In-medium jet shape from energy collimation in parton showers: Comparison with CMS PbPb data at 2.76 TeV

Abstract

We present the medium-modified energy collimation in the leading-logarithmic approximation (LLA) and next-to-leading-logarithmic approximation (NLLA) of QCD. As a consequence of more accurate kinematic considerations in the argument of the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi (DGLAP) fragmentation functions (FFs) we find a new NLLA correction ${\cal O}(\alpha_s)$ which accounts for the scaling violation of DGLAP FFs at small $x$. The jet shape is derived from the energy collimation within the same approximations and we also compare our calculations for the energy collimation with the event generators Pythia 6 and YaJEM for the first time in this paper. The modification of jets by the medium in both cases is implemented by altering the infrared sector using the Borghini-Wiedemann model. The energy collimation and jet shapes qualitatively describe a clear broadening of showers in the medium, which is further supported by YaJEM in the final comparison of the jet shape with CMS PbPb data at center-of-mass energy 2.76 TeV. The comparison of the biased versus unbiased YaJEM jet shape with the CMS data shows a more accurate agreement for biased showers and illustrates the importance of an accurate simulation of the experimental jet-finding strategy.