# Presentation: 122141

## Measurements of thermal photons and the dielectron continuum with PHENIX

- - Torsten Dahms -
- Stony Brook University
- Workshop on Hot & Dense Matter in the RHIC-LHC Era
- February 13th, 2008

- charm & bottom cross section

- low mass enhancement

- pT spectra

- mass spectra

- thermal photons

- p+p collisions

- centrality dependence

- Au+Au collisions

- chiral symmetry

- medium modification

## 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?

- 2008-02-13

## The Data

- 800M MinBias Au+Au events
- 2.25pb-1 of triggered p+p data
- Combinatorial background removed by mixed events (0.25% syst. uncertainty in Au+Au)
- additional correlated background:
- cross pairs from decays with four electrons in the final state
- particles in same jet (low mass)
- or back-to-back jet (high mass)
- well understood from MC

- p+p at √s = 200GeV

- 2008-02-13

## The Raw Subtracted Spectrum

- Torsten Dahms - Stony Brook University

- 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

- 2008-02-13

## 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 channels
- Other mesons are fit with:mT scaling of π0 parameterization  pT→√(pT2+mmeson2-mπ2) fit the normalization constant****** All mesons m****T**** scale!**

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## p+p Cocktail Comparison

- Data absolutely normalized
- Excellent agreement with Cocktail
- Filtered in PHENIX acceptance
- Cross Sections:
- Charm: integration after cocktail subtraction
- σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb
- Simultaneous fit of charm and bottom:
- σcc = 518 ± 47 (stat) ± 135 (syst) ± 190 (model) μb
- σbb = 3.9 ± 2.4 (stat) +3/-2 (syst) μb
- Charm cross section from single electron measurement:
- σcc = 567 ± 57 ± 193 μb

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## Cocktail Comparison

- Data and cocktail absolutely normalized
- Cocktail from hadronic sources
- Charm from
- PYTHIA
- Single electron non photonic spectrum w/o angular correlations
- σcc= Ncoll x 567±57±193b
- Predictions are filtered in ***PHENIX acceptance**** & resolution*
- Low-Mass Continuum:enhancement 150 < mee < 750 MeV3.4 ± 0.2 (stat) ± 1.3 (syst) ± 0.7 (model)
- Intermediate-Mass Continuum:
- Single e  pT suppression
- PYTHIA softer than p+p but coincide with Au+Au
- Angular correlations unknown
- Room for thermal contribution?

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## Au+Au & p+p Comparison

- p+p and Au+Au normalized to π0 region
- Agreement in intermediate mass and J/ψ just for ‘coincidence’(J/ψ happens to scale as π0 due to scaling with Ncoll + suppression)

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## Comparison with Theory

- Ingredients:
- Freeze-out Cocktail
- “random” charm
- ρ spectral function
- LMR
- m>0.4 GeV:some models describe data
- m<0.4 GeV:enhancement not reproduced
- IMR
- Randomized charm + thermal may work

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## 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

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## 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

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## Mass Spectra: pT dependency

- Study pT dependency of the low mass enhancement in Au+Au

- p+p in agreement with cocktail
- Au+Au low mass enhancement concentrated at low pT

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

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

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

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

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## pT Spectra

- p+p: follows the cocktail
- Au+Au: significantly deviates at low pT

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## Understanding the pT dependency

- Comparison with cocktail
- Single exponential fit:
- Low-pT: 0<mT<1 GeV
- High-pT: 1<mT<2 GeV
- 2-component fits
- 2 exponentials
- mT-scaling of 0 + exponential

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## Yields and Slopes

- SLOPES

- YIELDS

- Intermediate pT:
- inverse slope increase with mass
- consistent with radial flow
- Low pT:
- inverse slope of  ~120MeV
- accounts for most of the yield

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## Mass Spectra: pT dependency

- Study pT dependency of the low mass enhancement in Au+Au

- high pT region provides window for thermal photon measurement

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

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

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

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

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## Virtual Photons

**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**

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

- Now direct photons
- Any  source of real γ produces virtual γ with very low mass
- Rate and mass distribution given by same formula
- No phase space factor for *m***ee**<< *p***T **photon
- Improved S/B by measuring direct photon signal in mass region in which π0 are suppressed

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## Cocktail comparison

- QM2005
- Results from Au+Au
- QM2008
- long awaited result from p+p
- important confirmation of method
- p+p
- Agreement of p+p data and hadronic decay cocktail
- Small excess in p+p at large mee and high pT
- Au+Au
- data agree for mee <50MeV
- Clear enhancement visible above for all pT

**1 < p****T**** < 2 GeV**
**2 < p****T**** < 3 GeV**
**3 < p****T**** < 4 GeV**
**4 < p****T**** < 5 GeV**

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## Shape Comparison

- At m=0 Dalitz and internal conversion pairs have indistinguishable shape
- Shape differs as soon as π0 is suppressed due to phase space limitation
- Assume internal conversions of direct photons
- Fix absolute normalization of cocktail and direct photons by normalizing to data in mee<30MeV
- Fit paramater *r* is fraction of direct photons
- Two component fit in80 < mee < 300MeV gives: χ2/DOF=11.6/10
- It’s not the η:
- Independent measurement of η in Au+Au fixes π0/η ratio to: 0.48 ± 0.08
- Fit with eta shape gives: χ2/DOF = 21.1/10

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## Fraction of direct photons

- Compared to direct photons from pQCD
- p+p
- Consistent with NLO pQCD
- favors small μ
- Au+Au
- Clear excess above pQCD

- μ = 0.5pT
- μ = 1.0pT
- μ = 2.0pT

- p+p

- Au+Au (MB)

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## Comparison

- Agreement of all three methods within their errors
- Internal conversion method observes clear excess above decay photons
- Extract direct photon spectrum by multiplying with measured inclusive photon spectrum: Nγdirect = r · Nγinclusive

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## The spectrum

- Compare spectra to NLO pQCD
- p+p
-  consistent with pQCD
- Au+Au
- above binary scaled pQCD
- If excess of thermal origin:inverse slope is related to initial temperature

- 2008-02-13

## Conclusions

- p+p
- LOW MASS:
- Excellent agreement with hadronic decay cocktail
- INTERMEDIATE MASS:
- Extract charm and bottom cross sections
- σcc = 544 ± 39 (stat) ± 142 (syst) ± 200 (model) μb
- σbb= 3.9 ± 2.4 (stat) +3/-2 (syst) μb
- THERMAL PHOTONS
- p+p in agreement with pQCD

- 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
- Enhancement concentrated at low pT
- INTERMEDIATE MASS:
- Coincidence agreement with PYTHIA
- Room for thermal radiation?
- THERMAL PHOTONS:
- Dielectron mass shape for pT > 1 GeV and mee < 300MeV consistent with internal conversions of virtual photons
- Au+Au above pQCD

- First dielectron continuum measurement at RHIC
- Despite of low signal/BG
- Thanks to high statistics
- Very good understanding of background normalization

- HBD upgrade will reduce background great improvement of systematic and statistical uncertainty (LMR)
- Silicon Vertex detector will distinguish charm from prompt contribution (IMR)

- 2008-02-13

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

## Backup

## 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!

- 2008-02-13

## Theory Comparison II

- Cocktail not subtracted from data
- (necessary for comparison)
- Calculations from
- R. Rapp & H. van Hees
- K. Dusling & I. Zahed
- E. Bratovskaja & W. Cassing (in 4π)

- 2008-02-13

## Cu+Cu dN/dm – Minimum Bias

- Torsten Dahms - Stony Brook University

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

- 2008-02-13

## Dielectrons at RHIC – Intermediate Mass

- K. Gallmeister, B. Kämpfer and O. P. Pavlenko Phys. Rev. C **57**, 3276 (1998)Phys. Lett. B **419**, 412 (1998)
- see also e.g.:
- E. V. Shuryak, Phys. Rev. C **55,** 961 (1997)

- Transverse momentum spectra of dielectrons at constrained transverse masses:
- RHIC with PHENIX acceptance, pT > 1 GeV and 2 GeV < Mee < 3 GeV

- Mee hard chance to find thermal dileptons with Mee > 2 GeV.
- The double differential rate dNe+e−/dMT2 dQT2 with MT in a narrow interval and with a suitable pTmin cut on the individual leptons seems to allow for a window at large values of the pair pT where the thermal yield shines out

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## Dielectrons at RHIC

- Expected Sources:
**Light hadron decays**
- Dalitz decays π0, η
- Direct decays ρ, ω and φ
**Hard processes**
- Charm (beauty) production
- Important at high mass & high pT
- Much larger at RHIC than at the SPS
**Cocktail of known sources**
- Measure π0, η spectra & yields
- Use known decay kinematics
- Apply detector acceptance
- Fold with expected resolution

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## Relativistic Heavy Ion Collider

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## 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
- Central arm physics(|y|<0.35, p ≥ 0.2 GeV/c):
- charmonium  J/ψ, ψ’→ e+e-
- vector meson  ρ, ω, φ → e+e-
- high pT           π0, π+, π-
- direct photons
- open charm
- hadron physics
- Two muon arms at forward rapidity (1.2 < |η| < 2.4, p  2 GeV/c)

**e****+**

**e****-**

**p**

**g**

- Measure rare probes in heavy ion collisions (e.g. Au+Au) as well as in p+p (+spin program)

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## Electron Identification

- Charged particle tracking (δm: 1%)  **DC, PC1, PC3**
- PHENIX optimized for Electron ID
- Cherenkov light **RICH **+
- shower **EMCAL**

- Emission and measurement of Cherenkov light in the Ring Imaging Cherenkov detector→ measure of min. velocity
- Production and of em. shower in the Electro-Magnetic Calorimeter                  measure of energy E
- Electrons: E ≈ p
- Hadrons: E < p

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## The Double Challenge

- Torsten Dahms - Stony Brook University

- Need to detect a very weak source of e+e- pairshadron decays (m>200 MeV,  pT>200 MeV)	~ 4x10-6  / π0
- In the presence of hundreds of charged particlescentral Au+Au collision	dNch / dy ≈ 700
- And several pairs per event from trivial originπ0 Dalitz decays	~ 10-2 / π0+ γ conversions  (assume 0.5% radiation length)	~ 10-2 / π0
- huge combinatorial background   (dNch / dy)2
- pairing of tracks originating from unrecognized π0 Dalitz decays and γ conversions
- no means to reduce combinatorial backgroundbeyond reducing conversion length to 0.4% andpT cut at 200 MeV
-  Signal to background depending on mass up to1 : few hundred
- Electron pairs are emitted through the wholehistory of the collision:
- need to disentangle the different sources.
- need excellent reference p+p and d+Au data.

**Experimental Challenge**** **

**Analysis Challenge **

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**Notes:**
Can be summarized as: The search for the needle in the haystack. Limited acceptance w/o tracking before the magnetic field certainly do not provide the best conditions for such an analysis.

## Combinatorial Background 

- Torsten Dahms - Stony Brook University

- Which belongs to which? **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
- Produce like and unlike sign in the mixed events at the proper rate 			(B+- = 2√B++B--)
- Like sign used as a cross check for the shape  provide absolute normalization for unlike sign background
- Use same event topology (centrality, vertex, reaction plane)
- Remove every unphysical correlation

- Combinatorial Background

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**Notes:**
Because we don’t know which electrons come from the same parent, we combine all of them. This creates a large combinatorial background which overwhelms any signal (S/B ~ 1/100). Due to the two arm spectrometer acceptance, the invariant mass spectra of like sign and unlike sign pairs have a different shape. Therefore we have to rely on event mixing to reproduce the combinatorial background. Producing like and unlike sign pairs in mixed events at the proper rate, i.e. 2sqrt(), allows us to cross check the shape with the like sign pairs which then provides an absolute normalization for the unlike sign background....

## Background Normalization: N+-=2√N++N-- 

- Torsten Dahms - Stony Brook University

- Background Normalization: N+-=2√N++N--

**--- Foreground: same event**
**--- Background: mixed event**

- Small signal in like sign at low mass
- N++ and N-- estimated from the mixed events like signB++ and B-- normalized at high mass:B++/N++ = 1
- B--/N-- =1for mass > 700 MeV
-  Uncertainty due to statistics of N++ and N--: 0.12%

- Correction for asymmetry of pair cut
- Pair cut works differently in like and unlike sign pairs κ =κ+-/√ κ++ κ -- = 1.004
- estimated with mixed events
- Systematic error (conservative): 0.2%

**TOTAL SYSTEMATIC  ERROR = 0.25%**

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**Notes:**
The proper background normalization is given by 2sqrt(). Due to a small like sign signal at very low masses originating from pi0 double conversions, pi0 Dalitz + conversions etc,, we need to estimate the number of like sign pairs by normalizing the mixed events above 700MeV. Besides the statistical precision given by the like sign yield in the foreground, the systematic uncertainty comes from the different effects of pair cuts on like and unlike sign pairs. All in all we achieve a precision in the background normalization of 0.25%.

## Des einen Freud – des anderen Leid: Conversions

- Torsten Dahms - Stony Brook University

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

**Photon conversions**

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

- Inclusive
- Removed by phiV cut
- After phiV cut

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## Cocktail Ingredients

- Torsten Dahms - Stony Brook University

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

- 2008-02-13

## p+p Cocktail Tuning (ω & φ)

- ω and φ are fit with:
- modified Hagedorn (as on previous slide: all parameter free)
- π0 parameterization with modified Hagedorn + mT scaling (as on previous slide: A is only free parameter,  pT→√(pT2+mω2-mπ2))
- exponential in mT

- Fits of ω cross section
- mod. Hagedorn: χ2/NDF = 21.6/18
- mT scaled π0: χ2/NDF = 34.1/22
- expo in mT: χ2/NDF = 77.0/8
- Fits of φ cross section
- mod. Hagedorn: χ2/NDF = 30.5/13
- mT scaled π0: χ2/NDF = 32.4/17
- expo in mT: χ2/NDF = 73.3/15

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## p+p Cocktail Tuning (J/ψ)

- Published J/ψ is fit with:
- modified Hagedorn (all parameter free)
- π0 parameterization with modified Hagedorn + mT scaling (one free parameter)
- exponential in mT
- also shown is the published fit with a power law

- Fits of J/ψ cross section
- mod. Hagedorn: χ2/NDF = 9.86/12
- mT scaled π0: χ2/NDF = 12.5/16
- expo in mT: χ2/NDF = 15.0/14

- 2007-12-14

- Torsten Dahms - Stony Brook University

- 2008-02-13

## In practice

- Material conversion pairs removed by analysis cut
- Combinatorial background removed by mixed events
- Calculate ratios of various m**ee** bins to lowest one:  *R***data**
- If no direct photons: ratios correspond to Dalitz decays
- If excess:  direct photons
- Fit of virtual photon shape to data in principle also possible(done for d+Au)

- From conventional measurement

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## Slide 41

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## Low pT mass spectra

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## Direct Photons

- Direct photon sources:
- QCD Compton scattering
- Annihilation
- QCD Bremsstrahlung
- Hard photons from inelastic scattering of incoming partons
- Thermal photons are emitted via same processes but from thermalized medium carry information about the temperature of the medium

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