Dark Sector Effective Field Theory

Abstract

We introduce the effective field theory of two different light dark particles interacting with the standard model (SM) light states in a single vertex, termed dark sector effective field theory (DSEFT). We focus on the new light particles with spin up to 1 and being real in essence, namely, new real scalars $\varphi$ and $S$, Majorana fermions $\chi$ and $\psi$, and real vectors $X_{\mu }$ and $V_{\mu }$. In the framework of low energy effective field theory with QED and QCD symmetry, the DSEFT can be classified into six categories, including the scalar-scalar-SM ($\varphi S$-SM), fermion-fermion-SM ($\chi \psi$-SM), vector-vector-SM ($XV$-SM), scalar-fermion-SM ($\varphi \chi$-SM), scalar-vector-SM ($\varphi X$-SM), and fermion-vector-SM ($\chi X$-SM) cases. For each case, we construct the effective operator basis up to canonical dimension 7, which will cover most interesting phenomenology at low energy. As a phenomenological example, we investigate the longstanding neutron lifetime anomaly through the neutron dark decay modes $n\to \chi \varphi \text{ or }\chi X$ from the effective interactions in the fermion-scalar-SM or fermion-vector-SM case. When treating the light fermion as a dark matter candidate, we also explore the constraints from DM-neutron annihilation signal at Super-Kamiokande. We find the neutron dark decay in each scenario can accommodate the anomaly, at the same time, without contradicting with the Super-Kamiokande limit.

Comments: 33 pages, 4 figures, typos are corrected and several new references are included. To appear in JHEP