Examining a reduced jet-medium coupling in Pb+Pb collisions at the Large Hadron Collider

Abstract

Recent data on the nuclear modification factor RAA of jet fragments in 2.76 ATeV Pb+Pb collisions at the Large Hadron Collider (LHC) indicate that the jet-medium coupling in a Quark-Gluon Plasma (QGP) is reduced at LHC energies and not compatible with the coupling deduced from data at the Relativistic Hadron Collider (RHIC). We estimate the reduction factor from a combined fit to the available data on RAA( √ s, pT , b) and the elliptic flow v2( √ s, pT , b) at √ s = 0.2, 2.76 ATeV over a transverse momentum range pT ∼ 10 − 100 GeV and a broad impact parameter, b, range. We use a simple analytic "polytrop" model (dE/dx = −κE a x z T c ) to investigate the dynamical jet-energy loss model dependence. Varying a = 0 − 1 interpolates between weakly-coupled and strongly-coupled models of jet-energy dependence while z = 0 − 2 covers a wide range of possible jet-path dependencies from elastic and radiative to holographic string mechanisms. Our fit to LHC data indicates an approximate 60% reduction of the coupling κ from RHIC to LHC and excludes energy-loss models characterized by a jet-energy exponent with a > 1/3. In particular, the rapid rise of RAA with pT ≥ 10 GeV combined with the slow variation of the asymptotic v2(pT ) at the LHC rules out popular exponential geometric optics models (a = 1). The LHC data are compatible with 0 ≤ a ≤ 1/3 pQCD-like energy-loss models where the jet-medium coupling is reduced by approximately 10% between RHIC and LHC.

Event-by-EventHydrodynamics+JetEnergy Loss: A Solution to theRAA⊗vNoronha-Hostler1, Betz2, Noronha3 et al. 2016Phys. Rev. Lett.Self Cite11361573View full textAdd to dashboardCite

“…The soft sector is modeled via event-by-event hydrodynamic simulations performed using the v-USPhydro code [61][62][63][64] while the hard sector is described using the energy loss framework developed in [36,65,66]. We show that the positive contribution from low p T v 2 fluctuations overwhelms the small, negative contributions from event plane fluctuations, which in turn leads to an overall enhancement of high p T elliptic flow in comparison to previous calculations.…”

“…Energy loss model. The nuclear modification factor and the high p T azimuthal anisotropies are studied here using the BBMG jet-energy loss model developed in [36,65,66] supplemented with the energy density and flow profiles obtained from the event-by-event viscous hydrodynamic simulations for the soft sector described above. In this model, the energy loss per unit length experienced by a fast moving parton in the plasma, dE/dL, is modeled as…”

Azimuthal jet tomography at RHIC and LHC

“…We compare recent data on the nuclear modification factors measured at RHIC [1] and LHC [12][13][14][15][16] to a wide class of jet-energy loss models [10] based on perturbative QCD (pQCD), conformal and non-conformal AdS holography, and a phenomenological model with an enhanced energy loss around the transition temperature of T c ≈ 170 MeV, coupled to different recent bulk QGP collective fields [5,7,8]. We found that (1) a running coupling pQCD energy loss seems to be favored, (2) a realistic QGP flow backgrounds is essential to simultaneously describe both the nuclear modification factors and the high-p T elliptic flow, and (3) conformal AdS string-like jet holography appears to be ruled out by the LHC data while novel non-conformal generaliztaions of string models [17] may provide an alternative description.…”

“…is the local temperature along the jet path at time τ for a jet produced initially at time τ 0 and distributed according to a Glauber transverse initial profile [10]. For dimensionless couplings, c = 2+z−a.…”

“…The generic energy-loss model considered [9,10] is characterized by the jet energy, path length, and thermal-field dependence described via the three exponents (a, b, c): …”