# Presentation: 699327

## Dileptons at RHICe+e- pairs: a clock and a thermometer of relativistic heavy ion collisions 

- Alberica Toia
- Stony Brook University

**Dilepton Measurements at RHIC**
**p+p: ****charm and bottom cross section**
**Au+Au:****large enhancement**
**Mass**
**Centrality**
**Transverse momentum**

**In medium modifications**
**Chiral symmetry**
**Spectral function**
**Thermal radiation**
**Heavy quarks energy loss**

## Dileptons at RHIC

- Expected sources
**Light hadron decays**
- Dalitz decays 
- Direct decays  and 
**Hard processes**
- Charm (beauty) production
- Much larger at RHIC than at SPS

- Photons and dileptons: radiation from the media
- direct probes of any collision stages (no final-state interactions)
- large emission rates in hot and dense matter
- according to the VMD their production is mediated in the hadronic phase by the light neutral vector mesons (ρ, ω, and φ) which have short life-time
- Changes in position and width: signals of the chiral transition?

## HI low-mass dileptons at a glance

- time scale of experiments

- period of data taking

**(KEK E235)****           **

**CERES****           **

**DLS    ****       **

**NA60****           **

**CBM   ****       **

- energy scale of experiments

**(KEK E235)****           **

**CERES****           **

**DLS****           **

**NA60****           **

**HADES  ****         **

**CBM****          **

**PHENIX**

- First RHIC results:
- Au+Au: arXiv 0706.3034
- p+p: arXiv 0802:0050
- Others coming soon...

## 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 conservative 0.25% error we assigned)

**p+p**

**Au+Au**

- arXiv: 0802.0050

- arXiv: 0706.3034

## Cocktail Tuning (p+p)

**Cocktail Tuning (p+p)**

- Start from the π0 , assumption: π0 = (π+ + π-)/2
- parameterize PHENIX pion data:

- Other mesons well measured in electronic and hadronic channel
- Other mesons are fit with:
- mT scaling of π0 parameterization  pT→√(pT2+mmeson2-mπ2) fit the normalization constant
**All mesons m****T**** scale!!!**

- PHENIX Preliminary

- arXiv: 0802.0050

## p+p Cocktail Comparison

**p+p Cocktail Comparison**

- Data absolutely normalized
- Excellent agreement data-cocktail
- Extract charm and bottom cross section

- Charm: integration after cocktail subtraction
-  c=544 ± 39 (stat) ± 142 (sys) ± 200 (model) b
- Simultaneous fit of charm and bottom:
-  c=518 ± 47 (stat) ± 135 (sys) ± 190 (model) b
-  b= 3.9 ± 2.4 (stat) +3/-2 (sys) b

## Charm and bottom cross sections

- CHARM

- BOTTOM

- Dilepton measurement in agreement with single electron, single muon, and with FONLL (upper end)

- Dilepton measurement in agreement with measurement from e-h correlation and with FONLL (upper end)

- First measurements of bottom cross section at RHIC energies!!!

- Y.Morino
- Session XIV

## Au+Au Cocktail Comparison

**Au+Au Cocktail Comparison**

- Data absolutely normalized
- Cocktail filtered in PHENIX acceptance
- Charm from
- PYTHIA
- Single electron non photonic spectrum w/o angular correlations
- c= Ncoll x 567±57±193b

- Low-Mass Continuum:enhancement 150 <mee<750 MeV: 3.4±0.2(stat.) ±1.3(syst.)±0.7(model)
- Intermediate-Mass Continuum:
- Single-e  pt suppression & non-zero v2: charm thermalized?
- PYTHIA single-e pT spectra softer than p+p but coincide with Au+Au
- Angular correlations unknown
- Room for thermal contribution?

## pp – AuAu comparison

- pp and AuAu normalized to 0 region
- p+p: follows the cocktail
- Au+Au: large Enhancement in 0.15-0.75
- Agreement in intermediate mass and J/ just for ‘coincidence’(J/ happens to scale as 0 due to scaling with Ncoll + suppression)

**p+p NORMALIZED TO m****ee****<100 MeV**

## Centrality Dependency

**Centrality Dependency**

- LOW MASS

- INTERMEDIATE MASS

## pT dependency 

**p****T**** dependency **

**p+p: follows the cocktail **
**Au+Au: enhancement concentrated at low p****T**

**0<p****T****<0.7 GeV/c**

**0.7<p****T****<1.5 GeV/c**

**1.5<p****T****<8 GeV/c**

**0<p****T****<8.0 GeV/c**

**p+p**

**Au+Au**

- arXiv: 0802.0050

- arXiv: 0706.3034

## pT dependency II 

**p+p: follows the cocktail for all the mass bins**
**Au+Au: significantly deviate at low p****T**** **

**p+p**

**Au+Au**

## Understanding the pT dependency

**Comparison with cocktail**
**Single exponential fit:**
**Low-p****T****: 0<m****T****<1 GeV**
**High-p****T****: 1<m****T****<2 GeV**
**2-components fits**
**2exponentials**
**m****T****-scaling of ********0**** + exponential**

## Yields and Slopes

**Intermediate p****T****: inverse slope increase with mass, consistent with radial flow**
**Low p****T****: **
**inverse slope of ~ 120MeV **
**accounts for most of the yield **

**SLOPES**

## pT dependency 

**p****T**** dependency **

**0<p****T****<0.7 GeV/c**

**0.7<p****T****<1.5 GeV/c**

**1.5<p****T****<8 GeV/c**

**0<p****T****<8.0 GeV/c**

**p+p**

**Au+Au**

- arXiv: 0802.0050

- arXiv: 0706.3034

**p+p: follows the cocktail **
**Au+Au: enhancement concentrated at low p****T**

## Dileptons at low mass and high pT

**Dileptons at low mass and high p****T**

- m<2 only Dalitz contributions
- p+p: no enhancement at low pT
- Au+Au: large enhancement at low pT
- Any  source of *real * produces *virtual*  with very low mass
- Assuming internal conversion of direct photon  extract the fraction of direct photon
- p+p: follows pQCD
- Au+Au: clear excess above pQCD  signal of thermal photons?

- PHENIX Preliminary

- PHENIX Preliminary

- T.Dahms Session XV
- Y.Yamaguchi Poster 125

## Theory comparison

- Freeze-out Cocktail + “random” charm +  spectral function
*Low mass*
- M>0.4GeV/c2: some calculations OK
- M<0.4GeV/c2: not reproduced
*Intermediate mass*
- Random charm + thermal partonic may work

## Theory Comparison II

**Theory Comparison II**

- Calculations from
- R.Rapp & H.vanHees
- K.Dusling & I.Zahed
- E.Bratovskaja & W.Cassing (in 4)

## Summary

- First measurements of dielectron continuum at RHIC

- 	p+p
*Low mass*
- Excellent agreement with cocktail
*Intermediate mass*
- Extract charm and bottom
-  c = 544 ± 39 (stat) ± 142 (sys) ± 200 (model) b
-  b= 3.9 ± 2.4 (stat) +3/-2 (sys) b

- 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
- pT dependency: enhancement concentrated at low pT
*Intermediate mass*
- Agreement with PYTHIA: coincidence?

## Outlook

- PHENIX:
- New results coming soon
- d+Au: cold nuclear matter
- Cu+Cu: low Npart
- Di-muon continuum (forward rapidity)
- Upgrades:
- HBD: reduction of combinatorial background  LMR
- SVTX: tagging of charm  IMR
- Low-Energy Scans: search for the critical point
- STAR: upgrades (HFT and TOF)

- D.Dutta
- Poster 144

## Backup

## A Hadron Blind Detector (HBD) for PHENIX

- Dalitz & Conversion rejection via opening angle
- Identify electrons in field free region
- Veto signal electrons with partner
- HBD concept:
- windowless CF4 Cherenkov detector
- 50 cm radiator length
- CsI reflective photocathode
- Triple GEM with pad readout
- Reverse bias (to get rid of ionization electrons in the radiator gas)
- Status
- installed and taking data in RUN7
- x3 more statistics

## Background studies

**Background studies**

- HBD simulation
- Change acceptance filter in simulation
- Assume HBD rejects hadrons 1:10
- Assume single electron rejection 1:10
- Expect S/B 1:10 at m~ 500 MeV
- Not included
- Realistic hadron pT distribution
- Mass resolution
-  Additional conversion background

**total **

**reject hadrons **

**reject electron**

**charm**

## Projections

- N. events
- 1x109
- 10x109

## RHIC at low energy

- Npart = 109.1 (MinBias Au+Au)
- Nch/Npart and <pT> at mid-rapidity at sqrt(s_NN)=17.2 from SPS
- pT distributions and particle ratios according to CERES paramterization in EXODUS pt*(a1*exp(-1.0*mt/T1)+a2*exp(-1.0*mt/T2)+a3*exp(-1.0*mt/T3))
- other hadrons:
- T  = 0.175+0.115*mass .;
- pt*exp(-1.0*mt/T);
- PHENIX acceptance filter for +- magnetic field configuration
- The expected ZDC rates at beam energy of ~8.5GeV is 100Hz. * 75% to convert into a BBC rate* 50% for the lifetime event rate of 37.5Hz

**Expected whole vertex minbias event rate [Hz]**

**Which scaling**
**do we use?**

## Projections

- 50M events(=2 weeks runtime = 1.2*106 s)
- 500M events(+ electron cooling)

## The unfiltered calculations

- black: our standard cocktail
- red  : hadronic spectrum using the VACUUM rho spectral function
- green: hadronic spectrum using the IN-MEDIUM rho spectral function
- blue : hadronic spectrum using a rho spectral function with DROPPING MASS
- magenta : QGP spectrum using the HTL-improved pQCD rate

## CuCu dN/dm – Min Bias

**CuCu dN/dm – Min Bias**

- π
- η
- η’
- ω
- ρ
- φ
- cc
- J/ψ
- ψ’

## Unphysical correlations II

**Unphysical correlations II**

- Yield as a function of cut: at some point (when the cut removes all the ghosts) the yield should saturate
- The yield has a minimum then increases then saturates
- The saturation value is different for converter and non converter runs (different electron multiplicity)
- The minimum value and the saturation value are the same for converter and non converter runs

- Cut value

- Corrected yield in 0-140 MeV

- cut optimized for pions

- Cut optimized for electrons

- Converter
- Non converter

- UNLIKE

- mass

- pT

- This is not reproducible in mixed events cut differently in real and mixed events need a larger cut
- Cut applied as event cut
- Real events: discarded and never reused
- Mixed events: regenerated to avoid topology dependence

## Photon conversion rejection

- Conversion removed with  orientation angle of the pair in the magnetic field

**Photon conversion**

- e+e- at r≠0 have m≠0(artifact of PHENIX tracking:
- no tracking before the field)
-  effect low mass region
-  have to be removed

- Beampipe

- MVD support structures

- r ~ mee

- Inclusive
- Removed by phiV cut
- After phiV cut

## Photon conversion cut

- No cut
- M<30 MeV & V<0.25  &
- 	M<600 MeV & V<0.04
- 	M<600 MeV & V<0.06
- 	M<600 MeV & V<0.08
- 	M<600 MeV & V<0.10
- 	M<600 MeV & V<0.12
- 	M<600 MeV & V<0.14
- 	M<600 MeV & V<0.20
- 	M<600 MeV & V<0.40

## Physical background

**Semi-correlated Background**

- X

-  0*
- e+e-
- 	            e+e-
- “jets”

**Photon conversion**

- e+e- at r≠0 have m≠0(artifact of PHENIX tracking)
- Conversion removed with  orientation angle of the pair in the magnetic field

- Background is charge-independent
- Calculate the shape with MC
- Normalize to the **like-sign** spectra
-  Good description of the data

- arXiv: 0802.0050

## Combinatorial Background

- PHENIX 2 arm spectrometer acceptance:
- 	dNlike/dm ≠ dNunlike/dm  different shape  need event mixing
- (like/unlike differences preserved)Use Like sign as a cross check for the shape and to determine normalization
- Small signal in like sign at low mass
- N++ and N–- estimated from the mixed events like sign B++ and B-- normalized at high mass (> 700 MeV)
- Normalization: 2√N++ N--
- Uncertainty due to statistics of N++ and N--: 0.12%
- Correction for asymmetry of pair cut
- K=k+-/√k++ k-- = 1.004Systematic error (conservative): 0.2%

**TOTAL SYSTEMATIC  ERROR = 0.25%**

- LIKE SIGN SPECTRA

- Use same event topology
- (centrality, vertex, reaction plane)
- Remove every unphysical correlation

## Comparison of BG subtraction Methods

- to determine syst. uncertainty:
- spread of two extreme cases (blue & orange): 5-10%

**Monte Carlo method**
**Like sign method**** ****(with some variations)**
**give consistent results over the full invariant mass range**

## 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 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

## 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

## Acceptance

**charge/pT**

******0**

******0**

**z vertex**

-  Define acceptance filter (from real data)
-  Correct only for efficiency **IN** the acceptance
-  “Correct” theory predictions **IN** the acceptance

- Single electron pT > 200 MeV
- Pair mT > 400 MeV
- Not an analysis cut, but a constrain from the magnetic field

- mass

- pT

## Acceptance

- Comparison of PHENIX and NA60 acceptance (pT  vs. yCM)
- Experiments measure in different regions of phase space

- NA60

## Efficiency Correction

- mee (GeV/c2)

- Correction for RICH ghost cut

- Correction for eID cut

- Mee 0.8 MeV
- Pair eff = single eff 2
- Mee 0.4—0.8 pair eff decreases as consequence of single eff drop
- Mee < 0.4 MeV: pair eff increases as consequence of mT cutoff (from single pT cutoff which truncates the single distribution leading to larger pT

## Cross check Converter Method

- We know precise radiation length (X0) of each detector material
- The photonic electron yield can be measured by increase of
- additional material (photon converter was installed)
- The non-photonic electron yielddoes not increase
- Photonic single electron: x 2.3
- Inclusive single electron :x 1.6
- Combinatorial pairs :x 2.5

**Photon Converter (Brass: 1.7% X****0****)**

## Centrality Dependence

**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

- LOW MASS

- INTERMEDIATE MASS

## Extract 2 components

**2 EXPONENTIALS**

**HAGEDORN + EXPONENTIAL**

**We fit the sum of 2 exponentials (a*exponential1 + b*exponential2) **
**We fit Hagedorn to M****ee****<100MeV (********0****-dominated)**
**Then we fit (a*m****T****-scaling + exponential) to the other mass bins**
**Because of their different curvature, m****T****-scaling and the exponential account for more or less of the yield in the low-p****T**** region.  **

## Centrality dependence of background

## Dileptons at low mass and high pT

**Dileptons at low mass and high p****T**

- Below m=2 only Dalitz contributions, calculable with QED
- Any  source of *real * produces *virtual*  with very low mass
- Rate and mass distribution given by same formula
- No phase space factor for mee<< pTphoton
- Assuming internal conversion of direct photon  extract the fraction of direct photon
- Clear excess above pQCD  signal of thermal photons?

## Thermal photons?

- Direct measurement of photons  (EMCAL) in this energy region impaired by:
- Neutral hadron contamination
- Energy resolution in π0 reconstruction
-  Try to use dielectrons!

**No significant excess at low p****T**

## The idea

**phase space factor**

**form factor**

**invariant mass of virtual photon**

**invariant mass of Dalitz pair**

**phase space factor**

**form factor**

**invariant mass of Dalitz pair**

**invariant mass of virtual photon**

**The idea**

- Start from Dalitz decay
- Calculate inv. mass distribution of Dalitz pairs‘

- Now direct photons
- Any  source of real **** produces virtual **** with very low mass
- Rate and mass distribution given by same formula
- No phase space factor for *m***ee**<< *p***T **photon

## Low mass dielectron pairs

**Low mass dielectron pairs**

- Invariant mass distribution of Dalitz pairs fully calculable with QED
- Different contributions are well measured
- Any  source of real **** (on-shell) produces virtual **** with very low mass (~ mass shell)for *m***ee**<< *p***T **photon: Rate and mass distribution  same shape(no parent  no phase space factor  no cutoff)

- Experimental technique
- Measure yield in different mass windows (different rate of different contributions)
- Calculate ratios of various *M***inv** bins to lowest one:  *R***data**
- If no direct photons: ratios correspond to Dalitz decays
- If excess:  direct photons

## Comparison to conventional result

**Comparison to conventional result**

- From conventional measurement

- Measured ratio

## The spectrum

- Compare to NLO pQCD
- L.E. Gordon and W. Vogelsang
- Phys. Rev. D48, 3136 (1993)
- Above (questionable) pQCD
- Compare to thermal model
- D. d’Enterria, D. Peressounko
- nucl-th/0503054
- Data above thermal at high pT
- Data consistent with thermal+pQCD
- Needs confirmation from p+p measurement

- 2+1 hydro
- T**0****ave**=360 MeV (T**0****max**=570 MeV)
******0**=0.15 fm/c