Session J5: Invited Session: Exploring Low Energy QCD with Rare Meson Decays

Light Meson Decays at WASA-at-COSY

The WASA-at-COSY experiment consists of a 4-$\pi$ detector installed around an internal fixed target at the COSY storage ring at Forschungszentrum J\"ulich in Germany. A major component of the physics program is the study of decays of light mesons. Mesons are produced in proton-deuteron and proton-proton collisions at energies just above the production thresholds. The detector is capable of fully reconstructing all produced particles, including hadronic ejectiles and both charged and neutral meson decay products. This talk will focus on measurements of decays of the $\eta$ based on a sample of $3\times 10^{7}$ $\eta$ mesons collected in the $pd\rightarrow {^{3}He}\eta$ reaction. The decays $\eta\rightarrow\pi^{+}\pi^{-}\gamma$ and $\eta\rightarrow\pi^{+}\pi^{-}e^{+}e^{-}$ allow probes of anomalous QCD. The measurement of the decay dynamics in the isospin-violating reaction $\eta\rightarrow\pi^{+}\pi^{-}\pi^{0}$ could pave the way for stricter experimental limits on the ratio of the light quark masses. The reaction $\eta\rightarrow e^{+}e^{-}\gamma$ provides information on the contribution of virtual vector mesons to the decay process, which can be quantified by measuring the electromagnetic transition form factor. Additionally, the high statistics of $\eta$ mesons available allows observation of the rare process $\eta\rightarrow e^{+}e^{-}e^{+}e^{-}$. Results from the analyses of these channels will be presented. WASA-at-COSY has also collected over an order of magnitude more events in the $pp \rightarrow pp \eta$ reaction. This should enable greater sensitivity on physical observables in the rare decays. The analysis of this data is in progress. Additionally, data has been taken at different energies in order to study decays of the $\pi^{0}$ and $\omega$ mesons. An overview of these projects will be given.   

What Can We Learn from Hadronic and Radiative Decays of Light Mesons?

Chiral perturbation theory offers a powerful tool for the investigation of light pseudoscalar mesons. It incorporates the fundamental symmetries of QCD, interrelates various processes, and allows to link these to the light quark masses. Its shortcomings lie in a limited energy range: the radius of convergence of the chiral expansion is confined to below resonance scales. Furthermore, the strongest consequences of chiral symmetry are manifest for pseudoscalars (pions, kaons, eta) only: vector mesons, e.g., have a severe impact in particular for reactions involving photons. In this talk, I advocate dispersions relations as another model-independent tool to extend the applicability range of chiral perturbation theory. They even allow to tackle the physics of vector mesons in a rigorous way. It will be shown how dispersive methods can be used to resum large rescattering effects, and to provide model-independent links between hadronic and radiative decay modes. Examples to be discussed will include decays of the eta meson, giving access to light-quark-mass ratios or allowing to test the chiral anomaly; and meson transition form factors, which have an important impact on the hadronic light-by-light-scattering contribution to the anomalous magnetic moment of the muon.   

PrimEx Experiments and the Prospects of Rare Meson Decays at GlueX

There are two major challenges in contemporary physics: QCD confinement and new physics beyond the Standard Model. A study of not-so-rare and rare decays of light neutral pseudoscalar mesons $\pi^{0}$, $\eta $ and $\eta \prime $ provides a sensitive probe to investigate both fundamental issues. A comprehensive PrimEx experimental program at Jefferson Laboratory (Jlab) is aimed at gathering high precision measurements on the two-photon decay widths and transition form factors at low Q$^{2}$ on $\pi^{0}$, $\eta $ and $\eta \prime $ via the Primakoff effect. Completed experiments on the $\pi^{0}$ radiative decay width at Jlab 6 GeV, and planned measurements of $\eta $ and $\eta \prime $ at Jlab 12 GeV will provide a rich laboratory to test chiral anomaly and to study the origin and dynamics of symmetry breaking in QCD confinement. On the other hand, measurements of branching ratios or upper limits for various rare and forbidden $\eta $ decays are about five orders of magnitude more sensitive to high order QCD contributions or new interactions, because the lowest orders are filtered out by conserved symmetries. This circumstance presents a great opportunity to test high order chiral perturbation predictions, and to search for new sources of symmetry violations (such as C and CP) and new physics beyond the Standard Model. An experimental program to study $\eta $ rare decays with the GlueX apparatus has been developed at Jlab. Simulations show that the experimental approach, which combines a state of the art PbWO$_{4}$ calorimeter, a 12 GeV tagged photon beam, and recoil particle measurement, will reduce the background by almost two orders of magnitude compared to other competitors in the world. The updated result from the PrimEx experiments and the prospects of rare meson decays at GlueX will be presented.