Sterile Neutrino Dark Matter

This book is a new look at one of the hottest topics in contemporary science, Dark Matter. It is the pioneering text dedicated to sterile neutrinos as candidate particles for Dark Matter, challenging some of the standard assumptions which may be true for some Dark Matter candidates but not for all. So, this can be seen either as an introduction to a specialized topic or an out-of-the-box introduction to the field of Dark Matter in general. No matter if you are a theoretical particle physicist, an observational astronomer, or a ground based experimentalist, no matter if you are a grad student or an active researcher, you can benefit from this text, for a simple reason: a non-standard candidate for Dark Matter can teach you a lot about what we truly know about our standard picture of how the Universe works.

We present a comprehensive review of keV-scale sterile neutrino Dark Matter, collecting views and insights from all disciplines involved - cosmology, astrophysics, nuclear, and particle physics - in each case viewed from both theoretical and experimental/observational perspectives. After reviewing the role of active neutrinos in particle physics, astrophysics, and cosmology, we focus on sterile neutrinos in the context of the Dark Matter puzzle. Here, we first review the physics motivation for sterile neutrino Dark Matter, based on challenges and tensions in purely cold Dark Matter scenarios. We then round out the discussion by critically summarizing all known constraints on sterile neutrino Dark Matter arising from astrophysical observations, laboratory experiments, and theoretical considerations. In this context, we provide a balanced discourse on the possibly positive signal from X-ray observations. Another focus of the paper concerns the construction of particle physics models, aiming to explain how sterile neutrinos of keV-scale masses could arise in concrete settings beyond the Standard Model of elementary particle physics. The paper ends with an extensive review of current and future astrophysical and laboratory searches, highlighting new ideas and their experimental challenges, as well as future perspectives for the discovery of sterile neutrinos.

Sterile neutrinos can be generated in the early universe through oscillations with active neutrinos and represent a popular and well-studied candidate for our universe's dark matter. Stringent constraints from X-ray and gamma-ray line searches, however, have excluded the simplest of such models. In this letter, we propose a novel alternative to the standard scenario in which the mixing angle between the sterile and active neutrinos is a dynamical quantity, induced through interactions with a light axion-like field. As the energy density of the axion-like particles is diluted by Hubble expansion, the degree of mixing is reduced at late times, suppressing the decay rate and easily alleviating any tension with X-ray or gamma-ray constraints. We present a simple model which illustrates the phenomenology of this scenario, and also describe a framework in which the QCD axion is responsible for the production of sterile neutrinos in the early universe.

Non-standard interactions (NSI) between active neutrinos (neutrinos belonging to the Standard Model) have the potential to modify the production of a fourth type of keV-scale mass neutrino (called "sterile"), due to active-sterile neutrino oscillations in the early Universe, a process known as the Dodelson-Widrow mechanism (DW). Through this modified DW mechanism, enough sterile neutrinos could be produced to account for the whole of the dark matter. This type of dark matter production occurred before big bang nucleosynthesis (BBN), an era just before the temperature of the Universe settled to 5 MeV, which is currently unknown. Thus, this type of dark matter production can be explored with different pre-BBN cosmological models. Taking into consideration the uncertainties in the pre-BBN cosmology besides the effects of NSI, we may identify a viable parameter space for sterile neutrino searches allowed by astrophysical limits and assess their effect on the range of mixing angles for keV-scale mass sterile neutrino dark matter. Detection of a sterile neutrino could provide insight into the pre-BBN era of the Universe. We find that the combined uncertainties in the early Universe cosmologies, as well as in the active neutrino interactions, significantly expand the allowed parameter space for sterile neutrinos that could constitute the whole of the dark matter. This thesis work will be published in a paper with the same title by C. Chichiri, G. Gelmini, P. Lu and V. Takhistov, already posted as a preprint in the arXiv,e-Print: 2.111.04087[hep-ph], and submitted for publication to the Journal of Cosmology and Astroparticle Physics (JCAP).

We focus on two dark matter candidates: sterile neutrinos and primordial black holes (PBH). We explore the effects of non-standard pre-Big Bang Nucleosynthesis (pre-BBN) cosmologies, such scalar-tensor and kination cosmologies, on the abundance of sterile neutrinos over a large range of masses. In particular, sterile neutrinos of keV-scale mass represent a viable warm dark matter candidate whose decay can generate the putative 3.5 keV X-ray signal observed in galaxy and galaxy clusters. eV-scale sterile neutrinos can be the source of various accelerator/beam neutrino oscillation anomalies. Two production mechanisms are considered here, a collisional non-resonant Dodelson-Widrow (DW) mechanism and a resonant Shi-Fuller (SF) conversion (which requires a large lepton asymmetry). The DW mechanism is a freeze-in process, and the final abundance of sterile neutrinos using this production method is inversely proportional to the Hubble expansion rate. We find that in one of the scalar tensor models we consider, the sterile neutrino parameters necessary to generate the tentative 3.5 keV signal would be within reach of the TRISTAN upgrade to the ongoing KATRIN experiment as well as the planned upgrades to the HUNTER experiment, however the contribution to the dark matter density would be very small. In another scalar tensor model, sterile neutrinos could both generate the X-ray signal and comprise much of dark matter. In our study of resonant production, we find that the parameter space in which coherent and adiabatic resonant production can occur shifts with changing pre-BBN cosmology. We find that for a broad range of parameters (mass, mixing angle, lepton asymmetry), resonance can occur in the LSND/MiniBooNE and DANS/NEOSS experiments' preferred regions for at least one of the non-standard cosmologies we consider. With respect to PBH as dark matter candidates, we derive a new type of cosmology-independent bound. We consider the heating of the surrounding interstellar medium gas by dynamical friction and from the formation of accretion disks around intermediate mass $10-10^{5} M_\odot$ PBH. By estimating the cooling rate and assuming thermal equilibrium, we derive a new constraint. Light PBH with mass $10^{15}-10^{17}$ g emit significant Hawking radiation and are constrained by the same cooling argument. We extend this analysis to PBH with extreme spin, which results in stronger bounds compared to non-spinning PBH.

Based on a Simons Symposium held in 2018, the proceedings in this volume focus on the theoretical, numerical, and observational quest for dark matter in the universe. Present ground-based and satellite searches have so far severely constrained the long-proposed theoretical models for dark matter. Nevertheless, there is continuously growing astrophysical and cosmological evidence for its existence. To address present and future developments in the field, novel ideas, theories, and approaches are called for. The symposium gathered together a new generation of experts pursuing innovative, more complex theories of dark matter than previously considered.This is being done hand in hand with experts in numerical astrophysical simulations and observational techniques—all paramount for deciphering the nature of dark matter. The proceedings volume provides coverage of the most advanced stage of understanding dark matter in various new frameworks. The collection will be useful for graduate students, postdocs, and investigators interested in cutting-edge research on one of the biggest mysteries of our universe.