Exclusive   Electro-production from the Neutron in the Resonance Region Jixie Zhang Dissertation Defense Old Dominion University March 4th, 2010

Outline Who? ……………...BoNuS Collaboration When? …………… – present What (goal)? …….... Study neutron resonances Why (neutron)? How? Theory support Experiment setup Data Analysis Result Summary

History of Nuclear Physics

Resonances predicted by Quark Model vs. experimental measurements From Full lines: tentative assignment to observed states Dashed lines: so far no observed counterparts.

Resonances 6 families, depending on quark content (Isospin): N* (1/2),  (3/2),  (0),  (1),  (1/2),  (0) N*,  Resonance, combination of u and d only Notation: L 2I2J 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  +N   + N Possible for N not  Heisenberg Uncertainty Principle: short life time  wide energy Measured from the decay products I=1 I=1/2  +N   + N I=0 I=1/2 Possible for both N and 

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

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.

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   Production Kinematics  n   - p  * = polar angle of the outgoing   in C.M. frame  * = Azimuthal angle of the outgoing   in C.M. frame, q ** **  n p Hadronic plane E, k Leptonic plane

Exclusive   Cross Section Unpolarized virtual photon cross-section of n    +p Degree of transverse polarization:,

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′ e npnp  p psps  d

d p  psps n quasi-free primary background Possible Background d p 00 p p  d p p n  p d p  p n   p rescattering pp rescattering ep scattering primary background

FSI Prediction for D( ,   p)p R = ratio of the total to the quasi-free cross section  R = polar angle between photon and spectator proton P R = momentum of the spectator proton quasi-free  under  p rescattering peak region only

Model Independence?! How? Final State Interactions Off-shell Effects VIP s Select low P s and large backward  pq (angle between Ps and virtual photon) to minimize FIS. Off-shell effects are negligible for small P s. Choose P s <120 MeV/c as Very Important Spectator Protons (VIP) C. Atti et al, Eur. Phys. J. A 19, (2004). W. Melnitchouk et al, Phys. Lett. B377, 11 (1996).

Jefferson Lab Experiment E Barely off-shell Nucleon Structure (BoNuS) Electron beam energies: 1.1, 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

Continuous Electron Beam Accelerator Facility (CEBAF)

CLAS in Jefferson Lab, Hall B

Radial Time Projection Chamber (RTPC)

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

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

Cuts and corrections used in this analysis 1.Data calibrations and Quality Checks 2.Vertex z and theta-z cut 3.Exclusive electron identification (without E_tot/p cut) 4.Apply energy cut due to EC threshold for electrons 5.Fiducial cut for trigger electron,   and proton 6.Energy loss correction 7.Acceptance correction 8.Background subtraction 9.Radiation correction, using one single value (0.81) for all 10.Particle detection efficiency for e ,  , proton and RTPC proton

Fiducial Cut Fitting Function Fiducial cut is the geometry cut which is used to remove inefficiency part of the detector. Momentum dependent Electron:  = 0.35, Pion:  = 1.0 Ptoton :  = 3.0 Fitting function:

New Fiducial cut for e 

Fiducial cut for  

Fiducial cut for proton

Energy Loss Correction for CLAS Particles Particles loss energy when going through matter; Binned by measured f, q, z, p; Based on simulation, fit ‘P 0 -P vs P’ with an integral Bethe-Bloch function for all particles Pr ee   P 0 -P measured (Gev/c) P measured (Gev/c)

Energy Loss Correction for RTPC Proton

Missing Mass Cut:  5 GeV Missing Mass is the mass needed to account for the energy and momentum conservations laws. E 2 = P 2 + M 2  * + n    + X X =  * + n -   E x =E  + E n – E  ; P x =P  + P n - P  M x 2 =E x 2 – P x 2

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.Using my PID routines to identify the  , if not found, use a negative non-trigger particle as   4.Using my PID routines to identify protons, if not found, use a positive particle as proton 5.Apply vertex and energy loss corrections 6.Apply 2-  missing mass cut

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 can be done with simulation? Help to design the detector and choose the best configurations of HV and Drift Gas Debug/optimize reconstruction code for RTPC Generate energy loss correction tables, radiation length tables Detector’s acceptance and efficiency study, model the background…

D(e,e   p)p Acceptance Correction Binning information: W: 150 MeV each bin, [1.15,3.10); Q 2 : 6 bins, {0.1309, , , , , , }; 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 generated events in the same bin. Need to generate tables for D(e,e   p RTPC )p and D(e,e   p CLAS )p reaction separately. Applied event by event Need to have acceptance cut to ensure the reliabilities For CLAS channel: and less than 10% uncertainty (> 100 detected events) For RTPC channel: and less than 15% uncertainty (> 40 detected events)

Acceptance Correction, E=5.3 GeV

Background Subtraction f 1, f 2, f 3 and f 4 are the fitted background fraction functions for real data. f 1 sim, f 2 sim, f 3 sim and f 4 sim are the fitted background fraction functions for simulation data. Final background correction factor: where is the coverage of 2  in a gaussian distribution. Simulation average

Background of Simulation Data

Background of Real Data

CLAS trigger efficiency Trigger efficiency is the fraction that the trigger particle (electrons) is detected. It is obtained by scaling the simulation data to the real data, then calculating the ratio of real events counts in each p-  grid to the simulation events counts in the same grid.

CLAS proton efficiency

RTPC proton efficiency

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

Charge After Stopper Correction Target Contamination

Time integrated Luminosity

Cross Section Calculations 1.Select exclusive events 2.Apply corrections: acceptance, trigger,  , CLAS proton and RTPC proton efficiency, radiative and background correction.

Cross Section Fitting A0 A1 Cos  *A2 Cos2  * = ++

Cross Section: BoNuS Vs MAID and SAID 4 GeV, W = Cos    MAID 07SAID 08 D(e,e   p CLAS )pD(e,e   p RTPC )p

Cross Section: BoNuS Vs MAID and SAID 2 GeV, W = Cos    0.875

BoNuS Vs Models, 2 GeV, W = 1.23 MAID 07SAID 08 D(e,e   p CLAS )pD(e,e   p RTPC )p

BoNuS Vs Models, 5 GeV, W = MAID 07SAID 08 D(e,e   p CLAS )pD(e,e   p RTPC )p

BoNuS VIP Vs MAID, 5 GeV, W = VIP = o D(e,e   p RTPC )p VIPD(e,e   p CLAS )p VIP

A 0 : BoNuS VIP Vs MAID, 2 GeV W

A 0 : BoNuS VIP Vs MAID, 4 GeV

A 0 : BoNuS VIP Vs MAID, 5 GeV

Summary Measured absolute cross sections for D(e,e’pi-p)p reaction over a wide kinematic range. Huge increase in available data for neutron channel These data will be used to improve our understanding of neutron structure, as part of fits to world data (SAID, MAID…) Our results are very sensitive to the acceptance correction, which must be carefully checked. In most bins, the p RTPC and p CLAS channels are consistent. We see qualitative consistency in most bins between our results and model predictions. The VIP data (low p s and large  pq ) are mostly consistent with the full data set. We look forward to working with I. Strakovsky (SAID) and others to incorporate these data into the world database.

Thank you!

Resonances examples

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.

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

Vertex Cut

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)

CLAS Particle Identifications 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 

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

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

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 , and d  distributions. H. Fenker, et.al. Nucl.Instrum.Meth. A592: ,2008

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

Sim Vs Real Real 5G Simulation 5G

Beam Energy and EC Threshold Cut Using the nominal beam energy: , , and GeV, for 1, 2, 4, and 5 passes data respectively. Run Period EC Threshold (mv)Energy Cut (MeV/c) ~ ~ ~ Apply EC threshold cut to trigger electron

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 K+K+ ee   P 0 -P measured (Gev/c) P measured (Gev/c)

Missing Mass Cut Beam Energy MM Cut of D(e,e`   p)p RTPC MM Cut of D(e,e`   p RTPC )p 2.x / / x / / x / / GeV