Jets and parton energy loss at RHIC: experimental status

Abstract

Experimental evidence from RHIC indicates that matter having an energy density far in excess of the value required for the creation of a deconfined phase is produced in ultrarelativistic Au+Au collisions at √ s NN =200 GeV. This matter thermalizes rapidly, is strongly interacting, and displays hydrodynamic properties akin to a fluid with very low viscosity. Studies of the interaction of hard scattered partons with this matter provide an important probe of its properties. In fact, studies of the suppression of high-p T hadrons have been one of the key arguments used by the RHIC experiments that the matter created is something fundamentally new and exciting. I review the current experimental picture that has emerged from the data collected at RHIC, with an emphasis on the effort to extract quantitative measurements from the suppression of high-p T hadrons.Keywords: RHIC; Heavy-ion collisions; Jets; Energy loss I. JETS AS PARTONIC PROBESThe hard scattering of partons in the initial state of an ultrarelativistic heavy-ion collision holds great promise as a probe of the hot, dense matter created in these collisions. The initial hard scatterings occur very early in the collision process (τ < 1 fm/c) and are sensitive to the evolution of the surrounding, color-charged medium primarily through energy loss mechanisms (either collisional or radiative) [1]. These hard-scattered partons are not observed by themselves in the final state, however, as they fragment into collimated "jets" of color-singlet hadrons, clustered around the initial parton direction. Experimentally, jets can be identified and studied through the correlation of hadrons in the final state, or through the single inclusive spectra of high-p T particles. In this contribution I will limit the discussion to what we have learned about the properties of the medium created in heavy ion collisions through the use of single inclusive spectra. The properties of the medium that can be inferred through particle correlations are discussed in another contribution to this conference [2]. A. Hard Probes and pQCDIn order to use hard scattering as a probe of nuclear matter we need to have a solid understanding of three main components: initial state effects, the parton-level cross section, and final state effects (exclusive of the effect of the matter itself). In the collinear factorization ansatz, the cross section for the production of high-p T particles can be written as:Initial state effects include the distribution of partons in the initail nuclei, as represented by the function f a/n (x a , Q 2 ). For example, a common initial state effect is the multiple scattering of partons prior to the hard scattering event, which results in a significant additional transverse momentum of the final state hadrons (the "Cronin effect" [3]). These effects can be identified through the use of collisions like d+Au collisions at RHIC, where the final state effects are expected to be small. In a similar way, our understanding of the partonic level cross section dσ dt (ab → cd) can be e...

Summary talk for ISMD 06

“…Lajoie presented the latest data from Phenix concerning this energy loss [28]. One has a good phenomenological understanding of jet energy loss for light quarks, but a detailed quantitative theory is difficult [4].…”

Dissociation of Pseudotensor Mesons through Critical Temperature

“…Quarks and gluons condense to create a gas of nucleons and light mesons, the latter decay subsequently. Jet quenching for light mesons at RHIC is well explained by radiative energy loss calculations [10,11]. As a result, probing the variations of hadronic parameters depending on temperature can provide us valuable information on the new phase of matter, i.e.…”