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Register a new userA Review of Rare Pion and Muon Decays
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Dinko Pocanic
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After a decade of no measurements of pion and muon rare decays, PIBETA, a new experimental program is producing its first results. We report on a new experimental study of the pion beta decay, Pi(+) -> Pi(0) e(+) Nu, the Pi(e2 gamma) radiative decay, Pi(+) -> e(+) Nu Gamma, and muon radiative decay, Mu -> e Nu Gamma. The new results represent four- to six-fold improvements in precision over the previous measurements. Excellent agreement with Standard Model predictions is observed in all channels except for one kinematic region of the Pi(e2 gamma) radiative decay involving energetic photons and lower-energy positrons.
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Simple dynamics, few available decay channels, and highly controlled radiative and loop corrections, make pion and muon decays a sensitive means of exploring details of the underlying symmetries. We review the current status of the rare decays: pi+ -> e+ nu, pi+ -> e+ nu gamma, pi+ -> pi0 e+ nu, and mu+ -> e+ nu nu-bar gamma. For the latter we report new preliminary values for the branching ratio B(E_gamma >10 MeV, theta_(e-gamma) > 30deg) = 4.365 (9)_stat (42)_syst x 10^{-3}, and the decay parameter eta-bar = 0.006 (17)_stat (18)_syst, both in excellent agreement with standard model predictions. We review recent measurements, particularly by the PIBETA and PEN experiments, and near-term prospects for improvement. These and other similar precise low energy studies complement modern collider results materially.
I briefly review the theory status of exclusive rare radiative decays.
Simple dynamics, few available decay channels, and extremely well controlled radiative and loop corrections, make pion and muon decays a sensitive means for testing the underlying symmetries, the universality of weak fermion couplings, as well as for study of pion structure and chiral dynamics. We review the current state of experimental study of the allowed rare decays of charged pions: (a) electronic, $pi^+ to e^+ u_e$, or $pi_{e2}$, (b) radiative, $pi^+ to e^+ u_egamma$, or $pi_{e2gamma}$, and (c) semileptonic, $pi^+to pi^0 e^+ u$, or $pi_{e3}$, as well as muon radiative decay, $mu^+to e^+ u_{text{e}}bar{ u}_{mu}gamma$. Taken together, these data present an internally consistent picture that also agrees well with Standard Model (SM) predictions. However, even following the great strides of the recent decades, experimental accuracy is lagging far behind that of the theoretical description for all above processes. We review the implications of the present state of knowledge and prospects for further improvement in the near term.
We review the recent measurements of the rare pion decays: Pi+ -> Pi0 e+ Nu [pion beta, Pi_(e3), or Pi_beta decay], radiative decay Pi+ -> e+ Nu Gamma [Pi_(e2Gamma) or RPD], and Pi+ -> e+ Nu [Pi_(e2)] decay, as well as the radiative muon decay, Mu -> e Nu Nu-bar Gamma, their theoretical implications, and prospects for further improvement.
Building on the rare pion and muon decay results of the PIBETA experiment, the PEN collaboration has undertaken a precise measurement of B_{pi e2} = R^pi_{e/mu}, the pi^+ -> e^+ u(gamma) decay branching ratio, at the Paul Scherrer Institute, to reduce the present 40times experimental precision lag behind theory to ~ 6-7times. Because of large helicity suppression, R^pi_{e/mu} is uniquely sensitive to contributions from non-(V-A) physics, making this decay a particularly suitable subject of study. Even at current precision, the experimental value of B_{pi e2} provides the most accurate test of lepton universality available. During runs in 2008-10, PEN has accumulated over 2times 10^7 pi_{e2} events; a comprehensive maximum-likelihood analysis is currently under way. The new data will also lead to improved precision of the earlier PIBETA results on radiative pi and mu decays.
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