First measurement of the strange axial coupling constant using neutral-current quasielastic interactions of atmospheric neutrinos at KamLAND

TL;DR

This paper reports the first measurement of the strange axial coupling constant $g_A^s$ using atmospheric neutrino data at KamLAND, providing a new constraint on nucleon structure with minimal dependence on the axial mass.

Contribution

It introduces a novel measurement of $g_A^s$ using neutron multiplicity data from NCQE interactions, with a simulation of nuclear deexcitation effects not included in previous models.

Findings

Measured $g_A^s = -0.14^{+0.25}_{-0.26}$, the most stringent limit without $M_A$ constraints.

Developed a new simulation of nuclear deexcitation of $^{12}$C for neutrino interactions.

Provided insights into the two-body current contributions in neutrino-nucleon interactions.

Abstract

We report a measurement of the strange axial coupling constant $g_A^s$ using atmospheric neutrino data at KamLAND. This constant is a component of the axial form factor of the neutral-current quasielastic (NCQE) interaction. The value of $g_A^s$ significantly changes the ratio of proton and neutron NCQE cross sections. KamLAND is suitable for measuring NCQE interactions as it can detect nucleon recoils with low-energy thresholds and measure neutron multiplicity with high efficiency. KamLAND data, including the information on neutron multiplicity associated with the NCQE interactions, makes it possible to measure $g_A^s$ with a suppressed dependence on the axial mass $M_A$, which has not yet been determined. For a comprehensive prediction of the neutron emission associated with neutrino interactions, we establish a simulation of particle emission via nuclear deexcitation of $^{12}$C, a process not considered in existing neutrino Monte Carlo event generators. Energy spectrum fitting for each neutron multiplicity gives $g_A^s =-0.14^{+0.25}_{-0.26}$, which is the most stringent limit obtained using NCQE interactions without $M_A$ constraints. The two-body current contribution considered in this analysis relies on a theoretically effective model and electron scattering experiments and requires future verification by direct measurements and future model improvement.