Direct photon interferometry

1
Direct photon interferometry

• D.Peressounko
• RRC Kurchatov Institute

2
Outlook

• Photons are special
• Penetrating gt Specific R(KT) dependence
• Massless gt Unusual Rinv and linv
  interpretation
• Rare gt Strong background
• Experimental review
• Completed experiments
• TAPS,WA98
• Ongoing
• PHENIX,STAR
• Developing
• ALICE
• Conclusions

3
Accessing space-time dimensions of different
stages of the collision

• 31 hydro with first order phase transition.
• QGP phase includes pre-equilibrium pQCD
  contribution

PbPb _at_ 17.2 AGeV
Rout
Rside
Rlong
hadr
QGP
mixed
D.P. Phys.Rev.Lett.93022301,2004
4
KT dependence of photon correlation radii
D.Srivastava, Phys.Rev.C71034905,2005
RHIC AuAu _at_ 200 AGeV
D.P. Phys.Rev.Lett.93022301,2004
T.Renk, hep-ph/0408218
5
Predictions for correlation radii
RHIC, AuAu_at_200 AGeV, KT2GeV
System Rout(fm) Rside(fm) Rlong(fm) Rinv(fm)
gg 4.4 4.2 0.2 D.Srivastava, Phys.Rev.C71034905,2005
gg 4.3 3.9 1.2 3.0 D.Peressounko, Phys.Rev.Lett.93022301,2004
ee KT1 GeV 6.0 3.2 3.3 3.2 J.Alam et al., Phys.Rev.C70054901,2004
gg 5.5 3.0 1.6 3.0 J.Alam et al., Phys.Rev.C67054902,2003
gg 5.1 4.3 2.8 - T.Renk, hep-ph/0408218
Not LCMS system
6
Qinv parameterization for massless particles
S(x) exp( - t2/t2 x2/Ro2 - y2/Rs2 - z2/Rl2),
C2(qo,qs,ql)1 exp( -qo2(Ro2 t2b2) -qs2Rs2
-ql2Rl2)
?d3q/qe C2(qo,qs,ql) d(Qinv2q2)
C2(Qinv)
(integrate in CM frame of the pair)
?d3q/qe d(Qinv2q2)
1/(4p)?1 exp-Qinv2(K02/M2cos2q (Ro2b2t2)
Rs2 sin2qsin2f Rl2sin2qcos2f ) dW
1linvexp-Qinv2Rinv2)
linv 1/(4p) ?exp - 4KT2(Ro2 t2)cos2qdW
Rinv ltRs,Rlgt (not Ro!)

For massless particles (g,e) Qinv
parameterization is very special!
7
Qinv parameterization for massless particles (MC)
Set 1 Ro 6 Rs 6 Rl 6
Set 2 Ro 4 Rs 6 Rl 6
Set 3 Ro 2 Rs 6 Rl 6
Set 4 Ro 6 Rs 4 Rl 6
Set 5 Ro 6 Rs 2 Rl 6
Set 6 Ro 6 Rs 4 Rl 4
Set 7 Ro 4 Rs 4 Rl 4
Set 8 Ro 2 Rs 4 Rl 4
Set 9 Ro 6 Rs 2 Rl 2
linv Erf(2KTvRo2 t2)/(2KTvRo2 t2)
linv1/(2KTvRo2 t2)

8
Background photon correlations

• Bose-Einstein p0 correlations
• Resonance decays
• Collective flow

g
g
p0

g
p0
g
p0
h

p0
p0
9
p0 BE residual correlations
Rpp4 fm
Rpp5 fm
Rpp6 fm
C2pp1exp(-Qinv2Rpp2)
D.P. Phys.Rev.Lett.93022301,2004
10
p0 BE residual correlations
A.Deloff and T.Siemiarczuk, ALICE internal note
INT-98-50
C2pp(D)1l/(1D2Rpp2)2
dNp/dppepx(-p/3GeV)
D1/2(k1-k2)
11
p0 BE residual correlations
Varying strength
Varying width (and strength)
O.V.Utyuzh, G.Wilk, Nukleonika 49S15 (2004),
hep-ph/0312364
12
TAPS detector setup
BaF2 25 cm long (12 X0) prism of hexagonal cross
section, the diameter of the inner circle being
5.9 cm (69 of the Moliere radius).
Distance to IP 62 cm
Min angle cut between photons 8.30
Typical photon energy 10 MeV
13
TAPS mgg distribution and C2
86KrnatNi _at_ 60 AMeV
181Ta197Au _at_ 40 AMeV
Geant simulations
Comparison to BUU calculations
14
WA98 setup
Number of events collected Peripheral
(20 min bias) 3897935 Central (10 min bias)
5817217
15
Two photon correlation functions
16
WA98 apparatus effects
Lmin 20 cm (5 modules)
Lmin 25 cm (6 modules)
Lmin 30 cm (7 modules)
Lmin 35 cm (9 modules)
100 lt KT lt 200 MeV
100 lt KT lt 200 MeV
200 lt KT lt 300 MeV
200 lt KT lt 300 MeV
17
Hadrons and photon conversion
Contamination, (charged neutral)
pid
100ltKTlt200 200ltKTlt300
All (37 4) (22 4)
Narrow (16 1) (4 1)
Neutral ( 1 4) (1 4)
Narrow neutral (1 1) (1 1)
ltrue
1 (Ngdir)2
lobs
2 (Ngtot cont)2
(1 cont/ Ngtot)2
18
Photon background correlations
Simulations
p0p0 Bose-Einstein correlations
Slope -(4.50.4)10-3 (GeV-1)
Elliptic flow
Slope -(3.10.4)10-3 (GeV-1)
Decays of resonances
K0s?2p0?4g K0L?3p0?6g h?3p0?6g w?p0g?3g
19
Invariant correlation radius
C2(Qinv) 1 l/(4p) ? do exp - Qinv2 (Rs2 sin2q
sin2f Rl2 sin2q cos2f )
-
(Qinv2 4KT2)cos2q Ro2
Rpplong
Rgg
Rppside
(for massless particles!)
Rinv f(Rs,Rl)
Erf(2KTRo)
linv l
2KTRo
20
Yield of direct photons
Correlation method
The lowest yield (Ro0)
Most probable yield (Ro6 fm)
Ngdir Ngtotal v2l
Subtraction method
Subtraction method, upper limit
Predictions
Erf(2KTRo)
linv l
hadronic gas
2KTRo
QGP
pQCD
sum
Predictions S. Turbide, R. Rapp, and C. Gale,
hep-ph/0308085.
21
PHENIX setup
Lead Scintillator Lead scintillating plates
of 5.55.5 cm2 at a distance 510 cm from IP.
Lead Glass PbGl crystals 44 cm2 cross
section distance 550 cm from IP
22
PHENIX Comparison to data
dAu collisions at vsNN200 GeV
23
STAR
Use 1 gamma in TPC, 1 gamma in calorimeter.
Conclusions from the talk of J. Sandweiss on
RHIC-AGS users meeting, June 21, 2005, BNL

• A procedure has been developed which permits the
  measurement of gamma-gamma HBT signals despite
  the large background of gammas from p0 mesons
• Gamma energy gt 1.0 GeV is required for the
  residual p0 correlation to be small
• No HBT calculation may be needed but appears to
  be doable.

24
ALICE setup
PHOS crystals PbW04 22 cm cross section
Distance to IP 460 cm
25
ALICE unfolding and resolution
26
ALICE photon correlations in HIJING event
Kt200 MeV
27
Summary

• Direct photon and electron interferometry is
  rather special subject due to penetrating nature,
  zero mass and low yield.
• Two-photon correlations were observed in two
  experiments up to now.
• Photon correlations are analyzed now at PHENIX
  and STAR.
• PHOS detector at ALICE is very promising tool due
  to fine granularity and high spatial and energy
  resolutions.

28
PHENIX MC simulations
Kt 0.2 GeV
K?pp0
ct4.7 m
K0S?p0p0
ct0.02 m
K0L?3p0
ct15. m
h?3p0
Using measured spectra and yields for p0, kaons
and h
29
Jan-e Alam et al., ee correlations
KT1 GeV
Not LCMS
J.Alam et al., Phys.Rev.C70054901,2004
30
T.Renk
side
Side
out
Long
T.Renk, hep-ph/0408218
31
Penetrating probes probe all stages?
RHIC AuAu _at_ 200 AGeV
D.P. Phys.Rev.Lett.93022301,2004
32
Possible sources of distortion of correlation
function

• Apparatus effects (cluster splitting and merging)
• Hadron misidentification
• Photon conversion
• Photon background correlations
• Bose-Einstein correlations of parent p0
• Collective (elliptic) flow
• Residual correlations due to decays of
  resonances