Measurement of the hard component of jet fragmentation functions in PbPb collisions at 2.76 TeV

This note describes the analysis of jet fragmentation functions in PbPb collisions at 2.76 TeV center of mass energy collected in 2010 with the CMS detector at the LHC. Jets are reconstructed using the anti-$k_T$ sequential clustering algorithm~\cite{Cacciari:2008gp} with a resolution parameter of 0.3, using the particle flow method of event reconstruction. The heavy-ion background is subtracted using the iterative pileup subtraction method described in~\cite{Kodolova:2007hd}. Fragmentation functions were constructed with charged particle tracks of $\pt\ > 4 \GeVc$ in a sample of dijet events. Both the leading and subleading jets show fragmentation similar to that of a parton in a vacuum, as seen by comparisons to pp data and to simulations at 2.76 TeV of {\sc {pythia}}~\cite{bib_pythia} dijets embedded in a heavy ion background generated with {\sc {hydjet}}~\cite{Lokhtin:2005px}. The fragmentation function is studied as a function of collision centrality, as well as dijet $\pt$ asymmetry. The results provide evidence that, despite the large energy loss of partons as they traverse the nuclear medium, the hard component of the fragmentation function evaluated with respect to the reconstructed jet is not strongly modified in comparison to jet fragmentation in vacuum.