The Phenomenology of Right Handed Neutrinos

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.

Figures (15)

• Figure 1: The particle content of the SM. Are we missing the right handed partner of the neutrinos? Picture taken from Shaposhnikov:2013dra.
• Figure 2: A schematic illustration of the relation between $F$ and $M_M$ in the seesaw limit $m_D\ll M_M$. Individual elements of the matrices $F$ and $M_M$ can deviate considerably from this if there are cancellations in (\ref{['activeneutrinomasses']}). Plot taken from Abazajian:2012ys.
• Figure 3: Left plot: The region in the mass-mixing plane preferred by the LSND anomaly (coloured bands) is compared to the region allowed by MiniBooNe (coloured lines) and exclusion plots by other experiments as indicated in the plot; plot taken from Antonello:2012pq. Right plot: region allowed by the combined reactor and gallium anomalies, taken from Abazajian:2012ys. Data from both is e.g. combined in figure 4 in Kopp2013.
• Figure 4: Diagram for neutrinoless double beta decay. Here we have made the lepton number flow explicit by assigning arrows to fermion lines. The "clashing arrows" in the center of the diagram are allowed because neutrinos and antineutrinos are indistinguishable if they are Majorana particles. If some $N_I$ are light enough, they may also be exchanged instead of $\upnu_i$. The amplitude for this process vanishes in the limit $M_M\rightarrow 0$.
• Figure 5: Current bounds on $m_{ee}$ (here called $m_{\beta\beta}$) as a function of the smallest neutrino mass, as summarized in Bilenky:2012qi. "NS" refers to normal hierarchy, "IS" to inverted hierarchy. The cosmological limit has tightened with the publication of the Planck results Ade:2013lta.