Title: Measurements of low pT direct photons in PHENIX
1Measurements of low pT direct photons in PHENIX
- Yorito Yamaguchi
- for the PHENIX collaboration
- CNS, University of Tokyo
2Introduction
- Quark Gluon Plasma
- De-confined phase of quarks and gluons
- Experimental approach at RHIC
- vsNN 200GeV AuAu collisions
- Direct photons are an important probe to
investigate the characteristics of evolution of
the matter created by heavy ion collisions. - Penetrate the strong interacting matter
- Emitted from every stage of collisions
- Hard photons (High pT)
- Initial hard scattering, Pre-equilibrium
- Thermal photons (Low pT)
- Carry the thermodynamic information from QGP and
hadron gas
3Photon Measurement in PHENIX
- Hard photon
- Strong suppression of high pT hadrons helps to
improve the S/N ratio - Successfully measured for both pp and AuAu
collisions - Measured pT range
- up to 14GeV/c
- Good agreement with pQCD
- Thermal photon
- Thermal radiation from QGP
- Primary contributor in low pT range up to
3-6GeV/c - Direct evidence of thermal equilibration
- Thermal photon measurement is very challenging
because it is very hard due to a large background
from hadron decays.
AuAu
4Low pT Photons
- Long-awaited results for both pp and AuAu
- Experimental determination is very important
since applicability of pQCD is doubtable in low
pT region. - In real photon measurement
- Measured yield with a large systematic error
- Difficulty on measuring low pT real direct
photons - Finite energy resolution of the EMCal
- Large hadron background
- Alternative method to measure low pT direct
photons - Measure ee- pairs from virtual direct photons
- Advantages on measuring virtual photons
- High momentum resolution of the Drift Chamber
- Reliable estimation of the hadron decay
components using Kroll-Wada formula
5Virtual Photon Measurement
- Any source of real g can emit g with very low
mass. - Convert direct g fraction to real direct photon
yield
Kroll-Wada formula
S Process dependent factor
- Case of Hadrons
-
- Obviously S 0 at Mee Mhadron
- Case of g
- If pT2Mee2
-
- Possible to separate hadron decay components
from real signal in the proper mass window.
6Signal Extraction
AuAu
pp
arXiv 0706.3034
arXiv 0802.0050
- Real signal
- di-electron continuum
- Background sources
- Combinatorial background
- Material conversion pairs
- Additional correlated background
- Visible in pp collisions
- Cross pairs from decays with 4 electrons in the
final state - Pairs in same jet or back-to-back jets
7Hadronic Cocktail Calculation
- Remaining pairs after background subtraction
- Real signal Hadron decay components
- Estimate hadron components using hadronic
cocktail
arXiv 0802.0050
- Mass distributions from hadron decays are
simulated by Monte Carlo. - p0, h, h, w, f, r, J/y, y
- Effects on real data are implemented.
- PHENIX acceptance, detector effect, efficiencies
- Hadronic cocktail was well tuned to individually
measured yields of mesons in PHENIX for both pp
and AuAu collisions.
8Cocktail Comparison
AuAu
pp
arXiv 0802.0050
arXiv 0706.3034
- pp
- Excellent agreement with cocktail
- AuAu
- Large enhancement in low mass region
- Integrated yield in150MeV
- Real/cocktail 3.4 0.2(stat) 1.3(sys)
0.7(model)
9pT Sliced Mass Spectra
Normalized by the yield in mee
AuAu pp- Shape differences between pp and AuAu are
larger at lower pT.
10pT Dependence
pp
AuAu
- AuAu
- pT
- Large enhancement ? thermal qqbar and pp
annihilations?
11Cocktail Comparison
1 pT
pp Good agreement between real and cocktail Small excess at higher pT- AuAu
- Good agreement in Mee
- Enhancement is clearly seen above 100MeV.
12Determination of g fraction, r
Direct g/inclusive g is determined by fitting
the following function for each pT bin.
Reminder fdirect is given by Kroll-Wada formula
with S 1.
r direct g/inclusive g
- Fit in 80-300MeV gives
- Assuming direct g shape
- c2/NDF11.6/10
- Assuming h shape instead of direct g shape
- c2/NDF21.1/10
- Twice as much as measured h yield
- Assumption of direct g is favorable.
Mee (GeV/c2)
13direct g/inclusive g
pp
AuAu
Base line
Curves NLO pQCD calculations with different
theoretical scales done by W. Vogelsang.
- pp
- Consistent with NLO pQCD
- better agreement with small µ
- AuAu
- Clear enhancement above NLO pQCD
14Direct Photon Spectra
The virtual direct photon fraction is converted
to the direct photon yield.
- pp
- First measurement in 1-4GeV/c
- Consistent with NLO pQCD
- Serves as a crucial reference
- AuAu
- Above binary scaled NLO pQCD
- Excess comes from thermal photons?
15Theory Comparison
D.dEnterria, D.Peressounko, Eur.Phys.J.C 46
(2006)
S.Turbide, R.Rap, C.Gale, Phys.Rev.C 69 (2004)
T0ave360 MeV (T0max590 MeV) t00.15 fm/c
T0max370 MeV t00.33 fm/c
16Summary Outlook
- Direct photon measurements with virtual photon
method in pp and AuAu collisions have been done
at RHIC-PHENIX. - The fractions of direct g to inclusive g above
pT of 1GeV/c are obtained by making a shape
comparison between real pairs and a hadronic
cocktail. - This is the first time that direct photon
production in pp collisions has been measured in
1 - Direct photon yield in pp collisions is
consistent with NLO pQCD. - The result in pp serves as a crucial reference
to AuAu result. - Excess of direct photon yield above binary
scaled NLO pQCD in AuAu collisions is observed. - The paper on direct photon measurement with
virtual photon method will be submitted soon. - pp analysis with more statistics is now
ongoing. - Result with higher quality will be provided.
- pT region will be extended upward.
- Same analysis will be done in dAu collisions.
17backup
18PHENIX Detector
- Minimum Bias data sample (triggered by BBC)
Electron triggered data sample (pp) - BBC
- z-vertex
- DC, PC1
- Tracking
- RICH EMCal
- Electron ID
- Electron Trigger
19Material Conversion Pair Cut
The pairs from material conversion should be
removed.
These pairs can be recognized by its orientation
relative to the magnetic field.
No cut M M M M M
20Additional Correlated Background
Jet cross pair
Dalitz conversion cross pair
Correlated combinatorial background is very
good agreement with the real like sign mass
spectrum.
Systematic error due to background subtraction
2
21Combinatorial Background (AuAu)
- Normalization factor is determined by like sign
pairs. - N and N-- estimated from the mixed events like
sign B and B-- normalized at high mass ( 700
MeV) - Normalization 2vN N--
- Uncertainty due to statistics of N and N--
0.12
22Centrality Dependence
arXiv 0706.3034
? AuAu ? pp Cocktail
- Integrated yield divided by Npart/2
- 150MeV
- Strong centrality dependence
- Increases faster than Npart
- mee
- Agreement with cocktail
23Theory Comparison 2
- Freeze-out Cocktail random charm r
spectral function - Low mass
- M0.4GeV/c2 some calculations OK
- M