measurements of thermal photons and the dielectron continuum with phenix
DESCRIPTION
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 13 th , 2008. charm & bottom cross section. centrality dependence. p T spectra. chiral symmetry. mass spectra. - PowerPoint PPT PresentationTRANSCRIPT
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
low mass enhancement
pT spectra mass spectra
thermal photons
p+p collisions
centrality dependence
Au+Au collisions
chiral symmetry
medium modification
2008-02-13 2
Dileptons at RHICExpected sources• Light hadron decays
– Dalitz decays – Direct decays and
• Hard processes– Charm (beauty) production – Much larger at RHIC than at SPS
Possible modifications
suppression (enhancement)
Chiral symmetry restoration continuum enhancement modification of vector mesons
thermal radiationcharm modificationexotic bound states• 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
• Changes in position and width: signals of the chiral transition?
time
hard parton scattering
AuAu
hadronization
freeze-out
formation and thermalization of quark-gluonmatter?
Space
Time
expansion
Jet cc e p K
2008-02-13 3
The Data• 800M MinBias Au+Au events• 2.25pb-1 of triggered p+p data
• Combinatorial background removed by mixed events (0.25% syst. uncertainty in Au+Au)
• 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
π0
π0
e+
e-
e+
e-
γ
γ
π0
e-
γ
e+
p+p at √s = 200GeVp+p at √s = 200GeV
submitted to Phys. Lett.B
arXiv: 0802.0050
2008-02-13 42007-12-14 Torsten Dahms - Stony Brook University 4
The Raw Subtracted SpectrumSame analysis on data sample with additional conversion materialCombinatorial background increased by 2.5Good agreement within statistical error signal/signal = BG/BG * BG/signal
large!!!0.25%
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
300,000 pairs50,000 above 0
2008-02-13 5
Cocktail Tuning (p+p)• Start from the π0 , assumption: π0 = (π+ + π-)/2
• parameterize PHENIX pion data:
n0T2TT
3
3
pp)bpapexp(
A
pd
σdE
Other mesons•well measured in electronic and hadronic channels
•Other mesons are fit with:mT scaling of π0 parameterization pT→√(pT
2+mmeson2-mπ
2) fit the normalization constant All mesons mT scale!
PHENIX Preliminary
2008-02-13 6
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
• Charm cross section from single electron measurement:
– σcc = 567 ± 57 ± 193 μb
submitted to Phys. Lett.B
arXiv: 0802.0050
2008-02-13 7
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±193b
• Predictions are filtered in PHENIX acceptance & resolution
• Low-Mass Continuum:enhancement 150 < mee < 750 MeV3.4 ± 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 8
p+p normalized to mee<100 MeV
Au+Au & p+p Comparison
• 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 9
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 10
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 11
Centrality Dependenceπ0 production scales with NpartLow Mass:• If in-medium enhancement from ππ or
qq annihilationyield should increase faster than
proportional to NpartIntermediate Mass:• charm follows binary scalingyield should increase proportional to
Ncoll
LOW MASS
INTERMEDIATE MASS
submitted to Phys. Rev. Lett
arXiv:0706.3034
2008-02-13 12
Mass Spectra: pT dependency• Study pT dependency of the low mass enhancement in Au+Au
• p+p in agreement with cocktail
• Au+Au low mass enhancement concentrated at low pT
0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c
0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c
2008-02-13 13
pT Spectra
•p+p: follows the cocktail•Au+Au: significantly deviates at low pT
2008-02-13 14
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 0 + exponential
2008-02-13 15
Yields and SlopesSLOPES YIELDS
Total yield (DATA)
2expo fitmT-scaling +expo fit
Low-pT yield
• 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 16
Mass Spectra: pT dependency• Study pT dependency of the low mass enhancement in Au+Au
• high pT region provides window for thermal photon measurement
0 < pT < 8 GeV/c 0 < pT < 0.7 GeV/c
0.7 < pT < 1.5 GeV/c 1.5 < pT < 8 GeV/c
2008-02-13 17
Compton
q γ
g q
Compton
q γ*
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
Virtual Photons
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 γ produces virtual γ with very low mass
• Rate and mass distribution given by same formula
– No phase space factor for mee<< pT
photon
• Improved S/B by measuring direct photon signal in mass region in which π0 are suppressed
π0
γ
γπ0
γ
e+
e-γ*
2008-02-13 18
Cocktail comparison
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
1 < pT < 2 GeV2 < pT < 3 GeV3 < pT < 4 GeV4 < pT < 5 GeV
p+p Au+Au (MB)
PHENIX Preliminary
2008-02-13 19
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 in80 < mee < 300MeV gives: χ2/DOF=11.6/10
• It’s not the η:– 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)(mrf)(mr)f()f(m eedirecteecocktailee 1
2008-02-13 20
Fraction of direct photons• 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
μ = 0.5pT
μ = 1.0pT
μ = 2.0pT
μ = 0.5pT
μ = 1.0pT
μ = 2.0pT
p+p Au+Au (MB)
2008-02-13 21
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 22
The spectrum• Compare spectra to NLO pQCDp+p• consistent with pQCDAu+Au• above binary scaled pQCD• If excess of thermal origin:
inverse slope is related to initial temperature
2008-02-13 23
Conclusions
p+pLOW MASS:• Excellent agreement with hadronic decay cocktail
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
THERMAL PHOTONS• p+p in agreement with pQCD
Au+AuLOW MASS:• 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:• Coincidence agreement with PYTHIA• Room for thermal radiation?
THERMAL PHOTONS:• Dielectron mass shape for pT > 1 GeV and mee <
300MeV consistent with internal conversions of virtual photons
• Au+Au above pQCD
• First dielectron continuum measurement at RHIC– Despite of low signal/BG
– Thanks to high statistics– Very good understanding of background normalization
•HBD upgrade will reduce background great improvement of systematic and statistical uncertainty (LMR)•Silicon Vertex detector will distinguish charm from prompt contribution (IMR)
Backup
2008-02-13 25
Charm and bottom cross sections
CHARM BOTTOM
Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end)
Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end)
First measurements of bottom cross section at RHIC energies!
2008-02-13 26
Theory Comparison II
Cocktail not subtracted from data(necessary for comparison)
Calculations from• R. Rapp & H. van Hees• K. Dusling & I. Zahed• E. Bratovskaja & W. Cassing (in 4π)
2008-02-13 272007-12-14 Torsten Dahms - Stony Brook University 27
Cu+Cu dN/dm – Minimum Biasπηη’ωρφccJ/ψψ’
2008-02-13 28
Dielectrons at RHIC – Intermediate Mass
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)
• 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 dQT
2 with MT in a narrow interval and with a suitable pT
min 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 29
Dielectrons at RHICExpected 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
Possible modifications
suppression (enhancement)
Chiral symmetry restoration continuum enhancement modification of vector mesons
thermal radiationcharm modificationexotic bound states
R. Rapp nucl-th/0204003R. Rapp nucl-th/0204003
•Strong enhancement of low-mass pairs persists at RHIC
•Open charm contribution becomes significant
2008-02-13 30
Relativistic Heavy Ion Collider
2008-02-13 31
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• 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)
e+e-
pg
• Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program)
2008-02-13 32
Electron Identification• Charged particle tracking (δm: 1%)
DC, PC1, PC3• PHENIX optimized for Electron ID• Cherenkov light RICH + • shower EMCAL
• 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
Cerenkovphotons from e+ or e- are detected by array of PMTs
mirror
Most hadrons do not emit Cerenkov light
Electrons emit Cerenkovphotonsin RICH.
Central Magnet
RICH
PMT arrayPMT array
Cerenkovphotons from e+ or e- are detected by array of PMTs
mirror
Most hadrons do not emit Cerenkov light
Electrons emit Cerenkovphotonsin RICH.
Central Magnet
RICH
PMT arrayPMT array
RICH
Energy-Momentum
All charged tracks
Background
Net signal
RealRICH cut
2008-02-13 332007-12-14 Torsten Dahms - Stony Brook University 33
The Double Challenge• Need to detect a very weak source of e+e- pairs
hadron decays (m>200 MeV, pT>200 MeV) ~ 4x10-6 / π0
• In the presence of hundreds of charged particlescentral Au+Au collision dNch / dy ≈ 700
• And several pairs per event from trivial originπ0 Dalitz decays ~ 10-2 / π0
+ γ conversions (assume 0.5% radiation length) ~ 10-2 / π0 huge combinatorial background (dNch / dy)2
– 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% andpT cut at 200 MeV
Signal to background depending on mass up to1 : few hundred
• Electron pairs are emitted through the wholehistory of the collision:– need to disentangle the different sources. – need excellent reference p+p and d+Au data.
Experimental Challenge
Analysis Challenge
2008-02-13 342007-12-14 Torsten Dahms - Stony Brook University 34
Which belongs to which? Combinatorial backgroundγ → e+ e- γ → e+ e- γ → e+ e- γ → e+ e-
π0 → γ e+ e- π0 → γ e+ e- π0 → γ e+ e- π0 → γ e+ e-
PHENIX 2 arm spectrometer acceptance: dNlike/dm ≠ dNunlike/dm different shape need event mixinglike/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
Combinatorial Background
2008-02-13 352007-12-14 Torsten Dahms - Stony Brook University 35
Background Normalization: N+-
=2√N++N--
--- Foreground: same event--- Background: mixed event
Small signal in like sign at low massN++ and N-- estimated from the mixed events like signB++ and B-- normalized at high mass:B++/N++ = 1 B--/N-- =1for mass > 700 MeV Uncertainty due to statistics of N+
+ and N--: 0.12%
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%
TOTAL SYSTEMATIC ERROR = 0.25%
2008-02-13 362007-12-14 Torsten Dahms - Stony Brook University 36
Des einen Freud – des anderen Leid: Conversions
Conversion removed with orientation angle of the pair in the magnetic field
Photon 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
InclusiveRemoved by phiV cutAfter phiV cut
z
y
x e+e-
B
Conversion pairz
y
x e+
e-
B
Dalitz decay
r ~ mass
PHENIX Beam Pipe
MVD support structure
2008-02-13 372007-12-14 Torsten Dahms - Stony Brook University 37
Cocktail Ingredients• Start from the π0 , assumption: π0 = (π+ + π-)/2
• parameterize PHENIX pion data:
n0T2TT
3
3
pp)bpapexp(
A
pd
σdE
p+p at √s=200 GeV
π0 → γ γ (Phys. Rev. D 76, 051106 (2007))π± (Phys. Rev. C 74, 024904)
2008-02-13 38
p+p Cocktail Tuning (ω & φ)
38
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→√(pT
2+mω2-mπ
2))
• 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
• expo in mT: χ2/NDF = 73.3/15
2008-02-13 39
p+p Cocktail Tuning (J/ψ)
39
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
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
2007-12-14 Torsten Dahms - Stony Brook University
2008-02-13 40
In practice
0-30
• 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)
0
0
direct
data
incl.
direct
*
*
RR
RR
incl.
direct
÷
200-300 M
eV
140-200
Rdata
90-140
From conventional measurement
2008-02-13 41
2008-02-13 42
Low pT mass spectra
2008-02-13 43
Direct Photons• Direct photon sources:
– QCD Compton scattering
– Annihilation
– QCD Bremsstrahlung
• 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
T
1np
hard:
/ E Tethermal:
Decay photons(p0→g+g, h→g+g, …)
γqqg
γgqq