Experimental study of rare charged pion decays

Abstract

The combination of simple dynamics, small number of available decay channels, and extremely well controlled radiative and loop corrections, make charged pion decays a sensitive means for testing the underlying symmetries and the universality of weak fermion couplings, as well as for improving our understanding of pion structure and chiral dynamics. This paper reviews the current state of experimental study of the allowed rare decays of charged pions: (a) leptonic, ${\pi }^{+}\to e^{+}{\nu }_e$, or ${\pi }_{e2}$, (b) radiative, ${\pi }^{+}\to e^{+}{\nu }_e\gamma$, or ${\pi }_{e2\gamma }$, and ${\pi }^{+}\to e^{+}{\nu }_e{e}^{+}{e}^{-}$, or ${\pi }_{e2ee}$, and (c) semileptonic, ${\pi }^{+}\to {\pi }^0{e}^{+}\nu$, or ${\pi }_{e3}$. Taken together, the combined data set presents an internally consistent picture that also agrees well with standard model predictions. The internal consistency is illustrated well by the ${\pi }_{e2}$ branching ratio of $(R_{e/\mu }^{\pi }{)}^{\mathrm{PIBETA}}=(1.2366\pm 0.0064)\times 10^{-4}$ extracted in this work from the PIBETA measurement of the ${\pi }_{e3}$ decay and the current best value for the CKM matrix element $V_{ud}$. However, even after the great progress of the recent decades, experimental precision 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.

arXiv comment34 pages, 20 figures, 1 table; submitted to Journal of Physics G: Nuclear and Particle Physics, for the focus section on "The low energy precision frontier", guest edited by Barry Holstein

References in corpus (24)

• Big Bang Nucleosynthesis (in "The Review of Particle Properties" 2004)
• Chiral Perturbation Theory
• Superallowed 0+ to 0+ nuclear beta decays: A new survey with precision tests of the conserved vector current hypothesis and the standard model
• Improved Calculation of Electroweak Radiative Corrections and the Value of V_ud
• The Standard Model prediction for R_{e/mu}^{(pi,K)}
• Precision Measurements and CKM Unitarity
• Precise Measurement of the Pi+ -> Pi0 e+ nu Branching Ratio
• Form Factors in the radiative pion decay
• A New Measurement of the ${\pi }^0$ Radiative Decay Width
• New, high statistics measurement of the K+ -> pi0 e+ nu (Ke3) branching ratio
• New Precise Measurement of the Pion Weak Form Factors in the Pi+ -> e+ nu gamma Decay
• Radiative Corrections to pi_{l2} and K_{l2} Decays
• The NuTeV Anomaly, Lepton Universality, and Non-Universal Neutrino-Gauge Couplings
• Neutral Pion Lifetime Measurements and the QCD Chiral Anomaly
• Precise Measurement of the Pion Axial Form Factor in the Pi+ -> e+ nu gamma Decay
• The pionic beta decay in chiral perturbation theory
• Design, Commissioning and Performance of the PIBETA Detector at PSI
• Constraints on Scalar Couplings from ${\pi }^{\pm }\to l^{\pm }{\nu }_l$
• New precision measurements of free neutron beta decay with cold neutrons
• High Purity Pion Beam at TRIUMF
• Pion Leptonic Decays and Supersymmetry
• Study of a Large NaI(Tl) Crystal
• Observations on the radiative corrections to pion beta-decay
• Radiative corrections to radiative πe 2 decay

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• Beam particle tracking with a low-mass mini time projection chamber in the PEN experiment