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

• 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 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 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 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 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 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 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