Direct measurement of fragmentation photons in p+p collisions

Direct measurement of fragmentation photons in p+p collisions

Direct measurement of fragmentation photons in p+p collisions at
s = 200GeV with the PHENIX experiment
Ali Hanks, Columbia University, for the PHENIX Collaboration
[email protected]

p+p Motivation

The fragmentation component is significant ( 20%) at
low pT

Au+Au Motivation

How can we test this predictions?

Photons radiated by jets as they propagate do not
interact with the medium - make a good probe

- "isolation" cuts made to distinguish direct initial state
photons from fragmentation photons
difficult to incorporate these cuts into the theory
calculations
large uncertainties in comparison of direct photon yield
with and without this cut

Provide direct measure of radiation spectrum of jets
Expect strong final state interaction effects on
fragmentation (bremsstrahlung) photon cross-section

RAA predictions show enhancement highly
sensitive to jet quenching
Zakharov

hep-ph/0405101

no quenching

NLO pQCD describes the data well

A direct measurement of the fragmentation component
is necessary to provide a good test of the theory
Phys. Rev. Lett. 98, 012002 (2007): hep-ex/0609031

photon sources
Decay photons

Direct photons
Leading order(LO) - initial state:

h (0,,)

Decays are the dominant source of photons detected

Most (80%) from 0'ss
They are the major source of background for direct
photon measurment

with quenching

Next to leading order (NLO) fragmentation:

Medium induced modifications:
Occurs at NLO - modifies the fragmentation
component of direct photon spectrum only

Just bremsstrahlung/fragmentation photons

medium

Subtraction Method
Construct h- per trigger yields to select photons associated
with high pT hadrons
- Hadron trigger from 2-5 GeV/c
- Inclusive yield contains both fragmentation photons and the
hadronic decay background

Tagging method to calculate decay background
- Tag 0 and photons from invariant mass distribution of photon pairs
- Use simulations to calculated tagging efficiency
Scale Sys.

- Estimate remaining background from the yield

The Fragmentation photon signal can then be measured by
subtracting the decay background from the inclusive yield

Integrating subtracted near side
distribution shows a significant yield of
fragmentation photons
Flat non-zero background subtracted from
integral - included in systematic upper bound
Other direct photon sources will not be
correlated with the trigger hadron so do not
contribute to this signal

First result for ratio of
fragmentation to
inclusive photon per
trigger yield in 200
GeV p+p collisions

Applying similar
methods, additional jet
properties such as the jT
and pout distributions can
be measured.

Method Sys.

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