Measuring fragmentation photons in p+p collisions

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

2. frag/inc for direct photons Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031 Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031 INCNLO(v1.4): J. Ph. Guillet, M. Werlen et al pQCD predictions • 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

3. Zakharov hep-ph/0405101 no quenching with quenching Medium modifications • Expect strong final state effects such as significant enhancement for pT < 10 GeV/c • Improve our understanding of jet quenching: • direct measurement of radiation spectrum Just Bremsstrahlung photons • p+p cross section necessary for study of nuclear modifications Ali Hanks - APS

4. h (0,,) Photon Sources • Decays are dominant source of photons detected • most (80%) from 0's • Major source of background for direct photon measurement Decay photons 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

5.  h 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 Ali Hanks - APS

6. Assuming all decay photon correlations have the same shape as from 0s 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 Ali Hanks - APS

7.  π0 trigger  pair accepted single  accepted 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 Ali Hanks - APS

8. underestimate overestimate PYTHIA 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  Ali Hanks - APS

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

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