Direct photons. Data review.

1
Direct photons. Data review.

• Sergey Kiselev, ITEP Moscow
• Introduction
• Before QM06
• pp prompt ?
• AB experimental methods
• Subtraction method
• Momentum correlations method
• Internal conversion method
• QM06
• high-pt
• low-pt
• elliptic flow v2
• ?-h azimuthal correlation
• Conclusions

2
Introduction

• Direct photons not from hadron decays
• Quark gluon level
• qq ? g?, qg ? q?, qq(g) ? qq(g)?.
  Initial hard NN collisions, pQCD ?
  prompt ?.
• Thermalised QGP stage ? thermal ? from QGP.

3
Introduction cont.

• Hadron level
• meson scatterings pp ? ??, p? ? p?, pK ? K ?,
  K? ? K?, KK ? p?, pK ? K?, . . .
• Thermalised hadron stage ? thermal ? from hadron
  gas
• Decay photons
• Long lived (c? c?AB 50-100 fm)
  p0 ? ??, ? ? ??, ? ? ??/??/2?
• Shot lived (c? ? c?AB 50-100 fm)
  ? ? p?, a1 ? p?, ? ? N?, K ?
  K?, ? ? ??, ? ? pp?, . . .

4
Prompt g pp data

• A Compilation of Data, J. Phys. G, 23 (1997) A1
• Data fit D.Srivastava, Eur.Phys.J, C22 (2001)
  129

5
Prompt g data fit
6
Prompt g other data fit

• PHENIX hep-ph/0609037
• d3s/dyd2pT (vs)5
• F(xT,y)

7
Prompt g data/theory
P.Aurenche et.al, PR D73, 094007 (2006), with
PHENIX, D0 data The data can be described
(accuracy ?50) within the NLO QCD (JETPHOX
code), except E706 experiment.
8
Experimental methods for direct photons

• Last years large progress in experimental
  techniques to extract direct photons
• Direct photons have been revealed by methods
• Subtraction method, WA98/2000, PHENIX/2005
• Momentum correlation method, WA98/2004
• Internal conversion method, PHENIX/2005
• External conversion method, PHENIX/2006, QM06

9
Subtraction method

• Direct photon measurement by the subtraction
  method WA98 PRL 85 (2000) 3595, PHENIX PRL 94
  (2005) 232301
• WA98 subtracted from measured photons those from
  known hadronic source from decays of
  reconstructed po, h and other hadrons (with some
  assumption of its yield and spectrum)
• At pt gt 0,5 GeV/c
• from po - 80
• from h - 13
• from w - 2
• from h - 1.3

10
Subtraction WA98
11
Subtraction PHENIX
PRL 94, 232301 (2005)
12
Momentum correlations method

• gg momentum correlations, WA98 PRL 93 (2004)
  022301, STAR nucl-ex/0511055
• gg correlations ? direct photons
• direct photons correlation provide the system
  sizes at all stages of heavy-ion collisions
• Needs larger statistics

13
Momentum correlations WA98

• Rinv (gg) 6 fm Rinv (pp)
• emitted in the late, hadron gas, stage of the
  collision
• Thermal calculations substantialy underestimate
  the data.

14
Internal conversion method

• A novel technique (QM05) internal conversion of
  direct photons into ee-, PHENIX, nucl-ex/0511041
• any source of real g emits also virtual g with
  very low-mass
• gdirect ( gdirect /gincl.) gincl.
• Electrons in the central arms were
  identified by matching charged particle tracks to
  clusters in the ECAL and to rings in the RICH

15
Internal conversion - details
Main assumption the Kroll-Wada (KW) formula,
Phys.Rev.98(1955)1355 A given reaction produces,
instead of real ?, ? ? ee-. For decays
N? number of real ? po ? ?? po ? ?? ?
?ee- I?KW 0.0059 ? ? ?? ? ? ?? ?
?ee- I?KW 0.0081 ?direct ?direct
? ee- I?KW 0.0162
phase space factor 1, if peeT mee
16
Internal conversion cont.

• Measurement for 1 lt pT lt 5 GeV/c consistent with
  calculations when thermal photon emission is
  taken into account (at high pT gt 5 it is
  consistent with a NLO pQCD calculation).

17
RAA vs pT for direct photons
QM05 High pT direct photons are not suppressed
? strong suppression of mesons is not an
initial effect, but a final state effect
18
Quark Matter 2006

• PHENIX data
• High-pT direct photon
• Hard scattering
• Talk by T.Isobe
• Low-pT direct photon
• Thermal emission from QGP.
• Talk by D. Peressounko
• Azimuthal anisotropy of direct photon
• Talk by V.S. Panteuev
• ?- h azimuthal correlations
• Talk by J. Jin
• Enter STAR into the game

19
Systematic Study of High-pT Direct Photon
Production with the PHENIX Experiment at RHIC

• Tadaaki Isobe (CNS, Univ. of Tokyo)
• for the PHENIX Collaboration
• The 19th International Conference on
• Ultra-Relativistic Nucleus-Nucleus Collisions
• at Shanghai, China

20
High-pT direct photons where are we?

• pp agreement with NLO pQCD.
• Run2 high-pT direct photons in AuAu consistent
  with binary scaled NLO pQCD.
• Binary scaling is not obvious.
• 70 prompt photon 30 jet fragmentation at
  high-pT.
• Fragmentation photons are suppressed by
  jet-quenching effect at RHIC?
• Compensated by other contribution?

Phys. Rev. Lett. 94, (2005) 232301
30 contribution from jet frag.
21
Fraction of fragmentation photons

• Fraction of fragmentation photons is evaluated in
  terms of the fraction of isolated photons in
  200GeV pp collisions.
• If energy of cone around a target photon is much
  less than the energy of photon, the photon is
  defined as isolated photon.
• Isolation cut removes fragmentation
  (Bremsstrahlung) photons.
• Data are consistent with theoretical expectation
  within the error.
• hep-ex/0609031

Theoretical predictions where PHENIX acceptance
is taken into account
22
Parameterization of the pp data

• For the calculation of nuclear modification
  factor, pp reference is parameterized using a
  fitting function.
• The fitting works well to parameterize the pp
  photon data.
• The parameterized photon data show enhancement
  comparing with NLO pQCD expectation by a factor
  of more than 20.

23
Medium induced high-pT direct photons

• Interaction of hard-scattered parton with dense
  matter.
• Compton scattering (jet-photon conv.)
• R.J. Fries, Phys.Rev. C72 (2005) 041902
• Over estimation at high-pT
• S. Turbide, Phys.Rev. C72 (2005) 014906
• Reproduce data well
• Bremsstrahlung
• B.G. Zakharov, JETP Lett. 80 (2004) 1
• If it is observed, it can be used as hard probe
  of bulk matter.

24
Direct photon RAA

• Nuclear modification factor
• Yield from hard processes can be scaled with
  Ncoll ( of collisions).
• divided by expected (binary scaled) yield from
  pp data.
• RAA is consistent with Ncoll scaled pp
  reference.
• RAA of very high-pT direct photons is below one.
• A hint of jet-quenching effect for fragmentation
  photons?
• RAA using pQCD shows different tendency.

First data using pp data as reference!
Thanks to the more accurate pp/AuAu data.
25
Comparison of RAA with theoretical calculation

• Turbide et al.
• Phys. Rev. C72 (2005) 014906 Private
  communication.
• AMY formalism for jet-quenching effect for
  fragmentation photons.
• Systematically data points are below theoretical
  prediction.
• F. Arleo
• JHEP 0609 (2006) 015
• High-pT suppression due to isospin effect, in
  addition to jet-quenching and shadowing.
• BDMPS for jet-quenching.
• Medium induced jet-photon is not taken into
  account.
• The suppression of very high-pT photon is well
  reproduced.

26
Summary/T.Isobe

• Direct photons in 200GeV pp collisions.
• Published final year-3 pp data.
• Preliminary year-5 data.
• Consistent with NLO pQCD within uncertainty.
• Direct photons in 200GeV AuAu collisions.
• Clear signal of direct photon at high-pT thanks
  to the strong suppression of neutral hadrons.
• Spectra up to 20GeV/c.
• First direct photons RAA in 200GeV AuAu
  collisions with pp reference.
• Almost consistent with binary scaled pp data.
• RAA seems to be below one at very high-pT region.

27
Direct photons at low pt measured in PHENIX

• D.Peressounko
• RRC Kurchatov institute
• for the PHENIX collaboration

28
Subtraction and Tagging methods
Tagging
Subtraction

• Measure photon spectrum and remove those photons
  which make p0 mass with any other
• Correct direct photon candidate sample for
  contribution from p0 decay photons with missing
  partner and for direct photons with fake partners
• Subtract contribution from h, w etc. decays

• Measure yield of inclusive photons corrected for
  hadron contaminations, conversion, efficiency and
  acceptance.
• Measure spectrum of p0, h, w etc.
• Calculate yield of decay photons and subtract it
  from the inclusive yield.

Ngdir Ngincl - Ngdecay
29
dAu collisions Double ratio

• Internal conversion method provides smaller
  systematic errors
• But not as small as in the case of AuAu
  collisions in Run-4
• (Large background of external conversion on
  MVD detector in Run-3)

30
dAu collisions Spectrum
Internal conversion extends range of significant
points to pt gt 2 GeV
Data agrees with pQCD predictions in full pt
range gt No indication for nuclear
effects
31
AuAu collisions External conversion
External conversion
Tagging

• Use very tight PID cut on EMCAL photons to
  produce very clean photon sample

• Use external photon conversion in beam pipe to
  produce very clean photon sample

no pair cut with pair cut

• Combining this photon with others measured in
  EMCAL with loose PID cut, estimate proportion of
  photons coming from p0 decays
• Correct for missing p0 decay partners
• Subtract h, w, h decay photons
• Calculate ratio Ngall/Ngdecay

Dalitz
Conversion
avoid explicit calculation of p0 spectrum
Uses very pure photon sample
gtreduce systematic errors
32
Comparison AuAu, central collisions

• Systematic errors are still too large to extract
  thermal photons.
• There is room for thermal photons

This is consistent with excess above pQCD
reported previously
Spectra agree within errors in entire pt range
33
Two-photon correlations
e
g2
g2rec
g1
g1
e-

• External conversion
• No close cluster interference
• No hadron contamination

C2 calculated in EMCAL and convertedEMCAL agree
gt both effects are under control
34
Conclusions/D.Peressounko

• Measurement of direct photon yield at small pt is
  very interesting but extremely complicated task
• PHENIX has developed a variety of methods for
  direct photon extraction in pp, dAu and AuAu
  collisions producing consistent results
• Because of the large systematic errors comparison
  of binary scaled dAu spectrum with AuAu does
  not allow to make a statement on the origin of
  the excess above pQCD observed in AuAu.
• More precise data are coming

35
PHENIX measurements of reaction plane dependence
of high pT photons and pions in AuAu collisions

• Vladislav Pantuev, University at Stony Brook
• for PHENIX collaboration

36
In pp Hard photonsdirect component
Annihilation
Compton
In AA picture is much more complicated
Photons in AA
Direct Photons
Decay Photons
Preequilibriumphotons
hard
thermal
hardthermal
QGP
Hadron gas
direct
fragmentation
jet-g-conv.
Medium inducedg bremsstr.
V20
V2gt0
V2gt0
V2gt0
V2lt0
V2lt0
37
There are theoretical predictions with a sizable
azimuthal parameter v2
Jets lose more energy where the medium is thicker
- more jet-photon conversions (v2lt0), photons
from fragmentation of quenched jets (v2gt0)
Thermal photons are produced throughout the
expansion history and reflect quark anisotropy
S.Turbide, C.Gale, R.J.Fries, PRL 96 032303 (2006)
R. Chatterjee et al., PRL 96, 202302 (2006)
4
38
How does PHENIX measure direct photons and v2?

• Measure inclusive photon yield, Ninc
• Measure hadron contribution components p0 and h
• By Monte Carlo calculate p0 and h decay
  background in inclusive sample, Nbg
• Calculate direct photon excess over hadron
  decays, R
• Measure inclusive and hadron v2 by reaction plane
  method
• Calculate direct photon v2 as

R v2inc v2BG
v2dir
R 1
Use large statistics Run 4 AuAu data set
R N inc / NBG
See also poster 86 by Kentaro Miki
5
39
Step by step calculations
PHENIX preliminary
PHENIX preliminary
PHENIX preliminary
PHENIX preliminary
6
40
PHENIX direct photon v2 result
Systematic errors dominantly from R Enormous
background of decay photons at low pT
Within statistical and systematic errors v2 is
consistent with zero. v20 or cancellation of
different contributions?
7
41
PHENIX Measurement on High pT h-h and g-h
Azimuthal Correlations

• Jiamin Jin
• For the PHENIX collaboration
• Quark Matter 2006, Shanghai
• Nov. 18, 2006

42
Method
?

• Construct inclusive g-h yield
• Construct decay g-h yield via
• Pair by pair weighted summation
• convolutes all p0-h pair contributions from
  higher pT
• Weight reflects probability from kinematics for
  p0 at given pT to decay into photon in given pT
  range
• Subtraction via

?
pp
?
inclusive g decay g
R
43
First direct g-h yield in pp, cmp with PYTHIA!
pp important baseline measurement!
PYTHIA 6.1 kT 2.5 GeV

• A nice confirmation of our method

44
First direct g-h yield in AuAu!
AuAu study medium induced modification!
1/NtrigdN/dDf
1/NtrigdN/dDf
Df(rad)
Df(rad)

• v2 modulated bkgd for inclusive/decay g
  subtracted from each corr. fn under ZYAM
  assumption (v2 from single g/p measurements)
• Near side yield consistent with 0
• small fragmentation photon yield
• Away side yield also small, suppressed!?

45
Comparison btw AuAu and pp
Black AuAu Red pp
1/NtrigdN/dDf
1/NtrigdN/dDf
Df(rad)
Df(rad)

• Hint of away side modification in AuAu!

46
Away side integrated yield

• trigger g pT dependence
• assoc. h pT dependence
• centrality dependence pp

• pp shows a systematic trend of having higher
  yields in the
• away side than AuAu!

47
Conclusions/J.Jin

• First measurements of high pT direct g-h yields
  in AuAu/pp collisions
• Near side yield 0
• Hint of suppression of away side jets in AuAu
• Need more statistics to make conclusive statement
• Upcoming AuAu run

48
Conclusions

• High-pt RAA in 200GeV AuAu collisions seems to
  be below one at very high-pT region.
• Low-pt more methods, but errors are still too
  large. More precise data are coming
• Elliptic flow v2 within statistical and
  systematic errors v2 is consistent with zero.
• ?-h azimuthal correlations near side yield 0,
  hint of suppression of away side jets in AuAu,
  need more statistics.
• Multi-purpose Direct Photons