Scaling patterns of the suppression ofπ0yields inAu+Aucollisions at

Abstract

Suppression measurements for neutral pions (π 0 ) are used to investigate the predicted path length (L) and transverse momentum (pT ) dependent jet quenching patterns of the hot QCD medium produced in Au+Au collisions at √ sNN = 200 GeV. The observed scaling patterns show the predicted trends for jet-medium interactions dominated by radiative energy loss. They also allow simple estimates of the transport coefficientq and the ratio of viscosity to entropy density η/s. These estimates indicate that the short mean free path (λ) in the QCD medium leading to hydrodynamic-like flow with a small value of η/s, is also responsible for the strong suppression observed.One of the important discoveries at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL), has been the observation that highp T hadron yields are suppressed in central and midcentral A+A collisions when compared to the binaryscaled yields from p+p collisions [1]. This observation has been attributed to jet-quenching [2] -the process by which hard scattered partons interact and loose energy in the hot and dense quark gluon plasma (QGP) produced in the collisions. Subsequent to such interactions, the partons which do emerge, then fragment into topologically aligned hadrons (jets) which provide the basis for the π 0 suppression measurements. There is considerable current interest in the use of jet quenching as a quantitative tomographic probe of the QGP. Recent theoretical efforts have centered on investigations of the energy loss mechanism for scattered partons which propagate through this medium. Two such mechanisms are; (i) scatterings off thermal partons in binary elastic collisions and (ii) Gluon bremsstrahlung with the Landau-Pomeranchuk-Migdal (LPM) [3] effect. The latter has been investigated via different formalisms [4,5,6,7,8,9,10]. Studies of the relative importance of both jet quenching mechanisms have also been made [11,12,13,14]. To date, a conclusive mechanistic picture has not yet emerged.Initial quantitative studies with models which incorporate the time evolution of the QGP medium via relativistic ideal (3+1)-dimensional hydrodynamical simulations, are currently underway [15,16]. However, the value of such studies rests heavily on accurate knowledge of the dominant mechanism/s for jet quenching. Therefore, it is important to pursue validation tests which can lend insight or provide a clear distinction between different energy loss mechanisms.The experimental probe commonly exploited for jetquenching studies in AA collisions is the nuclear modification factor (R AA );where σ pp is the particle production cross section in p+p collisions and T AA is the nuclear thickness function averaged over the impact parameter range associated with a given centrality selectionThe corresponding average number of nucleon-nucleon collisions, N coll = σ inel pp T AA , is routinely obtained via a Monte-Carlo Glauber-based model calculation [18,19].In this letter we use R AA measurements to perform validation tests which addresses the q...

Constraints on models for the initial collision geometry in ultrarelativistic heavy ion collisions

“…Such a trend has been observed in the preliminary and final data sets reported in Refs. [10,21,35] and is consistent with the conclusions reached in Refs. [10,36] that the N part and impact parameter dependence of the eccentricity scaled flow coefficients v 2 ε 2 and v 4 ε 4 favor fKLN-like initial collision geometries.…”

“…A constant ratio for the flow coefficients v 4 (v 2 ) 2 ≈ 0.5 is also predicted [21]. It is well established that initial eccentricity fluctuations also influence the magnitude of v 2,4 significantly [5,10,[21][22][23], i.e., the presence of these fluctuations serve to increase the value of v 2,4 . Therefore, one avenue to search for new experimental constraints, is to use ε 2,4 as a proxy for v 2,4 and study the model dependencies of their magnitudes and trends vs. N part .…”

“…and v 2 1) [9,10]. The magnitudes and trends of v 2,4 are known to be sensitive to the transport properties of the expanding partonic matter [3,4,6,[11][12][13][14][15][16][17].…”

“…Consequently, there is considerable current interest in their use for quantitative extraction of the specific shear viscosity, i.e., the ratio of shear viscosity η to entropy density s of the plasma. Such extractions are currently being pursued via comparisons to viscous relativistic hydrodynamic simulations [16][17][18], transport model calculations [14,15] and hybrid approaches which involve the parametrization of scaling violations to ideal hydrodynamic behavior [10,12,13]. In all cases, accurate knowledge of the initial eccentricity ε 2,4 of the collision zone, is a crucial unresolved prerequisite for quantitative extraction of η s .…”

Deviation from quark number scaling of the anisotropy parameterv2of pions, kaons, and protons inAu+Aucollisions atAdare1, Afanasiev2, Aidala3 et al. 2012Phys. Rev. CSelf Cite827490View full textAdd to dashboardCite

“…Similarly, models that consider recombination between "thermal" partons (soft partons thermalized in the medium) and "shower" partons (partons fragmented from jets) predict centrality-dependent deviations from n q scaling that are particle-species dependent [30]. Understanding the limits of the recombination domain is important in relation to viscous hydrodynamics and the extraction of the shear viscosity over entropy density (η/s) from the data [31][32][33], as well as for developing a unified approach in describing jet energy loss and high p T v 2 [34][35][36]. Searches for deviations from n q scaling are also important for the low-energy scan program at RHIC as they have been considered as a signature of the transition between sQGP formation and a hadronic system.…”