1. Possibility for Double DVCS measurement in Hall A Alexandre Camsonne SBS Meeting June 4 th 2013

2. Double DVCS    p  ‘  p’ l + + l - Guidal and Vanderhaegen : Double deeply virtual Compton scattering off the nucleon (arXiv:hep-ph/0208275v1 30 Aug 2002) Belitsky Radyushkin : Unraveling hadron structure with generalized parton distributions (arXiv:hep-ph/0504030v3 27 Jun 2005)

3. Double Deeply Virtual Compton Scattering scattered electron scattered proton outgoing virtual photon lepton pair from virtual photon  = p1-p2 = q2-q1  = x bj = p = p1+p2 q = ½ (q1+q2)  q pqpq QQ 2p 1 q 1  = QQ 2p  q Q   q 2 Q   -(k-k’) 2

4. Double DVCS Detect dilepton pair instead of real photon Allow to vary skewness  of the reaction Charged particle in final state can use spectrometer to measure momentum ( less requirement on calorimeter energy resolution) Muon channel can go through large amount of material, possibly clean trigger with coincidence

5. Double DVCS and Virtual Bethe Heitler Interference of Double DVCS and virtual Bethe Heitler

6.   Kinematical coverage DVCS  =  JLab 11 GeV 25 GeV 40 GeV H u ( ,  ) 2 2 DVCS only probes  =  line Example with model of GPD H for up quark Jlab : Q 2 >0 Kinematical range increases with beam energy ( larger dilepton mass )

7. DDVCS cross section VGG model Order of ~0.1 pb = 10 -36 cm 2 Virtual Beth and Heitler Interference term enhanced by BH Contributions from mesons small when far from meson mass

8. DDVCS measurement Need high luminosity – Hall B : 10 35 - 10 36 cm -2 s -1 – mEIC : 1.5x 10 34 cm -2 s -1 – Want 10 38 cm -2 s -1 ideally 10 39 cm -2 s -1 Pair detection : clean trigger Ideally look at muons channel to avoid ambiguity with initial electron ( muon source low luminosity )

9. DDVCS measurement Large acceptance to get the whole angular coverage of the pair Forward angle for increase of Bethe and Heitler interference

10. Hall A DVCS experiment 13x16 PbF2 Calorimeter 39 cm x 48 cm at 110 cm Energy resolution Centered on Virtual photon Energy resolution about 3% Exclusivity by missing mass technique detecting photon only

11. Dimuon detection No ambiguity Can go through material : clean trigger

12. Lepton pairs

13. Possible similar setup with SBS HRS+SBS Pros – Virtual photon very well defined – High momentum resolution – Good vertex resolution Cons – Small acceptance : need to run at high luminosity

14. Option with BigBite and SBS BigBite as electron arm : – Pros Larger acceptance – Cons Lower momentum resolution

15. Double DVCS Challenges – Pion muon discrimination Record shower profile – Vertex reconstruction – Need detector without shielding for accurate momenta and vertex resolution ( GEM )

16. Muons and pion shower

17. Muon identification Add material to stop other electromagnetic process Scintillator planes for muon trigger Use sampling calorimeter to look at shower : layers of material + GEM or Micromegas with pads and digital readout ( CALICE, SdHCAL, dHCAL ) GEM or MicroMegas   Pion showers Muon only does energy loss

18. Micromegas readout

19. Cost electronics For 32 cm x 48 cm = 1536 cm2 3m x 3m = 90 000 cm2 MicroROC Digital chips ASICs : 320 chips = 20 480 channels for 25 K$ about 100 K$ per plane Compatible with SRS Micromegas detector 32 cm x 48 cm about 3000 $

20. Other possible readouts RPC GEMs Silicium detectors…

21. Possible detectors to reuse Gen neutron detector SRC neutron detector Any layered detector : add pad planes

22. Conclusion Opportunity to measure double DVCS at JLab12 GeV in dimuon channel with high statistics SBS can make a first measurement on limited kinematic range Simulation work and detector R&D Early Career Grant for SBS to build MicroMegas Workshop Proposal next PAC