Neutrinos with magnetic moments emit photons into inhomogeneous media
This paper has been withdrawn by the author. The spinor matrix element was incorrect, and orders of magnitude too big.
hep-ph↗
arXiv subjects
R. F. Sawyer
Publications and source records attributed to R. F. Sawyer.
The multi-angle instability in dense neutrino systems
We calculate rates of flavor exchange within clouds of neutrinos interacting with each other through the standard model coupling, assuming a conventional mass matrix. For cases in which there is an angular dependence in the relation among intensity, flavor and spectrum, we find instabilities in the evolution equations and greatly speeded-up flavor exchange. The instabilities are categorized by examining linear perturbations to simple solutions, and their effects are exhibited in complete numerical solutions to the system. The application is to the region just under the neutrino surfaces in the supernova core.
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Hydrodynamics of an electron-positron plasma near a black hole; applications to jet formation
We investigate some features of the hydrodynamics and neutrino physics in the (predominantly) electron-positron plasma above a hyperaccreting disk (or torus) around a black hole, a conjectured engine for a short gamma ray burst. We suggest a possible scenario in which plasma in the region very near the black hole, energetically driven by neutrino annihilation, emerges as a subsonic wind, which in a spherically symmetrical case would decelerate as it moves out. In this case we argue that the plasma heating will be primarily through neutrino-electron and neutrino-positron scattering, and that this process will be important throughout a region considerably larger than that of the neutrino annihilation process. In simple solutions a relatively gentle anisotropy in the heating through this process can create an approximately conical sonic surface, aligned with the system's axis. Inside this cone the fluid accelerates upwards as in standard jet models.
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Bulk viscosity of a gas of neutrinos and coupled scalar particles, in the era of recombination
Bulk viscosity may serve to damp sound waves in a system of neutrinos coupled to very light scalar particles, in the era after normal neutrino decoupling but before recombination. We calculate the bulk viscosity parameter in a minimal scheme involving the coupling of the two systems. We add some remarks on the bulk viscosity of a system of fully ionized hydrogen plus photons.
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Effects of ion and electron correlations on neutrino scattering in the infall phase of a supernova
Many authors have used one-component plasma simulations in discussing the role of ion-ion correlations in reducing neutrino opacities during the collapse phase of a supernova. In a multicomponent plasma in which constituent ions have even a small range of N/Z ratios neutrino opacities are much larger, in some regions of parameters, than for the case of a one component plasma.
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Speed-up of neutrino transformations in a supernova environment
When the neutral current neutrino-neutrino interaction is treated completely, rather than as an interaction among angle-averaged distributions, or as a set of flavor-diagonal effective potentials, the result can be flavor mixing at a speed orders of magnitude faster than that one would anticipate from the measured neutrino oscillation parameters. It is possible that the energy spectra of the three active species of neutrinos emerging from a supernova are nearly identical.
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"Classical" instabilities and "quantum" speed-up in the evolution of neutrino clouds
We study some examples of collective behavior in neutrino clouds governed by the neutral-current neutrino-neutrino interaction. The standard equations for analyzing such systems are rederived in a two-step process: first, a replacement of the full interaction Hamiltonian with a "forward" Hamiltonian that contains only the momentum states that were initially occupied by a neutrino of one flavor or another; second, a factorization assumption that reduces the time evolution problem to the solution of coupled equations for the expectations of various bilinear forms in the neutrino fields. We designate the latter as the "classical" equations. We analyze some solutions of these equations in cases in which the initial momentum and flavor distributions of neutrinos are strongly anisotropic in space. In some cases we find an instability that leads to rapid evolution of the flavor-angle distribution, even when it is seeded by a very small initial flavor mixing (or alternatively by a very small neutrino mass$^2$ term). Turning to the more complete case in which we do not assume the classical factorization, but instead solve for the evolution under the influence of the full "forward" Hamiltonian, we find the possibility of rapid evolution, under our definition, even when there is no seeding from conventional neutrino mixing.
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Collective polarization exchanges in collisions of photon clouds
The one-loop "vacuum" Heisenberg-Euler coupling of four electromagnetic fields can lead to interesting collective effects in the collision of two photon clouds, on a time scale orders of magnitude faster than one estimates from the cross-section and density. We estimate the characteristic time for macroscopic transformation of positive to negative helicity in clouds that are initially totally polarized and for depolarization of a polarized beam traversing an unpolarized cloud.
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Evolution speed in some coupled-spin models
We investigate the time evolution of some models with N spins and pairwise couplings, for the case of large N, in order to compare evolution times with "speed limit" minima derived in the literature. Both in a (symmetric) case with couplings of the same strength between each pair and in a case of broken symmetry, the times necessary for evolution to a state in which the simplest initial state has evolved into a nearly orthogonal state are proportional to 1/N, as is the speed limit time. However the coefficient in the broken symmetry case comes much closer to the speed limit value. Introducing a different criterion for evolution speed, based on macroscopic changes in occupation, we find a corresponding enhancement in rates in the asymmetric case as compared to the symmetric case.
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Absence of resonant enhancements in some inclusive rates
A toy model is defined and solved perturbatively with the aim of examining some claimed "resonant" enhancements of certain reaction rates that enter popular models of leptogenesis. We find: a) that such enhancements are absent; and b) that the perturbative solution, as done correctly using finite- temperature field theory, is well defined without the "resumming" procedures found in the literature. The pathologies that led to the perceived need for these procedures are an artifact of uncritical use of weighted vacuum cross- sections in the determination of rates, without adequate attention to the effects of the medium upon the single particle states within it.
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Polarization precession in photon-photon encounters
We calculate the rate of precession of the direction of polarization of a photon traversing a sea of plane-polarized photons moving in the opposed direction, where the interaction is the one-loop "vacuum" Heisenberg-Euler coupling of four fields. Substantial precession can take place in a distance many orders of magnitude shorter than the free path for photon-photon scattering, mediated by the same interaction. We consider briefly the possibility of some interesting collective effects in the case in which instead of a particle and a sea, two seas are caused to collide.
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Neutrino transport in accretion disks
We test approximate approaches to solving a neutrino transport problem that presents itself in the analysis of some accretion-disk models. Approximation #1 consists of replacing the full, angular- dependent, distribution function by a two-stream simulation, where the streams are respectively outwardly and inwardly directed, with angles $\cos θ=\pm 1/\sqrt{3}$ to the vertical. In this approximation the full energy dependence of the distribution function is retained, as are the energy and temperature dependences of the scattering rates. Approximation #2, used in recent works on the subject, replaces the distribution function by an intensity function and the scattering rates by temperature-energy-averaged quantities. We compare the approximations to the results of solving the full Boltzmann equation. Under some interesting conditions, approximation #1 passes the test; approximation #2 does not. We utilize the results of our analysis to construct a toy model of a disc at a temperature and density such that relativistic particles are more abundant than nucleons, and dominate both the opacity and pressure. The nucleons will still provide most of the energy density. In the toy model we take the rate of heat generation (which drives the radiative transfer problem) to be proportional to the nucleon density. The model allows the simultaneous solution of the neutrino transport and hydrostatic equilibrium problems in a disk in which the nucleon density decreases approximately linearly as one moves from the median plane of the disk upwards, reaching zero on the upper boundary.
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Speed-up through entanglement -- many-body effects in neutrino processes
We study a system containing many particles of identical kinematics with a zero range interaction that scatters one from the other, and with the possible exchange of an attribute. Taking an initial condition in which the attribute is asymmetrically distributed in the regions of momentum space occupied by the particles, we study the rate at which it becomes uniformly distributed, through collisions. We find, in some circumstances, a rate that is much faster than that which would be estimated from cross-sections. This behavior is attributable in some general sense to N-particle entanglement. We suggest applications to neutrino physics, where the attribute is neutrino flavor.
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State permutations from manipulation of near level-crossings
We discuss some systematic methods for implementing state manipulations in systems formally similar to chains of a few spins with nearest-neighbor interactions, arranged such that there are strong and weak scales of coupling links. States are permuted by means of bias potentials applied to a few selected sites. This generic structure is then related to an atoms-in-a-cavity model that has been proposed in the literature as a way of achieving a decoherence free subspace. A new method using adiabatically varying laser detuning to implement a CNOT gate in this model is proposed.
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Generation of entangled states and error protection from adiabatic avoided level crossings
We consider the environment-affected dynamics of $N$ self-interacting particles living in one-dimensional double wells. Two topics are dealt with. First, we consider the production of entangled states of two-level systems. We show that by adiabatically varying the well biases we may dynamically generate maximally entangled states, starting from initially unentangled product states. Entanglement degradation due to a common type of environmental influence is then computed by solving a master equation. However, we also demonstrate that entanglement production is unaffected if the system-environment coupling is of the type that induces ``motional narrowing''. As our second but related topic, we construct a different master equation that seamlessly merges error protection/detection dynamics for quantum information with the environmental couplings responsible for producing the errors in the first place. Adiabatic avoided crossing schemes are used in both topics.
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Entanglement and quantal coherence: a study of two limiting cases of rapid system--bath interactions
We consider the dynamics of a system coupled to a thermal bath, going beyond the standard two-level system through the addition of an energy excitation degree of freedom. Further extensions are to systems containing many fermions, with the master equations modified to take Fermi-Dirac statistics into account, and to potentials with a time-dependent bias that induce resonant avoided crossing transitions. The limit $Q \to \infty$, where the interaction rate with the bath is much greater than all free oscillation rates for the system, is interrogated. Two behaviors are possible: freezing (quantum Zeno effect) or synchronization (motional narrowing). We clarify the conditions that give rise to each possibility, making an explicit connection with quantum measurement theory. We compare the evolution of quantal coherence for the two cases as a function of $Q$, noting that full coherence is restored as $Q \to \infty$. Using an extended master equation, the effect of system-bath interactions on entanglement in bipartite system states is computed. In particular, we show that the sychronization case sees bipartite system entanglement fully preserved in the large $Q$ limit.
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Neutrino Propagation in Dense Astrophysical Systems
Even the elusive neutrinos are trapped in matter, albeit transiently, in several astrophysical circumstances. Their interactions with the ambient matter not only reveal the properties of such exotic matter itself, but also shed light on the fundamental properties of the neutrinos themselves. The physical sites of interest include the early universe, supernovae, and newly-born neutron stars. Detection of neutrinos from these vastly different eras using the new generation of neutrino detectors holds great promise for enhancing our understanding of neutrino-matter interactions and astrophysical phenomena.
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Synchronisation and MSW sharpening of neutrinos propagating in a flavour blind medium
We consider neutrino oscillations in a medium in which scattering processes are blind to the neutrino flavour. We present an analytical derivation of the synchronised behaviour obtained in the limit where the average scattering rate is much larger than the oscillation frequency. We also examine MSW transitions in these circumstances, and show that a sharpening of the transition can result.
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