New neutrino experiments

Following incredible recent progress in understanding neutrino oscillations, many new ambitious experiments are being planned to study neutrino properties. The most important may be to find a non-zero value of θ₁₃. The most promising way to do this appears to be to measurev _μ →v _e oscillations with anE/L near Δm _atmo ² . Future neutrino experiments are great.     

Flavor symmetry L μ-L τ and quasidegenerate neutrinos

Current data implies three simple forms of the neutrino mass matrix, each corresponding to the conservation of a nonstandard lepton charge. While models based on L _e and L _e-L _μ-L _τ are well known, little attention has been paid to L _μ-L _τ. A low-energy mass matrix conserving L _μ-L _τ implies quasidegenerate light neutrinos. Moreover, it is μ-τ symmetric and therefore (in contrast to L _e and L _e-L _μ-L _τ) automatically predicts maximal atmospheric neutrino mixing and zero U _e3. A seesaw model based on L _μ-L _τ is investigated and testable predictions for the neutrino mixing observables are given. Renormalization group running below and in between the seesaw scales is taken into account in our analysis, both numerically and analytically. The text was submitted by the authors in English.     

Links between neutrino oscillations,leptogenesis, and proton decay within supersymmetric grand unification

Evidence in favor of supersymmetric grand unification including that based on the observed family multiplet-structure, gauge coupling unification, neutrino oscillations, baryogenesis, and certain intriguing features of quark-lepton masses and mixings is noted. It is argued that attempts to understand (a) the tiny neutrino masses (especially Δm ²(v ₂ – v₃)), (b) the baryon asymmetry of the Universe (which seems to need leptogenesis), and (c) the observed features of fermion masses such as the ratiom _b/m_τ, the smallness ofV _cb and the maximality of seem to select out the route to higher unification based on an effective string-unifiedG(224) =SU(2)_L ×SU(2)_R ×SU(2)^c orSO(10)-symmetry that should be operative in 4D, as opposed to other alternatives. A predictiveSO(10)/G(224)-framework possessing supersymmetry is presented that successfully describes the masses and mixings of all fermions including neutrinos. It also accounts for the observed baryon asymmetry of the Universe by utilizing the process of leptogenesis, which is natural to this framework. It is argued that a conservative upper limit on the proton lifetime within thisSO(10)/G(224)-framework, which is so far most successful, is given by x 10³⁴ years. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten (compared to SuperK) ought to reveal proton decay. Implications of this prediction for the next-generation nucleon decay and neutrino-detector are noted.     

Lepton flavour symmetry and the neutrino magnetic moment

With the standard model gauge group and the three standard left-handed Weyl neutrinos, two minimal scenarios are investigated where an arbitrary non-Abelian lepton flavour symmetry groupG _H is responsible for a light neutrino with a large magnetic moment. In the first case, with scalar fields carrying lepton flavour, some finetuning is necessary to get a small enough neutrino mass for _v =O(10)^–11 _B. In the second scenario, the introduction of heavy charged gauge singlet fermions with lepton flavour allows for a strictly massless neutrino to one-loop order. In both cases, the interference mechanism for smallm and large _v is unique, independently ofG _H . In explicit realizations of the two scenarios, the horizontal groups are found to be non-Abelian extensions of a Zeldovich-Konopinski-Mahmoud lepton number symmetry. Only a discrete part ofG _H is spontaneously broken leading to a light Dirac neutrino with a large magnetic moment.     

<Emphasis Type="Italic">SU</Emphasis><Subscript><Emphasis Type="Italic">L</Emphasis></Subscript>(4)×<Emphasis Type="Italic">U</Emphasis>(1) model for electroweak unification and sterile neutrinos

Some of the basic problems in neutrino physics, such as new energy scales, the enormous gap between the neutrino masses and the lightest charged fermion mass, and the possible existence of sterile neutrinos in the eV mass range are studied in the local gauge group SU _L (4)×U(1) for electroweak unification, which does not contain fermions with exotic electric charges. It is shown that the neutrino mass spectrum can be decoupled from that of the other fermions. The further normal seesaw mechanism for neutrinos, with right-handed neutrino Majorana masses of order M≫M _weak as well a new eV-scale can be accommodated. The eV-scale seesaw may manifest itself in experiments like the Liquid Scintillation Neutrino Detector (LSND) and MiniBooNE (MB) experimental results and future neutrino experiments.     

On fermion mass hirerachies in MSSM-like quiver models with stringy corrections

Using instanton effects, we discuss the problem of fermion mass hierarchies in an MSSM-like Type IIA orientifolded model with U(3) × Sp(1) × U(1) × U(1) gauge symmetry obtained from intersecting D6-branes. In the corresponding four-stack quiver, the different scales of the generated superpotential couplings offer a partial solution to fermion mass hierarchies. Using the known data with neutrino masses m_vt <~2 eVm_{v_\tau } \lesssim 2 eV, we give the magnitudes of the relevant scales.     

Interpretation of the recent Kolar events

We give plausible interpretations of the unusual events seen in the proton decay detector at Kolar Gold Fields indicating the existence of a massive (≳2GeV) long lived (10⁻⁸−10⁻⁹s) particle. We show that it is possible to accommodate the particle in the standard model as a fourth generation neutrino, or inE ₆ grand unified theory as a neutral fermion occurring in27 representation or in supersymmetric theory as a scalar neutrino. However, there is a difficulty in explaining the large production rate for the particle.     

Probing grand unification with fermion masses,neutrino oscillations and proton decay

It is noted that a set of facts points to the relevance in four dimensions of conventional supersymmetric unification based on minimally a string-unifiedG(224) symmetry, or maximallySO(10). These include: (i) the observed family structure, (ii) quantization of electric charge, (iii) meeting of the three gauge couplings, (iv) neutrino oscillations (in particular the value of δm ²(νμ−δ _τ), suggested by SuperK), (v) the intricate pattern of the masses and mixings of the fermions, including the smallness ofV _cb and the largeness ofθ _μνμτ ^osc , and (vi) the need for B-L as a generator to implement baryogenesis (via leptogenesis). A concrete proposal is presented within a predictiveSO(10)/G(224) framework that successfully describes the masses and mixings of all fermions, including the neutrinos — with eight predictions, all in agreement with observation. Within this framework, a systematic study of proton decay is carried out, which (a) pays special attention to its dependence on the fermion masses, (b) limits the threshold corrections so as to preserve natural coupling unification, and (c) uses recently improved values of the matrix element and renormalization effects. Allowing for both minimal supersymmetric standard model (MSSM) and its proposed variant, the so-called extended supersymmetric standard model (ESSM), as effective low-energy the ories, the study shows that a conservative upper limit on the proton lifetime is about (l–2)× 10³⁴ years, with ·⁻K⁺ being the dominant decay mode, and quite possibly μ⁺ K ⁰ ande ⁺π⁰ being prominent. This in turn strongly suggests that an improvement in the current sensitivity by a factor of five to ten ought to reveal proton decay. For comparison, some alternatives to the conventional approach to unification pursued here are mentioned at the end. Invited paper presented at the International Summer School held at ICTP, Trieste (June, 2001) and at WHEPP-7 Conference, Allahabad, India (January, 2002). This is an updated version of the paper presented at the Erice School (September, 2000), hep-ph/0106082.     

Two-photon resonant four-wave mixing in atomic sodium vapour

Using the 4d- and 5s-states of sodium, two different types of two-photon resonance enhanced four-wave mixing have been observed. In the first type 2v _L =v _IR +v _pIR radiation near 2.34 μm and UV radiation near 320 nm has been detected. In the second typev _P −v _S =v _L +v _IR the UV radiation produced in the first parametric process acted as a pump for stimulated electronic Raman scattering via the 4p-state of sodium. Resonantly enhanced by the two-photon transition 3s–4s, radiation near 1.18 μm was produced in this scheme. The realisation of a tunable coherent IR source based on these four-wave mixing processes is discussed.     

e -μ+→e +μ- process in a model with theSU(2)L×SU(2)R×U(1)B−L gauge group

A model based on theSU(2)_L×SU(2)_R×U(1)_B−L gauge group is used to study the lepton recharging processe ^-μ⁺→e ⁺μ^-. It should be possible to observe this process on a muon collider in the fixed-electron-target regime or the electron-beam regime. It is shown that the given process can be used to measure not only the characteristics of physical Higgs bosons, but also the parameters of neutrino oscillations. Grodno University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 83–88, November, 1998.     

What can we learn from high precision measurements of neutrino mixing angles?

Many experiments are being planned to measure the neutrino mixing angles more precisely. In this note, the theoretical significance of a high precision measurement of these parameters is discussed. It is emphasized that they can provide crucial information about different ways to understand the origin of large atmospheric neutrino mixing and move us closer towards determining the neutrino mass matrix. They may also be able to throw light on the question of lepton-quark unification as well as the existence of any leptonic symmetries. For instance if exact μ↔ τ symmetry in the neutrino mass matrix is assumed to be the reason for maximalv _μ-v_gt mixing, one gets θ₁₃ = 0 and {ie1295-01} can provide information about the way the μ↔ τ symmetry breaking manifests in the case of normal hierarchy.     

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).     

Is realization of Majorana neutrino and neutrinoless double beta decay possible in the framework of standard weak interactions?

Usually it is supposed that Majorana neutrino produced in the superposition state χ _L = ν _L + (ν _L ) ^c and then follows the neutrinoless double beta decay. But since the standard weak interactions are chiral invariant then neutrino at production has definite helicity (ν _L and (ν _L ) ^c have opposite spirality). Then these neutrinos are separately produced and their superposition state cannot appear. Thus we see that for unsuitable helicity the neutrinoless double β decay is not possible even if it is supposed that neutrino is a Majorana particle (i.e. there is not a lepton number which is conserved). Also transition of Majorana neutrino ν _L into antineutrino (ν _L ) ^c at their oscillations is forbidden since helicity in vacuum holds. Transition Majora neutrino ν _L into (ν _R ) ^c (i.e., ν _L → (ν _R ) ^c ) at oscillations is unobserved since it is supposed that mass of (ν _R ) ^c is very big. If neutrino is a Dirac particle there can be transition of ν _L neutrino into (sterile) antineutrino $ \bar v_R $ \bar v_R (i.e., ν _L → $ \bar v_R $ \bar v_R ) at neutrino oscillations if there takes place double violation of lepton number. It is necessary also to remark that introducing of a Majorana neutrino implies violation of global and local gauge invariance in the standard weak interactions.     

Remarks to the standard scheme (theory) of neutrino oscillations. Corrected scheme of neutrino oscillations

In the standard theory of neutrino oscillations, it is supposed that physically observed neutrino states ν _e , ν_μ, ν_τ, have no definite masses, that they are initially produced as a mixture of the ν₁, ν₂, ν₃ neutrino states (i.e., they are produced as a wave packet), and that neutrino oscillations are the real ones. Then, this wave packet must decompose at a definite distance into constituent parts and neutrino oscillations must disappear. It was shown that these suppositions lead to violation of the law of energy and momentum conservation. An alternative scheme of neutrino oscillations obtained within the framework of particle physics has been considered, where the above mentioned shortcomings are absent, the oscillations of neutrinos with equal masses are the real ones, and the oscillations of neutrinos with different masses are the virtual ones. Expressions for probabilities of neutrino transitions (oscillations) in the alternative (corrected) scheme are given. The text was submitted by the author in English.     

Renormalization of seesaw neutrino masses in the standard model with two-Higgs doublets

Using the theoretical ambiguities inherent in the seesaw mechanism, we derive the new analytic expressions for both quadratic and linear seesaw formulae for neutrino masses at low energies, with either up-type quark masses or charged lepton masses. This is possible through full radiative corrections arising out of the renormalizations of the Yukawa couplings, the coefficients of the neutrino-mass-operator in the standard model with two-Higgs doublets, and also the QCD-QED rescaling factors below the top-quark mass scale, at one-loop level. We also investigate numerically the unification of top-b-τ Yukawa couplings at the scale M ₁=0.59×10⁸ GeV for a fixed value of tan β=58.77, and then evaluate the seesaw neutrino masses which are too large in magnitude to be compatible with the presently available solar and atmospheric neutrino oscillation data. However, if we consider a higher but arbitrary value of M ₁=0.59×10¹¹ GeV, the predictions from linear seesaw formulae with charged lepton masses, can accommodate simultaneousely both solar atmospheric neutrino oscillation data.     

Symmetries of the standard model without and with a right-handed neutrino

Given the particle content of the standard model without and with a right-handed neutrino, the requirement that all anomalies cancel singles out a set of possible global symmetries which can be gauged. I review this topic and propose a new gauge symmetryB — 3L _T in the context of the minimal standard model consisting of the usual three families of quarks and leptons plus just onev _R. The many interesting phenomenological consequences of this hypothesis are briefly discussed.     

Small Dirac neutrino mass and left-right symmetry

We analyze the possibility of generating light Dirac neutrinos at the tree level in a left-right symmetric scenario. We present a minimal extension of the standard SU(2) _L × SU(2) _R × U(1) _Y′ model where the above result is achieved through a “see-saw” like mechanism induced by the minimization of the Higgs potential. The Dirac neutrinos thus obtained are naturally light; indeed we show that the scheme is stable under radiative corrections. The neutrino mass is inversely related to the scale of parity breaking, which may naturally be in the TeV range, leading to new phenomenology in an interesting energy domain.     

Growth velocity and the topography of Ni-Zn binary alloy electrodeposits

We show that the electrodeposition of Ni-Zn alloys at the lowest growth velocities, v < 0.5μm/s, exclusively proceeds from an abnormal co-deposition phenomenon. The growth process in this v region greatly depends on the initial [Co²⁺] concentration of the film deposition bath. A theoretical approach of this process including the role of the saturation surface roughness of the alloy, , leads to an estimation of the transport properties of the ad-atoms involved during the deposit formation. Their surface diffusion coefficient varying between 1.76×10^-10 and 2.40×10^-8 cm^-2/s exhibits a minimal value, D _s = 2.10×10^-10 cm^-2/s located between v = 0.17 and 0.35μm/s. The spatial scaling analysis of the local roughness, σ, examined according to the power-law σ≈L ^α reveals that the resulting roughness exponents concurs with the Kardar-Parisi-Zhang dynamics including the restricted surface diffusion. Two main v regions leads to different fractal textural features of the alloy film surface. Below 0.10 μm/s, the roughness exponent obtained is α≈ 0.6, depicting a limited ad-atom mobility. Over v = 0.30μm/s, this exponent stabilises at α≈ 0.82, indicating an increase of the surface diffusion. Received 16 August 2000 and Received in final form 20 June 2001     

Sterile neutrino in a minimal three-generation see-saw model

We investigate symmetries in Dirac and Majorana mass matrices of neutrinos in a three-generation scenario. We show that if we invokeL _e +L _μ-L _τ x S _2R symmetry, one combination of right-handed neutrino states remains massless which can be interpreted as a sterile neutrino. Next we consider a SU2_L x U(1)_y x U(l)_R gauge model and show how higher-dimensional operators can induce mixing between left- and right-handed states which explains solar, atmospheric and LSND experimental results.     

O(5)×U(1) electroweak gauge theory and the neutrino oscillations in matter

A study is made of the groupO(5)×U(1). The group is economical in the number of gauge bosons, which we associate with each of its generators, and is anomaly-free. The left-handed leptonsL _L ^T (v _e ,e,,v ) _L are assigned to the four-dimensional spinorial representations ofO(5). The right-handed particles are taken to be the singlets of the group. The theory has three sets of gauge bosons: (1) analogues of the GWS model, (2) additional charged gauge bosons, and (3) a set of three additional neutral gauge bosons as compared to the GWS model. We introduce neutrino mixing by mixing the additional charged gauge bosons. We develope a theory of neutrino oscillations in matter in such a way that in the absence of matter the scattering length reduces to the usual scattering length in vacuum. Even if the neutrino masses are equal or the neutrinos are massless, we still have neutrino oscillations in matter, a result already noted by Wolfenstein.