Dileptons with PHENIX

Dilepton analysisPHENIX setup with and w/o HBD Key: background subtraction

Results Well understood baseline pp w & w/o HBD, dAu A new handle on charm/bottom separation in dAuDilepton puzzle in AuAu Comment on STAR data

HBD analysisHBD performance in central AuAu collisionsStatus at QM2012Recent progress

Summary

Axel Drees, May 22nd 2013, Trento

Axel Drees

PHENIX Setup without HBD

background p0 e-e+ g g e-e+

e-

e+

PC1

DC

magnetic field &tracking detectors

e+e- pairsE/p and RICH

2

e- and e+ acceptanceare different

No background rejection!

dileptonS/B < 1:150

200 GeVRun-4 AuAu

Run-5 pp, CuCuRun8 dAu

e+

e-

Axel Drees

PHENIX Setup with HBD

background p0 e-e+ g g e-e+

e-

e+

PC1

DC

magnetic field &tracking detectors

e+e- pairsE/p and RICH

3

background rejection in field free region!

S/B improves by 5-10

200 GeVRun-9 pp

Run-10 AuAu

62.4 GeVRun-10 AuAu

e+

e-

HBD

field free region

PHENIX Setup with HBD

Axel Drees

Combinatorial background: e+ and e- from different uncorrelated source

Need event mixing because of acceptance differences for e+ and e-

Use like sign pairs to check event mixingUnphysical correlated background

Track overlaps in detectorsNot reproducible by mixed events: removed from event sample (pair cut)

Correlated background: e+ and e- from same source but not “signal”“Cross” pairs “jet” pairs

Use Monte Carlo simulation and like sign data to estimate and subtract background

0 e e e ep g g+ - + -

0

e e

e e

p gg+ -

+ -

Xπ0

π0e+

e-

e+

e-γ

γ

π0

e- γe+

Subtractions dominate systematic uncertainty

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Key: Understanding the Background Subtraction

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Component-by-Component Bkg Subtraction

( , ) 22T T

BGFG m p FG FGBG BG

+-+- - - ++

- - ++

Mixed events

Correlated Signal = Data-Mix

Cross pairsSimulate cross pairs with decay generator Normalize to like sign data for small mass

Jet pairsSimulate with PYTHIANormalize to like sign data

2N N N+- ++ - -

Like Sign Data

Unlike Sign DataUnlike sign pairs

same simulationsnormalization from like sign pairs

Alternative methodeCorrect like sign

correlated background with mixed pairs

Signal: S/B 1

5

pp raw data

2 10-7/MeV

2 10-8/MeV

Key to dilepton analysis is to understand the background

1 10-10/MeV

1 10-9/MeV

Correlated background roughly consistent with expectation

p0: NpAA/Np

pp*eAA/epp

jet: Ncoll*RAA*eAA/epp

Alternative Method: Relative Acceptance Correction

FG1122 =

S = FG12 – a FG1122

Method not usable for AuAu

A large number of simulations show that relative acceptance correction can not be controlled to better than a few %

PHENIX d+Au run-8

Sys. Uncertainties on Relative AcceptanceRelative acceptance correction depends on

Physics source, thus mixed events not perfect descriptionDifference in number of electron and positronsDifferences in pt spectra of electrons & positronsz-vertex position Variations in active area (within similar run groups)

Case study: variation of active area in dAu setupIn MC simulation remove randomly drift chamber readout cards (1/80 of acceptance)

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Axel Drees

Dilepton Continuum in p+p Collisions PHENIX Phys. Lett. B 670, 313 (2009)

Data and Cocktail of known sources represent pairs with e+ and e- PHENIX acceptanceData are efficiency corrected

Excellent agreement of data and hadron decay contributionswith 30% systematic

uncertainties

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Consistent with PHENIX single electron measurement

sc= 567±57±193mb *

* PYTHIA, 1st order, D/B semileptonic decay

p+p Data with the HBD

Cocktail updatedBR, trans. form factors, r-shape, bremsstrahlung, c/b

New Charm/Bottom simulation

MC@NLO using cross sections determined in d+AuLike sign contribution subtracted in data and simulation

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Consistent with known sources!

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Dilepton Continuum in d+Au Collisions

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PHENIX preliminary

PYTHIA

3 data sets consistent with cocktail within 20-30%pp, dAu w/o HBD; and pp w HBD

Baseline at RHIC energies well understood!

Detailed double differential data Subtract hadron decay contributionRemaining yield dominated by heavy flavor production

d+Au Data: pT Dependence

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Common wisdom: charm bottom(but not quite true!)

What can we learn about Charm/Bottom

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Any like sign subtraction in data must be accounted for if

compared to simulation!

Use MC@NLO 2nd order pQCD Monte-CarloSignificant improvement over 1st PYTHIA (gg-fusion)Subtle differences compared to “tuned” PYTHIA

New features of bottom production not relevant for charmFeed-down: BR (B→e) ~ BR (B→D→e) ~ 10%B0B0 oscillations 60% of pairs unlikesign

ee-pairs from bottom production

ee-pairs from charm & bottom production

in PHENIX acceptance

Charm/Bottom Cross Section from d+Au Data

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Hadron decay cocktail and like-sign pairs subtractedMC@NLO normalized to double differential data (m vs pT)Extrapolated heavy flavor cross sections:

scc = 710 62 (stat) 183 (syst) 80 (model) mbsbb = 4.5 0.7 (stat) 1.1 (syst) 0.2 (model) mbDY not considered, bottom might be overestimated

Double Differential View

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Relative importance of c/b changes with mass and pT

c/b discrimination powerLow pT low mass → charmHigh pT low mass → bottom

Possible new opportunity for thermal radiation (Au+Au)

Mass – pT independentInverse slope in mass reflects temperature of ~ few 100 MeVShould be distinguishable from c/bWill be effected by heavy flavor modification

d+Au at 200 GeV

c > b

c ~ b

c < b

Axel Drees

Au+Au Dilepton ContinuumExcess 150 <mee<750 MeV: 4.7 ± 0.4(stat.) ± 1.5(syst.) ± 0.9(model)

Charm from PYTHIA filtered by acceptance scc= Ncoll × 567±57±193mb

Charm “thermalized” filtered by acceptancescc= Ncoll × 567±57±193mb

Intermediate-mass continuum: consistent with PYTHIAsince charm is modified room for thermal radiation

hadron decay cocktail tuned to AuAu

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Need better handleon heavy flavor

PHENIX Phys. Rev. C 81 (2010) 034911

Soft Low Mass Dilepton Puzzle

mT spectrum of excess dileptonsSubtract cocktail Correct for pair acceptanceFit two exponentials in mT –m0

1st component T ~ 260 MeV Consistent with thermal photon yield

2nd “soft” component T ~ 100 MeVIndependent of massMore than 50% of yieldSoft component also seen at SPS in NA60 and CERES data

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92 11 9 MeV

258 37 10 MeV

300 < m < 750 MeV

Soft component eludes theoretical explanation

acceptance corrected

PHENIX Phys. Rev. C 81 (2010) 034911

Soft Low Mass Dilepton Puzzle

mT spectrum of excess dileptonsSubtract cocktail Correct for pair acceptanceFit two exponentials in mT –m0

1st component T ~ 260 MeV Consistent with thermal photon yield

2nd “soft” component T ~ 100 MeVIndependent of massMore than 50% of yieldSoft component also seen at SPS in NA60 and CERES data

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92 11 9 MeV

258 37 10 MeV

300 < m < 750 MeV

acceptance corrected

PHENIX Phys. Rev. C 81 (2010) 034911

CERES

Soft component eludes theoretical explanation

A look at STAR p+p Dilepton Data

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STAR arXiv:1204.1890

charm cross section: STAR s = 920 mb PHENIX(MC@NLO) s = 710 mb*

acceptance: STAR Df2p |Dh|=2 PHENIX cocktail in STAR acceptance

PHENIX cocktail and STAR cocktail consistent

*consistent with PYTHIA 570mb

A look at STAR pp and AuAu Data

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Compared to PHENIX cocktailno appreciable enhancement

On a different note: comparing PHENIX data to cocktailNot acceptance correctedSoft component emphasized20%-30% increase of enhancement

Ratio STAR data/ PHENIX cocktail in STAR acceptance

Data from PHENIX HBD Upgrade

HBD fully operational:Single electron ~ 20 P.E.Conversion rejection ~ 90% Dalitz rejection ~ 80%Improvement of S/B factor 5-10 to published results

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Window less CF4 Cherenkov detectorGEM/CSI photo cathode readoutOperated in B-field free region

p+p data in 2008/9Au+Au data in 2009/10

Improve S/B by rejecting combinatorial background

Effect of CF4 Scintillation on HBD PerformanceMaterial of HBD 2-3% conv. probability

3-4x larger conversion background Most material behind radiatorReduced p-rejection in RICHEffect centrality dependent

Scintillation ~ 10pe/pad in central AuAuSubtract statistically; multiple algorithms; 2 presented at QM11Fluctuation in scintillation results in centrality dependent rejection In central collisions estimate 90% rejection at 65% efficiency

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Rely on veto by HBD

Scintillation light limitsHBD veto

HBD module in AuAu before subtraction:

HBD module after subtraction:

10% central, 62 GeV:

efficiency 60%Rejection 90%

e-

e-

Status at Quark Matter 2012 Background subtraction with rel. acceptance corrected like-sign pairs

Systematic uncertainty to large to obtain result in central collisionsObserve over subtraction in central collisions

Ongoing analysis improvementsComponent-by-component background subtraction (run-4 AuAu)Improved pion rejectionIncreased statistics

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Peripheral (60-92%) Au+Au Semi central (20-40%) Au+Au

Analysis Improvements Since QM2012Improved data analysis of RICH information

Issue: parallel track point to same ring in RHICNew algorithm resolves ring MC simulation: Purity 70% 90% at 80% pair efficiency

Include TOF information for hadron rejection

EMCal 450 ps; TOF ~120 psSmall improvement of S/B

Increased statistics by 1.25

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PbSc timing central AuAu

New run of data production completed

Quantitative Understanding of BackgroundSimulation of background from p0 production

p0 e-e+ g & g e-e+

Full PHENIX Geant simulation Unlike sign pairsLike sign cross pairs

Work in progressAbsolute normalizationQuantitative understanding of rejectionSimulation of jet background

Once completed look at unlike sign data

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0

e e

e e

p gg+ -

+ -

X

Blind analysis

conversions

Dalitz

data

dito with HBD rejection

Central (10-20%) AuAu

Dat

a an

d M

C n

orm

aliz

ed to

eac

h ot

her

Late conversionsrejection factor 6

late conversion

early conversion

no HBD rejection

early conversionrejection factor 4

Summary and OutlookPHENIX measured ee-pairs at √s=200 GeV

Well understood baseline in pp and dAu collisionsWithin 20-30% consistent with hadron decays & heavy flavor productionConstrains heavy flavor production

Puzzles in AuAu collisonslarger excess beyond contribution from hadronic phase with medium modified r-meson properties soft momentum distribution

Thermal photon puzzle (see talk by Gabor David)Large thermal yield with T > 220 MeV (10-20% of decay photons)Large elliptic flow (v2)

HBD analysis moving towards completionExpect results this year

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Backup slides

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Centrality Dependence of Enhancement

CERES 95/96

Warning this is for illustration only!!!

0-30% cross sectionNpart NchCERES

pT > 200 MeV/c

PHENIX

In+In

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Acceptance Function

Polarized unpolarized: difference smaller than 10%

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Acceptance for Virtual PhotonsData presented as e+ and e- in acceptance, this is not the same as virtual photon in acceptance! Physical distribution requires that virtual photon is in acceptance!

detector

g*

e+

e-

Virtual photon and electron and positron in the acceptance

B-field

g*

e+

e-

Virtual photon in acceptanceelectron and/or positron NOT in the acceptance

detector

B-field

Case A

Case B

Case APair acceptance = Case A + Case B

Acceptance depends on pair dynamics!

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Combinatorial Background: Like Sign Pairs

--- Foreground: same evt N++--- Background: mixed evt B++

Shape from mixed events Excellent agreements for like

sign pairs also with centrality and pT

Normalization of mixed pairs Small correlated background at

low masses from double conversion or Dalitz+conversion

normalize B++ and B- - to N++ and N- - for m > 0.7 GeV

Normalize mixed + - pairs to

Subtract correlated BG

Systematic uncertainties statistics of N++ and N--: 0.12 % different pair cuts in like and

unlike sign: 0.2 %Normalization of mixed events:systematic uncertainty = 0.25%

2N N N+- ++ - -

Au-Au

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Axel Drees

Au-Au Raw Unlike-Sign Mass Spectrum

Mixed unlike sign pairs normalized to:

2N N N+- ++ - -

Unlike sign pairs data

ssignal/signal = sBG/BG * BG/signal

as large as 200!! 0.25%

Systematic errors from background subtraction:

up to 50% near 500 MeV

arXiv: 0706.3034

Run with addedPhoton converter

2.5 x background

Excellent agreement within errors!

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Centrality Dependence of Background Subtraction

For all centrality binsmixed event background and like sign data agree withinquoted systematic errors!!

Evaluation in 0.2 to 1 GeV range

Compare like sign data and mixed background

Similar results for background evaluation as function pT

Axel Drees

Background Description of Function of pT

Good agreement

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