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

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, 012002 (2007): hep-ex/0609031 Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031  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

Medium modifications Improve our understanding of jet quenching: Zakharov hep-ph/0405101 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

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

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

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

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

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

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

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