Anomaly-induced Quadrupole Moment of the Neutron in Magnetic Field

TL;DR

This paper predicts a new type of quadrupole moment for neutrons induced by magnetic fields, arising from the chiral anomaly, which could be experimentally detected and has implications for astrophysical objects.

Contribution

It introduces a novel chiral magnetic quadrupole moment for neutrons, linking the chiral anomaly to observable electromagnetic properties.

Findings

Estimated neutron quadrupole moment: $oxed{ ext{~}1.35 imes 10^{-2} ext{ fm}^4}$

Proposes experimental detection via magnetic and inhomogeneous electric fields

Implications for neutron stars and magnetars

Abstract

The neutrons cannot possess a quadrupole moment in the vacuum. Nevertheless, we show that in the presence of an external magnetic field the neutrons acquire a new type of quadrupole moment $Q^{ij}= \chi\,\sigma^i B^j$ involving the components of spin and magnetic field. This "chiral magnetic" quadrupole moment arises from the interplay of the chiral anomaly and the magnetic field; we estimate its value for the neutron in the static limit, and find $\chi \simeq 1.35\cdot10^{-2}\,{\rm fm}^4$. The detection of the quadrupole moment of the neutron would provide a novel test of the role of the chiral anomaly in low-energy QCD and can be possible in the presence of both magnetic and inhomogeneous electric fields. The quadrupole moment of the neutron may affect e.g. the properties of neutron stars and magnetars.