A Measurement of the Bethe-Heitler PairProduction Asymmetries and Positronium

ProductionLetter of Intent to the HIgS PAC

June 9, 2011

Spokespeople:G. Ron (HUJI)D. Higinbotham (JLab)B. Sawatzky (JLab)B. Norum (UVa)

Bethe-Heitler Like Processes

& Interfere with

&

Which is the T-reverse for

1st order

Born

2nd order

BornCompton

DVCS

So WHAT?• The interference manifests itself as an asymmetry in

the e+/e- pair. Either energy (equal angle) or angle (equal energy) should work.

• For example, to 1st order, the asymmetry in energy is:

Bethe-Heitler cross section is actually suppressed for E(e+)=E(e-) Beth-Heitler

Calculation for 12C

These Asymmetries are NOT Small

Theory300-550MeV

On Lead12C on Rho

Resonance

But not well measuredBrodsky, et al. Phys Rev 173 (1968) 1011

Brodsky, et al. Phys Rev 187 (1969) 1899

Asymmetry Physics• Measure Real part of Compton amplitude:

• At high energy measures the J=0 fixed pole.• Corrections to forward Compton amplitude

• (which is an input to GDH sum rule).

• Related to 2g in p(e,e’p), potentially easier to measure than the asymmetry in p(e±,e±p) (OLYMPUS/JLab 2-photon exchange).

• Analogous to the QCD pomeron/odderon interference.

Positronium Physics• Sometimes the e+/e- pair is created in a bound

state (positronium).• Test production of positronium,• Disassociation analogous to pion diffractive

dissociation (Brodsky).• Positronium is relativistic, effectively on light cone.

• Light-front wavefunctions modern method for QCD/QED calculations.

Courtesy of S. Brodsky

Courtesy of S. Brodsky

Courtesy of S. Brodsky

Courtesy of S. Brodsky

Courtesy of S. Brodsky

Courtesy of S. Brodsky

Experimental Method

• Positronium atoms disassociate on thin downstream foil.

• Saskatchewan Pair-Spectrometer magnet + VDC/MWDC.

• Beam time request: 21 Calendar days

• Bethe-Heitler and Positronium production using HIgS real photon beam on different targets (p for GDH, Heavy targets for higher cross section).

MANPOWER &Future Plans

• Lead institutes: Hebrew U., JLab, UVa.• Theory support: Budjer Institute, SLAC.• Large interested collaboration.• Most of the experimental apparatus is already available now.

• Magnet, tracking chambers, DAQ exist now.

• Plans underway to develop target ladder design and necessary support structures.

• Continued work on simulation, optics, and background studies.

• Will return with full proposal at next HIgS PAC