Medium modification of jet shapes and jet multiplicities

Medium-induced parton energy loss is widely considered to underlie the suppression of high-pt leading hadron spectra in square root sNN = 200 GeV Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC). Its description implies a characteristic kt broadening of the subleading hadronic fragments associated with the hard parton. However, this latter effect is more difficult to measure and has remained elusive so far. Here, we discuss how it affects genuine jet observables, which are accessible at the Large Hadron Collider and possibly at RHIC. We find that the kt broadening of jet multiplicity distributions provides a very sensitive probe of the properties of dense QCD matter, whereas the sensitivity of jet energy distributions is much weaker. In particular, the sensitive kinematic range of jet multiplicity distributions is almost unaffected by the high multiplicity background.