The Phenomenology of Right Handed Neutrinos (1303.6912v3)

Abstract: Neutrinos are the only particles in the Standard Model of particle physics that have only been observed with left handed chirality to date. If right handed neutrinos exist, they could be responsible for several phenomena that have no explanation within the Standard Model, including neutrino oscillations, the baryon asymmetry of the universe, dark matter and dark radiation. After a pedagogical introduction, we review recent progress in the phenomenology of right handed neutrinos. We in particular discuss the mass ranges suggested by hints for neutrino oscillation anomalies and dark radiation (eV), sterile neutrino dark matter scenarios (keV) and experimentally testable theories of baryogenesis (GeV to TeV). We summarize constraints from theoretical considerations, laboratory experiments, astrophysics and cosmology for each of these.

• The paper demonstrates how right-handed neutrinos can generate small neutrino masses via the seesaw mechanism while constraining their mass ranges.
• It highlights the potential of keV-scale right-handed neutrinos as warm dark matter candidates that influence cosmic structure formation.
• The study shows that heavy right-handed neutrinos can drive leptogenesis to explain the baryon asymmetry, despite significant experimental challenges.

The Phenomenology of Right-Handed Neutrinos

The paper "The Phenomenology of Right-Handed Neutrinos" provides a comprehensive exploration of the potential roles and implications of right-handed neutrinos within and beyond the Standard Model (SM) of particle physics. It addresses the significant gaps in our current understanding of particle physics, such as neutrino mass, dark matter (DM), the baryon asymmetry of the universe (BAU), and possible deviations from the established model in cosmological studies.

The existence of right-handed (RH) neutrinos is one of the simplest extensions to the SM, motivated by the observation that left-handed neutrinos have been experimentally detected, yet their right-handed counterparts remain elusive. RH neutrinos are presumed to be singlet under all SM gauge interactions and could demonstrate extremely weak interactions due to their hypothesized heavy Majorana masses. The paper scrutinizes the plausible mass ranges for RH neutrinos and evaluates their potential to bridge critical gaps in our understanding of neutrino oscillations, baryogenesis, and dark matter.

Neutrino Mass and Oscillation

Neutrinos, within the SM, are conceptualized as massless. However, neutrino oscillations, an experimentally verified phenomenon, necessitate the existence of mass differences among neutrino generations. The paper discusses how RH neutrinos can partake in the seesaw mechanism, providing a natural explanation for the smallness of neutrino masses. This model suggests that if RH neutrinos are substantially heavier than the electroweak scale, the induced effective mass for the active neutrinos pushes them into the observable range of oscillations.

In terms of mass, RH neutrinos could span several orders of magnitude, from very light (eV scale) to extremely heavy (up to GUT scale around $10^{15}$ GeV). Each mass range implicates different aspects of phenomenology; for instance, keV-scale RH neutrinos are compelling dark matter candidates, whereas those at the GeV to TeV scale may influence baryogenesis and might be testable at current colliding facilities.

Dark Matter and Cosmological Implications

RH neutrinos are compelling candidates for dark matter if they exist at keV masses, potentially produced in the early universe via a process akin to the Dodelson-Widrow mechanism. Such neutrinos would be warm dark matter (WDM), having implications on the large-scale structures of the universe. This aspect is particularly appealing as it offers a potential resolution to the aforementioned small-scale issues with cold dark matter (CDM), such as the core-cusp problem and satellite galaxy abundance discrepancies.

Moreover, RH neutrinos with eV masses could contribute to the effective number of neutrino species ($N_{\text{eff}}$) affecting the cosmic microwave background (CMB) and big bang nucleosynthesis (BBN). The plausibility of eV-scale sterile neutrinos filling this role remains a hot topic as recent observations challenge this notion, suggesting they might not thermalize in the early universe as once thought.

Baryogenesis and Leptogenesis

Beyond the SM, RH neutrinos provide a robust mechanism for generating the baryon asymmetry of the universe via leptogenesis. The paper discusses how the heavy Majorana neutrinos can induce a lepton asymmetry through CP-violating decays in the early universe, a fraction of which gets converted into baryon asymmetry via weak sphalerons. This mechanism requires the masses of RH neutrinos to be quite heavy, typically above the electroweak scale, to ensure the asymmetry is substantial enough and survives washout processes.

Experimental Prospects and Challenges

While the indirect influence of RH neutrinos is extensively discussed, direct observation poses a significant challenge. The paper explores the practicalities and prospects of detecting RH neutrinos through current and planned experimental setups, including neutrino oscillation experiments, collider searches at the LHC, and astrophysical observations. These remain pivotal in either sighting such particles or further constraining their parameter space.

Conclusion

Overall, this paper underscores the multifaceted role that RH neutrinos could play in addressing pivotal unresolved questions in modern physics. While theoretical motivations are compelling, it also underscores the need for experimental confirmation. Detecting or precisely constraining RH neutrinos is paramount in either affirming their hypothesized existence or refining our theories beyond the Standard Model. Future experimental endeavors combined with astrophysical observations hold the promise of potentially observing these elusive particles or, in their absence, paving the way toward new physics paradigms. However, challenges persist, primarily due to their weakly interacting nature, requiring innovative experimental strategies and precise astronomical observations.