Low-pTcollective flow induces high-pTjet quenching

Abstract

Data on low-p T hadronic spectra are widely regarded as evidence of a hydrodynamic expansion in nucleusnucleus collisions. In this interpretation, different hadron species emerge from a common medium that has built up a strong collective velocity field. Here we show that the existence of a collective flow field implies characteristic modifications of high-p T parton fragmentation. We generalize the formalism of parton energy loss to the case of flow-induced oriented momentum transfer. We also discuss how to embed this calculation in hydrodynamic simulations. Flow effects are found to result generically in characteristic asymmetries in the η × φ plane of jet-energy distributions and of multiplicity distributions associated with high-p T trigger particles. However, collective flow also contributes to the medium-induced suppression of single inclusive high-p T hadron spectra. In particular, we find that low-p T elliptic flow can induce a sizable additional contribution to the high-p T azimuthal asymmetry by selective elimination of those hard partons that propagate with significant inclination against the flow field. This reduces, at least partially, the recently observed problem that models of parton energy loss tend to underpredict the large azimuthal asymmetry v 2 of high-p T hadronic spectra in semiperipheral Au + Au collisions.

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“…of the modeling of this idea, one arrives at estimates between dN ch PbPb /dη ≃ 1700 [8] and dN ch PbPb /dη ≃ 1800 − 2100 [9]. Similar values are also obtained by invoking other mechanisms to tame the perturbative growth [10].…”

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“…Most of the empirical properties of the quenching factor for light-flavor hadrons (magnitude, p T -, centrality-, √ s NN -dependences of the suppression) [ 34] are in quantitative agreement with the predictions of non-Abelian parton energy loss models which assume that the parent parton loses energy by gluonstrahlung while traversing a medium with a large color density. Very large initial gluon rapidity densities, dN g /dy ≈ 1000 [ 35], or equivalently, transport coefficients 3 , q ≈ 14 GeV 2 /fm [ 37,38], are needed to explain the amount of hadron suppression. However, the fact that the quenching factor for high-p T electrons from semi-leptonic D and B decays is as suppressed as the light hadrons in central AuAu (Fig.…”

“…Left: Data versus models for dN ch /dη| η=0 in AuAu at √ s NN = 200 GeV [ 13]. Right: Energy and centrality dependences (in terms of the number of nucleons participating in the collision, N part ) of dN ch /dη| η=0 (normalized by N part ): PHOBOS AuAu data [ 11] versus the predictions of the saturation approach [ 15] .…”

“…1, left). On the other hand, approaches [ 14,15] based on gluon saturation [ 4], which take into account a reduced initial number of scattering centers in the nuclear parton distribution functions,…”