Supervisors: Prof. G. Susinno, Dr. M. Capua

1. Università della Calabria Dipartimento di Fisica g* g p′ p Measurement of the cross sectionsfor Deeply Virtual Compton Scattering at HERAand A theoretical model in the framework of the Regge theory Supervisors: Prof. G. Susinno, Dr. M. Capua PhD thesis presentation Salvatore Fazio - Università della Calabria - 19 Dec. 2007 PhD Coordinator: Prof. G. Falcone

2. Plan of the talk • Introduction to diffractive physics and DVCS • Description of the Hera accelerator and the ZEUS experiment • DVCS event selection • Measurements of • Direct measurement of • Description of our new theoretical model for the DVCS amplitude • Comparison of the model to the HERA DVCS data • Conclusions and outlook

3. X e’(k’) Q2 *(q) e (k) X W IP rapidity gap P (p) t P’(p’) Diffractive DIS Deep Inelastic Scattering (DIS) Q2 HERA studies the proton structure function s W Main kinematical variables DIS

4. Q2 W t DVCS process (1) Results: s(Q2) and s(W) in a larger phase space w.r.t. DESY-03-059 ds/dt with LPS detector Why to study DVCS? Frankfurt, Freund and Strikman (FFS) [1999] t dependence never measured directly!

5.  Generalized Parton Distributions (GPD) g* GPD x2 x1 p p GPD DVCS process (2) DVCS cross section is sensitive to the gluon distribution inside the proton: GPDs are important for the diffractive Higgs production at the future LHC experiments at CERN in Geneva

6. H1 • Circumference: 6.3 km • Electron energy: 27.5 GeV • Proton Energy: 920 GeV • Centre of mass energy: ZEUS The HERA accelerator The first and the only ep collider built in the world

7. The ZEUS Detector • Main Components: • Central Tracking Detector • Uranium calorimeter • Muon chambers • Forward Plug Calorimeter • Small Rear Tracking Detector • Luminosity Monitor • Leading Proton Spectrometer • Properties: • Ermeticity • Asimmetricity • Compensated LUMI SRTD p e+ LPS FPC

8. xL The Leading Proton Spectrometer (LPS) • Main properties: • Detection of scattered protons at small • angles (θ  1 mrad) • Very high precision measurement • of the scattered proton momentum diffractive peak LPS is made by 6 stations S1 ÷ S6 and 9 pots between 24 m and 90 m from the I.P. One pot contains 6 layers of silicon micro-strips The S4 ÷ S6 stations are twins and contain a seventh plane used by the trigger system

9. Q2 W 2nd 1st t QED compton Selection criteria • 99e+ - 00 ZEUS data • Two em candidates • First candidate in RCAL • Second candidate in RCAL • or in BCAL • 1 or 0 tracks • H1 box cut • Elasticity cut • Energy in FCAL < 1 GeV • and in FPC < 1GeV • -100 < Zvtx < 50 cm • DST 93 bit • E1 > 15 GeV • E2 > 2.5 GeV • q2 < 2.85 L = 61.14 pb-1 After all selection criteria 13084 events

10. ′ e g ′ e g* ′ ′ ′ p p DVCS BH Strategy ′ Data/MC • Sample: no tracks matching to the second cluster (4570 events) (DVCS+BH) Data/MC e+ Sample: a track match to the second cluster (8027 events) (BH + Dilepton + J/y) Data/MC e- Sample: a negative charged track match to the 2nd cluster(487 events) (Dilepton + J/y) Data/MC New kinematic region: 3 < Q2 < 100 GeV2 40 < W < 170 GeV Pub. kinematic region: 5 < Q2 < 100 GeV2 40 < W < 140 GeV

11. Data vs MC Back g sample compared with DVCS MC New kinematic region Pub. kinematic region

12. Data vs MC e- sample compared with dilepton and J/y MCs Back Pub. kinematic region New kinematic region J/ycontibution relevant at high W and low Q2 Dilepton = 31 % J/y = 69 % Dilepton = 85 % J/y = 15 %

13. Data vs MC e+ sample compared with BH+background MC (dilep. and J/y) Back Pub. kinematic region New kinematic region Dil.+ J/y= 8 % Dil.+ J/y= 5 % We can subtract J/Y and dilepton contributions from the e+ sample to have a clean BH sample and normalize the BH MC to this one

14. Data vs MC Clean DVCS sample compared with DVCS MC Pub. kinematic region New kinematic region GOOD AGREEMENT

15. Comparison with published analysis Pub. kinematic region: 5 < Q2 < 100 GeV2 40 < W < 140 GeV Very good agreement with published analysis

16. <W> = 107 GeV <Q2> = 5.9 GeV2 + 0.08 – 0.06 ZEUS pub. Value: 0.75 ± 0.15 (stat.) <Q2>=9.6 GeV2 (syst.) New cross sections New kinematic region: 3 < Q2 < 100 GeV2 40 < W < 170 GeV 3 more bins at higher W 1 more bin at lower Q2 n= 1.56 ± 0.08 Fit: Q2n ZEUS pub. Value: 1.54 ± 0.07 (stat.) ± 0.06 (syst.) <W>=89 GeV d = 0.58 ± 0.11 Fit: Wd

17. Q2 dipendence for the W slope NEW ZEUS PRELIMINARY Presented at: [EPS07, Manchester (UK), 19-25 July 07] New set of points added at Q2 = 3.8 GeV2 No Q2 dipendence for the W slope

18. Systematic uncertainties Systematic uncertainties for each bin • List of the most relevant • systematic checks: • min. appr.(MA) < 25 cm & MA < 15 cm • E1 > 8 GeV & E1 > 12 GeV • E2 > 2 GeV & E2 > 3 GeV • PSi1 > 0.95 & PSi1> 0.85 • PSi2 > 0.75 & PSi2 > 0.65 • q2 < 2.75 rad & q2 < 3.14 rad • E-pz > 35 GeV (12) & E-pz > 45 GeV • E-pz < 65 GeV • EFPC < 0.9 GeV & EFPC < 1.1 GeV • |Zvtx| < 50 cm • box cuts variation • elasticity cuts variation • trigger efficiency variation • elastic/inelastic mix variation Q2 bins W bins

19. LPS selection cuts: • 0.96 < xL < 1.04 • 0.08 < |t| < 0.53 GeV2 • E+pz +1840·xL<1865 GeV • docap > 0.04 cm • X(S4 station) > - 33 cm LPS selection criteria • 3 < Q2 < 100 GeV2 • 40 < W < 170 GeV • ZEUS selection criteria • 2000 data only L = 31 pb-1 After all selection criteria 33 events in g sample

20. xL t LPS data/MC e+ sample g sample DVCS sample The estimated fraction of BH events in the g-sample after the LPS selection is: ~ 22 % (e+ -sample)

21. LogL b ds/dt with LPS NEW ZEUS PRELIMINARY Presented at: [EPS07, Manchester (UK), 19-25 July 07] b slope determination by a maximum Log-likelihood unbinned fit b = 4.5 ± 1.8 (stat.) ± 0.4 (sys.) GeV-2

22. Systematic uncertainties (LPS selection) • LPS systematic check list: • Minimum approach to the beam pipe > 0.1 cm • E+pz +1840·xL< 1855 GeV • 0.09 < t < 0.5 GeV2 • X (S4 station) > - 32 cm Total LPS syst. uncert.: ~ 7 % Systematics for b calculation: D BH ratio: 4% D MC binning in Llikel. fit: 4% Total b syst. uncert.: ~ 9 % b = 4.5 ± 1.8 (stat.) ± 0.4 (sys.)GeV-2

23. G. Watt hep-ph 0712.2670v1 17 Dec 2007!

24. A new theoretical model [M. Capua, S. Fazio, R. Fiore, L. Jenkowsky, F. Paccanoni] Published in:Physics Letters B645 (Feb. 2007) 161-166 • Applications for the model can be: • Guide for the experimentalists • Study of various extreme regimes of the • scattering amplitude vs Q2, W, t • (perturbative –> unperturbative QCD) • Study of GPDs A new variable is introduced: z = t - Q2 DVCS amplitude: the t dependence at the vertex pIPp is introduced by: the vertex γ*pγ is introduced by the trajectory: DVCS amplitude can be written as: where: (z) (z) (z) hep-ph/0605319 inviato a: Phys. Lett. B Presentato alla Conferenza Internazionale:Diffraction06, settembre 2006, Milos, Greece hep-ph/0605319 inviato a: Phys. Lett. B Presentato alla Conferenza Internazionale:Diffraction06, settembre 2006, Milos, Greece hep-ph/0605319 inviato a: Phys. Lett. B Presentato alla Conferenza Internazionale:Diffraction06, settembre 2006, Milos, Greece Introduciamo in questo modello la nuova variabile: z = t-Q2 Introduciamo in questo modello la nuova variabile: z = t-Q2 Ampiezza DVCS: Ampiezza DVCS: la dipendenza da t del vertice pIPp è introdotta tramite: Il vertice γ*pγ è introdotto tramite la traiettoria: L’ampiezza può essere scritta come: la dipendenza da t del vertice pIPp è introdotta tramite: Il vertice γ*pγ è introdotto tramite la traiettoria: L’ampiezza può essere scritta come: Dove: Dove:

25. A fit was performed with two parameters A0 e b on HERA 1996-2000 data: H1 Coll. Eur. Phys. J. C. 44 (2005) ZEUS Coll. Phys. Lett B 573 (2003) It was assumed: α(0)=1.25(effective Reggeon intercepta: mix of a “soft+hard” Pomeron) a′ = α1*α2 = 0.38 GeV-2(slope for the effective Reggeon, ~0.2 for a soft Pomeron) a1 = 1 (α2 = α′/1) A new theoretical model where

26. b = 1.08 ± 0.10 c2/ndf = 1.15 b = 0.93 ± 0.05 c2/ndf = 0.57 Results from the fits Fits were performed over the HERA data Good agreement between the three fitted b values! Paper with fits including the new data is ongoing Presented at: [DIFFRACTION06, Milos (Greece), Sep. 06] [EDS07, Hamburg (Germany), 21-25 May 07] b = 1.04 ± 0.91 c2/ndf = 0.15

27. F2 stucture function Comparison between HERA data and the model for F2(s,Q2) DIS structure function All parameter fixed like in our model Really good agreement! The model reproduces experimental data at small x and moderate Q2

28. b = 1.08 ± 0.10 c2/ndf = 1.15 b = 0.93 ± 0.05 c2/ndf = 0.57 b = 1.04 ± 0.91 c2/ndf = 0.15 Summary & outlook • DVCS analysis: • New ZEUS measurements of s(Q2) and s(W) for DVCS events in a larger phase space • A new Q2 bin measurement of s(W)slope was performed • First ZEUS measurement of by a direct measurement of the outgoing proton momentum using the LPS detector was achieved • Measurement of the DVCS b slope going to be published • The knowledge got during this work will help to analyze the HERA II data (L = 400 pb-1) leading to more precise cross section measurements • The model: • A New model for the DVCS amplitude has been proposed and fitted to the HERA DVCS data • The model reproduce the DIS F2 shape very well at small x and moderate Q2 • A new paper with fits including the new published measurement and an evolution of the model is ongoing

29. PhD activity summary • DVCS analysis: • 1 Paper at ZEUS editorial board • Results presented at: • EPS07, Manchester (UK), 19-25 July 07 • Talks at ZEUS collab. Meetings: • Florence (Italy), 19 Oct. 06 • Hamburg (Germany), 08 Mar. 07 • Hamburg (Germany), 27 Jun. 07 • Hamburg (Germany), 02 July 07 • Other ZEUS activities: • Data taking shifts at the ZEUS detector • Coordination of Data Quality Monitoring for • the ZEUS diffractive group • Participation at several group and • collaboration meetings • 26 papers published with the ZEUS collab. • Several papers at ZEUS editorial board • DVCS model: • 1 paper published on: • Physics Letters B645 (Feb. 2007) 161-166 • 1 paper ongoing • Results presented at: • DIFFRACTION06, Milos (Greece), Sep. 06 • EDS07, Hamburg (Germany), 21-25 May 07 • Other schools & talks at conferencies: • Scuola nazionale di Otranto: • XVIII seminario nazionale di fisica nucleare • e subnucleare, Otranto (Italy), 22–28 Sep. 05 • Invited talk “Diffraction at HERA” at: • Fysikermøtet 07, Conference of the • Norwegian Society of Physics, Tromsø • (Norway), 08-10 Aug. 07

30. RESERVE

31. at HERA only! The HERA phase space

32. The HERA tunnel Protons at 920 GeV (Superconductive magnets) Elettrons or positrons at 27.5 GeV (Conventional magnets)

33. g* ′ g elastic diffractionep e (VM,g) p ′ ′ ,g X g* single diffractive dissociation ep e (VM,g) Y VM ′ Y x2 x1 Y p p double diffractive dissociation ep e X Y Why diffractive physics at HERA? Before HERA diffractive physics was studied in hadron-hadron interactions only VDM IPomeron as a couple of gluons

34. The Roman Pots technique MOBILE STATIONS LOW RADIATION DOSE • The motion is in three steps: • Detectors are in the safe position • Detectors are inserted in the pots and moved to the beam • Detector are in their final data taking position

35. Fraction of BH in the g sample bin by bin vs W BH ratio (%) g sample We must subtract the BH contribution Data vs MC g sample vs BH MC Pub. kinematic region New kinematic region The estimated fraction of BH events in the g-sample is: ~ 42 % The estimated fraction of BH events in the g-sample is: ~ 63 %

36. Syst. checks for the s(Q2)

37. Syst. checks for the s(W)

38. ds/dt comparison with H1 H1 calculation of t:

39. FLT62 trigger efficiency The study of the FLT62 trigger efficiency measuring photon energy was performed for our larger kinematic sample Photon energy Cut: E2=2.0 GeV (in the analysis) E2=2.0 GeV (in our Bethe Heitler MC) Pure BH sample selection cuts: • 1 track from the vertex • |zvtx|< 50 cm • 0.5 < p < 10 GeV (p calculated from the track) • Q2 > 3 GeV2 • dE/dx > 1.2 GeV (energy loss per unit of a track) • 0 < Zwidth < 0.7 (energy weighted average of the cluster width in the z direction) To measure trigger efficiency: • Pure BH selection cuts • Analysis selection cuts was done Bothfor data and MC, and then we calculate the weight: and with this we correct the photon energy E2

40. FLT62 Trigger efficiency Weight applied in RCAL and in BCAL and a comparison with the published analysis Trigger efficiencies for data and MC separately in BCAL & RCAL BCAL RCAL

41. Df12 distributions MC BH inel. BH data MC BH elastic. % Inelastic BH contribution Elastic BH ratio BH el. = 87.8 ± 7.7 %

42. Beam halo background Il fondo di beam halo è dovuto all'interazione di protoni dell'alone del fascio con le stazioni dell'LPS. Eventi di questo tipo hanno xL~1 e possono essere confusi con eventi di fisica diffrattiva Per la conservazione dell’energia gli eventi con: sono dominati beam halo, quindi vanno rimossi! Bisogna valutare la contaminazione!!! Dopo un dettagliato studio si è stimata una contaminazione dovuta agli eventi di beam halo pari al4%

43. LPS selection DVCS sample vs DVCS MC

44. LPS selection