Sterile neutrino dark matter within the vSMEFT

Sterile neutrino dark matter within the vSMEFT

ABSTRACT: Sterile neutrinos with masses at the keV scale and mixing to the active neutrinos offer an elegant explanation of the observed dark matter density. However, the very same mixing inevitably leads to radiative photon emission and the non-observation of such peaked X-ray lines rules out this minimal sterile neutrino dark matter hypothesis. We show that in the context of the Standard Model effective field theory with sterile neutrinos (vSMEFT), higher dimensional operators can produce sterile neutrino dark matter in a broad range of parameter space. In particular, vSMEFT interactions can open the large mixing parameter space due to their destructive interference, through operator mixing or matching, in the X-ray emission. We also find that, even in the zero mixing limit, the dark matter density can always be explained by vSMEFT operators. The testability of the studied vSMEFT operators in searches for electric dipole moments, neutrinoless double beta decay, and pion decay measurements is discussed.

One Introduction

The observation of neutrino oscillations establishes more than fifty evidence for beyond the Standard Model physics. Furthermore, astrophysical and cosmological observations demonstrate that approximately twenty-six percent of the energy density of our Universe is attributed to a beyond the Standard Model matter component. It is well known that the introduction of sterile neutrinos in the Standard Model (SM) can solve both problems simultaneously, while being minimal in the theoretical realization. The light neutrino mass spectrum in the arguably most minimal framework, the type-One seesaw, can be generated by two sterile neutrinos, which mix to the active sector via Yukawa interactions. It was first pointed out by Dodelson and Widrow that the inclusion of a third sterile neutrino (Vs) with mass at the keV scale and small mixing can account for the dark matter density. Its non-thermal production in the minimal realization fixes a unique relation between the active-sterile neutrino mixing, zero, and the sterile neutrino mass mys:

However, for the vs to be dark matter, the resulting mixing is experimentally excluded due to non-observations of X-rays arising from the radiative decay vs to Va Y. A plethora of proposals extending the Dodelson and Widrow mechanism to circumvent the X-ray constraint exist in literature and they can be divided into the following two classes: i) Lower the mixing of equation one point one required to meet the correct dark matter abundance. This can be achieved for example by introducing additional interactions of the sterile neutrino or via its resonant production in a background of large lepton asymmetries. ii) For mixings as large as those of equation one point one additional sterile neutrino interactions lead to a destructive interference on the amplitude level of vs to Va Y.

Current analyses in the literature so far have been limited to specific models and assumptions. In this paper we propose an agnostic approach to the sterile neutrino dark matter phenomenology within the context of effective field theories. In particular, we consider the Standard Model Effective Field Theory (SMEFT), extended by sterile neutrinos (vSMEFT), and study the effects of sterile neutrino interactions including up to dimension-six operators. Both modifications of the original Dodelson and Widrow mechanism as pointed out above in i) and ii) are found in the vSMEFT context. Special attention is paid to vSMEFT operators leading to neutrino photon dipoles at low energies. By generalizing to dipole operators the one-loop renormalization group equations and one-loop matching corrections at the electroweak scale, we find that seven out of sixteen of the dimension-six vSMEFT operators can induce sizable neutrino photon dipoles. For each of them, we identify the new physics scale at which X-ray constraints are evaded while the sterile neutrino can be dark matter.

The paper is organized as follows. In Section two we first provide a concise review of neutrino masses and discuss the diagonalization of the neutrino mass matrix. Then we introduce the vSMEFT basis, derive the renormalization group equations for the mixing of dimension-six operators onto dipoles in vSMEFT, calculate matching corrections at the electroweak scale, and discuss additional renormalization group evolution from the electroweak scale to low energy in the sterile neutrino extended Low Energy Effective Field Theory (vLEFT). In Section three the sterile neutrino dark matter production via mixing and the associated constraints are discussed. Then we derive the dark matter production via dimension-six vSMEFT interactions and confront the found parameter space to X-ray constraints on sterile neutrino dark matter. Special attention is paid to scenarios in which X-ray emission i) is suppressed due to destructive interference effects and ii) arises solely from tree-level vSMEFT interactions. For both cases we demonstrate that the sterile neutrinos can match the observed dark matter abundance. Section four considers possible laboratory probes for the vSMEFT interactions that can lead to sterile neutrino dark matter. In Section five we conclude.