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Glennys R. Farrar

Publications and source records attributed to Glennys R. Farrar.

At least 109 records · Page 6Linked to original sources
arXiv2003

Interacting Dark Matter and Dark Energy

We discuss models for the cosmological dark sector in which the energy density of a scalar field approximates Einstein's cosmological constant and the scalar field value determines the dark matter particle mass by a Yukawa coupling. A model with one dark matter family can be adjusted so the observational constraints on the cosmological parameters are close to but different from what is predicted by the Lambda CDM model. This may be a useful aid to judging how tightly the cosmological parameters are constrained by the new generation of cosmological tests that depend on the theory of structure formation. In a model with two families of dark matter particles the scalar field may be locked to near zero mass for one family. This can suppress the long-range scalar force in the dark sector and eliminate evolution of the effective cosmological constant and the mass of the nonrelativistic dark matter particles, making the model close to Lambda CDM, until the particle number density becomes low enough to allow the scalar field to evolve. This is a useful example of the possibility for complexity in the dark sector.

astro-ph↗
arXiv2003

Nuclear and nucleon transitions of the H di-baryon

We consider 3 types of processes pertinent to the phenomenology of an H di-baryon: conversion of two $Λ$'s in a doubly-strange hypernucleus to an H, decay of the H to two baryons, and -- if the H is light enough -- conversion of two nucleons in a nucleus to an H. We compute the spatial wavefunction overlap using the Isgur-Karl and Bethe-Goldstone wavefunctions, and treat the weak interactions phenomenologically. The observation of $Λ$ decays from doubly-strange hypernuclei puts a constraint on the H wavefunction which is plausibly satisfied. In this case the H is very long-lived as we calculate. An absolutely stable H is not excluded at present. SuperK can provide valuable limits.

hep-ph↗
arXiv2003

Air Shower Simulations in a Hybrid Approach using Cascade Equations

A new hybrid approach to air shower simulations is described. At highest energies, each particle is followed individually using the traditional Monte Carlo method; this initializes a system of cascade equations which are applicable for energies such that the shower is one-dimensional. The cascade equations are solved numerically down to energies at which lateral spreading becomes significant, then their output serves as a source function for a 3-dimensional Monte Carlo simulation of the final stage of the shower. This simulation procedure reproduces the natural fluctuations in the initial stages of the shower, gives accurate lateral distribution functions, and provides detailed information about all low energy particles on an event-by-event basis. It is quite efficient in computation time.

astro-ph↗
arXiv2003

Transitions of two baryons to the H dibaryon in nuclei

We calculate the suppression in the rate at which two baryons in a nucleus (viz., nucleons or $Λ$'s) convert to an H dibaryon, using an Isgur-Karl wavefunction for quarks in the baryons and H, and a Bethe-Goldstone wavefunction for the baryons in the nucleus. If $r_H \lsi 1/3~ r_N$, we find $τ_{A_{ΛΛ}\to A'_H}\gsi τ_Λ$ and the observation of $Λ$ decays from double-$Λ$ hypernuclei does not exclude the existence of the H. If $m_H < 2 m_p$, nuclei are unstable but have very long lifetimes. For reasonable values of $r_H$ and the nuclear wavefunction, the lifetime can be long enough to evade anticipated SuperK limits $τ_{A_{NN}\to A'_H}\gsi {\rm few} 10^{29}$ yr, or short enough to be observed. An analysis of SuperK data to look for this possibility should be undertaken.

hep-ph↗
arXiv2003

Non-binding of Flavor-Singlet Hadrons to Nuclei

Strongly attractive color forces in the flavor singlet channel may lead to a stable H dibaryon. Here we show that an H or other compact, flavor singlet hadron is unlikely to bind to nuclei, so that bounds on exotic isotopes do not exclude their stability. Remarkably, a stable H appears to evade other experimental constraints as well, when account is taken of its expected compact spatial wavefunction.

hep-ph↗
arXiv2002

Dominant Contributions to Lateral Distribution Functions in Ultra-High Energy Cosmic Ray Air Showers

In hadron induced air showers of highest energies (E> 1e18 eV), the lateral distribution functions of electrons and muons are a superposition of many separate electromagnetic sub-showers, initiated by meson decay at different altitudes and energies. The lateral distribution function is the primary tool for reconstructing the energy of the primary in a UHE cosmic ray shower, so understanding it in detail is a prerequisite for having confidence in the energy determination. We analyze in this paper the dominant contributions to the ground level lateral distribution functions, as a function of the altitude and energy at which the sub-showers are initiated. Far from the core, the dominant contribution to the density of electrons comes from sub-showers initiated at low altitudes and low energies (E<100 GeV). The dominant sub-showers are initiated at large radial distance from the core and at a large angle with respect to the main shower axis. This demonstrates the need for careful treatment of low energy hadron physics modeling even for ultrahigh energy primaries.

hep-ph↗
arXiv2001

Neutron Stars with a Stable, Light Supersymmetric Baryon

If a light gluino exists, the lightest gluino-containing baryon, the \OSO, is a possible candidate for self-interacting dark matter. In this scenario, the simplest explanation for the observed ratio $Ω_{dm}/Ω_b \approx 6-10$ is that $m_{S^0} \sim 900$\MeVcs; this is not at present excluded by particle physics. Such an \OSO could be present in neutron stars, with hyperon formation serving as an intermediate stage. We calculate equilibrium compositions and equation of state for high density matter with the \OSO, and find that for a wide range of parameters the properties of neutron stars with the \OSO are consistent with observations. In particular, the maximum mass of a nonrotating star is $1.7-1.8 M_\odot$, and the presence of the \OSO is helpful in reconciling observed cooling rates with hyperon formation.

astro-ph↗
arXiv2000

Deducing the Source of Ultrahigh Energy Cosmic Rays

The apparent lack of suitable astrophysical sources for cosmic rays with E > 10^{19.7} eV (UHECRs) is the "GZK Paradox". We argue that whatever mechanism produces them must also account for events down to ~10^{18.7} eV, including their isotropy and spectral smoothness. This rules out galactic sources, distributed sources such as topological defects, and Gamma Ray Bursts (GRBs). We are lead to identify the powerful radio galaxy Cen A, at 3.4 Mpc, as the probable source of most UHECRs observed at Earth today, and to estimate the extragalactic magnetic field to be ~0.3 microG.

astro-ph↗
arXiv2000

Masses of Flavor Singlet Hybrid Baryons

We study the possibility that four iso-singlet baryons $Λ_s(1405)$ $J^P=1/2^{-}$, $Λ_s(1520)$ $J^P=3/2^{-}$, $Λ_c(2593)$ $J^P=1/2^{-}$ and $Λ_c(2625)$ $J^P=3/2^{-}$ are hybrids: three quark one gluon states (udsg). We calculate the mass separations of the candidates, using a degeneracy-lifting hyperfine interaction from an effective single colored gluon exchange between the constituents. The correct ordering of masses is obtained (contrary to the case for the conventional interpretation as 3 quarks with L=1) and the splittings are plausible. The parity of these states is not measured, only assumed to be negative. In the hybrid picture, the lightest states are parity even and the parity odd counterparts lie about 300 MeV higher. Thus the hybrid ansatz predicts that either the parity of the $Λ(1405)$ etc is positive, or that there are undiscovered positive parity states about 300 MeV lower. We also remark that in this picture, the H-dibaryon mass may be around 1.5 GeV.

hep-ph↗
arXiv2000

Self-Interacting Dark Matter

Spergel and Steinhardt have recently proposed the concept of dark matter with strong self-interactions as a means to address numerous discrepancies between observations of dark matter halos on subgalactic scales and the predictions of the standard collisionless dark matter picture. We review the motivations for this scenario and discuss some recent, successful numerical tests. We also discuss the possibility that the dark matter interacts strongly with ordinary baryonic matter, as well as with itself. We present a new analysis of the experimental constraints and re-evaluate the allowed range of cross-section and mass.

astro-ph↗
arXiv2000

GZK Violation - a Tempest in a (Magnetic) Teapot?

The apparent lack of suitable astrophysical sources for the observed highest energy cosmic rays (UHECRs) within $\approx 20$ Mpc is the "GZK Paradox". We constrain representative models of the extra-galactic magnetic field structure by Faraday Rotation measurements; limits are at the $μ$G level rather than the nG level usually assumed. In such fields, even the highest energy cosmic rays experience large deflections. This allows nearby AGNs (possibly quiet today) or GRBs to be the source of ultra-high energy cosmic rays without contradicting the GZK distance limit.

astro-ph↗
arXiv2000

Soft Yukawa couplings in supersymmetric theories

The possibility of radiatively generated fermion masses arising from chiral flavor violation in soft supersymmetry-breaking terms is explored. Vacuum stability constraints are considered in various classes of models, and allow in principle all of the first- and second-generation quarks and leptons and the $b$-quark to obtain masses radiatively. Radiatively induced Higgs-fermion couplings have non-trivial momentum-dependent form factors, which at low momentum are enhanced with respect to the case of tree-level Yukawa couplings. These form factors may be probed by various sum rules and relations among Higgs boson decay widths and branching ratios to fermion final states. An apparent, large, hard violation of supersymmetry also results for Higgsino couplings. Mixing between left- and right-handed scalar superpartners is enhanced. A radiative muon mass is shown to lead to a relatively large and potentially measurable contribution to the muon anomalous magnetic moment. If the light-quark masses arise radiatively, the neutron electric dipole moment is suppressed by a natural phase alignment between the masses and dipole moment, and is below the current experimental bound. The possibility of neutrino masses arising from softly broken lepton number, and concomitant enhanced sneutrino-antisneutrino oscillations, is briefly discussed.

hep-ph↗
arXiv1998

Exotic massive hadrons and ultra-high energy cosmic rays

We investigate the proposal that primary cosmic rays of energy above the Greisen-Zatsepin-Kuzmin cutoff are exotic massive strongly interacting particles (uhecrons). We study the properties of air showers produced by uhecrons and find that masses in excess of about 50 GeV are inconsistent with the highest energy event observed. We also estimate that with sufficient statistics a uhecron of mass as low as 10 GeV may be distinguished from a proton.

hep-ph↗
arXiv1998

Correlation between Compact Radio Quasars and Ultra-High Energy Cosmic Rays

Some proposals to account for the highest energy cosmic rays predict that they should point to their sources. We study the five highest energy events (E>10^20 eV) and find they are all aligned with compact, radio-loud quasars. The probability that these alignments are coincidental is 0.005, given the accuracy of the position measurements and the rarity of such sources. The source quasars have redshifts between 0.3 and 2.2. If the correlation pointed out here is confirmed by further data, the primary must be a new hadron or one produced by a novel mechanism.

astro-ph↗
arXiv1998

Can Ultra High Energy Cosmic Rays be Evidence for New Particle Physics?

Candidate astrophysical acceleration sites capable of producing the highest energy cosmic rays (E > 10^{19.5} eV) appear to be at far greater distances than is compatible with their being known particles. The properties of a new particle which can account for observations are discussed and found to be tightly constrained. In order to travel 100's or 1000's of Mpc through the cosmic microwave background radiation without severe energy loss and yet produce a shower in Earth's atmosphere which is consistent with observations, it must be a hadron with mass of order a few GeV and lifetime greater than about 1 week. A particle with the required properties was identified years ago in the context of supersymmetric theories with a very light gluino. Laboratory experiments do not exclude it, as is discussed briefly.

astro-ph↗
arXiv1997

Are ultrahigh energy cosmic rays signals of supersymmetry?

We investigate the possibility that cosmic rays of energy larger than the Greisen-Zatsepin-Kuzmin cutoff are not nucleons, but a new stable, massive, hadron that appears in many extensions of the standard model. We focus primarily on the S^0, a uds-gluino bound state. The range of the S^0 through the cosmic background radiation is significantly longer than the range of nucleons, and therefore can originate from sources at cosmoglogical distances.

astro-ph↗
arXiv1997

e+ e- Cross Section and Exclusion of Massless Electroweak Gauginos

Measurements of the total hadronic cross section in e+e- annihilation are shown to be capable of severely limiting the possibility that gauginos have negligible tree level masses. A combined analysis of 1997 and earlier LEP data, considering simultaneously conventional SUSY signatures and purely hadronic final states, should achieve a 95% cl sensitivity to the case that the SU(2) and U(1) gauginos are massless. If integrated luminosity targets are achieved, it should also be possible to exclude the case that the wino or wino and gluino are light while the bino is heavy, except possibly for a small region of mu, tan beta. The analysis applies whether or not R-parity is conserved, and can also be used to reduce the model-dependence of conventional SUSY searches.

hep-ph↗