Neutrino masses are clear evidence for physics beyond the standard model and much more remains to be understood about the neutrino sector. We highlight some of the outstanding questions and research opportunities in neutrino theory. We show that most
of these questions are directly connected to the very rich experimental program currently being pursued (or at least under serious consideration) in the United States and worldwide. Finally, we also comment on the state of the theoretical neutrino physics community in the U.S.
We study the capabilities of the MAJORANA DEMONSTRATOR, a neutrinoless double-beta decay experiment currently under construction at the Sanford Underground Laboratory, as a light WIMP detector. For a cross section near the current experimental bound,
the MAJORANA DEMONSTRATOR should collect hundreds or even thousands of recoil events. This opens up the possibility of simultaneously determining the physical properties of the dark matter and its local velocity distribution, directly from the data. We analyze this possibility and find that allowing the dark matter velocity distribution to float considerably worsens the WIMP mass determination. This result is traced to a previously unexplored degeneracy between the WIMP mass and the velocity dispersion. We simulate spectra using both isothermal and Via Lactea II velocity distributions and comment on the possible impact of streams. We conclude that knowledge of the dark matter velocity distribution will greatly facilitate the mass and cross section determination for a light WIMP.
We point out that stars in the mass window ~ 8-12 Msun can serve as sensitive probes of the axion-photon interaction, g_{Agammagamma}. Specifically, for these stars axion energy losses from the helium-burning core would shorten and eventually elimina
te the blue loop phase of the evolution. This would contradict observational data, since the blue loops are required, e.g., to account for the existence of Cepheid stars. Using the MESA stellar evolution code, modified to include the extra cooling, we conservatively find g_{Agammagamma} <~ 0.8 * 10^{-10} GeV^{-1}, which compares favorably with the existing bounds.
We examine the prospects of probing nonstandard interactions (NSI) of neutrinos in the e-tau sector with upcoming long-baseline nu_mu -> nu_e oscillation experiments. First conjectured decades ago, neutrino NSI remain of great interest, especially in
light of the recent 8B solar neutrino measurements by SNO, Super-Kamiokande, and Borexino. We observe that the recent discovery of large theta_13 implies that long-baseline experiments have considerable NSI sensitivity, thanks to the interference of the standard and new physics conversion amplitudes. In particular, in some parts of NSI parameter space, the upcoming NOvA experiment will be sensitive enough to see ~ 3sigma deviations from the SM-only hypothesis. On the flip side, NSI introduce important ambiguities in interpreting NOvA results as measurements of CP-violation, the mass hierarchy and the octant of theta_23. In particular, observed CP violation could be due to a phase coming from NSI, rather than the vacuum Hamiltonian. The proposed LBNE experiment, with its longer ~ 1300 km baseline, may break many of these interpretative degeneracies.
Monojet events at colliders have been used to probe models of dark matter and extra dimensions. We point out that these events also probe extensions of the Standard Model modifying neutrino-quark interactions. Such nonstandard interactions (NSI) have
been discussed in connection with neutrino oscillation experiments. Assuming first that NSI remain contact at LHC energies, we derive stringent bounds that approach the levels suggested by the Boron-8 solar data. We next explore the possibility that the mediators of the NSI can be produced at colliders. The constraints are found to be strongest for mediator masses in the 10^2-10^3 GeV range, with the best bounds above ~ 200 GeV coming from ATLAS and below from CDF. For mediators with masses below 30 GeV the monojet bounds are weaker than in the contact limit. These results also directly apply to light dark matter searches. Lastly, we discuss how neutrino NSI can be distinguished from dark matter or Kaluza-Klein states with charged lepton searches.
Recently, it has been demonstrated that neutrinos in a supernova oscillate collectively. This process occurs much deeper than the conventional matter-induced MSW effect and hence may have an impact on nucleosynthesis. In this paper we explore the eff
ects of collective neutrino oscillations on the r-process, using representative late-time neutrino spectra and outflow models. We find that accurate modeling of the collective oscillations is essential for this analysis. As an illustration, the often-used single-angle approximation makes grossly inaccurate predictions for the yields in our setup. With the proper multiangle treatment, the effect of the oscillations is found to be less dramatic, but still significant. Since the oscillation patterns are sensitive to the details of the emitted fluxes and the sign of the neutrino mass hierarchy, so are the r-process yields. The magnitude of the effect also depends sensitively on the astrophysical conditions - in particular on the interplay between the time when nuclei begin to exist in significant numbers and the time when the collective oscillation begins. A more definitive understanding of the astrophysical conditions, and accurate modeling of the collective oscillations for those conditions, is necessary.
We investigate collective flavor oscillations of supernova neutrinos at late stages of the explosion. We first show that the frequently used single-angle (averaged coupling) approximation predicts oscillations close to, or perhaps even inside, the ne
utrinosphere, potentially invalidating the basic neutrino transport paradigm. Fortunately, we also find that the single-angle approximation breaks down in this regime; in the full multiangle calculation, the oscillations start safely outside the transport region. The new suppression effect is traced to the interplay between the dispersion in the neutrino-neutrino interactions and the vacuum oscillation term.
