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R. R. Volkas

Publications and source records attributed to R. R. Volkas.

At least 37 records · Page 2Linked to original sources
arXiv2000

Coherent oscillation in a linear quantum system coupled to a thermal bath

We consider the time development of the density matrix for a system coupled to a thermal bath, in models that go beyond the standard two-level systems through addition of an energy excitation degree of freedom and through the possibility of the replacement of the spin algebra by a more complex algebra. We find conditions under which increasing the coupling to the bath above a certain level decreases the rate of entropy production, and in which the limiting behavior is a dissipationless sinusoidal oscillation that could be interpreted as the synchronization of many modes of the uncoupled system.

quant-ph↗
arXiv2000

Discovering mirror particles at the Large Hadron Collider and the implied cold universe

The Mirror Matter or Exact Parity Model sees every standard particle, including the physical neutral Higgs boson, paired with a parity partner. The unbroken parity symmetry forces the mass eigenstate Higgs bosons to be maximal mixtures of the ordinary and mirror Higgs bosons. Each of these mass eigenstates will therefore decay 50% of the time into invisible mirror particles, providing a clear and interesting signature for the Large Hadron Collider (LHC) which could thus establish the existence of the mirror world. However, for this effect to be observable the mass difference between the two eigenstates must be sufficiently large. In this paper, we study cosmological constraints from Big Bang Nucleosynthesis on the mass difference parameter. We find that the temperature of the radiation dominated (RD) phase of the universe should never have exceeded a few 10's of GeV if the mass difference is to be observable at the LHC. Chaotic inflation with very inefficient reheating provides an example of how such a cosmology could arise. We conclude that the LHC could thus discover the mirror world and simultaneously establish an upper bound on the temperature of the RD phase of the universe.

hep-ph↗
arXiv2000

Unbroken versus broken mirror world: a tale of two vacua

If the Lagrangian of nature respects parity invariance then there are two distinct possibilities: either parity is unbroken by the vacuum or it is spontaneously broken. We examine the two simplest phenomenologically consistent gauge models which have unbroken and spontaneously broken parity symmetries, respectively. These two models have a Lagrangian of the same form, but a different parameter range is chosen in the Higgs potential. They both predict the existence of dark matter and can explain the MACHO events. However, the models predict quite different neutrino physics. Although both have light mirror (effectively sterile) neutrinos, the ordinary-mirror neutrino mixing angles are unobservably tiny in the broken parity case. The minimal broken parity model therefore cannot simultaneously explain the solar, atmospheric and LSND data. By contrast, the unbroken parity version can explain all of the neutrino anomalies. Furthermore, we argue that the unbroken case provides the most natural explanation of the neutrino physics anomalies (irrespective of whether evidence from the LSND experiment is included) because of its characteristic maximal mixing prediction.

hep-ph↗
arXiv2000

Oscillating Neutrinos from the Galactic Center

It has recently been demonstrated that the $γ$-ray emission spectrum of the EGRET-identified, central Galactic source 2EG J1746-2852 can be well fitted by positing that these photons are generated by the decay of $π^0$'s produced in p-p scattering at or near an energizing shock. Such scattering also produces charged pions which decay leptonically.The ratio of $γ$-rays to neutrinos generated by the central Galactic source may be accurately determined and a well-defined and potentially-measurable high energy neutrino flux at Earth is unavoidable. An opportunity, therefore, to detect neutrino oscillations over an unprecedented scale is offered by this source. In this paper we assess the prospects for such an observation with the generation of neutrino Čerenkov telescopes now in the planning stage. We determine that the next generation of detectors may find an oscillation signature in the Galactic Center (GC) signal.

astro-ph↗
arXiv2000

Geophysical constraints on mirror matter within the Earth

We have performed a detailed investigation of geophysical constraints on the possible admixture of mirror matter inside the Earth. On the basis of the Preliminary Reference Earth Model (PREM) -- the `Standard Model' of the Earth's interior -- we have developed a method which allows one to compute changes in various quantities characterising the Earth (mass, moment of inertia, normal mode frequencies etc.)due to the presence of mirror matter. As a result we have been able to obtain for the first time the direct upper bounds on the possible concentration of the mirror matter in the Earth. In terms of the ratio of the mirror mass to the Earth mass a conservative upper bound is $3.8\times 10^{-3}$. We then analysed possible mechanisms (such as lunar and solar tidal forces, meteorite impacts and earthquakes) of exciting mirror matter oscillations around the Earth centre. Such oscillations could manifest themselves through global variations of the gravitational acceleration at the Earth's surface. We conclude that such variations are too small to be observed. Our results are valid for other types of hypothetical matter coupled to ordinary matter by gravitation only (e.g. the shadow matter of superstring theories).

hep-ph↗
arXiv1999

Maximal $ν_e \to ν_s$ solution to the solar neutrino problem: just-so, MSW or energy independent?

We examine the maximal $ν_e \to ν_s$ solution to the solar neutrino problem. This solution can be motivated by the exact parity model and other theories. The $ν_e$ survival probability exhibits one of three qualitatively different behaviours depending on the value of $Δm^2$, viz. approximately energy independent, just-so or MSW. By the last of these we mean an enhanced night-time event rate due to regeneration in the Earth. We study all of these possibilities in the context of the recent SuperKamiokande data.

hep-ph↗
arXiv1999

Neutrino Physics and the Mirror World

Improper Lorentz Transformations can be retained as exact symmetries of Nature if the particle content and gauge group of the Standard Model are doubled. The resulting ``Exact Parity Model (EPM)'' sees each ordinary particle paired with a mirror analogue. If neutrinos have mass and if they mix, then the EPM predicts that each ordinary neutrino will be maximally mixed with its mirror neutrino partner. This provides a very simple explanation for the very large mixing angle observed for atmospheric muon neutrinos by SuperKamiokande and other experiments. Maximal mixing for electron neutrinos is also well motivated by the solar neutrino problem. If small interfamily mixing is switched on, then the LSND anomaly can also be accomodated by the EPM. The EPM thus provides a unified, simple and to some extent predictive framework for explaining all of the anomalous neutrino data. This talk will briefly review the EPM or mirror neutrino scenario.

hep-ph↗
arXiv1999

Implications of mirror neutrinos for early universe cosmology

The Exact Parity Model (EPM) is, in part, a theory of neutrino mass and mixing that can solve the atmospheric, solar and LSND anomalies. The central feature of the neutrino sector is three pairs of maximally mixed ordinary and mirror neutrinos. It has been shown that ordinary-mirror neutrino oscillations can generate large neutrino asymmetries in the epoch of the early universe immediately prior to Big Bang Nucleosynthesis (BBN). The large neutrino asymmetries generically suppress the production of mirror neutrinos, and a sufficiently large $ν_e$ asymmetry can directly affect light element synthesis through nuclear reaction rates. In this paper we present a detailed calculation of neutrino asymmetry evolution driven by the six-flavour EPM neutrino sector, focusing on implications for BBN.

hep-ph↗
arXiv1998

Relic neutrino asymmetries and big bang nucleosynthesis in a four neutrino model

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.

hep-ph↗
arXiv1998

Confronting solutions to the atmospheric neutrino anomaly involving large angle $ν_μ \to ν_e$ oscillations with SuperKamiokande and CHOOZ

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 $4 \times 10^{-4} ~< Δm^2/eV^2 ~< 1.5 \times 10^{-3}$.

hep-ph↗
arXiv1998

Comparing and contrasting the $ν_μ \to ν_τ$ and $ν_μ \to ν_s$ solutions to the atmospheric neutrino problem with SuperKamiokande data

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.

hep-ph↗
arXiv1997

Testing maximal electron and muon neutrino oscillations with sub-GeV SuperKamiokande atmospheric neutrino data

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.

hep-ph↗
arXiv1997

Up-down atmospheric neutrino flux asymmetry predictions for various neutrino oscillation scenarios

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.

hep-ph↗
arXiv1997

Effects for atmospheric neutrino experiments from electron neutrino oscillations

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.

hep-ph↗
arXiv1997

How neutrino oscillations can induce an effective neutrino number of less than 3 during Big Bang Nucleosynthesis

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.

hep-ph↗
arXiv1996

Neutrino-antineutrino asymmetry in the early universe

In this talk I discuss the use of a relic neutrino-antineutrino asymmetry in suppressing active-sterile neutrino oscillations in the early universe. This phenomenon can serve to greatly loosen the well known Big Bang Nucleosynthesis constraints on active-sterile neutrino mixing, and is thus important in some proposed solutions to the solar and atmospheric neutrino anomalies which employ large-angle active-sterile mixing. I first discuss the necessary conditions for the survival of a pre-existing neutrino asymmetry. I then consider the much more interesting phenomenon whereby such an asymmetry can actually be created by active-sterile neutrino oscillations.

hep-ph↗