# Presentation: 870351

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

- - Torsten Dahms -
- Stony Brook University
- 2007 Annual Meeting of the Division of Nuclear Physics
- October 11, 2007

## 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

- 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

- Dielectrons at RHIC

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
Dielectrons are a penetrating probe of the medium created in HI collisions, because they don’t interact via the strong force. Therefore they carry information from their production time. The graph shows the expected sources of dielectrons as a function of their invariant mass. At low masses the dominant production is due to pi0 and eta Dalitz decays. Towards higher masses Dalitz decays of heavier mesons become more important. There is additional contribution due to resonsancs decays of vector masses. The IMR is dominated by open charm decays, correlated due to the ccbar production.
Possible medium modificiations are:
...

## 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******

****

****

- 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
For the p+p dataset a single electron trigger wit a pT threshould of 400MeV was used, which requires a hit in RICH and a shower in the EMC associated to a charged track.

## p+p Raw Spectra

- Combinatorial Background
- from mixed events:
- normalize like sign above 600MeV (signal free region)
- BG+- =2√N++N--
- Subtract conversions
- Subtract cross pairs

- cocktail

- FG ERT
- BG MinBias

- FG ERT
- BG MinBias
- Subtracted

- Unlike sign

- Like sign

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
The p+p measurement benefits from the lower background than in HI collisions, but faces an additional challenge due to ERT trigger bias.

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
Cocktail comparison... Cocktail from measurements where available, mT scaling is assumed otherwise. We use PYTHIA for the open charm contribution using the cross section extracted from the single electron measurement. But we know that the single electron spectrum is harder than PYTHIA, therefore we may expect to also have a harder mass spectrum then predicted by PYTHIA.

Since the validity of the assumption of mT scaling is clear, the goal is to sub sequentially improve or substitute the cocktail with measurements of all ingredients. We have started with the omega and phi (next slide)

## 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)

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
We have measured the cross section of omega and phi in p+p collisions at 200GeV. The left plot shows the comparison to various hadronic decay channels of the omega. The omegae+ e- decay channel allows to extend the cross section measurement to very low pT. Good agreement in overlapping region. We can see the same in case of the phi cross section. The comparison to the KK channel will be particular interesting in HI collisions considering the fact that the phi is very close to twice the kaon mass. If there is a mass shift this should lead to change of the KK decay w.r.t. to the dielectron channel.

## 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/c**2**, and it will be dominant as we go to higher p**T**...*

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
Now we have also measured the dielectron spectrum in Au+Au collision. Here I am showing the inv. mass spectrum for min. bias Au+Au collisions at 200GeV. The data are compared to a cocktail of hadronic sources, which has been generated very much like in the case of p+p. While the cocktail reproduces very much the spectrum below 100MeV which is dominated by pi0 Dalitz decays, as well as the resonances like omega, phi and J/psi, we see a huge enhancement in the low mass region.

From single electrons measurement in Au+Au we know that charm is modified by the medium and suppressed at high pT. Therefore the medium may also affect the angular correlation of ccbar pairs which should be reflected in dielelctron spectrum. The dashed line shows the prediction from PYTHIA while the dotted line shows the exptected contribution from open charm in the case of a complete randomization of the angular correlation of the ccbar pair. This would give some room for additional thermal contribution

## 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)

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
Instead of comparing the data to the cocktail, we can compare the dielectron spectra from Au+Au and p+p collisions directly. Here we have scaled the p+p spectrum to the yield in Au+Au below 100MeV, so to the pi0. Again, we observe a very good agreement not only in the pi0 region but also for omega and phi. But the low mass enhancement persists. The impression that the J/psi scales as the pi0 is in agreement with the observed J/psi suppression in Au+Au. If we scale the p+p spectrum instead of to the pi0 by the number of binary collisions, we can clearly see the suppression of the J/psi. The question remains what precisely is happening to the IMR. A look at the pT dependence may reveal more information.

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
For now we will look at the centrality dependence of the yield in different mass regions:...

## Centrality Dependence

- π0 production scales with Npart
- Low Mass:
- If in-medium enhancement from ππ or qq  annihilation
- yield should increase faster than proportional to Npart
- Intermediate Mass:
- charm follows binary scaling
- yield should increase proportional to Ncoll

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
Shown here is the Yield per participating nucleon pair as function of centrality. For the pi0 region we find the expected scaling with the number of participants and a very good agreement with the hadr. cocktail. Now if the low mass region enhancement was from either pi+ pi- annhiliation in a hadronic medium or q qbar annihilation in a partonic medium we would expect and increase of the yield that goes faster than proportional to the number of participants. Indeed the data show an increase that does not scale with Npart but increases faster.

Because charm follows binary scaling, we show in case of the IMR we the yield per binary collision. The syst. uncertainties are large but we may see a hint of suppression in the IMR towards more central Au+Au collision.

## Conclusions

- First dielectron continuum measurement at RHIC
- p+p
- Very precise measurement: will fix cocktail components and charm
- Au+Au
- LOW 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

**Notes:**
This brings me to my conclusion...

## Backup

## φ→e+e- in d+Au and Au+Au

**d+Au at ****√s****NN****=200 GeV**** **

**Au+Au at ****√s****NN****=200 GeV**** **

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Relation to previous analyses

- Torsten Dahms - Stony Brook University

- 2007-05-25

## ω & φ meson: mT Spectra

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## The Raw Subtracted Spectrum

- Same analysis on data sample with additional conversion material
- Combinatorial background increased by 2.5
- Good 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)

- 300,000 pairs
- 50,000 above 0

- Torsten Dahms - Stony Brook University

- 2007-05-25

## 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

## 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

## 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

- Torsten Dahms - Stony Brook University

- 2007-05-25

## Continuum in p+p: More Challenges

- 2. 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

- Torsten Dahms - Stony Brook University

- 2007-05-25