Analysis of the dielectron continuum in Au Au @ 200 GeV with PHENIX

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Analysis of the dielectron continuum in AuAu _at_
200 GeVwith PHENIX
Alberica Toiafor the PHENIX Collaboration

• Physics Motivation
• Analysis Strategy (870M events)
• Cuts
• Single electron cuts
• Electron pair cuts remove hit sharing
• Spectra Foreground, Background (mix events),
  Subtracted
• Efficiency / Acceptance
• Cocktail theory comparison
• Different centrality classes

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Physics Motivation em probes
time
e-
e
g
? Expansion ?
space
electro-magnetic radiation g, ee-, mm- rare,
emitted any time reach detector unperturbed by
strong final state interaction
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ee- Pair Continuum at RHIC

• Expected sources
• Light hadron decays
• Dalitz decays p0, h
• Direct decays r/w and f
• Hard processes
• Charm (beauty) production
• Important at high mass high pT
• Much larger at RHIC than at the SPS
• Cocktail of known sources
• Measure p0,h spectra yields
• Use known decay kinematics
• Apply detector acceptance
• Fold with expected resolution

Possible modifications
Chiral symmetry restoration continuum
enhancement modification of vector mesons
thermal radiation charm modification exotic bound
states
suppression (enhancement)
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Electron Identification

• PHENIX optimized for Electron ID
• track
• Cherenkov light RICH
• shower EMCAL

Charged particle tracking DC, PC1, PC2, PC3
and TEC Excellent mass resolution (1)
p
e
e-
Pair cuts (to remove hit sharing)
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Combinatorial Background
Which belongs to which? Combinatorial
background g? e e- g? e e- g? e e- g? e
e- p0 ? g e e- p0 ? g e e- p0 ? g e e- p0
? g e e- PHENIX 2 arm spectrometer acceptance
dNlike/dm ? dNunlike/dm ? different shape ?
need event mixing like/unlike differences
preserved in event mixing ? Same normalization
for like and unlike sign pairs
RATIO
--
--- Foreground same evt --- Background mixed evt
BG fits to FG 0.1
In all centrality bins
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Combinatorial Background

• Different independent normalizations used to
  estimate sys error
• Measured like sign yield Real,-- / Mixed,--
• Geometrical mean N 2vNN
• Event counting Nevent / Nmixed events
• Track counting N NN-
• After all corrections are applied, all the
  normalizations agree within 0.5

Systematic uncertainty ?0.25
--- Foreground same evt --- Background mixed evt
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Photon conversion rejection

• g?ee- at r?0 have m?0(artifact of PHENIX
  tracking)
• effect low mass region
• have to be removed
• For conversion photons
• Mass B dl radius

Conversion removed with orientation angle of the
pair in the magnetic field
--- inclusive --- removed by conversion cut ---
after conversion cut
Photon conversion
beampipe
air
T.Dahms
Support structures
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Subtracted spectrum
Integral180,000 above p015,000
BG normalized to Measured like sign yield
All the pairs Combinatorix Signal
PHENIX PRELIMINARY
PHENIX PRELIMINARY

• 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

ssignal/signal sBG/BG BG/signal
0.25
PHENIX PRELIMINARY
large!!!
Yellow band error on combinatorial background
normalization
Green band other systematics
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Cocktail ingredients (pp) p0

• most important get the p0 right (gt80 ),
  assumption p0 (p p-)/2
• parameterize PHENIX pion data

• most relevant the h meson (Dalitz conversion)
• also considered r, w, h, f
• use mT scaling for the spectral shape, i.e.

• normalization from meson/p0 at high pT as
  measured (e.g. h/p0 0.450.10)

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Cocktail comparison
PHENIX PRELIMINARY

• Data and cocktail absolutely normalized
• Cocktail from hadronic sources
• Charm from PYTHIA
• Predictions are filtered in PHENIX acceptance
• Good agreement in p0 Dalitz
• Continuumhint for enhancement not significant
  within systematics
• What happens to charm?
• Single e ? pt suppression
• angular correlation???
• LARGE SYSTEMATICS!

PHENIX PRELIMINARY
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Comparison with theory

• calculations for min bias
• QGP thermal radiation included

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

R.Rapp, Phys.Lett. B 473 (2000) R.Rapp,
Phys.Rev.C 63 (2001) R.Rapp, nucl/th/0204003
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Different centralities
20-40
0-10
10-20
PHENIX PRELIMINARY
60-100
40-60
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Mass ratios (A-B)/(0-100 MeV)
Ratio of different mass intervals to p0 yield
(0-100 MeV)
150-300 MeV
300-450 MeV
PHENIX PRELIMINARY
PHENIX PRELIMINARY
450-600 MeV
1.1-2.9 GeV
PHENIX PRELIMINARY
PHENIX PRELIMINARY
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A Hadron Blind Detector (HBD) for PHENIX
signal electron
Cherenkov blobs
e-
partner positron needed for rejection
e
qpair opening angle
1 m
S/B 100x

• 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
• Prototype just installed!

Irreducible charm background
S/B increased by factor 100
J.Kamin
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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
• Hint for enhancement not significant
• Improvement of the systematic uncertainty
• Centrality dependency (though not strong)
• 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 lt 25
• Single electron cuts
• Pt 150 MeV 20 GeV
• Ecore gt 150 MeV
• Match PC3 EMC
• PC3 (Phiz) lt 3 sigma
• EMC (Phiz) lt 3 sigma
• Dispmax lt 5 (ring displacement)
• N0min gt 3 tubes
• dep gt -2 sigma (overlapping showers NOT
  removed)
• chi2/npe0 lt 10
• Quality 63, 51, 31

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Pair cuts
DC ghosts (like sign) fabs(dphi) lt 0.1 rad
fabs(dz) lt 1.0 cm
--- Foreground same evt --- Background mixed evt
RICH ghosts (like and unlike sign)Post Field
Opening Angle lt 0.988
like
Cos(PFOA)
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Systematic error

• Systematic error of simulation (from AN415)
• Acceptance difference between real/simulation is
  less than 1.5.
• Systematic error of real data (from AN415)
• Single e eID efficiency difference between
  real/simulation dep lt 2, emc match lt 3, n0 lt
  5, chi2/npe0 lt 8.5, disp lt 5.
• n0?n1 double sys error
• Other correction factor (as AN415)
• Embedding efficiency (from Run2).
• Background Normalization

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Acceptance filter

• 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
q0
charge/pT
f0
z vertex
Roughly parametrized from data
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Efficiency

• 2 sets of simulations of dielectron pairs
• White in mass (0-4GeV)
• White in pT (0-4GeV)
• Vertex(-30,30), rapidity (1unit), phi (0,2p)

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

2D efficiency corrections Mass vs pT
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Single e distribution Poisson
0-10
10-20
20-30
30-40
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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

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A closer look at resonances
phi
Agreement with other analyses
A. Kozlov, K. Ozawa
J/psi
Upsilon???
H. Pereira, T.Gunji
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Mass spectra
10-20
0-10
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Mass spectra
40-60
20-40
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Mass spectra
60-100
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Cocktail
10-20
0-10
30
Cocktail
40-60
20-40
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Cocktail
60-100
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Theoretical Calculation of p-p Annihilation
gtgt 100 publications since 1995

• Low mass enhancement due to pp annihilation
• Spectral shape dominated r meson
• Vacuum r propagator
• Vacuum values of width and mass
• In medium r propagator
• Brown-Rho scaling
• Dropping masses as chiral symmetry is restored
• Rapp-Wambach melting resonances
• Collision broadening of spectral function
• Only indirectly related to chiral symmetry
  restoration
• Medium modifications driven by baryon density
• Model space-time evolution of collision
• Different approaches
• Consistent with hadron production data
• Largest contribution from hadronic phase