Observation of the destructive interference in the radiative kaon decay K- --> mu-nugamma

TL;DR

This study reports the first measurement of destructive interference in the radiative decay of kaons, revealing form factor differences that challenge existing theoretical predictions and exploring exotic tensor interactions.

Contribution

It provides the first experimental measurement of the destructive interference in K- --> mu-nu gamma decay and examines potential contributions from exotic tensor interactions.

Findings

Measured Fv-Fa=0.126 with uncertainties

Value exceeds ChPT prediction by two standard deviations

Constraints on tensor form factor Ft consistent with zero and theory

Abstract

Using data collected with the ISTRA+ spectrometer at U70 proton synchrotron of IHEP, we report the first measurement of the destructive interference in the radiative kaon cecay K- -->mu-nugamma. We find the difference of the vector and axial form factors Fv-Fa=0.126+/-0.027+/-0.043. The measured value is two standard deviations above the O(p**4) ChPT prediction equal to 0.055. Inclusion of exotic tensor interaction gives Fv-Fa=0.144+/-0.044+/-0.035 and Ft=-0.0079+/-0.0113+/-0.0073, i.e. -0.03<Ft<0.01 at 90% CL,consistent both with zero and with recent theoretical prediction equal to |Ft|=0.022.