Ali Hanks - APS 2008 1 Direct measurement of fragmentation photons in p+p collisions at √s =...
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Ali Hanks - APS 2008 1 Direct measurement of Direct measurement of fragmentation photons in fragmentation photons in p+p collisions at p+p collisions at √ √ s s = 200GeV with the PHENIX = 200GeV with the PHENIX experiment experiment Ali Hanks for the PHENIX collaboration Hard Probes 2008 Illa da Toxa, Galacia- Spain June 12 th , 2008
Ali Hanks - APS 2008 1 Direct measurement of fragmentation photons in p+p collisions at √s = 200GeV with the PHENIX experiment Ali Hanks for the PHENIX
Ali Hanks - APS 2008 1 Direct measurement of fragmentation
photons in p+p collisions at s = 200GeV with the PHENIX experiment
Ali Hanks for the PHENIX collaboration Hard Probes 2008 Illa da
Toxa, Galacia-Spain June 12 th, 2008
Slide 2
Ali Hanks - HP 2008 2 12/06/08 Photon Sources h ( 0, , ) Direct
photons Leading order(LO) - prompt: Next to leading order (NLO) -
fragmentation: Decay photons Decays are dominant source of photons
detected most (80%) from 0 's Major source of background for direct
photon measurement medium Medium induced modifications: Occurs at
NLO - modifies the fragmentation component of direct photon
spectrum only
Slide 3
Ali Hanks - HP 2008 3 12/06/08 Photons in heavy ion collisons
Many sources of photons which contribute to total Au+Au direct
photon cross-section difficult to test predictions of modification
of any single component from the inclusive spectrum Energy loss
effects insignificant within current experimental limitations
Turbide et al; hep-ph/0502248
Slide 4
Ali Hanks - HP 2008 4 12/06/08 Predicting modification
Measuring R AA could highlight any modification to direct photon
spectrum Various predictions for affects of energy loss range from
enhancement in to suppression Modification to production of
fragmentation photons is dominant source of variation from unity
Complication due to possible cold nuclear matter affects playing a
role I. Vitev; Winter Workshop 08
Slide 5
Ali Hanks - HP 2008 5 12/06/08 Why fragmentation photons? p+p
cross section necessary for study of nuclear modifications Improve
our understanding of jet quenching: direct measurement of radiation
spectrum Modification very sensitive to energy loss of the jet
significant enhancement for p T
Slide 6
Ali Hanks - HP 2008 6 12/06/08 Phys. Rev. Lett. 98, 012002
(2007): hep-ex/0609031 frag / inc for direct photons INCNLO(v1.4):
J. Ph. Guillet, M. Werlen et al Phys. Rev. Lett. 98, 012002 (2007):
hep-ex/0609031 pQCD predictions NLO pQCD describes data well
fragmentation component significant (> 20%) at low p T 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
Slide 7
Ali Hanks - HP 2008 7 12/06/08 Finding Fragmentation Photons
Use jet correlations to single out fragmentation photons Trigger on
high p T hadrons, 2-5 GeV/c, and calculate distribution of
associated photons, focusing on the near side (- /2 to /2) h Use
event mixing to correct for acceptance
Slide 8
Ali Hanks - HP 2008 8 12/06/08 Tagging 0 and decay photons
Tagged correlations are needed for subtracting decay photons tag
photons in pairs that fall within the 0 or peak h tag To measure
total decay yield, efficiency of tagging method is needed
Slide 9
Ali Hanks - HP 2008 9 12/06/08 Tagging efficiency Use
simulation of 0 / decays to obtain p T and dependent efficiency
correction give 0 s and s and p T dependent distributions around a
"trigger" hadron Kinematics of simulated 0 s and s are estimated
from data fit 0 and correlations to simulate their distributions
use input p T distribution from hadron trigger data to get
conditional yield Compare yield when both photons are accepted to
yield for all single 0 / photons accepted 00 trigger pair accepted
single accepted
Slide 10
Ali Hanks - HP 2008 10 12/06/08 Putting it all together The
final decay yield is obtained by combining everything we've seen so
far : Assuming heavier decay photon correlations can be estimated
using 's
Slide 11
Ali Hanks - HP 2008 11 12/06/08 We see fragmentation photons!
Final subtraction yields significant signal in most p T ranges
Ratio of near-side fragmentation photon yield to inclusive ~0.1 for
intermediate p T systematicScale systematic
Slide 12
Ali Hanks - HP 2008 12 12/06/08 Comparison with previous
results Can compare with range of allowed values based on isolated
photon measurement See agreement within systematic uncertainties
Should be cautious: very different conditions (biases) went into
these two measurements
Slide 13
Ali Hanks - HP 2008 13 12/06/08 Summary and Outlook pQCD NLO
calculations predict a frag spectrum > 20% of the inclusive
spectrum in pp collisions In Au-Au significant nuclear modification
to the fragmentation contribution may be seen Preliminary p+p
measurement shows fractional yield of fragmentation photons of
~0.1GeV/c at intermediate p T Success of method is promising for
future measurements p out and j T measurements will provide
information about the jet properties and may be more directly
comparable with theory calculations fragmentation photons in d+Au
and Au+Au Possibility for improvements in systematics at low and
high p T more sophisticated tagging efficiency estimates Careful
study of trigger p T dependence
Slide 14
Ali Hanks - HP 2008 14 12/06/08 Backup Slides
Slide 15
Ali Hanks - HP 2008 15 12/06/08 Reconstructing 0 's and 's
Reconstruct 0 's and 's from photon pairs, use event mixing to
estimate combinatoric background Fit remaining background to get
S/B correction
Slide 16
Ali Hanks - HP 2008 16 12/06/08 Subtraction method in Pythia
Measure 0 and photons directly to obtain decay background test
subtraction in PYTHIA Use to estimate remaining decay background (
, ') Compare with what PYTHIA gives for actual frag yield and
distribution PYTHIA
Slide 17
Ali Hanks - HP 2008 17 12/06/08 Tagging efficiency - 2nd method
Can also calculate tagging efficiency from full GEANT simulation
using PYTHIA input tag 0 photons as in the data, using simulated
kinematics construct true 0 photon distribution from PYTHIA
information extract tagging efficiency by calculating how often
true 0 photons are successfully tagged Compare this with what is
calculated using the fastmc Any differences are indications of
systematic uncertainties in how well the efficiency can be
determined Full simulation can be used to understand them
Slide 18
Ali Hanks - HP 2008 18 12/06/08 Tagging efficiency Comparison
shows a small systematic difference that needs to be understood
several possibilities currently being explored differences in
single particle efficiencies that contribute to loss of pair
photons would effect efficiencies differences in the energy
smearing