1. RAA predictions show enhancement highly sensitive to jet quenching
Columbia University, for the PHENIX Collaboration
Ali Hanks,
Direct measurement of fragmentation photons in p+p collisions at s = 200GeV with the PHENIX experiment
Phys. Rev. Lett. 98, (2007): hep-ex/
NLO pQCD describes the data well
The fragmentation component is significant (≥ 20%) at low pT
How can we test this predictions?
"isolation" cuts made to distinguish direct initial state photons from fragmentation photons
difficult to incorporate these cuts into the theory calculations
large uncertainties in comparison of direct photon yield with and without this cut
A direct measurement of the fragmentation component is necessary to provide a good test of the theory
p+p Motivation
Au+Au Motivation
Photons radiated by jets as they propagate do not interact with the medium - make a good probe
Provide direct measure of radiation spectrum of jets
Expect strong final state interaction effects on fragmentation (bremsstrahlung) photon cross-section
Zakharov hep-ph/
no quenching
with quenching
Just bremsstrahlung/fragmentation photons
RAA predictions show enhancement highly sensitive to jet quenching
Direct photons
Leading order(LO) - initial state:
Next to leading order (NLO) - fragmentation:
medium
Medium induced modifications:
Occurs at NLO - modifies the fragmentation component of direct photon spectrum only
h (0,,)
Decay photons
Decays are the dominant source of photons detected
Most (80%) from 0's
They are the major source of background for direct photon measurment
photon sources
Subtraction Method
Construct h- per trigger yields to select photons associated with high pT hadrons
Hadron trigger from 2-5 GeV/c
Inclusive yield contains both fragmentation photons and the hadronic decay background
Tagging method to calculate decay background
Tag 0 and  photons from invariant mass distribution of photon pairs
Use simulations to calculated tagging efficiency
Estimate remaining background from the  yield
Scale Sys.
Method Sys.
Integrating subtracted near side  distribution shows a significant yield of fragmentation photons
Flat non-zero background subtracted from integral - included in systematic upper bound
Other direct photon sources will not be correlated with the trigger hadron so do not contribute to this signal
The Fragmentation photon signal can then be measured by subtracting the decay background from the inclusive yield
First result for ratio of fragmentation to inclusive photon per trigger yield in 200
GeV p+p collisions
Applying similar methods, additional jet properties such as the jT and pout distributions can be measured.