42818934
Automatic scanning of nuclear emulsions with wide-angle acceptance for nuclear fragment detection
Nuclear emulsion, a tracking detector with sub-micron position resolution, has played a successful role in the field of particle physics and the analysis speed has been substantially improved by the development of automated scanning systems. This paper describes a newly developed automated scanning system and its application to the analysis of nuclear fragments emitted almost isotropically in nuclear evaporation. This system is able to recognize tracks of nuclear fragments up to |tan θ| < 3.0 (where θ is the track angle with respect to the perpendicular to the emulsion film), while existing systems have an angular acceptance limited to |tan θ| < 0.6. The automatic scanning for such a large angle track in nuclear emulsion is the first trial. Furthermore the track recognition algorithm is performed by a powerful Graphics Processing Unit (GPU) for the first time. This GPU has a sufficient computing power to process large area scanning data with a wide angular acceptance and enough flexibility to allow the tuning of the recognition algorithm. This new system will in particular be applied in the framework of the OPERA experiment: the background in the sample of τ decay candidates due to hadronic interactions will be reduced by a better detection of the emitted nuclear fragments.
118836819
A new method to correct deformations in emulsion using a precise photomask
Abstract A new method to correct the emulsion deformation, mainly produced in the development process, is developed to recover the high accuracy of nuclear emulsion as a tracking device. The method is based on a precise photomask and a careful treatment of the emulsion films. A position measurement accuracy of 0.6 μ m is obtained over an area of 5 cm×7 cm. The method allows to measure positions of track segments with submicron accuracy in an ECC brick with as few as 10 reference tracks for alignment. Such a performance can be important for hybrid emulsion experiments at underground laboratories where only a small number of reference tracks for alignment are available.
119118987
Flavour mixing of neutrinos and baryon asymmetry of the universe
Abstract We investigate baryogenesis in the ν MSM, which is the Minimal Standard Model (MSM) extended by three right-handed neutrinos with Majorana masses smaller than the weak scale. In this model the baryon asymmetry of the universe (BAU) is generated via flavour oscillation between right-handed neutrinos. We consider the case when BAU is solely originated from the CP violation in the mixing matrix of active neutrinos. We perform analytical and numerical estimations of the yield of BAU, and show how BAU depends on mixing angles and CP violating phases. It is found that the asymmetry in the inverted hierarchy for neutrino masses receives a suppression factor of about 4% comparing with the normal hierarchy case. It is, however, pointed out that, when θ 13 = 0 and θ 23 = π / 4 , baryogenesis in the normal hierarchy becomes ineffective, and hence the inverted hierarchy case becomes significant to account for the present BAU.
56141242
Partial mass degeneracy and spontaneous CP violation in the lepton sector
Inspired by the small mass-squared difference measured in the solar neutrino oscillation experiments and by the testability, we suggest that a limit of the partial mass degeneracy, in which masses of the first two generation fermions are degenerate, may be a good starting point for understanding the observed fermion mass spectra and mixing patterns. The limit indicates the existence of a two-dimensional rotation symmetry, such as O(2), D N and so on, in flavor space of the first two generations. We propose simple models for the lepton sector based on D N and show that the models can successfully reproduce the experimental data without imposing unnatural hierarchies among dimensionless couplings, although at least 10% tuning is necessary in order to explain a large atmospheric mixing. It is especially found that the Z 2 subgroup of the D N symmetry plays an important role in understanding the smallness of the electron mass and θ PMNS 13 . We also discuss testability of the models by the future neutrinoless-double-beta-decay experiments and cosmological observations.