Sterile neutrino portal to Dark Matter I: the U(1) B−L case

Published for SISSA by Springer Received: June 27, 2016 Revised: January 12, 2017 Accepted: January 31, 2017 Published: February 8, 2017 Miguel Escudero,a Nuria Riusa and Verónica Sanzb a Departamento de Fı́sica Teórica and IFIC, Universidad de Valencia-CSIC, C/ Catedrático José Beltrán, 2, E-46980 Paterna, Spain b Department of Physics and Astronomy, University of Sussex, Falmer Campus, Brighton BN1 9QH, U.K. E-mail: , , Abstract: In this paper we explore the possibility that the sterile neutrino and Dark Matter sectors in the Universe have a common origin. We study the consequences of this assumption in the simple case of coupling the dark sector to the Standard Model via a global U(1)B−L , broken down spontaneously by a dark scalar. This dark scalar provides masses to the dark fermions and communicates with the Higgs via a Higgs portal coupling. We find an interesting interplay between Dark Matter annihilation to dark scalars — the CP-even that mixes with the Higgs and the CP-odd which becomes a Goldstone boson, the Majoron — and heavy neutrinos, as well as collider probes via the coupling to the Higgs. Moreover, Dark Matter annihilation into sterile neutrinos and its subsequent decay to gauge bosons and quarks, charged leptons or neutrinos lead to indirect detection signatures which are close to current bounds on the gamma ray flux from the galactic center and dwarf galaxies. Keywords: Beyond Standard Model, Neutrino Physics ArXiv ePrint: 1606.01258 Open Access, c The Authors. Article funded by SCOAP3 . doi:10.1007/JHEP02(2017)045 JHEP02(2017)045 Sterile neutrino portal to Dark Matter I: the U(1)B−L case Contents 1 2 A dark sector with U(1)B−L 2 3 Parametrization of the physical states 3.1 Neutrino masses 5 6 4 Phenomenology 4.1 Constraints from Higgs decays 4.2 Direct detection 4.3 Dark Matter relic abundance 4.4 Constraints from indirect searches and CMB 4.5 Self-interacting Dark Matter 7 8 10 11 14 17 5 Results 19 6 Conclusions and outlook 21 1 Introduction The study of the dark Universe is one of the best handles to understand what lies beyond the Standard Model (SM), particularly possible connections between Dark Matter and other sectors. The SM neutrino sector is especially interesting, as the observation of neutrino masses already points to new physics beyond the SM, possibly in the form of massive right-handed neutrinos. This raises the question whether these two new forms of massive particles, Dark Matter and right-handed neutrinos, are somewhat linked. A very minimal possibility would be that of right-handed neutrinos constituting the Dark Matter of the Universe [1]. Yet, this option is tightly constrained in a region of small mixing with active neutrinos and mass around the keV, which will be explored in upcoming experiments and potentially excluded, see [2] for a recent review on the subject. In this paper we propose a different scenario, where sterile neutrinos and a fermionic Dark Matter particle would have a common origin within a dark sector. These dark fermions would exhibit couplings to a dark scalar, which would bring a source of Majorana masses. The right-handed neutrinos would mix with active neutrinos, providing a link to the SM, which Dark Matter would inherit via exchanges of the dark scalar. Additionally, the dark scalar could couple to the SM via a Higgs portal, providing Dark Matter yet another mechanism to communicate with the SM. In this paper we choose the rather natural option of charging the dark sector under U(1)B−L , but another minimal choice would be to assume an exact symmetry of the dark sector which stabilizes the lightest dark –1– JHEP02(2017)045 1 Introduction 2 A dark sector with U(1)B−L We consider the following set-up: we extend the SM with a complex scalar field, φ and n chiral (RH) fermion fields, ΨR . All these new fields are SM singlets, and charged under a global U(1) symmetry which can be identified with U(1)B−L , so that Lφ = 2 and LΨR = 1.1 SM U (1)B L L H Dark N Moreover, we assume that (for the reasons explained below) some of the dark fermions have vanishing or suppressed coupling to the SM singlet operator LL H, so they could be stable (or cosmologically stable); we will denote such stable fermion(s) by χR , as opposed to the rest of the dark fermions, which we will call NR . Communication between the Standard Model fields and the new singlet sector (φ, ΨR ) is determined by the U(1)B−L charges and the requirement of renormalizability of the interactions. The relevant part of the Lagrangian reads: L ⊃ µ2H H † H − λH (H † H)2 + µ2φ φ† φ − λφ (φ† φ)2 − λHφ (H † H) (φ† φ) (2.1)     λχab λN ab α c − √ φ χRa χcRb + h.c. − √ φ N Ra NRb + h.c. − (Yαa LL HNRa + h.c.) 2 2 1 Note that U(1)B−L is the only anomaly-free global symmetry in the SM. Therefore, extensions of the SM including a gauged U(1)B−L have been considered in various contexts, and in particular in scenarios where the breaking appears at low-scale (e.g. [6–9]). –2– JHEP02(2017)045 particle and allows to communicate with the SM via the right-handed neutrinos, singlets under both the SM and the dark group, see [3, 4] and [5]. The paper is organized as follows. After presenting the set-up of our model in section 2, and the consequences of the breaking of U(1)B−L in the scalar sector in section 3, we move onto the phenomenology of the model in section 4. The study of Higgs decays and direct detection in sections 4.1 and 4.2, does lead to strong contraints on the mixing between the dark scalar and the Higgs. How Dark Matter can satisfy the observed relic abundance is explored in section 4.3, and the correlation with indirect detection in section 4.4. We discuss the implications of a strongly self-interacting Dark Matter in this model in section 4.5, just before moving onto summarizing our findings in section 5. We conclude in section 6 by providing a summary of the results and outlook of possible new directions of investigation. Fermionic Dark Matter: χR could be: possible mechanisms to ensure stability of the dark fermions 1. Z2 symmetry: the simplest possibility is that the fermions χR are odd under an exact Z2 symmetry, while all the SM particles, the singlet scalar φ and the remaining fermions NR are even. Then, the Yukawa coupling of χR to SM leptons will be forbidden, resulting on a stable sterile neutrino Dark Matter. 2. Compositeness: the dark sector is a low-energy description of a new strongly coupled sector (charged under the global U(1)B−L ), with the dark particles bound states of the α strong dynamics. Mixing between the SM operator LL H and fermionic bound states Oa with lepton number are allowed, but the strength of this mixing is determined by the anomalous dimension of Oa . One could also describe this set-up in terms of a holographic dual, where operators from a strongly coupled sector like Oa are represented by states living in more than 4D, Oa (x) → χR (x, z), with z is the extra dime (...truncated)