Sigma term update

This article reviews the current theoretical and experimental status of the field of muon decay and its potential to search for new physics beyond the standard model. The importance of rare muon processes with lepton flavor violation is highly stressed, together with precision measurements of normal muon decay. Recent up-to-date motivations of lepton flavor violation based on supersymmetric models, in particular supersymmetric grand unified theories, are described along with other theoretical models. Future prospects of experiments and muon sources of high intensity for further progress in this field are also discussed.

Muon decay and physics beyond the standard model

The nuclear physics of muon capture

Neutrino–nuclear responses for astro-neutrinos, single beta decays and double beta decays

In this review the quenching of the weak axial-vector coupling constant, $g_{\rm A}$, is discussed in nuclear $\beta$ and double-$\beta$ decays. On one hand, the nuclear-medium and nuclear many-body effects are separated, and on the other hand the quenching is discussed from the points of view of different many-body methods and different $\beta$-decay and double-$\beta$-decay processes. Both the historical background and the present status are reviewed and contrasted against each other. The theoretical considerations are tied to performed and planned measurements, and possible new measurements are urged, whenever relevant and doable. Relation of the quenching problem to the measurements of charge-exchange reactions and muon-capture rates is pointed out.

/pdf/value-of-the-axial-vector-coupling-strength-in-b-and-bb-1zkevdorex.pdf

Value of the Axial-Vector Coupling Strength in β and ββ Decays: A Review

We report measurements of capture rates and hyperfine dependences in muon capture on {sup 23}Na to various states in Ne and F isotopes. We also report comparisons of the capture rates and hyperfine dependences for six {sup 23}Na {r_arrow} {sup 23}Ne transitions with the 1{ital s}-0{ital d} shell model with the empirical effective interaction of Brown and Wildenthal and the realistic effective interaction of Kuo and Brown. Fits to the data with the Brown and Wildenthal interaction yield an effective coupling {ital {tilde g}}{sub {ital a}} = {minus}1.01 {plus_minus} 0.07 and an effective coupling ratio {ital {tilde g}}{sub {ital p}}/{ital {tilde g}}{sub {ital a}} = 6.5 {plus_minus} 2.4. The value of {ital {tilde g}}{sub {ital a}} is consistent with values of {ital {tilde g}}{sub {ital a}} extracted from {beta}{sup +}/{beta}{sup {minus}} decay and ({ital p},{ital n})/({ital n},{ital p}) charge exchange data, and the value of {ital {tilde g}}{sub {ital p}}/{ital {tilde g}}{sub {ital a}} is consistent with the predictions of PCAC and pion-pole dominance. We evaluate the nuclear model dependence of these values of {ital {tilde g}}{sub {ital a}} and {ital {tilde g}}{sub {ital p}}/{ital {tilde g}}{sub {ital a}} and examine the role of the Gamow-Teller and other matrix elementsmore » in the {sup 23}Na {r_arrow} {sup 23}Ne transitions. {copyright} {ital 1996 The American Physical Society.}« less

Observables in muon capture on 23Na and the effective weak couplings g-tildea and g-tildep.

The branching ratio for radiative muon capture (RMC), relative to the nonradiative process, is sensitive to {ital g}{sub {ital p}}, the induced pseudoscalar coupling constant of the weak hadronic current. The photon energy spectra from RMC on {sup 27}Al, {sup 28}Si, {sup 40}Ca, {sup nat}Mo, {sup nat}Sn, and {sup nat}Pb have been measured using a high-acceptance pair spectrometer. The measured partial branching ratios, {ital R}{sub {gamma}}, for photons of {ital E}{sub {gamma}}{gt}57 MeV are 1.43{plus minus}0.13, 1.93{plus minus}0.18, 2.09{plus minus}0.19, 1.11{plus minus}0.11, 0.98{plus minus}0.09, and 0.60{plus minus}0.07 respectively, in units of 10{sup {minus}5}. The results confirm the previously observed suppression of {ital R}{sub {gamma}} with increasing {ital Z} for {ital Z}{gt}20. For {sup 40}Ca the present result is in good agreement with previous measurements. For the heavier nuclei, the results are compared with two recent calculations performed in the Fermi-gas model. In one case the data indicate a complete quenching of {ital g}{sub {ital p}}, but the more recent calculation does not reproduce the data for any value of {ital g}{sub {ital p}}.

Radiative muon capture on Al, Si, Ca, Mo, Sn, and Pb

We report measurements of the {pi}{sup -}p{yields}{pi}{sup 0}n differential cross sections at six momenta (104-143 MeV/c) and four angles (0 deg. - 40 deg.) by detection of {gamma}-ray pairs from {pi}{sup 0}{yields}{gamma}{gamma} decays using a photon-pair spectrometer at TRIUMF. This kinematic region exhibits a vanishing zero-degree cross section from destructive interference between s and p waves, thus yielding special sensitivity to pion-nucleon dynamics. Our data and previous data do not agree, with important implications for earlier claims of large isospin-violating effects.

Test of isospin symmetry via low-energy 1H(pi{-},pi{0})n charge exchange.

The reaction ..pi../sup -/p..--> pi../sup 0/n has been studied using a large NaI crystal to detect the decay ..gamma.. rays of the ..pi../sup 0/, at pion beam energies of 45.6, 62.2, 76.4, 91.7, 106.8, and 121.9 MeV. The NaI was set at nine different laboratory angles, but at almost every angle the angular distribution of the ..pi../sup 0/ can be derived from the energy spectrum of the ..gamma.. rays. The results are compared with recent phase-shift analyses and with various calculations. New values of the charge exchange scattering length are derived.

Reaction pi -p--> pi 0n below the Delta resonance.

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Papers

Observables in muon capture on 23Na and the effective weak couplings g-tildea and g-tildep.

Radiative muon capture on Al, Si, Ca, Mo, Sn, and Pb

Test of isospin symmetry via low-energy 1H(pi{-},pi{0})n charge exchange.

Reaction pi -p--> pi 0n below the Delta resonance.