Cross Section of Exclusive   Electro-production from Neutron Jixie Zhang (CLAS Collaboration) Old Dominion University Sep. 2009

2 Exclusive   electro-production Detect e`,   and at least ONE of the two final state protons in D(e,e `   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. Low momentum spectator proton Novel approach by the BoNuS collaboration: detect the spectator proton directly. e` e npnp  p psps  d

3 Q 2 = -(q µ ) 2 = 4  sin 2 (  e /2) W 2 = (q µ + n µ ) 2 = (q µ + d µ - p s µ ) 2 = (  µ + p µ ) 2 Exclusive   Production Kinematics  n   - p ,q ** **  n p Hadronic plane E,kE,k E,k Leptonic plane  * = polar angle of the emitted negative pion in C.M. frame  * = Azimuth angle of the emitted negative pion in C.M. frame

4 Exclusive   Cross Section Unpolarized virtual photon cross-section of n    +p == Degree of transverse polarization: Converted to W’, Q 2 frame:

5 d p -- psps n quasi-free d p 00 p p -- This final state interaction is small for low momentum p s and has a very strong angular dependence. primary background Primary Background

6 Model Independence?! How? Final State Interactions Binding Effects VIP s Select low P s and large backward  pq (angle between Ps and virtual photon) to minimize FIS. Binding effect is negligible for small P s. Choose P s <120 MeV/c as Very Important Spectator Protons (VIP s )

Jefferson Lab Experiment E Barely 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 D 2 gas, 20 cm long Data were taken from Sep. to Dec. in 2005

8 CLAS in Jefferson Lab, Hall B

9 Radial Time Projection Chamber (RTPC)

10 Helium/DME at 80/20 ratio Sensitive to protons with momenta of MeV/c 3 layers of GEM foils 3200 pads (channels) 5 Tesla B field Particles ID by dE/dx 140 µm 3-D tracking: time of drift -> r pad position -> , z dE/d X 7Atm. D 2 gas target, 20 cm in length Trigger Electron BoNuS RTPC Detector

11 The Drift Path of An Ionized Electron The red lines show the drift path of each ionization electron that would appear on a given channel. In green is the spatial reconstruction of where the ionization took place. In reconstruction, hits which are close to each other in space are linked together and fit to a helical trajectory. This resulting helix tells us the vertex position and the initial three momentum of the particle. A MAGBOLTZ simulation of the crossed E and B fields, together with the drift gas mixture, determines the drift path and the drift velocity of the electrons.

12 FC RTPC Sits in the Center of CLAS CLAS BoNuS RTPC stopper

Analysis Outline 1.Quality checks 2.Target thickness analysis (N b ) 3.Beam charge analysis (q) 4.Kinematic corrections CLAS and RTPC PID, RTPC Gain Calibration and Drift Velocity Calibration RTPC Energy Loss Correction CLAS Energy Loss Correction Beam Line and Vertex Correction 5.Simulation Overview 6.Acceptance and efficiency analysis 7.Background subtraction 8.Cross section (  )

14 Run Selection: Quality Checks 4 GeV5 GeV Without Stopper Bad Runs

15 Target Thickness Calculate the effective target pressure for a data set by weighting the pressure of each run with the number of good electrons in that run.

16 Charge After Stopper Correction Without Stopper

17 CLAS Particle Identification Negative non-trigger particles:  above purple curve   (2)Among the rest,  between purple and light blue curve K  (3)Among the rest,  below light blue curve anti-proton Positive particles:  between light blue and green curve proton  below green curve Deuteron (3)Among the rest,  above purple curve   (4)Among the rest,  between purple and light blue curve K 

18 RTPC Proton Identification proton Deuteron Helium dQ/dx protons Nuclei Real data

19 RTPC Gain Calibration Channel by Channel gain multipliers can be determined for each run by comparing the track’s expected energy loss to the measured value. After applying the gain corrections, a clear separation of protons and heavier particles through dE/dx has been achieved. Gain constants (vs phi and z) determined independently for two different runs. Before and After Gain Calibration

20 Energy Loss Correction for RTPC Proton

21 RTPC Drift Velocity Calibration Trigger electrons measured by CLAS are compared to the same electrons measured in BoNuS during High Gain Calibration runs. The drift velocity parameters are chosen which best improve the centroid and width of the dz, d θ, and d φ distributions. H. Fenker, et.al. Nucl.Instrum.Meth. A592: ,2008

22 Energy Loss Correction for CLAS Particles Based on simulation Using uniform integral Bethe-Bloch function to fit ‘P 0 -P vs P’ for all particles Binned by measured , , z, p Pr Kaons ee   P 0 -P measured (Gev/c) P measured (Gev/c)

23 Z el -Z 2 (mm) Sector 2 Sector 6 Sector 4 Sector 1 Sector 3 Sector 5 Z el -Z 2 (mm) Before and After Vertex Correction Using real data, find the best beam position to minimize the vertex difference among coincident particles

24 Missing Mass Cut

25 Background Subtraction (1)

26 Background Subtraction (2)

27 Simulation Overview + Evgen (fsgen or other event generators)  RTPC (BONUS)  CLAS(gsim)  Gsim Post Processing (gpp)  Reconstruction (user_ana)  Skim  Higher Level Simulation Ntuple What I have done using simulation? Help to design the detector and choose the best configurations of HV and Drift Gas Debug/optimize reconstruction code of RTPC Generate energy loss correction tables, radiation length tables Detector’s acceptance and efficiency study

28 RTPC Proton Threshold Momentum Proton

29 17X 13X 5X 100X 50X Exclusive Simulation Status Generated 5 GeV exclusive events with the following distribution: Flat W Q 2 with shape 1/x 2.7 t with shape -1/e 1.3 Flat  *

30 Sim. Vs Real Real 5G Simulation 5G Sim. 5G

31 D(e,e   p)p Acceptance Correction Binning information: W: 120 MeV each bin, [1.10,3.5); Q 2 : 6 bins, { , , , , , , }; cos  *: 8 bins, 0.25 each bin, [-1.0,1.0);  *: 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 simulated events in the same bin. Need to generate tables for D(e,e   p RTPC )p reaction and D(e,e   p CLAS )p reaction separately. Applied event by event, or bin by bin if data analysis binning is identical to acceptance binning.

32 Acceptance Correction, E=5.3 GeV

33 Acceptance Correction, E=5.3 GeV

34 Exclusive   Events Selection 1.Select good trigger (scattered electron) q<0, CC + Osipenko (CC geometry match cut) EC (Ein>0.06, without Etot/p Cut) Theta_Z_Cut 2.Vertex Z correlation cut |z_el –Z_i| < 2.71 cm, I could be  , fast proton or RTPC proton. 3.By default using my PID routines to identify pions, if no pion found, use a negative non-trigger particle as   4.Using my PID routines to identify protons, if no proton found, use a positive particle as proton 5.Apply kinematics corrections 6.Apply 2-  missing mass cut 7.Apply background subtraction 8.Apply acceptance and efficiency correction

35 Cross Section Fitting

36 Cross Section Fitting

37 Cross Section: BoNuS Vs MAID Q2Q2 Cos  *  TT  LT  T     L

38 Cross Section: Ps 120  T     L

39 Thank you!

40 Cross Section: BoNuS Vs MAID

41 Cross Section: Ps 120

Summary The BoNuS RTPC detector, together with CLAS, allows us to study the neutron resonances clearly We have taken data for D(e,e   p)p over a large kinematic range in  * and  *, Q 2 and W We presented preliminary results for the   p invariant mass showing resonance structure Additional analysis underway for cross sections

43 Meson spectrum, E = 5.3 GeV, Acceptance and momentum not corrected yet PRELIMINARY D(e,e   p rtpc p CLAS )X      

44 Data on the proton - nothing comparable exists for the neutron CLAS p(e,e   )n

45 Electron Identification rtr 1.Negative charge. 2.Number of photo electrons (Nphe) > E_inner > 0.06 and 0.016*P+0.15 < E_total/P < Pass Osipenko cut (geometry matching between SC and CC)

46 Proton and Kaon Identification I am using dt to identify particles which have heavier mass than the mass of a pion. Good protons (kaons): plot the dt distribution for proton (kaons) candidates by setting m equal to the known mass (0.938 GeV or GeV), then fit it with a gaussian function. All candidates under the 2 sigma cut will be good proton (kaons).

47   Identification Define the upper limit and lower limit for dt/t, where Upper_limit = 90% * f(m=0.494 GeV) Lower_limit = 35%* f(m= MeV). All candidates locate in between these 2 limits are good pions. dt/t Vs p 2 for   candidates   candidates: Negative charge. Can not pass good electron cut Can not pass good K  cut

48 Quality Checks in epics event level 4 sigma cut El_Ratio is the number of D(e, e´)X counts normalized to the beam charge during a EPICS event (20-seconds).

49 Baryonic resonances, D(e,e   p)p E = 5.3 GeV, No acceptance correction yet PRELIMINARY N(1520)D13, N(1535)S11  (1620)S31, N(1650)S11 N(1675)D15, N(1680)D15  (1700)D33, N(1710)P11 N(1720)P13  (1232)P33 Invariant Mass of   and fast proton

50 Definition Differential Cross Section: the probability to observe a scattered particle in a given quantum state per solid angle unit Integral (Total) Cross Section: the integral of the differential cross section on the whole sphere of observation (4π) Luminosity: the number of particles per unit area per unit time times the number of the target Integrated Luminosity: the integral of the luminosity with respect to time.

51 Sim Vs Real Real 5G Simulation 5G

52 Cross Section Fitting

53 Cross Section: BoNuS Vs MAID

54 Cross Section: Ps 120

Detector Overview(2) Generate a proton of 70 MeV/c momentum, Z=0,Theta=90 degree, Phi=0. Beam View (left) and Top View(Right) Blue color means the detector layer has energy deposited.

Detector Overview(3) Proton:Momentum 180 Mev/c, Theta=90, random Z and Phi

57 RTPC GEM 50 µm 5 µm 55 µm 70 µm        

58 Run Selection: Quality Checks Procedure: 1.Remove known bad runs 2.Group data by the following Beam energy Beam Tune Existence of stopper 3.Plot N_el/Charge for each run in the group, then fit them with gaussian function 4.Choose 3 sigma cut, go to CLAS_ONLINE logbook to check each run which is close to the cut position. Adjust the cut position as needed.

59 Charge Calculation Difficulty: we used stopper to protect Faraday cup, which affect the fcup reading with a ‘stopper penetration efficiency (  )'. R is the ratio of charge measured by Fcup to BPM current. Life-Time Gated charge for with and without stopper runs:

60 Charge calculation Procedure: 1.Remove bad runs using qulity check result 2.Group data by the following Beam energy Beam Tune Target type Existence of stopper 3.Get the mean R (ratio of Fcup/Current) for each run 4.Fit R with a constant value for each data set 5.Verify the result by looking into N_el/charge time history

61 Energy Loss Correction for CLAS Particles Based on simulation Using uniform integral Bethe-Bloch function for all particles Binned by measured , , z, p Top: before correction Bottom: after correction Pr K+ El  