Is the Unitarity of the quark-mixing-CKM-matrix violated in neutron $\beta$-decay?

TL;DR

This paper presents a new measurement of neutron beta-decay asymmetry leading to a calculation of the CKM matrix element V_{ud} and finds a 3-sigma deviation from unitarity, suggesting potential new physics.

Contribution

It provides the first determination of V_{ud} from neutron decay asymmetry measurements and tests CKM unitarity with high precision.

Findings

Measured neutron beta-decay asymmetry A_0 = -0.1189(7)

Derived V_{ud} from neutron decay and found deviation from CKM unitarity

Observed a 3-sigma deviation indicating possible new physics or unaccounted systematic errors.

Abstract

We report on a new measurement of neutron $\beta$-decay asymmetry. From the result \linebreak $A_0$ = -0.1189(7), we derive the ratio of the axial vector to the vector coupling constant $\lambda$ = ${\it g_A/g_V}$ = -1.2739(19). When included in the world average for the neutron lifetime $\tau$ = 885.7(7)s, this gives the first element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix $V_{ud} $. With this value and the Particle Data Group values for $V_{us}$ and $V_{ub}$, we find a deviation from the unitarity condition for the first row of the CKM matrix of $\Delta$ = 0.0083(28), which is 3.0 times the stated error.