arXiv subjects
Publications and source records attributed to George M. Fuller.
We give a geometric interpretation of the neutrino 'oscillation length resonance' recently discovered by Petcov. We use this picture to identify two new solutions for oscillation length resonances in a 3-layer earth model.
We discuss Big Bang Nucleosynthesis constraints on maximal $ν_μ\leftrightarrowν_s$ mixing. Vacuum $ν_μ\leftrightarrowν_s$ oscillation has been proposed as one possible explanation of the Super Kamiokande atmospheric neutrino data. Based on the most recent primordial abundance measurements, we find that the effective number of neutrino species for Big Bang Nucleosynthesis (BBN) is $N_ν\la 3.3$. Assuming that all three active neutrinos are light (with masses $\ll 1$ MeV), we examine BBN constraints on $ν_μ\leftrightarrowν_s$ mixing in two scenarios: (1) a negligible lepton asymmetry (the standard picture); (2) the presence of a large lepton asymmetry which has resulted from an amplification by $ν_τ\leftrightarrowν_{s'}$ mixing ($ν_{s'}$ being $ν_s$ or another sterile neutrino species). The latter scenario has been proposed recently to reconcile the BBN constraints and large-angle $ν_μ\leftrightarrowν_s$ mixing. We find that the large-angle $ν_μ\leftrightarrowν_s$ mixing in the first scenario, which would yield $N_ν\approx 4$, is ruled out as an explanation of the Super Kamiokande data. It is conceivably possible for the $ν_μ\leftrightarrowν_s$ solution to evade BBN bounds in the second scenario, but only if 200 eV$^2\la m^2_{ν_τ}-m^2_{ν_{s'}}\la 10^4$ eV$^2$ is satisfied, and if $ν_τ$ decays non-radiatively with a lifetime $\la 10^3$ years. This mass-squared difference implies 15 eV$\la m_{ν_τ}\la 100$ eV if $ν_{s'}$ is much lighter than $ν_τ$. We conclude that maximal (or near maximal) $ν_μ\leftrightarrowν_τ$ mixing is a more likely explanation of the Super Kamiokande data.
In the paper "Big Bang Nucleosynthesis and Active-Sterile Neutrino Mixing: Evidence for Maximal Muon-Neutrino/Sterile-Neutrino Mixing in Super Kamiokande" (astro-ph/9810075), we suggested that to evade the Big Bang Nucleosynthesis exclusion of the muon neutrino to sterile neutrino oscillation explanation of the Super Kamiokande data, the tau neutrino must have a mass over about 15 eV and it must mix with a lighter sterile neutrino. A stable tau neutrino with this mass is inconsistent with cosmological structure formation. In a comment on our paper (astro-ph/9811067), Foot and Volkas argued that our result is incorrect and that the required tau neutrino mass should be much lower. Here we back up our original result with a more detailed calculation. We show that the argument of Foot and Volkas is invalid, most likely due to an insufficient energy resolution in the low energy part of the neutrino spectrum.
The formation of supermassive black holes through the gravitational collapse of supermassive objects ($M \ga 10^4 M_\odot$) has been proposed as a source of cosmological gamma-ray bursts. The major advantage of this model is that such collapses are far more energetic than stellar-remnant mergers. The major drawback of this idea is the severe baryon loading problem in one-dimensional models. We can show that the observed log N - log P (number vs. peak flux) distribution for gamma-ray bursts in the BATSE database is not inconsistent with an identification of supermassive object collapse as the origin of the gamma-ray bursts. This conclusion is valid for a range of plausible cosmological and gamma-ray burst spectral parameters.
We investigate the effects of baryon/anti-baryon inhomogeneity on primordial nucleosynthesis. Recent work claims that electroweak baryogenesis could give rise to distinct regions of net baryon and anti-baryon number, which could survive until the nucleosynthesis epoch. We discuss neutron diffusion effects on nucleosynthesis yields in these models and speculate on the prospects for obtaining constraints.
We explore neutrino capture during r-process nucleosynthesis in neutrino-driven ejecta from nascent neutron stars. We focus on the interplay between charged-current weak interactions and element synthesis, and we delineate the important role of equilibrium nuclear dynamics. During the period of coexistence of free nucleons and light and/or heavy nuclei, electron neutrino capture inhibits the r-process. At all stages, capture on free neutrons has a larger impact than capture on nuclei. However, neutrino capture on heavey nuclei by itself, if it is very strong, is also detrimental to the r-process until large nuclear equilibrium clusters break down and the classical neutron-capture phase of the r-process begins. The sensitivity of the r-process to neutrino irradiation means that neutrino-capture effects can strongly constrain the r-process site, neutrino physics, or both. These results apply also to r-process scenarios other than neutrino-heated winds.
We propose that the gravitational collapse of supermassive objects ($ M\ga 10^4 M_\odot$), either as relativistic star clusters or as single supermassive stars (which may result from stellar mergers in dense star clusters), could be a cosmological source of $γ$-ray bursts. These events could provide the seeds of the supermassive black holes observed at the center of many galaxies. Collapsing supermassive objects will release a fraction of their huge gravitational binding energy as thermal neutrino pairs. We show that the accompanying neutrino/antineutrino annihilation-induced heating could drive electron/positron ``fireball'' formation, relativistic expansion, and associated $γ$-ray emission. The major advantage of this model is its energetics: supermassive object collapses are far more energetic than solar mass-scale compact object mergers; therefore, the conversion of gravitational energy to fireball kinetic energy in the supermassive object scenario need not be highly efficient, nor is it necessary to invoke directional beaming. The major weakness of this model is difficulty in avoiding a baryon loading problem for one dimensional collapse scenarios.
It has recently been suggested that cosmologically significant numbers of black holes could form during a first-order QCD phase transition. Further, it has been asserted that these black holes would have masses corresponding naturally to the inferred mass ($\sim 1 M_{\odot}$) of the MACHOs responsible for the observed gravitational microlensing events. In this model, the underlying spectrum of primordial density perturbations provides the fluctuations that give rise to black holes at the epoch of the QCD transition. We employ a simplified model to estimate the reduction in the critical overdensity of a horizon-sized primordial perturbation required for collapse to a black hole. We find that a first-order QCD transition does indeed produce a sharp peak in the black hole mass spectrum, but that this peak corresponds to the horizon mass at an epoch somewhat earlier than the cosmological transition itself. Assuming a COBE normalized primordial density perturbation spectrum with constant spectral index, for the black holes so produced to be cosmologically significant would require an extremely finely tuned ``blue'' primordial density perturbation spectrum. Specifically, in the context of our simplified model, a spectral index in the range $n=1.37-1.42$ corresponds to the range $Ω\sim 10^{-5}-10^3$ of the black hole contribution to the present-day density parameter.
We calculate the luminosity and energy spectrum of the neutrino emission from electron-positron pair annihilation during the collapse of a supermassive star (${M} \ga 5\times10^4 {M_\odot}$). We then estimate the cumulative flux and energy spectrum of the resulting neutrino background as a function of the abundance and redshift of supermassive stars and the efficiency of these objects in converting gravitational energy into neutrino energy. We estimate the expected signal in some of the new generation of astrophysical neutrino detectors from both a cumulative background of supermassive stars and single collapse events associated with these objects.
We describe how thermally produced anti-electron neutrinos ($\barν_e$) from the homologously collapsing core of a supermassive star ($M \ga 5\times10^4 M_\odot$) can lead to significant deuterium enrichment in the ejected envelope of such a star. Deuterium-enriched material at high redshift might then serve as a clue to the existence of pregalactic supermassive stars. Conceivably, the ejected deuterium-enriched material could intercept the line of sight to a distant QSO and mimic a Lyman limit absorber. In such a case, the deuterium abundance inferred from absorption lines might not reflect the true primordial abundance of deuterium. We discuss relevant theoretical uncertainties in supermassive star physics as well as potential observational signatures in Lyman $α$ absorber clouds for processing by stars of this kind.
Three-neutrino mixing schemes suggested by Cardall & Fuller and Acker & Pakvasa are compared and contrasted. Both of these schemes seek to solve the solar and atmospheric neutrino problems and to account for the possible neutrino oscillation signal in the LSND experiment. These neutrino oscillation schemes have different atmospheric and solar neutrino signatures that will be discriminated by Super-Kamiokande and SNO. They will also have different signatures in proposed long-baseline accelerator and reactor experiments. In particular, both of these schemes would give dramatic (and dramatically different) signals in an ``intermediate baseline'' experiment, such as the proposed ICARUS detector in the Jura mountains 17 km from CERN.
Limits can be placed on nonstandard neutrino physics when big bang nucleosynthesis (BBN) calculations employing standard neutrino physics agree with the observationally inferred primordial abundances of deuterium (D), $^3$He, $^4$He, and $^7$Li. These constraints depend most sensitively on the abundances of D and $^4$He. New observational determinations of the primordial D and/or $^4$He abundances could force revisions in BBN constraints on nonstandard neutrino physics.
We discuss general relativistic effects in the steady-state neutrino-driven ``wind'' which may arise from nascent neutron stars. In particular, we generalize previous analytic estimates of the entropy per baryon $S$, the mass outflow rate $\dot M$, and the dynamical expansion time scale $τ_{dyn}$. We show that $S$ increases and $τ_{dyn}$ decreases with increasing values of the mass-to-radius ratio describing the supernova core. Both of these trends indicate that a more compact core will lead to a higher number of neutrons per iron peak seed nucleus. Such an enhancement in the neutron-to-seed ratio may be required for successful r-process nucleosynthesis in neutrino-heated supernova ejecta.
We discuss neutrino oscillations in curved spacetime. Our heuristic approach can accomodate matter effects and gravitational contributions to neutrino spin precession in the presence of a magnetic field. By way of illustration, we perform explicit calculations in the Schwarzschild geometry. In this case, gravitational effects on neutrino oscillations are intimately related to the redshift. We discuss how spacetime curvature could affect the resonance position and adiabaticity of matter-enhanced neutrino flavor conversion.
Recent determinations of the deuterium abundance, $^2$H/H, in high redshift Lyman limit hydrogen clouds challenge the usual picture of primordial nucleosynthesis based on \lq\lq concordance\rq\rq\ of the calculated light element ($^2$H, $^3$He, $^4$He, $^7$Li) nucleosynthesis yields with the observationally-inferred abundances of these species. Concordance implies that all light element yields can be made to agree with the observationally-inferred abundances (within errors) for single global specifications of the baryon-to-photon ratio, $η$; lepton number; neutron lifetime; and expansion rate (or equivalently, effective number of light neutrino degrees of freedom $N_ν $). Though one group studying Lyman limit systems obtains a high value of $^2$H/H ($\sim 2\times {10}^{-4}$), another group finds consistently low values ($\sim 2\times {10}^{-5}$). In the former case, concordance for $N_ν =3$ is readily attained for the current observationally-inferred abundances of $^4$He and $^7$Li. But if the latter case represents the primordial deuterium abundance, then concordance for {\it any} $N_ν$ is impossible unless the primordial value of $^7$Li/H is considerably larger than the abundance of lithium as measured in old, hot Pop II halo stars. Furthermore, concordance with $N_ν=3$ is possible for low $^2$H/H only if either (1) the primordial $^4$He abundance has been significantly underestimated, or (2) new neutrino sector physics is invoked. We argue that systematic underestimation of both the $^7$Li and $^4$He primordial abundances is the likely resolution of this problem, a conclusion which is strengthened by new results on $^4$He.
The reported signal at the LSND experiment, when interpreted as neutrino mixing with $δm^2 = 6 \;\rm{eV}^2$, provides evidence for neutrinos with a cosmologically significant mass. However, attempts to reconcile this interpretation of the experiment with other hints about neutrino properties require a (sterile) fourth neutrino and/or an ``inverted'' neutrino mass hierarchy. An interpretation of the LSND experiment employing $δm^2 = 0.3\; \rm{eV}^2$, with three-generation mixing and a ``normal'' neutrino mass hierarchy, can just barely be reconciled with the negative results of other laboratory neutrino oscillation experiments and the positive hints of neutrino oscillation from the solar and atmospheric neutrino problems. Though subject to test by by future experiments, such a solution allows (but does not demand) neutrino masses relevant for dark matter.
Studies of limits on active-sterile neutrino mixing derived from big bang nucleosynthesis considerations are extended to consider the dependance of these constraints on the primordial deuterium abundance. This study is motivated by recent measurements of D/H in quasar absorption systems, which at present yield discordant results. Limits on active-sterile mixing are somewhat relaxed for high D/H. For low D/H ($\approx 2 \times 10^{-5}$), no active-sterile neutrino mixing is allowed by currently popular upper limits on the primordial $^4$He abundance $Y$. For such low primordial D/H values, the observational inference of active-sterile neutrino mixing by upcoming solar neutrino experiments would imply that $Y$ has been systematically underestimated, unless there is new physics not included in standard BBN.
Two dimensional concordance plots involving the baryon-to-photon ratio, $η$, and an effective number of light neutrinos, $N_ν$, are used to discuss the overall consistency of standard big-bang nucleosynthesis in light of recent determinations of the primordial deuterium ($^2$H) abundance. Observations of high-redshift Ly-$α$ clouds have provided discordant $^2$H/H determinations: one cloud with $^2$H/H high compared with the previously accepted upper limit on ($^2$H + $^3$He)/H; and one system with a significantly lower upper bound on $^2$H/H than those previously obtained. The high value of $^2$H/H agrees well with the current observationally-inferred primordial abundances of $^4$He and $^7$Li for $N_ν=3$. The low value of $^2$H/H does not fit well with the current observationally-inferred primordial abundance of $^4$He for $N_ν=3$. In addition, if the low value of $^2$H/H is indicative of the primordial deuterium abundance, then significant depletion of $^7$Li in old, hot Pop II halo stars is probably required to obtain a concordant range of $η$, for {\em any} effective number of neutrino flavors. The use of conservative ranges for the primordial abundances of $^2$H, $^4$He, and $^7$Li allow success of the standard picture for $N_ν=3$.