Highest Q 2 Polarized Measurement of the Electric Form Factor of the Neutron – E Presented by Jonathan Miller For the Hall A E Collaboration 6/14/2008

Outline Collaboration Electric Form Factor of the Neutron Measurement Technique Apparatus used Charge Identification Missing Mass Quasi Elastic Selection Asymmetry Calculation Gen Calculation To Do Conclusion 2

Overview Introduction Electric Form Factor of the Neutron Quasi-elastic Process Selection New Techniques Asymmetry Calculation and Results Conclusions and Future 3

Collaborators Bogdan Wojtsekhowski – Spokeperson Gordon Cates – Spokesperson Nilanga Liyanage – Spokesperson Sergey Abrahamyan - Brandon Craver – Graduating Summer ‘08 Aidan Kelleher – Graduating Fall ‘08 Ameya Kolarkar – Graduated Winter ‘07 Jonathan Miller – Graduating Fall ‘08 Seamus Riordan – Graduated Spring ‘08 Tim Ngo – Masters graduated Summer ‘07 4

Collaborators NameInstitutionResponsibility Gordon CatesUniversity of VirginiaSpokesperson Nilanga LiyanageUniversity of VirginiaSpokesperson Bogdan WojtsekhowskiJefferson LabSpokesperson Robert FeuerbachWilliam and MaryPost Doc and Analysis Coordinator Current Students Sergey AbrahamyanUniversity of YerevanMonte Carlo, Shower and Analysis of Q 2 = 1.2 GeV 2 Brandon CraverUniversity of VirginiaDrift Chambers Aidan KelleherWilliam and MaryTarget and Analysis of Q 2 = 1.7 and 2.5 GeV 2 Jonathan MillerUniversity of MarylandNeutron Arm and Analysis of Q 2 = 1.7, 2.5, and 2.5 GeV 2 Graduated Seamus Riordan, PhDCarnegie Mellon E Post Doc Analysis Software and Analysis of Q 2 = 1.7 and 3.5 GeV 2 and Post Doc Tim Ngo, MSUniversity of CaliforniaNeutron Arm Geometry Ameya Kolarkar, PhDUniversity of KentuckyTarget 5

The Electric Form Factor of the Neutron Neutron Electron 6

Generalized Parton Distributions 7 Using GPDs, the ratio of the up and down flavor components of the Dirac form factor are constrained by a measurement of the Sachs electric form factor. A smaller value of G e n relates to a smaller value of the ratio.

Polarized helium 3 acts as a polarized neutron target. Recoiling neutron and electron are detected in coincidence. 8

What are we using to make the measurement? CEBAF provides a polarized (83%) electron beam. The Target provides high polarization throughout the experiment (45-50%). Big Bite provides the trigger, and selects for scattered electron events. The Neutron Arm provides selection of quasi-elastic events (using time of flight and hit location) and charge identification. 9

How are we measuring G E n ? 10 Coefficients averaged over acceptance.

Experiment Overview Beam Energy (GeV)Q 2 (GeV 2 )Neutron Momentum (GeV/c)Flight Path (m) Challenges: High Q 2 High rate in the detectors Determination of charged Due to large fringe magnetic field caused by Big Bite 11

Experiment Observations The time spectrum for quasi-elastics currently has a  <500 ps. Even clean events (like the one pictured) still contain many hits not relating to the event. Most events are much messier. Neutron Arm Event – Shows hits within the neutron arm (veto and neutron detectors). 12

Charge Identification Adjust minimum amplitude to remove lower energy hits. Removes accidentals in the dead region (up to 110 ns) and in the coincidence region. Using a value of 200 safely removes accidentals. Veto Rate is accidental rate per paddle (average). 13

Backup Slides How the rates change in the veto detectors at different cuts on the amplitude. Placing a cut on the amplitude in the veto of 200, removes accidentals both at the time of the coincidence, and hits coming in before the event in the neutron bars. The threshold was set to be inclusive in hardware, however, restricting amplitude serves the same function (Excluding computer dead time) as having a higher threshold. We can adjust the threshold to not consider lower energy hits as hits. Large cuts on missing mass are needed at Q 2 = 3.4 GeV 2 to remove pion electroproduction. These cuts are still necessary at Q 2 = 1.8 GeV 2. The cuts used are for a missing mass of 2 GeV. 14

Quasi-Elastic Selection Main selection of Quasi-elastic Events is via Time of Flight Perpendicular Momentum Invariant Mass For quasi-elastics,  of the Time Spectrum is 500 ps for Q 2 = 1.7 GeV 2. 15

To select quasi-elastic neutrons, time of flight and missing q perpendicular is used in addition to the invariant mass. After the time of flight and q perpendicular cuts are in place, the familiar missing parallel and perpendicular momentum. The Time of flight sigma is 400 ns (for elastics) for Q 2 = 1.8 GeV Run 4427(Elastic)

Accidental Background Technique Green is region to determine the ratio of charged to uncharged. Red is the region used to select (quasi)elastics. Blue is the region used to determine total background counts. To determine the ratio of charged to uncharged events, events far from the quasi-elastic region are used so that they are not effected by the quasi-elastic events. 17

Missing Mass Large amounts of data need removed at Q 2 = 3.5 GeV 2 to remove pion electroproduction. These cuts are still necessary at Q 2 = 1.7 GeV 2. The cuts used are for a missing mass of <=2 GeV. 18

The three boxed regions are those that are used to determine background events (blue), those that are used to determine background ratio (red), and those that are used to determine (quasi)elastic events (green). The background ratio of charged to neutrals are determined far from the quasi-elastic hit location. The Number of background is determined by shifting the time window close to the quasi-elastic selected region. Calculate proton asymmetry based upon proton form factor ratio. Account for background. Account for proton to neutron conversion via dilution factors. Account for corrections in the background and conversion corrected asymmetry. 19

Asymmetry Calculation To determine the physical asymmetry: – The accidental background is subtracted from the raw asymmetry. – Proton physical asymmetry is calculated from the known proton form factors. – Proton to neutron conversion between the target and the detector is accounted. – Various other dilution corrections are included in the Table below. CorrectionsValue Beam Polarization83.5% ± 1.1% Target Polarization48.7% ± 2% Neutron Polarization86% ± 2% Nitrogen Dilution94.3% ± 0.9% 20

G E n Calculation NameQ 2 = 1.7 GeV 2 (± Sta ± Sys)Q 2 = 3.5 GeV 2 (± Sta ± Sys) Raw Asymmetry ± ± Number of QE Q2Q Physical Asymmetry ± ± ± ± Lambda (form factor ratio) ± ± G E n (not including FSI) ± ± ± ±

Results 22 Preliminary

To Do Final State Interaction calculations in progress. Implement Ole Hansen’s new Big Bite tracking code. Finalize Target, Big Bite and Neutron Arm calibrations. Results for Q 2 = 1.2 and 2.5 GeV 2. Improve Monte Carlo to account for Pion Electroproduction. Members of the analysis group will be working to have published all kinematics by early

Conclusion Preliminary values of G E n for Q 2 = 1.7 and 3.5 GeV 2 achieved. The Q 2 = 3.5 GeV 2 point shows the discriminating power of our results, and suggests a reevaluation of our understanding of GPDs and the orbital momentum of the quarks. 24 Preliminary