Theoretically palatable flavor combinations of astrophysical neutrinos

The flavor composition of high-energy astrophysical neutrinos can reveal the physics governing their production, propagation, and interaction. The IceCube Collaboration has published the first experimental determination of the ratio of the flux in each flavor to the total. We present, as a theoretical counterpart, new results for the allowed ranges of flavor ratios at Earth for arbitrary flavor ratios in the sources. Our results will allow IceCube to more quickly identify when their data imply standard physics, a general class of new physics with arbitrary (incoherent) combinations of mass eigenstates, or new physics that goes beyond that, e.g., with terms that dominate the Hamiltonian at high energy.

Spallation Backgrounds in Super-Kamiokande Are Made in Muon-Induced Showers

Crucial questions about solar and supernova neutrinos remain unanswered. Super-Kamiokande has the exposure needed for progress, but detector backgrounds are a limiting factor. A leading component is the beta decays of isotopes produced by cosmic-ray muons and their secondaries, which initiate nuclear spallation reactions. Cuts of events after and surrounding muon tracks reduce this spallation decay background by $simeq 90%$ (at a cost of $simeq 20%$ deadtime), but its rate at 6--18 MeV is still dominant. A better way to cut this background was suggested in a Super-Kamiokande paper [Bays {it et al.}, Phys.~Rev.~D {bf 85}, 052007 (2012)] on a search for the diffuse supernova neutrino background. They found that spallation decays above 16 MeV were preceded near the same location by a peak in the apparent Cherenkov light profile from the muon; a more aggressive cut was applied to a limited section of the muon track, leading to decreased background without increased deadtime. We put their empirical discovery on a firm theoretical foundation. We show that almost all spallation decay isotopes are produced by muon-induced showers and that these showers are rare enough and energetic enough to be identifiable. This is the first such demonstration for any detector. We detail how the physics of showers explains the peak in the muon Cherenkov light profile and other Super-K observations. Our results provide a physical basis for practical improvements in background rejection that will benefit multiple studies. For solar neutrinos, in particular, it should be possible to dramatically reduce backgrounds at energies as low as 6 MeV.

Cosmic neutrino cascades from secret neutrino interactions

The first detection of high-energy astrophysical neutrinos by IceCube provides new opportunities for tests of neutrino properties. The long baseline through the Cosmic Neutrino Background (C$ u$B) is particularly useful for directly testing secret neutrino interactions ($ u$SI) that would cause neutrino-neutrino elastic scattering at a larger rate than the usual weak interactions. We show that IceCube can provide competitive sensitivity to $ u$SI compared to other astrophysical and cosmological probes, which are complementary to laboratory tests. We study the spectral distortions caused by $ u$SI with a large s-channel contribution, which can lead to a dip, bump, or cutoff on an initially smooth spectrum. Consequently, $ u$SI may be an exotic solution for features seen in the IceCube energy spectrum. More conservatively, IceCube neutrino data could be used to set model-independent limits on $ u$SI. Our phenomenological estimates provide guidance for more detailed calculations, comparisons to data, and model building.

First calculation of cosmic-ray muon spallation backgrounds for MeV astrophysical neutrino signals in Super-Kamiokande

When muons travel through matter, their energy losses lead to nuclear breakup (spallation) processes. The delayed decays of unstable daughter nuclei produced by cosmic-ray muons are important backgrounds for low-energy astrophysical neutrino experiments, e.g., those seeking to detect solar neutrino or Diffuse Supernova Neutrino Background (DSNB) signals. Even though Super-Kamiokande has strong general cuts to reduce these spallation-induced backgrounds, the remaining rate before additional cuts for specific signals is much larger than the signal rates for kinetic energies of about 6 -- 18 MeV. Surprisingly, there is no published calculation of the production and properties of these backgrounds in water, though there are such studies for scintillator. Using the simulation code FLUKA and theoretical insights, we detail how muons lose energy in water, produce secondary particles, how and where these secondaries produce isotopes, and the properties of the backgrounds from their decays. We reproduce Super-Kamiokande measurements of the total background to within a factor of 2, which is good given that the isotope yields vary by orders of magnitude and that some details of the experiment are unknown to us at this level. Our results break aggregate data into component isotopes, reveal their separate production mechanisms, and preserve correlations between them. We outline how to implement more effective background rejection techniques using this information. Reducing backgrounds in solar and DSNB studies by even a factor of a few could help lead to important new discoveries.

Observing the Next Galactic Supernova

We model the distance, extinction, and magnitude probability distributions of a successful Galactic core-collapse supernova (ccSN), its shock breakout radiation, and its massive star progenitor. We find, at very high probability (~100%), that the next Galactic SN will easily be detectable in the near-IR and that near-IR photometry of the progenitor star very likely (~92%) already exists in the 2MASS survey. Most ccSNe (~98%) will be easily observed in the optical, but a significant fraction (~43%) will lack observations of the progenitor due to a combination of survey sensitivity and confusion. If neutrino detection experiments can quickly disseminate a likely position (~3 deg), we show that a modestly priced IR camera system can probably detect the shock breakout radiation pulse even in daytime (~64% for the cheapest design). Neutrino experiments should seriously consider adding such systems, both for their scientific return and as an added and internal layer of protection against false triggers. We find that shock breakouts from failed ccSNe of red supergiants may be more observable than those of successful SNe. We review the process by which neutrinos from a Galactic ccSN would be detected and announced. We provide new information on the EGADS system and its potential for providing instant neutrino alerts. We also discuss the distance, extinction, and magnitude probability distributions for the next Galactic Type Ia SN. Based on our modeled observability, we find a Galactic ccSN rate of 3.2 (+7.3/-2.6) per century and a Galactic Type Ia SN rate of 1.4 (+1.4/-0.8) per century for a total Galactic SN rate of 4.6 (+7.4/-2.7) per century is needed to account for the SNe observed over the last millennium.

The next-generation liquid-scintillator neutrino observatory LENA

We propose the liquid-scintillator detector LENA (Low Energy Neutrino Astronomy) as a next-generation neutrino observatory on the scale of 50 kt. The outstanding successes of the Borexino and KamLAND experiments demonstrate the large potential of liquid-scintillator detectors in low-energy neutrino physics. LENAs physics objectives comprise the observation of astrophysical and terrestrial neutrino sources as well as the investigation of neutrino oscillations. In the GeV energy range, the search for proton decay and long-baseline neutrino oscillation experiments complement the low-energy program. Based on the considerable expertise present in European and international research groups, the technical design is sufficiently mature to allow for an early start of detector realization.

The Diffuse Supernova Neutrino Background is detectable in Super-Kamiokande

The Diffuse Supernova Neutrino Background (DSNB) provides an immediate opportunity to study the emission of MeV thermal neutrinos from core-collapse supernovae. The DSNB is a powerful probe of stellar and neutrino physics, provided that the core-collapse rate is large enough and that its uncertainty is small enough. To assess the important physics enabled by the DSNB, we start with the cosmic star formation history of Hopkins & Beacom (2006) and confirm its normalization and evolution by cross-checks with the supernova rate, extragalactic background light, and stellar mass density. We find a sufficient core-collapse rate with small uncertainties that translate into a variation of +/- 40% in the DSNB event spectrum. Considering thermal neutrino spectra with effective temperatures between 4-6 MeV, the predicted DSNB is within a factor 4-2 below the upper limit obtained by Super-Kamiokande in 2003. Furthermore, detection prospects would be dramatically improved with a gadolinium-enhanced Super-Kamiokande: the backgrounds would be significantly reduced, the fluxes and uncertainties converge at the lower threshold energy, and the predicted event rate is 1.2-5.6 events /yr in the energy range 10-26 MeV. These results demonstrate the imminent detection of the DSNB by Super-Kamiokande and its exciting prospects for studying stellar and neutrino physics.

Conservative Constraints on Dark Matter Annihilation into Gamma Rays

Using gamma-ray data from observations of the Milky Way, Andromeda (M31), and the cosmic background, we calculate conservative upper limits on the dark matter self-annihilation cross section to monoenergetic gamma rays, <sigma_A v>_{gamma gamma}, over a wide range of dark matter masses. (In fact, over most of this range, our results are unchanged if one considers just the branching ratio to gamma rays with energies within a factor of a few of the endpoint at the dark matter mass.) If the final-state branching ratio to gamma rays, Br(gamma gamma), were known, then <sigma_A v>_{gamma gamma} / Br(gamma gamma) would define an upper limit on the total cross section; we conservatively assume Br(gamma gamma) > 10^{-4}. An upper limit on the total cross section can also be derived by considering the appearance rates of any Standard Model particles; in practice, this limit is defined by neutrinos, which are the least detectable. For intermediate dark matter masses, gamma-ray-based and neutrino-based upper limits on the total cross section are comparable, while the gamma-ray limit is stronger for small masses and the neutrino limit is stronger for large masses. We comment on how these results depend on the assumptions about astrophysical inputs and annihilation final states, and how GLAST and other gamma-ray experiments can improve upon them.

Probing new physics with long-lived charged particles produced by atmospheric and astrophysical neutrinos

As suggested by some extensions of the Standard Model of particle physics, dark matter may be a super-weakly interacting lightest stable particle, while the next-to-lightest particle (NLP) is charged and meta-stable. One could test such a possibility with neutrino telescopes, by detecting the charged NLPs produced in high-energy neutrino collisions with Earth matter. We study the production of charged NLPs by both atmospheric and astrophysical neutrinos; only the latter, which is largely uncertain and has not been detected yet, was the focus of previous studies. We compute the resulting fluxes of the charged NLPs, compare those of different origins, and analyze the dependence on the underlying particle physics setup. We point out that even if the astrophysical neutrino flux is very small, atmospheric neutrinos, especially those from the prompt decay of charmed mesons, may provide a detectable flux of NLP pairs at neutrino telescopes such as IceCube. We also comment on the flux of charged NLPs expected from proton-nucleon collisions, and show that, for theoretically motivated and phenomenologically viable models, it is typically sub-dominant and below detectable rates.

Characterizing Supernova Progenitors via the Metallicities of their Host Galaxies, from Poor Dwarfs to Rich Spirals

We investigate how the different types of supernovae are relatively affected by the metallicity of their host galaxy. We match the SAI Supernova Catalog to the SDSS-DR4 catalog of star-forming galaxies with measured metallicities. These supernova host galaxies span a range of oxygen abundance from 12 + log(O/H) = 7.9 to 9.3 (~ 0.1 to 2.7 solar) and a range in absolute magnitude from MB = -15.2 to -22.2. To reduce the various observational biases, we select a subsample of well-characterized supernovae in the redshift range from 0.01 to 0.04, which leaves us with 58 SN II, 19 Ib/c, and 38 Ia. We find strong evidence that SN Ib/c are occurring in higher-metallicity host galaxies than SN II, while we see no effect for SN Ia relative to SN II. We note some extreme and interesting supernova-host pairs, including the metal-poor (~ 1/4 solar) host of the recent SN Ia 2007bk, where the supernova was found well outside of this dwarf galaxy. To extend the luminosity range of supernova hosts to even fainter galaxies, we also match all the historical supernovae with z < 0.3 to the SDSS-DR6 sky images, resulting in 1225 matches. This allows us to identify some even more extreme cases, such as the recent SN Ic 2007bg, where the likely host of this hypernova-like event has an absolute magnitude MB ~ -12, making it one of the least-luminous supernova hosts ever observed. This low-luminosity host is certain to be very metal poor (~ 1/20 solar), and therefore this supernova is an excellent candidate for association with an off-axis GRB. The two catalogs that we have constructed are available online and will be updated regularly. Finally, we discuss various implications of our findings for understanding supernova progenitors and their host galaxies.