Crossed Channel Compton ScatteringCrossed Channel Compton Scattering

Michael Düren and George Serbanut, II. Phys. Institut,

- some remarks on cross sections and background processes

p

06.09.2004

Michael Düren, Univ. Gießen

Process of interest: GPDs from the QCD handbag diagram

Timelike wide angle Compton scattering

(large pT)

Q2 large: DVCS Q2 small: Wide angle Compton

scattering (large pT)

e-

e+

Hard exclusive

meson production (large pT)

hard gluo

n

Michael Düren, Univ. Gießen

The experimentalist‘s point of view:

2.:Tag one photon

with large pT

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 pT

Michael Düren, Univ. Gießen

Experimental requirements:

Estimates for pbeam=15 GeV/c Photon kinematics:

E = 15.5 ... 0.5 GeV @0°... 180° Photon angle in CMS and

transverse momentum are „large“ for wide angle Compton: pT = few 100 MeV ... 2.7 GeV

Interesting range in LAB around E = 8 GeV @ =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

pT

pbeam=15 GeV/c, s=30 GeV²

Michael Düren, Univ. Gießen

Calculated cross section

2

2222

sin

)(cos)(2

cos

sRsR

sd

d AVem

/month10 few/s100.5Rate

fm100.25

:Result

scm102L

GeV 10s

GeV/c 5p

:sAssumption

33-

29-

1-2-32

2

beam

pp

vector

axial vector

Simple model by Freund, Radyushkin, Schäfer, Weiss PRL 90, 092001 (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 pp

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 pp

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 0.51.5 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

- : ~20 000 pb : ~200 pb : 0.5...5 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

– ...

%4.1/%6/ EEE

2/* sE

180*

45*

Michael Düren, Univ. Gießen

Some first simulationsby 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 10000

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

pp

s