4 Search Results

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π0 into two photons (γγ), leading to its unusually short lifetime. We precisely measured the decay width Γ of the π⁰ → γγ process. The differential cross sections for π0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Γ(π⁰ → γγ) = 7.798±0.056(stat.) ± 0.109(syst.) eV, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earliermore » experiment to generate a weighted average of Γ(π⁰ → γγ) = 7.802±0.052(stat.) ± 0.105(syst.) eV. Lastly, our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.« less

Exclusive meson electroproduction off protons is a powerful tool to probe the effective degrees of freedom in excited nucleon states at the varying distance scale where the transition from the contributions of both quark core and meson-baryon cloud to the quark core dominance. During the past decade, the CLAS collaboration has executed a broad experimental program to study the excited states of the proton using polarized electron beam and both polarized and unpolarized proton targets. The measurements covered a broad kinematic range in the invariant mass $$W$$ and photon virtuality $Q^2$ with nearly full coverage in polar and azimuthal anglesmore » in the hadronic CM system. As results, several low-lying nucleon resonance states in particular from pion threshold to $W < 1.6$ GeV have been explored. These include $$\Delta$$(1232)$$\frac{3}{2}^+$$, $$N(1440)\frac{1}{2}^+$$, $$N(1520)\frac{3}{2}^-$$, and $$N(1535)\frac{1}{2}^-$$ states. In addition, we recently published the differential cross sections and helicity amplitudes of the reaction $$\gamma^*p\to n\pi^+$$ at higher $$W$$ (1.6 to 2.0 GeV) which are the $$N(1675)\frac{5}{2}^-$$, $$N(1680)\frac{5}{2}^+$$, and $$N(1710)\frac{1}{2}^+$$ states. These excited states with isospin $1/2$ and with masses near 1.7 GeV can be accessed in single $$n\pi^+$$ production as there are no isospin $3/2$ states present in this mass range with the same spin-parity assignments. As a result, I will briefly discuss these states from CLAS results of the single charged pion electroproduction data.« less

We report on the first measurement of cross sections for exclusive deeply virtual pion electroproduction off the proton, ep -> e'n pi(+), above the resonance region at backward pion center-of-mass angles. The phi(pi)*-dependent cross sections were measured, from which we extracted three combinations of structure functions of the proton. Our results are compatible with calculations based on nucleon-to-pion transition distribution amplitudes (TDAs). These non-perturbative objects are defined as matrix elements of three-quark-light-cone-operators and characterize partonic correlations with a particular emphasis on baryon charge distribution inside a nucleon

The poster summarizes results from the FY14/15 JLab LDRD project "Physics potential of polarized light ions with EIC@JLab" regarding possible measurements of neutron structure functions (unpolarized and polarized) in deep-inelastic scattering with spectator nucleon tagging at a future Electron-Ion Collider. Also included are some results by Cosyn et al. regarding the theoretical description of the JLab 6 GeV BONuS data. Presented by W. Cosyn at 11th European Research Conference "Electromagneitc Interactions with Nucleons and Nuclei," Paphos, Cyprus, 1-7 November, 2015