Search

Precision measurements in allowed nuclear beta decays and neutron decay are reviewed and analyzed both within the Standard Model and looking for new physics. The analysis incorporates the most recent experimental and theoretical developments. The results are interpreted in terms of Wilson coefficients describing the effective interactions between leptons and nucleons (or quarks) that are responsible for beta decay. New global fits are performed incorporating a comprehensive list of precision measurements in neutron decay, superallowed $0^+\rightarrow 0^+$ transitions, and other nuclear decays that include, for the first time, data from mirror beta transitions. The results confirm the $V$-$A$ character of the interaction and translate into updated values for $V_{ud}$ and $g_A$ at the $10^{-4}$ level. We also place new stringent limits on exotic couplings involving left-handed and right-handed neutrinos, which benefit significantly from the inclusion of mirror decays in the analysis., Comment: 35 pages; v2: analysis updated, comments and references added; v3: compared to the JHEP version, typos in Eqs. 2.1, A3, A4 corrected, Eq. 4.4 updated, comment on aCORN update added in footnote 12

The document presents a summary of discussions at recent workshops at the Amherst Center for Fundamental Interactions at Amherst, MA, and at the European Centre for Theoretical Studies in Nuclear Physics and Related Areas at Trento, Italy, on the potential sensitivity of precision beta-decay experiments towards new physics., Comment: 11 pages, no figures, white paper

The status of tests of the standard electroweak model and of searches for new physics in allowed nuclear $\beta$ decay and neutron decay is reviewed including both theoretical and experimental developments. The sensitivity and complementarity of recent and ongoing experiments are discussed with emphasis on their potential to look for new physics. Measurements are interpreted using a model-independent effective field theory approach enabling to recast the outcome of the analysis in many specific new physics models. Special attention is given to the connection that this approach establishes with high-energy physics. A new global fit of available $\beta$-decay data is performed incorporating, for the first time in a consistent way, superallowed $0^+\to 0^+$ transitions, neutron decay and nuclear decays. The constraints on exotic scalar and tensor couplings involving left- or right-handed neutrinos are determined while a constraint on the pseudoscalar coupling from neutron decay data is obtained for the first time as well. The values of the vector and axial-vector couplings, which are associated within the standard model to $V_{ud}$ and $g_A$ respectively, are also updated. The ratio between the axial and vector couplings obtained from the fit under standard model assumptions is $C_A/C_V = -1.27510(66)$. The relevance of the various experimental inputs and error sources is critically discussed and the impact of ongoing measurements is studied. The complementarity of the obtained bounds with other low- and high-energy probes is presented including ongoing searches at the Large Hadron Collider., Comment: 70 pages, 14 figures. Invited review article for Prog. Part. Nucl. Phys. v2: Journal version (clarifications, references, and 2 new figures added)

Background: The accurate determination of atomic final states following nuclear $\beta$ decay plays an important role in many experiments. In particular, the charge state distributions of ions following nuclear $\beta$ decay are important for determinations of the $\beta-\nu$ angular correlation with improved precision. Purpose: Our measurement aims at providing benchmarks to test theoretical calculations. Method: The kinematics of Li$^{n+}$ ions produced following the $\beta$ decay of $^6{\rm He}$ within an electric field were measured using $^6{\rm He}$ atoms in the metastable $(1s2s,~{^3S_1})$ and in the $(1s2p,~{^3P_2})$ states confined by a magneto-optical trap. The electron shake-off probabilities were deduced including their dependence on ion energy. Results: We find significant discrepancies on the fractions of Li ions in the different charge states with respect to a recent calculation and discuss a plausible explanation. We also point out that there is no explanation for a large discrepancy between the same calculation and a previous measurement of the Li-ion energy dependence of the charge distribution from decays of the electronic ground state.

The current most stringent constraints on exotic scalar or tensor couplings in neutron and nuclear $\beta$ decay, involving left-handed neutrinos, are obtained from the Fierz interference term. The sensitivity to this term in a correlation coefficient is usually driven by an energy-averaged kinematic factor that increases monotonically toward smaller values of the $\beta$ endpoint energies. We first point out here that this property does not hold for certain differential observables that are directly sensitive to the Fierz term, such as the $\beta$ or the recoil energy spectrum. This observation is relevant for the selection of sensitive transitions in searches for exotic couplings through spectrum shape measurements. We then point out previous errors in the exploitation of measurements of the $\beta-\nu$ angular correlation coefficient and discuss their impact on the extraction of constraints on exotic couplings., Comment: 7 pages, 5 figures

Precision measurements in nuclear beta decay offer a sensitive window to search for new physics beyond the standard electroweak model and allow also the determination of the fundamental weak vector coupling in processes involving the lightest quarks. Searches for new physics are also a strong motivation for experiments carried out at the high energy frontier reached at the most powerful particle colliders. It is instructive to confront results from the low energy and the high energy frontiers in order to look for possible complementarities and orient new avenues for experiments at low energies. We review here the status of constraints on new physics obtained from nuclear and neutron decays and compare them to those from other semi-leptonic processes and from the LHC. We stress the requirements of new precision experiments in beta decay in order to impact the search for new physics at the light of current and projected LHC results. We describe recent experimental results and ongoing developments in nuclear and neutron beta decay, with emphasis on their planned goals to improve present limits on exotic weak couplings., Comment: Submitted to Annalen der Physik for the Special Issue "Precision experiments and fundamental physics at low energies"

We review the determination of Vud from nuclear mirror transitions and describe current experimental and theoretical efforts aimed at improving its precision., Comment: Proceedings of CKM 2012, the 7th International Workshop on the CKM Unitarity Triangle, University of Cincinnati, USA, 28 September - 2 October 2012