arXiv subjects

V. B. Semikoz

Publications and source records attributed to V. B. Semikoz.

50 records · Page 3Linked to original sources

arXiv1998

Neutrino magnetic moments and low-energy solar neutrino-electron scattering experiments

The scattering of solar neutrinos on electrons is sensitive to the neutrino magnetic moments through an interference of electromagnetic and weak amplitudes in the cross section. We show that future low-energy solar neutrino experiments with good angular resolution can be sensitive to the resulting azimuthal asymmetries in event number and should provide useful information on non-standard neutrino properties such as magnetic moments. We compare asymmetries expected at HELLAZ (mainly pp neutrinos) with those at the Kamiokande and Super-Kamiokande experiments (Boron neutrinos), both for the case of Dirac and Majorana neutrinos and discuss the advantages of low energies. Potentially interesting information on the solar magnetic fields may be accessible.

hep-ph↗

arXiv1998

Neutrino electron scattering and electroweak gauge structure: Probing the masses of a new Z boson

Low-energy high-resolution neutrino-electron scattering experiments may play an important role in testing the gauge structure of the electroweak interaction. We discuss the use of strong radioactive neutrino sources (e.g. $^{51}$Cr) in underground experiments such as BOREXINO, HELLAZ and LAMA. We display the sensitivity of these detectors in testing the possible existence of extra neutral $E_6$ gauge bosons.

hep-ph↗

arXiv1998

Neutrino conversions in random magnetic fields and $\tildeν_e$ from the Sun

The magnetic field in the convective zone of the Sun has a random small-scale component with the r.m.s. value substantially exceeding the strength of a regular large-scale field. For two Majorana neutrino flavors $\times$ two helicities in the presence of a neutrino transition magnetic moment and nonzero neutrino mixing we analyze the displacement of the allowed ($Δm^2- \sin^22θ$)-parameter region reconciled for the SuperKamiokande(SK) and radiochemical (GALLEX, SAGE, Homestake) experiments in dependence on the r.m.s. magnetic field value $b$, or more precisely, on a value $μb$ assuming the transition magnetic moment $μ= 10^{-11}μ_B$. In contrast to RSFP in regular magnetic fields we find an effective production of electron antineutrinos in the Sun even for small neutrino mixing through cascade conversions $ν_{eL}\to ν_{μL}\to \tildeν_{eR}$, $ν_{eL}\to ν_{μR}\to \tildeν_{eR}$ in a random magnetic field that would be a signature of the Majorana nature of neutrino if $\tildeν_{eR}$ will be registered. Basing on the present SK bound on electron antineutrinos we have also found an excluded area in the same $Δm^2,~\sin^22θ$-plane and revealed a strong sensitivity to the random magnetic field correlation length $L_0$.

hep-ph↗

arXiv1998

Low-energy solar anti-neutrinos

If neutrino conversions within the Sun result in partial polarization of initial solar neutrino fluxes, then a new opportunity arises to observe the anti-ν_e's in future neutrino experiments in the low energy region (such as BOREXINO or HELLAZ) and thus to probe the Majorana nature of the neutrinos. The ν_e -> anti-ν_e conversions may take place for low energy solar neutrinos while being unobservable at the Kamiokande and Super-Kamiokande experiments.

hep-ph↗

arXiv1998

Aperiodic Spin-Flavor Conversions and $\tildeν_e$ from the Sun with Random Magnetic Field

The magnetic field in the solar convective zone has a random small-scale component with the r.m.s. value substantially exceeding the strength of a regular large-scale field. For two Majorana neutrino flavors $\times$ two helicities in the presence of a neutrino transition magnetic moment and nonzero neutrino mixing we analyze the displacement of the allowed ($Δm^2 - \sin^22θ$) - parameter region reconciled for the SuperKamiokande(SK) and radiochemical (GALLEX, SAGE, Homestake) experiments in dependence on the r.m.s. magnetic field value $b$. We find an effective production of electron antineutrinos in the Sun even for small neutrino mixing through the cascade conversions in the random magnetic field that would be a signature of the Majorana nature of neutrino if $\tildeν_{eR}$ will be registered. Basing on the present SK bound on electron antineutrinos we also estimated the corresponding excluded area in the plane $Δm^2, \sin^22θ$.

hep-ph↗

arXiv1998

Low-energy Anti-neutrinos from the Sun

We consider the sensitivity of future neutrino experiments in the low energy region, such as BOREXINO or HELLAZ, to a solar electron antineutrino signal. We show that, if neutrino conversions within the Sun result in partial polarization of initial solar neutrino fluxes, then a new opportunity arises to observe the electron antineutrinos and thus to probe the Majorana nature of the neutrinos. This is achieved by comparing the slopes of the energy dependence of the differential neutrino electron scattering cross section for different neutrino conversion scenarios. We also show how the ν_e -> \barν_e conversions may take place for low energy solar neutrinos while being unobservable at the Kamiokande and Super-Kamiokande experiments.

hep-ph↗

arXiv1997

Can we distinguish Majorana and Dirac neutrinos in solar neutrino experiments ?

If neutrino conversions within the Sun result in partial polarization of initial solar neutrino fluxes then a new opportunity to distinguish Majorana and Dirac neutrinos by measuring the differential $ν_ee$-scattering cross section in solar neutrino detectors arises. The experiment like HELLAZ would be preferable in testing of recoil electron spectra differences initiated by Majorana and Dirac neutrinos since low energy recoil electrons ($T\geq 100 keV$) can be detected and electron energy and direction can be determined with good precision.

hep-ph↗

arXiv1997

Neutrino Conversions in a Polarized Medium

Electron polarization induced by magnetic fields can modify the potentials relevant for describing neutrino conversions in media with magnetic fields. The magnitudes of polarization potentials are determined for different conditions. We show that variations of the electron polarization along the neutrino trajectory can induce resonant conversions in the active-sterile neutrino system, but cannot lead to level crossing in the active-active neutrino system. For neutrino flavour conversions the polarisation leads only to a shift of the standard MSW resonance. For polarizations $λ\lsim 0.04$ the direct modifications of the potential (density) due to the magnetic field pressure are smaller than the modifications due to the polarization effect. We estimate that indeed the typical magnitude of the polarization in the sun or in a supernova are not expected to exceed $10^{-2}$. However even such a small polarization may lead to interesting consequences for supernova physics and for properties of neutrino signals from collapsing stars.

hep-ph↗

arXiv1996

The MSW conversion of solar neutrinos and random matter density perturbations

We present a generalization of the resonant neutrino conversion in matter, including a random component in the matter density profile. The study is focused on the effect of such matter perturbations upon both large and small mixing angle MSW solutions to the solar neutrino problem. This is carried out both for the active-active $ν_e \to ν_{μ,τ}$ as well as active-sterile $ν_e \to ν_s$ conversion channels. We find that the small mixing MSW solution is much more stable (especially in $δm^2$) than the large mixing solution. Future solar neutrino experiments, such as Borexino, could probe solar matter density noise at the few percent level.

hep-ph↗

arXiv1996

The effect of random matter density perturbations on the MSW solution to the solar neutrino problem

We consider the implications of solar matter density random noise upon resonant neutrino conversion. The evolution equation describing MSW-like conversion is derived in the framework of the Schrödinger approach. We study quantitatively their effect upon both large and small mixing angle MSW solutions to the solar neutrino problem. This is carried out both for the active-active $ν_e \ra ν_{μ,τ}$ as well as active-sterile $ν_e \ra ν_s$ conversion channels. We find that the small mixing MSW solution is much more stable (especially in $Δm^2$) than the large mixing solution. The possible existence of solar matter density noise at the few percent level could be tested at future solar neutrino experiments, especially Borexino.

hep-ph↗

arXiv1995

A New Type of Resonant Neutrino Conversions Induced by Magnetic Fields

We consider resonant neutrino conversions in magnetised matter, such as a degenerate electron gas. We show how magnetisation effects caused by axial vector interactions of neutrinos with the charged leptons in the medium can induce a new type of resonant neutrino conversion which may occur even in situations where the MSW effect does not occur, such as the case of degenerate or inverted neutrino mass spectra. Our new resonance may simultaneously affect anti-neutrino $\bar{ν_a} \leftrightarrow \barν_b$ as well as neutrino $ν_{a} \leftrightarrow ν_b$ flavour conversions, and therefore it may substantially affect supernova neutrino energy spectra. Using SN1987A data we conclude that only laboratory experiments with long baseline such as ICARUS or MINOS are likely to find neutrino oscillations due to their sensitivity to small $Δm^2$. We also comment on the possibility of resonant conversions induced by Majorana neutrino transition moments and mention the case of sterile neutrinos $ν_s$.

hep-ph↗

arXiv1995

Bounds on Neutrino Transition Magnetic Moments in Random Magnetic Fields

We consider the conversions of active to sterile Majorana neutrinos $ν_{a}$ and $ν_{s}$, due to neutrino transition magnetic moments in the presence of random magnetic fields (r.m.f.) generated at the electroweak phase transition. From a simple Schrödinger-type evolution equation, we derive a stringent constraint on the corresponding transition magnetic moments and display it as a function of the domain size and field geometry. For typical parameter choices one gets limits much stronger than usually derived from stellar energy loss considerations. These bounds are consistent with the hypothesis of seeding of galactic magnetic fields by primordial fields surviving past the re-combination epoch. We also obtain a bound on active-sterile neutrino transition magnetic moments from supernova energy loss arguments. For r.m.f. strengths in the range $10^7$ to $10^{12}$ Gauss we obtain limits varying from $μ_{as}^ν \lsim 10^{-13}μ_B$ to $μ_{as}^ν \lsim 10^{-18}μ_B$, again much stronger than in the case without magnetic fields.

hep-ph↗

arXiv1995

Neutrino conversions in hot plasma

We discuss the excitation of sterile neutrinos in the early universe using general quantum kinetic equations, which also incorporate a possible primordial magnetic field $B$. We find a new contribution to the excitation propability, which has its origin in the shrinkage of the spin vector. In the absence of $B$ nucleosynthesis implies the constraint $\mid Δm^2\mid\sin^22θ_0\lsim 1.6\times 10^{-6}~{\rm eV}^2$ which is more restrictive than previous estimates. We also present examples of possible stringent limits for $B\ne 0$.

hep-ph↗

arXiv1994

"Nucleosynthesis constraints on active-sterile neutrino conversions in the early universe with random magnetic field"

We consider active-sterile neutrino conversions in the early universe hot plasma in the presence of a random magnetic field generated at the electroweak phase transition. Within a random field domain the magnetization asymmetry of the lepton antilepton plasma produced by a uniform constant magnetic field is huge in contrast to their small density asymmetry, leading to a drastic change in the active-sterile conversion rates. Assuming that the random field provides the seed for the galactic field one can estimate the restrictions from primordial nucleosynthesis. Requiring that the extra sterile \neu does not enter in equilibrium with the active ones before nucleosynthesis we find limits of the oscillation parameters which are stronger than in the isotropic case.

hep-ph↗