Transcript Folie 1 - Wayne State University

Direct-Photon Production in
PHENIX
Oliver Zaudtke
for the
Collaboration
Winter Workshop on Nuclear Dynamics 2006
What are Direct Photons?
Definition:
Photons that emerge directly from a particle collision
inclusive photons =
decay photons
+ direct photons
challenge: large background from hadronic decays
measured direct-photons in various reaction systems
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PHENIX Data
Data presented in this talk:
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Direct Photons in p+p
at √s = 200 GeV
Why Direct Photons in p+p?
p+p
test of pQCD
baseline for direct photons in A+A
constrain gluon distribution in
proton (gluon contributes at LO)
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Direct-Photon Production in p+p
produced in hard scatterings:
q
γ
g
q
q
γ
q
g
rates can be calculated in pQCD
prompt/pQCD photons
quark-gluon Compton scattering
quark-antiquark annihilation
Bremsstrahlung
g
γ
direct component
g
fragmentation component
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q
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q
γ
q
g
q
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Analysis Techniques (I)
Cocktail Method
start with inclusive photons
subtract photons from
hadronic decays
main contribution from
p0 decays (p0 → gg)
spectrum of decay photons can
be simulated
sum
p0 → g g (80 %)
signal/background ratio very
small
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h→gg
w → p0 g
h’ → g g, g X
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Results in p+p (I)
p  p at s  200 GeV
two independent
analyses
PbGl
PbSc
different detectors
NLO pQCD
CTEQ6M PDF
µ=pT/2, pT, 2pT
(by W. Vogelsang)
measured spectra
agree with each other
good agreement with NLO pQCD
important baseline for Au+Au measuement
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Analysis Techniques (II)
Isolation Method
What fraction of direct photons
is isolated?
q
γ
g
q
q
γ
q
g
Compton
isolated
photons
Annihilation
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Analysis Techniques (II)
g
q
Isolation Method
What fraction of direct photons
is isolated?
Bremsstrahlung
γ
q
g
γ
q
isolation cut should remove
Bremsstrahlung
difficult to estimate efficiency
of isolation cut:
underlying event (soft physics)
limited acceptance
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g
q
Jet
g
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Results in p+p (II)
isolation of direct
photons:
isolation cut
no cut
includes underlying
event
NLO pQCD calculation
by W. Vogelsang
direct photon sample
with cocktail method
effect on p0 decay
photons
PHENIX Preliminary
data is better described by pQCD + underlying event
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Direct Photons in d+Au
at √s = 200 GeV
Why Direct Photons in d+Au?
p+p
test of pQCD
baseline for direct photons in A+A
constrain gluon distribution in
proton (gluon contributes at LO)
d+Au
study cold nuclear matter effects
kT broadening (Cronin)
nuclear shadowing
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Results in d+Au
in good agreement with binary
scaled pQCD
no indication of cold nuclear
matter effects (RdA ≈ 1)
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Direct Photons in Au+Au
at √s = 200 GeV
Why Direct Photons in Au+Au?
p+p
Au+Au
test of pQCD
baseline for direct photons in A+A
constrain gluon distribution in
proton (gluon contributes at LO)
photons do not interact
strongly with medium
hard photons
test of binary scaling
(Ncoll scaling)
understand high-pT hadron
suppression
thermal photons
d+Au
study cold nuclear matter effects
constrain temperature of
the collision system
QGP signature
kT broadening (Cronin)
nuclear shadowing
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Hard-Scattering in Au+Au
direct photons produced in early hard scatterings at high-pT
signal should scale with number of binary collisions
PRL 94, 232301 (2005) (Run 2)
binary scaling holds for all
centralities
nuclear modification factor
PRL
p0 suppression caused by parton
energy loss in the medium
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Contribution of Bremsstrahlung
maybe it is not that simple:
is Bremsstrahlung production modified by medium?
calculation by W. Vogelsang
direct
Bremsstrahlung
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large contribution
from Bremsstrahlung
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Modification of Bremsstrahlung?
is Bremsstrahlung suppressed by
parton energy loss in the medium
(induced gluon radiation)?
possible net effect: RAA ≈ 1
direct photon RAA
enhancement of direct-photon
production via medium induced
Bremsstrahlung?
Jeon, Jalilian-Marian, Sarcevic,
Zakharov,
Nucl. Phys.hep-ph/0405101
A 715, 795 (2003)
issue not solved yet!
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Thermal Photons in Au+Au
Photon Spectrum in Au+Au
hot and dense medium in
thermal equilibrium
schematic view
thermal:
Compton
e
 Eg /T
Decay photons
Annihilation
hard:
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pTn
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Realistic Calculation
thermal photons from
QGP dominant source in
pT range ≈1-3 GeV/c
Turbide, Rapp, Gale, Phys. Rev. C 69 (014902), 2004
potential signature for
QGP
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Thermal Signal?
published spectrum
PRL 94, 232301 (2005) (Run 2)
new data set (Run4)
no significant excess at low pT with
conventional methods
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Internal Conversion (I)
A New Approach:
direct photons via internal conversion
g
any source of real photons can emit virtual
photons with very low mass (e.g. p0 Dalitz)
virtual photons can subsequently decay into
e+e- pairs
same is true for direct-photon production
p0
g*
q
g
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e+
e-
e+
e-
g*
q
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Internal Conversion (II)
mass distribution of Dalitz pairs is given by Kroll-Wada Formula
QED
phase
space
form
factor
at very low mass (mee ≈ 0) mass distribution process
independent (only governed by QED part)
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Internal Conversion (III)
0-30
direct photons:
90-300 MeV
hadronic form factor
becomes unity
no phase space
limitation for photons
in chosen mass range
S/B ≈ 1
p0 Dalitz pairs highly suppressed
in this mass range
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The Method
Measure ratio:
0-30
90-300 MeV
Calculate:
from Kroll-Wada formula
÷
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The Method
Measure ratio:
0-30
90-300 MeV
Calculate:
÷
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measured with EMCal
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Comparison to Conventional Result
(+1)
significant signal below
pT = 3 GeV/c
systematic errors:
~20% measured h/p0
~10% inclusive photons
~5% acceptance
~25 % total systematic error
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The Direct-Photon Spectrum
NLO pQCD
Phys. Rev. D48, 3136 (1993)
L.E. Gordon, W. Vogelsang
Thermal Model
2+1 hydro
T0ave=360 MeV(T0max=570 MeV)
t0=0.15 fm/c
nucl-th/0503054
D. d’Enterria, D. Perresounko
The data are consistent with
thermal + pQCD
(only one possible interpretation!)
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The Direct-Photon Spectrum
Signal of thermal origin?
analysis of p+p and d+Au
using the same technique is
needed
if excess in Au+Au is of
thermal origin the reference
will show a much smaller
effect
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Summary
p+p
direct photons show good agreement with NLO pQCD
isolated signal better described by pQCD + underlying event
d+Au
good agreement with scaled NLO pQCD
no indication of cold nuclear matter effects
Au+Au
hard scattering contribution (pT>4 GeV/c) follows binary scaling
for all centralities
significant signal obtained at low pT (<4 GeV/c) using internal
conversion
spectrum agrees with NLO pQCD + thermal model
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Backup
The PHENIX Experiment I
photon measurement:
EMCal (PbGl, PbSc)
electron measurement:
Tracking, RICH, EMCal
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Analysis Techniques (II)
Tagging Method
increase signal/background
ratio by tagging p0 photons
start with sample of direct
photon candidates
tagging efficiency determind in
Monte-Carlo
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Centrality dependence
more peripheral
indication for centrality dependence
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