dielectrons with phenix: from p+p to au+au

Dielectrons with PHENIX:from p+p to Au+Au

- Torsten Dahms -Stony Brook University

2007 Annual Meeting of the Division of Nuclear PhysicsOctober 11, 2007

2007-05-25 Torsten Dahms - Stony Brook University 2

Why dielectrons• Don’t interact via strong force• Signal integrated over full

evolution of the systemExpected 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

Dielectrons at RHIC

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

•Strong enhancement of low-mass pairs persists at RHIC

•Open charm contribution becomes significant


The PHENIX experiment• Charged particle tracking:

– DC, PC1, PC2, PC3• Electron ID:

– Cherenkov light RICH– shower EMCal

• Photon ID:– shower EMCal

• Lead scintillator calorimeter (PbSc)• Lead glass calorimeter (PbGl)

– charged particle veto• Remove π contamination with pair cut

on parallel tracks in RICH• Remove conversions in detector

material with cut on orientation in magnetic field

e+e

EMCRICH

• ERT (EMC & RICH Trigger):– select events that were triggered by a track which

fires RICH and showers in EMC (i.e. single electron trigger with pT threshold: 0.4GeV


p+p Raw SpectraCombinatorial Background

from mixed events:

• normalize like sign above 600MeV (signal free region)

• BG+- =2√N++N--

• Subtract conversions

• Subtract cross pairs

cocktail

FG ERTBG MinBias

FG ERTBG MinBiasSubtracted

Unlike sign

Like sign


p+p Cocktail Comparison• Data abs. normalized to J/ψ (acceptance correct our J/ψ yield and normalize to published yield)

• Cocktail tuned for p+p• π0: Hagedorn parameterization of measured spectrum

• η: mT scaling• φ and J/ψ adjusted to measurement• Filtered in PHENIX acceptance

• Very good agreement over all mass range

• cc contribution from PYTHIA(567±57±193mb)

• Known from single e analysis that PYTHIA is softer than the data


ω & φ meson• Measured cross section of ω & φ → e+ e- in p+p at √s = 200 GeV• Goal:

– comparison to Au+Au measurement

– improving the hadronic cocktail components

dσ/dy (mbarn)

Extracted by integration

ω: 3.50 +/- 0.18 (stat) +/- 0.88 (sys)

φ: 0.37 +/- 0.02 (stat) +/- 0.09 (sys)


Au+Au Cocktail Comparison

• Data and cocktail absolutely normalized

• Cocktail from hadronic sources• Charm from

– PYTHIA – Single electron non photonic spectrum

w/o angular correlations• Predictions are filtered in PHENIX

acceptance & resolution

• Low-Mass Continuum:enhancement 150 < mee < 750 MeV

• Intermediate-Mass Continuum:– Single e pT suppression– PYTHIA softer than p+p but coincide

with Au+Au – Angular correlations unknown– Room for thermal contribution?A prediction (Rapp, nucl-th/0204003) says direct thermal radiation is about the same as charm

contribution in 1-2GeV/c2, and it will be dominant as we go to higher pT…

submitted to Phys. Rev. Lett

arXiv:0706.3034


p+p normalized to mee<100 MeV

Au+Au & p+p Comparison• p+p and Au+Au

normalized to π0 region

• Agreement at the resonances (ω, 2x φ)

• Enhancement in 0.2-0.8

• Agreement in intermediate mass and J/ψ just for ‘coincidence’(J/ψ happens to scale as π0 due to scaling with Ncoll + suppression)

p+p multiplied by Ncoll


Yield in Different Mass Ranges

0-100 MeV: π0 dominated; approximately scales with Npart

150-750 MeV: continuum

1.2-2.8 GeV: charm dominated;scales with Ncoll

Study yield in these mass regions as a function of centrality


Centrality Dependenceπ0 production scales with NpartLow Mass:• If in-medium enhancement from ππ or

qq annihilationyield should increase faster than

proportional to NpartIntermediate Mass:• charm follows binary scalingyield should increase proportional to

Ncoll

LOW MASS

INTERMEDIATE MASS


arXiv:0706.3034


Conclusions• First dielectron continuum measurement at RHIC

p+p• Very precise measurement: will fix cocktail components and charm

Au+AuLOW MASS:• Enhancement above the cocktail expectations:

3.4±0.2(stat.) ±1.3(syst.)±0.7(model)• Centrality dependency: increase faster than Npart

INTERMEDIATE MASS:• Coincident agreement with PYTHIA• Room for thermal radiation?

• Soon to come: study of pT shape of enhancement and comparison to p+p• HBD upgrade will reduce background

great improvement of systematic and statistical uncertainty• Silicon Vertex detector will distinguish charm from prompt contribution

Backup


φ→e+e- in d+Au and Au+Aud+Au at √sNN=200 GeV Au+Au at √sNN=200 GeV


Relation to previous analyses


ω & φ meson: mT Spectra


p+p Analysis• Used ERT triggered dataset to increase statistics:

– Run5 ERT data set corresponds to ~50B MinBias events– 10B analyzed so far, more to come– select events that were triggered by a track which fires RICH and

showers in EMC (i.e. single electron trigger with pT threshold: 0.4GeV) Trigger bias on combinatorial background– Remove random benefit (only accept pairs in which at least one

electron has fired the trigger)– Generate mixed events from MinBias dataset, with same requirement

on the pair• Need to correct for trigger efficiency:

– Hadron cocktail– Depends on trigger dead area– Determine trigger efficiency from MinBias (triggered electron/all

electrons)– Simulate trigger efficiency for every EMC sector– Project into mass vs. pT

MBpERTp TT


The Raw Subtracted SpectrumSame analysis on data sample with additional conversion materialCombinatorial background increased by 2.5Good agreement within statistical error signal/signal = BG/BG * BG/signal

large!!!0.25%

From the agreement converter/non-converter and the decreased S/B ratio scale error < 0.1%(well within the 0.25% error we assigned)


arXiv:0706.3034

300,000 pairs50,000 above 0


Background Normalization• Background shape well reproduced• Four independent normalization factors:

– like sign yield (no like sign signal):FG+-/BG+- = (FG++/BG++ + FG--/BG--)/2(needs to exclude low mass region, due to signal from double conversions)

– pair production (geometrical mean):N+- = 2√N++N--

– number of mixed events: Nevt/Nmix

– number of tracks: <N+-> = <N+><N->

• Very good agreement:within 0.5% syst. uncertainty of ±0.25%


Which belongs to which?γ e+ e- γ e+ e- γ e+ e- γ e+ e-

π0 γ e+ e- π0 γ e+ e- π0 γ e+ e- π0 γ 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

RATIO

BG fits to FG 0.1%

like sign

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

unlike sign • Normalization of unlike sign needs to be corrected for pair cuts bias between like and unlike sign pairs(i.e. pair cut on RICH ghosts)

• Statistical uncertainty: 0.1%• + uncertainty on pair cut bias lead to total syst. uncertainty of ±0.25%

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


Dielectrons in p+p:Advantages and Challenges

• Advantages: improved signal/background

• Challenges:1. triggered data complications for the mixed-event technique

• Used triggered dataset to increase statistics i.e. event triggered by track that hit RICH and EMC (pT threshold: 0.4GeV)

– Trigger bias on combinatorialbackground

– Remove random benefit (only acceptpairs in which at least one electronhas fired the trigger)

– Generate mixed events from MinBiasdataset, with same requirement onthe pair

EMC

RICH


Continuum in p+p: More Challenges2. Need to correct for trigger efficiency

– Hadron cocktail– Depends on triggered dead area– Determine trigger efficiency from MinBias

(triggered electron/all electrons)– Simulate trigger efficiency EMC sector by sector– Project into mass vs. pT

electrons from MB events+ ERT triggered electrons from MB events+ ERT triggered electrons from ERT triggered events

dielectrons with phenix: from p+p to au+au

Documents

au measurementimproving

intermediate mass

j acceptance

single e analysis

mass rangecc contribution

pythia single electron

mevintermediatemass

j yield