Narrow Results

We consider all possible scalar bilinears, which couple to two fermions of the standard model. The various baryon and lepton number violating couplings allowed by these exotic scalars are studied. We then discuss which are constrained by limits on proton decay (to a lepton and a meson as well as to three leptons), neutron–antineutron oscillations, and neutrinoless double beta decay.

We assume that lepton number is explicitly broken at low energy scale (M) in the framework of the Higgs triplet (Δ) model. The scalar sector of the model is developed considering the particular assumption M = v_Δ ≈ eV. We show that such assumption infers a particular mass spectrum for the scalars that compose the triplet and cause a decoupling of these scalars from those that compose the standard scalar doublet.

We study lepton number violation in little Higgs model and find that the choice of putting triplet Higgs vev equal to zero so as not to have any tree level neutrino Majorana mass is not natural in the sense that such a term is generated at the one-loop level. We investigate the contribution of exotic lepton number violating terms on neutrinoless double beta decay, K meson decay and on trilepton production in ν–N scattering.

Black holes at the TeV scale are investigated in the extra large dimension scenario. We interpret the lightest black hole excitation as a singlet scalar field, and show how interaction terms can be appended to the standard model at the dimension five non-renormalizable level. Lepton family number violation is natural in this model. Muon magnetic moment, and neutrino masses are investigated. We also present a quantization scheme in n dimensions.

A successful baryogenesis theory requires a baryon-minus-lepton number violation if it works before the electroweak phase transition. The leading dimension-6 baryon number violating interactions conserve baryon-minus-lepton number, which dissociated baryogenesis from baryon number violation. We show that in some models, in which the baryon-minus-lepton number is violated in the proton and neutron decays, the baryogenesis and the nucleon decay could have a common origin. We extend the canonical seesaw model with an isotriplet leptoquark scalar and two isotriplet Higgs scalars and allow the Higgs triplet to have a quartic coupling with three leptoquark triplets and a cubic coupling with two Higgs doublets. The decays of the Higgs triplets can thus generate a baryon-minus-lepton asymmetry. The tiny vacuum expectation values of the Higgs triplets can naturally induce a testable proton decay even if the leptoquark is around the TeV scale. The leptoquark associated with any flavor neutrinos can mediate a neutrinoless double beta decay.

The neutrinoless double-beta $(0\nu \beta \beta )$ decay is a lepton-number-violating process which is experimentally unique for identifying the Majorana nature of massive neutrinos. We give a brief overview of some theoretical aspects of this process. In particular, a novel “coupling-rod” diagram is introduced to describe the effective Majorana mass ${〈m〉}_{ee}^{}$ in the complex plane. Possible contributions of new physics to ${〈m〉}_{ee}^{}$ are also discussed.

A simple model is constructed based on the gauge symmetry $\mathrm{\text{SU}}{(3)}_c\times \mathrm{\text{SU}}{(2)}_L\times \mathrm{\text{U}}{(1)}_Y\times \mathrm{\text{SU}}{(2)}_{\ell }$, with only the leptons transforming nontrivially under $\mathrm{\text{SU}}{(2)}_{\ell }$. The extended symmetry is broken down to the Standard Model gauge group at TeV-scale energies. We show that this model provides a mechanism for baryogenesis via leptogenesis in which the lepton number asymmetry is generated by $\mathrm{\text{SU}}{(2)}_{\ell }$ instantons. The theory also contains a dark matter candidate — the $\mathrm{\text{SU}}{(2)}_{\ell }$ partner of the right-handed neutrino.

We calculate the exact tree-level scalar mass matrices resulting from symmetry breaking using the most general gauge-invariant scalar potential of the 331 model, both with and without the condition that the lepton number is conserved. Physical masses are also obtained in some cases, as well as couplings to standard and exotic gauge bosons.

Motivated by effective low energy models of string origin, we discuss the neutrino masses and mixing within the context of the Minimal Supersymmetric Standard Model supplemented by a U(1) anomalous family symmetry and additional Higgs singlet fields charged under this extra U(1). In particular, we interpret the solar and atmospheric neutrino data assuming that there are only three left-handed neutrinos which acquire Majorana masses via a lepton number violating dimension-five operator. We derive the general form of the charged lepton and neutrino mass matrices when two different pairs of singlet Higgs fields develop nonzero vacuum expectation values and show how the resulting neutrino textures are related to approximate lepton flavor symmetries. We perform a numerical analysis for one particular case and obtain solutions for masses and mixing angles, consistent with experimental data.

The fate of R-parity in supersymmetric theories is discussed in detail. We make a strong case for R-parity violation showing that the simplest theories based on the local B - L symmetry predict the spontaneous breaking of R-parity at the SUSY scale. The possible implications for the searches at the Large Hadron Collider are discussed.

In the Standard Model the total lepton number is conserved. Thus, neutrinoless double-β decay, in which the total lepton number is violated by two units, is a probe of physics beyond the Standard Model. In this review we consider the basic mechanism of neutrinoless double-β decay induced by light Majorana neutrino masses. After a brief summary of the present status of our knowledge of neutrino masses and mixing and an introduction to the seesaw mechanism for the generation of light Majorana neutrino masses, in this review we discuss the theory and phenomenology of neutrinoless double-β decay. We present the basic elements of the theory of neutrinoless double-β decay, our view of the present status of the challenging problem of the calculation of the nuclear matrix element of the process and a summary of the experimental results.

Neutrinoless double beta decay, lepton number violating collider processes and the Baryon Asymmetry of the Universe (BAU) are intimately related. In particular, lepton number violating processes at low energies in combination with sphaleron transitions will typically erase any preexisting BAU. In this contribution, we briefly review the tight connection between neutrinoless double beta decay, lepton number violating processes at the LHC and constraints from successful baryogenesis. We argue that far-reaching conclusions can be drawn unless the baryon asymmetry is stabilized via some newly introduced mechanism.

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN designed to search for the rare neutrinoless double beta decay ($0\nu \beta \beta$) of ${}^{76}$Ge. In the first phase (Phase I) of the experiment, high purity germanium diodes were operated in a “bare” mode and immersed in liquid argon. The overall background level of $10^{-2}\mathrm{\text{cts}}/(\mathrm{\text{keV}}\cdot \mathrm{\text{kg}}\cdot \mathrm{\text{yr}})$ was a factor of ten better than those of its predecessors. No signal was found and a lower limit was set on the half-life for the $0\nu \beta \beta$ decay of ${}^{76}$Ge $T_{1/2}^{0\nu }>2.1\times 10^{25}$ yr (90% CL), while the corresponding median sensitivity was $2.4\times 10^{25}$ yr (90% CL). A second phase (Phase II) started at the end of 2015 after a major upgrade. Thanks to the increased detector mass and performance of the enriched germanium diodes and due to the introduction of liquid argon instrumentation techniques, it was possible to reduce the background down to $10^{-3}\mathrm{\text{cts}}/(\mathrm{\text{keV}}\cdot \mathrm{\text{kg}}\cdot \mathrm{\text{yr}})$. After analyzing 23.2 kg⋅yr of these new data no signal was seen. Combining these with the data from Phase I a stronger half-life limit of the ${}^{76}$Ge $0\nu \beta \beta$ decay was obtained: $T_{1/2}^{0\nu }>8.0\times 10^{25}$ yr (90% CL), reaching a sensitivity of $5.8\times 10^{25}$ yr (90% CL). Phase II will continue for the collection of an exposure of 100 kg$\cdot$yr. If no signal is found by then the GERDA sensitivity will have reached $1.4\times 10^{26}$ yr for setting a 90% CL. limit. After the end of GERDA Phase II, the flagship experiment for the search of $0\nu \beta \beta$ decay of ${}^{76}$Ge will be LEGEND. LEGEND experiment is foreseen to deploy up to 1-ton of ${}^{76}$Ge. After ten years of data taking, it will reach a sensitivity beyond 10${}^{28}$ yr, and hence fully cover the inverted hierarchy region.

The abundances of baryons and leptons are not only closely related to each other and to the generation of neutrino masses but may also be linked to the dark matter in the Universe. In this paper we review how a consistent physics beyond the Standard Model framework for cosmology and neutrino masses could arise by studying these interrelations.

Nuclear double beta decay provides an extraordinarily broad potential to search for beyond-standard-model physics. The occurrence of the neutrinoless decay(0νββ) mode has fundamental consequences: first, the total lepton number is not conserved, and second, the neutrino is a Majorana particle. Furthermore, the measured effective mass provides an absolute scale of the neutrino mass spectrum. In addition, double beta experiments yield sharp restrictions for other beyond-standard-model physics. These include SUSY models (R-parity breaking and conserving), leptoquarks (leptoquark-Higgs coupling), compositeness, left-right symmetric models (right-handed W boson mass), test of special relativity and of the equivalence principle in the neutrino sector and others. First evidence for neutrinoless double beta decay was reported by the HEIDELBERG–MOSCOW experiment in 2001. The HEIDELBERG–MOSCOW experiment is by far the most sensitive0νββ experiment since more than 10 years. It is operating 11 kg of enriched ⁷⁶Ge in the GRAN SASSO Underground Laboratory. The analysis of the data taken from 2 August 1990–20 May 2003 is presented here. The collected statistics is 71.7 kg y. The background achieved in the energy region of the Q value for double beta decay is 0.11 events/kg y keV. The two-neutrino accompanied half-life is determined on the basis of more than 100,000 events to be years. The confidence level for the neutrinoless signal has been improved to a 4.2σ level. The half-life is years. The effective neutrino mass deduced is (0.2–0.6) eV (99.73% C.L.), with the consequence that neutrinos have degenerate masses. The sharp boundaries for other beyond SM physics, mentioned above, are comfortably competitive to the corresponding results from high-energy accelerators like TEVATRON, HERA, etc.

Neutrino-less double beta decay (0νββ) is currently known to be only experiment to verify whether lepton number conservation is violated. The violation is the key to create matter dominated universe by the so-called Leptogenesys. We have been studying double beta decay of ⁴⁸Ca. First stage of the experiment was carried out by using the ELEGANT VI detector system. We gave the best lower limit of the half life of 0νββ of ⁴⁸Ca. We have been developing CANDLES detector system to sense much longer life-time region. We have developed techniques to reduce further backgrounds. One of the techniques has been used in the ELEGANTS VI system and improved further the half life limit. The CADLES detector system will be installed Kamioka underground laboratory next year. I describe these studies.

A simple model is constructed based on the gauge symmetry SU(3)_c×SU(2)_L×U(1)_Y × SU(2)_l, with only the leptons transforming nontrivially under SU(2)_l. The extended symmetry is broken down to the Standard Model gauge group at TeV-scale energies. We show that this model provides a mechanism for baryogenesis via leptogenesis in which the lepton number asymmetry is generated by SU(2)_l instantons. The theory also contains a dark matter candidate — the SU(2)_l partner of the right-handed neutrino.

Nuclear double beta decay, an extremely rare radioactive decay process, is - in one of its variants - one of the most exciting means of research into particle physics beyond the standard model. The large progress in sensitivity of experiments searching for neutrinoless double beta decay in the last two decades - based largely on the use of large amounts of enriched source material in "active source experiments" - has lead to the observation of the occurrence of this process in nature (on a 6.4 sigma level), with the largest half-life ever observed for a nuclear decay process (2.2×10²⁵ y). This has fundamental consequences for particle physics - violation of lepton number, Majorana nature of the neutrino. These results are independent of any information on nuclear matrix elements (NME)*. It further leads to sharp restrictions for SUSY theories, sneutrino mass, right-handed W-boson mass, superheavy neutrino masses, composite-ness, leptoquarks, violation of Lorentz invariance and equivalence principle in the neutrino sector.

The masses of light-neutrinos are found to be degenerate, and to be at least 0.22±0.02eV. This fixes the contribution of neutrinos as hot dark matter to ≥4.7% of the total observed dark matter. The neutrino mass determined might solve also the dark energy puzzle.

Neutrinoless nuclear double 0νββ decay, recently experimentally observed for ⁷⁶Ge on a confidence level of > 6 σ, has fundamental consequences for particle physics - violation of (total) lepton number, Majorana nature of the neutrino. It further leads to sharp restrictions for SUSY theories, sneutrino mass, right-handed W-boson mass, superheavy neutrino masses, compositeness, leptogenesis, violation of Lorentz violation and equivalence principle in the neutrino sector.

The masses of light neutrinos follow to be degenerate, and to be at least 0.22 ± 0.02 eV. This fixed the contribution of neutrinos as hot dark matter to ≥ 4.7% of the total observed dark matter. The neutrino mass determined might also solve the dark energy puzzle.

The observation of 0νββ decay is naturally a great challenge for future experiments, in particular also with other 0νββ - emitter isotopes. There are several important experiments under construction. Present experiments have hardly a chance to reach the sensitivity required for confirmation - although sometimes this impression is given (e.g. by unproperly comparing 1.5 σ limits (of CUORICINO etc.) with the 6σ result obtained for ⁷⁶Ge.

The neutrinoless double-beta (0νββ) decay is a lepton-number-violating process which is experimentally unique for identifying the Majorana nature of massive neutrinos. We give a brief overview of some theoretical aspects of this process. In particular, a novel “coupling-rod” diagram is introduced to describe the effective Majorana mass 〈m〉_ee in the complex plane. Possible contributions of new physics to 〈m〉_ee are also discussed.