Reaction pp $ \rightarrow$ pp $ \pi$ $ \pi$ $ \pi$ as a background for hadronic decays of the $ \eta^{{\prime}}_{}$ meson

Abstract

Abstract. Isospin violating hadronic decays of the η and η mesons into 3π mesons are driven by a term in the QCD Lagrangian proportional to the mass difference of the d and u quarks. The source giving large yield of the mesons for such decay studies are pp interactions close to the respective kinematical thresholds. The most important physics background for η, η → πππ is coming from direct three-pion production reactions. In case of the η meson the background for the decays is relatively low (≈ 10%). The purpose of this article is to provide an estimate of the direct pion production background for the η → 3π decays. Using the inclusive data from the COSY-11 experiment we have extracted the differential cross-section for the pp → pp-multipion production reactions with the invariant mass of the pions equal to the η meson mass and estimated an upper limit for the signal to background ratio for studies of the η → π + π − π 0 decay. Motivation Three-pion decays of the η and η mesonsThe η and η decays into three pions violate isospin and occur only due to u and d quark mass difference. The decay width is sensitive to the mass difference:, where the Γ 0 term can be calculated in the isospin limit m d = m u . The decays might provide a precise constraint for the light-quark mass ratios [1].In the case of the η meson the existence of the isospin conserving decays into three pseudoscalars (ππη) implies that instead of the decay width for the isospin violating πππ channel one can measure the ratioas it was proposed by Gross, Treiman and Wilczek [2]. Such measurement is self-contained since it does not require normalization for the partial decay width from other experiments. Additionally, simultaneous measurement of the two decays modes, with similar final states, ensures that many systematic uncertainties will cancel. a

Production of $η$ and $η'$ Mesons on Nucleons and Nuclei

“…The COSY-WASA tests for the production of the η meson through the pp → ppη reaction were less promising since, in the initial runs, no clear η signal was identified [233]. Multipion production will certainly provide a very significant background [234].…”