Title: 
       Direct photon and two particle correlations - on the mission to LHC
Speaker: 
                               Jan Rak
Affiliation:
                Jyvaskyla University and HIP, Finland

Abstract:
Measurement of the direct photon yield provides a rich information
about the exited QCD medium created in heavy ion collisions. High
transverse momentum direct photons are produced in hard parton-parton
scattering by inverse QCD Compton scattering and $q\bar{q}$
annihilation. Unlike scattered quarks or gluons, direct photons do not
interact strongly with the QCD medium. Furthermore, direct photons
carry, in the lowest approximation, the same energy as the outgoing
quark on the opposite side and thus the away-side particle
distribution associated to the direct photon can be used as tool to
study the modification of the fragmentation function in heavy ion
collisions. However the outgoing partons produced in the hard process
carry also an initial transverse momentum $k_{T}$. This results in a
transverse momentum imbalance and acoplanarity. Therefore a completed
understanding of this effect in $p+p$ collisions is vital for
comparison to the results in heavy ions. An analysis of the expected
influence of the $k_{T}$ effect on the direct photon-hadron
correlations will be presented. Various aspects of direct photon
physics at LHC will be discussed as well.