We report recent results of high-pt measurements in Pb--Pb collisions at $sqrt{s_{NN}}=2.76$ TeV by the ALICE experiment and discuss the implications in terms of energy loss of energetic partons in the strongly interaction medium formed in the collisions.
We review the currently available formalisms for radiative energy loss of a high-momentum parton in a dense strongly interacting medium. The underlying theoretical framework of the four commonly used formalisms is discussed and the differences and co
mmonalities between the formalisms are highlighted. A quantitative comparison of the single gluon emission spectra as well as the energy loss distributions is given for a model system consisting of a uniform medium with a fixed length of L=2 fm and L=5 fm (the `Brick). Sizable quantitative differences are found. The largest differences can be attributed to specific approximations that are made in the calculation of the radiation spectrum.
The similarities and differences between three commonly used formalisms for radiative parton energy loss in hot strongly interacting matter are discussed. The single gluon emission spectra are evaluated for a model system consisting of a homogeneous
medium with a fixed length, the `TECHQM brick. Sizable quantitative differences are found and the origins of these differences are discussed.
Yields, correlation shapes, and mean transverse momenta pt{} of charged particles associated with intermediate to high-pt{} trigger particles ($2.5 < pt < 10$ GeVc) in d+Au and Au+Au collisions at $snn=200$ GeV are presented. For associated particles
at higher $pt gtrsim 2.5$ GeVc, narrow correlation peaks are seen in d+Au and Au+Au, indicating that the main production mechanism is jet fragmentation. At lower associated particle $pt < 2$ GeVc, a large enhancement of the near- ($dphi sim 0$) and away-side ($dphi sim pi$) associated yields is found, together with a strong broadening of the away-side azimuthal distributions in Au+Au collisions compared to d+Au measurements, suggesting that other particle production mechanisms play a role. This is further supported by the observed significant softening of the away-side associated particle yield distribution at $dphi sim pi$ in central Au+Au collisions.
High-pT particle production is suppressed in heavy ion collisions due to parton energy loss in dense QCD matter. Here we present a systematic comparison of two different theoretical approximations to parton energy loss calculations: the opacity expan
sion and the multiple-soft scattering approximation for the simple case of a quark traversing a homogeneous piece of matter with fixed length (the TECHQM brick problem), with focus on the range of parameters that is relevant for interpreting RHIC measurements of high-pT hadron suppression.
We present selected recent results of multi-hadron correlation measurements in azimuth and pseudorapidity at intermediate and high pt{} in Au+Au collisions at $sqrt{s_{NN}}=200$ GeV, from the STAR experiment at RHIC. At intermediate pt, measurements
are presented that attempt to determine the origin of the associated near-side (small dphi) yield at large pseudo-rapidity difference deta{} that is found to be present in heavy ion collisions. In addition, results are reported on new multi-hadron correlation measures at high-pt{} that use di-hadron triggers and multi-hadron cluster triggers with the goal to constrain the underlying jet kinematics better than in the existing measurements of inclusive spectra and di-hadron correlations.
These proceedings present a brief overview of the main results on jet-modifications in heavy ion collisions at RHIC. In heavy ion collisions, jets are studied using single hadron spectra and di-hadron correlations with a high-pt{} trigger hadrons. At
high pt, a suppression of the yields due to parton energy loss is observed. A quantitative confrontation of the data with various theoretical approaches to energy loss in a dense QCD medium is being pursued. First results from $gamma$-jet events, where the photon balances the initial jet energy, are also presented and compared to expectations from models based on di-hadron measurements. At intermediate pt, two striking modifications of the di-hadron correlation structure are found in heavy ion collisions: the presence of a long-range {it ridge} structure in deta{}, and a large broadening of the recoil jet. Both phenomena seem to indicate an interplay between hard and soft physics.
