Measuring Bremsstrahlung Photons in ?s = 200GeV p-p Collisions

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Measuring Bremsstrahlung Photons in ?s 200GeV
p-p Collisions
Motivation
Photons radiated will not interact with the
medium - make a good probe

• In heavy ion collisions as jets propogate through
  the medium they lose energy through gluon
  radiation

Radiated gluons are lost through interaction with
the medium

• Provide direct measure of radiation spectrum
• Expect strong final state interaction effects on
  bremsstrahlung photon cross-section

• Gluon radiation spectrum cannot be measured
  directly

RAA predictions show significant enhancement for
pTlt10GeV/c
p-p reference
In pp collisions the "bremsstrahlung" photons are
produced as the jets fragment

• At next-to-leading-order pQCD describes the
  proton-proton photon cross-section well
• Includes fragmentation component 20
• The distinction between direct and fragmentation
  photons blurs
• Isolation cuts can be used to distinguish direct
  photons experimentally
• however it is difficult to incorporate these cuts
  into the theory calculations
• A direct measurement of the fragmentation
  component will provide a good test of the theory

INCNLO(v1.4) J. Ph. Guillet, M. Werlen et al

• trigger hadrons are detected using DC-PC1 tracks
  and PC3 projections
• photons are detected using the the EMC

Method

• Tagging efficiency calculated from fastmc
• ?? and pT dependent input distribution taken from
  data to simulate ?0's correlated with high pT
  hadrons

• Fragmentation photons are picked out by selecting
  photons associated with a high pT hadron
• Hadron trigger from 2-20 GeV/c, ?assoc from 1.8 -
  15 GeV/c
• Inclusive yield contains both fragmentation
  photons and the hadronic decay background

• ?0 tagging method to calculate decay background

photons that fall in the ?0 mass peak window are
tagged

• This gives us the corrected ?0 decay yield

• Assuming a similar shape for other hadronic
  decays, the ?0 decay yield can be used to
  calculate total decay yield

• The yield for tagged photons now needs to be
  corrected for efficiency

• h-?frag per trigger yield calculated using fully
  corrected ?decay per trigger yield

• The tagged ? yield, including corrections, can be
  subtracted from the inclusive yield, leaving only
  the fragmentation photons remaining

19thInternational Conference on
Ultra-Relativistic Nucleus-Nucleus Collisions
14-20 November, 2006 Shanghai, China