999 research outputs found
Gauged Dimension Bubbles
Some of the peculiar electrodynamical effects associated with gauged
``dimension bubbles'' are presented. Such bubbles, which effectively enclose a
region of 5d spacetime, can arise from a 5d theory with a compact extra
dimension. Bubbles with thin domain walls can be stabilized against total
collapse by the entrapment of light charged scalar bosons inside the bubble,
extending the idea of a neutral dimension bubble to accommodate the case of a
gauged U(1) symmetry. Using a dielectric approach to the 4d dilaton-Maxwell
theory, it is seen that the bubble wall is almost totally opaque to photons,
leading to a new stabilization mechanism due to trapped photons. Photon
dominated bubbles very slowly shrink, resulting in a temperature increase
inside the bubble. At some critical temperature, however, these bubbles
explode, with a release of radiation.Comment: 14 pages, no figures; to appear in Phys.Rev.
Generalized Parton Distributions of the Pion
Off-forward structure functions of the pion are investigated in twist-two and
twist-three approximation. A simple model is used for the pion, which allows to
introduce finite size effects, while preserving gauge invariance. Results for
the imaginary parts of the gamma^* pi -> gamma^* pi off-forward amplitude and
of the structure functions are presented. Generalized Callan-Gross relations
are obtained.Comment: 4 pages, 5 figures, LaTeX, uses espcrc2.sty (included), presented at
QCD03 Conference, Montpellier, France, July 200
On the reheating stage after inflation
We point out that inflaton decay products acquire plasma masses during the
reheating phase following inflation. The plasma masses may render inflaton
decay kinematicaly forbidden, causing the temperature to remain frozen for a
period at a plateau value. We show that the final reheating temperature may be
uniquely determined by the inflaton mass, and may not depend on its coupling.
Our findings have important implications for the thermal production of
dangerous relics during reheating (e.g., gravitinos), for extracting bounds on
particle physics models of inflation from Cosmic Microwave Background
anisotropy data, for the production of massive dark matter candidates during
reheating, and for models of baryogenesis or leptogensis where massive
particles are produced during reheating.Comment: 8 pages, 2 figures. Submitted for publication in Phys. Rev.
Fermionic and Bosonic Stabilizing Effects for Type I and Type II Dimension Bubbles
We consider two types of "dimension bubbles", which are viewed as 4d
nontopological solitons that emerge from a 5d theory with a compact extra
dimension. The size of the extra dimension varies rapidly within the domain
wall of the soliton. We consider the cases of type I (II) bubbles where the
size of the extra dimension inside the bubble is much larger (smaller) than
outside. Type I bubbles with thin domain walls can be stabilized by the
entrapment of various particle modes whose masses become much smaller inside
than outside the bubble. This is demonstrated here for the cases of scalar
bosons, fermions, and massive vector bosons, including both Kaluza-Klein zero
modes and Kaluza-Klein excitation modes. Type II bubbles expel massive particle
modes but both types can be stabilized by photons. Plasma filled bubbles
containing a variety of massless or nearly massless radiation modes may exist
as long-lived metastable states. Furthermore, in contrast to the case with a
"gravitational bag", the metric for a fluid-filled dimension bubble does not
exhibit a naked singularity at the bubble's center.Comment: 17 pages, no figs; to appear in Phys.Rev.
Social Media and Well-being: A Methodological Perspective
Due to the methodological challenges inherent in studying social media use (SMU), as well as the methodological choices that have shaped research into the effects of SMU on well-being, clear conclusions regarding relationships between SMU and well-being remain elusive. We provide a review of five methodological developments poised to provide increased understanding in this domain: (1) increased use of longitudinal and experimental designs; (2) the adoption of behavioural (rather than self-report) measures of SMU; (3) focusing on more nuanced aspects of SMU; (4) embracing effect heterogeneity; and (5) the use of formal modelling and machine learning. We focus on how these advances stand to bring us closer to understanding relations between SMU and well-being, as well as the challenges associated with these developments
Bremsstrahlung simulation in K to pi l^pm nu_l (gamma) decays
In physics simulation chains, the PHOTOS Monte Carlo program is often used to
simulate QED effects in decays of intermediate particles and resonances. The
program is based on an exact multiphoton phase space. In general, the matrix
element is obtained from iterations of a universal kernel and approximations
are involved. To evaluate the program precision, it is necessary to formulate
and implement within the generator the exact matrix element, which depends on
the decay channel. Then, all terms necessary for non-leading logarithms are
taken into account.
In the present letter we focus on the decay K to pi l^pm nu_l and tests of
the PHOTOS Monte Carlo program. We conclude a 0.2% relative precision in the
implementation of the hard photon matrix element into the emission kernel,
including the case where approximations are used.Comment: 1+20 pages, 8 figure
Association of HLA types A1-B8-DR3 and B27 with rapid and slow progression of HIV disease
We examined how HLA types A1-B8-DR3 and B27 were related to progression of clinical disease and rate of loss of CD4 lymphocytes in the Edinburgh City Hospital cohort of HIV-positive patients, mainly injection drug users. Patients (n = 692) were prospectively followed from 1985 through March 1994. Accurately estimated seroconversion times were determined retrospectively for a subgroup of 313 (45%). Of 262 patients (39%) who were fully or partially HLA typed, 155 (50%) had known seroconversions. Of 34 patients typed positive for A1-B8-DR3, 29 progressed to CDC stage IV, 22 to AIDS and 20 died. Twelve patients were typed positive for B27; six of these progressed to CDC stage IV, one to AIDS and none died. In a proportional hazards analysis of the 313 patients with known seroconversions, A1-B8-DR3 was significantly associated with covariate-adjusted relative risks of 3.7 (95% CI 1.9-7.2), 3.1 (1.6-6.0) and 1.9 (1.1-3.2) for progression from seroconversion to death, AIDS and CDC stage IV, respectively. Events for B27 were too rare to include B27 in analyses to death and AIDS, but B27 was significantly associated with slower progression to CDC stage IV (0.3, CI 0.1-0.9). Random effects growth curve models were used to estimate individual rates of loss of square root CD4 count and loss of CD4 percentage, for 603 and 617 patients, respectively. A1-B8-DR3 was associated with rapid loss of both markers (p=0.02 and p = 0.01, respectively); B27 was associated with slow loss of both markers (p=0.04 and p<0.005
Nearly degenerate neutrinos, Supersymmetry and radiative corrections
If neutrinos are to play a relevant cosmological role, they must be
essentially degenerate with a mass matrix of the bimaximal mixing type. We
study this scenario in the MSSM framework, finding that if neutrino masses are
produced by a see-saw mechanism, the radiative corrections give rise to mass
splittings and mixing angles that can accommodate the atmospheric and the
(large angle MSW) solar neutrino oscillations. This provides a natural origin
for the hierarchy. On the other hand,
the vacuum oscillation solution to the solar neutrino problem is always
excluded. We discuss also in the SUSY scenario other possible effects of
radiative corrections involving the new neutrino Yukawa couplings, including
implications for triviality limits on the Majorana mass, the infrared fixed
point value of the top Yukawa coupling, and gauge coupling and bottom-tau
unification.Comment: 32 pages, 12 Postscript figures, uses psfig.st
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