# Presentation: 192708

## Interferometry of direct photons in Pb+Pb collisions at 158 AGeV

***D. Peressounko***
*** ******for WA98 collaboration.***

## Experimental setup

**       ****Number of events collected: **
**Peripheral (20% min bias) 3897935**
**Central (10% min bias)      5817217**

## Two-photon correlation function.

****** ****~ 2·****10****-3**

## 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 0;
- Collective (elliptic) flow;
- Residual correlations due to decays of  resonances;

## Apparatus effects

**L****min**** = 20 cm (5 modules)**

**L****min**** = 25 cm (6 modules)**

**L****min**** = 30 cm (7 modules)**

**L****min**** = 35 cm (9 modules)**

**200 < K****T**** < 300 MeV**

**100 < K****T**** < 200 MeV**

**100 < K****T**** < 200 MeV**

**200 < K****T**** < 300 MeV**

## Hadrons and photon conversion

**“****Narrow”    (16 + 1)%         (4 + 1)%**

**“****Neutral”     ( 1 + 4)%         (1 + 4)%**

**“****All”            (37 + 4)%       (22 + 4)%**

**“****Narrow **
**  ****neutral”      (1 + 1)%         (1 + 1)%**

- obs =                          =

- 1      (Ndir)2

- 2 (Ntot + cont)2

**Contamination, (charged + neutral)**

**100<K****T****<200    200<K****T****<300**

- pid

- true

- (1+ cont/ Ntot)2

## Photon background correlations

******0********0**** 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→20→4****
- K0L→30→6****
- →30→6****
- →0****→3****

- Simulations

## Invariant correlation radius

-                                                                       - (Qinv2 + 4KT2)cos2 Ro2 }

- R

- Rlong

- Rside

- Rinv = f(Rs,Rl)

- inv = 

- Erf(2KTRo)

- 2KTRo

- (for massless particles!)

**Pion correlation radii:**
**M.M. Aggarwal, et al., (WA98collbration),**
**Phys. Rev. C67 (2003) 014906. **

## Yield of direct photons

**Subtraction method,**
**upper limit**

**Correlation method:**

**Subtraction method**

**Predictions**

**hadronic gas**

**QGP**

**sum**

**pQCD**

**Predictions:**
**S. Turbide, R. Rapp, and C. Gale, **
**hep-ph/0308085.**

**N********dir**** = N********total**** ****√****2******

******inv ****= ******

**Erf(2K****T****R****o****)**

**2K****T****R****o**

**Most probable yield (R****o****=6 fm)**

**The lowest yield (R****o****=0)**

## Direct photon correlations were measured for the first time in ultrarelativistic heavy ion collisions.
We measured photon invariant correlation radius Rinv for two KT bins, 100<KT<200 MeV and 200<KT<300 MeV. In both cases Rinv was very close to the pion correlation radii. 
Using invariant correlation strength parameter inv we extracted lower limit on direct photon yield.  Even this lower limit is considerably larger than existing theoretical predictions.

- We measured photon invariant correlation radius Rinv for two KT bins, 100<KT<200 MeV and 200<KT<300 MeV. In both cases Rinv was very close to the pion correlation radii.
- Using invariant correlation strength parameter inv we extracted *lower limit* on direct photon yield.  Even this lower limit is considerably larger than existing theoretical predictions.

- Conclusions

## WA98 collaboration

- M.M.Aggarwal,1 Z.Ahammed,2 A.L.S.Angelis,3 V.Antonenko,4 V.Arefiev,5 V.Astakhov,5 V.Avdeitchikov,5T.C.Awes,6
- P.V.K.S.Baba,7 S.K.Badyal,7 S.Bathe,8 B.Batiounia,5 T.Bernier,9 K.B.Bhalla,10 V.S.Bhatia,1 C. Blume,8 D.Bucher,8
- H.B.usching,8 L.Carlen,11 S.Chattopadhyay,2 M.P.Decowski,12 H.Delagrange,9 P.Donni,3 M.R.Dutta Majumdar,2
- K.El Chenawi,11 A.K.Dubey,13 K.Enosawa,14 S.Fokin,4 V.Frolov,5 M.S.Ganti,2 S.Garpman,11 O.Gavrishchuk,5
- F.J.M.Geurts,15 T.K.Ghosh,16 R.Glasow,8B.Guskov,5 H.A.Gustafsson,11 H.H.Gutbrod,17 I.Hrivnacova,18 M.Ippolitov,4
- H.Kalechofsky,3 K.Karadjev,4 K.Karpio,19B.W.Kolb,17I.Kosarev,5I.Koutcheryaev,4A.Kugler,18P.Kulinich,12M.Kurata,14
- A.Lebedev,4H.Lohner,16 L. Luquin,9D.P.Mahapatra,13V.Manko,4 M.Martin,3 G.Martınez,9 A.Maximov,5 Y.Miake,14
- G.C.Mishra,13 B.Mohanty,13 M.-J.Mora,9 D.Morrison,20 T.Moukhanova,4 D.S.Mukhopadhyay,2 H.Naef,3 B.K.Nandi,13
- S.K.Nayak,7 T.K.Nayak,2 A.Nianine,4 V.Nikitine,5 S.Nikolaev,4 P.Nilsson,11 S.Nishimura,14 P.Nomokonov,5
- J.Nystrand,11 A.Oskarsson,11 I.Otterlund,11 T.Peitzmann,15 D.Peressounko,4 V.Petracek,18 S.C.Phatak,13
- W.Pinganaud,9 F.Plasil,6 M.L.Purschke,17 J.Rak,18 R.Raniwala,10 S.Raniwala,10 N.K.Rao,7 F.Retiere,9 K.Reygers,8
- G.Roland,12 L.Rosselet,3 I.Roufanov,5 C.Roy,9 J.M.Rubio,3 S.S.Sambyal,7 R.Santo,8 S.Sato,14 H. Schlagheck,8
- H.-R.Schmidt,17 Y.Schutz,9 G.Shabratova,5 T.H.Shah,7 I.Sibiriak,4 T.Siemiarczuk,19 D.Silvermyr,11 B.C.Sinha,2
- N.Slavine,5 K.Soderstrom,11 G.Sood,1 S.P.Sorensen,20 P.Stankus,6 G.Stefanek,19 P.Steinberg,12 E.Stenlund,11
- M.Sumbera,18 T.Svensson,11 A.Tsvetkov,4 L.Tykarski,19 E.C.v.d.Pijll,15 N.v.Eijndhoven,15 G.J.v.Nieuwenhuizen,12
- A.Vinogradov,4 Y.P.Viyogi,2 A.Vodopianov,5 S.Voros,3 B.Wyslouch,12 and G.R.Young6

**1**University of Panjab,                                               **2**Variable Energy Cyclotron Centre
- 3University of Geneva                                              4RRC “Kurchatov Institute”
- 5Joint Institute for Nuclear Research,Dubna            6Oak Ridge National Laboratory
- 7University of Jammu                                               8University of Munster
- 9SUBATECH                                                           10University of Rajasthan
- 11University of Lund                                                 12MIT
- 13Institute of Physics, Bhubaneswar                        14University of Tsukuba
- 15Universiteit Utrecht/NIKHEF                                 16KVI, University of Groningen
- 17Gesellschaft fur Schwerionenforschung (GSI)      18Nuclear Physics Institute, Rez
- 19Institute for Nuclear Studies, Warsaw                   20University of Tennessee