measurement of the dielectron continuum in p+p & au+au collisions at √s nn = 200 gev with...

Measurement of the dielectroncontinuum in p+p & Au+Au

collisions at √sNN = 200 GeV with PHENIX

- Torsten Dahms -Stony Brook University

2nd Berkeley School of Collective Dynamicsin High-Energy Collisions

May 24, 2007

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

Outline

• Motivation: probe the medium• Preliminary Au+Au result• Status of the p+p measurement• Summary


MotivationWhy 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

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


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


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


Au+Au Result

PHENIX Preliminary

Systematic and Normalization Error

870 Million

MinBias events


Au+Au: MB comparison to Cocktail

PHENIX Preliminary

• Data have been corrected for single electron ID efficiency

• Data and Cocktail absolutely normalized

• Cocktail from hadronic sources (i.e. light mesons via mT scaling)

• open charm from PYTHIA

• Cocktail filtered into PHENIX acceptance

• Indication of enhancement in region:150<mee<740MeV


Au+Au Mass Ratios• Ratio of yield in various mass regions to the yield within the

π0 mass range, 0-100 MeV

• Yield in mass window 450-600 MeV increases faster with centrality than π0 mass range

PHENIX Preliminary


Intermediate Mass Region

PeripheralColl

CentralCollCP NYield

NYieldR

)/(

)/(

• IMR dominated by open charm

• c-cbar produced in pairs• mee = p1p2(1- cos(θ12))• Suppression of IMR in addition to observed high pT suppression of charm in single electron measurement

• If decrease true, it could indicate modified dynamical correlation of c-cbar pair

• But within errors constant with Npart


p+p Status

• Important to provide reference beyond “cocktail”

• Much smaller combinatorial background (S/B~1, compared to 1:100 in Au+Au collisions)

• But also much smaller multiplicity triggered data

Au+Au

p+p


p+p Analysis

• Used ERT triggered dataset to increase statistics:– 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


First estimate of trigger effects

• Inv. mass spectrum for e+e- pairs from hadronic cocktail:– for MinBias events

– with trigger condition on at least one of the electrons

• Small mass dependence (up to the φ)

• Difference mainly due to dead area of trigger

MinBiasERT Implementation


p+p Combinatorial Background• To get shape of combinatorial background correct:

– Mixed events from MinBias sample– Apply trigger condition as pair cut, i.e. at least one of the electrons has fired the

ERT trigger• BG normalized to like sign above 700MeV

Like Sign Pairs:FG ERTBG MinBias

Signal due to double conversion


p+p Raw Spectra• Resonances very well reproduced• Photon conversions in detector material removed after BG

subtraction

Unlike Sign Pairs:FG ERTBG MinBiasSubtracted


The ω and φ


Summary

• Background shape and normalization well understood• Au+Au:

– Hint of low mass excess– Centrality dependent supression in charm region – Medium breaks opening angle correlation of c-cbar?

• p+p:– Benefit of much smaller background: S/B~1– Combinatorial BG from triggered data understood– Early stage of analysis– Could give answers to:

• pT distribution of hadrons/input for cocktail• Baseline for charm• Identify thermal photons contribution to Au+Au• IMR: RCP RAA

Backup


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%


Trigger Efficiency

W0

W3 E3

E0

• pT spectra for each sector

• Good agreement of ERT triggered tracks from MinBias and ERT triggered events

• Ratio of ERT triggered/MinBias trigger efficiency

all tracks (after eID cuts) from MB events+ ERT triggered tracks from MB events+ ERT triggered tracks from ERT triggered events


Trigger Efficiency


“Wrong” Combinatorial BG

• Mixed events from ERT triggered events

• What if we apply a trigger condition on the mixed event– require every MinBias event

that is used for mixed events to have at least one trigger particle

– this should reproduce a ERT trigger sampleBG should have same trigger bias

like sign:FG ERTBG ERTBG MinBias

measurement of the dielectron continuum in p+p & au+au collisions at √s nn = 200 gev with...

Documents

e e e e e e e e

e ephenix

p p au au collisions

au collisionsbut

mass window

single electron trigger

high mass high ptmuch

various mass regions