1. Exclusive Production of Hadron Pairs in Two-Photon Interactions Bertrand Echenard University of Geneva on behalf of the LEP collaborations Hadronic Physics Session EPS, Aachen, 17-23 July 2003 contact: bertrand.echenard@cern.ch

2. 17-23 july 2003 B.EchenardEPS 2003, Aachen Outline q  and K pair production ?  and K meson pair production [ALEPH: CERN-EP:2003-030] q Baryon pair production ? pp [L3: CERN-EP:2003-014 OPAL: Eur. Phys. J. C28 (2003) 45 ] ?  and     [L3: PLB 536 (2002) 24] q     and     pair production ?     and     pair production at Q 2  0 [L3: submitted to EPS] ?     pair production at high Q 2 [L3: CERN-EP:2003-020]

3. 17-23 july 2003 B.EchenardEPS 2003, Aachen Hadron pair production in  collisions q  and K pair production Tests of QCD calculations Handbag model Amplitude   HH factorized into: ? Hard part   qq scattering ? Soft part: qq  HH transition (1) M. Diehl, P. Kroll and C. Vogt, PLB 532 (2002) 99, hep-ph/0112274. (2) S. J. Brodsky and J. P. Lepage, PRD 22 (1980) 2157. ? Handbag model (1) based on the Brodsky and Lepage approach (2)

4. 17-23 july 2003 B.EchenardEPS 2003, Aachen Hadron pair production in  collisions q Baryon pair production Tests of QCD calculations ? Handbag model (1) ? Three quark model (2) ? Quark-diquark model (3) Three quark model Quark-diquark model Diquark Three quark & Diquark models (1) M. Diehl, P. Kroll and C. Vogt, hep-ph/0206288. (2) G. Farrar et al., Nucl Phys B 259 (1985) 702. (3) C. F. Berger, B. Lechner and W. Schweiger, Fizika B8 (1999) 371; C. F. Berger and W. Schweiger hep-ph/0212066.

5. 17-23 july 2003 B.EchenardEPS 2003, Aachen  and K pair selection ? 2 tracks of opposite charge ? Electron rejection criteria ? Muon rejection criteria ? Cut on dE/dx to select pions and kaons ? Exclusive event selection cut ALEPH event selection:

6. 17-23 july 2003 B.EchenardEPS 2003, Aachen       and       cross sections q       ) =  (        within uncertainty q Shape well described by QCD predictions  Disagreement in normalization and ratio K/  (QCD:  (             )  2  ) Fit with power law: AW -6 ?     : A=200±40 nb GeV 6      : A=220±40 nb GeV 6            

7. 17-23 july 2003 B.EchenardEPS 2003, Aachen Proton-antiproton pair selection q L3: ? Antiproton Neural network: P, dE/dx, E t /p t & shower shape ? Proton dE/dx and E t /p t q OPAL: ? dE/dx and E/P

8. 17-23 july 2003 B.EchenardEPS 2003, Aachen   pp cross section Results compatible with previous measurements Extend W  range qqqq Good agreement with quark-diquark model Three quark model excluded qqqq

9. 17-23 july 2003 B.EchenardEPS 2003, Aachen   pp angular distribution  Agreement in the intermediate and high W  region  Disagreement with diquark model in the low W  region q Low mass region well described by spherical harmonics : 92% Y 2 0 and 8% Y 0 0 Low massIntermediate massHigh mass

10. 17-23 july 2003 B.EchenardEPS 2003, Aachen  and    pair  selection  Decay channel:   p     p      

11. 17-23 july 2003 B.EchenardEPS 2003, Aachen    and       cross sections q Good agreement with diquark model predictions q Three quark model excluded 

12. 17-23 july 2003 B.EchenardEPS 2003, Aachen    and       cross sections q Agreement with the handbag model predictions using   pp as input data 

13. 17-23 july 2003 B.EchenardEPS 2003, Aachen m(     ) spectrum     mass spectrum     mass spectrum m(     ) spectrum     and     pair production at Q 2  0 q     channel ? High statistics (~70000 events) ? Clear     peak q     channel ? Less statistics (~5000 events)  ? Clear     peak L3

14. 17-23 july 2003 B.EchenardEPS 2003, Aachen     and     pair production at Q 2  0  Broad enhancement near threshold of        Ratio of     /     cross sections incompatible with formation of one Isospin 0 or 1 resonance Model with  in different spin-parity- helicity states and isotropic 4  A   BW 12 BW 34  J,Jz + permutations A 4  =1 ? (J P,J z ) = (2 +,2) dominant state ? (0 +,0) small contribution ? (0 -,0), (2 +,0), (2 -,0(1,2)) negligible Partial Wave Analysis: p L3

15. 17-23 july 2003 B.EchenardEPS 2003, Aachen     pair production at high Q 2 q Select 4  + tagged electron: ?        ?          ?            non-resonant ? Separated by a box method q Q 2 interval: 1.2 GeV 2 < Q 2 < 30 GeV 2 m(     ) spectrum     mass spectrum L3 00 0000

16. 17-23 july 2003 B.EchenardEPS 2003, Aachen     pair production at high Q 2 Broad enhancement near threshold of       as observed in Q 2  0 measurement ? 1.2 GeV 2 < Q 2 < 8.5 GeV 2 8.8 GeV 2 < Q 2 < 30 GeV 2         NR   

17. 17-23 july 2003 B.EchenardEPS 2003, Aachen     pair production at high Q 2 q Fit with 1/(Q m (Q 2 +  W  2 ) 2 ): ? QCD prediction (1) : m=2 ? Data: m=2.4 ± 0.3 (  W   =1.95) (1) M. Diehl et al. PRD 62 073014 (2000); hep-ph/0003233 Low and high Q 2 data are well described by GDVM form factor  -pole not sufficient q q 161-209 GeV 91 GeV

18. 17-23 july 2003 B.EchenardEPS 2003, Aachen Conclusions q  and K pair production ?       ) =  (        ? Only W  dependence agrees with QCD predictions ? Normalization and K/  ratio disagree with QCD predictions q Baryon pair production ? Data can be described by diquark model and handbag model ? Three quark model excluded q     and     pair production ? Dominance of (J P,J z ) = (2 +,2) for Q 2  0 ? Broad enhancement near threshold of       ? Good agreement with QCD predictions for d  e+e- /dQ 2 ? GVDM form factor also describes well the Q 2 dependence of  