Constraints to the radiative lifetime of a light neutral fermion in the galactic halo from IUE

Summary We present the negative result of a search for the radiative decay of a light neutral fermion, that might be the neutrino (if massive) or the photino, gravitationally bounded to our Galaxy. The limit obtained for the radiative lifetime of a particle of mass between 12.5 and 21.5 eV is ≈10¹⁸ years. These new data on UV background in the range (1250÷2000) ? show the presence of continuum emission and diffuse lines emission at high galactic latitude (|b ^II|>45°). The lines are identifiable with C IV, λ-1549 ? and N III, λ=1749 ?. The continuum level sharply rises at 1680 ? to the level of (314±136) photons/(cm²s sr ?) and remains nearly constant up to 2000 Å. Below 1680 Å we found no emission with an upper limit of ≈100 units. These new data are briefly discussed in comparison with the results of previous experiments and theoretical expectations. Paper presented at the Congress ?Galactic and Extragalactic Dark Matter?, Roma, 28 to 30 June 1983.     

Flavor-changing neutral currents and top-quark properties in Standard Model extensions

The structure and the generation of flavor-changing neutral currents are considered in a Standard Model extension involving a singlet quark of the up type. Anomalous top-quark properties caused by singlet-ordinary mixing is described in a phenomenological way. It is shown that, for single top production and for asymmetry in electron-positron annihilation, there is an enhancement in relation to predictions of the Standard Model.     

Goldstone fermion couplings and supersymmetry breaking in superstring models

We re-examine supersymmetry breaking in the observable sectors of superstring-inspired supergravity models by computing Goldstone fermion couplings at the one-loop level. We find that a single global U (1) phase invariance is sufficient to forbid masses for gauge non-singlet chiral scalar bosons, and that Heisenberg symmetry is not necessary.     

A radiative model with a naturally mild neutrino mass hierarchy

Many neutrino mass models postulate the existence of at least two extra fermions in order to account for the measured solar and atmospheric mass splittings. In these models, however, the predicted hierarchy between the two mass splittings is generically much larger than the observed one, unless extra flavor symmetries are introduced. We present in this Letter a radiative neutrino mass model consisting of the Standard Model extended by one heavy fermionic singlet and two scalars which predicts, under very general conditions, a neutrino mass hierarchy in qualitative agreement with the experimental value.     

Implications of a natural fermion mass hierarchy

The hierarchical struture of the fundamental fermion mass spectra is required to arise in a non-accidental way from a unified model G_family with a horizontal symmetry factor group G_generation. A quark or lepton must then not be in the same representation of G_family as its anti-particle. Models for G_family of the type SU(4)_C × SU(2)_L × SU(2)_R are favoured over SU(5) or SO(10).     

Closing in on the radiative weak chiral couplings

We point out that, given the current experimental status of radiative kaon decays, a subclass of the \(\mathcal{O} (p^4)\) counterterms of the weak chiral lagrangian can be determined in closed form. This involves in a decisive way the decay \(K^\pm \rightarrow \pi ^\pm \pi ^0 l^+ l^-\), currently being measured at CERN by the NA48/2 and NA62 collaborations. We show that consistency with other radiative kaon decay measurements leads to a rather clean prediction for the \(\mathcal{{O}}(p^4)\) weak couplings entering this decay mode. This results in a characteristic pattern for the interference Dalitz plot, susceptible to be tested already with the limited statistics available at NA48/2. We also provide the first analysis of \(K_S\rightarrow \pi ^+\pi ^-\gamma ^*\), which will be measured by LHCb and will help reduce (together with the related \(K_L\) decay) the experimental uncertainty on the radiative weak chiral couplings. A precise experimental determination of the \(\mathcal{{O}}(p^4)\) weak couplings is important in order to assess the validity of the existing theoretical models in a conclusive way. We briefly comment on the current theoretical situation and discuss the merits of the different theoretical approaches.     

On the quantum behavior of a neutral fermion in a pseudoscalar potential barrier

In this Letter we have studied the quantum behavior of a spin half neutral fermion interacting with a pseudoscalar potential barrier in (1+1$1+1$)-dimensional spacetime. Exact solutions for the corresponding Dirac equation are obtained both for bound and scattering states. The exact energy levels are obtained from the solutions of Dirac equation. The validity of the quasi-classical quantization rule is examined. For the scattering process the transmission and reflection coefficients are exactly calculated. The absence of the Klein?s paradox is also discussed.     

The fermion mass matrices near a fixed point

The structure of the fermion mass matrices is investigated in the neighbourhood of an infrared fixed point with special emphasis on the mixing angles which determine the charged current couplings. We find that it is necessary to introduce a fourth generation of quarks whose masses are predicted. The mass of the top quark is close to 35 GeV. The solution for a two family problem is found, which predicts very small charged current couplings of the very heavy to the lighter quarks.     

Vortex-core structure in neutral fermion superfluids with population imbalance

Quantized vortex-core structure is theoretically investigated in fermion superfluids with population imbalance for two atom species of neutral atom clouds near a Feshbach resonance. In contrast with the vortex core in balance case where the quantum depletion makes a vortex visible through the density profile measurement, the vortex core is filled in and becomes less visible because the quantized discrete bound states are occupied exclusively by the majority species. Yet it is shown that the core can be visible through the minority density profile experiment using phase contrast imaging, revealing an interesting opportunity to examine low-lying fermionic core bound states unexplored so far.     

Probability of creating a neutral vector boson by a fermion in an arbitrary plane-wave field

The probability is obtained of creating a neutral vector boson by a fermion in an arbitrary plane-wave electromagnetic field. The superposition of a constant crossed field and the field of a planar monochromatic wave of an elliptic polarization is considered as the correct field. Some particular cases of the indicated superposition are studied.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 53–59, July, 1990.