Abstract
The η meson provides a laboratory to study isospin violation and search for new flavor-conserving sources of C and CP violation with a sensitivity approaching 10−6 of the isospin-conserving strong amplitude. Some of the most interesting rare η decays are the neutral modes, yet the effective loss of photons from the relatively common decay η → 3π0 → 6γ (33%) has largely obscured rare decays producing 3-5γ's. Particularly important relevant branches include the highly suppressed η → π0 2γ → 4γ, which provides a rare window on testing models of O (p 6 ) contributions in ChPTh, and η → 3γ and η → 2π0 γ → 5γ which provide direct constraints on C violation in flavor-conserving processes. The substitution of lead tungstate in the forward calorimeter of the GluEx setup in Jefferson Lab's new Hall D would allow dramatically improved measurements. The main niche of this facility, which we call the JLab Eta Factory (JEF), would be η decay neutral modes. However, this could likely be expanded to rare η′(958) decays for low energy QCD studies as well as η decays involving muons for new physics searches.
Highlights
IntroductionThe problem is that when photons from this dominant branch are lost out of the acceptance (or showers merge in the calorimeter), rare decays to 3-5 ’s can be overwhelmed by background
The meson provides a laboratory to study isospin violation and search for new flavor-conserving sources of C and CP violation. [1] Because the has an unusually small decay width [3], a branching ratio upper limit of O(10−6) corresponds to a partial width 11 orders of magnitude smaller than for example the total decay width
The substitution of lead tungstate in the forward calorimeter of the GluEx setup in Jefferson Lab’s new Hall D would allow dramatically improved measurements
Summary
The problem is that when photons from this dominant branch are lost out of the acceptance (or showers merge in the calorimeter), rare decays to 3-5 ’s can be overwhelmed by background. The appropriate figure of merit (FOM) to maximize is F OM ≡ N /fbkg. Note that an order of magnitude reduction in a BR upper limit requires an improvement in FOM of 100! The addition of a high resolution lead tungstate core to the GluEx forward lead-glass calorimeter would allow dramatically improved neutral mode measurements (a reduction of 1–1.5 orders of magnitude in the BR). The missing energy cut prevents feed-down from higher invariant masses into the invariant mass window, while a cut on co-planarity between the and recoil proton suppresses multistep decays like + p → 0 + X+ → 2 0 + p
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