Crossed Channel Compton Scattering Michael Düren and George Serbanut, II. Phys. Institut, - some remarks on cross sections and background processes  

Michael Düren, Univ. Gießen Process of interest: GPDs from the QCD handbag diagram Timelike wide angle Compton scattering (large p T ) Q 2 large: DVCS Q 2 small: Wide angle Compton scattering (large p T )  e -   e +   Hard exclusive meson production (large p T ) hard gluon

Michael Düren, Univ. Gießen The experimentalist‘s point of view: 2.:Tag one photon with large p T 3.: Observe second particle which balances momentum and total energy ,  * , , ,... Compare the production of real photons, virtual photons and all kind of neutral mesons (and in case of a deuteron target also charged mesons) 4.: Compare the differential x-sections of real photons, virtual photons and all kind of neutral mesons (and in case of a deuteron target also charged mesons) 1.: Reject all events with more than 2 primary final state particles Ask theoreticians for predictions (  *,  even give polarization observables) 2.:Tag one photon with large p T

Michael Düren, Univ. Gießen Experimental requirements: Estimates for p beam =15 GeV/c  Photon kinematics: E  = °  Photon angle in CMS and transverse momentum are „large“ for wide angle Compton: p T = few 100 MeV GeV  Interesting range in LAB around E  = 8  =20°  4  calorimeter needed!  Background suppression by –Large acceptance charged particle detector veto –Good resolution calorimeter for check of exclusivity (momentum balance) –Possibly large acceptance neutral particle veto (neutrons) EE pTpT  p beam =15 GeV/c, s=30 GeV²

Michael Düren, Univ. Gießen Calculated cross section vector axial vector Simple model by Freund, Radyushkin, Schäfer, Weiss PRL 90, (2003) Data from e + e - suggest that the model underestimates the real rate by a large factor

Michael Düren, Univ. Gießen Comparison of model with e + e - -data

Michael Düren, Univ. Gießen Measured cross section Armstrong et al., PRD 56 (1997) 2509 Fermilab experiment E760 has already some first measurements of this channel! Fermilab data are all background dominated!

Michael Düren, Univ. Gießen Cross section Model by Weiss (03) Model by Weiss (03) *10 to fit e+e- data (Cleo, LEP) Cleo data 60 pb E760 upper limits 28/13 pb E760 fit result 0.5  1.5 pb Feed down limit from  and  E760 feed down limit from  and  Panda lumi limit (100 ev/month) s (GeV²)  (pb)

Michael Düren, Univ. Gießen Feed down background Example from E760:  The cross section at 13 GeV² is -  : ~ pb -  : ~200 pb -  : pb -  feed-down: 25 pb (similar contribution from  and  )  E760 detector: –1280 lead glass counter –E>20 MeV threshold –6-11 mrad resolution –  Necessary selections: –no charged tracks –exactly 2 neutral clusters –Each cluster has –Two clusters are collinear in CMS –Minimal angle cut –Additional hits in calorimeter below threshold may not match to asymmetric  0 decay –...

Michael Düren, Univ. Gießen Some first simulations by G. Serbanut Reconstructed invariant mass of 5.5 GeV  events Feed-down of 5.5 GeV  events are strongly suppressed;  is zero in this simulation of ev.

Michael Düren, Univ. Gießen Conclusions  The cross section is roughly known  PANDA luminosity is sufficient to measure it over the energy range up to ~ 4.5 GeV  The feed-down background from  and  requires a large suppression factor  It has to be about a factor of 10 better than at E760  The PANDA detector needs a calorimeter with good granularity, good resolution and very low detection threshold  Detailed Monte Carlo background studies are needed (and have started by G. Serbanut)  All other handbag channels (mesonic and leptonic) have to be studied in future