Understanding LHC jets in the light of RHIC data

Hard probes are a cornerstone in the ongoing program to determine the properties of hot and dense QCD matter as created in ultrarelativistic heavy ion collisions. LHC measurements have so far resulted in a wealth of high P_T data, opening new kinematic windows with high statistics. Yet on first glance, several observations are counter-intuitive and seem to contradict results from the RHIC high P_T program. This calls for a combined analysis of high P_T hadrons and reconstructed jets at RHIC and LHC in a unified framework testing a large number of theoretical models for both medium evolution and shower medium interactions against the systematics of the data. A consistent picture of shower-medium interaction emerges from this analysis which explains where and why results appear counter-intuitive.Comment: 4 pages, 1 figure, talk given at the Quark Matter 2012 International Conference, August 13-18, Washington D.C., US

From R_AA via correlations to jets - the long road to tomography

The main motivation to investigate hard probes in heavy ion collisions is to do tomography, i.e. to infer medium properties from the in-medium modification of hard processes. Yet while the suppression of high P_T hadrons has been measured for some time, solid tomographic information is slow to emerge. This can be traced back to theoretical uncertainties and ambiguities in modelling both medium evolution and parton-medium interaction. Ways to overcome these difficulties are to constrain models better and to focus on more differential observables. Correlations of high P_T hadrons offer non-trivial information beyond what can be deduced from single hadron suppression. They reflect not only the hard reaction being modified by the medium, but also the back reaction of the medium to the hard probe. Models for hard back-to-back correlations are now very well constrained by a wealth of data and allow insights into the nature of the parton-medium interaction as well as first true tomographic results. Models of full in-medium jet evolution are being actively developed, but have yet to make substantial contact with data. Progress is slower in the understanding of low P_T correlations, the ridge and the cone, although a qualitative understanding of the nature of the physics behind these correlations starts to emerge.Comment: 8 pages, 3 figures- To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Focus talk on interactions between jets and medium

The energy and momentum lost by a hard parton propagating through hot and dense matter has to be redistributed during the nuclear medium evolution. Apart from heating the medium, there is the possibility that collective modes are excited leading to the emergence of Mach cones or Cherenkov radiation. Recent two-particle correlation measurements by STAR \cite{Wang:2004kf} and PHENIX \cite{Adler:2005ee} at RHIC indicate that such phenomena may play an important role in understanding the jet-medium interactions. Possible collective modes are discussed and it is demonstrated that Mach cones as created by colorless or colored sound are a possible explanation of the hardronic two-particle correlation data.Comment: Talk given at Quark Matter 2005, 18th Internationl Conference on Nucleus-Nucleus Interactions, August 4-9, 2005, Budapest, Hungar