Parity - Violating Pion - Nucleon Coupling h ( 1 ) πNN from π + - Electroproton Production Near the Threshold

We study the possibility of measuring parity-violating pion-nucleon coupling h (1) πNN from π+-electroproton production in the threshold region. We calculate the electronhelicity asymmetry in terms of h (1) πNN and the Z-boson exchange between the electron and proton in leading-order heavy-baryon chiral perturbation theory. From the result, we demonstrate that the coupling can be determined to an accuracy of ∼ 2× 10−7 in an experiment with incident electron energy 250 MeV, luminosity 1038 sec−1 cm−2, and a running time of 107 sec. Weak interaction effects in hadron structure and reactions are usually masked by much stronger strong and electromagnetic interactions. Only under special circumstances can they be studied through high-precision measurements of observables vanishing in the absence of weak processes. The cross section asymmetry originating from a single longitudinal polarization is such an observable. At low energy, parity-violating hadronic interactions can be systematically classified in the framework of effective field theories [1–3]. In chiral power counting, the most important is the isovector P-odd pion-nucleon coupling h (1) πNN which is responsible for the longest range part of the parityviolating ∆I = 1 NN forces [1,4,5], the nucleon and nuclear anapole moments [6–11]. In quantum chromodynamics (QCD), its value is believed to be dominated by the s-quark contribution generated from the neutral current interaction [12]. Clearly, h (1) πNN is one of the most important observables that characterize the size of the nonleptonic weak interactions in hadron systems. In the last two decades, serious attempts have been made to pin down h (1) πNN , and as of to date, considerable uncertainty remains. Reviews of the experimental and theoretical status at the different stages of investigation can be found in [5,13,14]. In many-body systems, parityviolating effects can be enhanced significantly by important many-body correlations and have been detected experimentally. However, because of the underlying theoretical uncertainty, they cannot be translated easily into the corresponding results for the fundamental parameters. A large discrepancy exists on the extracted parity-violating coupling from the gamma asymmetry Aγ from the polarized F decay [15] and from the anapole moment measurement in Cs atom [16]. In few-body systems, the theory is under better control; but the parity-violating effects are generally small. The existing experimental results, and hence the extracted parameters, have very large error bars [17]. New