Sterile Neutrinos (2106.05913v1)

Abstract: Neutrinos, being the only fermions in the Standard Model of Particle Physics that do not possess electromagnetic or color charges, have the unique opportunity to communicate with fermions outside the Standard Model through mass mixing. Such Standard Model-singlet fermions are generally referred to as "sterile neutrinos''. In this review article, we discuss the theoretical and experimental motivation for sterile neutrinos, as well as their phenomenological consequences. With the benefit of hindsight in 2020, we point out potentially viable and interesting ideas. We focus in particular on sterile neutrinos that are light enough to participate in neutrino oscillations, but we also comment on the benefits of introducing heavier sterile states. We discuss the phenomenology of eV-scale sterile neutrinos in terrestrial experiments and in cosmology, we survey the global data, and we highlight various intriguing anomalies. We also expose the severe tension that exists between different data sets and prevents a consistent interpretation of the global data in at least the simplest sterile neutrino models. We discuss non-minimal scenarios that may alleviate some of this tension. We briefly review the status of keV-scale sterile neutrinos as dark matter and the possibility of explaining the matter-antimatter asymmetry of the Universe through leptogenesis driven by yet heavier sterile neutrinos.

Essay on "Sterile Neutrinos"

The review paper under analysis provides a comprehensive exploration of sterile neutrinos—hypothetical elementary particles that extend beyond the conventional expectations within the Standard Model of particle physics. Sterile neutrinos are unique in that they do not interact via standard weak interactions and are considered to interact through mass mixing with existing neutrino flavors. The concept of sterile neutrinos has grown in significance due to various experimental anomalies and their potential contributions to several cosmological phenomena.

Theoretical Framework and Motivation

The paper explores various theoretical models that incorporate sterile neutrinos, starting with the basic Type-I seesaw mechanism that demands heavy sterile neutrino masses to explain the smallness of active neutrino masses. It further discusses the Neutrino Minimal Standard Model ($\nu$MSM), which proposes the introduction of one keV-scale sterile neutrino as a candidate for dark matter and eV-scale sterile neutrinos to participate in neutrino oscillations.

The paper outlines more sophisticated models such as the Inverse Seesaw and Extended Seesaw, where smaller mass scales for sterile neutrinos are naturally generated. These frameworks are theorized to accommodate sterile neutrinos and are interesting for experiments focusing on oscillations, astrophysical observations, and cosmology.

Experimental Evidence and Discrepancies

Significant portions of the research paper focus on experimental anomalies suggesting the presence of sterile neutrinos—these are primarily rooted in anomalies observed in short-baseline reactor experiments, the LSND, and MiniBooNE experiments, and neutrino experiments utilizing intense radioactive sources such as GALLEX and SAGE. Sterile neutrinos have emerged as one of the possible solutions to reconcile these anomalies, although fitting them consistently with all experimental data, especially constraints from $\nu_\mu$ disappearance, presents challenges.

While $\nu_\mu \to \nu_e$ conversions seem consistent among short-baseline experiments, sharp conflict arises when reconciling these with $\nu_\mu$ disappearance data. This tension is highlighted with global analysis, demonstrating that simple three-neutrino models with additional sterile neutrino mixing cannot fully explain the observed anomalies without further theoretical adjustments.

Cosmological Implications

Further complicating their existence, sterile neutrinos face stringent constraints from cosmological observations, such as the Cosmic Microwave Background (CMB) and Big Bang Nucleosynthesis (BBN). The paper stresses that a simple extension involving sterile neutrinos would generally lead to unacceptable contributions to the effective number of neutrino species ($N_\text{eff}$) and sum of the neutrino masses ($\sum m_\nu$) affecting the cosmic abundance and structure formation.

Nevertheless, there exist scenarios like 'secret interactions', where sterile neutrinos are hypothesized to interact via a new dark gauge boson, possibly creating cosmologically acceptable models. These interactions could help reconcile short-baseline experimental hints with mainstream cosmological data by suppressing the production of sterile neutrinos in the early Universe.

Heavy Sterile Neutrinos and Beyond

The analysis extends beyond eV-scale sterile neutrinos, hypothesizing keV-scale sterile neutrinos as dark matter candidates due to their sufficient stability and absence of interactions coupled to predictions from a series of observations, including X-ray signals and constraints from the Tremaine-Gunn limits on phase space distribution in galactic halos. Although intriguing, confirmed detection in cosmic X-ray signals remains contentious and requires further investigation.

In addition, the role of heavy sterile neutrinos in explaining the baryon asymmetry through leptogenesis is addressed. The paper suggests that the decay of heavy sterile neutrinos may have generated an excess of matter over antimatter in the early Universe, fitting the Sakharov conditions for baryogenesis. However, these mechanisms often involve very high energies, which pose a challenge for practical experimental interrogation.

Conclusion

In summary, the review underscores the rich tapestry of sterile neutrinos from theoretical, experimental, and cosmological perspectives. Despite challenges in reconciliation across these domains, sterile neutrinos remain a vibrant subject of inquiry with potential implications for several fundamental questions, including dark matter composition and the origins of matter asymmetry. Ultimately, further advances in experimental techniques and cosmological observations will determine the viability of sterile neutrinos as a component of our understanding of the Universe.