Measurement of Exclusive  - Electro-production from the Neutron in the Resonance Region Jixie Zhang Physics Department Old Dominion University 11/30/2006

J.Zhang, ODU Outline Motivation Experiment Progress During the Past Year Bonus Geant4 Simulation Outlook

J.Zhang, ODU Motivation Purpose: Study neutron resonances (excited states of the neutron) Strategy: Study the n  p  - reaction channel by tagging the spectator proton in the BoNuS detector Difficulty: No free neutron target, need to use deuteron instead.

J.Zhang, ODU 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 - even with perfect knowledge of the wave function information is lost

J.Zhang, ODU Solution Conservation of energy and momentum allows one to access “ free ” neutron! Electron scattered from the neutron of the deuteron, detect the spectator proton. npnp  p psps e` e  spectator proton, detected by BoNus detector

J.Zhang, ODU quasi-free This final state interaction is small for low momentum p s and has a very strong angular dependence. primary background Final States Interaction D(e,e ’  - p s )p process

Calculations by J.M.Laget  r = angle between p s and q R = ratio of total to quasi-free cross section for D( ,  - p)p R R momentum of p s

J.Zhang, ODU Strategy Detect p s in BoNuS detector and look at the cross section for the D(e,e ’  - p s )p process as a function of  R. If it is flat, we can assume negligible contribution from FSI. Extract n  - p cross section. Do this for bins in spectator proton momentum and look for the onset of FSI effects.

BoNuS Experiment (E03-012) BoNuS = Barely off-shell Nucleon Scattering Title: The Structure of the Free Neutron Via Spectator Tagging Data run taken from Sep. to Dec. in 2005 Experiment setup: Recoil protons are detected by BoNus Radial Time Projection Chamber (RTPC) detector; scattered electrons and other final state particles are detected by CEBAF Large Acceptance Spectrometer (CLAS) Data analysis is ongoing

J.Zhang, ODU CEBAF Large Acceptance Spectrometer (CLAS) CLAS Torus e-e- backwards p

J.Zhang, ODU BoNus RTPC Detector Thin-walled gas target (deterium, 7 atm., room temperature) Radial Time Projection Chamber (RTPC) with Gaseous Electron Multipliers (GEMs) Detect low energy protons (between 66 and 250 MeV/c) 3-Dimension: Drift time (R) + 2D channel position (3200 channels in total)

J.Zhang, ODU Bonus Detector

Progress During the Past Year CLAS Analysis 1.Quality Checks 2.Electron Identification(cuts) Cerenkov Cut Calorimeter Cut 3.Beam Line & Vertex Correction 4.EC Time Calibration RTPC Analysis 1.Geant4 Simulation

J.Zhang, ODU Quality Checks for D(e, e´)X El_Ratio is the number of D(e, e´)X counts normalized to the beam charge during a 20 seconds time interval (we call this time interval as an EPICS event) 4 sigma cut

J.Zhang, ODU Quality Checks for D(e, e´)X 4 sigma cut

J.Zhang, ODU Quality Checks Sector by Sector

J.Zhang, ODU Electron Identification: Cerenkov Cut Nphe: the number of photoelectrons from the Cerenkov detector. Cut: if momentum less than 3.0 GeV/c, we require nphe>1.5; otherwise require nphe>1.0. nphe for negative particles Nphe x 10 Good data region

J.Zhang, ODU Electron Identification: Cerenkov Cut Cc_c2: the angle (in radian) between the CC hit and the nearest SC hit. We require 0<cc_c2<0.08 Good data region

J.Zhang, ODU Electron Identification: Electromagnetic Calorimeter Cut We require E_total>0.10 and E_inner>0.06. Good data region

J.Zhang, ODU Beam Position CLAS detector reconstruction software assumes that the beam is on the z axis. This is not true! The beam position is very crucial in BONUS experiment since it has a long target and the reconstructed momentum is deeply dependant on position and angle! Beam comes out of page

J.Zhang, ODU How to Find Beam Position? Coefficients a and b will tell you how much is the angle between the beam line and the z axis: tan  = square root of (a 2 + b 2 ) Fit for true beam position by minimizing the vertex z difference of 2 particles (scattered electron and the 2nd particle) in a event Beam position is determined by a linear function of z: X = x0 + a (z + z0); Y = y0 + b (z + z0); where z0 is the center of Bonus detector at CLAS coordinate. I choose z0 = mm.

J.Zhang, ODU Beam Line Position a and b are not zero, therefore the beam line is tilted: tan  = square root of (a2 + b2)   0.16 degree For a long target of 200 mm, this is none neglectable! Coef a Coef b

J.Zhang, ODU Beam Line Position X X0 is the beam position at vertex z0 = mm, from my fit I showed above X_2H01 is the beam position measured by CLAS at position “2H01”( a few meters in front of the entrance window of Bonus detector) X0 X_2H01

J.Zhang, ODU Beam Line Position Y Y0 is the beam position at vertex z = mm, from my fit I showed above Y_2H01 is the beam position measured by CLAS at position “2H01” Y0 Y_2H01

J.Zhang, ODU Vertex Correction Result Using new beam position to correct the vertex z for all particles

J.Zhang, ODU Vertex Correction Result (2) Red:after Black:before

J.Zhang, ODU EC Time Calibration Why? EC time is very critical since it is used to identified neutral particles. How? Neutral particles do not have too much signal in Scintillate Counter (SC), therefore we must choose a particle which will give enough signal both in SC and EC to calibrate the time information. The scattered electron is the best choice. Calibrate EC time against SC time for electron by minimize the difference between them.

J.Zhang, ODU EC Time Calibration Result Before(left) and After(right) EC time calibration ECt -SCt

J.Zhang, ODU Geant4 Simulation for Bonus Detector 1.Overview 2.Momentum threshold 3.Momentum correction 4.Future work

Detector Overview

Detector Overview(1) Beam View Top View

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

J.Zhang, ODU Minimum Detected Proton Momentum Proton

J.Zhang, ODU Momentum Correction How? 1.In my simulation, I know both generated and reconstructed momentum. So I can make a correction. 2.Momentum depends on reconstructed radius, theta, and z. 3.Generate a momentum map by fitting the momentum for each bin of reconstructed radius, theta and z. Use this map to correct the momentum(energy). Reconstructed momentum Generated momentum

J.Zhang, ODU After Momentum Correction

J.Zhang, ODU Future work 1.Create BOS format output which can be read by CLAS reconstruction software and CLAS geant3 simulation code. 2.Integral Bonus simulation( Geant4 ) with CLAS simulation( Geant3 ) 3. Study the Bonus experiment in detail by comparing the simulation data to the real data 4. Acceptance calculation, and so on.

J.Zhang, ODU Outlook Data calibrations are still on the road. Identify the  - in CLAS. Study the n  p  - channel. Study the background. Study the FSI effects. Lot of jobs need to be done…

Thank You! Question?

J.Zhang, ODU Kinematics q 2 = -Q 2 = (  2 – q 2 ) = 4  i  f sin 2 (  e /2). W = (m n 2 + 2m n  - Q 2 ) 1/2

J.Zhang, ODU Electron Cut Result

J.Zhang, ODU Bonus Detector