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34th International Conference on High Energy Physics, Philadelphia, 2008
Direct Photons at RHIC
G. David (for the PHENIX Collaboration)
BNL, Upton, NY 11973, USA
Direct photons are ideal tools to investigate kinematical and thermodynamical conditions of heavy ion collisions since
they are emitted from all stages of the collision and once produced they leave the interaction region without further
modification by the medium. The PHENIX experiment at RHIC has measured direct photon production in p+p
and Au+Au collisions at 200 GeV over a wide transverse momentum (PT) range. The p+p measurements allow a
fundamental test of QCD, and serve as a baseline when we try to disentangle more complex mechanisms producing
high PT direct photons in Au+Au. As for thermal photons in Au+Au we overcome the difficulties due to the large
background from hadronic decays by measuring "almost real" virtual photons which appear as low invariant mass
e+e- pairs: a significant excess of direct photons is measured above the above next-to-leading order perturbative
quantum chromodynamics calculations. Additional insights on the origin of direct photons can be gained with the
study of the azimuthal anisotropy which benefits from the increased statistics and reaction plane resolution achieved
in RHIC Year-7 data.
Direct photons - defined as those not originating from (final state) hadron decays - are an important tool with
unique capabilities to study both elementary particle and heavy ion collisions. By not interacting strongly they
have a large mean free path and leave the interaction region unaltered even if they propagate through the hot,
dense medium assumed to be formed in relativistic heavy ion collisions. The lowest order QCD processes generating
photons - quark-antiquark annihilation (qq -> gy) and quark-gluon Compton scattering (qg -> q-) - are reasonably
well understood  thus photons are a sensitive probe of modifications of the parton distribution functions in nuclei
(nPDFs). Furthermore, in p+p collisions at higher transverse momenta (PT) the Compton process dominates, thus
the photon "calibrates" the total energy of the opposing gluon jet (apart of the uncertainty stemming from the
intrinsic kT of partons). Setting the energy scale of the hard scattered parton (and the ensuing jet) is even more
important in heavy ion collisions, where the partons lose a significant fraction of their energy while crossing the
At higher order fragmentation (or Bremsstrahlung) photons contribute both in p+p and heavy ion collisions; the
calculated rates in p+p are consistent with recent measurements but the uncertainties are large both on the theoretical
and experimental side . It is very important to understand all p+p processes as well as possible because in heavy
ion collisions several new photon production mechanisms emerge or become significant, and often it is exactly the
difference from p+p behavior that reveals the new physics. Case in point: the nuclear modification factor RAA, the
ratio of a cross-section measured in A+A collisions and the corresponding p+p cross-section scaled by the nuclear
thickness function. If RAA is different from unity, it invariably signals some new physics in A+A with respect to p+p
collisions. If RAA 1 the interpretation is somewhat less clear: either the physics mechanisms producing photons
are exactly the same as in p+p, or there might possibly be a "conspiracy" of mechanisms, some increasing, others
decreasing RAA but ultimately balancing each other. Also, one has to be careful when interpreting direct photon
RAA. For high PT hadrons (thought to be mostly leading fragments of jets) scaling the p+p cross-sections with the
nuclear thickness to obtain the expected A+A rates (colloquially "scaling with the number of binary nucleon-nucleon
collisions" or "N~o01 scaling") is legitimate due to the isospin symmetry of protons and neutrons in the nucleus. The
same is not true for direct photons: due to the electromagnetic coupling the photon cross section is proportional to
the sum Ee of quark charges squared, different for p and n, causing a trivial deviation of RAA from unity ("isospin
effect" ) which may either mask or enhance other effects changing RAA. In fact, the cleanest way to generate
direct photon RAA would be to compare to a properly scaled mixture of pp, pn and nn cross-sections at the same
energy, which could (and in the author's private opinion, should) be measured at RHIC from d+d collisions, since
the above three reactions could be tagged event-by-event.
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Gabor,D. Direct Photons at RHIC, article, July 29, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc899490/m1/3/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.