65,826 research outputs found
Higgs Boson Mass Bounds in the Standard and Minimal Supersymmetric Standard Model with Four Generations
We study the question of distinguishability of the Higgs sector between the
standard model with four generations(SM4) and the minimal supersymmetric
standard model with four generations (MSSM4). We find that a gap exists between
the SM4 and MSSM4 Higgs boson masses for a range of the fourth generation
fermion mass considered in the analysis at a fixed top quark mass. We also
compare the Higgs boson mass bounds in these models with those in the standard
and the minimal supersymmetric standard models.Comment: 11 pages, Revtex, 3 postscript figures, accepted for publication in
Mod. Phys. Lett.
Cluster Accretion Shocks as Possible Acceleration Sites for Ultra High Energy Protons below the Greisen Cutoff
Three-dimensional hydrodynamic simulations of large scale structure in the
Universe have shown that accretion shocks form during the gravitational
collapse of one-dimensional caustics, and that clusters of galaxies formed at
intersections of the caustics are surrounded by these accretion shocks.
Estimated speed and curvature radius of the shocks are 1000-3000 \kms and about
5 Mpc, respectively, in the CDM universe. Assuming that energetic
protons are accelerated by these accretion shocks via the first-order Fermi
process and modeling particle transport around the shocks through Bohm
diffusion, we suggest that protons can be accelerated up to the {\it Greisen
cutoff energy} near eV, provided the mean magnetic field
strength in the region around the shocks is at least of order a microgauss. We
have also estimated the proton flux at earth from the Virgo cluster. Assuming a
few (1-10) \% of the ram pressure of the infalling matter would be transferred
to the cosmic-rays, the estimated flux for eV is consistent
with observations, so that such clusters could be plausible sources of the UHE
CRs.Comment: 14 pages, uuencoded compressed postscript file. Accepted for Jan. 1,
1996 issue of Ap
Self-Similar Evolution of Cosmic-Ray Modified Shocks: The Cosmic-Ray Spectrum
We use kinetic simulations of diffusive shock acceleration (DSA) to study the
time-dependent evolution of plane, quasi-parallel, cosmic-ray (CR) modified
shocks. Thermal leakage injection of low energy CRs and finite Alfv\'en wave
propagation and dissipation are included. Bohm diffusion as well as the
diffusion with the power-law momentum dependence are modeled. As long as the
acceleration time scale to relativistic energies is much shorter than the
dynamical evolution time scale of the shocks, the precursor and subshock
transition approach the time-asymptotic state, which depends on the shock sonic
and Alfv\'enic Mach numbers and the CR injection efficiency. For the diffusion
models we employ, the shock precursor structure evolves in an approximately
self-similar fashion, depending only on the similarity variable, x/(u_s t).
During this self-similar stage, the CR distribution at the subshock maintains a
characteristic form as it evolves: the sum of two power-laws with the slopes
determined by the subshock and total compression ratios with an exponential
cutoff at the highest accelerated momentum, p_{max}(t). Based on the results of
the DSA simulations spanning a range of Mach numbers, we suggest functional
forms for the shock structure parameters, from which the aforementioned form of
CR spectrum can be constructed. These analytic forms may represent approximate
solutions to the DSA problem for astrophysical shocks during the self-similar
evolutionary stage as well as during the steady-state stage if p_{max} is
fixed.Comment: 38 pages, 12 figures, ApJ accepte
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