arXiv subjects

Petr Vogel

Publications and source records attributed to Petr Vogel.

At least 19 recordsLinked to original sources

arXiv2020

Muon capture rates: Evaluation within the Quasiparticle Random Phase Approximation

The Quasiparticle Random Phase Approximation (QRPA) is used in evaluation of the total muon capture ratesfor the final nuclei participating in double-beta decay. Several variants of the method are used, depending on the size of the single particle model space used, or treatment of the initial bound muon wave function. The resulting capture rates are all reasonably close to each other. In particular, the variant that appears to be most realistic, results in rates in good agreement with the experimental values. There is no necessity for an empirical quenching of the axial current coupling constant $g_A$. Its standard value $g_A$ = 1.27 seems to be adequate.

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arXiv2019

Reactor Neutrinos: Toward Oscillations

I shall sketch the history of reactor neutrino physics over five decades since the Reines-Cowan proof of neutrino existence in the late 50s, till the advent of the present era of precision reactor neutrino oscillation experiments. There are three chapters of this story: i) Exploration of possibilities of the reactor neutrinos in the 60s and 70s; ii) Looking for oscillations under the streetlamp in the 80s and 90s; iii) Exploring oscillations in detail with known (almost) $Δm^2_{atm}$ and $Δm^2_{sol}$.}

hep-ph↗

arXiv2018

$0νββ$ nuclear matrix elements, neutrino potentials and $\mathrm{SU}(4)$ symmetry

Intimate relation between the Gamow-Teller part of the matrix element $M^{0ν}_\mathrm{GT}$ and the $2νββ$ closure matrix element $M^{2ν}_\mathrm{cl}$ is explained and explored. If the corresponding radial dependence $C^{2ν}_\mathrm{cl}(r)$ would be known, $M^{0ν}$ corresponding to any mechanism responsible for the $0νββ$ decay can be obtained as a simple integral. However, the $M^{2ν}_\mathrm{cl}$ values sensitively depend on the properties of higher lying $1^+$ states in the intermediate odd-odd nuclei. We show that the $β^-$ and $β^+$ amplitudes of such states typically have opposite relative signs, and their contributions reduce severally the $M^{2ν}_\mathrm{cl}$ values. Vanishing values of $M^{2ν}_\mathrm{cl}$ are signs of a partial restoration of the spin-isospin $\mathrm{SU}(4)$ symmetry. We suggest that demanding that $M^{2ν}_\mathrm{cl}$ = 0 is a sensible way, within the method of the Quasi-particle Random Phase Approximation (QRPA), of determining the amount of renormalization of isoscalar particle-particle interaction strength $g^{T=0}_{pp}$. Using such prescription, the matrix elements $M^{0ν}$ are evaluated; their values are not very different ($\le$ 20\%) from the usual QRPA values when $g^{T=0}_{pp}$ is related to the known $2νββ$ half-lives.

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arXiv2016

Reactor Neutrino Spectra

We present a review of the antineutrino spectra emitted from reactors. Knowledge of these and their associated uncertainties are crucial for neutrino oscillation studies. The spectra used to-date have been determined by either conversion of measured electron spectra to antineutrino spectra or by summing over all of the thousands of transitions that makeup the spectra using modern databases as input. The uncertainties in the subdominant corrections to beta-decay plague both methods, and we provide estimates of these uncertainties. Improving on current knowledge of the antineutrino spectra from reactors will require new experiments. Such experiments would also address the so-called reactor neutrino anomaly and the possible origin of the shoulder observed in the antineutrino spectra measured in recent high-statistics reactor neutrino experiments.

hep-ph↗

arXiv2016

Evaluation of reactor neutrino flux: issues and uncertainties

Evaluation of the reactor $\barν_e$ flux and spectrum is an essential ingredient of their application in the neutrino oscillation studies. Two anomalies, i.e. discrepancies between the observed and expected count rates, are widely discussed at the resent time. The total rate is $\sim$ 6\% lower than the expectation at all distances $>$ 10 m from the reactor. And there is a shoulder (often referred to as "bump") at neutrino energies 5-7 MeV, not predicted in the calculated spectrum. I review the ways the flux and spectrum is evaluated and concentrate on the error budget. I argue that far reaching conclusions based on these anomalies should await a thorough understanding of the uncertainties of the spectrum, and point out possible standard physics sources of the anomalies.

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arXiv2015

The light neutrino exchange mechanism of the $0νββ$-decay with left- and right-handed leptonic and hadronic currents revisited

The extension of Majorana neutrino mass mechanism of the neutrinoless double-beta decay ($0νββ$) with the inclusion of right-handed leptonic and hadronic currents is revisited. While only the exchange of light neutrinos is assumed, the $s_{1/2}$ and $p_{1/2}$-states of emitted electrons as well as recoil corrections to the nucleon currents are taken into account. Within the standard approximations the decay rate is factorized into a sum of products of kinematical phase-space factors, nuclear matrix elements and the fundamental parameters that characterize the lepton number violation. Unlike in the previous treatments the induced pseudoscalar term of hadron current is included, resulting in additional nuclear matrix elements. An improved numerical computation of the phase-space factors is presented, based on the exact Dirac wave functions of the $s_{1/2}$ and $p_{1/2}$ electrons with finite nuclear size and electron screening taken into account. The dependence of values of these phase-space factors on the different approximation schemes used in evaluation of electron wave functions is discussed. The upper limits for effective neutrino mass and the parameters $\langleλ\rangle$ and $\langleη\rangle$ characterizing the right-handed current mechanism are deduced from data on the $0νββ$-decay of $^{76}$Ge and $^{136}$Xe using nuclear matrix elements calculated within the nuclear shell model and quasiparticle random phase approximation. The differential decay rates, i.e. the angular correlations and the single electron energy distributions for various combinations of the total lepton number violating parameters that can help to disentangle the possible mechanism are described and discussed.

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arXiv2015

Neutrino Oscillation Studies with Reactors

Nuclear reactors are one of the most intense, pure, controllable, cost-effective, and well-understood sources of neutrinos. Reactors have played a major role in the study of neutrino oscillations, a phenomenon that indicates that neutrinos have mass and that neutrino flavors are quantum mechanical mixtures. Over the past several decades reactors were used in the discovery of neutrinos, were crucial in solving the solar neutrino puzzle, and allowed the determination of the smallest mixing angle $θ_{13}$. In the near future, reactors will help to determine the neutrino mass hierarchy and to solve the puzzling issue of sterile neutrinos.

hep-ex↗

arXiv2013

Lepton Flavor and Number Conservation, and Physics Beyond the Standard Model

The physics responsible for neutrino masses and lepton mixing remains unknown. More experimental data are needed to constrain and guide possible generalizations of the standard model of particle physics, and reveal the mechanism behind nonzero neutrino masses. Here, the physics associated with searches for the violation of lepton-flavor conservation in charged-lepton processes and the violation of lepton-number conservation in nuclear physics processes is summarized. In the first part, several aspects of charged-lepton flavor violation are discussed, especially its sensitivity to new particles and interactions beyond the standard model of particle physics. The discussion concentrates mostly on rare processes involving muons and electrons. In the second part, the status of the conservation of total lepton number is discussed. The discussion here concentrates on current and future probes of this apparent law of Nature via searches for neutrinoless double beta decay, which is also the most sensitive probe of the potential Majorana nature of neutrinos.

hep-ph↗

arXiv2013

$0νββ$ and $2νββ$ nuclear matrix elements, QRPA, and isospin symmetry restoration

Within QRPA we achieve partial restoration of the isospin symmetry and hence fulfillment of the requirement that the $2νββ$ Fermi matrix element $M^{2ν}_F$ vanishes, as it should, unlike in the previous version of the method. This is accomplished by separating the renormalization parameter $g_{pp}$ of the particle-particle proton-neutron interaction into the isovector and isoscalar parts. The isovector parameter $g_{pp}^{T=1}$ need to be chosen to be essentially equal to the pairing constant $g_{pair}$, so no new parameter is needed. For the $0νββ$ decay the Fermi matrix element $M^{0ν}_F$ is substantially reduced, while the full matrix element $M^{0ν}$ is reduced by $\approx$ 10%. We argue that this more consistent approach should be used from now on in the proton-neutron QRPA and in analogous methods.

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arXiv2012

Nuclear structure and double beta decay

Study of the neutrinoless double beta decay, $0νββ$, includes a variety of problems of nuclear structure theory. They are reviewed here. The problems range from the mechanism of the decay, i.e. exchange of the light Majorana neutrino neutrino versus the exchange of some heavy, so far unobserved particle. Next, the proper expressions for the corresponding operator are described that should include the effects of the nucleon size and of the recoil order terms in the hadronic current. The issue of proper treatment of the short range correlations, in particular for the case of the heavy particle exchange, is discussed also. The variety of methods employed these days in the theoretical evaluation of the nuclear matrix elements $M^{0ν}$ is briefly described and the difficulties causing the spread and hence uncertainty in the values of $M^{0ν}$ are discussed. Finally, the issue of the axial current quenching, and of the resonance enhancement in the case of double electron capture are described.

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arXiv2011

Relation between the $2νββ$ and $0νββ$ nuclear matrix elements

A formal relation between the GT part of the nuclear matrix elements $M^0ν}_{GT}$ of $0νββ$ decay and the closure matrix elements $M^{2ν}_{cl}$ of $2νββ$ decay is established. This relation is based on the integral representation of these quantities in terms of their dependence on the distance $r$ between the two nucleons undergoing transformation. We also discuss the difficulties in determining the correct values of the closure $2νββ$ decay matrix elements.

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arXiv2010

Relation between the $0νββ$ and $2νββ$ nuclear matrix elements revisited

We show that the dominant Gamow-Teller part, $M^{0ν}_{GT}$, of the nuclear matrix element governing the neutrinoless $ββ$ decay is related to the matrix element $M^{2ν}_{cl}$ governing the allowed two-neutrino $ββ$ decay. That relation is revealed when these matrix elements are expressed as functions of the relative distance $r$ between the pair of neutrons that are transformed into a pair of protons in the $ββ$ decay. Analyzing this relation allows us to understand the contrasting behavior of these matrix elements when $A$ and $Z$ is changed; while $M^{0ν}_{GT}$ changes slowly and smoothly, $M^{2ν}$ has pronounced shell effects. We also discuss the possibility of phenomenological determination of the $M^{2ν}_{cl}$ and from them of the $M^{0ν}_{GT}$ values from the experimental study of the $β^{\pm}$ strength functions.

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arXiv2010

On a theory of neutrino oscillations with entanglement

We show that, despite appearances, a theoretical approach to neutrino oscillation in which the neutrino and its interaction partners are entangled yields the standard result for the neutrino oscillation wavelength. We also shed some light on the question of why plane-wave approaches to the neutrino oscillation problem can yield the correct oscillation wavelength even though they do not explicitly account for the localization of the neutrino source and the detector.

hep-ph↗

arXiv2008

$0νββ$ nuclear matrix elements and the occupancy of individual orbits

The measured occupancies of valence orbits in $^{76}$Ge and $^{76}$Se are used as a guideline for modification of the effective mean field energies that results in better description of these quantities. With them, in combination with the selfconsitent renormalized quasiparticle random phase approximation (SRQRPA) method that ensures conservation of the mean particle number in the correlated ground state, we show that the resulting $0νββ$ nuclear matrix element for the $^{76}$Ge $\to$ $^{76}$Se transition is reduced by $\sim$25% compared to the previous QRPA value, and therefore the difference between the present approach and the interacting shell model predictions becomes correspondingly smaller. Analogous modification of the mean field energies for the A=82 system also results in a reduction of $0νββ$ matrix element for the $^{82}$Se $\to$ $^{82}$Kr transition, making it also closer to the shell model prediction.

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arXiv2008

Nuclear physics aspects of double beta decay

Comprehensive description of the phenomenology of the $ββ$ decay is given, with emphasis on the nuclear physics aspects. After a brief review of the neutrino oscillation results and of motivation to test the lepton number conservation, the mechanism of the $0νββ$ is discussed. Its relation to the lepton flavor violation involving charged leptons and its use as a diagnostic tool of the $0νββ$ mechanism is described. Next the basic nuclear physics of both $ββ$-decay modes is presented, and the decay rate formulae derived. The nuclear physics methods used, the nuclear shell model and the quasiparticle random phase approximation, are described next, and the choice of input parameters is discussed in the following section. Finally, the numerical values of the nuclear matrix elements, and their uncertainty, are presented. In the appendix the relation of the search for the neutrino magnetic moment to the Dirac versus Majorana nature of neutrinos is described.

hep-ph↗

arXiv2008

Neutrino Mass and Neutrinoless Double Beta Decay

The motivation for the search for $0νββ$ decay is briefly reviewed. It is stressed that the exchange of light Majorana neutrinos is not the only possible mechanism of the decay. The link between lepton number and lepton flavor violation is described and its role in elucidating the $0νββ$-decay mechanism is discussed. The main topic of the talk is the evaluation of the nuclear matrix elements and their uncertainty. Various physics effects that influence the value of the matrix elements are described and the results of the two main methods, the quasiparticle random phase approximation and the nuclear shell model, are compared.

hep-ph↗

arXiv2008

Anatomy of nuclear matrix elements for neutrinoless double-beta decay

We show that, within the Quasiparticle Random Phase Approximation (QRPA) and the renormalized QRPA (RQRPA) based on the Bonn CD nucleon-nucleon interaction, the competition between the pairing and the neutron-proton particle-particle and particle-hole interactions causes contributions to the neutrinoless double-beta decay matrix element to nearly vanish at internucleon distances of more than 2 or 3 fermis. As a result, the matrix element is more sensitive to short-range/high-momentum physics than one naively expects. We analyze various ways of treating that physics and quantify the uncertainty it produces in the matrix elements, with three different treatments of short-range correlations.

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arXiv2007

Conversion of electron spectrum associated with fission into the antineutrino spectrum

The accuracy of the procedure that converts the experimentally determined electron spectrum associated with fission of the nuclear fuels ^{235}U, ^{239}Pu, ^{241}Pu, and ^{238}U into the $\barν_e$ spectrum is examined. By using calculated sets of mutually consistent spectra it is shown that the conversion procedure can result in a small $\sim$1% error provided several conditions are met. Chief among them are the requirements that the average nuclear charge as a function of the $β$ decay endpoint energy is independently known and that the $\barν_e$ spectrum is binned into bins that are several times larger than the width of the slices used to fit the electron spectrum.

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