arXiv subjects

Nicole F. Bell

Publications and source records attributed to Nicole F. Bell.

At least 91 records · Page 5Linked to original sources

arXiv2006

Cosmological Signatures of Interacting Neutrinos

We investigate signatures of neutrino scattering in the Cosmic Microwave Background (CMB) and matter power spectra, and the extent to which present cosmological data can distinguish between a free streaming or tightly coupled fluid of neutrinos. If neutrinos have strong non-standard interactions, for example, through the coupling of neutrinos to a light boson, they may be kept in equilibrium until late times. We show how the power spectra for these models differ from more conventional neutrino scenarios, and use CMB and large scale structure data to constrain these models. CMB polarization data improves the constraints on the number of massless neutrinos, while the Lyman--$α$ power spectrum improves the limits on the neutrino mass. Neutrino mass limits depend strongly on whether some or all of the neutrino species interact and annihilate. The present data can accommodate a number of tightly-coupled relativistic degrees of freedom, and none of the interacting-neutrino scenarios considered are ruled out by current data -- although considerations regarding the age of the Universe disfavor a model with three annihilating neutrinos with very large neutrino masses.

astro-ph↗

arXiv2006

Magnetic Moments of Dirac Neutrinos

The existence of a neutrino magnetic moment implies contributions to the neutrino mass via radiative corrections. We derive model-independent "naturalness" upper bounds on the magnetic moments of Dirac neutrinos, generated by physics above the electroweak scale. The neutrino mass receives a contribution from higher order operators, which are renormalized by operators responsible for the neutrino magnetic moment. This contribution can be calculated in a model independent way. In the absence of fine-tuning, we find that current neutrino mass limits imply that $|μ_ν| < 10^{-14}$ Bohr magnetons. This bound is several orders of magnitude stronger than those obtained from solar and reactor neutrino data and astrophysical observations.

hep-ph↗

arXiv2005

How Magnetic is the Dirac Neutrino?

We derive model-independent, "naturalness" upper bounds on the magnetic moments μ_νof Dirac neutrinos generated by physics above the scale of electroweak symmetry breaking. In the absence of fine-tuning of effective operator coefficients, we find that current information on neutrino mass implies that |μ_ν| < 10^(-14) Bohr magnetons. This bound is several orders of magnitude stronger than those obtained from analyses of solar and reactor neutrino data and astrophysical observations.

hep-ph↗

arXiv2005

Cosmological Lepton Asymmetry, Primordial Nucleosynthesis, and Sterile Neutrinos

We study post weak decoupling coherent active-sterile and active-active matter-enhanced neutrino flavor transformation in the early universe. We show that flavor conversion efficiency at Mikheyev-Smirnov-Wolfenstein resonances is likely to be high (adiabatic evolution) for relevant neutrino parameters and energies. However, we point out that these resonances cannot sweep smoothly and continuously with the expansion of the universe. We show how neutrino flavor conversion in this way can leave both the active and sterile neutrinos with non-thermal energy spectra, and how, in turn, these distorted energy spectra can affect the neutron-to-proton ratio, primordial nucleosynthesis, and cosmological mass/closure constraints on sterile neutrinos. We demonstrate that the existence of a light sterile neutrino which mixes with active neutrinos can change fundamentally the relationship between the cosmological lepton numbers and the primordial nucleosynthesis He-4 yield.

astro-ph↗

arXiv2005

Measuring Flavor Ratios of High-Energy Astrophysical Neutrinos

We discuss the prospects for next generation neutrino telescopes, such as IceCube, to measure the flavor ratios of high-energy astrophysical neutrinos. The expected flavor ratios at the sources are $ϕ_{ν_e}:ϕ_{ν_μ}:ϕ_{ν_τ} = 1:2:0$, and neutrino oscillations quickly transform these to $1:1:1$. The flavor ratios can be deduced from the relative rates of showers ($ν_e$ charged-current, most $ν_τ$ charged-current, and all flavors neutral-current), muon tracks ($ν_μ$ charged-current only), and tau lepton lollipops and double-bangs ($ν_τ$ charged-current only). The peak sensitivities for these interactions are at different neutrino energies, but the flavor ratios can be reliably connected by a reasonable measurement of the spectrum shape. Measurement of the astrophysical neutrino flavor ratios tests the assumed production mechanism and also provides a very long baseline test of a number of exotic scenarios, including neutrino decay, CPT violation, and small-$δm^2$ oscillations to sterile neutrinos.

hep-ph↗

arXiv2004

Gamma-Ray Constraint on Galactic Positron Production by MeV Dark Matter

The Galactic positrons, as observed by their annihilation gamma-ray line at 0.511 MeV, are difficult to account for with astrophysical sources. It has been proposed that they are produced instead by dark matter annihilation or decay in the inner Galactic halo. To avoid other constraints, these processes are required to occur "invisibly," such that the eventual positron annihilation is the only detectable signal. However, electromagnetic radiative corrections to these processes inevitably produce real gamma rays (``internal bremsstrahlung"); this emission violates COMPTEL and EGRET constraints unless the dark matter mass is less than about 20 MeV.

astro-ph↗

arXiv2004

Neutrinoless Universe

We consider the consequences for the relic neutrino abundance if extra neutrino interactions are allowed, e.g., the coupling of neutrinos to a light (compared to $m_ν$) boson. For a wide range of couplings not excluded by other considerations, the relic neutrinos would annihilate to bosons at late times, and thus make a negligible contribution to the matter density today. This mechanism evades the neutrino mass limits arising from large scale structure.

astro-ph↗

arXiv2004

Pseudo-Dirac Neutrinos, a Challenge for Neutrino Telescopes

Neutrinos may be pseudo-Dirac states, such that each generation is actually composed of two maximally-mixed Majorana neutrinos separated by a tiny mass difference. The usual active neutrino oscillation phenomenology would be unaltered if the pseudo-Dirac splittings are $δm^2 \alt 10^{-12}$ eV$^2$; in addition, neutrinoless double beta decay would be highly suppressed. However, it may be possible to distinguish pseudo-Dirac from Dirac neutrinos using high-energy astrophysical neutrinos. By measuring flavor ratios as a function of $L/E$, mass-squared differences down to $δm^2 \sim 10^{-18}$ eV$^2$ can be reached. We comment on the possibility of probing cosmological parameters with neutrinos.

hep-ph↗

arXiv2003

Neutrino Mixing and Cosmology

We review the consequences of neutrino mixing in the early universe. For both active-sterile mixing or mixing between three active neutrinos only, the consequences of oscillations depend crucially upon the size of the universe's lepton number (relic neutrino asymmetry.)

hep-ph↗

arXiv2003

Sensitivity to $θ_{13}$ and $δ$ in the Decaying Astrophysical Neutrino Scenario

We have previously shown that the decay of high-energy neutrinos from distant astrophysical sources would be revealed by flavor ratios that deviate strongly from the $ϕ_{ν_e}:ϕ_{ν_μ}:ϕ_{ν_τ} = 1:1:1$ expected from oscillations alone. Here we show that the deviations are significantly larger when the mixing angle $θ_{13}$ and the CP phase $δ$ are allowed to be nonzero. If neutrinos decay, this could allow measurement of $θ_{13}$ and $δ$ in IceCube and other near-term neutrino telescopes.

hep-ph↗

arXiv2003

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.

hep-ph↗

arXiv2003

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.

quant-ph↗

arXiv2003

Decay of High-Energy Astrophysical Neutrinos

Existing limits on the non-radiative decay of one neutrino to another plus a massless particle (e.g., a singlet Majoron) are very weak. The best limits on the lifetime to mass ratio come from solar neutrino observations, and are $τ/m \agt 10^{-4}$ s/eV for the relevant mass eigenstate(s). For lifetimes even several orders of magnitude longer, high-energy neutrinos from distant astrophysical sources would decay. This would strongly alter the flavor ratios from the $ϕ_{ν_e}:ϕ_{ν_μ}:ϕ_{ν_τ} = 1:1:1$ expected from oscillations alone, and should be readily visible in the near future in detectors such as IceCube.

hep-ph↗

arXiv2002

Stringent Constraints on Cosmological Neutrino-Antineutrino Asymmetries from Synchronized Flavor Transformation

We assess a mechanism which can transform neutrino-antineutrino asymmetries between flavors in the early universe, and confirm that such transformation is unavoidable in the near bi-maximal framework emerging for the neutrino mixing matrix. We show that the process is a standard Mikheyev-Smirnov-Wolfenstein flavor transformation dictated by a synchronization of momentum states. We also show that flavor ``equilibration'' is a special feature of maximal mixing, and carefully examine new constraints placed on neutrino asymmetries. In particular, the big bang nucleosynthesis limit on electron neutrino degeneracy xi_e < 0.04 does not apply directly to all flavors, yet confirmation of the large-mixing-angle solution to the solar neutrino problem will eliminate the possibility of degenerate big bang nucleosynthesis.

astro-ph↗

arXiv2002

Do solar neutrinos decay?

Despite the fact that the solar neutrino flux is now well-understood in the context of matter-affected neutrino mixing, we find that it is not yet possible to set a strong and model-independent bound on solar neutrino decays. If neutrinos decay into truly invisible particles, the Earth-Sun baseline defines a lifetime limit of $τ/m \agt 10^{-4}$ s/eV. However, there are many possibilities which must be excluded before such a bound can be established. There is an obvious degeneracy between the neutrino lifetime and the mixing parameters. More generally, one must also allow the possibility of active daughter neutrinos and/or antineutrinos, which may partially conceal the characteristic features of decay. Many of the most exotic possibilities that presently complicate the extraction of a decay bound will be removed if the KamLAND reactor antineutrino experiment confirms the large-mixing angle solution to the solar neutrino problem and measures the mixing parameters precisely. Better experimental and theoretical constraints on the $^8$B neutrino flux will also play a key role, as will tighter bounds on absolute neutrino masses. Though the lifetime limit set by the solar flux is weak, it is still the strongest direct limit on non-radiative neutrino decay. Even so, there is no guarantee (by about eight orders of magnitude) that neutrinos from astrophysical sources such as a Galactic supernova or distant Active Galactic Nuclei will not decay.

hep-ph↗

arXiv2002

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.

quant-ph↗

arXiv2001

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.

quant-ph↗

arXiv2001

Neutrino oscillations and Big Bang Nucleosynthesis

We outline how relic neutrino asymmetries may be generated in the early universe via active-sterile neutrino oscillations. We discuss possible consequences for big bang nucleosynthesis, within the context of a particular 4-neutrino model.

hep-ph↗