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Analysis of the dielectron continuum in Au+Au @ 200

GeVwith PHENIX Alberica Toiafor the PHENIX Collaboration

• Physics Motivation • Analysis Strategy (765M events)

– Cuts• Single electron cuts • Electron pair cuts: remove hit sharing

– Spectra: Foreground, Background (mix events), Subtracted

– Efficiency / Acceptance• Cocktail & theory comparison

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Physics Motivation: em probes

space

time

Hard Scattering

AuAu

Exp

ansi

on

Hadronization

Freeze-out

QGPThermaliztion

e- e+

electro-magnetic radiation: , e+e-, +-

rare, emitted “any time”; reach detector unperturbed by strong final state interaction

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e+e Pair Continuum at RHIC

Expected sources• Light hadron decays

– Dalitz decays – Direct decays and

• Hard processes– Charm (beauty)

production – Important at high mass &

high pT

– Much larger at RHIC than at the SPS

• Cocktail of known sources– Measure , 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

e-

e+

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Electron IdentificationPHENIX optimized for Electron ID• track + • Cherenkov light RICH + • shower EMCAL

Charged particle tracking: DC, PC1, PC2, PC3 and TECExcellent mass resolution (1%)

Pair cuts (to remove hit sharing)

Energy-Momentum

All charged tracks

Background

Net signalReal

RICH cut

PC1

PC3

DC

magnetic field &tracking detectors

e+e

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Which belongs to which? Combinatorial background e+ e- eeeeee0 e+ e- 0 e+ e- 0 e+ e- 0 e+ e-PHENIX 2 arm spectrometer acceptance:

dNlike/dm ≠ dNunlike/dm different shape need event mixinglike/unlike differences preserved in event mixing Same

normalization for like and unlike sign pairs

Combinatorial Background

--- Foreground: same evt--- Background: mixed evt BG fits to FG

0.1%

RATIO

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• Different independent normalizations used to estimate sys error– Measured like sign yield – Event counting: Nevent / Nmixed events– Poisson assumption: N± = 2√N++N—– Track counting: ‹N±› = ‹N+›‹N-›

• All the normalizations agree within 0.5%

Combinatorial Background

Systematic uncertainty: 0.25%Systematic uncertainty: 0.25% e+ e -

e+ e -

--- Foreground: same evt--- Background: mixed evt

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Photon conversion rejection

Mass [GeV/c2]

--- without conversion --- with conversion

air

Support structures

Conversion removed with orientation angle of the pair in the magnetic field

Photon conversion

e+e- at r≠0 have m≠0(artifact of PHENIX tracking)• effect low mass region• have to be removed

r ~ mee

beampipe

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Subtracted spectrum

All the pairsCombinatorixSignal

BG normalized to Measured like sign yield

Integral:180,000 above 0:15,000

Green band: systematic uncertainty• Acceptance• Efficiency• Run-by-run

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Signal to Background• Very low signal to

background ratio in the interesting region main systematic uncertainty

Yellow band: error on combinatorial background normalization

Green band: other systematics

signal/signal = BG/BG * BG/signal0.25% large!!!

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A closer look at resonances

Upsilon???

J/psi

phiAgreement with other analyses

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Cocktail comparison

• Data and cocktail absolutely normalized

•Cocktail from hadronic sources•Charm from PYTHIAPredictions are filtered in PHENIX acceptance

•Good agreement in 0 Dalitz•Continuum:hint for enhancement not significant within systematics

•What happens to charm?•Single e pt suppression•angular correlation???

• LARGE SYSTEMATICS!

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Data/cocktail

Measurement [10-5 counts/event]

Predictions[10-5 counts/event]

0.15-0.7 GeV/c2 17.8 ± 3.8 ± 1.50 12.3

1.1-2.5 GeV/c2 0.67 ± 0.50 ± 0.11 1.16

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Comparison with theory

R.Rapp, Phys.Lett. B 473 (2000)R.Rapp, Phys.Rev.C 63 (2001)R.Rapp, nucl/th/0204003

• Systematic error too large to distinguish predictions• Mainly due to S/B• Need to improve

HBD

• calculations for min bias• QGP thermal radiation included

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A Hadron Blind Detector (HBD) for PHENIX

S/B ~ 100x

Irreducible charm background

signal electron

Cherenkov blobs

partner positronneeded for rejection e+

e-

pair opening angle

~ 1 m

• Dalitz rejection via opening angle – Identify electrons in field free region– Veto signal electrons with partner

• HBD concept: – windowless CF4 Cherenkov detector – 50 cm radiator length– CsI reflective photocathode – Triple GEM with pad readout

• Construction/installation 2005/2006

S/B increased by factor 100

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Summary & Outlook

• First dielectron continuum measurement at RHIC– Despite of low signal/BG– Thanks to high statistics– Good understanding of background normalization

• Measurement consistent with cocktail predictions within the errors– Improvement of the systematic uncertainty

• HBD upgrade will reduce background great improvement of systematic and statistical uncertainty

“The most beautiful sea hasn't been crossed yet. And the most beautiful words I wanted to tell you I haven't said yet ...“ (Nazim Hikmet)

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Backup

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Single electron cuts• Event cut:

– zvertex <= 25• Single electron cuts:

– Pt: = 150 MeV – 20 GeV– Ecore >= 150 MeV– Match PC3 & EMC

• PC3 (Phi+z) < 3 sigma• EMC (Phi+z) < 3 sigma

– Dispmax < 5 (ring displacement)– N0min >= 3 tubes– dep >= -2 sigma (overlapping

showers NOT removed)– chi2/npe0 < 10– Quality = 63, 51, 31

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Pair cuts

RICH ghosts (like and unlike sign)Post Field Opening Angle < 0.988

--- Foreground: same evt--- Background: mixed evt

like

Cos(PFOA)

DC ghosts (like sign): fabs(dphi) < 0.1 rad fabs(dz) < 1.0 cm

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Systematic error

• Systematic error of simulation – Acceptance difference between real/simulation is

less than: 3%.– Single e eID efficiency difference between

real/simulation is less than 8.8%.• Dep < 1%, emcsdphie < 1%, emcsdze < 1%, n0 <

7%, chi2/npe0 < 1%, disp < 5%.

• Systematic error of real data– Stability of acceptance: 5%– Stability of eID efficiency: 5%

• Other correction factor– Embedding efficiency < 10% (Run2 7%).

• Background Normalization

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Acceptance filter

Roughly parametrized from data

char

ge/

pT

0

0

z vertex

• Decoupling acceptance – efficiency corrections• Define acceptance filter (from real data)• Correct only for efficiency IN the acceptance• “Correct” theory predictions IN the acceptance• Compare

ACCEPTANCE FILTER

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Efficiency2 sets of simulations of dielectron pairs

•White in mass (0-4GeV)•White in pT (0-4GeV)•Vertex(-30,30), rapidity (1unit), phi (0,2)

•Linearly falling mass (0-1GeV)•Linearly falling pT (0-1GeV)•Vertex(-30,30), rapidity (1unit), phi (0,2)

2D efficiency corrections:Mass vs pT

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Single e distribution: Poisson

0-10%

20-30%

10-20%

30-40%

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Normalization of combinatorial backgroundSame normalization used for like and unlike sign pairs 4 different (independent) normalizations:

• Akiba : Nevent / Nmixed events• Hemmick: N± = 2√N++N—• Zajc: ‹N±› = ‹N+›‹N-›• Drees: 0.5*( (IntegralFG++(0.15-4 GeV) / IntegralBG++(0.15-4 GeV))  + (IntegralFG-- (0.15-4 GeV) / IntegralBG-- (0.15-4 GeV)) )

Normalization factors [10-2]• akiba: 8.74• hemmick: 8.69• zajc: 8.71• drees: 8.70

upper limit: Integral in charm region=0 Normalization factor 8.75

All normalizations agree within 0.5%

All normalizations agree within 0.5%

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The unfiltered calculations

•black: our standard cocktail•red : hadronic spectrum using the VACUUM rho spectral function•green: hadronic spectrum using the IN-MEDIUM rho spectral function•blue : hadronic spectrum using a rho spectral function with DROPPING MASS•magenta : QGP spectrum using the HTL-improved pQCD rate