Biomedical subjects

John F Beacom

Publications and source records attributed to John F Beacom.

8 recordsLinked to original sources

PubMed2006

Stringent constraint on galactic positron production.

The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare it with the observed diffuse Galactic gamma-ray data. Even with a simplified but conservative treatment, we find that the positron injection energies must be less than or similar to 3 MeV, which strongly constrains models for Galactic positron production.

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PubMed2005

Detection of neutrinos from supernovae in nearby galaxies.

While existing detectors would see a burst of many neutrinos from a Milky Way supernova, the supernova rate is only a few per century. As an alternative, we propose the detection of approximately 1 neutrino per supernova from galaxies within 10 Mpc, in which there were at least 9 core-collapse supernovae since 2002. With a future 1 Mton scale detector, this could be a faster method for measuring the supernova neutrino spectrum, which is essential for calibrating numerical models and predicting the redshifted diffuse spectrum from distant supernovae. It would also allow a > or approximately 10(4) times more precise trigger time than optical data alone for high-energy neutrinos and gravitational waves.

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PubMed2005

Revealing the supernova-gamma-ray burst connection with TeV neutrinos.

Gamma-ray bursts (GRBs) are rare, powerful explosions displaying highly relativistic jets. It has been suggested that a significant fraction of the much more frequent core-collapse supernovae are accompanied by comparably energetic but mildly relativistic jets, which would indicate an underlying supernova-GRB connection. We calculate the neutrino spectra from the decays of pions and kaons produced in jets in supernovae, and show that the kaon contribution is dominant and provides a sharp break near 20 TeV, which is a sensitive probe of the conditions inside the jet. For a supernova at 10 Mpc, 30 events above 100 GeV are expected in a 10 s burst in the IceCube detector.

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PubMed2005

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.

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PubMed2004

Antineutrino spectroscopy with large water Cerenkov detectors.

We propose modifying large water C erenkov 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 summation operatorE(gamma)=8 MeV, this would make neutrons visible for the first time in such detectors, allowing antineutrino tagging by the coincidence detection reaction nu (e)+p-->e(+)+n (similarly for nu (mu)). 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.

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PubMed2004

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(nu)) 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.

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PubMed2004

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 deltam(2) less, similar 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 deltam(2) approximately 10(-18) eV(2) can be reached. We comment on the possibility of probing cosmological parameters with neutrinos.

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PubMed2003

Decay of high-energy astrophysical neutrinos.

Existing limits on the nonradiative 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 tau/m greater, similar 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 phi(nu(e)):phi(nu(mu)):phi(nu(tau))=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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