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\begin{center}
{\Large Extracting jet fragmentation functions using photon-tagged jet events in Pb+Pb collisions with the CMS experiment\\ }
Christof Roland for the CMS Collaboration\\
\end{center}
The energy loss of fast partons traversing the strongly interacting matter
produced in high-energy nuclear collisions is one of the most interesting 
observables to probe the nature of the produced medium. The collisional and 
radiative energy loss of the partons will modify their fragmentation functions 
depending on the path length of the partons in the medium and the medium density. 
Pb+Pb collisions at $\sqrt{s_{\scriptscriptstyle{{\rm NN}}}}=5.5$~TeV at the LHC 
will allow us to perform the first detailed measurements of the in-medium modifications 
of fragmentation functions of parton initiated jets.

The fragmentation functions can be studied using the $\gamma$-jet 
channel~\cite{Wang:1996yh,Arleo:2006xb}, where the  initial transverse energy of the 
fragmenting parton can be determined from the photon transverse energy, since the 
photon does not strongly interact with the medium.

The CMS experiment features large acceptance electromagnetic and hadronic calorimeters,
a high precision tracking system. 
To extract fragmentation functions of parton initiated jets in $\gamma$-jet events, 
isolated photons reconstructed in the electromagnetic calorimeter ($\abs{\eta}<2.0$) are selected and 
correlated with back-to-back reconstructed calorimeter jets ($\abs{\eta}<2.0$) in \PbPb\ events. 
For the selected photon jet pairs, charged particles reconstructed in the tracking system ($\abs{\eta}<2.5$)
that lie within a $0.5$ radius cone in $\eta$-$\phi$ around the reconstructed jet axis 
are selected. The fragmentation function is constructed correlating the transverse 
energy of the photons, with the transverse momentum of the tracks in the jet cone.

This study based on a full geant simulation of a data set of $\gamma$-jet events, 
containing a $\gamma$ with \et\ $> 70$~GeV, embedded in central \PbPb\ events 
corresponding to a full year of data taking with an integrated luminosity of 0.5~nb$^{-1}$. 
The capability to measure jet fragmentation functions in the $\gamma$ jet channel with high 
statistics is demonstrated. The systematic uncertainties are estimated to be on the order of 
30\% (10\%) for quneched events containing a $\gamma$ with \et\ $> 70$~GeV ($100$~GeV) mostly 
due to a bias induced by the finite efficiency of the jet reconstruction. Overall the 
fragmentation function can be reconstructed with high significance for 0.2 $< \xi <$ 5 
for both, \et\ $> 70$~GeV and $> 100$~GeV.

Reconstructed $\gamma$-jet events can be used to study the dependence of 
high-$\pt$ parton fragmentation on the medium and will provide 
sensitivity to the foreseeable changes in the fragmentation functions 
relative to fragmentation in vacuum. This measurement will allow a 
quantitative test of proposed mechanisms for parton energy loss in the 
medium, testing fundamental properties of the high-density QCD medium
produced in high-energy nuclear collisions.
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\bibitem{Wang:1996yh}
  X.~N.~Wang, Z.~Huang and I.~Sarcevic, Phys.\ Rev.\ Lett.\  {\bf 77} (1996) 231.

\bibitem{Arleo:2006xb}
  F.~Arleo, JHEP {\bf 0609} (2006) 015.
\end{thebibliography}
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