We review the current status of precision measurements in allowed nuclear beta decay, including neutron decay, with emphasis on their potential to look for new physics beyond the standard electroweak model. The experimental results are interpreted in the framework of phenomenological model-independent descriptions of nuclear beta decay as well as in some specific extensions of the standard model. The values of the standard couplings and the constraints on the exotic couplings of the general beta decay Hamiltonian are updated. For the ratio between the axial and the vector couplings we obtain C A /C V = −1.26992(69) under the standard model assumptions. Particular attention is devoted to the discussion of the sensitivity and complementarity of different precision experiments in direct beta decay. The prospects and the impact of recent developments of precision tools and of high intensity low energy beams are also addressed.

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The V{ud} element of the Cabibbo-Kobayashi-Maskawa quark mixing matrix has traditionally been determined from the analysis of data in nuclear superallowed 0;{+}-->0;{+} transitions, neutron decay, and pion beta decay. After providing a new test of the conserved vector current hypothesis, we present here a new independent determination of |V{ud}| from a set of five T=1/2 nuclear mirror transitions. The extracted value, |V{ud}|=0.9719+/-0.0017, is at 1.2 combined standard deviations from the value obtained from superallowed 0;{+}-->0;{+} transitions and has a precision comparable to the value obtained from neutron decay experiments.

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The final analysis of the experiment determining both components of the transverse polarization of electrons (σT1, σT2) emitted in the β decay of polarized, free neutrons is presented. The T-odd, P-odd correlation coefficient quantifying σT2, perpendicular to the neutron polarization and electron momentum, was found to be R= 0.004 ± 0.012 ± 0.005. This value is consistent with time-reversal invariance and significantly improves both the earlier result and limits the relative strength of the imaginary scalar couplings in the weak interaction. The value obtained for the correlation coefficient associated with σT1, N= 0.067 ± 0.011 ± 0.004, agrees with the standard model expectation, providing an important sensitivity test of the experimental setup. The present result sets constraints on the imaginary part of scalar and tensor couplings in weak interactions. Implications for parameters of the leptoquark exchange model and the minimal supersymmetric model with R-parity violation are discussed

Both components of the transverse polarization of electrons (σT 1 , σT 2 ) emitted in the β-decay of polarized, free neutrons have been measured. The T-odd, P-odd correlation coefficient quantifying σT 2 , perpendicular to the neutron polarization and electron momentum, was found to be R = 0.008±0.015±0.005. This value is consistent with time reversal invariance, and significantly improves limits on the relative strength of imaginary scalar couplings in the weak interaction. The value obtained for the correlation coefficient associated with σT 1 , N = 0.056±0.011±0.005, agrees with the Standard Model expectation, providing an important sensitivity test of the experimental setup.PACS numbers: 24.80.+y, 23.40.Bw, 24.70.+s, 11.30.Er Despite the great success of the Standard Model (SM) of elementary particles and their interactions, several important questions remain open. One of these is the incomplete knowledge of physics of CP-violation, or via the CPT theorem, time reversal symmetry violation (TRV). The SM with the Cabbibo-Kobayashi-Maskawa (CKM) mixing scheme accounts for CP violation discovered in kaon [1] and B-meson [2, 3] systems. It fails, however, to explain the basic observation of the dominance of matter in the present Universe. The discovery of new CPor T-violating phenomena, especially in systems built of light quarks with vanishingly small contributions of CKM matrix induced mechanisms, would be a major breakthrough. Nuclear beta decay experiments test these systems and free neutron decay plays a particular role: due to its simplicity it is free of corrections associated with the nuclear and atomic structure. Further, final state interaction effects, which can mimic T violation, are minimal and can, in addition, be calculated with a relative precision better than 1% [4].In this Letter we report on the first experiment searching for the real and imaginary parts of scalar and tensor couplings via the measurement of the transverse polarization of electrons emitted in the decay of free neutrons. There exist very few measurements of this observable in general [5,6], and only two in nuclear beta decays. One of them, for the 8 Li system [7], provides the most stringent limit on tensor coupling constants of the weak interaction.According to [8], the decay rate distribution as a function of electron energy (E) and momentum (p), from polarized neutrons is proportional to:where J is the neutron spin,σ is a unit vector onto which the electron spin is projected and A is beta decay asymmetry parameter. N and R are correlation coefficients which, for neutron decay with the SM assumptions, and allowing for a small admixture of scalar and tensor couplings C S , C T , C ′ S , C ′ T , can be expressed as:whereT )/C A and m is the electron mass. The R correlation value vanishes to the lowest order within the SM. Including final state interactions it becomes different from zero, R F SI ≈ 0.0006, still below the sensitivity of the present experiment. A larger measured value would provide a hint for the existence of exo...

The β-asymmetry parameter A for the Gamow-Teller decay of 60 Co was measured by polarizing the radioactive nuclei with the brute force low-temperature nuclear-orientation method. The 60 Co activity was cooled down to milliKelvin temperatures in a 3 He-4 He dilution refrigerator in an external 13 T magnetic field. The β particles were observed by a 500 µm thick Si PIN diode operating at a temperature of about 10 K in a magnetic field of 0.6 T. Extensive GEANT4 Monte-Carlo simulations were performed to gain control over the systematic effects. Our result, A = −1.014 (12)stat (16)syst, is in agreement with the Standard-Model value of −0.987(9), which includes recoil-order corrections that were addressed for the first time for this isotope. Further, it enables limits to be placed on possible tensor-type charged weak currents as well as other physics beyond the Standard Model.