Optimization of a neutrino factory oscillation experiment

We discuss the optimization of a neutrino factory experiment for neutrino oscillation physics in terms of muon energy, baselines, and oscillation channels (gold, silver, platinum). In addition, we study the impact and requirements for detector technology improvements, and we compare the results to beta beams. We find that the optimized neutrino factory has two baselines, one at about 3000 to 5000km, the other at about 7500km (``magic'' baseline). The threshold and energy resolution of the golden channel detector have the most promising optimization potential. This, in turn, could be used to lower the muon energy from about 50GeV to about 20GeV. Furthermore, the inclusion of electron neutrino appearance with charge identification (platinum channel) could help for large values of \sin^2 2 \theta_{13}. Though tau neutrino appearance with charge identification (silver channel) helps, in principle, to resolve degeneracies for intermediate \sin^2 2 \theta_{13}, we find that alternative strategies may be more feasible in this parameter range. As far as matter density uncertainties are concerned, we demonstrate that their impact can be reduced by the combination of different baselines and channels. Finally, in comparison to beta beams and other alternative technologies, we clearly can establish a superior performance for a neutrino factory in the case \sin^2 2 \theta_{13} < 0.01.Comment: 51 pages, 25 figures, 6 tables, references corrected, final version to appear in Phys. Rev.

keV sterile neutrino dark matter in gauge extensions of the standard model

It is known that a keV scale sterile neutrino is a good warm dark matter candidate. We study how this possibility could be realized in the context of gauge extensions of the standard model. The na\"ive expectation leads to large thermal overproduction of sterile neutrinos in this setup. However, we find that it is possible to use out-of-equilibrium decay of the other right-handed neutrinos of the model to dilute the present density of the keV sterile neutrinos and achieve the observed dark matter density. We present the universal requirements that should be satisfied by the gauge extensions of the standard model, containing right-handed neutrinos, to be viable models of warm dark matter, and provide a simple example in the context of the left-right symmetric model.Comment: RevTex, 13 pages, 5 figures; journal version (corrected typos

Microscopic and Macroscopic Effects in the Decoherence of Neutrino Oscillations

We present a generic structure (the layer structure) for decoherence effectsin neutrino oscillations, which includes decoherence from quantum mechanicaland classical uncertainties. The calculation is done by combining the conceptof open quantum system and quantum field theory, forming a structure composedof phase spaces from microscopic to macroscopic level. Having information lossat different levels, quantum mechanical uncertainties parameterize decoherenceby an intrinsic mass eigenstate separation effect, while decoherence forclassical uncertainties is typically dominated by a statistical averagingeffect. With the help of the layer structure, we classify the former as statedecoherence (SD) and the latter as phase decoherence (PD), then furtherconclude that both SD and PD result from phase wash-out effects of differentphase structures on different layers. Such effects admit for simple numericalcalculations of decoherence for a given width and shape of uncertainties. Whileour structure is generic, so are the uncertainties, nonetheless, a few notableones are: the wavepacket size of the external particles, the effectiveinteraction volume at production and detection, the energy reconstruction modeland the neutrino production profile. Furthermore, we estimate the experimentalsensitivities for SD and PD parameterized by the uncertainty parameters, forreactor neutrinos and decay-at-rest neutrinos, using a traditional ratemeasuring method and a novel phase measuring method.<br

Spin wave excitations: The main source of the temperature dependence of Interlayer exchange coupling in nanostructures

Quantum mechanical calculations based on an extended Heisenberg model are compared with ferromagnetic resonance (FMR) experiments on prototype trilayer systems Ni_7/Cu_n/Co_2/Cu(001) in order to determine and separate for the first time quantitatively the sources of the temperature dependence of interlayer exchange coupling. Magnon excitations are responsible for about 75% of the reduction of the coupling strength from zero to room temperature. The remaining 25% are due to temperature effects in the effective quantum well and the spacer/magnet interfaces.Comment: accepted for publication in PR

Prospects of accelerator and reactor neutrino oscillation experiments for the coming ten years

We analyze the physics potential of long baseline neutrino oscillation experiments planned for the coming ten years, where the main focus is the sensitivity limit to the small mixing angle $\theta_{13}$. The discussed experiments include the conventional beam experiments MINOS, ICARUS, and OPERA, which are under construction, the planned superbeam experiments J-PARC to Super-Kamiokande and NuMI off-axis, as well as new reactor experiments with near and far detectors, represented by the Double-Chooz project. We perform a complete numerical simulation including systematics, correlations, and degeneracies on an equal footing for all experiments using the GLoBES software. After discussing the improvement of our knowledge on the atmospheric parameters $\theta_{23}$ and $\Delta m^2_{31}$ by these experiments, we investigate the potential to determine $\theta_{13}$ within the next ten years in detail. Furthermore, we show that under optimistic assumptions and for $\theta_{13}$ close to the current bound, even the next generation of experiments might provide some information on the Dirac CP phase and the type of the neutrino mass hierarchy.Comment: 38 pages, 13 figures, Eqs. (1) and (5) corrected, small corrections in Figs. 8, 9, and Tab. 4, discussion improved, ref. added, version to appear in PRD, high resolution figures are available at http://www.sns.ias.edu/~winter/figs0403068.htm

Prospects for Finding Sterile Neutrino Dark Matter at KATRIN

We discuss under what circumstances a signal in upcoming laboratory searchesfor keV-scale sterile neutrinos would be compatible with those particles beinga sizable part or all of dark matter. In the parameter space that will beexperimentally accessible by KATRIN/TRISTAN, strong X-ray limits need to berelaxed and dark matter overproduction needs to be avoided. We discusspostponing the dark matter production to lower temperatures, a reduced sterileneutrino contribution to dark matter, and a reduction of the branching ratio inphotons and active neutrinos through cancellation with a new physics diagram.Both the Dodelson-Widrow and the Shi-Fuller mechanisms for sterile neutrinodark matter production are considered. As a final exotic example, potentialconsequences of CPT violation are discussed.<br

Probing New Physics at Future Tau Neutrino Telescopes

We systematically investigate new physics scenarios that can modify theinteractions between neutrinos and matter at upcoming tau neutrino telescopes,which will test neutrino-proton collisions with energies \gtrsim 45~{\rmTeV}, and can provide unique insights to the elusive tau neutrino. At suchhigh energy scales, the impact of parton distribution functions of second andthird generations of quarks (usually suppressed) can be comparable to thecontribution of first generation with small momentum fraction, hence making tauneutrino telescopes an excellent facility to probe new physics associated withsecond and third families. Among an inclusive set of particle physics models,we identify new physics scenarios at tree level that can give competitivecontributions to the neutrino cross sections while staying within laboratoryconstraints: charged/neutral Higgs and leptoquarks. Our analysis is close tothe actual experimental configurations of the telescopes, and we perform a$\chi^2$-analysis on the energy and angular distributions of the tau events. Bynumerically solving the propagation equations of neutrino and tau fluxes inmatter, we obtain the sensitivities of representative upcoming tau neutrinotelescopes, GRAND, POEMMA and Trinity, to the charged Higgs and leptoquarkmodels. While each of the experiments can achieve a sensitivity better than thecurrent collider reaches for certain models, their combination is remarkablycomplementary in probing the new physics. In particular, the new physics willaffect the energy and angular distributions in different ways at thosetelescopes.<br

Connections between the Seesaw and Dark Matter Searches

In some dark matter models, the coupling of the dark matter particle to the standard model Higgs determines the dark matter relic density while it is also consistent with dark matter direct detection experiments. On the other hand, the seesaw for generating the neutrino masses probably arises from a spontaneous symmetry breaking of global lepton number. The dark matter particle thus can significantly annihilate into massless Majorons when the lepton number breaking scale and hence the seesaw scale is near the electroweak scale. This leads to an interesting interplay between neutrino physics and dark matter physics and the annihilation mode has an interesting implication on dark matter searches.Comment: 4 pages. Major revision. To appear in PR