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Understanding η Background in Coherent Photoproduction: A Study by David Lawrence

This presentation by David Lawrence at JLab in November 2009 delves into the background effects associated with η photoproduction in the PrimEx experiment. The focus is on coherent production being understood while addressing the challenges posed by inclusive h production that may introduce undetected particles in the final state. Lawrence uses Pythia to model these background effects, examining issues such as photon energy incidents, cross-section measurements, and the impact of cut parameters. The findings suggest discrepancies in event rates compared to predictions and emphasize the importance of refining criteria to enhance experimental feasibility.

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Understanding η Background in Coherent Photoproduction: A Study by David Lawrence

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  1. hPhotoproduction David Lawrence, JLab Nov. 20, 2009 PrimEx eta background - David Lawrence

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

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

  4. Elasticity and Invariant mass cuts Aram Dave PrimEx eta background - David Lawrence

  5. Number of photons Incident on FCAL PrimEx eta background - David Lawrence

  6. Elasticity for h n.b. exclusive channel excluded PrimEx eta background - David Lawrence

  7. 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 PrimEx eta background - David Lawrence

  8. Cross-section vs. elasticity cut n.b. Primakoff-only cross-section for 0-0.5o is 0.58nb total cross-section for 0-0.5o is 0.72nb From Aram’s numbers for reconstructed h’s: Corresponds to: q< 8o <6s cut (Allowing only 2g in FCAL reduces this to ~ 6.5nb) q< 0.5o >8s cut PrimEx eta background - David Lawrence

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

  10. Summary • Analysis of pythia generated events confirms Aram’s initial finding that events introduce forward-going h’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 (q<0.5o) • 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 h’s PrimEx eta background - David Lawrence

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