36 research outputs found
Standard Model Neutrinos as Warm Dark Matter
Standard Model neutrinos are not usually considered plausible dark matter
candidates because the usual treatment of their decoupling in the early
universe implies that their mass must be sufficiently small to make them
``hot'' dark matter. In this paper we show that decoupling of Standard Model
neutrinos in low reheat models may result in neutrino densities very much less
than usually assumed, and thus their mass may be in the keV range. Standard
Model neutrinos may therefore be warm dark matter candidates.Comment: 5 pages, 5 figures, LaTeX file uses revtex packag
New Upper Limits on the Tau Neutrino Mass from Primordial Helium Considerations
In this paper we reconsider recently derived bounds on tau neutrinos,
taking into account previously unaccounted for effects. We find that, assuming
that the neutrino life-time is longer than , the constraint
rules out masses in the range
for Majorana neutrinos and
for Dirac neutrinos. Given that the present
laboratory bound is 35 MeV, our results lower the present bound to and
for Majorana and Dirac neutrinos respectively.Comment: 9 pages (2 figures available upon request), UM-AC-93-0
Cosmological Implications of Neutrinos
The lectures describe several cosmological effects produced by neutrinos.
Upper and lower cosmological limits on neutrino mass are derived. The role that
neutrinos may play in formation of large scale structure of the universe is
described and neutrino mass limits are presented. Effects of neutrinos on
cosmological background radiation and on big bang nucleosynthesis are
discussed. Limits on the number of neutrino flavors and mass/mixing are given.Comment: 41 page, 7 figures; lectures presented at ITEP Winter School,
February, 2002; to be published in the Proceeding
Bounds on Dirac Neutrino Masses from Nucleosynthesis
We derive new bounds on the Dirac mass of the tau and muonic neutrinos. By
solving the kinetic equation for the rate of energy deposition due to helicity
flipping processes and imposing the constraint that the number of effective
species contributing to the energy density at the time of nucleosynthesis be
, we find the bounds KeV and
KeV for MeV. The constraint leads to the much stronger bound KeV for both species
of neutrinos.Comment: 10 pages, UM-TH-94-21, UMN-TH-1303-94, FERMILAB-Pub-94/199-
Scattering theory and ground-state energy of Dirac fermions in graphene with two Coulomb impurities
We study the physics of Dirac fermions in a gapped graphene monolayer containing two Coulomb impurities. For the case of equal impurity charges, we discuss the ground-state energy using the linear combination of atomic orbitals (LCAO) approach. For opposite charges of the Coulomb centers, an electric dipole potential results at large distances. We provide a nonperturbative analysis of the corresponding low-energy scattering problem
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Hadron and photon experiments at fixed-target accelerators
Possible hadron and photon experiments at 20 TeV stationary-target proton accelerator have been considered in order to see typical limitations and possibilities of the experiments in this new energy domain
The probability of muon sticking to helium in the muon-catalyzed fusion dtμ → μ-4He + n
.