Direct photon interferometry
D.Peressounko
RRC “Kurchatov Institute”
Outlook

Photons are special:
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


=>
=>
=>
Specific R(KT) dependence
Unusual Rinv and linv interpretation
Strong background
Experimental review




Penetrating
Massless
Rare
Completed experiments
 TAPS,WA98
Ongoing
 PHENIX,STAR
Developing
 ALICE
Conclusions
D.Peressounko, WPCF, Kromeriz, 2005
2
Accessing space-time dimensions of different
stages of the collision
•3+1 hydro with first order phase transition.
•QGP phase includes pre-equilibrium pQCD contribution
Pb+Pb @ 17.2 AGeV
Rout
hadr
Rside
Rlong
QGP
mixed
D.P. Phys.Rev.Lett.93:022301,2004
D.Peressounko, WPCF, Kromeriz, 2005
3
KT dependence of photon correlation radii
D.Srivastava, Phys.Rev.C71:034905,2005
RHIC Au+Au @ 200 AGeV
D.P. Phys.Rev.Lett.93:022301,2004
T.Renk, hep-ph/0408218
D.Peressounko, WPCF, Kromeriz, 2005
4
Predictions for correlation radii
RHIC, Au+Au@200 AGeV, KT=2GeV
System Rout(fm)Rside(fm) Rlong(fm) Rinv(fm)
gg
4.4
4.2
0.2
gg
4.3
3.9
1.2
3.0
D.Peressounko, Phys.Rev.Lett.93:022301,2004
ee
KT=1 GeV
6.0
3.2
3.3*
3.2
J.Alam et al., Phys.Rev.C70:054901,2004
gg
5.5
3.0
1.6*
3.0
J.Alam et al., Phys.Rev.C67:054902,2003
gg
5.1
4.3
2.8
-
D.Srivastava, Phys.Rev.C71:034905,2005
T.Renk, hep-ph/0408218
*Not LCMS system
D.Peressounko, WPCF, Kromeriz, 2005
5
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)
C2(Qinv)=
∫d3q/qe C2(qo,qs,ql) d(Qinv2+q2)
∫d3q/q
2
2
e d(Qinv +q )
(integrate in CM frame of the pair)
= 1/(4p)∫[1+ exp{-Qinv2(K02/M2cos2q (Ro2+b2t2) + Rs2 sin2qsin2f + Rl2sin2qcos2f ) }] dW
= 1+linvexp{-Qinv2Rinv2)
Rinv = <Rs,Rl> (not Ro!)
linv = 1/(4p) ∫exp{ - 4KT2(Ro2 + t2)cos2q}dW
For massless particles (g,e) Qinv parameterization is very special!
D.Peressounko, WPCF, Kromeriz, 2005
6
Qinv parameterization for massless particles (MC)
linv = Erf(2KT√Ro +
2
t2)/(2KT√Ro2
+
linv=1/(2KT√Ro2 + t2)
t 2)
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
D.Peressounko, WPCF, Kromeriz, 2005
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Background photon correlations

Bose-Einstein p0 correlations
g
p0
g
g
p0
}
g

Resonance decays
p0
h
p0
}
p0

Collective flow
D.Peressounko, WPCF, Kromeriz, 2005
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p0 BE residual correlations
Rpp=4 fm
Rpp=5 fm
Rpp=6 fm
C2pp=1+exp(-Qinv2Rpp2)
D.P. Phys.Rev.Lett.93:022301,2004
D.Peressounko, WPCF, Kromeriz, 2005
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p0 BE residual correlations
A.Deloff and T.Siemiarczuk,
ALICE internal note
INT-98-50
C2pp(D)=1+l/(1+D2Rpp2)2
dNp/dp=p·epx(-p/[3GeV])
D=1/2(k1-k2)
D.Peressounko, WPCF, Kromeriz, 2005
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p0 BE residual correlations
Varying width (and strength)
Varying strength
O.V.Utyuzh, G.Wilk, Nukleonika 49:S15 (2004), hep-ph/0312364
D.Peressounko, WPCF, Kromeriz, 2005
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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
D.Peressounko, WPCF, Kromeriz, 2005
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TAPS: mgg distribution and C2
86Kr+natNi
@ 60 AMeV
181Ta+197Au
@ 40 AMeV
Geant
simulations
Comparison to BUU calculations
D.Peressounko, WPCF, Kromeriz, 2005
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WA98 setup
Number of events collected:
Peripheral (20% min bias) 3897935
Central (10% min bias)
5817217
D.Peressounko, WPCF, Kromeriz, 2005
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Two photon correlation functions
D.Peressounko, WPCF, Kromeriz, 2005
15
WA98: apparatus effects
Lmin = 20 cm (5 modules)
Lmin = 25 cm (6 modules)
Lmin = 30 cm (7 modules)
100 < KT < 200 MeV
200 < KT < 300 MeV
Lmin = 35 cm (9 modules)
100 < KT < 200 MeV
200 < KT < 300 MeV
D.Peressounko, WPCF, Kromeriz, 2005
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Hadrons and photon conversion
Contamination, (charged + neutral)
pid
100<KT<200
200<KT<300
“All”
(37 + 4)%
(22 + 4)%
“Narrow”
(16 + 1)%
(4 + 1)%
“Neutral”
( 1 + 4)%
(1 + 4)%
“Narrow
neutral”
(1 + 1)%
(1 + 1)%
dir)2
ltrue
1
(N
g
lobs =
=
2 (Ngtot + cont)2 (1+ cont/ Ngtot)2
D.Peressounko, WPCF, Kromeriz, 2005
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Photon background correlations
p0p0 Bose-Einstein correlations:
Slope: -(4.5±0.4)·10-3 (GeV-1)
Elliptic flow:
Slope: -(3.1±0.4)·10-3 (GeV-1)
Decays of resonances:
K0s→2p0→4g
K0L→3p0→6g
h→3p0→6g
w→p0g→3g
D.Peressounko, WPCF, Kromeriz, 2005
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Invariant correlation radius
C2(Qinv) =1 + l/(4p) ∫ do exp{ - Qinv2 (Rs2 sin2q sin2f + Rl2 sin2q cos2f )
Rgg
Rpplong
Rppside
- (Qinv2 + 4KT2)cos2q Ro2 }
(for massless particles!)
Rinv = f(Rs,Rl)
linv = l
D.Peressounko, WPCF, Kromeriz, 2005
Erf(2KTRo)
2KTRo
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Yield of direct photons
Correlation method:
The lowest yield (Ro=0)
Most probable yield (Ro=6 fm)
Subtraction method
Subtraction method,
upper limit
Predictions
hadronic gas
QGP
pQCD
sum
Ngdir = Ngtotal √2l
linv = l
Erf(2KTRo)
2KTRo
Predictions:
S. Turbide, R. Rapp, and C. Gale,
hep-ph/0308085.
D.Peressounko, WPCF, Kromeriz, 2005
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PHENIX setup
Lead Scintillator
Lead + scintillating
plates of 5.5*5.5 cm2
at a distance 510 cm
from IP.
Lead Glass
PbGl crystals
4*4 cm2 cross section
distance 550 cm
from IP
D.Peressounko, WPCF, Kromeriz, 2005
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PHENIX: Comparison to data
d+Au collisions at √sNN=200 GeV
D.Peressounko, WPCF, Kromeriz, 2005
22
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
π0 mesons
Gamma energy > 1.0 GeV is required
for the residual π0 correlation to be
“small”
“No HBT” calculation may be needed
but appears to be doable.
D.Peressounko, WPCF, Kromeriz, 2005
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ALICE setup
PHOS:
crystals PbW04
2*2 cm cross section
Distance to IP 460 cm
D.Peressounko, WPCF, Kromeriz, 2005
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ALICE: unfolding and resolution
D.Peressounko, WPCF, Kromeriz, 2005
25
ALICE: photon correlations in HIJING event
Kt=200 MeV
D.Peressounko, WPCF, Kromeriz, 2005
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Summary


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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.
D.Peressounko, WPCF, Kromeriz, 2005
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PHENIX: MC simulations
Kt = 0.2 GeV
K+→p+p0
ct=4.7 m
K0S→p0p0
ct=0.02 m
K0L→3p0
ct=15. m
h→3p0
Using measured spectra and yields for p0, kaons and h
D.Peressounko, WPCF, Kromeriz, 2005
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Jan-e Alam et al., ee correlations
KT=1 GeV
Not LCMS
J.Alam et al., Phys.Rev.C70:054901,2004
D.Peressounko, WPCF, Kromeriz, 2005
29
side
T.Renk
Side
out
Long
T.Renk, hep-ph/0408218
D.Peressounko, WPCF, Kromeriz, 2005
30
Penetrating probes: probe all stages?
RHIC Au+Au @ 200 AGeV
D.P. Phys.Rev.Lett.93:022301,2004
D.Peressounko, WPCF, Kromeriz, 2005
31
Possible sources of distortion of correlation function

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Apparatus effects (cluster splitting and merging)
Hadron misidentification
Photon conversion
Photon background correlations:
0
 Bose-Einstein correlations of parent p ;
 Collective (elliptic) flow;
 Residual correlations due to decays of resonances;
D.Peressounko, WPCF, Kromeriz, 2005
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