Kellogg Seminar 10/2006-7/2007

The physics potentials of a very long baseline neutrino experiment using a wide band muon neutrino beam will be discussed. An intense neutrino source combined with a massive detector will allow a precise measurement of the disappearance parameters and has a much improved sensitivity to the mixing angle theta_13 compared to current limits. If this angle is large enough, the CP violating phase as well as the mass hierarchy can be determined. Besides the physics reach, the options for neutrino beams originating from either BNL or FNAL will be compared, as well as proposed detectors at the two remaining DUSEL candidate sites, the Henderson and Homestake mines.

Parity violation in elastic electron scattering is sensitive to possible strange quark contributions to the vector structure of the nucleon, and this provides an opportunity to isolate effects of the qq-bar sea. The HAPPEX collaboration in Hall A at Jefferson Lab has measured the parity-violating asymmetry in the scattering of longitudinally-polarized 3 GeV electrons from Hydrogen and Helium cryogenic targets at a scattering angle of 6 degrees and low Q^2. The asymmetry for Hydrogen is sensitive to a linear combination of the strange quark contributions to the electric (G_E^s) and magnetic (G_M^s) form factors of the proton while the Helium asymmetry is senstive only to G_E^s. The combination of the two measurements allows G_E^s and G_M^s to be separately determined. Final results will be presented.

The tiny neutrino mass implies a very high scale of B-L breaking, which is the necessary condition for the scenario of baryogenesis via leptogenesis to happen. I will talk about how a realistic model of SO(10) grand unification theory (GUT), which provides a natural scheme of B-L breaking, can be constructed to fit all the quark and lepton masses and mixing and give the baryon asymmetry of the Universe. This model successfully avoids problems associated with previous models in the literature and gives a sizable prediction of theta13 within the reach of the next generation of experiments.

We have recently performed calculations of the coherent neutrino flavortransformations in the supernovae environment. These calculations take into account forward scattering of neutrinos and anti-neutrinos, and for the first time include the complete energy and angular dependence of the scattering amplitude. We find that this coherent scattering can initiate collective behavior and potentially alter supernovae nucleosynthesis and the expected neutrino signal in terrestrial detectors.

Precise measurements of low-energy processes can offer a window into much higher energy scales. Specifically, we examine the signals of electroweak-scale supersymmetry that could appear in muon, beta, and pion decays. In particular, leptonic pion decay provides a very clean proble into the slepton and chargino sectors, which may provide signals at upcoming experiments at PSI and TRIUMF.

The progress in establishing a Deep Underground Scientific and Engineering Laboratory by the NSF, will be reviewed along with the recent events and advancements by the Homestake Collaboration. I will include in my seminar a discussion of Nuclear Astrophysics, Neutrinoless Double Beta Decay, and Dark Matter Experiments appropriate for Homestake.

Sterile neutrinos with keV mass and a small mixing angle can make up the dark matter. There are some additional indications that such particles may exist. Their emission from a supernova is anisotropic, and this anisotropy can also account for the observed velocities of pulsars. Radiative decays of relic sterile neutrinos can speed up the formation of the first stars. While x-ray astronomy offers the best prospects for detection of sterile dark matter via radiative decays, the existence of sterile neutrinos at the keV scale has implications for new physics at the electroweak scale and the upcoming experiments at the LHC.

Dark Matter, an invisible and elusive substance originally proposed by Zwicky to explain the mass to light ratio of galaxies and galaxy clusters, still evades revealing its true identity. While SUSY inspired heavy Cold Dark Matter candidates are studied extensively, many others have also been proposed, spanning many orders of magnitude in mass. I will focus on two particular scenarios of Dark Matter candidates on the lighter side: (1) The bulge of our Galaxy is illuminated by the 0.511 MeV gamma-ray line flux from annihilations of nonrelativistic positrons, which inspired many dark matter motivated mechanisms (MeVDM, XDM etc.). Central to resolving the origin of the positrons is their injection energies which can be tightly constrained by higher-energy gamma rays from inflight annihilation of the same positrons (as they loose energy in the interstellar medium) before they annihilate at rest and produce the 0.511 MeV gamma-ray line. (2) Sterile neutrinos, as Warm Dark Matter candidate, might more easily account for small scale clustering measurements than the heavier particles typically invoked in Cold Dark Matter cosmologies. Mass of sterile neutrinos can be directly constrained by searching for X-rays from their radiative decays in dark matter halos of galaxies, galaxy clusters and cosmic X-ray background.

By numerically solving the appropriate Boltzmann equations, we study the production of sterile neutrinos at low reheating temperatures. We take into account the production in oscillations as well as in direct decays and compute the sterile neutrino primordial spectrum, the effective number of neutrino species, and the sterile neutrino contribution to the mass density of the Universe as a function of the mixing and the reheating parameters. It is shown that sterile neutrinos with non-negligible mixing angles may still lead to $N_\nu\sim 3$. For both production mechanisms, regions where sterile neutrinos account for the dark matter are identified. We also point out that the direct decay opens up a new production mechanism for sterile neutrino dark matter in which cosmological constraints can be satisfied.

Cosmogenic neutrinos are created in the collisions between cosmic rays and microwave background photons. While the existance is almost certain, the flux is very small. New instruments, like ANITA and ARIANNA are designed to detect cosmogenic, or GZK, neutrinos. ANITA launched from McMurdo, Antarctica on Dec. 15, 2005 and remained aloft for 35 days. We discuss the ANITA concept, calibration procedures, and a new idea to determine the neutrino cross-section at extremely high energies. The ARIANNA concept utilizes the Ross Ice Shelf near the coast of Antarctica to increase the sensitivity to cosmogenic neutrinos by roughly an order of magnitude when compared to the sensitivity of existing detectors and those under construction. Therefore, ARIANNA can test a wide variety of scenarios for cosmogenic neutrino production, and probe for physics beyond the standard model by measuring the neutrino cross-section at center of momentum energies near 100 TeV.

I will present the first oscillation results of the Mini Booster Neutrino Experiment (MiniBooNE) at Fermilab. MiniBooNE seeks to confirm or refute the neutrino oscillation signal observed by the LSND experiment. The LSND measurement, when taken together with the solar and atmospheric oscillation data, implies the existence of physics beyond the standard model in the neutrino sector. MiniBooNE has performed two independent and blind oscillation searches for electron neutrino appearance in a muon neutrino beam. In both, analysis selections and fitting procedures were determined before candidate electron neutrino events were examined. I will report on the results of these two analyses.