W. A. Horowitz

Comparison of jet quenching formalisms for a quark-gluon plasma brick

We review the currently available formalisms for radiative energy loss of a high-momentum parton in a dense strongly interacting medium. The underlying theoretical framework of the four commonly used formalisms is discussed and the differences and commonalities between the formalisms are highlighted. A quantitative comparison of the single-gluon emission spectra as well as the energy-loss distributions is given for a model system consisting of a uniform medium with a fixed length of L = 2 fm and L = 5 fm (the “Brick”). Sizable quantitative differences are found. The largest differences can be attributed to specific approximations that are made in the calculation of the radiation spectrum.

Quenching and tomography from the RHIC to the LHC

We compare fully perturbative and fully nonperturbative pictures of high-pT energy loss calculations to the first results from the LHC. While oversuppressed compared to published ALICE data, parameter-free pQCD predictions based on the WHDG energy loss model constrained to RHIC data simultaneously describe well the preliminary CMS hadron suppression, ATLAS charged hadron v2, and ALICE D meson suppression; we also provide for future reference WHDG predictions for B meson RAA. However, energy loss calculations based on AdS/CFT also qualitatively describe well the RHIC pion and non-photonic electron suppression and LHC charged hadron suppression. We propose the double-ratio of charm to bottom quark RAA as a qualitative distinguisher between these two energy loss pictures.

Heavy Quark Jet Quenching with Collisional plus Radiative Energy Loss and Path Length Fluctuations

Abstract With the QGP opacity computed perturbatively and with the global entropy constraints imposed by the observed d N c h / d y ≈ 1000 , radiative energy loss alone cannot account for the observed suppression of single non-photonic electrons. We show that collisional energy loss, which previously has been neglected, is comparable to radiative loss for both light and heavy jets and may in fact be the dominant mechanism for bottom quarks. Predictions taking into account both radiative and collisional losses significantly reduce the discrepancy with data. In addition to elastic energy loss, it is critical to include jet path length fluctuations to account for the observed pion suppression.

Testing pQCD and AdS/CFT Energy Loss at RHIC and LHC

We compare calculations of jet quenching observables at √s = 2.76 ATeV to preliminary LHC data from weakcoupling pQCD and strong-coupling AdS/CFT drag energy loss models constrained to √s = 200 AGeV RHIC data. While somewhat overpredicting the suppression of hadrons, the pQCD-based WHDG model reproduces the hadron azimuthal anisotropy (over many centrality classes) and the suppression of D mesons at LHC. The drag predictions shown here compare poorly to the suppression of D mesons, but the current experimental uncertainties are large. The double ratio of D to B meson RAA(pT) should provide a robust experimental measurement to distinguish between the very different assumptions of the strength of interactions in the QGP produced in heavy ion collisions; i.e., whether, from a jet quenching standpoint, the medium is either weakly-or strongly-coupled.

Testing AdS/CFT drag and pQCD heavy quark energy loss

We present charm and bottom nuclear modification factors for RHIC and LHC using standard model perturbative QCD and recent AdS/CFT string drag energy loss models. We find that extreme extrapolations to LHC mask potential experimentally determinable differences in the individual RAAs but that their ratio, RcAA/RbAA, as a function of transverse momentum is a remarkably robust observable for finding deviations from either theoretical framework.

Running coupling corrections to high energy inclusive gluon production

Abstract We calculate running coupling corrections for the lowest-order gluon production cross section in high energy hadronic and nuclear scattering using the BLM scale-setting prescription. In the final answer for the cross section the three powers of fixed coupling are replaced by seven factors of running coupling, five in the numerator and two in the denominator, forming a ‘septumvirate’ of running couplings, analogous to the ‘triumvirate’ of running couplings found earlier for the small- x BFKL/BK/JIMWLK evolution equations. It is interesting to note that the two running couplings in the denominator of the ‘septumvirate’ run with complex-valued momentum scales, which are complex conjugates of each other, such that the production cross section is indeed real. We use our lowest-order result to conjecture how running coupling corrections may enter the full fixed-coupling k T -factorization formula for gluon production which includes nonlinear small- x evolution.

Heavy Quark Production and Energy Loss

Abstract Heavy flavor research is a vigorous and active topic in high-energy QCD physics. Comparing theoretical predictions to data as a function of flavor provides a unique opportunity to tease out properties of quark–gluon plasma. We explicitly demonstrate this utility with energy loss predictions based on the assumption of 1) a weakly-coupled plasma weakly coupled to a high- p T probe using pQCD and 2) a strongly-coupled plasma strongly coupled to a high- p T probe using AdS/CFT; we find that while the former enjoys broad qualitative agreement with data, it is difficult to reconcile the latter with experimental measurements.

Systematic theoretical uncertainties in jet quenching due to gluon kinematics

We find that the current radiative energy loss kernels obtained from the opacity expansion dramatically violate the collinear approximation used in their derivation. By keeping only the lowest order in collinearity terms, models based on the opacity expansion have ∼ 50% systematic uncertainty in the calculation of π RAA in 0-5% most central RHIC collisions resulting in a systematic uncertainty of ∼ 200% in the extracted medium density. Surprisingly, the inclusion of a thermal gluon mass on the order of the Debye screening scale affects RAA at only about the 5% level due to non-intuitive coherence effects. For some observables such as RAA, the effect of these uncertainties decreases with increasing jet energy; for others, such as the average number of radiated gluons, the effect is energy independent. We note that it is likely that the differences reported in the extracted values of medium parameters such as q̂ by various jet energy loss models will fall within this collinear approximation systematic uncertainty; it is imperative for the quantitative extraction of medium parameters or the possible falsification of the hypothesis of weak coupling between the hard probes and soft modes of the quark gluon plasma medium that future radiative energy loss research push beyond the lowest order collinear approximation.

Strong-coupling jet energy loss from AdS/CFT

A bstractWe propose a novel definition of a holographic light hadron jet and consider the phenomenological consequences, including the very first fully self-consistent, completely strong-coupling calculation of the jet nuclear modification factor RAA, which we find compares surprisingly well with recent preliminary data from LHC. We show that the thermalization distance for light parton jets is an extremely sensitive function of the a priori unspecified string initial conditions and that worldsheets corresponding to non-asymptotic energy jets are not well approximated by a collection of null geodesics. Our new string jet prescription, which is defined by a separation of scales from plasma to jet, leads to the re-emergence of the late-time Bragg peak in the instantaneous jet energy loss rate; unlike heavy quarks, the energy loss rate is unusually sensitive to the very definition of the string theory object itself. A straightforward application of the new jet definition leads to significant jet quenching, even in the absence of plasma. By renormalizing the in-medium suppression by that in the vacuum we find qualitative agreement with preliminary CMS RAAjet(pT) data in our simple plasma brick model. We close with comments on our results and an outlook on future work.

pQCD versus AdS/CFT tested by heavy quark energy loss

We predict the charm and bottom quark nuclear modification factors using weakly coupled perturbative quantum chromodynamics (pQCD) and strongly coupled AdS/CFT drag methods. The log(pT/MQ)/pT dependence of pQCD loss and the momentum independence of drag loss lead to different momentum dependences for the RAA predictions. This difference is enhanced by examining a new experimental observable, the double ratio of charm to bottom nuclear modification factors, Rcb = RcAA/RbAA. At LHC the weakly coupled theory predicts Rcb → 1, whereas the strongly coupled theory predicts Rcb ~ 0.2 independent of pT. At RHIC the differences are less dramatic, as the production spectra are harder, but the drag formula is applicable to higher momenta, due to the lower medium temperature.