arXiv subjects

Evgeny Akhmedov

Publications and source records attributed to Evgeny Akhmedov.

At least 19 recordsLinked to original sources

arXiv2026

Resonant Induced Orbital Electron Capture: Novel method for low-energy $\barν_e$ detection

We propose a novel approach to detecting low-energy electron antineutrinos based on the induced capture of orbital electrons by nuclei. This process is resonant, requiring the antineutrino energy to precisely match the energy difference between the final and initial atomic systems. For continuous-spectrum sources, the resonance conditions can be satisfied without fine-tuning, and the effective cross sections depend on the spectral intensity of the $\barν_e$ flux at the resonance rather than on the neutrino energy itself. This opens the possibility of detecting neutrinos of never previously probed low energies. We identify a number of candidate nuclides that allow transitions to excited states of the daughter nuclei corresponding to resonant $\barν_e$ energies below the inverse $β$ decay threshold of 1.8 MeV, and we consider a distinctive atomic-nuclear coincidence signature for background rejection. We discuss implications of the proposed method for detecting low-energy reactor neutrinos, geoneutrinos, and keV-scale thermal solar neutrinos. Applications to neutrino oscillation experiments and to reactor monitoring are also briefly discussed.

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arXiv2026

Bound-state beta decay of tritium: Path to first observation and novel approach to direct neutrino mass measurement

Bound-state $β$-decay of tritium, the process, in which the final-state electron is created in a bound atomic state of the produced ${\rm ^3He}$ atom instead of freely flying away, is predicted by the standard theory of weak interactions but has not been observed so far. We study the possibility of its experimental observation through the detection of photons from radiative decay of the excited atomic states of neutral ${\rm ^3He}$ populated by this process. We also propose a novel approach to direct neutrino mass measurement and sterile neutrino search based on accurate determination of the speed of the produced ${\rm ^3He}$ atoms through Doppler broadening of the emitted photon lines.

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arXiv2025

On chirality and chiral neutrino oscillations

It has been claimed in a number of publications that neutrinos can exhibit chirality oscillations. In this note we discuss the notion of chirality and show that chiral neutrino oscillations in vacuum do not occur. We argue that the incorrect claims to the contrary resulted from a failure to clearly discriminate between quantum fields, states and wave functions. We also emphasize the role played in the erroneous claims on the possibility of chirality oscillations by the widely spread misconceptions about negative energies.

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arXiv2024

Non-relativistic neutrinos and the question of Dirac vs. Majorana neutrino nature

Finding out if neutrinos are Dirac or Majorana particles is known to be extremely difficult. This is generally believed to be due to the smallness of the neutrino mass compared to typical neutrino energies, and to the fact that in the limit of vanishing mass all distinctions between Dirac and Majorana neutrinos disappear. This, however, does not necessarily mean that for non-relativistic neutrinos distinguishing between them is an easy task. We consider this problem in detail. The issues discussed include the possibilities of studying neutrino nature with non-relativistic neutrinos produced in $β$ decay in direct neutrino mass measurement experiments, with relic neutrinos, as well as with slow neutrinos in pair production processes and in scattering, both elastic and inelastic. We also discuss processes allowed for only one of the two neutrino types as a means of uncovering neutrino nature.

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arXiv2024

BBN catalysis by doubly charged particles

We consider primordial nucleosynthesis in the presence of hypothetical quasi-stable doubly charged particles. Existence of $X^{--}$ with macroscopic lifetimes will lead to the formation of its bound states with $^4$He and other light elements, significantly facilitating the subsequent formation of lithium nuclei. From observational constraints on maximum allowable amount of lithium, that we update in this work, we derive strong constraints on the abundance and lifetime of $X^{--}$. In a likely cosmological freeze-out scenario with temperatures initially exceeding the mass of $X^{--}$, the BBN constrains the lifetime of these particles to be less than about 100 seconds. For parametrically long lifetimes, lithium abundance data constrain $X^{--}$ abundance to be less than $10^{-9}$ relative to protons, regardless of whether these particles decay or remain stable. Stable particles could saturate the dark matter density only if their mass is comparable to or in excess of $10^{10}$ GeV, and most of $X^{--}$ will be found in bound states with beryllium nuclei, so that chemically they would appear as abnormally heavy helium isotopes.

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arXiv2024

Long-lived doubly charged scalars in the left-right symmetric model: catalyzed nuclear fusion and collider implications

We show that the doubly charged scalar from the $SU(2)_R$-triplet Higgs field in the Left-Right Symmetric Model has its mass governed by a hidden symmetry so that its value can be much lower than the $SU(2)_R$ breaking scale. This makes it a long-lived particle while being consistent with all existing theoretical and experimental constraints. Such long-lived doubly charged scalars have the potential to trigger catalyzed fusion processes in light nuclei, which may have important applications for energy production. We show that it could also bear consequences on the excess of large ionization energy loss ($dE/dx$) recently observed in collider experiments.

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arXiv2024

Can quantum statistics help distinguish Dirac from Majorana neutrinos?

Finding out if neutrinos are Dirac or Majorana particles is known to be extremely difficult due to the smallness of neutrino mass and the fact that in the limit $m_ν=0$ both Dirac and Majorana neutrinos become Weyl particles, i.e. are indistinguishable. There have been suggestions in the literature that in the case of processes with production of a neutrino-antineutrino pair (if neutrinos are Dirac particles) or two neutrinos (if they are of Majorana nature) quantum statistics may be of help. This is because for Majorana neutrinos quantum indistinguishability of identical particles requires the amplitude of the process to be antisymmetrized with respect to the interchange of the final-state neutrinos, whereas no such antisymmetrization must be done for Dirac neutrinos. It has been claimed that the resulting differences between the cross sections for Dirac and Majorana neutrinos persist even for arbitrarily small but not exactly vanishing neutrino mass. We demonstrate that, at least in the framework of the Standard Model, this is not the case. We also give a general proof that within the Standard Model quantum statistics does not help tell Dirac and Majorana neutrinos apart in the limit of negligibly small $m_ν/E$.

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arXiv2022

Solar $\barν_e$ flux: Revisiting bounds on neutrino magnetic moments and solar magnetic field

The interaction of neutrino transition magnetic dipole moments with magnetic fields can give rise to the phenomenon of neutrino spin-flavour precession (SFP). For Majorana neutrinos, the combined action of SFP of solar neutrinos and flavour oscillations would manifest itself as a small, yet potentially detectable, flux of electron antineutrinos coming from the Sun. Non-observation of such a flux constrains the product of the neutrino magnetic moment $μ$ and the strength of the solar magnetic field $B$. We derive a simple analytical expression for the expected $\barν_e$ appearance probability in the three-flavour framework and we use it to revisit the existing experimental bounds on $μB$. A full numerical calculation has also been performed to check the validity of the analytical result. We also present our numerical results in energy-binned form, convenient for analyses of the data of the current and future experiments searching for the solar $\barν_e$ flux. In addition, we give a comprehensive compilation of other existing limits on neutrino magnetic moments and of the expressions for the probed effective magnetic moments in terms of the fundamental neutrino magnetic moments and leptonic mixing parameters.

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arXiv2022

Damping of neutrino oscillations, decoherence and the lengths of neutrino wave packets

Spatial separation of the wave packets (WPs) of neutrino mass eigenstates leads to decoherence and damping of neutrino oscillations. Damping can also be caused by finite energy resolution of neutrino detectors or, in the case of experiments with radioactive neutrino sources, by finite width of the emitted neutrino line. We study in detail these two types of damping effects using reactor neutrino experiments and experiments with radioactive $^{51}$Cr source as examples. We demonstrate that the effects of decoherence by WP separation can always be incorporated into a modification of the energy resolution function of the detector and so are intimately entangled with it. We estimate for the first time the lengths $σ_x$ of WPs of reactor neutrinos and neutrinos from a radioactive $^{51}$Cr source. The obtained values, $σ_x = (2\times 10^{-5} - 1.4\times 10^{-4})$ cm, are at least six orders of magnitude larger than the currently available experimental lower bounds. We conclude that effects of decoherence by WP separation cannot be probed in reactor and radioactive source experiments.

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arXiv2022

Reply to "Comment on "Damping of neutrino oscillations, decoherence and the lengths of neutrino wave packets""

In arXiv:2209.00561 our treatment arXiv:2208.03736 of effects of particles emitted together with neutrinos on neutrino wave packets is criticized on several grounds. We show here that this criticism is based on misinterpretation of our results and is invalid. Our conclusions and, in particular, the conclusion that neutrino wave packet separation effects are unobservable in reactor and neutrino source experiments, remain unchanged.

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arXiv2022

Nuclear fusion catalyzed by doubly charged scalars: Implications for energy production

A number of popular extensions of the Standard Model of particle physics predict the existence of doubly charged scalar particles $X^{\pm\pm}$. Such particles may be long-lived or even stable. If exist, $X^{--}$ could form atomic bound states with light nuclei and catalyze their fusion by essentially eliminating the Coulomb barrier between them. Such an $X$-catalyzed fusion ($X$CF) process does not require high temperatures or pressure and may have important applications for energy production. A similar process of muon-catalyzed fusion ($μ$CF) has been shown not to be a viable source of energy because of the sticking of negative muons to helium nuclei produced in the fusion of hydrogen isotopes, which stops the catalytic process. We analyze $X$CF in deuterium environments and show that the $X$-particles can only stick to $^6$Li nuclei, which are produced in the third-stage reactions downstream in the catalytic cycle. The corresponding sticking probability is very low, and, before getting bound to $^6$Li, each $X$-particle can catalyze $\sim 3.5\cdot 10^{9}$ fusion cycles, producing $\sim 7\cdot 10^{4}$ TeV of energy. We also discuss the ways of reactivating the $X$-particles from the Coulomb-bound (${\rm ^6Li}X$) states, which would allow re-using them in $X$CF reactions.

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arXiv2020

Neutrino oscillations in matter: from microscopic to macroscopic description

Neutrino flavour transmutations in nonuniform matter are described by a Schrödinger-like evolution equation with coordinate-dependent potential. In all the derivations of this equation it is assumed that the potential, which is due to coherent forward scattering of neutrinos on matter constituents, is a continuous function of coordinate that changes slowly over the distances of the order of the neutrino de Broglie wavelength. This tacitly assumes that some averaging of the microscopic potential (which takes into account the discrete nature of the scatterers) has been performed.The averaging, however, must be applied to the microscopic evolution equation as a whole and not just to the potential. Such an averaging has never been explicitly carried out. We fill this gap by considering the transition from the microscopic to macroscopic neutrino evolution equation through a proper averaging procedure. We discuss some subtleties related to this procedure and establish the applicability domain of the standard macroscopic evolution equation. This, in particular, allows us to answer the question of when neutrino propagation in rarefied media (such as e.g.\ low-density gases or interstellar or intergalactic media) can be considered within the standard theory of neutrino flavour evolution in matter.

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arXiv2019

Relic neutrino detection through angular correlations in inverse $β$-decay

Neutrino capture on beta-decaying nuclei is currently the only known potentially viable method of detection of cosmic background neutrinos. It is based on the idea of separation of the spectra of electrons or positrons produced in captures of relic neutrinos on unstable nuclei from those from the usual $β$-decay and requires very high energy resolution of the detector, comparable to the neutrino mass. In this paper we suggest an alternative method of discrimination between neutrino capture and $β$-decay, based on periodic variations of angular correlations in inverse beta decay transitions induced by relic neutrino capture. The time variations are expected to arise due to the peculiar motion of the Sun with respect to the C$ν$B rest frame and the rotation of the Earth about its axis and can be observed in experiments with both polarized and unpolarized nuclear targets. The main advantage of the suggested method is that it does not depend crucially on the energy resolution of detection of the produced $β$-particles and can be operative even if this resolution exceeds the largest neutrino mass.

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arXiv2019

Quantum mechanics aspects and subtleties of neutrino oscillations

Neutrino oscillations appear to be a simple quantum mechanical phenomenon. However, a closer look at them reveals a number of subtle points and apparent paradoxes. Some of the basic issues of the theory of neutrino oscillations are still being debated. I discuss, from a historical perspective, how the role of quantum mechanical aspects of neutrino oscillations was realized in the course of the development of the theory of this phenomenon.

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arXiv2018

Coherent scattering and macroscopic coherence: Implications for neutrino, dark matter and axion detection

We study the question of whether coherent neutrino scattering can occur on macroscopic scales, leading to a significant increase of the detection cross section. We concentrate on radiative neutrino scattering on atomic electrons (or on free electrons in a conductor). Such processes can be coherent provided that the net electron recoil momentum, i.e. the momentum transfer from the neutrino minus the momentum of the emitted photon, is sufficiently small. The radiative processes is an attractive possibility as the energy of the emitted photons can be as large as the momentum transfer to the electron system and therefore the problem of detecting extremely low energy recoils can be avoided. The requirement of macroscopic coherence severely constrains the phase space available for the scattered particle and the emitted photon. We show that in the case of the scattering mediated by the usual weak neutral current and charged current interactions this leads to a strong suppression of the elementary cross sections and therefore the requirement of macroscopic coherence results in a reduction rather than an increase of the total detection cross section. However, for the $νe$ scattering mediated by neutrino magnetic or electric dipole moments coherence effects can actually increase the detection rates. Effects of macroscopic coherence can also allow detection of neutrinos in 100 eV -- a few keV energy range, which is currently not accessible to the experiment. A similar coherent enhancement mechanism can work for relativistic particles in the dark sector, but not for the conventionally considered non-relativistic dark matter.

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arXiv2017

Collective neutrino oscillations and neutrino wave packets

Effects of decoherence by wave packet separation on collective neutrino oscillations in dense neutrino gases are considered. We estimate the length of the wave packets of neutrinos produced in core collapse supernovae and the expected neutrino coherence length, and then proceed to consider the decoherence effects within the density matrix formalism of neutrino flavour transitions. First, we demonstrate that for neutrino oscillations in vacuum the decoherence effects are described by a damping term in the equation of motion of the density matrix of a neutrino as a whole (as contrasted to that of the fixed-momentum components of the neutrino density matrix). Next, we consider neutrino oscillations in ordinary matter and dense neutrino backgrounds, both in the adiabatic and non-adiabatic regimes. In the latter case we study two specific models of adiabaticity violation -- one with short-term and another with extended non-adiabaticity. It is demonstrated that, while in the adiabatic case a damping term is present in the equation of motion of the neutrino density matrix (just like in the vacuum oscillation case), no such term in general appears in the non-adiabatic regime.

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arXiv2017

Do non-relativistic neutrinos oscillate?

We study the question of whether oscillations between non-relativistic neutrinos or between relativistic and non-relativistic neutrinos are possible. The issues of neutrino production and propagation coherence and their impact on the above question are discussed in detail. It is demonstrated that no neutrino oscillations can occur when neutrinos that are non-relativistic in the laboratory frame are involved, except in a strongly mass-degenerate case. We also discuss how this analysis depends on the choice of the Lorentz frame. Our results are for the most part in agreement with Hinchliffe's rule.

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arXiv2017

Decoherence by wave packet separation and collective neutrino oscillations

In dense neutrino backgrounds present in supernovae and in the early Universe, neutrino oscillations may exhibit complex collective phenomena, such as synchronized oscillations, bipolar oscillations and spectral splits and swaps. In this Letter we consider for the first time the effects of decoherence by wave packet separation on these phenomena. We derive the evolution equations that govern neutrino oscillations in a dense medium in the presence of decoherence and consider the evolution of several simple neutrino systems in detail. We show that decoherence may modify the oscillation pattern significantly and lead to qualitatively new effects. In particular, contrary to the no-decoherence case, strong flavor conversion becomes possible even in the case of constant or nearly constant density of the neutrino background.

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