What to learn from dilepton transverse momentum spectra in heavy-ion collisions?

Recently the NA60 collaboration has presented high precision measurements of dimuon spectra double differential in invariant mass $M$ and transverse pair momentum $p_T$ in In-In collisions at $158 {\rm AGeV}$. While the $M$-dependence is important for an understanding of in-medium changes of light vector mesons and is $p_T$ integrated insensitive to collective expansion, the $p_T$-dependence arises from an interplay between emission temperature and collective transverse flow. This fact can be exploited to derive constraints on the evolution model and in particular on the contributions of different phases of the evolution to dimuon radiation into a given $M$ window. We present arguments that a thermalized evolution phase with $T > 170 {\rm MeV}$ leaves its imprint on the spectra.Comment: Contributed to 19th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions: Quark Matter 2006 (QM 2006), Shanghai, China, 14- 20 Nov 200

Note on proton-antiproton suppression in 200 AGeV Au-Au collisions

We discuss the measured nuclear suppression of p + pbar production in 200 AGeV Au-Au collisions at RHIC within radiative energy loss. For the AKK set of fragmentation functions, proton production is dominated by gluons, giving rise to the expectation that the nuclear suppression for p + pbar should be stronger than for pions due to the stronger coupling of gluons to the quenching medium. Using a hydrodynamical description for the soft matter evolution, we show that this is indeed seen in the calculation. However, the expected suppression factors for pions and protons are sufficiently similar that a discrimination with present data is not possible. In the high p_T region above 6 GeV where the contributions of hydrodynamics and recombination to hadron production are negligible, the model calculation is in good agreement with the data on p + pbar suppression.Comment: 3 pages, 2 figures, slightly expanded versio

On the sensitivity of the dijet asymmetry to the physics of jet quenching

The appearance of monojets is among the most striking signature of jet quenching in the context of ultrarelativistic heavy-ion collisions. Experimentally, the disappearance of jets has been quantified by the ATLAS and CMS collaborations in terms of the dijet asymmetry observable A_J. While the experimental findings initially gave rise to claims that the measured A_J would challenge the radiative energy loss paradigm, the results of a systematic investigation of A_J in different models for the medium evolution and for the shower-medium interaction presented here suggest that the observed properties of A_J arise fairly generically and independent of specific model assumptions for a large class of reasonable models. This would imply that rather than posing a challenge to any particular model, the observable prompts the question what model dynamics is not compatible with the data.Comment: 8 pages, 6 figures, added computations of jet R_AA and R=0.2 result

Energy deposition in hard dihadron triggered events in heavy-ion collisions

The experimental observation of hadrons correlated back-to-back with a (semi-)hard trigger in heavy ion collisions has revealed a splitting of the away side correlation structure in a low to intermediate transverse momentum (P_T) regime. This is consistent with the assumption that energy deposited by the away side parton into the bulk medium produced in the collision excites a sonic shockwave (a Mach cone) which leads to away side correlation strength at large angles. A prediction following from assuming such a hydrodynamical origin of the correlation structure is that there is a sizeable elongation of the shockwave in rapidity due to the longitudinal expansion of the bulk medium. Using a single hadron trigger, this cannot be observed due to the unconstrained rapidity of the away side parton. Using a dihadron trigger, the rapidity of the away side parton can be substantially constrained and the longitudinal structure of the away side correlation becomes accessible. However, in such events several effects occur which change the correlation structure substantially: There is not only a sizeable contribution due to the fragmentation of the emerging away side parton, but also a systematic bias towards small energy deposition into the medium and hence a weak shockwave. In this paper, both effects are addressed.Comment: 5 pages, 2 figure

Hard dihadron correlations in heavy-ion collisions at RHIC and LHC

High transverse momentum (P_T) processes are considered to be an important tool to probe and understand the medium produced in ultrarelativistic heavy-ion collisions via the interaction of hard, perturbatively produced partons with the medium. In this context, triggered hard dihadron correlations constitute a class of observables set between hard single inclusive hadrons (dominated by the leading jet fragments) and fully reconstructed jets - while they probe some features of the perturbative QCD evolution of a parton shower in the medium, they do not suffer from the problem of finding a suitable separation between soft perturbative (jet-like) and soft non-perturbative (medium-like) physics as the identification of full jets does. On the other hand, the trigger requirement introduces non-trivial complications to the process, which makes the medium-modification of the correlation pattern difficult and non-intuitive to understand. In this work, we review the basic physics underlying triggered dihadron correlations and make a systematic comparison of several combinations of medium evolution and parton-medium interaction models with the available data from 200 AGeV Au-Au collisions at RHIC. We also discuss the expected results for 2.76 ATeV Pb-Pb collisions at the LHC.Comment: 13 pages, 7 figures, submitted to PR

Systematics of parton-medium interaction from RHIC to LHC

Despite a wealth of experimental data for high-P_T processes in heavy-ion collisions, discriminating between different models of hard parton-medium interactions has been difficult. A key reason is that the pQCD parton spectrum at RHIC is falling so steeply that distinguishing even a moderate shift in parton energy from complete parton absorption is essentially impossible. In essence, energy loss models are effectively only probed in the vicinity of zero energy loss and, as a result, at RHIC energies only the pathlength dependence of energy loss offers some discriminating power. At LHC however, this is no longer the case: Due to the much flatter shape of the parton p_T spectra originating from 2.76 AGeV collisions, the available data probe much deeper into the model dynamics. A simultaneous fit of the nuclear suppression at both RHIC and LHC energies thus has great potential for discriminating between various models that yield equally good descriptions of RHIC data alone.Comment: Talk given at Quark Matter 2011, 22-28 May 2011, Annecy, Franc