Flow effects on jet profiles and multiplicities

We study the effects of low-pT collective flow on radiative energy loss from high-pT partons traversing the QCD medium created in high-energy nucleus-nucleus collisions. We illustrate this idea through three examples. Due to longitudinal flow, jet profiles at the LHC present marked asymmetries in the η×φ-plane, and widths in η and φ of particle distributions associated with a high-pT trigger at RHIC become different. Finally, transverse flow implies an increase of high-pT v2 at RHIC. PACS. 25.75.Ld Collective flow in relativistic heavy-ion collisions – 24.85.+p Quarks, gluons, and QCD in nuclei and nuclear processes – 25.75.Gz Particle correlations in relativistic heavy-ion collisions 1 Motivation and formalism Low transverse momentum inclusive spectra and azimuthal correlations measured in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) indicate that different hadron species emerge from a common medium which has built up a strong collective velocity field [1,2,3,4]. These measurements are broadly consistent with calculations based on ideal hydrodynamics, whose success is regarded as strong evidence [5] that the medium produced in nucleus-nucleus collisions has a very small mean free path, shows a very rapid thermalization at a time less than 1 fm/c after initial impact, and behaves like an almost ideal fluid with vanishing viscosity. This suggests strong position-momentum correlations in the medium. In this contribution we explore possible effects of this collective flow on high-pT observables [6,7]. At collider energies, the production of high-pT hadrons and jets provides a novel independent characterization of the produced medium. This is so since the gluon radiation off parent partons is sensitive to the interaction between the partonic projectile and the medium, see the reviews in [8,9,10,11,12]. The radiative energy loss of a fast parton traversing a QCD medium is determined by momentum exchanges perpendicular to the trajectory of the parton. Thus, if the hard parton (jet) is produced in a frame not co-moving with the collective flow, momentum exchanges become anisotropic and an additional contribution to energy loss comes from flow, see Fig. 1. At a given energy density ǫ, the dynamic behaviour of the medium is fully specified by its equation of state (EOS) a Present address: Departamento de F́ısica de Part́ıculas and Instituto Galego de Altas Enerx́ıas, Facultade de F́ısica, Campus Sur, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain. Fig. 1. Sketch of the radiation from a fast parton traversing the vacuum (left), a static medium (center) and a flowing medium (right). p = p(ǫ, T, μB) which enters the energy momentum tensor T (x) = (ǫ+ p) u u − p g . (1) Here, u = γ(1,β) is the flow velocity field. On the other hand, quenched high-pT hadroproduction is sensitive to the transport coefficient q̂, which is proportional to the density of scattering centres and characterizes the squared average momentum transfer from the medium to the hard parton per unit path length. This transport coefficient is related to ǫ [13], q̂ [ GeV/fm ] = c ǫ [ (GeV/fm)3/4 ] . Here, c is a proportionality constant of order unity [13,14]. In order to take into account the effect of the anisotropy in momentum exchanges due to the presence of flow on radiative energy loss, we modify the Yukawa-like scattering 2 Néstor Armesto: Flow effects on jet profiles and multiplicities potential usually employed to model the medium [15],