arXiv subjects
Publications and source records attributed to R. Foot.
Motivated by the atmospheric and solar neutrino anomalies, we study neutrino masses in a parity invariant SU(5)xSU(5)' grand unified model. Constraints from photon-mirror photon kinetic mixing are discussed.
Oscillations between ordinary and sterile neutrinos can generate large neutrino asymmetries in the early universe. These asymmetries can significantly affect big bang nucleosynthesis (BBN) through modification of nuclear reaction rates. We study this phenomenon within a model consisting of the three ordinary neutrinos plus one sterile neutrino that can be motivated by the neutrino anomalies and the dark matter problem. We calculate how the lepton asymmetries produced evolve at temperatures where they impact on BBN. The effect of the asymmetries on primordial helium production is determined, leading to an effective number of neutrino flavours during BBN of either about 2.7 or 3.1 depending on the sign of the lepton asymmetry.
We show that the SuperKamiokande atmospheric neutrino results explain electric charge quantisation, provided that the oscillation mode is $ν_μ \to ν_τ$ and that the neutrino mass is of Majorana type.
Neutrino oscillation scenarios involving large angle $ν_μ \to ν_e$ oscillations are disfavoured in the parameter range $Δm^2/eV^2 >~ 10^{-3}$ by recent results from the CHOOZ reactor-based $\barν_e$ disappearance experiment. For this reason we extend our previous work on up-down asymmetries for various oscillation scenarios by computing up-down asymmetries and the R ratio for the entire conceivable range $10^{-4} - 10^{-1} eV^2$ of $Δm^2$. Matter effects in the Earth play a crucial role. We perform a $χ^2$ fit to the data. We find that, because of the matter effect, the three-flavour maximal mixing model provides a reasonable fit to SuperKamiokande and CHOOZ data provided that the relevant $Δm^2$ is in the range $4 \times 10^{-4} ~< Δm^2/eV^2 ~< 1.5 \times 10^{-3}$.
The $ν_μ \to ν_τ$ and $ν_μ \to ν_s$ solutions to the atmospheric neutrino problem are compared with SuperKamiokande data. The differences between these solutions due to matter effects in the Earth are calculated for the ratio of $μ$-like to $e$-like events and for up-down flux asymmetries. These quantities are chosen because they are relatively insensitive to theoretical uncertainties in the overall neutrino flux normalisation and detection cross-sections and efficiencies. A $χ^2$ analysis using these quantities is performed yielding $3σ$ ranges which are approximately given by $(0.725 - 1.0, 4 \times 10^{-4} - 2 \times 10^{-2} eV^2)$ and $(0.74 - 1.0, 1 \times 10^{-3} - 2 \times 10^{-2} eV^2)$ for $(\sin^2 2θ,Δm^2)$ for the $ν_μ \to ν_τ$ and $ν_μ \to ν_s$ solutions, respectively. Values of $Δm^2$ smaller than about $2 \times 10^{-3}$ eV$^2$ are disfavoured for the $ν_μ \to ν_s$ solution, suggesting that future long baseline experiments should see a positive signal if this scenario is the correct one.
Motivated by the Exact Parity Model and other theories, the hypothesis that each of the known neutrinos oscillates maximally with a sterile partner has been put forward as an explanation of the atmospheric and solar neutrino anomalies. We provide detailed predictions for muon and electron flux ratios induced in the Kamiokande and SuperKamiokande detectors by sub-GeV atmospheric neutrinos. Several different, carefully chosen cuts on momentum and zenith angle are proposed, emphasizing the role of up-down flux asymmetries.
We compute up-down asymmetries for atmospheric neutrino induced electron and muon events in the SuperKamiokande detector for the following neutrino oscillation models: (A) maximally mixed $ν_μ - ν_τ$ or $ν_μ - ν_s$, (B) maximally mixed $ν_μ - ν_e$, (C) threefold maximal mixing between $ν_e, ν_μ$ and $ν_τ$, and (D) neutrino oscillations via Equivalence Principle or Lorentz invariance violation. We emphasise the role of different momentum cuts.
The minimal interpretation of the atmospheric neutrino data suggests that the muon neutrino oscillates into another species with a mixing angle close to the maximal $π/4$. In the Exact Parity Symmetric Model, both the muon and electron neutrinos are expected to be maximally mixed with essentially sterile partners ($ν'_μ$ and $ν'_e$ respectively). We examine the impact of maximal $ν_e - ν'_e$ oscillations on the atmospheric neutrino experiments. We estimate that maximal $ν_e - ν'_e$ oscillations will have effects on atmospheric neutrino data for $|δm^2 (ν_e - ν_e')| > 7 \times 10^{-5} eV^2$. For $δm^2$ in this range, a slight but distinctive rise in the ratio of muon-like to electron-like events is predicted for the low-energy sample. Furthermore, the ratio of low-energy electron-like events with zenith angles less than $90°$ to those with zenith angles greater than $90°$ should be greater than 1.
A simple alternative to the usual Pati-Salam model is proposed. The model allows quarks and leptons to be unified with gauge group $SU(4) \otimes SU(2)_L \otimes SU(2)_R$ at a remarkably low scale of about 1 TeV. Neutrino masses in the model arise radiatively and are naturally light.
Ordinary-sterile neutrino oscillations can generate significant neutrino asymmetry in the early Universe. In this paper we extend this work by computing the evolution of neutrino asymmetries and light element abundances during the Big Bang Nucleosynthesis (BBN) epoch. We show that a significant electron-neutrino asymmetry can be generated in a way that is approximately independent of the oscillation parameters $δm^2$ and $\sin^2 2θ$ for a range of parameters in an interesting class of models. The numerical value of the asymmetry leads to the prediction that the effective number of neutrino flavours during BBN is either about 2.5 or 3.4 depending on the sign of the asymmetry. Interestingly, one class of primordial deuterium abundance data favours an effective number of neutrino flavours during the epoch of BBN of less than 3.
The assumption of exact, unbroken parity symmetry leads directly to a simple predictive resolution of the atmospheric and solar neutrino puzzles. This is because the existence of this symmetry implies the existence of a set of mirror neutrinos which must mix maximally with the known neutrinos if neutrinos have mass. The maximal mixing of the electron neutrino with the mirror electron neutrino with $3 \times 10^{-10} eV^2 < |δm^2| < 10^{-3} eV^2$ eads to a predicted reduction of the solar neutrino flux by a factor of 2, which is in quite good agreement with the experiments. The maximal mixing of the muon neutrino with the mirror muon neutrino with $|δm^2| \simeq 10^{-2} eV^2$ also solves the atmospheric neutrino puzzle. We show that there is a significant range of parameters where these solutions are not in conflict with standard Big Bang Nucleosynthesis when the creation of lepton asymmetry due to neutrino oscillations is taken into account.
Ordinary-sterile neutrino oscillations can generate a significant lepton number asymmetry in the early Universe. We study this phenomenon in detail. We show that the dynamics of ordinary-sterile neutrino oscillations in the early Universe can be approximately described by a single integro-differential equation which we derive from both the density matrix and Hamiltonian formalisms. This equation reduces to a relatively simple ordinary first order differential equation if the system is sufficiently smooth (static limit). We study the conditions for which the static limit is an acceptable approximation. We also study the effect of the thermal distribution of neutrino momenta on the generation of lepton number. We apply these results to show that it is possible to evade (by many orders of magnitude) the Big Bang Nucleosynthesis (BBN) bounds on the mixing parameters, $δm^2$ and $\sin^2 2θ_0$, describing ordinary-sterile neutrino oscillations. We show that the large angle or maximal vacuum oscillation solution to the solar neutrino problem does not significantly modify BBN for most of the parameter space of interest, provided that the tau and/or mu neutrinos have masses greater than about 1 eV. We also show that the large angle or maximal ordinary-sterile neutrino oscillation solution to the atmospheric neutrino anomaly does not significantly modify BBN for a range of parameters.
In gauge theories like the standard model, the electric charges of the fermions can be heavily constrained from the classical structure of the theory and from the cancellation of anomalies. There is however mounting evidence suggesting that these anomaly constraints are not as well motivated as the classical constraints. In light of this we discuss possible modifications of the minimal standard model which will give us complete electric charge quantisation from classical constraints alone. Because these modifications to the standard model involve the consideration of baryon number violating scalar interactions, we present a complete catalogue of the simplest ways to modify the standard model so as to introduce explicit baryon number violation. This has implications for proton decay searches and baryogenesis.
The exact parity model (EPM) is a simple extension of the Standard Model which reinstates parity invariance as an unbroken symmetry of nature. The mirror matter sector of the model can interact with ordinary matter through gauge boson mixing, Higgs boson mixing and, if neutrinos are massive, through neutrino mixing. The last effect has experimental support through the observed solar and atmospheric neutrino anomalies. In this paper we show that the exact parity model can be formulated in a quaternionic framework. This suggests that the idea of mirror matter and exact parity may have profound implications for the mathematical formulation of quantum theory.
Evidence for $\bar ν_μ \rightarrow \bar ν_e$ oscillations has been reported at LAMPF using the LSND detector. Further evidence for neutrino mixing comes from the solar neutrino deficit and the atmospheric neutrino anomaly. All of these anomalies require new physics. We show that all of these anomalies can be explained if the standard model is enlarged so that an unbroken parity symmetry can be defined. This explanation holds independently of the actual model for neutrino masses. Thus, we argue that parity symmetry is not only a beautiful candidate for a symmetry beyond the standard model, but it can also explain the known neutrino physics anomalies.
We re-examine the big bang nucleosynthesis (BBN) bounds on the mixing of neutrinos with sterile species. These bounds depend on the assumption that the relic neutrino asymmetry $L_ν$ is very small. We show that for $L_ν$ large enough (greater than about $10^{-5}$) the standard BBN bounds do not apply. We apply this result to the sterile neutrino solution to the atmospheric neutrino anomaly and show that for $L_ν > 7 \times 10^{-5}$ it is consistent with BBN. The BBN bounds on sterile neutrinos mixing with electron neutrinos can also be weakened considerably.
We discuss a simple predictive solution to the solar neutrino problem based on maximal vacuum neutrino oscillations. The solution can be motivated by the exact parity symmetric model which predicts that the neutrino mass eigenstates are maximal mixtures of ordinary and mirror weak eigenstates (if neutrinos are massive). We show that this proposed solution to the solar neutrino problem is in reasonable agreement with the experiments, and that in the near future this scheme may be either ruled out or tested more precisely as statistics improve for the SAGE and GALLEX experiments. Predictions are also given for the upcoming Superkamiokande, SNO and Borexino experiments.
We re-examine neutrino oscillations in the early universe. Contrary to previous studies, we show that large neutrino asymmetries can arise due to oscillations between ordinary neutrinos and sterile neutrinos. This means that the Big Bang Nucleosynthesis (BBN) bounds on the mass and mixing of ordinary neutrinos with sterile neutrinos can be evaded. Also, it is possible that the neutrino asymmetries can be large (i.e. $\stackrel{>}{\sim} 10\%$), and hence have a significant effect on BBN through nuclear reaction rates.