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Search for Thermal Photons in PHENIX. - Torsten Dahms - Stony Brook University 23 rd Winter Workshop On Nuclear Dynamics February 13, 2007. Outline. Motivation: signature of QGP Conventional direct photon measurements New attempts at low p T : Tagging of EMCal photons - PowerPoint PPT Presentation
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Search for Thermal Photonsin PHENIX
- Torsten Dahms -Stony Brook University
23rd Winter Workshop On Nuclear DynamicsFebruary 13, 2007
Torsten Dahms - Stony Brook University 2
Outline
• Motivation: signature of QGP• Conventional direct photon measurements• New attempts at low pT:
– Tagging of EMCal photons– External conversions in detector material– Low mass internal conversions
• Summary
Torsten Dahms - Stony Brook University 3
The “Little Bang” in the lab
space
time
Hard Scattering
AuAu
Exp
ansi
on
Hadronization
Freeze-out
QGPThermalization
electro-magnetic radiation: γ, e+e-, +-
rare, emitted “any time”; reach detector unperturbed by strong final state interaction
Final state probes
Soft probes
Penetrating probes
e- e+
Torsten Dahms - Stony Brook University 4
Thermal photons?
T
1np
hard:
/ E Tethermal:
Decay photons(0→, →, …)
No significant excess at low pT
Torsten Dahms - Stony Brook University 5
Direct photons in Au+AuConventional method:• Measure inclusive photons
γincl = γdecay + γdirect
• Calculate double ratio:(γincl/π0)measured / (γdecay/ π0)background = γincl/ γdecay =1+ γdirect/ γdecay
• If double ratio > 1 direct photons
• Run4: more statistics, but still no conclusive measurement
Torsten Dahms - Stony Brook University 6
The PHENIX experiment
• Charged particle tracking:– DC, PC1, PC2, PC3
• Electron ID:– Cherenkov light RICH
– shower EMCal
• Photon ID:– shower EMCal
•Lead scintillator calorimeter (PbSc)
•Lead glass calorimeter (PbGl)
– charged particle veto
e+e
Torsten Dahms - Stony Brook University 7
New Idea
• Measure inclusive photon spectrum, but a very clean sample
• Tag all photons, which combined with a photon from a second (less clean) sample can be identified as pion decay product
)(p γεaε)(pN
)(pγaε)(pN
Tπ
γ'T tagπ
TTinclγ
00
incl
f
DA
TA
a = acceptance
= efficiency
’= loose efficiency
f = conditional probability of having a loose photon (’) in the acceptance, once you already have a clean the acceptance
)(p γa)(pN
)(pγa)(pN
Tπ
T tagπ
TThadronγ
00
hadr
f
SIM
UL
AT
ION
sim
tagπγTγ
data
tagπγTγ
γ'hadr
incl0
hadr
0incl
N)(pN
N)(pNε
γ
γ
DOUBLE
RATIO
everything cancels out except for ’minimal systematics
Torsten Dahms - Stony Brook University 8
Clean Photon Sample
• Method I:– Only use EMCal clusters which
fulfill very strict PID cuts
• Method II:– Identify conversion photons in
beam pipe– Additional advantage:
• very good momentum resolution of charged tracks at low pT
• No detector artifacts
no pair cutwith pair cut
Dal
itz
Conversions
r~mass
PHENIX Beam Pipe
Torsten Dahms - Stony Brook University 9
0 signal extraction
Real eventsMixed event
•BG subtraction within pT bins•Normalized outside the π0 peak
• Clean EMCal sample has better S/B ratio
Clean EMCal γ sample
Beam pipe conversions
Torsten Dahms - Stony Brook University 10
Results in Au+Au
• Agreement of all three results within their errors
• There seems to be an excess above the decay photons at low pT
Torsten Dahms - Stony Brook University 11
Compton
q
g q
Compton
q
g q
e+
e-
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
Another Idea
32
2
)1(M
meeeeee
e
ee
e
mm
m
m
m 1)
21(
41
3
22
2
2
2
22 )( eemF
• Start from Dalitz decay
• Calculate inv. mass distribution of Dalitz pairs‘
• Now direct photons
• Any source of real produces
virtual with very low mass
• Rate and mass distribution given by same formula– No phase space factor for
mee<< pT photon
0
0
e+
e-
Torsten Dahms - Stony Brook University 12
In practice
0-3
0
• Material conversion pairs removed by analysis cut
• Combinatorial background removed by mixed events
• Calculate ratios of various mee bins to lowest one: Rdata
• If no direct photons: ratios correspond to Dalitz decays
• If excess: direct photons
• Fit of virtual photon shape to data in principle also possible(done for d+Au)
From conventional measurement
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
÷
200-3
00 M
eV
÷
140-2
00
Rdata
÷
90-1
40
Torsten Dahms - Stony Brook University 13
Comparison
• Agreement of all three methods within their errors• Also internal conversions favor an excess above decay photons
Torsten Dahms - Stony Brook University 14
The spectrum• Compare to NLO pQCD
– L.E. Gordon and W. Vogelsang
– Phys. Rev. D48, 3136 (1993)
• Above (questionable) pQCD
• Compare to thermal model– D. d’Enterria, D. Peressounko
– nucl-th/0503054
• Data above thermal at high pT
• Data consistent with thermal+pQCD
• Needs confirmation from p+p measurement
2+1 hydro
T0ave=360 MeV (T0
max=570 MeV)
0=0.15 fm/c
Torsten Dahms - Stony Brook University 15
Internal Conversions: d+Au
• Internal conversion method provides smaller systematic errors • But not as small as in the case of Au+Au collisions in Run-4
(Large background of external conversion on MVD detector in Run-3)
Torsten Dahms - Stony Brook University 16
Internal Conversion: d+Au
• Internal conversion extends range of significant points to pt > 2 GeV
• Data agrees with pQCD predictions in full pt range
No indication for nuclear effects
Torsten Dahms - Stony Brook University 17
Comparison: d+Au & Au+Au
Direct photon spectrum measured in d+Au collisions and scaled with <Ncoll> agrees pretty well with spectrum measured in Au+Au at high pT
• There is room for thermal photons
• Systematic errors are still too big to decide whether excess seen in Au+Au can be assign to thermal source
Torsten Dahms - Stony Brook University 18
Summary• Photons are penetrating hard and soft probes for relativistic
heavy ion collisions• Conventional Calorimeter measurement
– Systematic uncertainties at low pT too large to make definite statement about thermal photon contribution
• New methods show excess above decay photons• Consistent with each other• Internal conversions
– Promising new technique to measure direct photons– Thermal photon scenario consistent for pT<3GeV/c– Because of the large systematic errors comparison of binary scaled
d+Au spectrum with Au+Au does not allow to make a statement on the origin of the excess above pQCD observed in Au+Au
– Same analysis of p+p is needed as confirmation
Torsten Dahms - Stony Brook University 19
Backup
Torsten Dahms - Stony Brook University 21
The PHENIX experiment
Beam Pipe
West Arm East Arm
γ
e+
e-
e+
e-
γ
Collision Vertexe+
e-
γ
•electrons:•momentum reconstruction (1% resolution) •particle ID: RICH (loose cuts because clean signature of conversion peak)
•same or opposite arms: different pT acceptance
•photons: EMCal (loose cuts high efficiency ~ 98%)
track reconstruction assumes vertex in the interaction point conversion at radius r≠0: e+e- pairs ‘acquire’ an opening angle
they acquire an invariant mass m = B dl ~ r > 0if r=4 cm (beam pipe) m =20 MeV
Torsten Dahms - Stony Brook University 22
Invariant e+e- mass spectrum of
Run 4 Au+Au:Dalitz decays
beampipeconversions
e eγ
e e γ γγπ0
e e γπ0
air conversions & combinatorial background
GeV 002sNN
•Conversion pairs are created off-vertex•Track reconstruction produces apparent opening angle•Leads to apparent mass ~20MeV/c2
Torsten Dahms - Stony Brook University 23
• Dalitz decays have a real opening angle due to the π0 mass
• Conversion pairs have small intrinsic opening angle– magnetic field produces opening of the pair in azimuth
direction
– orientation perpendicular to the magnetic field
Pair properties
0ee Δ 0-
00
z
y
x e+
e-
BConversion pair
z
y
x
e+
e-
BDalitz decay
MC Simulationall pairsdalitz decaybeam pipe conversions
MC Simulationall pairsdalitz decaybeam pipe conversions
Torsten Dahms - Stony Brook University 24
Simulations: Nγhadr(pT) and Nγ
π0
tag(pT)• Inclusive photon spectrum
– π0, η → γe+e-
• π0 parameterization from measured data
• η from mT scaling, yield normalized at high pT (0.45 from measurement)
– Use Dalitz decay (π0→ γ γ ~ π0 → γ γ* → γe+e- for pT > 0.8 GeV/c)
• All e+e- (from π0, η) in the acceptance pT spectrum of e+e-
• If γ from π0 is also in acceptancepT spectrum of e+e- from π0 all e+e- pairs
e+e- pairs from π0 1
/Nev
t dN
/dp
T [c
/Ge
V]
Torsten Dahms - Stony Brook University 25
Cocktail ingredients (pp): 0
• most important: get the 0 right (>80 %), assumption: 0 = (+ + -)/2
• parameterize PHENIX pion data:n
TTT p
pbpap
c
pd
dE
0
23
3
)exp(
• most relevant: the meson (Dalitz & conversion)• also considered: ’• use mT scaling for the spectral shape, i.e.
• normalization from meson/0 at high pT as measured (e.g. 0 = 0.45±0.10)
222mmpp mesonTT
Torsten Dahms - Stony Brook University 26
•Gale QM05