap equations. Based on this, we obtain the two-loop HTL-resummed thermodynamic functions of
N
= 4 SYM. We compare our final result with known results obtained in the weak- and strong-coupling limits. We also compare to previously obtained approximately self-consistent HTL resummations and Padé approximants. We find that the two-loop HTL resummed results for the scaled entropy density is a quantitatively reliable approximation to the scaled entropy density for 0 ≤ λ ≲ 2 and is in agreement with previous approximately self-consistent HTL resummation results for λ ≲ 6.
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