Electron- and Positron-proton elastic scattering in clas larry Weinstein

Electron- and Positron-proton elastic scattering in CLAS

• Larry Weinstein

• Old Dominion University

The Proton Charge Form Factor

How to measure Two Photon Exchange

Recap:

Positrons to the rescue?

JLab CLAS

CLAS in Maintenance Position

Experiment Features/Bugs

• Identical mixed e+/e− beam (250 pA each)

• Continuous beam energy distribution

• Wide Q2 and angle (ε) coverage

• Simultaneous cross section measurements

• Minimize systematic uncertainty
• Allows 1-2% measurement of e+/e− cross section ratio

• Reverse magnetic fields (chicane/Torus) to reduce acceptance effects

• Overdetermined ep kinematics allows background rejection

• Unprecedented photon luminosity (1000 times!)

Oct 2006 test run

• 3-Dipole chicane with photon blocker

• Remove material from tagger exit line

• Shield tagger exit line (bunker)

• Tagger dump cooling

• Lepton-beam fiber monitor (FIU)

• LOTS of new shielding and collimation

• Collimate after chicane
• Collimate before CLAS

• LOTS of modifications during the test run

2006 Test Run Results: Luminosity

• Maximum luminosity achieved:

• 80 nA 3.2 GeV electrons

• 0.5% radiator, 5% convertor

• Figure of Merit: 20 pA (80nA*0.5%*5%)

• Limited by drift chamber occupancies (3% limit)

• Region 1 DC occupancy 2.3%
• Occupancy entirely lepton-beam related
• Lepton beam too large - 80% of hits not target related
• Region 3 DC occupancy 0.7%

• Test run achieved 4% of proposal luminosity

• Luminosity and backgrounds agreed with simulations.

• Factor of 10-20 improvement on previous test runs!

How to do better

• Limitations:

• Target and heat exchanger apertures

• Moller electrons trapped by the CLAS magnetic field

• Background from the tagger vacuum box

• Background from other (diffuse) sources

• Limited beam monitoring

• Improvements:

• Reduce pre-CLAS collimator aperture from 6 to 4 cm

• Add minitorus and Moller catcher

• Shield tagger vacuum box (6 mm Pb)

• Large concrete pipe and disk shield

• Double target length

• Build a position sensitive removable beam-line calorimeter for beam characterization

New beamline shielding

Target shielding and Moller electrons

Luminosity vs time

2006 Test Run Data Analysis

Kinematic cuts:

Kinematic cuts II:

Kinematic Cuts III:

Data Analysis: Acceptance

Data Analysis: Kinematics

Data Analysis: Binning

Results: e+p/e-p double ratios

Test Run vs the World!

What next?

Summary:

• GEp measured by Rosenbluth and Polarization transfer expts differ by a factor of 3 at Q2 = 6

• Two Photon Exchange might explain the discrepancy

• The e+p/e-p ratio is the only way to measure the real part of the TPE amplitude

Summary:

• The TPE Engineering Test Run

• Produced a mixed identical electron/positron beam
• Validated beam line simulations
• Measured the e+p to e-p elastic scattering ratio at low Q2
• small systematic uncertainties

• The TPE experiment

• Will be able to achieve the proposed luminosity
• Will cover a wide kinematic range
• Will determine precisely the TPE contribution and resolve the GE problem
• Will be ready to run in 2010

• Anti-matter beams are cool!