Transcript Measurements of thermal photons and the dielectron continuum with PHENIX

Measurements of thermal photons and
the dielectron continuum with PHENIX
- Torsten Dahms Stony Brook University
Workshop on Hot & Dense Matter in the RHIC-LHC Era
February 13th, 2008
charm & bottom cross section
centrality dependence
pT spectra
chiral symmetry
thermal photons
mass spectra
Au+Au collisions
p+p collisions
low mass enhancement
medium modification
Time
time
g
Dileptons at RHIC
p f Jet p K g p
e L cc m
Expected sources
• Light hadron decays
– Dalitz decays p0, h
freeze-out
– Direct decays r/w and f
• Hard processes
– Charm (beauty) production
hadronization
– Much larger at RHIC than at SPS
formation and
thermalization
of quark-gluon
matter?
hard parton scattering
Space
Au
Au
• Photons and dileptons: radiation from the media
– direct probes of any collision stages (no final-state interactions)
– large emission rates in hot and dense matter
– according to the VMD their production is mediated in the
hadronic phase by the light neutral vector mesons (ρ, ω, and φ)
which have short life-time
Possible modifications
Chiral symmetry restoration
continuum enhancement
modification of vector mesons
thermal radiation
charm modification
exotic bound states
suppression (enhancement)
• Changes in position and width: signals of the chiral
transition?
2008-02-13
2
The
Data
p+p at √s = 200GeV
• 800M MinBias Au+Au events
• 2.25pb-1 of triggered p+p data
• Combinatorial background
removed by mixed events
• additional correlated background:
– cross pairs from decays with four
electrons in the final state
– particles in same jet (low mass)
– or back-to-back jet (high mass)
– well understood from MC
e- γ
e+
e+
π0
2008-02-13
submitted to Phys. Lett.B
arXiv: 0802.0050
e-
γ
e+
π0
π0
γ
e3
The Raw Subtracted Spectrum
Same analysis on data sample with additional conversion material
Combinatorial background increased by 2.5
Good agreement within statistical error
s
/signal = s
signal
BG/BG
0.25%
300,000 pairs
50,000 above p0
* BG/signal
large!!!
From the agreement
converter/non-converter
and the decreased S/B
ratio scale error < 0.1%
(well within the 0.25%
error we assigned)
submitted to Phys. Rev. Lett
arXiv:0706.3034
2008-02-13
2007-12-14
Torsten Dahms - Stony Brook University
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Cocktail Tuning (p+p)
• Start from the π0 , assumption: π0 = (π+ + π-)/2
• parameterize PHENIX pion data:
3
dσ
A
E 3 
d p exp( ap T  bp T2 )  p T p 0

PHENIX Preliminary
2008-02-13
Other mesons
• well measured in electronic and
hadronic channels
• Other mesons are fit with:
mT scaling of π0 parameterization
pT→√(pT2+mmeson2-mπ2)
fit the normalization constant
 All mesons mT scale!
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
n
p+p Cocktail Comparison
• Data absolutely normalized
• Excellent agreement with Cocktail
• Filtered in PHENIX acceptance
Cross Sections:
• Charm: integration after cocktail
subtraction
– σcc = 544 ± 39 (stat) ± 142 (syst) ±
200 (model) μb
• Simultaneous fit of charm and
bottom:
– σcc = 518 ± 47 (stat) ± 135 (syst) ±
190 (model) μb
– σbb = 3.9 ± 2.4 (stat) +3/-2 (syst) μb
submitted to Phys. Lett.B
arXiv: 0802.0050
2008-02-13
• Charm cross section from single
electron measurement:
– σcc = 567 ± 57 ± 193 μb
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Cocktail Comparison
• Data and cocktail absolutely normalized
• Cocktail from hadronic sources
• Charm from
– PYTHIA
– Single electron non photonic spectrum w/o
angular correlations
– σcc= Ncoll x 567±57±193mb
• Predictions are filtered in PHENIX
acceptance & resolution
• Low-Mass Continuum:
enhancement 150 < mee < 750 MeV
3.4 ± 0.2 (stat) ± 1.3 (syst) ± 0.7 (model)
• Intermediate-Mass Continuum:
– Single e  pT suppression
– PYTHIA softer than p+p but coincide with
Au+Au
– Angular correlations unknown
– Room for thermal contribution?
submitted to Phys. Rev. Lett
arXiv:0706.3034
2008-02-13
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Au+Au & p+p Comparison
p+p normalized to mee<100 MeV
• p+p and Au+Au normalized
to π0 region
• Agreement in intermediate
mass and J/ψ just for
‘coincidence’
(J/ψ happens to scale as π0
due to scaling with Ncoll +
suppression)
2008-02-13
8
Comparison with Theory
Ingredients:
• Freeze-out Cocktail
• “random” charm
• ρ spectral function
LMR
• m>0.4 GeV:
some models describe data
• m<0.4 GeV:
enhancement not reproduced
IMR
• Randomized charm + thermal
may work
2008-02-13
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Yield in Different Mass Ranges
0-100 MeV: π0 dominated;
approximately scales with Npart
150-750 MeV: continuum
1.2-2.8 GeV: charm dominated;
scales with Ncoll
Study yield in these mass regions as a function of centrality
2008-02-13
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Centrality Dependence
LOW MASS
π0 production scales with Npart
Low Mass:
• If in-medium enhancement from ππ or
qq annihilation
yield should increase faster than
proportional to Npart
Intermediate Mass:
• charm follows binary scaling
yield should increase proportional to
Ncoll
INTERMEDIATE MASS
2008-02-13
submitted to Phys. Rev. Lett
arXiv:0706.3034
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Mass Spectra: pT dependency
• Study pT dependency of the low mass enhancement in Au+Au
0 < pT < 8 GeV/c
0 < pT < 0.7 GeV/c
0.7 < pT < 1.5 GeV/c
1.5 < pT < 8 GeV/c
• p+p in agreement with cocktail
• Au+Au low mass enhancement concentrated at low pT
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pT Spectra
•p+p: follows the cocktail
•Au+Au: significantly deviates at low pT
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Understanding the pT dependency
• Comparison with cocktail
• Single exponential fit:
– Low-pT: 0<mT<1 GeV
– High-pT: 1<mT<2 GeV
• 2-component fits
– 2 exponentials
– mT-scaling of p0 + exponential
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Yields and Slopes
SLOPES
YIELDS
Low-pT yield
2expo fit
mT-scaling +expo fit
Total yield (DATA)
• Intermediate pT:
– inverse slope increase with mass
– consistent with radial flow
• Low pT:
– inverse slope of ~120MeV
– accounts for most of the yield
2008-02-13
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Mass Spectra: pT dependency
• Study pT dependency of the low mass enhancement in Au+Au
0 < pT < 8 GeV/c
0 < pT < 0.7 GeV/c
0.7 < pT < 1.5 GeV/c
1.5 < pT < 8 GeV/c
• high pT region provides window for thermal photon measurement
2008-02-13
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Virtual Photons
Compton
π0
• Start from Dalitz decay
• Calculate inv. mass distribution of Dalitz pairs
γ q
π0
γ* γ
γ*
γ
g
e+
γe
e+
e
q
2
2
4me2
2me2 1
mee
1 dN ee 2
2

1  2 (1  2 )
F (mee ) (1  2 )3
N g dm ee 3p
mee
mee mee
M
invariant mass of
Dalitz pair
invariant mass of
virtual photon
form factor
phase space factor
• Now direct photons
• Any source of real γ produces
virtual γ with very low mass
• Rate and mass distribution given
by same formula
2008-02-13
– No phase space factor for
mee<< pT photon
• Improved S/B by measuring direct
photon signal in mass region in which
π0 are suppressed
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Cocktail comparison
PHENIX Preliminary
p+p
2008-02-13
Au+Au (MB)
1 < pT < 2 GeV
2 < pT < 3 GeV
3 < pT < 4 GeV
4 < pT < 5 GeV
QM2005
• Results from Au+Au
QM2008
• long awaited result from p+p
• important confirmation of method
p+p
• Agreement of p+p data and
hadronic decay cocktail
• Small excess in p+p at large mee
and high pT
Au+Au
• data agree for mee <50MeV
• Clear enhancement visible above
for all pT
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Shape Comparison
• At m=0 Dalitz and internal conversion
pairs have indistinguishable shape
• Shape differs as soon as π0 is
suppressed due to phase space
limitation
• Assume internal conversions of direct
photons
– Fix absolute normalization of cocktail
and direct photons by normalizing to
data in mee<30MeV
– Fit paramater r is fraction of direct
photons
– Two component fit in
80 < mee < 300MeV gives:
χ2/DOF=11.6/10
• It’s not the η:
f(mee )  ( 1  r)f cocktail(mee )  rf direct(mee )
2008-02-13
– Independent measurement of η in
Au+Au fixes π0/η ratio to: 0.48 ± 0.08
– Fit with eta shape gives:
χ2/DOF = 21.1/10
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Fraction of direct photons
p+p
Au+Au (MB)
μ = 0.5pT
μ = 1.0pT
μ = 2.0pT
2008-02-13
• Fraction of direct photons
• Compared to direct photons
from pQCD
p+p
• Consistent with NLO pQCD
• favors small μ
Au+Au
• Clear excess above pQCD
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Comparison
• Agreement of all three methods within their errors
• Internal conversion method observes clear excess above decay photons
• Extract direct photon spectrum by multiplying with measured inclusive photon
spectrum: Nγdirect = r · Nγinclusive
2008-02-13
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The spectrum
• Compare spectra to NLO pQCD
p+p
• consistent with pQCD
Au+Au
• above binary scaled pQCD
• If excess of thermal origin:
inverse slope is related to initial
temperature
2008-02-13
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Conclusions
• First dielectron continuum measurement at RHIC
– Despite of low signal/BG
– Thanks to high statistics
– Very good understanding of background normalization
Au+Au
LOW MASS:
p+p
LOW MASS:
• Excellent agreement with hadronic decay cocktail
• Enhancement above the cocktail expectations:
3.4±0.2(stat.) ±1.3(syst.)±0.7(model)
• Centrality dependency: increase faster than Npart
• Enhancement concentrated at low pT
INTERMEDIATE MASS:
INTERMEDIATE MASS:
• Extract charm and bottom cross sections
• σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb
• σbb= 3.9 ± 2.4 (stat) +3/-2 (syst) μb
• Coincidence agreement with PYTHIA
• Room for thermal radiation?
THERMAL PHOTONS
THERMAL PHOTONS:
• p+p in agreement with pQCD
• Dielectron mass shape for pT > 1 GeV and mee <
300MeV consistent with internal conversions of
virtual photons
• Au+Au above pQCD
2008-02-13
•HBD upgrade will reduce background
 great improvement of systematic and statistical
uncertainty (LMR)
•Silicon Vertex detector will distinguish charm from
prompt contribution (IMR)
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Backup
Charm and bottom cross sections
CHARM
Dilepton measurement in agreement with
single electron, single muon, and with
FONLL (upper end)
2008-02-13
BOTTOM
Dilepton measurement in agreement with
measurement from e-h correlation and with
FONLL (upper end)
First measurements of bottom cross
section at RHIC energies!
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Theory Comparison II
Cocktail not subtracted from data
(necessary for comparison)
2008-02-13
Calculations from
• R. Rapp & H. van Hees
• K. Dusling & I. Zahed
• E. Bratovskaja & W. Cassing (in 4π)26
Cu+Cu dN/dm – Minimum Bias
π
η
η’
ω
ρ
φ
cc
J/ψ
ψ’
2008-02-13
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Dielectrons at RHIC – Intermediate Mass
• Transverse momentum spectra of dielectrons at
constrained transverse masses:
• RHIC with PHENIX acceptance,
pT > 1 GeV and 2 GeV < Mee < 3 GeV
• Mee hard chance to find thermal dileptons with
Mee > 2 GeV.
• The double differential rate dNe+e−/dMT2 dQT2
with MT in a narrow interval and with a
suitable pTmin cut on the individual leptons
seems to allow for a window at large values of
the pair pT where the thermal yield shines out
2008-02-13
K. Gallmeister, B. Kämpfer and O. P. Pavlenko
Phys. Rev. C 57, 3276 (1998)
Phys. Lett. B 419, 412 (1998)
see also e.g.:
E. V. Shuryak, Phys. Rev. C 55, 961 (1997)
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Dielectrons at RHIC
Expected Sources:
• Light hadron decays
– Dalitz decays π0, η
– Direct decays ρ, ω and φ
• Hard processes
– Charm (beauty) production
– Important at high mass & high pT
– Much larger at RHIC than at the
SPS
• Cocktail of known sources
– Measure π0, η spectra & yields
– Use known decay kinematics
– Apply detector acceptance
– Fold with expected resolution
2008-02-13
Possible modifications
Chiral symmetry restoration
continuum enhancement
modification of vector mesons
thermal radiation
charm modification
exotic bound states
R. Rapp nucl-th/0204003
suppression (enhancement)
•Strong enhancement of low-mass pairs
persists at RHIC
•Open charm contribution becomes
significant
29
Relativistic Heavy Ion Collider
2008-02-13
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The PHENIX Experiment
• Measure rare probes in heavy ion
collisions (e.g. Au+Au) as well as in
p+p (+spin program)
• 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)
p
g
e-
e+
– charged particle veto
• Central arm physics
(|y|<0.35, p ≥ 0.2 GeV/c):
– charmonium J/ψ, ψ’→ e+e– vector meson ρ, ω, φ → e+e– high pT
π0, π+, π– direct photons
– open charm
– hadron physics
• Two muon arms at forward rapidity
(1.2 < |η| < 2.4, p 2 GeV/c)
2008-02-13
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Electron Identification
• Charged particle tracking (δm: 1%)
DC, PC1, PC3
• PHENIX optimized for Electron ID
• Cherenkov light RICH +
• shower EMCAL
Most hadrons
do not emit
Cerenkov light
mirror
Cerenkov
photons from
e+ or e- are
detected by
array of PMTs
• Emission and measurement of
Cherenkov light in the Ring Imaging
Cherenkov detector→ measure of min.
velocity
• Production and of em. shower in the
Electro-Magnetic Calorimeter
 measure of energy E
• Electrons: E ≈ p
• Hadrons: E < p
ＲＩＣＨ
RICH
All charged tracks
RICH cut
PMT array
PMT array
Electrons
Central Magnet emit
Cerenkov
photons
in RICH.
Real
Net signal
Background
Energy-Momentum
2008-02-13
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The Double Challenge
Experimental Challenge
• Need to detect a very weak source of e+e- pairs
hadron decays (m>200 MeV, pT>200 MeV)
• In the presence of hundreds of charged particles
central Au+Au collision
• And several pairs per event from trivial origin
π0 Dalitz decays
+ γ conversions (assume 0.5% radiation length)
huge combinatorial background  (dNch / dy)2
~ 4x10-6 / π0
dNch / dy ≈ 700
~ 10-2 / π0
~ 10-2 / π0
– pairing of tracks originating from unrecognized π0 Dalitz decays and γ conversions
– no means to reduce combinatorial background
beyond reducing conversion length to 0.4% and
pT cut at 200 MeV
 Signal to background depending on mass up to
1 : few hundred
Analysis Challenge
• Electron pairs are emitted through the whole
history of the collision:
– need to disentangle the different sources.
– need excellent reference p+p and d+Au data.
2008-02-13
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Combinatorial Background
Which belongs to which? Combinatorial background
γ → e+ eγ → e+ eγ → e+ eπ0 → γ e+ eπ0 → γ e+ eπ0 → γ e+ e-
γ → e+ eπ0 → γ e+ e-
PHENIX 2 arm spectrometer acceptance:
dNlike/dm ≠ dNunlike/dm  different shape  need event mixing
like/unlike differences preserved in event mixing
Produce like and unlike sign in the mixed events at the proper rate
(B+- = 2√B++B--)
Like sign used as a cross check for the shape  provide absolute normalization for
unlike sign background
Use same event topology (centrality, vertex, reaction plane)
Remove every unphysical correlation
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Background Normalization: N+-=2√N++N--
--- Foreground: same event
--- Background: mixed event
Correction for asymmetry of pair cut
• Pair cut works differently in like and unlike sign pairs
 κ =κ+-/√ κ++ κ -- = 1.004
• estimated with mixed events
• Systematic error (conservative): 0.2%
2008-02-13
Small signal in like sign at low mass
N++ and N-- estimated from the
mixed events like sign
B++ and B-- normalized at high mass:
B++/N++ = 1
B--/N-- =1
for mass > 700 MeV
 Uncertainty due to statistics of
N++ and N--: 0.12%
TOTAL
SYSTEMATIC
ERROR = 0.25%
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Des einen Freud – des anderen Leid:
Conversions
γ→e+e- at r ≠ 0 have m ≠ 0
(artifact of PHENIX tracking:
i.e. no tracking before the field)
• effect low mass region
• have to be removed
Conversion removed with
orientation angle of the pair in the
magnetic field
Conversion pair
z
Dalitz decay
z
B
B
y
Photon conversions
MVD support structure
r ~ mass
f
e-
x
e+
y
ex
e+
Inclusive
Removed by phiV cut
After phiV cut
PHENIX Beam Pipe
2008-02-13
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Cocktail Ingredients
• Start from the π0 , assumption: π0 = (π+ + π-)/2
• parameterize PHENIX pion data:
d 3σ
E
3
dp

exp( ap
A
T
 bp )  p T p 0
2
T
p+p at √s=200 GeV
π0 → γ γ (Phys. Rev. D 76, 051106 (2007))
π± (Phys. Rev. C 74, 024904)
2008-02-13
37

n
p+p Cocktail Tuning (ω & φ)
ω and φ are fit with:
• modified Hagedorn (as on previous slide:
all parameter free)
• π0 parameterization with modified
Hagedorn + mT scaling (as on previous
slide: A is only free parameter,
pT→√(pT2+mω2-mπ2))
• exponential in mT
Fits of ω cross section
• mod. Hagedorn: χ2/NDF = 21.6/18
• mT scaled π0: χ2/NDF = 34.1/22
• expo in mT: χ2/NDF = 77.0/8
Fits of φ cross section
• mod. Hagedorn: χ2/NDF = 30.5/13
• mT scaled π0: χ2/NDF = 32.4/17
2008-02-13 2
• expo
in mT: χ /NDF = 73.3/15
38
p+p Cocktail Tuning (J/ψ)
Fits of J/ψ cross section
• mod. Hagedorn: χ2/NDF = 9.86/12
• mT scaled π0: χ2/NDF = 12.5/16
• expo in mT: χ2/NDF = 15.0/14
2008-02-13
2007-12-14
Published J/ψ is fit with:
• modified Hagedorn (all
parameter free)
• π0 parameterization with
modified Hagedorn + mT
scaling (one free
parameter)
• exponential in mT
• also shown is the published
fit with a power law
Torsten Dahms - Stony Brook University
39
In practice
200-300 MeV
÷
÷
÷
140-200
90-140
0-30
Rdata
• 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)
g direct
g *direct Rdata  Rp h


g *incl. Rdirect  Rp h
g incl.
0
0
2008-02-13
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From conventional measurement
2008-02-13
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Low pT mass spectra
2008-02-13
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Direct Photons
• Direct photon sources:
– QCD Compton scattering
qg  γq
– Annihilation
thermal:
e
qq  γg
 Eg /T
– QCD Bremsstrahlung
Decay photons
(p0→g+g, h→g+g, …)
hard:
2008-02-13
1
pTn
• Hard photons from inelastic
scattering of incoming partons
• Thermal photons are emitted
via same processes but from
thermalized medium
 carry information about the
temperature of the medium 43