Transcript Measuring bremsstrahlung photons in pp collisions (update)

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 12th, 2008
Ali Hanks - APS 2008
1
Photon Sources
Decay photons
h (0,,)
• 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 - HP 2008
12/06/08
2
Photons in heavy ion collisons
Turbide et al; hep-ph/0502248
• 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
Ali Hanks - HP 2008
12/06/08
3
Predicting modification
• Measuring RAA 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
Ali Hanks - HP 2008
12/06/08
4
Why fragmentation photons?
• Improve our understanding
of jet quenching:
– direct measurement of radiation
spectrum
• Modification very sensitive
to energy loss of the jet
p+p cross section necessary for
study of nuclear modifications
– significant enhancement for
pT<10 GeV/c
Ali Hanks - HP 2008
12/06/08
5
pQCD predictions
• NLO pQCD describes data
well
• fragmentation component
significant (> 20%) at low pT
• How can we test this
prediction?
frag/inc for direct photons
– “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 - HP 2008
Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031
INCNLO(v1.4):
J. Ph.
Guillet,
M. Werlen
et al
Phys.
Rev. Lett. 98,
012002
(2007):
hep-ex/0609031
12/06/08
6
Finding Fragmentation Photons
• Use jet correlations to single out fragmentation photons
– Trigger on high pT hadrons, 2-5 GeV/c, and calculate  distribution of
associated photons, focusing on the near side (-/2 to /2)

Use event mixing to
correct for acceptance


h
Ali Hanks - HP 2008
12/06/08
7
Tagging 0 and  decay photons
• Tagged correlations are needed for subtracting decay photons
– tag photons in pairs that fall within the 0 or  peak


tag


h
To measure total decay yield, efficiency of tagging method is needed
Ali Hanks - HP 2008
12/06/08
8
Tagging efficiency
• Use simulation of 0/ decays to
obtain pT and  dependent
efficiency correction

– give 0’s and ’s  and pT
dependent distributions around a
"trigger" hadron
π0
trigger
• Kinematics of simulated 0’s and
’s are estimated from data
– fit 0 and  correlations to simulate
their  distributions
 pair accepted
– use input pT distribution from
hadron trigger data to get
conditional yield
• Compare yield when both
photons are accepted to yield for
all single 0/ photons accepted
Ali Hanks - HP 2008
single  accepted
12/06/08
9
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
Ali Hanks - HP 2008
12/06/08
10
We see fragmentation photons!
 systematic
Scale systematic
• Final subtraction yields significant signal in most pT ranges
• Ratio of near-side fragmentation photon yield to inclusive ~0.1 for
intermediate pT
Ali Hanks - HP 2008
12/06/08
11
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
Ali Hanks - HP 2008
12/06/08
12
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 pT
• Success of method is promising for future measurements
– pout and jT 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 pT
– more sophisticated tagging efficiency estimates
– Careful study of trigger pT dependence
Ali Hanks - HP 2008
12/06/08
13
Backup Slides
Ali Hanks - HP 2008
12/06/08
14
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
Ali Hanks - HP 2008
12/06/08
15
Subtraction method in Pythia
• Measure 0 and  photons directly to obtain decay background
• test subtraction in PYTHIA
Use  to estimate
remaining decay
background (, ')
PYTHIA
PYTHIA
Compare with what PYTHIA gives for
actual frag yield and  distribution
Ali Hanks - HP 2008
12/06/08
16
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
Ali Hanks - HP 2008
12/06/08
17
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
Ali Hanks - HP 2008
12/06/08
18