arXiv subjects

John F. Beacom

Publications and source records attributed to John F. Beacom.

158 records · Page 9Linked to original sources

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

GADZOOKS! Antineutrino Spectroscopy with Large Water Cerenkov Detectors

We propose modifying large water Čerenkov detectors by the addition of 0.2% gadolinium trichloride, which is highly soluble, newly inexpensive, and transparent in solution. Since Gd has an enormous cross section for radiative neutron capture, with $\sum E_γ= 8$ MeV, this would make neutrons visible for the first time in such detectors, allowing antineutrino tagging by the coincidence detection reaction $\barν_e + p \to e^+ + n$ (similarly for $\barν_μ$). Taking Super-Kamiokande as a working example, dramatic consequences for reactor neutrino measurements, first observation of the diffuse supernova neutrino background, Galactic supernova detection, and other topics are discussed.

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

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.

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arXiv2003

Neutral-Current Atmospheric Neutrino Flux Measurement Using Neutrino-Proton Elastic Scattering in Super-Kamiokande

Recent results show that atmospheric $ν_μ$ oscillate with $δm^2 \simeq 3 \times 10^{-3}$ eV$^2$ and $\sin^2{2θ_{atm}} \simeq 1$, and that conversion into $ν_e$ is strongly disfavored. The Super-Kamiokande (SK) collaboration, using a combination of three techniques, reports that their data favor $ν_μ\to ν_τ$ over $ν_μ\to ν_{sterile}$. This distinction is extremely important for both four-neutrino models and cosmology. We propose that neutrino-proton elastic scattering ($ν+ p \to ν+ p$) in water Čerenkov detectors can also distinguish between active and sterile oscillations. This was not previously recognized as a useful channel since only about 2% of struck protons are above the Čerenkov threshold. Nevertheless, in the present SK data there should be about 40 identifiable events. We show that these events have unique particle identification characteristics, point in the direction of the incoming neutrinos, and correspond to a narrow range of neutrino energies (1-3 GeV, oscillating near the horizon). This channel will be particularly important in Hyper-Kamiokande, with $\sim 40$ times higher rate. Our results have other important applications. First, for a similarly small fraction of atmospheric neutrino quasielastic events, the proton is relativistic. This uniquely selects $ν_μ$ (not $\barν_μ$) events, useful for understanding matter effects, and allows determination of the neutrino energy and direction, useful for the $L/E$ dependence of oscillations. Second, using accelerator neutrinos, both elastic and quasielastic events with relativistic protons can be seen in the K2K 1-kton near detector and MiniBooNE.

hep-ph↗

arXiv2002

Supernovae and Neutrinos

A long-standing problem in supernova physics is how to measure the total energy and temperature of $ν_μ$, $ν_τ$, $\barν_μ$, and $\barν_τ$. While of the highest importance, this is very difficult because these flavors only have neutral-current detector interactions. We propose that neutrino-proton elastic scattering, $ν+ p \to ν+ p$, can be used for the detection of supernova neutrinos in scintillator detectors. It should be emphasized immediately that the dominant signal is on {\it free} protons. Though the proton recoil kinetic energy spectrum is soft, with $T_p \simeq 2 E_ν^2/M_p$, and the scintillation light output from slow, heavily ionizing protons is quenched, the yield above a realistic threshold is nearly as large as that from $\barν_e + p \to e^+ + n$. In addition, the measured proton spectrum is related to the incident neutrino spectrum. The ability to detect this signal would give detectors like KamLAND and Borexino a crucial and unique role in the quest to detect supernova neutrinos. These results are now published: J. F. Beacom, W. M. Farr and P. Vogel, Phys. Rev. D {\bf 66}, 033001 (2002) [arXiv:hep-ph/0205220]; the details are given there.

astro-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

Potential for Supernova Neutrino Detection in MiniBooNE

The MiniBooNE detector at Fermilab is designed to search for $ν_μ\to ν_e$ oscillation appearance at $E_ν\sim 1 {\rm GeV}$ and to make a decisive test of the LSND signal. The main detector (inside a veto shield) is a spherical volume containing 0.680 ktons of mineral oil. This inner volume, viewed by 1280 phototubes, is primarily a Čerenkov medium, as the scintillation yield is low. The entire detector is under a 3 m earth overburden. Though the detector is not optimized for low-energy (tens of MeV) events, and the cosmic-ray muon rate is high (10 kHz), we show that MiniBooNE can function as a useful supernova neutrino detector. Simple trigger-level cuts can greatly reduce the backgrounds due to cosmic-ray muons. For a canonical Galactic supernova at 10 kpc, about 190 supernova $\barν_e + p \to e^+ + n$ events would be detected. By adding MiniBooNE to the international network of supernova detectors, the possibility of a supernova being missed would be reduced. Additionally, the paths of the supernova neutrinos through Earth will be different for MiniBooNE and other detectors, thus allowing tests of matter-affected mixing effects on the neutrino signal.

hep-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

Detection of Supernova Neutrinos by Neutrino-Proton Elastic Scattering

We propose that neutrino-proton elastic scattering, $ν+ p \to ν+ p$, can be used for the detection of supernova neutrinos in scintillator detectors. Though the proton recoil kinetic energy spectrum is soft, with $T_p \simeq 2 E_ν^2/M_p$, and the scintillation light output from slow, heavily ionizing protons is quenched, the yield above a realistic threshold is nearly as large as that from $\barν_e + p \to e^+ + n$. In addition, the measured proton spectrum is related to the incident neutrino spectrum, which solves a long-standing problem of how to separately measure the total energy and temperature of $ν_μ$, $ν_τ$, $\barν_μ$, and $\barν_τ$. The ability to detect this signal would give detectors like KamLAND and Borexino a crucial and unique role in the quest to detect supernova neutrinos.

hep-ph↗

arXiv2001

Enhanced signal of astrophysical tau neutrinos propagating through Earth

Earth absorbs $\nue$ and $\numu$ of energies above about 100 TeV. As is well-known, although $\nutau$ will also disappear through charged-current interactions, the $\nutau$ flux will be regenerated by prompt tau decays. We show that this process also produces relatively large fluxes of secondary $\nube$ and $\nubmu$, greatly enhancing the detectability of the initial $\nutau$. This is particularly important because at these energies $\nutau$ is a significant fraction of the expected astrophysical neutrino flux, and only a tiny portion of the atmospheric neutrino flux.

astro-ph↗

arXiv2001

On the Normalization of the Neutrino-Deuteron Cross Section

As is well-known, comparison of the solar neutrino fluxes measured in SuperKamiokande (SK) by $ν+ e^- \to ν+ e^-$ and in the Sudbury Neutrino Observatory (SNO) by $ν_e + d \to e^- + p + p$ can provide a ``smoking gun'' signature for neutrino oscillations as the solution to the solar neutrino puzzle. This occurs because SK has some sensitivity to all active neutrino flavors whereas SNO can isolate electron neutrinos. This comparison depends crucially on the normalization and uncertainty of the theoretical charged-current neutrino-deuteron cross section. We address a number of effects which are significant enough to change the interpretation of the SK--SNO comparison.

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