1. Measuring fragmentation photons in p+p collisions
Ali Hanks
for the PHENIX collaboration
APS Conference
April 15, 2007
Ali Hanks - APS
4/15/07

2. pQCD predictions  NLO pQCD describes data well
frag/inc for direct photons
INCNLO(v1.4): J. Ph. Guillet, M. Werlen et al
Phys. Rev. Lett. 98, (2007): hep-ex/
Phys. Rev. Lett. 98, (2007): hep-ex/ 
NLO pQCD describes data well
fragmentation component significant (> 20%) at low pT
How can we test this prediction?
“isolation” cuts made to distinguish prompt photons
difficult to match to theory
direct measurement of fragmentation contribution necessary to really test theory
“direct” photons should be excluded
Ali Hanks - APS
4/15/07

3. Medium modifications Improve our understanding of jet quenching:
Zakharov hep-ph/
no quenching
with quenching
Improve our understanding of jet quenching:
direct measurement of radiation spectrum
Just Bremsstrahlung photons
Expect strong final state effects such as significant enhancement for pT < 10 GeV/c
p+p cross section necessary for study of nuclear modifications
Ali Hanks - APS
4/15/07

4. Photon Sources Decay photons Direct photons
Decays are dominant source of photons detected
most (80%) from 0's
Major source of background for direct photon measurement
Direct photons
Medium induced modifications:
Leading order(LO) - prompt:
medium
Next to leading order (NLO) - fragmentation:
Occurs at NLO - modifies the fragmentation component of direct photon spectrum only
Ali Hanks - APS
4/15/07

5. Finding Fragmentation Photons
Use jet correlations to single out fragmentation photons
Trigger on high p_T hadrons, 2-15 GeV/c, and calculate  distribution of associated photons
Use event mixing to correct for acceptance  h
Ali Hanks - APS
4/15/07

6. Estimating 0 background
Tag 0 decay photons using invariant mass of photon pairs
Use simulations to calculate tagging efficiency
Use π0s to obtain full background
Assuming all decay photon correlations have the same shape as from 0s
Ali Hanks - APS
4/15/07

7. Tagging efficiency
Use fast MC to simulate 0 decays to obtain pT and  dependent efficiency correction
generate 0s in gaussian distribution around "trigger" hadron
Compare yield when both photons are accepted to yield for all single 0 photons accepted  π0
trigger
 pair accepted
single  accepted
Ali Hanks - APS
4/15/07

8. Remaining decay background
Can we use π0 decays to estimate all decays?
Use PYTHIA to study difference in shape of  distribution for other decays from that of π0 decays - first try 
PYTHIA
underestimate
overestimate
Ali Hanks - APS
4/15/07

9. Getting the shape right
How well can we do measuring the eta directly?
Compare with what PYTHIA gives for actual frag  distribution and yield
PYTHIA
Use  to estimate remaining decay background (, ')
h - frag near side  distributions
PYTHIA
Ali Hanks - APS
4/15/07

10. Summary
pQCD NLO calculations predict a frag spectrum > 20% of the inclusive spectrum in pp collisions
In Au-Au significant nuclear modification to the bremsstrahlung contribution may be seen
Looking for photons correlated with a hadron has the advantage of maximizing the fragmentation signal while removing uncorrelated prompt photons and random background sources
PYTHIA shows a difference in correlation shape between 0 and other sources that needs to be accounted for to do final background subtraction
may be possible to estimate this using simulation
Ali Hanks - APS
4/15/07