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 mixing
Unphysical correlated backgroundTrack 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 e
0
e e
e e
Xπ0
π0e+
e-
e+
e-
γ
γ
π0
e-γ
e+
Subtractions dominate systematic uncertainty
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Key: Understanding the Background Subtraction
Axel Drees
Component-by-Component Bkg Subtraction
( , ) 22
T T
BGFG m p FG FG
BG 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 Data
Unlike sign pairssame 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)
Axel Drees8
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
Axel Drees10
Consistent with known sources!
Axel Drees
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
Axel Drees12
Common wisdom: charm bottom(but not quite true!)
What can we learn about Charm/Bottom
Axel Drees13
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
Axel Drees14
Hadron decay cocktail and like-sign pairs subtracted
MC@NLO normalized to double differential data (m vs pT)
Extrapolated heavy flavor cross sections:cc = 710 62 (stat) 183 (syst) 80 (model) mbbb = 4.5 0.7 (stat) 1.1 (syst) 0.2 (model) mbDY not considered, bottom might be overestimated
Double Differential View
Axel Drees15
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
Axel Drees17
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
Axel Drees18
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
Axel Drees19
STAR arXiv:1204.1890
charm cross section: STAR s = 920 mb PHENIX(MC@NLO) s = 710 mb*
acceptance: STAR =2Df p |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
Axel Drees20
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
Axel Drees21
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 Performance
Material of HBD 2-3% conv. probability3-4x 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
Axel Drees22
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
Axel Drees23
Peripheral (60-92%) Au+Au Semi central (20-40%) Au+Au
Analysis Improvements Since QM2012
Improved 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
Axel Drees24
PbSc timing central AuAu
New run of data production completed
Quantitative Understanding of Background
Simulation 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
Axel Drees25
0
e e
e e
X
Blind analysis
conversions
Dalitz
data
dito with HBD rejection
Central (10-20%) AuAu
Dat
a an
d M
C n
orm
aliz
ed t
o ea
ch o
ther
Late conversionsrejection factor 6
late conversion
early conversion
no HBD rejection
early conversionrejection factor 4
Summary and Outlook
PHENIX 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
Axel Drees26
Backup slides
Axel Drees27
Axel Drees28
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
Axel Drees29
Acceptance Function
Polarized unpolarized: difference smaller than 10%
Axel Drees30
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!
Axel Drees
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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Axel Drees33
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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