BRIEF NEUTRINO PHYSICS UPDATE

We demonstrate that an SU (2)L× U (1)Y model with the same particle content as Standard Model (SM) and discrete reflection symmetry between second and third generations of leptons gives rise to charged lepton and neutrino mass matrices which can accommodate the present solar, atmospheric, WMAP neutrino experimental results. The model predicts the value of |U13| which could be tested in neutrino factories and the effective Majorana neutrino mass <mee> which is at the lower end of the present experimental value. Neutrino masses are generated through dim=5 operators and the scale of which are constrained by the value of <mee>. If, in future neutrinoless double beta decay experiments namely, MOON, EXO, GENIUS shift the lower bound on <mee> by one order, the present model will fail to accommodate the solar neutrino mixing angle due to LMA solution.

The modern theories of Grand Unification (GUT) and supersymmetric (SUSY) extensions of standard model (SM) suppose that the conservation laws of the SM may be violated to some small degree. The nuclei are well-suited as a laboratory to test fundamental symmetries and fundamental interactions like lepton flavor (LF) and lepton number (LN) conservation. A prominent role between experiments looking for LF and total LN violation play yet not observed processes of neutrinoless double beta decay. The GUT's and SUSY models offer a variety of mechanisms which allow this process to occur. They are based on mixing of Majorana neutrinos and/or R-parity violation hypothesis. Although the neutrinoless double beta decay has not been seen it is possible to extract from the lower limits of the lifetime upper limits for the effective electron Majorana neutrino mass, effective right handed weak interaction parameters, the effective Majoron coupling constant, R-parity violating SUSY parameters etc. In this work the limits on the LN violating parameters extracted from current neutrinoless double beta decay experiments are listed. Studies in respect to future neutrinoless double beta decay experimental projects are also presented.

Neutrino mass spectrum and future beta decay experiments

A brief overview is given of theoretical analyses with neutrinoless double\nbeta decay experiments. Theoretical bounds on the ``observable'', <m>_betabeta,\nare presented. By using experimental bounds on <m>_betabeta, allowed regions\nare obtained on the m_l-cos{2theta_12} plane, where m_l stands for the lightest\nneutrino mass. It is shown that Majorana neutrinos can be excluded by combining\npossible results of future neutrinoless double beta decay and {}^3H beta decay\nexperiments. A possibility to constrain one of two Majorana phases is discussed\nalso.\n

FLARES (a Flexible scintillation Light Apparatus for Rare Event Searches) is a project for an innovative detector technology to be applied to rare event searches, and in particular to neutrinoless double beta decay experiments. Its novelty is the enhancement and optimization of the collection of the scintillation light emitted by ultra-pure crystals through the use of arrays of high performance silicon photodetectors cooled to 120 K. This would provide scintillation detectors with ~1% level energy resolution, with the advantages of a technology offering relatively simple low cost mass scalability and powerful background reduction handles, as requested by future neutrinoless double beta decay experimental programs.

We present effective Majorana neutrino mass limits obtained from the joint analysis of the recently published results of 76Ge and 136Xe neutrinoless double beta decay (0νββ) experiments, which was carried out by using the Bayesian calculations. Nuclear matrix elements (NMEs) used for the analysis are taken from the works, in which NMEs of 76Ge and 136Xe were simultaneously calculated. This reduced systematic errors connected with NME calculation techniques. The new effective Majorana neutrino mass limits less than [85.4–197.0] meV are much closer to the inverse neutrino mass hierarchy region.

In some classes of flavour models based on unified theories with a type I see-saw mechanism, the prediction for the mass of the lightest right-handed neutrino is in conflict with the lower bound from the requirement of successful thermal leptogenesis. We investigate how lifting the absolute neutrino mass scale by adding a type II see-saw contribution proportional to the unit matrix can solve this problem. Generically, lifting the neutrino mass scale increases the prediction for the mass of the lightest right-handed neutrino while the decay asymmetry is enhanced and washout effects are reduced, relaxing the lower bound on the mass of the lightest right-handed neutrino from thermal leptogenesis. For instance in classes of unified theories where the lightest right-handed neutrino dominates the type I see-saw contribution, we find that thermal leptogenesis becomes possible if the neutrino mass scale is larger than about 0.15 eV, making this scenario testable by neutrinoless double beta decay experiments in the near future.

The extension of the Standard Model of particle physics by sterile neutrinos can naturally explain the smallness of neutrino masses as observed by neutrino oscillation and nuclear beta decay experiments. These hypothetical particles, also referred to as heavy neutrinos in the mass eigenbasis, can give rise to a testable phenomenology when they have masses around the electroweak scale. Hence they are actively searched for at, for instance, colliders such as the Large Hadron Collider. The proposed future colliders, which are currently in the design phase, will be more powerful than the operated colliders to date. The new possibilities which they provide to search for sterile neutrinos and to test the neutrino mass mechanism in the not too far future have therefore to be assessed. In this thesis, various aspects of the sterile neutrino phenomenology as well as various searches for sterile neutrinos at colliders are investigated. In particular, we study the contributions from sterile neutrinos to the Higgs boson production mechanism at colliders, the implications of long-lived heavy neutrinos that lead to displaced vertex events, lepton-number violation as the manifestation of heavy neutrino-antineutrino oscillations, lepton-flavour violation as the consequence of leptonic mixing, the possibilities to resolve heavy neutrino-antineutrino oscillations, how to test the viable leptogenesis parameter space, and their implications to collider searches. These collider studies are investigated in the context of low scale seesaw scenarios featuring ns = 2 sterile neutrinos with masses in the range of O(1 GeV) and O(1 TeV), which constitutes the benchmark scenario. Within the benchmark scenario, analytical calculations, and analyses of Monte Carlo generated event samples are performed. The investigated collider studies demonstrate promising avenues to test sterile neutrinos at future colliders. This contains novel possible search strategies by the search for Higgs bosons produced from heavy neutrinos and by probing the effects from heavy neutrino-antineutrino oscillations via the displaced vertex search. The assessed capabilities of the future colliders with respect to the sterile neutrino searches contribute to the physics case of the future colliders.

Neutrino mass requires new physics beyond the Standard Model: sterile neutrinos are one such example. In general the mass scale of such particles is unknown, so that different model building scenarios arise. The aim of this thesis is to accommodate light sterile neutrinos in working
\nmodels and study the phenomenological consequences. Indeed, sterile neutrinos at the eV scale could explain observed short-baseline anomalies, whereas keV-scale warm dark matter particles could resolve tensions in the standard cosmological model. Different A4 flavour symmetry models
\nare modified to include sterile neutrinos, with the Froggatt-Nielsen mechanism controlling their mass spectrum and higher-order effects explicitly taken into account. The resulting signatures in neutrinoless double beta decay are focussed on. In addition, the connection between that process and neutrino mass is studied in the left-right symmetric model: the multitude of contributions are categorised and various special cases are studied numerically. The role of mixed helicity diagrams is emphasised, and the inverse neutrinoless double beta decay process at a linear collider is analysed in detail. These and other experimental signatures will be crucial in deciphering the nature and origin of neutrino mass.

Neutrino masses are yet unknown. We discuss the present state of effective electron antineutrino mass fromβdecay experiments; effective Majorana neutrino mass from neutrinoless double-beta decay experiments; neutrino mass squared differences from neutrino oscillation: solar, atmospheric, reactor, and accelerator-based experiments; sum of neutrino masses from cosmological observations. Current experimental challenges in the determination of neutrino masses are briefly discussed. The main focus is devoted to contemporary experiments.

Concluding talk at NOW 2006

Sterile massive neutrinos are a natural extension of the standard model of elementary particles. The energy density of the extra sterile massive states affects cosmological measurements in an analogous way to that of active neutrino species. We perform here an analysis of current cosmological data and derive bounds on the masses of the active and the sterile neutrino states, as well as on the number of sterile states. The so-called (3 + 2) models, with three sub-eV active massive neutrinos plus two sub-eV massive sterile species, is well within the 95% CL allowed regions when considering cosmological data only. If the two extra sterile states have thermal abundances at decoupling, big bang nucleosynthesis bounds compromise the viability of (3 + 2) models. Forecasts from future cosmological data on the active and sterile neutrino parameters are also presented. Independent measurements of the neutrino mass from tritium beta-decay experiments and of the Hubble constant could shed light on sub-eV massive sterile neutrino scenarios.

Current measurements of the large-scale structure of galaxies are able to place an $\sim 0.5 ~ \rm eV$ upper limit on the absolute mass scale of neutrinos. An order-of-magnitude improvement in raw sensitivity, together with an insensitivity to systematic effects, is needed to reach the lowest value allowed by particle physics experiments. We consider the prospects of determining both the neutrino mass scale and the number of of massive neutrinos using future redshift surveys, specifically those undertaken with the Square Kilometre Array (SKA), with and without additional constraints from the upcoming Planck CMB experiment. If the sum of the neutrino masses $\sum m_i \gtsimeq 0.25 ~ \rm eV$ then the imprint of neutrinos on large-scale structure (LSS) should be enough, on its own, to establish the neutrino mass scale and, considered alongside CMB constraints, it will also determine the number of massive neutrinos $N_{\nu}$, and hence the mass hierarchy. If $\sum m_i \sim 0.05 ~ \rm eV$, at the bottom end of the allowed range, then a combination of LSS, CMB and particle physics constraints should be able to determine $\sum m_i$, $N_{\nu}$ and the hierarchy. If $\sum m_i$ is in the specific range $0.1-0.25 ~ \rm eV$, then a combination of LSS, CMB and particle physics experiments should determine $\sum m_i$, but not $N_{\nu}$ or the hierarchy. Once an SKA-like LSS survey is available there are good prospects of obtaining a full understanding of the conventional neutrino sector, and a chance of finding evidence for sterile neutrinos.

Recent results from neutrino oscillation experiments have convincingly demonstrated that neutrinos have mass and they can mix. The neutrinoless double beta decay is the most sensitive process to determine the absolute scale of the neutrino masses, and the only one that can distinguish whether neutrino is a Dirac or a Majorana particle. A key ingredient for extracting the absolute neutrino masses from neutrinoless double beta decay experiments is a precise knowledge of the nuclear matrix elements (NME) for this process. Newly developed shell model approaches for computing the NME and half‐lifes for the two‐neutrino and neutrinoless double beta decay modes using modern effective interactions are presented. The implications of the new results on the experimental limits of the effective neutrino mass are discussed by comparing the decays of 48Ca and 76Ge.

Although the Standard Model (SM) is the best theory in describing phenomena among elementary particles, it su􏰄ers from several problems, such as the gauge hierarchy problem, origin of the electroweak symmetry breaking, non-zero neutrino mass, and no candidate of dark matter. For solving these problems, we consider minimal U(1) extension of the SM with the classically conformal invariance, where an anomaly-free U(1) gauge symmetry is introduced along with a U(1) Higgs 􏰅eld and three right- handed neutrinos (RHNs) for the seesaw mechanism generating neutrino masses. With no mass term allowed by the classically conformal invariance, the U(1) gauge symme- try is broken through the Coleman-Weinberg mechanism, which subsequently triggers the electroweak symmetry breaking. We perform parameter scan and identify regions resolving the SM Higgs vacuum instability, while satisfying the LHC Run-2 bound on the U(1) gauge boson production and the naturalness constraint. We also investigate cosmological aspects of the model. Introducing Z2 parity, one RHN being an unique parity-odd particle in the model serves as dark matter. A successful in􏰀ation sce- nario is possible by identifying the U(1) Higgs boson with in􏰀aton and introducing its non-minimal gravitational coupling. Interestingly, the LHC physics and cosmological observations are complementary narrowing down the model parameter space.