First direct constraints on Fierz interference in free-neutron β decay

Abstract

Precision measurements of free-neutron β decay have been used to precisely constrain our understanding of the weak interaction. However, the neutron Fierz interference term b n , which is particularly sensitive to beyond-standard-model tensor currents at the TeV scale, has thus far eluded measurement. Here we report the first direct constraints on this term, finding b n = 0.067 ± 0.005 stat +0.090 −0.061 sys , consistent with the standard model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the β spectrometer energy response. DOI: 10.1103/PhysRevC.96.042501 Precision measurements in nuclear β decay including lifetimes, angular/spin correlations, and energy spectra can be used to test predictions of the electroweak sector of the standard model [1][2][3][4][5][6][7]. Current efforts are underway to measure many of these quantities in nuclear β decay as well as in free-neutron decay. The Fierz interference term vanishes in the standard model but serves as a probe for new physics in scalar and tensor couplings [1,4] (here denoted b n ) is presented for the first time. This term is particularly sensitive to tensor couplings that previous extractions of the Fierz term in superallowed 0 + → 0 + β decay are not.The Fierz interference term appears in the full form of the differential neutron decay rate parametrized in terms of correlation coefficients of neutron spin, σ n = J n /| J n |, and momenta, p e , p ν , and total energies, E e ,E ν , of the final-state particles [8]where W(E e ) includes the total decay rate (e.g., 1/τ n ) and the phase space along with recoil-order, radiative, and Coulomb corrections. The correlation coefficients also include recoilorder corrections. The dimensionless parameter b n is the only spin-and momentum-direction-independent coefficient and thus survives summation over spin and integration over the final-state angular distributions leaving a distribution dependent only on the electron energy, E:

Using Nab to determine correlations in unpolarized neutron decay

“…The limit −0.041 < b F < 0.225 (90% C.L.) has been obtained for the first time in the neutron system from its influence on A β over the measured spectrum [4]. There is a compelling case for improving these limits by investigating the impact of b F on nuclear and neutron beta spectrum shape measurements [5,6].…”

Decay microscope for trapped neon isotopes

“…Due to the already monumental work on superallowed Fermi transitions over the last decades, which is still ongoing, it is difficult to imagine that beta-spectrum measurements of Fermi transitions will be competitive in b F extraction. On the other hand, there is a strong potential for improvement for the GT case, and it is the focus of several new betaspectrum campaigns utilizing nuclear [55][56][57][58][59][60][61], and neutron [62][63][64] decays, and aiming at σ(b GT ) 10 −3 (see table II). Correspondingly, theoretical corrections to the beta-spectrum shape are now at the level of a few 10 −4 [65].…”

Recent design studies for the novel momentum spectrometer NoMoS

“…Numerous experiments have been built or are under construction to measure these observables with high precision: conducted measurements of a have achieved an accuracy of ∆a/a = 2.6% [8] (PDG) whereas b was determined with an accuracy of ∆b = 0.09 [9]. Currently, various experiments try to improve on this.…”