On some singularities of the correlation functions that determine neutrino opacities

Certain perturbation graphs in the calculation of the effects of the medium on neutrino scattering in supernova matter have a nonintegrable singularity in a physical region. A number of papers have addressed the apparent pathology through an ansatz that invokes higher order (rescattering) effects. Taking the Gamow-Teller terms as an example, we display an expression for the spin-spin correlation function that determines the cross-sections. It is clear from the form that there are no pathologies in the order by order perturbation expansion. Explicit formulae are given for a simple case, leading to an answer that is very different from one given by other authors.Comment: 8 page

Red giant bound on the axion-electron coupling reexamined

If axions or other low-mass pseudoscalars couple to electrons (``fine structure constant'' $\alpha_a$) they are emitted from red giant stars by the Compton process $\gamma+e\to e+a$ and by bremsstrahlung $e+(Z,A)\to (Z,A)+e+a$. We construct a simple analytic expression for the energy-loss rate for all conditions relevant for a red giant and include axion losses in evolutionary calculations from the main sequence to the helium flash. We find that \alpha_a\lapprox0.5\mn(-26) or m_a\lapprox 9\,\meV/\cos^2\beta lest the red giant core at helium ignition exceed its standard mass by more than 0.025\,\MM_\odot, in conflict with observational evidence. Our bound is the most restrictive limit on $\alpha_a$, but it does not exclude the possibility that axion emission contributes significantly to the cooling of ZZ~Ceti stars such as G117--B15A for which the period decrease was recently measured.Comment: 11 pages, uuencoded and compressed postscript fil

Lepton asymmetry and primordial nucleosynthesis in the era of precision cosmology

We calculate and display the primordial light-element abundances as a function of a neutrino degeneracy parameter \xi common to all flavors. It is the only unknown parameter characterizing the thermal medium at the primordial nucleosynthesis epoch. The observed primordial helium abundance Y_p is the most sensitive cosmic ``leptometer.'' Adopting the conservative Y_p error analysis of Olive and Skillman implies -0.04 \alt \xi \alt 0.07 whereas the errors stated by Izotov and Thuan imply \xi=0.0245+-0.0092 (1 sigma). Improved determinations of the baryon abundance have no significant impact on this situation. A determination of Y_p that reliably distinguishes between a vanishing or nonvanishing \xi is a crucial test of the cosmological standard assumption that sphaleron effects equilibrate the cosmic lepton and baryon asymmetries.Comment: 5 pages, 2 figures; minor changes, references added, replaced to match the published version in PRD (Brief Reports

Decoherence - Fluctuation Relation and Measurement Noise

We discuss fluctuations in the measurement process and how these fluctuations are related to the dissipational parameter characterising quantum damping or decoherence. On the example of the measuring current of the variable-barrier or QPC problem we show there is an extra noise or fluctuation connected with the possible different outcomes of a measurement. This noise has an enhanced short time component which could be interpreted as due to ``telegraph noise'' or ``wavefunction collapses''. Furthermore the parameter giving the the strength of this noise is related to the parameter giving the rate of damping or decoherence.Comment: 6 pages, no figures, for Okun Festschrift, Physics Report

Compatibility of CAST search with axion-like interpretation of PVLAS results

The PVLAS collaboration has results that may be interpreted in terms of a light axion-like particle, while the CAST collaboration has not found any signal of such particles. We propose a particle physics model with paraphotons and with a low energy scale in which this apparent inconsistency is circumvented.Comment: 10 pages, 3 figures, version accepted for publication in Physical Review Letter

Electron-, Mu-, and Tau-Number Conservation in a Supernova Core

We study if the neutrino mixing parameters suggested by the atmospheric neutrino anomaly imply chemical equilibrium between mu- and tau-flavored leptons in a supernova (SN) core. The initial flavor-conversion rate would indeed be fast if the nu_mu-nu_tau-mixing angle were not suppressed by second-order refractive effects. The neutrino diffusion coefficients are different for nu_mu, anti-nu_mu, nu_tau and anti-nu_tau so that neutrino transport will create a net mu and tau lepton number density. This will typically lead to a situation where the usual first-order refractive effects dominate, further suppressing the rate of flavor conversion. Altogether, neutrino refraction has the nontrivial consequence of guaranteeing the separate conservation of e, mu, and tau lepton number in a SN core on the infall and cooling time scales, even when neutrino mixing angles are large.Comment: Slightly expanded version with improved presentation, no changes of substanc

Adiabatic Faraday effect in a two-level Hamiltonian formalism

The helicity of a photon traversing a magnetized plasma can flip when the B-field along the trajectory slowly reverses. Broderick and Blandford have recently shown that this intriguing effect can profoundly change the usual Faraday effect for radio waves. We study this phenomenon in a formalism analogous to neutrino flavor oscillations: the evolution is governed by a Schroedinger equation for a two-level system consisting of the two photon helicities. Our treatment allows for a transparent physical understanding of this system and its dynamics. In particular, it allows us to investigate the nature of transitions at intermediate adiabaticities.Comment: 8 pages, 2 eps figures, and a note added. Title changed. Matches published versio

A Self-Consistent Approach to Neutral-Current Processes in Supernova Cores

The problem of neutral-current processes (neutrino scattering, pair emission, pair absorption, axion emission, \etc) in a nuclear medium can be separated into an expression representing the phase space of the weakly interacting probe, and a set of dynamic structure functions of the medium. For a non-relativistic medium we reduce the description to two structure functions S_A(\o) and S_V(\o) of the energy transfer, representing the axial-vector and vector interactions. $S_V$ is well determined by the single-nucleon approximation while $S_A$ may be dominated by multiply interacting nucleons. Unless the shape of S_A(\o) changes dramatically at high densities, scattering processes always dominate over pair processes for neutrino transport or the emission of right-handed states. Because the emission of right-handed neutrinos and axions is controlled by the same medium response functions, a consistent constraint on their properties from consideration of supernova cooling should use the same structure functions for both neutrino transport and exotic cooling mechanisms.Comment: 33 pages, Te

Pulsar Bound on the Photon Electric Charge Reexamined

If photons had a small electric charge $Q_\gamma$ their path in the galactic magnetic field would be curved, leading to a time delay between photons of different frequency from a distant source. Cocconi's previous application of this argument led to a limit which is too restrictive by a factor of about 200; the corrected bound is Q_\gamma/e\lapprox10^{-29}.Comment: 3 pages, LaTe