Steiner t-designs for large t

One of the most central and long-standing open questions in combinatorial design theory concerns the existence of Steiner t-designs for large values of t. Although in his classical 1987 paper, L. Teirlinck has shown that non-trivial t-designs exist for all values of t, no non-trivial Steiner t-design with t > 5 has been constructed until now. Understandingly, the case t = 6 has received considerable attention. There has been recent progress concerning the existence of highly symmetric Steiner 6-designs: It is shown in [M. Huber, J. Algebr. Comb. 26 (2007), pp. 453-476] that no non-trivial flag-transitive Steiner 6-design can exist. In this paper, we announce that essentially also no block-transitive Steiner 6-design can exist.Comment: 9 pages; to appear in: Mathematical Methods in Computer Science 2008, ed. by J.Calmet, W.Geiselmann, J.Mueller-Quade, Springer Lecture Notes in Computer Scienc

Electron neutrino tagging through tertiary lepton detection

We discuss an experimental technique aimed at tagging electron neutrinos in multi-GeV artificial sources on an event-by-event basis. It exploits in a novel manner calorimetric and tracking technologies developed in the framework of the LHC experiments and of rare kaon decay searches. The setup is suited for slow-extraction, moderate power beams and it is based on an instrumented decay tunnel equipped with tagging units that intercept secondary and tertiary leptons from the bulk of undecayed \pi^+ and protons. We show that the taggers are able to reduce the \nue contamination originating from K_e3 decays by about one order of magnitude. Only a limited suppression (~60%) is achieved for \nue produced by the decay-in-flight of muons; for low beam powers, similar performance as for K_e3 can be reached supplementing the tagging system with an instrumented beam dump.Comment: 19 pages, 7 figures; minor changes, version to appear in EPJ

Non-standard interactions using the OPERA experiment

We investigate the implications of non-standard interactions on neutrino oscillations in the OPERA experiment. In particular, we study the non-standard interaction parameter $\epsilon_{\mu\tau}$. We show that the OPERA experiment has a unique opportunity to reduce the allowed region for this parameter compared with other experiments such as the MINOS experiment, mostly due to the higher neutrino energies in the CNGS beam compared to the NuMI beam. We find that OPERA is mainly sensitive to a combination of standard and non-standard parameters and that a resulting anti-resonance effect could suppress the expected number of events. Furthermore, we show that running OPERA for five years each with neutrinos and anti-neutrinos would help in resolving the degeneracy between the standard parameters and $\epsilon_{\mu\tau}$. This scenario is significantly better than the scenario with a simple doubling of the statistics by running with neutrinos for ten years.Comment: 15 pages, 5 figures, REVTeX

μ−e\mu-e conversion in nuclei within the CMSSM seesaw: universality versus non-universality

In this paper we study $\mu-e$ conversion in nuclei within the context of the Constrained Minimal Supersymmetric Standard Model, enlarged by three right handed neutrinos and their supersymmetric partners, and where the neutrino masses are generated via a seesaw mechanism. Two different scenarios with either universal or non-universal soft supersymmetry breaking Higgs masses at the gauge coupling unification scale are considered. In the first part we present a complete one-loop computation of the conversion rate for this process that includes the photon-, $Z$-boson, and Higgs-boson penguins, as well as box diagrams, and compare their size in the two considered scenarios. Then, in these two scenarios we analyse the relevance of the various parameters on the conversion rates, particularly emphasising the role played by the heavy neutrino masses, $\tan \beta$, and especially $\theta_{13}$. In the case of hierachical heavy neutrinos, an extremely high sensitivity of the rates to $\theta_{13}$ is indeed found. The last part of this work is devoted to the study of the interesting loss of correlation between the $\mu-e$ conversion and $\mu \to e \gamma$ rates that occurs in the non-universal scenario. In the case of large $\tan \beta$ and light $H^0$ Higgs boson an enhanced ratio of the $\mu-e$ to $\mu \to e \gamma$ rates, with respect to the universal case is found, and this could be tested with the future experimental sensitivities.Comment: 48 pages, 15 figures. Minor typos corrected and some references adde

Neutrino hierarchy from CP-blind observables with high density magnetized detectors

High density magnetized detectors are well suited to exploit the outstanding purity and intensities of novel neutrino sources like Neutrino Factories and Beta Beams. They can also provide independent measurements of leptonic mixing parameters through the observation of atmospheric muon-neutrinos. In this paper, we discuss the combination of these observables from a multi-kton iron detector and a high energy Beta Beam; in particular, we demonstrate that even with moderate detector granularities the neutrino mass hierarchy can be determined for $\theta_{13}$ values greater than 4$^\circ$.Comment: 16 pages, 7 figures. Added a new section discussing systematic errors (sec 5.2); sec.5.1 and 4 have been extended. Version to appear in EPJ

Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets

A theoretical study has been carried out to analyse the available results from the inelastic neutron scattering experiment performed on a quasi-two dimensional spin-1/2 ferromagnetic material $K_2CuF_4$. Our formalism is based on a conventional semi-classical like treatment involving a model of an ideal gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg ferromagnet on a square lattice. The results for dynamical structure functions for our model corresponding to spin-1/2, show occurrence of negative values in a large range of energy transfer even encompassing the experimental range, when convoluted with a realistic spectral window function. This result indicates failure of the conventional theoretical framework to be applicable to the experimental situation corresponding to low spin systems. A full quantum formalism seems essential for treating such systems.Comment: 16 pages, 6 figures, 1 Table Submitted for publicatio