# Presentation: 660460

## Measurement of the dielectroncontinuum 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

## Outline

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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Motivation

- Why dielectrons
- Don’t interact via strong force
- Signal integrated over full evolution of the system
- Expected 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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******

****

****

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Combinatorial Background

- 	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 mixing
- 	like/unlike differences preserved in event mixing  Same normalization  						for like and unlike sign pairs

**RATIO**

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Au+Au Result

- 870 Million
- MinBias events

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
delete

## Au+Au: MB comparison to Cocktail

- 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Intermediate Mass Region

- 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- MinBias
- ERT Implementation

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## p+p Raw Spectra

- Resonances very well reproduced
- Photon conversions in detector material removed after BG subtraction

- Torsten Dahms - Stony Brook University

- 2007-05-25

## The ω and φ

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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%

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Trigger Efficiency

- Torsten Dahms - Stony Brook University

- 2007-05-25

## “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 sampleBG should have same trigger bias

- _like sign:_FG ERT
- BG ERT
- BG MinBias

- Torsten Dahms - Stony Brook University

- 2007-05-25