Transcript No Slide Title

Dileptons with PHENIX
Axel Drees, May 22nd 2013, Trento
Dilepton analysis
PHENIX 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 dAu
Dilepton puzzle in AuAu
Comment on STAR data
HBD analysis
HBD performance in central AuAu collisions
Status at QM2012
Recent progress
Summary
PHENIX Setup without HBD
background
p0  e-e+ g
g  e-e+
e+e- pairs
E/p and RICH
e- and e+ acceptance
are different
eee+
e+
DC
No background
rejection!
dilepton
S/B < 1:150
PC1
magnetic field &
tracking detectors
200 GeV
Run-4 AuAu
Run-5 pp, CuCu
Run8 dAu
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PHENIX Setup with HBD
background
p0  e-e+ g
g  e-e+
e+e- pairs
E/p and RICH
field
e- free region
ee+
HBD
e+
background rejection
in field free region!
S/B improves by 5-10
DC
200 GeV
Run-9 pp
Run-10 AuAu
PC1
magnetic field &
tracking detectors
62.4 GeV
Run-10 AuAu
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Key: Understanding the Background Subtraction
Combinatorial background: e+ and e- from different uncorrelated source
p 0  e+ e - g
g  e+ e-
Need event mixing because of acceptance differences for e+ and eUse like sign pairs to check event mixing
Unphysical correlated background
Track overlaps in detectors
Not reproducible by mixed events: removed from event sample (pair cut)
Correlated background: e+ and e- from same source but not “signal”
 “jet” pairs
“Cross” pairs
e- γ
p 0  gg
+
e e
e+
-
e+
X
+ -
e-
e e
γ
π0
π0
π0
γ
e+
e-
Use Monte Carlo simulation and like sign data to estimate and subtract background
Subtractions dominate systematic uncertainty
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Component-by-Component Bkg Subtraction
Cross pairs
Simulate cross pairs
with decay generator
Normalize to like sign
data for small mass
Like Sign Data
pp raw data
Correlated
Signal = Data-Mix
Jet pairs
Simulate with PYTHIA
Normalize to like sign data
Unlike sign pairs
same simulations
normalization from
like sign pairs
Mixed
events
Unlike Sign Data
Alternative methode
Correct like sign
correlated background
with mixed pairs
FG+- (mT , pT )  2 FG-- FG++ 
N +-  2
N ++
N --
Signal: S/B  1
BG+2 BG-- BG++
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Key to dilepton analysis is to understand the background
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2 10-7/MeV
10-9/MeV
1 10-10/MeV
p0:
NpAA/Nppp*eAA/epp
2 10-8/MeV
jet: Ncoll*RAA*eAA/epp
Correlated background roughly consistent with expectation
Alternative Method: Relative Acceptance Correction
S = FG12 – a FG1122
FG1122 =
PHENIX d+Au run-8
A large number of simulations show
that relative acceptance correction
can not be controlled to better than a few %
Method not usable for AuAu
Sys. Uncertainties on Relative Acceptance
Relative acceptance correction depends on
Physics source, thus mixed events not perfect description
Difference in number of electron and positrons
Differences in pt spectra of electrons & positrons
z-vertex position
Variations in active area (within similar run groups)
Case study: variation of active area in dAu setup
In MC simulation remove randomly drift chamber readout cards
(1/80 of acceptance)
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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
acceptance
Data are efficiency corrected
Excellent agreement
of data and hadron
decay contributions
with 30% systematic
uncertainties
Consistent with PHENIX
single electron
measurement
sc= 567±57±193mb *
* PYTHIA, 1st order, D/B semileptonic decay
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p+p Data with the HBD
Cocktail updated
BR, trans. form factors,
r-shape, bremsstrahlung,
c/b
New Charm/Bottom
simulation
MC@NLO using cross
sections determined in
d+Au
Like sign contribution
subtracted in data and
simulation
Consistent with known sources!
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Dilepton Continuum in d+Au Collisions
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!
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d+Au Data: pT Dependence
Detailed double differential data
Subtract hadron decay contribution
Remaining yield dominated by heavy flavor production
Common wisdom:
(but not quite true!)
charm
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bottom
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What can we learn about Charm/Bottom
Use MC@NLO 2nd order pQCD Monte-Carlo
Significant improvement over 1st PYTHIA (gg-fusion)
Subtle differences compared to “tuned” PYTHIA
New features of bottom production not relevant for charm
Feed-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
Any like sign subtraction in
data must be accounted for if
compared to simulation!
in PHENIX
acceptance
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Charm/Bottom Cross Section from d+Au Data
Hadron decay cocktail and like-sign pairs subtracted
MC@NLO normalized to double differential data (m vs pT)
Extrapolated heavy flavor cross sections:
scc = 710  62 (stat)  183 (syst)  80 (model) mb
sbb = 4.5  0.7 (stat)  1.1 (syst)  0.2 (model) mb
DY not considered, bottom might be overestimated
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Double Differential View
d+Au at 200 GeV
Relative importance of c/b
changes with mass and pT
c/b discrimination power
Low pT low mass → charm
High pT low mass → bottom
c>b
Possible new opportunity for
thermal radiation (Au+Au)
Mass – pT independent
Inverse slope in mass
reflects temperature of ~
few 100 MeV
Should be distinguishable
from c/b
Will be effected by heavy
flavor modification
c<b
c~b
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Au+Au Dilepton Continuum
Excess 150 <mee<750 MeV:
4.7 ± 0.4(stat.) ± 1.5(syst.) ± 0.9(model)
hadron decay cocktail tuned to AuAu
PHENIX Phys. Rev. C 81 (2010) 034911
Charm from PYTHIA
filtered by acceptance
scc= Ncoll × 567±57±193mb
Charm “thermalized”
filtered by acceptance
scc= Ncoll × 567±57±193mb
Need better handle
on heavy flavor
Intermediate-mass continuum: consistent with PYTHIA
since charm is modified 16
room for thermal radiation
Axel Drees
Soft Low Mass Dilepton Puzzle
acceptance corrected
mT spectrum of excess dileptons
Subtract cocktail
Correct for pair acceptance
Fit two exponentials in mT –m0
300 < m < 750 MeV
1st component T ~ 260 MeV
Consistent with thermal photon
yield
258  37  10 MeV
2nd “soft” component T ~ 100 MeV
Independent of mass
More than 50% of yield
Soft component also seen at SPS
in NA60 and CERES data
92  11  9 MeV
PHENIX Phys. Rev. C 81 (2010) 034911
Soft component eludes
theoretical explanation
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Soft Low Mass Dilepton Puzzle
acceptance corrected
mT spectrum of excess dileptons
Subtract cocktail
Correct for pair acceptance
Fit two exponentials in mT –m0
300 < m < 750 MeV
1st component T ~ 260 MeV
Consistent with thermal photon
yield
258  37  10 MeV
2ndCERES
“soft” component T ~ 100 MeV
Independent of mass
More than 50% of yield
Soft component also seen at SPS
in NA60 and CERES data
92  11  9 MeV
PHENIX Phys. Rev. C 81 (2010) 034911
Soft component eludes
theoretical explanation
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A look at STAR p+p Dilepton Data
STAR arXiv:1204.1890
charm cross section: STAR
s = 920 mb
PHENIX(MC@NLO) s = 710 mb*
acceptance:
STAR
Df2p
|Dh|=2
PHENIX cocktail in STAR acceptance
PHENIX cocktail and STAR cocktail consistent
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*consistent with PYTHIA 570mb
Axel Drees
A look at STAR pp and AuAu Data
Ratio STAR data/ PHENIX cocktail
in STAR acceptance
Compared to PHENIX cocktail
no appreciable enhancement
On a different note: comparing PHENIX data to cocktail
Not acceptance corrected
Soft component emphasized
20%-30% increase of enhancement
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Data from PHENIX HBD Upgrade
Window less CF4 Cherenkov detector
GEM/CSI photo cathode readout
Operated in B-field free region
Improve S/B by rejecting
combinatorial background
HBD fully operational:
Single electron ~ 20 P.E.
Conversion rejection ~ 90%
Dalitz rejection ~ 80%
Improvement of S/B factor 5-10 to
published results
p+p data
in 2008/9
Au+Au data in 2009/10
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Effect of CF4 Scintillation on HBD Performance
HBD module in AuAu
before subtraction:
Material of HBD 2-3% conv. probability
3-4x larger conversion background
Most material behind radiator
Reduced p-rejection in RICH
Effect centrality dependent
e-
Rely on veto by HBD
HBD module after
subtraction:
Scintillation ~ 10pe/pad in central AuAu
e-
Subtract statistically; multiple algorithms;
2 presented at QM11
Fluctuation in scintillation results in
centrality dependent rejection
In central collisions estimate 90%
rejection at 65% efficiency
10% central, 62 GeV:
efficiency 60%
Rejection 90%
Scintillation light limits
HBD veto
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Status at Quark Matter 2012
Background subtraction with rel. acceptance corrected like-sign pairs
Systematic uncertainty to large to obtain result in central collisions
Observe over subtraction in central collisions
Semi central (20-40%) Au+Au
Peripheral (60-92%) Au+Au
Ongoing analysis improvements
Component-by-component background subtraction (run-4 AuAu)
Improved pion rejection
Increased statistics
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Analysis Improvements Since QM2012
Improved data analysis of RICH
information
Issue: parallel track point to
same ring in RHIC
New algorithm resolves ring
MC simulation: Purity 70%
90% at 80% pair efficiency
Include TOF information for
hadron rejection
EMCal 450 ps; TOF ~120 ps
Small improvement of S/B
PbSc timing central AuAu
Increased statistics by 1.25
New run of data production completed
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Quantitative Understanding of Background
Central (10-20%) AuAu
Simulation of background from
p0 production
no HBD rejection
Data and MC normalized to each other
early
conversion
p0  e-e+ g & g  e-e+
Full PHENIX Geant simulation
Unlike sign pairs
Like sign cross pairs p 0  gg
data
late conversion
conversions
e+ e-
X
e+ e-
Dalitz
Work in progress
Absolute normalization
Quantitative understanding of
rejection
Simulation of jet background
dito with
HBD rejection
Late conversions
rejection factor 6
Once completed  look at unlike
sign data
early conversion
rejection factor 4
Blind analysis
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Summary and Outlook
PHENIX measured ee-pairs at √s=200 GeV
Well understood baseline in pp and dAu collisions
Within 20-30% consistent with hadron decays & heavy flavor production
Constrains heavy flavor production
Puzzles in AuAu collisons
larger 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 completion
Expect results this year
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Backup slides
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Centrality Dependence of Enhancement
PHENIX
Warning this is for illustration only!!!
CERES
0-30% cross section
Npart  Nch
pT > 200 MeV/c
CERES 95/96
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In+In
Axel Drees
Acceptance Function
Polarized unpolarized: difference smaller than 10%
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Acceptance for Virtual Photons
Data 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
e+
Case A
Virtual photon and electron and
positron in the acceptance
g*
eB-field
detector
e+
Case B
Virtual photon in acceptance
electron and/or positron NOT in
the acceptance
g*
e-
Pair acceptance =
B-field
Case A
Case A + Case B
Acceptance depends on pair dynamics!
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Combinatorial Background: Like Sign Pairs
 Shape from mixed events
--- Foreground: same evt N++
--- Background: mixed evt B++


Au-Au
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
N +-  2

N ++ N --
Subtract correlated BG
 Systematic uncertainties

Normalization of mixed events:
systematic uncertainty = 0.25%
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
statistics of N++ and N--: 0.12 %
different pair cuts in like and
unlike sign: 0.2 %
Axel Drees
Au-Au Raw Unlike-Sign Mass Spectrum
arXiv: 0706.3034
Run with added
Photon converter
Unlike sign pairs
data
2.5 x background
Excellent
agreement within
errors!
Mixed unlike sign
pairs normalized
to:
N +-  2
N ++
N --
Systematic errors from background subtraction:
ssignal/signal = sBG/BG * BG/signal
0.25%
 up to 50% near 500 MeV
as large as 200!!
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Centrality Dependence of Background Subtraction
Compare like sign data and mixed
background
Evaluation in 0.2 to 1 GeV range
For all centrality bins
mixed event background
and like sign data agree within
quoted systematic errors!!
Similar results for background
evaluation as function pT
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Background Description of Function of pT
Good agreement
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