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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