1. 4/23/06 Ali Hanks - APS 1 A method for directly measuring bremsstrahlung photons from jets Ali Hanks APS Conference April 23, 2006

2. 4/23/06 Ali Hanks - APS 2 Motivation Provides direct measurement of radiation spectrum Expect strong final state interaction effects on bremsstrahlung contribution May be significant enhancement for p T < 10GeV/c Zakharov hep-ph/0405101 no quenching with quenching pp collisions important for study of nuclear modifications Understanding jet quenching: – a probe of the QGP Just Bremsstrahlung photons

3. 4/23/06 Ali Hanks - APS 3 pQCD predictions NLO pQCD describes data well At NLO distinction between direct photons and bremsstrahlung (fragmentation) photons blurs –“isolation” cuts made to distinguish direct photons –difficult to match to theory fragmentation component > 20% for p T < 10 GeV/c direct measurement of fragmentation contribution good test of theory Curves: INCNLO 1.4 INCNLO(v1.4): J. Ph. Guillet, M. Werlen et al fragmentation = bremsstrahlung

4. 4/23/06 Ali Hanks - APS 4 Methodology Pick out bremsstrahlung photons by selecting photons associated with a jet – Trigger on high p T hadrons – Calculate ∆  distribution for associated high p T photons – Correct for PHENIX acceptance by Remove background sources – Tag photons coming from π 0 decays – Correct for tagging efficiency – Estimate contribution from other decay sources

5. 4/23/06 Ali Hanks - APS 5 Inclusive h-  correlations Run 3 d-Au data was looked at as a first attempt Initial cuts made to select good runs and remove misidentified photons/tracks PHENIX acceptance corrected for by calculating ∆  distribution for mixed events (no correlation) Raw hadron-  ∆  distribution ∆∆ Inclusive hadron-  correlation function ∆∆

6. 4/23/06 Ali Hanks - APS 6 Removing π 0 decay background - 1 st attempt π 0 decay is most significant source of background –reduced by isolation cuts in case of direct photons –Jet-associated photons don’t have this advantage Calculate invariant mass for photon pairs Tag pairs with mass of 120-160MeV/c 2 Produce new correlation function Tagging efficiency must be included Invariant mass distribution GeV/c 2 hadron-  (π 0 ) correlation ∆∆

7. 4/23/06 Ali Hanks - APS 7 Evaluating the π 0 tagging efficiency Simulations of π 0 decays provide efficiency correction ( p T > 3GeV/c ) –π 0 s generated with p T dependent Gaussian distribution around “trigger” hadron –∆  distribution of decay photons calculated –Compare to photons from pairs where both photons accepted –Ratio gives tagging efficiency p T dependence of input π 0 distribution implies p T dependence in efficiency Input π 0 ∆  distribution ∆∆ no pair cut π0π0 trigger  hadron-  ∆  distribution ∆∆ pair acceptance

8. 4/23/06 Ali Hanks - APS 8 Evaluating the π 0 tagging efficiency Tagging efficiency p T < 3.5 GeV/c ∆ø Tagging efficiency 4 < p T < 4.5 GeV/c ∆ø Tagging efficiency p T > 6 GeV/c ∆ø Separate efficiency into several bins in p T of decay photon

9. 4/23/06 Ali Hanks - APS 9 Other sources of background  decay also source of background –distribution of decay photons may differ from that of π 0 s Try simulation with  kinematics –Same input p T spectrum weighted by  /π 0 ratio Fit decay distributions to check similarity Similarity means  decays can be subtracted using π 0 distribution hadron-  ∆  distribution π 0 photons  photons ∆∆ ∆ ∆  ~.1153+/-.0006 hadron-  (π 0 ) ∆  distribution hadron-  (  ) ∆  distribution  ~.1062+/-.0007 ∆ ∆

10. 4/23/06 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 Brehmsstrahlung contribution may be seen The Run 3 data was short on statistics, but the measurement should be possible with Run 5 pp Looking for photons associated with a hadron has the advantage of removing random background source as well as other sources of direct photons The challenge will be in removing the π 0 background, which is largest around jets

11. 4/23/06 Ali Hanks - APS 11 Results from Pythia for pp