 Photoproduction David Lawrence, JLab Nov. 20, /20/091PrimEx eta background - David Lawrence

“  ” production channels Coherent production is “understood” and so provides a defeatable background The background of concern (at the moment) is from inclusive  production which has additional, possibly undetected, particles in the final state. This is currently modeled using Pythia. Multiple  ‘s from the same event that pass both the  mass cut and the elasticity cut are rare enough to not be a problem. (Accidentals from multiple events has yet to be looked at.) 11/20/092PrimEx eta background - David Lawrence

Incident Photon Energy Aram modified bggen to sample an incoherent spectrum I used the default bggen configuration which samples a coherent bremstrahlung spectrum Aram’s data set contained more than 10 times as many events as what I’ll be showing. 11/20/093PrimEx eta background - David Lawrence

Elasticity and Invariant mass cuts Aram Dave 11/20/094PrimEx eta background - David Lawrence

Number of photons Incident on FCAL 11/20/095PrimEx eta background - David Lawrence

Elasticity for  n.b. exclusive channel excluded 11/20/096PrimEx eta background - David Lawrence

Cutting on elasticity Error on elasticity can be calculated from errors on input photons and tagger energy For FCAL photons, I used: Errors from 2 photons added in quadrature to get total error on reconstructed energy Constant error of 0.1% used for tagger energy 11/20/09PrimEx eta background - David Lawrence7

Cross-section vs. elasticity cut  < 8 o  < 0.5 o <6  cut >8  cut From Aram’s numbers for reconstructed  ’s: Corresponds to: 11/20/098PrimEx eta background - David Lawrence (Allowing only 2  in FCAL reduces this to ~ 6.5nb) n.b. Primakoff-only cross-section for o is 0.58nb total cross-section for o is 0.72nb

Cross-section 11/20/099PrimEx eta background - David Lawrence Here is plotted the cross-section for exclusive  events as produced by pythia. It gets the right order of magnitude (roughly), but the angular distribution is more forward than current theory suggests.

Summary Analysis of pythia generated events confirms Aram’s initial finding that events introduce forward-going  ’s at a rate several times the predicted Primakoff cross-section. (A tighter elasticity cut may bring this down to only a few times larger.) Pythia also predicts exclusive cross-section several times the predicted nuclear coherent cross- section in the forward direction (  <0.5 o ) Veto-ing on charged particles or additional photons in the event may help Feasibility of experiment hinges on confidence in pythia’s angular distribution of  ’s 11/20/09PrimEx eta background - David Lawrence10