Jixie Zhang (Jlab)

1. Nucleon Structure Function, An Introduction of the BoNuS Program at Hall B Jefferson Laband the G2P Program at Hall A Jixie Zhang (Jlab) March. 13, 2013

2. Outline • Thesis Work: extract D(e,e`p-p)p cross section • BoNuS Project • Data Analysis • Result • Current Work: • G2P Project • Status • Future Work

3. Nucleon Structure Functions Callan-Gross Relation Structure Function Definition (in the quark-parton model)

4. Strong Model Dependence SU(6) symmetry uv(x) = 2dv(x) Sz = 0 dominant, helicity conservation Farrar and Jackson, Phys Rev Lett35 (1975) Brodsky, Burkardt and Schmidt, Nuc PhysB441 (1995) S = 0 dominant, one-gluon exchange Close, Phys LettB43 (1973) Carlitz, Phys LettB58 (1975) Close and Thomas Phys LettB212 (1988) Isgur, Phys RevD59 (1999), Phys Rev Lett41 (1978) SLAC E139 (L. W. Whitlow, et al.), and E140 (J. Gomez, et al.)

5. Cross Section in the Resonance Region • Data on the Proton: Clear resonant structure, separation from the non-resonant background is possible • Data on the deuteron: Kinematically smeared due to binding, off-shell, final state interactions (FSI), etc.

6. Exclusive p- electro-production D(e,e`p-p)p Detect e′, p- and at least ONE of the two final state protons in D(e,e`p-p)p to ensure exclusivity and select events where the “spectator” proton has low, backwards momentum. Conservation of energy and momentum allows to determine the initial state of the neutron. e′ d p- g* n p p e ps Low momentum “spectator” proton Novel approach by the BoNuS collaboration: detect the spectator proton directly.

7. p- E, k f* q* n, q E, k n p Leptonic plane Hadronic plane p-Production Kinematics g* n p- p Q2 = -(qµ)2 = 4sin2(qe/2) W2 = (qµ + nµ )2 = (qµ + dµ - psµ)2 = (pµ + pµ )2 q* = polar angle of the outgoing p- in C.M. frame f* = Azimuthal angle of the outgoing p- in C.M. frame

8. Final State Interactions Off-Shell and FSI for D(e,e′ps)X Select low Ps (<120 MeV/c) and large backward qpq (>100o), angle between Ps and virtual photon, to minimize FIS. Off-shell Effects VIPs W. Melnitchouk et al, Phys. Lett. B377, 11 (1996). Off-shell effects are negligible for small Ps. Choose Ps<120 MeV/c as Very Important Spectator Protons (VIP) C. Atti et al, Eur. Phys. J. A 19,133-137 (2004).

9. p- p n d ps p- p p n p p- n d p- p d p Final State Interations primary background pp rescattering quasi-free (IA) • No theory calculation for g* n  p- p FSI yet. Try to estimate this from FSI of • n  p- p ....... • V. Tarasov, A. Kudryavtsev, W. Briscoe, H. Gao, IS, arXiv: 1105.0225 primary background pp rescattering

10. FSI Prediction for D(g,p-p)p, by Laget under pp rescattering peak region only R = ratio of the total to the quasi-free cross section qR = polar angle between photon and spectator proton PR = momentum of the spectator proton Strong momentum and anglular dependence

11. Cross Section Calculations • Select exclusive events • Apply corrections: background corr., radiative corr., trigger eff., p-, CLAS proton and RTPC proton detection eff., and acceptance correction.

12. Jefferson Lab Experiment E03-012Barely off-shell Nucleon Structure (BoNuS) Electron beam energies: 2.1, 4.2, 5.3 GeV Spectator protons were detected by the newly built Radial Time Projection Chamber (RTPC) Scattered electrons and other final state particles were detected by CEBAF Large Acceptance Spectrometer (CLAS) Target: 7 atm D2 gas, 20 cm long Data were taken from Sep. to Dec. in 2005 N.Baillie, S. Tkachenko, J. Zhang, K. Griffioen, S. Kuhn, Gail Dodge, C. Keppel, M.E. Christy, W. Melnitchouk, H. Fenker, S. Bültmann

13. FC RTPC Sits in the Center of CLAS BoNuS RTPC stopper CLAS

14. Sensitive to protons with momenta of 67-250 Mev/c 3 layers of GEM 3200 pads (channels) 5 Tesla B field Particles ID by dE/dx 7Atm. D2 gas target, 20cm in length proton Trigger Electron Deuteron / Helium 3-D tracking: time of drift -> r pad position -> f, z Helium/DME at 80/20 ratio 100 µm dE/dX Radial Time Projection Chamber (RTPC)

15. RTPC Resolution DZ Trigger electrons measured by CLAS are compared to the same electrons measured in BoNuS during High Gain Calibration runs. Df Dq H. Fenker, et.al. Nucl.Instrum. Meth. A592:273-286,2008

16. Analysis Outline Quality Checks Vertex correction and vertex z cuts Particle identifications (electron, p- and proton ) Fiducial cut for trigger electron, p- and proton Energy loss correction for electron, p- and proton Exclusive cut (2-s Missing Mass cut) Acceptance cut and acceptance correction Background subtraction Trigger efficiency correction, CLAS proton and RTPC proton detection efficiency correction Radiation correction Luminosity analysis Extract the cross section and structure functions Systematic error estimation

17. Simulation Overview RTPC(Geant4) CLAS(geant3) Reconstruction  Analysis  • What have been done with simulation? • Debug/optimize RTPC reconstruction packages • Generate energy loss correction tables, radiation length tables • Study Detector’s acceptance for D(e,e`p-pCLAS)p and D(e,e`p-pRTPC)p • Study particle detection efficiency • Model the background…

18. Kinematic coverage and binning, 5 GeV • W: 150 MeV each bin, [1.15,2.95) • Q2 : 6 bins with boundaries at 0.1309, 0.3790, 0.7697, 1.0969, 1.5632, 2.6594, 4.5243 • cosq*: 8 bins with boundaries at -1.0, -0.5, -0.1, 0.3, 0.55, 0.7, 0.8, 0.9,1.0 • f*: 15 bins, 24 degrees each bin, [0.0,360.0)

19. Missing Mass Cut: 2s from Gaussian Center 5 GeV 5 GeV background

20. Background Subtraction average f1, f2, f3 and f4 are the fitted background fraction functions for real data. f1sim, f2sim, f3sim and f4sim are the fitted background fraction functions for simulated data. Simulated Data Final background correction factor: where 0.9545 is the coverage of 2s in a gaussian distribution.

21. Background of D(e,e`p-pCLAS)p, 5G Real f2 f1 f4 f3

22. Background of D(e,e`p-pCLAS)p, 5G Sim. f1sim f2sim f3sim f4sim

23. Radiation Correction,E=5.3 GeV s / sborn W`=1.23 f* Leading Log approximation Calculation Based on Maid07 Using upraded “Exclurad”. Updated with MAID07 multipole model for both neutron and proton. A dedicate clas-note will be vesy soon.

24. CLAS trigger efficiency, 5G P (GeV) Data outside the curve will not be considered! q (deg) Trigger efficiency is the fraction that the trigger particle (electron) is detected. It is obtained by scaling the simulation data to the real data, then calculating the ratio of real event counts in each p-q grid to the simulated event counts in the same grid.

25. CLAS proton efficiency

26. RTPC proton efficiency

27. D(e,ep-p)p Acceptance Correction • Binning information: • W: 150 MeV each bin, [1.15,3.10); • Q2 : 6 bins, {0.1309, 0.3790, 0.7697, 1.0969, 1.5632, 2.6594, 4.5243 }; • cosq*: 8 bins, 0.25 each bin, [-1.0,1.0); • f*: 15 bins, 24 degrees each bin, [0.0,360.0). • Acceptance is the ratio of the number of events detected in a given bin to the number of generated events in the same bin. • Need to generate tables for D(e,ep-pRTPC)pandD(e,ep-pCLAS)preaction separately. • Applied event by event • Need to have acceptance cut to ensure the reliabilities • For CLAS channel: 0.008 and less than 10% uncertainty (> 100 detected events) • For RTPC channel: 0.003 and less than 15% uncertainty (> 40 detected events)

28. Cross Section Calculations • Select exclusive events • Apply corrections: background corr., radiative corr., trigger eff., p-, CLAS proton and RTPC proton detection eff., and acceptance correction.

29. Cross Section: BoNuS Vs MAID and SAID GeV2 Cosq* 0.95 preliminary 0.85 0.75 0.625 4 GeV, W` = 1.23 GeV f* MAID 07 SAID 08 D(e,ep-pRTPC)p D(e,ep-pCLAS)p

30. Minimizing Final State Interactions2 GeV, W` = 1.23 GeV Without VIP Cut Cosq*=0.95 Cosq*=0.85 preliminary f* Cosq*=0.95 With VIP Cut Cosq*=0.85 f* Prediction: FSI + Binding < 20% under VIP cut Data: 1)Better agreement between CLAS and RTPC channel for low Q2; 2) No obvious change other than removing data points to the results with larger Q2 VIP Cut= qpq>100o and 70<ps<120 MeV

31. Systematic Error 4 GeV, W` = 1.23 GeV GeV2 Cosq*=0.95 Cosq*=0.85 f* D(e,ep-pRTPC)p D(e,ep-pCLAS)p 15% in average, mainly come from the following (values are all in average): FSI is estimated to be 15%, not included in the figure yet

32. + A1 Cosf* + A2 Cos2f* = A0 Fit for the Structure Functions

33. Structure Functions: BoNuS Vs MAID5 GeV, W` = 1.525 GeV preliminary D(e,ep-pCLAS)p + VIP D(e,ep-pRTPC)p + VIP VIP = 70<ps<120 MeV/c, and qpq>100o FSI + Binding < 20% under VIP cut

34. A0: BoNuS Vs MAID, 4 GeV with VIP cut preliminary

35. BoNuS Kinematic Correction • Very Important Protons 70<ps<100 MeV/c • Corrections make resonances stand out • F2n/F2p can be measured at high x* ps distribution BoNuS Region 70, 100, 200 MeV/c VIPs

36. Accidental Backgrounds • Fits to the triangular background allows us to measure backgrounds underneath the peak • Rbg = Blue area / Pink area, Rbg is independent of kinematics

37. Two Analysis Methods • The Ratio Method • measure tagged counts divided by inclusive counts • correct this ratio for backgrounds • one scale factor gives F2n/F2d • The Monte Carlo Method • measure tagged counts • divide by spectator model Monte Carlo results • multiply by F2n used in the model • The two methods have different systematic errors, but give very similar results.

38. Comparison of These 2 Methods 1, by S. Tkachenko, MC method 2, by N. Baillie, Ratio method

39. BoNuS F2n

40. BoNuS F2n/F2p • F2n/F2n vs. x • Curves are CETQ error bands • CETQ cuts off at low x because Q2 is too low • Lower cuts in W* imply higher x but the inclusion of resonance contributions. • Results are consistent with CETQ trends at high x.

41. BoNuS Plans for 12 GeV • Data taking: • 35 days on D2 • 5 days on H2 • L = 2 x 1034 cm-2 sec-1 • DIS region: • Q2 > 1 GeV2 • W* > 2 GeV • ps < 100 MeV/c • θpq > 110° • x*max = 0.80 • W* > 1.8 GeV: x*max = 0.83 E12-06-113

42. Summary (I) • Measured absolute cross sections for D(e,e′p-p)p reaction over a wide kinematic range, 1.1<W`<2.8 GeV, 0.13<Q2<4.5 GeV2, full range of cosq* and f*. • We see qualitative consistency in most bins between our results and model predictions. In most bins, the pRTPC and pCLAS channels are consistent. • Huge increase in available data points (about 5000) for g* n  p- p. Currently only ~900 points available in the world database. BoNuS data will be used to improve our understanding of neutron structure, as part of fits to world data (SAID, MAID…). • Systematic error is estimated to be ~15% (not include FSI). • Plan to repeat this analysis using other deuteron data, i.e. CLAS E6 data. Will also use BoNuS 12GeV data to increase statistics.

43. Summary (II) • We have measured F2n on a “free” neutron target • No effects from Fermi motion and final-state interactions • No evidence for off-shell structure for ps<100 MeV/c • F2n/F2p behaves at high x much like CETQ high-x fits • F2n resonance data will significantly improve the world data set, which up to now came from d with nuclear corrections

44. Backup Slices

45. Motivation • Purpose: In order to understand the structure of the nucleon (neutron and proton) we need to study the excited states (resonances). • We have a lot of data on the proton but almost nothing on the neutron – both are needed for a complete understanding. • Strategy is to use pion production: g* + n p p- • We have some real photon data, almost no electroproduction (virtual photon) • Difficulty: No free neutron target, need to use deuteron instead.

46. Deep Inelastic Electron Scattering • Provided first experimental evidence for the “partonic” structure of the nucleon • As we increase the momentum transferred to the target, the resolving power increases Q2 > 1 GeV2, W > 2 GeV e- e- * q q N Deep Inelastic Scattering Need to know this ratio to extract structure function information from inclusive processes (Rosenbluth separation)

47. Resonances predicted by Quark Model vs. experimental measurements Full lines: tentative assignment to observed states Dashed lines: so far no observed counterparts. From http://pdg.lbl.gov/2009/reviews/rpp2009-rev-quark-model.pdf

48. Heisenberg Uncertainty Principle: short life time  wide energy Measured from the decay products Resonances 6 families, depending on quark content (Isospin): N*(1/2), D(3/2),L(0), S(1), X(1/2), (0) • +N  p + N N*, D Resonance, combination of u and d only Notation: L2I2J L: orbital angular momentum I: Isospin J: total angular momentum J = L+S S,P,D,F,G,H  L= 0,1,2,3,4,5 I=1 I=1/2 Possible for both N and D • +N  h + N I=0 I=1/2 Possible for N not D

49. Resonances examples

50. , Unpolarized virtual photon cross-section of g*+n  p-+p Degree of transverse polarization: Exclusive p- Cross Section