Measurements of Light Vector Mesons through the e+e-

Channel at the PHENIX

Xinhua Li for the PHENIX Collaboration

University of California – Riverside

Main goal of the Relativistic Heavy Ion Program at RHIC is detection of signals from potential

phase transition in strongly interacting matter associated with deconfinement or chiral

symmetry restoration

Medium modifications of the light vector

mesons should encode information on chiral

restoration

Introduction

Decay to di-electrons is particularly interesting, since electrons do not re-scatter in the medium and their invariant mass reflects the mass of the vector meson in the altered

vacuum state.

Broadening of at high T

Ralf Rapp, Physical Review C, 63 (2001)054907

Central arms of the PHENIX Spectrometer

Drift Chambermeasures particle

momentum

Ring Imaging

Cherenkov detects

electrons

Electro Calorimeter measures particle energy

Electrons in PHENIX photon converter runs

Mass of e+e- pair

Number of photo-electrons in RICH

E/p Chi2 in RICH

RHIC Au-Au Luminosity

1) Small cross section Br(e+e- )dN/dy ~ 10-

4;2) Search for rare events in the environment of high multiplicity;3) Large combinatorial background from photon conversions and Dalitz decays.

Challenge

Strategy

1. Select electron candidates.2. Require positron and electron to be from different PHENIX central arms.3. Transverse momentum above 300 MeV/c.4. Reject electrons created from photon conversion.5. Generate combinatorial background, using event mixing technique.6. Normalize the background by

, where is the number of e+e+ pairs from same

events, is the number of e-e- pairs from same

events, is the number of e+e- pairs from mixed

events.

7. Subtract the background.

mixeeeeNNN /2

eeN

eeN

mixN

Reconstructed meson

Statistics:25.8 million minimum biasevents of 200 AGeV AuAu

syst)((stat) 47 101 Signal 5620-

201

BackgroundSignal

Number of reconstructed ’s:

Simulation

Parameters e+e-

Generated events Ngen for |y|<0.6

3.70 M

Generated events Nrec for |y|<0.5

3.08 M

Reconstructed ’s 14920

Correction factor Ngen/Nrec

(T=320MeV)201

Embedding efficiency 0.70

Preliminary results

Parameters e+e-

Events analyzed 25.8 M

Branching ratio 2.96x10-4

Run-to-run fluctuation efficiency

0.71

Rapidity density dN/dy .)syst(.)stat(5.24.5 4.38.2

ncyRunEfficietoRun1

atioBranchingRFactorCorrection

N

N

dydN

events

corrected

,

where

correctedN is the number of ‘s corrected by embedding efficiency

Outlook

In the next RHIC physics run:

1. Higher luminosity.2. PHENIX Transition Radiation Detector will further improve

electron identification.3. EMCal-RICH electron trigger.