Lepton Capture Research Articles

Effects of an intermediate mass sterile neutrino population on the early Universe

The hot and dense early Universe combined with the promise of high-precision cosmological observations provide an intriguing laboratory for Beyond Standard Model (BSM) physics. We simulate the early Universe to examine the effects of the decay of thermally populated sterile neutrino states into Standard Model products around the time of weak decoupling. These decays deposit a significant amount of entropy into the plasma as well as produce a population of high-energy out-of-equilibrium active neutrinos. As a result, we can constrain these models by their inferred value of $N_{\rm eff}$, the effective number of relativistic degrees of freedom. In this work, we explore a variety of models with $N_{\rm eff}{}$ values consistent with CMB observations, but with vastly different active neutrino spectra which will challenge the standard cosmological model, affect lepton capture rates on free nucleons, and may significantly affect Big Bang Nucleosynthesis (BBN).

Bulk viscosity from Urca processes: npeμ matter in the neutrino-trapped regime

In this work, we extend our previous study of the bulk viscosity of hot and dense $npe$ matter induced by the Urca processes in the neutrino trapped regime to $npe\ensuremath{\mu}$ matter by adding the muonic Urca processes as well as the purely leptonic electroweak processes involving electron-muon transition. The nuclear matter is modeled in a relativistic density functional approach with two different parametrizations which predict neutrino dominated matter (DDME2 model) and antineutrino dominated matter (NL3 model) at temperatures for which neutrinos/antineutrinos are trapped. In the case of neutrino-dominated matter, the main equilibration mechanism is lepton capture, whereas in the case of antineutrino-dominated matter this is due to neutron decay. We find that the equilibration rates of Urca processes are higher than that of the pure leptonic processes, which implies that the Urca-process-driven bulk viscosity can be computed with the leptonic reactions assumed to be frozen. We find that the bulk viscosity decreases with temperature as $\ensuremath{\zeta}\ensuremath{\sim}{T}^{\ensuremath{-}2}$ at moderate temperatures. At high temperatures this scaling breaks down by sharp drops of the bulk viscosity close to the temperature where the proton fraction is density-independent and the matter becomes scale-invariant. This occurs also when the matter undergoes a transition from the antineutrino-dominated regime to the neutrino-dominated regime where the bulk viscosity attains a local maximum. We also estimate the bulk viscous dissipation timescales and find that these are in the $\mathrm{range}\ensuremath{\gtrsim}1\text{ }\text{ }\mathrm{s}$ for temperatures above the neutrino trapping temperature. These timescales would be relevant only for long-lived objects formed in binary neutron star mergers and hot proto-neutron stars formed in core-collapse supernovas.

A Study of $\tau^-$ and $\mu^-$ in the Field of Nuclei Using Neural Network Techniques

The rate of a heavy lepton (muon or tau) capture by nuclei as well as the heavy lepton to electron conversion rate can be calculated when the heavy lepton wavefunction is known. Analytical calculation of the wavefunction of any of these leptons around any nucleus is not feasible owning to their small Bohr radii, on the one hand, and to the finite nuclear extend on the other. A new numerical calculation algorithm is proposed hereby, which makes use of the concept of neural networks. The main advantage of this new technique is that the wave function is produced analytically as a sum of sigmoid functions.

Lepton emission rates of [formula omitted]v isotopes under stellar conditions

In astrophysical conditions prevalent during the late times of stellar evolution, lepton (e− and e+) emission processes compete with the corresponding lepton capture processes. Prior to the collapse, lepton emissions significantly affect the cooling of the core and reduce its entropy. Therefore the lepton emission rates for Fe-group nuclei serve as an important input for core-collapse simulations of high-mass stars. From earlier simulation studies, isotopes of vanadium (V) have great astrophysical significance in regard to their weak-decay rates which substantially affect Ye (fraction of lepton to baryon number) during the final developmental stages of massive stars. The current study involves the computation of the weak lepton emission (LE) rates for V-isotopes by employing the improved deformed proton-neutron Quasi-particle Random Phase Approximation (pn-QRPA) model. The mass numbers of the selected isotopes range from 43 to 64. The LE rates on these isotopes have been estimated for a broad spectrum of density and temperature under astrophysical conditions. The ranges considered for density and temperature are 101 to 1011 (g/cm3) and 107 to 3 × 1011 (K), respectively. The lepton emission rates from the present study were also compared to the rates previously estimated by using the independent-particle model (IPM) and large-scale shell model (LSSM). IPM rates are generally bigger than QRPA rates, while LSSM rates are overall in good comparison with the reported rates. We attribute these differences to correct placement of GT centroid in LSSM and QRPA models.

Probing dark photons in the early universe with big bang nucleosynthesis

We perform calculations of dark photon production and decay in the early universe for ranges of dark photon masses and vacuum coupling with standard model photons. Simultaneously and self-consistently with dark photon production and decay, our calculations include a complete treatment of weak decoupling and big bang nucleosynthesis (BBN) physics. These calculations incorporate all relevant weak, electromagnetic, and strong nuclear reactions, including charge-changing (isospin-changing) lepton capture and decay processes. They reveal a rich interplay of dark photon production, decay, and associated out-of-equilibrium transport of entropy into the decoupling neutrino seas. Most importantly, the self-consistent nature of our simulations allows us to capture the magnitude and phasing of entropy injection and dilution. Entropy injection-induced alteration of the time-temperature-scale factor relation during weak decoupling and BBN leads to changes in the light element abundance yields and the total radiation content (as parametrized by Neff). These changes suggest ways to extend previous dark photon BBN constraints. However, our calculations also identify ranges of dark photon mass and couplings not yet constrained, but perhaps accessible and probable, in future Stage-4 cosmic microwave background experiments and future high precision primordial deuterium abundance measurements.

Lepton capture rates due to isotopes of vanadium in astrophysical environment

Lepton (electron and positron) capture rates on iron-regime nuclei are an essential element for modeling the late stages of progression of massive stars that become core collapse and thermonuclear supernova. As per previous simulation studies, lepton capture (LC) rates on isotopes of vanadium are believed to have a substantial effect in regulating the Ye (lepton to baryon fraction) during the final evolutionary phases. The present work involves the calculation of lepton capture rates for 22 isotopes of vanadium by making use of the proton-neutron (pn-) quasiparticle random phase approximation (QRPA) model. The covered mass range is from A = 43 to 64. The LC rates have been computed over stellar densities ranging from $10^{1}$ to $10^{11}$ (g/cm3) and for the temperature range $10^{7} - 3 \times 10^{10}$ (K). A comparison of our calculated LC rates to the rates computed using other models (IPM and LSSM) has also been presented. As compared to the rates calculated by other models, pn-QRPA rates at high temperature ( $3 \times 10^{10}$ K) are larger by up to 1-2 orders of magnitude.

Nuclear Neutrino Spectra in Late Stellar Evolution

Neutrinos are the principle carriers of energy in massive stars, beginning from core carbon burning and continuing through core collapse and after the core bounce. In fact, it may be possible to detect neutrinos from nearby pre-supernova stars. Therefore, it is of great interest to understand the neutrino energy spectra from these stars. Leading up to core collapse, beginning around core silicon burning, nuclei become dominant producers of neutrinos, particularly at high neutrino energy, so a systematic study of nuclear neutrino spectra is desirable. We have done such a study, and we present our sd-shell model calculations of nuclear neutrino energy spectra for nuclei in the mass number range A = 21 – 35. Our study includes neutrinos produced by charged lepton capture, charged lepton emission, and neutral current nuclear deexcitation. Previous authors have tabulated the rates of charged current nuclear weak interactions in astrophysical conditions, but the present work expands on this not only by providing neutrino energy spectra, but also by including the heretofore untabulated neutral current de-excitation neutrino pairs.

Ground-state Nuclear Properties of Neutron-rich Copper Isotopes and Lepton Capture Rates in Stellar Matter

Ground-state Nuclear Properties of Neutron-rich Copper Isotopes and Lepton Capture Rates in Stellar Matter

Gamow-Teller strength and lepton captures rates on 66−71Ni in stellar matter

Charge-changing transitions play a significant role in stellar weak-decay processes. The fate of the massive stars is decided by these weak-decay rates including lepton (positron and electron) captures rates, which play a consequential role in the dynamics of core collapse. As per previous simulation results, weak interaction rates on nickel (Ni) isotopes have significant influence on the stellar core vis-à-vis controlling the lepton content of stellar matter throughout the silicon shell burning phases of high mass stars up to the presupernova stages. In this paper, we perform a microscopic calculation of Gamow–Teller (GT) charge-changing transitions, in the [Formula: see text]-decay and electron capture (EC) directions, for neutron-rich Ni isotopes ([Formula: see text]Ni). We further compute the associated weak-decay rates for these selected Ni isotopes in stellar environment. The computations are accomplished by employing the deformed proton–neutron quasiparticle random phase approximation (pn-QRPA) model. A recent study showed that the deformed pn-QRPA theory is well suited for the estimation of GT transitions. The astral weak-decay rates are determined over densities in the range of 10–10[Formula: see text][Formula: see text]g/cm3and temperatures in the range of 0.01[Formula: see text]–30[Formula: see text][Formula: see text]K. The calculated lepton capture rates are compared with the previous calculation of Pruet and Fuller (PF). The overall comparison demonstrates that, at low stellar densities and high temperatures, our EC rates are bigger by as much as two orders of magnitude. Our results show that, at higher temperatures, the lepton capture rates are the dominant mode for the stellar weak rates and the corresponding lepton emission rates may be neglected.

Weak interaction rate Coulomb corrections in big bang nucleosynthesis

We have applied a fully relativistic Coulomb wave correction to the weak reactions in the full Kawano/Wagoner big bang nucleosynthesis (BBN) code. We have also added the zero-temperature radiative correction. We find that using this higher accuracy Coulomb correction results in good agreement with previous work, giving only a modest $\ensuremath{\sim}0.04%$ increase in helium mass fraction over correction prescriptions applied previously in BBN calculations. We have calculated the effect of these corrections on other light element abundance yields in BBN, and we have studied these yields as functions of electron neutrino lepton number. This has allowed insights into the role of the weak neutron-proton interconversion processes in the setting of the neutron-to-proton ratio during the BBN epoch. We find that the lepton capture processes' contributions to this ratio are only second order in the Coulomb correction.

Estimates of Stellar Weak Interaction Rates for Nuclei in the Mass Range A = 65–80

We estimate lepton capture and emission rates, as well as neutrino energy loss rates, for nuclei in the mass range A = 65-80. These rates are calculated on a temperature/density grid appropriate for a wide range of astrophysical applications including simulations of late time stellar evolution and X-ray bursts. The basic inputs in our single-particle and empirically inspired model are (i) experimentally measured level information, weak transition matrix elements, and lifetimes, (ii) estimates of matrix elements for allowed experimentally unmeasured transitions based on the systematics of experimentally observed allowed transitions, and (iii) estimates of the centroids of the GT resonances motivated by shell model calculations in the fp shell as well as by (n, p) and (p, n) experiments. Fermi resonances (isobaric analog states) are also included, and it is shown that Fermi transitions dominate the rates for most interesting proton-rich nuclei for which an experimentally determined ground state lifetime is unavailable. For the purposes of comparing our results with more detailed shell model based calculations we also calculate weak rates for nuclei in the mass range A = 60-65 for which Langanke & Martinez-Pinedo have provided rates. The typical deviation in the electron capture and β-decay rates for these ≈30 nuclei is less than a factor of 2 or 3 for a wide range of temperature and density appropriate for presupernova stellar evolution. We also discuss some subtleties associated with the partition functions used in calculations of stellar weak rates and show that the proper treatment of the partition functions is essential for estimating high-temperature β-decay rates. In particular, we show that partition functions based on unconverged Lanczos calculations can result in errors in estimates of high-temperature β-decay rates.

The Influence of Nuclear Composition on the Electron Fraction in the Post–Core Bounce Supernova Environment

We study the early evolution of the electron fraction (or, alternatively, the neutron-to-proton ratio) in the region above the hot proto-neutron star formed after a supernova explosion. We study the way in which the electron fraction in this environment is set by a competition between lepton (electron, positron, neutrino, and antineutrino) capture processes on free neutrons and protons and nuclei. Our calculations take explicit account of the effect of nuclear composition changes, such as formation of particles (the alpha effect) and the shifting of nuclear abundances in nuclear statistical equilibrium associated with cooling in near-adiabatic outFLow. We take detailed account of the process of weak interaction freezeout in conjunction with these nuclear composition changes. Our detailed treatment shows that the effect can cause significant increases in the electron fraction, while neutrino and anti-neutrino capture on heavy nuclei tends to have a buffering effect on this quantity. We also examine the effect on weak rates and the electron fraction of fluctuations in time in the neutrino and antineutrino energy spectra arising from hydrodynamic waves. Our analysis is guided by the Wilson and Mayle supernova code numerical results for the neutrino energy spectra and density and velocity profiles.

Lattice calculation for lepton capture from vacuum-pair production in relativistic heavy-ion collisions

The new colliding-beam heavy-ion accelerators, designed to investigate nuclear matter at high temperatures and densities, have motivated great interest during the last decade concerning possible new electromagnetic phenomena. One of the most interesting of these processes is the sparking of the vacuum to produce lepton-antilepton pairs. The process of electron capture from vacuum pair production is a principal beam-loss mechanism for highly charged relativistic ions in a storage ring, and thus must be considered in the design and the operation of these machines. In this paper, we present calculated impact-parameter-dependent probabilities and cross sections for muon capture from vacuum production in collisions of relativistic heavy ions by solving the time-dependent Dirac equation in unrestricted three-dimensional space using lattice-collocation techniques. Calculations are performed for muon-pair production with capture into the K-shell in collisions of 197Au + 197Au at collider kinetic energies of 2 GeV per nucleon.

Relativistic multipole operators for semileptonic weak and electromagnetic nuclear reactions.

We discuss multipole operators that arise in a relativistic analysis of semileptonic weak and electromagnetic interactions with nuclei. These single-particle operators are evaluated between relativistic nucleon bound states that are solutions to the Dirac equation with potentials of the form produced by the \ensuremath{\sigma}-\ensuremath{\omega} model. The reduced matrix elements are given in terms of easily programmable radial integrals and can be applied to a number of reactions such as elastic and inelastic electron scattering, real photon processes, \ensuremath{\beta} decay, and charged lepton capture as well as more exotic interactions such as charged and neutral current neutrino reactions. As a specific example, we calculate the elastic and inelastic magnetic form factors of p-shell nuclei.

Stellar weak interaction rates for intermediate-mass nuclei. IV - Interpolation procedures for rapidly varying lepton capture rates using effective log (ft)-values

Simple expressions for continuum electron and positron capture phase space factors and the associated neutrino energy loss integrals are presented in terms of standard Fermi integrals. Continuous approximations to the relevant Fermi integrals and their first derivatives are made. These allow the computation of effective log (ft)--values, at each temperature and density point, for the continuum lepton capture rates considered in the earlier papers in this series. Since the effective log (ft)--values have most of the rapid temperature and density dependence associated with the phase space integrals removed, interpolation in temperature and density to obtain stellar rates is greatly facilitated in speed and accuracy. Computer simulations of stellar evolution will be able to implement more accurately our calculations of the stellar nuclear weak interaction rates of intermediate-mass nuclei. Generalization of the Fermi integral expressions for the lepton continuum capture phase space factors are given for astrophysical environments where there exists an equilibrium distribution of electron-type neutrinos. These allow rough estimates of the effect of neutrino blocking on our tabulated rates and estimates of total neutrino capture rates.

Direct measurement of the helicity of the muonic neutrino

We describe a simple experiment for measuring the helicity of the muonic neutrino, hνμ, directly. This experiment is the muonic analog of the celebrated Brookhaven experiment for the electron neutrino. Both are based on measuring the longitudinal polarization PL of the recoils in a 0→1 lepton capture; here we exploit 12C(μ−, νμ)12B(g.s.), determining the polarization of 12B through its β-decay asymmetry. Recoils going into a specific hemisphere are chosen by the trick of ’’selective implantation.’’ The paper includes an elementary derivation of the requisite theoretical expressions. We find hνμ = −1.06(11), i.e., −1.0 with considerable accuracy.

Measurement of the μ− + He 3 → H 3 + ν reaction rate

The reaction mu /sup -/ + He/sup 3/ yields T + nu was investigated using a statistical material giving results an order of magnitude better than those obtained previously (O. A. Zaimidoroga et al., J. Exptl. Theor. Phys., 41: 1804(1961)); previous experimental techniques were used. The rate of the reaction was measured to study muon-electron symmetry in interactions of these particles with nucleons; data agree with calculations within theoretical and experimental inaccuracies. Muon-electron symmetry in lepton capture by nucleons is implied. (L.N.N.)