Photons at RHIC Henner Büsching FIAS – University of Frankfurt Jyväskylä - March 2007.
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Transcript of Photons at RHIC Henner Büsching FIAS – University of Frankfurt Jyväskylä - March 2007.
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Photons at RHIC
Henner BüschingFIAS – University of Frankfurt
Jyväskylä - March 2007
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In a perfect world…
RHIC: 200 GeV pp Run over
RHIC: 200 GeV Run Started
Hoax! News: RHIC is running!
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…temperature of the QGP can be measured by thermal photon radiation…
…photons don’t interact with the medium…
…good probe of the medium…
… gamma- jet correlations…
XXX’s Diploma Thesis
…in a simple world…
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…in the real world…
PHENIX
Experiment: Problem to extractLarge uncertainties
Theory: Do we understand all sources?
measured / backgroundmeasured / (background
Difficult measurement !
Experiment: Need statisticsUnderstand detector?
Theory: Do we understand all sources?
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In the real world…a wide field!
PHENIX Direct v2 -jet Shapes
Virtual
Intermediate pT Studies
Statistical MethodExt. Conversions
Photon Tagging
Reaction Plane Dependence
Thermal
Bremsstrahlung
Jet Medium Interactions
Isolation Cut
Precision!
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Status
So far unfulfilled:
Initial temperature via thermal photonsEnergy loss via g-jet correlations
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7Run4 data
Direct photon spectrum for 1 < pT < 4.5 GeV
via internal conversion
Punchline first: Where we are 1
Low pT
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Punchline first: Where we are 2
Run2 dataHigh pT
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Punchline first: Where we are 3
Run4 dataHigh pT - Proton Reference
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Who ordered all these photons?
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Compton Annihilation
Hard direct photons:Direct component
q + g + q q + q + g
q
g q
q
q_
g
photons in A+A
direct photons decay photons
hard
direct fragmentationbremsstrahlung
Photon Sources in Au+Au
Un-affected by medium
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Hard direct photons:bremsstrahlung and fragmentation
Affected by medium
q
g
g
q
q
g
q
photons in A+A
direct photons decay photons
hard
direct fragmentationbremsstrahlung
Photon Sources in Au+Au
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photons in A+A
direct photons decay photons
hard thermal jet-plasma-interact.
QGP hadron gasdirect fragmentation
pre-equilibrium
jet--conv.
medium-induc. bremsstr.
Photon Sources in Au+Au
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Jet-plasma interactionjet-photon-conversion
hard QGPq + q + g
hard QGPq + g + q
photons in A+A
direct photons decay photons
hard thermal jet-plasma-interact.
QGP hadron gasdirect fragmentation
pre-equilibrium
jet--conv.
medium-induc. bremsstr.
Photon Sources in Au+Au
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Jet-plasma interaction:medium-induced bremsstrahlung q
QGP
photons in A+A
direct photons decay photons
hard thermal jet-plasma-interact.
QGP hadron gasdirect fragmentation
pre-equilibrium
jet--conv.
medium-induc. bremsstr.
Photon Sources in Au+Au
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photons in A+A
direct photons decay photons
hard thermal jet-plasma-interact.
QGP hadron gasdirect fragmentation
pre-equilibrium
jet--conv.
medium-induc. bremsstr.
Photon Sources in Au+Au
Created in all phases of the collisionOnce created, they survive
But this makes measurements hard to interpret
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Spectra - Schematic
• Thermal photons relevant below pT ~ 3 GeV/c
• Hard Photons for pT > ~ 6 GeV/c
T
1np
hard:
/ E Tethermal:
Decay photons(0→, →, …)
Jet-Plasma-Konv.
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C. Gale, hep-ph/0609301
Spectra – Recent Theory
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Let’s measure
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202005 p+p
0STAR: Welcome to the Club
2003 d+Au
0
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• Get clean inclusive photon sample
• Measure pT spectrum of and mesons with high accuracy
• Calculate number of decay photons per
– Done with Monte-Carlo
– mT scaling for (), ’, , …
• Finally:Subtract decay background from inclusive photon spectrum
The traditional wayall1backgroundalldirect )R1(
calc0all
meas0all
background
all
)/(
)/(R
/0
Subtraction
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Inclusive Photon Spectrum
Fraction of charged clusters
Fraction of neutral background(neutron, anti-neutrons)
efficiency
acceptancephoton conversion
Subtraction
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Uncertainties
Many systematic uncertainties of and photon measurements are highly correlated! Subtraction
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New ways I
• Advantage– Good momentum resolution
for low pT charged particles (ee) allows photon measurement at low pT
– Less trouble with background from hadrons
• Disadvantage– Radiator needs to be thin:
low photon detection probability
– Necessary to separate conversion pairs from 0 and Dalitz decays (0 e+e)
Conversion
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•Tag all photons, which combined with a photon from a second (less clean) sample can be identified as pion decay product•Correct direct photon candidate sample for contribution from decay photons with missing partner and for direct photons with fake partners•Subtract contribution from , etc. decays
TaggingNew ways II
• In central A+A collisions the number of random associations is too high for this approach
• Can work at high pT
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Results in Au+Au
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Direct photons in d+Au
It works…
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Internal Conversion
differentminv-distribution
• Any source of real produces virtual with very low mass
• minv distribution depends on process
• Rate and mass distribution given by same formula
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phase space factorform factorinvariant mass of virtual photon
invariant mass of Dalitz pair
phase space factorform factorinvariant mass of Dalitz pair
invariant mass of virtual photon
32
222
2
2
2
2
)1()(1
)2
1(4
13
21
M
mmF
mm
m
m
m
dm
dN
Nee
eeeeee
e
ee
e
ee
ee
ee
ee
dm
dN
N
1 32
2
)1(M
meeeeee
e
ee
e
mm
m
m
m 1)
21(
41
3
22
2
2
2
22 )( eemF
Internal Conversion
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“Thermal”
Discussion of results
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Low pT – internal conversion
2+1 hydro,d’Enterria, Perresounko
Good p+p is missingTi ~ 300 – 400 MeV
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Int. Conversions: d+Au
Not as successful as inAu+Au:
High background in Run3
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Comparison: d+Au & Au+Au
We still have to wait for Run5 p+p
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p+p and d+Au
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Direct photons in p+p
Run 3Run 5 Phys. Rev. Lett. 98, 012002 (2007)
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Direct photons in p+p
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Initial state: d+Au
No indication for nuclear effects
NLO pQCD from W.Vogelsang
Run 3
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High pT – Au+Au
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RAA at high pT
Energy loss
No energy loss
Reference for photons: pQCD
ddp/NdT
ddp/Nd)p(R
TNN2
AA
TAA2
TAA
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RAA at high pT
Reference for photons: measurement
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RAA with pQCD
RAA with p+p data
RAA at high pT
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RAA– Centrality Dependence
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Scenario: Isospin • Shadowing
• Isospin (p+n vs. p+p)
• Energy loss: • Suppression of direct photons
from fragmentation/bremsstrahlung
• BDMPS
• No medium induced jet-photon conv.
From fragmentation:~ 30% at pT = 15 GeV/c
F. Arleo, JHEP09 (2006) O15
direct
fragment
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Scenario: Isospin II
)Z)-(A Z)-2Z(A (Z)(1/A /N nn2
pnpp22
collAA
No problem?“primordial” photons unaltered?“medium-induced” photons can be enhanced or suppressedHelp from 62 GeV?Lower pT
Unclear effects? Jet- conversion
Vogelsang
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Scenario: Jet--Conversion + Energy loss
• AMY formalism for jet-quenching effect for fragmentation photons.• Systematically data points are below theoretical prediction.
Turbide et al.Phys. Rev. C72 (2005) 014906 + Private communication
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isolation
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Isolation Cut Expectation for direct photons:
N+N: isolated
Jet-Photon conversion: isolated
Jet-fragmentation: non isolated
Bremsstrahlung: non isolated
Jet
at 200 GeVNNp p s hep-ex/0609031
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Flow
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Direct-Photon v2
reaction plane
Spatial anisotropy Momentum anisotropy
v2 > 0 for hadrons
Expectation for direct photons:
N+N: v2 > 0
Jet-fragmentation: v2 > 0
Jet-Photon conversion: v2 < 0
In-medium bremsstr.: v2 < 0
Net effect: v2 < 0
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Expectation for v2: negative, |v2| 3% − 5%
Direct Photon v2 Turbide, Gale, Fries, Phys.Rev.Lett.96:032303,2006
Here: Assuming no energy loss: jet-g conversion dominates v2 : v2<0
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PHENIX PreliminarysNN=200GeV Au+Au
Direct Photon v2
inclusive decay direct 2 22 1
Rv vv
R
g gg -
=-
decay directinclusive
decay decay
Rg gg
g g
+= =
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PHENIX PreliminarysNN=200GeV Au+Au
Direct Photon v2
Run7: PHENIX new Reaction Plane Detector
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53Statistical errors only.
Direct Photon v2
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hadrons
The ‘royal’ analysis
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decay
inclusivehdecayhinclusivehdirect RY
RY
RY
)1
(/11
1
Inclusive -h
Decay -h contribution(via 0-hadron)
Direct -h !
p+p at 200 GeV
hdirectinclusive
directhdecay
inclusive
decayhinclusive YYY
-jet
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-jet
First results in p+p
Consistent with expectations from Pythia
First results in Au+Au
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Summary
• Direct photons are a difficult measurement– New analysis techniques– STAR started contributing
• High pT reach in p+p: agrees with pQCD within uncertainty.
• Direct photons in 200GeV Au+Au: spectra up to 20GeV/c.
• First direct photons RAA in 200GeV Au+Au collisions with p+p reference.– RAA seems to be below one in very high-pT region– Isospin?– Strong Jet-Medium enhancement ruled out
• First results on direct photon v2 (v2>0 ?)
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backup
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Systematic error on direct photon spectra in AuAu 200GeV
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Systematic error on direct photon spectra in p+p 200GeV
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More direct photon sources
dir
ect
photo
n R
AA
Bremsstrahlung photons through bulk matterB.G. Zakharov, JETP Lett. 80 (2004) 1