direct-photon production in phenix oliver zaudtke for the collaboration winter workshop on nuclear...
Post on 21-Dec-2015
215 views
TRANSCRIPT
Direct-Photon Production in PHENIX
Oliver Zaudtke
for the CollaborationWinter Workshop on Nuclear Dynamics 2006
WWND06 Oliver Zaudtke 2
direct photons
What are Direct Photons?
decay photonsinclusive photons = +
Definition:
Photons that emerge directly from a particle collision
challenge: large background from hadronic decays
measured direct-photons in various reaction systems
WWND06 Oliver Zaudtke 5
p+ptest of pQCDbaseline for direct photons in A+Aconstrain gluon distribution in proton (gluon contributes at LO)
Why Direct Photons in p+p?
WWND06 Oliver Zaudtke 6
produced in hard scatterings:rates can be calculated in pQCD
quark-gluon Compton scatteringquark-antiquark annihilationBremsstrahlung
Direct-Photon Production in p+p
fragmentation component
prompt/pQCD photons
q
q
q
q
g
g
g
γγ
direct component
q
qg
γ
q
q g
γ
WWND06 Oliver Zaudtke 7
Cocktail Methodstart with inclusive photons
subtract photons from hadronic decays
main contribution from0 decays (0 → )spectrum of decay photons can be simulated
signal/background ratio very small
Analysis Techniques (I)
0 → (80 %)
→ → 0
’ → X
sum
WWND06 Oliver Zaudtke 8
Results in p+p (I)GeV 200s at pp
PbGlPbScNLO pQCD CTEQ6M PDF µ=pT/2, pT, 2pT
(by W. Vogelsang)
two independent analyses
different detectors
measured spectra agree with each other
good agreement with NLO pQCD
important baseline for Au+Au measuement
WWND06 Oliver Zaudtke 9
What fraction of direct photons is isolated?
Analysis Techniques (II)
Isolation Method q
qg
γ
q
q g
γ
isolatedphotons
Compton
Annihilation
WWND06 Oliver Zaudtke 10
What fraction of direct photons is isolated?
isolation cut should remove Bremsstrahlung
difficult to estimate efficiency of isolation cut:
underlying event (soft physics)limited acceptance
Analysis Techniques (II)
Isolation Method
Jet
q
q
q
q
g
g
g
γγBremsstrahlung
WWND06 Oliver Zaudtke 11
Results in p+p (II)isolation of direct photons:
direct photon sample with cocktail method
effect on 0 decay photons
data is better described by pQCD + underlying event
isolation cutno cut
PHENIX Preliminary
includes underlyingevent
NLO pQCD calculationby W. Vogelsang
WWND06 Oliver Zaudtke 13
d+Austudy cold nuclear matter effects
kT broadening (Cronin)
nuclear shadowing
p+ptest of pQCDbaseline for direct photons in A+Aconstrain gluon distribution in proton (gluon contributes at LO)
Why Direct Photons in d+Au?
done
WWND06 Oliver Zaudtke 14
Results in d+Au
no indication of cold nuclearmatter effects (RdA ≈ 1)
in good agreement with binary scaled pQCD
WWND06 Oliver Zaudtke 16
d+Austudy cold nuclear matter effects
kT broadening (Cronin)
nuclear shadowing
Au+Auphotons do not interact strongly with mediumhard photons
test of binary scaling(Ncoll scaling)
understand high-pT hadron suppression
thermal photonsconstrain temperature of the collision systemQGP signature
Why Direct Photons in Au+Au?
done
p+ptest of pQCDbaseline for direct photons in A+Aconstrain gluon distribution in proton (gluon contributes at LO)
done
WWND06 Oliver Zaudtke 17
Hard-Scattering in Au+Audirect photons produced in early hard scatterings at high-pT
signal should scale with number of binary collisions
PRL 94, 232301 (2005)
binary scaling holds for all centralities
0 suppression caused by parton energy loss in the medium
PRLnuclear modification factor
(Run 2)
WWND06 Oliver Zaudtke 18
Contribution of Bremsstrahlung
maybe it is not that simple:is Bremsstrahlung production modified by medium?
calculation by W. Vogelsang
direct
Bremsstrahlung
large contributionfrom Bremsstrahlung
WWND06 Oliver Zaudtke 19
dir
ect
photo
n R
AA
Jeon, Jalilian-Marian, Sarcevic,Nucl. Phys. A 715, 795 (2003)Zakharov, hep-ph/0405101
Modification of Bremsstrahlung?
is Bremsstrahlung suppressed by parton energy loss in the medium (induced gluon radiation)?
enhancement of direct-photon production via medium induced Bremsstrahlung?
possible net effect: RAA ≈ 1
issue not solved yet!
WWND06 Oliver Zaudtke 21
Decay photons
Photon Spectrum in Au+Au
hot and dense medium inthermal equilibrium
nT
1
phard:
/ E Tethermal:
schematic view
Compton
Annihilation
WWND06 Oliver Zaudtke 22
Realistic Calculation
thermal photons from QGP dominant source in pT range ≈1-3 GeV/c
potential signature for QGP
Turbide, Rapp, Gale, Phys. Rev. C 69 (014902), 2004
WWND06 Oliver Zaudtke 23
Thermal Signal?published spectrum
new data set (Run4)
no significant excess at low pT with conventional methods
PRL 94, 232301 (2005)(Run 2)
WWND06 Oliver Zaudtke 24
Internal Conversion (I)
direct photons via internal conversion
any source of real photons can emit virtual photons with very low mass (e.g. 0 Dalitz)virtual photons can subsequently decay into e+e- pairs
same is true for direct-photon production
A New Approach:
0
e+
e-
q
g q
e+
e-
WWND06 Oliver Zaudtke 25
Internal Conversion (II)
mass distribution of Dalitz pairs is given by Kroll-Wada Formula
QED phasespace
formfactor
at very low mass (mee ≈ 0) mass distribution process independent (only governed by QED part)
WWND06 Oliver Zaudtke 26
Internal Conversion (III)
direct photons:hadronic form factor becomes unityno phase space limitation for photons in chosen mass range
90-300 MeV
0-3
0
0 Dalitz pairs highly suppressedin this mass range
S/B ≈ 1
WWND06 Oliver Zaudtke 27
The Method
Measure ratio:
Calculate:
0-3
0 90-300 MeV
÷
from Kroll-Wada formula
WWND06 Oliver Zaudtke 29
Comparison to Conventional Result
significant signal belowpT = 3 GeV/c
systematic errors:~20% measured /0
~10% inclusive photons~5% acceptance
~25 % total systematic error
(+1)
WWND06 Oliver Zaudtke 30
The Direct-Photon Spectrum
NLO pQCD
Thermal Model2+1 hydroT0
ave=360 MeV(T0max=570
MeV)0=0.15 fm/c
The data are consistent with thermal + pQCD(only one possible interpretation!)
Phys. Rev. D48, 3136 (1993)L.E. Gordon, W. Vogelsang
nucl-th/0503054
D. d’Enterria, D. Perresounko
WWND06 Oliver Zaudtke 31
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
The Direct-Photon Spectrum
WWND06 Oliver Zaudtke 32
p+pdirect photons show good agreement with NLO pQCDisolated signal better described by pQCD + underlying event
d+Augood agreement with scaled NLO pQCDno indication of cold nuclear matter effects
Au+Auhard scattering contribution (pT>4 GeV/c) follows binary scaling for all centralitiessignificant signal obtained at low pT (<4 GeV/c) using internal conversion
spectrum agrees with NLO pQCD + thermal model
Summary
WWND06 Oliver Zaudtke 35
The PHENIX Experiment I
photon measurement:EMCal (PbGl, PbSc)
electron measurement:Tracking, RICH, EMCal
~5 m
WWND06 Oliver Zaudtke 36
Tagging Method
increase signal/background ratio by tagging 0 photonsstart with sample of direct photon candidates
tagging efficiency determind in Monte-Carlo
Analysis Techniques (II)