Light Sterile Neutrinos: A White Paper (1204.5379v1)

Abstract: This white paper addresses the hypothesis of light sterile neutrinos based on recent anomalies observed in neutrino experiments and the latest astrophysical data.

• The paper presents detailed reviews of theoretical models predicting light sterile neutrinos beyond the Standard Model and their experimental implications.
• It analyzes key experimental findings, including the LSND 3.8σ anomaly and MiniBooNE observations, evidencing neutrino oscillation discrepancies.
• The study highlights cosmological constraints on N_eff and potential dark matter connections, fostering new avenues for future research.

Overview of Sterile Neutrinos in Neutrino Physics and Cosmology

The white paper on Light Sterile Neutrinos reviews various facets of theoretical and experimental investigations regarding the hypothesized light sterile neutrino states beyond the Standard Model (SM). These states are not active in weak interactions but can have implications in diverse areas from explaining anomalies in neutrino oscillation to potential contributions to dark matter.

At the theoretical level, several models predict the existence of sterile neutrinos primarily through extensions of the neutrino mass sector. These are often realized via seesaw mechanisms, including the type I seesaw where the introduction of heavy Majorana neutrinos generates small active neutrino masses. The exact role and mass scale of light sterile neutrinos can vary significantly depending on specific theoretical models, such as low-energy seesaws or split seesaws, each offering distinct motivations for the richness of the neutrino sector.

Cosmology presents another arena where sterile neutrinos might manifest effects. Observations of cosmic microwave background (CMB) anisotropies and Big Bang nucleosynthesis suggest constraints on the effective number of neutrino species, typically denoted as $N_{\text{eff}}$. Deviations from the SM prediction, where $N_{\text{eff}}=3.046$, could indicate sterile neutrino thermalization that adds relativistic degrees of freedom beyond the known three flavors. Moreover, sterile neutrinos might have implications for matter-antimatter asymmetry in the universe, interacting neutrinos in core-collapse supernovae, and impacts on the formation of large-scale structures in the universe.

Several experimental anomalies suggestive of sterile neutrinos include the LSND experiment, where a 3.8\sigma anomaly for $\bar{\nu}_{\mu} \to \bar{\nu}_e$ transitions was observed, and the MiniBooNE results, which appear consistent with LSND when running in antineutrino mode. These experiments provide compelling hints of new physics at new mass scales, thus offering potential evidence of sterile neutrinos. Furthermore, reactor antineutrino and Gallium source experiments have presented anomalies where observed neutrino fluxes are below theoretical expectations, possibly due to oscillations involving sterile states.

Experimental and observational developments continue as the neutrino research community pursues more extensive investigations into these anomalies. Proposed future experiments, ranging from short-baseline neutrino experiments to large-scale cosmological surveys, hold the potential to definitively confirm or rule out sterile neutrino scenarios. Moreover, the potential interactions of sterile neutrinos with new force carriers are being explored to understand their possible role in long-range interactions and their contributions as dark matter candidates.

Overall, the sterile neutrino hypothesis presents a tantalizing extension to the SM, bearing implications across particle physics, astroparticle physics, and cosmology. While empirical data imply promising hints, crafting a coherent picture requires resolving current discrepancies and achieving consistent evidence across various domains of physics. Future prospects and multidisciplinary collaboration will be crucial in resolving the mystery and potentially reshaping our understanding of the neutrino sector and beyond.