Direct Photons at RHIC Page: 4 of 6
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34th International Conference on High Energy Physics, Philadelphia, 2008
While direct photons carry a plethora of (almost) unbiased information, measuring them is very challenging due
to the large background from hadron decays. Substantial statistics and a thorough understanding of systematic
errors are required. If in addition to the total direct photon yield one wants to disentangle the contributions from
different physics sources (with minimal theoretical input and assumptions) the difficulties are even higher - but not
insurmountable. Studying the emergence and evolution of new phenomena and at the same time tight control of the
systematics is crucial. In specific, measuring various colliding systems (from p+p to the heaviest A+A) at different
c.m.s. energies in the very same experiment goes a long way toward this goal. The flexibility of the accelerator and
of the experiments at RHIC offers such an opportunity.
2. DIRECT PHOTONS AT RHIC - HIGH PT AND THE THERMAL REGION
Direct photon cross-sections in Vs 200GeV p+p collisions at RHIC were first published in the limited 5 <
PT < 8GeV/c range in , followed by a measurement in the 3 < PT < 16GeV/c range with considerably smaller
systematic errors and detailed comparison to NLO pQCD calculations . It was found that above PT > 5GeV/c
(where the systematic errors of the experiment and theory were comparable) the data are well described by the
theory, including the fraction of isolated photons. This was welcome news after the "controversial situation"  of
the past decade. Since RHIC collides polarized protons, a measurement of the polarized gluon structure functions is
now within reach. The data also provide a measured - rather than calculated - baseline for the nuclear modification
in heavy ion collisions (modulo the isospin-effect).
The first direct photon invariant yields up to PT 13GeV/c from sNN 200GeV Au+Au collisions were
published in , covering all collision centralities (impact parameter ranges) and in the same paper the direct photon
RAA, using an NLO pQCD reference and integrated above PT > 5GeV/c was shown to be unity within errors for
all centralities. This was a landmark result because it retroactively validated the concept of "N,0ii-scaling" and
RAA itself in studying "jet-quenching" at high PT with single inclusive hadron spectra [7, 8]. Although recent
developments in theory and better data help to draw a more nuanced picture of how, where and when high PT
photons are produced (in addition to primordial hard scattering), the above conclusion is still sane, not the least
thanks to the large differences between the effects: 7r -s are suppressed by a factor of 5, while direct photons (still
mostly from hard scattering) are suppressed much less, or not at all.
More recent (and still preliminary) data on direct photon RAA and comparisons to theoretical calculations are
shown on Fig. 1. With the PT range now extended to 20GeV/c photons appear to be somewhat suppressed. As
mentioned before, some of this might be of trivial origin; the solid line predicts a 15% drop due to the isospin effect
alone. Counteracting to this - particularly at medium PT, i.e. in the 0.1 < x =2pT/Vs < 0.2 region - is the
anti-shadowing (dash-dotted curve). On the other hand, if parton energy loss is added, direct photon production is
suppressed in the entire range (band at the bottom of the figure).
While the energy loss in the medium suppresses the yield of high PT particles, possibly including photons, the
"jet-photon conversion" mechanism  may increase high PT photon production. In this process a hard scattered
quark (proto-jet) interacts with a gluon or antiquark of the medium. The collision has a collinear singularity thus
the outcoming photon carries the full momentum of the original parton. Note that since antiquarks abund in the
thermalized medium, qq annihilation is no longer suppressed with respect to Compton-scattering of gluons thus a
new channel to direct photon production opens up.
An up-to-date overview of the known/assumed direct photon production mechanisms in heavy ion collisions is
given in  and detailed, centrality-dependent calculations are shown in . The tantalizing question is whether
the contributions from the individual components can be disentangled experimentally? In other words: can the
theory be tested? The answer is a tentative "yes" with a possible path laid out in  and briefly repeated here.
First, one has to realize that future, substantially larger data volumes not only decrease the statistical errors, but
they also help to apply more sophisticated analysis techniques that reduce the systematic errors. Also, they allow
meaningful analyses of multiple-differential quantities like azimuthal asymmetries of photon production. While such
asymmetries are not expected from the primordial hard scattering, medium-related photon sources will exhibit them.
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Gabor,D. Direct Photons at RHIC, article, July 29, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc899490/m1/4/: accessed April 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.