1,035 research outputs found
On the Inelastic Collapse of a Ball Bouncing on a Randomly Vibrating Platform
We study analytically the dynamics of a ball bouncing inelastically on a
randomly vibrating platform, as a simple toy model of inelastic collapse. Of
principal interest are the distributions of the number of flights n_f till the
collapse and the total time \tau_c elapsed before the collapse. In the strictly
elastic case, both distributions have power law tails characterised by
exponents which are universal, i.e., independent of the details of the platform
noise distribution. In the inelastic case, both distributions have exponential
tails: P(n_f) ~ exp[-\theta_1 n_f] and P(\tau_c) ~ exp[-\theta_2 \tau_c]. The
decay exponents \theta_1 and \theta_2 depend continuously on the coefficient of
restitution and are nonuniversal; however as one approches the elastic limit,
they vanish in a universal manner that we compute exactly. An explicit
expression for \theta_1 is provided for a particular case of the platform noise
distribution.Comment: 32 page
Three-loop HTL Free Energy for QED
We calculate the free energy of a hot gas of electrons and photons to three
loops using the hard-thermal-loop perturbation theory reorganization of
finite-temperature perturbation theory. We calculate the free energy through
three loops by expanding in a power series in m_D/T, m_f/T, and e^2, where m_D
and m_f are thermal masses and e is the coupling constant. We demonstrate that
the hard-thermal-loop perturbation reorganization improves the convergence of
the successive approximations to the QED free energy at large coupling, e ~ 2.
The reorganization is gauge invariant by construction, and due to cancellation
among various contributions, we obtain a completely analytic result for the
resummed thermodynamic potential at three loops. Finally, we compare our result
with similar calculations that use the Phi-derivable approach.Comment: 23 pages, 10 figures; v3 - typos corrected, additional discussions of
systematics added; corresponds with published versio
Three-loop HTL gluon thermodynamics at intermediate coupling
We calculate the thermodynamic functions of pure-glue QCD to three-loop order
using the hard-thermal-loop perturbation theory (HTLpt) reorganization of
finite temperature quantum field theory. We show that at three-loop order
hard-thermal-loop perturbation theory is compatible with lattice results for
the pressure, energy density, and entropy down to temperatures .
Our results suggest that HTLpt provides a systematic framework that can used to
calculate static and dynamic quantities for temperatures relevant at LHC.Comment: 24 pages, 13 figs. 2nd version: improved discussion and fixing typos.
Published in JHE
Screened Perturbation Theory to Three Loops
The thermal physics of a massless scalar field with a phi^4 interaction is
studied within screened perturbation theory (SPT). In this method the
perturbative expansion is reorganized by adding and subtracting a mass term in
the lagrangian. We consider several different mass prescriptions that
generalize the one-loop gap equation to two-loop order. We calculate the
pressure and entropy to three-loop order and the screening mass to two-loop
order. In contrast to the weak-coupling expansion, the SPT-improved
approximations appear to converge even for rather large values of the coupling
constant.Comment: 30 pages, 10 figure
Three-loop HTL QCD thermodynamics
The hard-thermal-loop perturbation theory (HTLpt) framework is used to
calculate the thermodynamic functions of a quark-gluon plasma to three-loop
order. This is the highest order accessible by finite temperature perturbation
theory applied to a non-Abelian gauge theory before the high-temperature
infrared catastrophe. All ultraviolet divergences are eliminated by
renormalization of the vacuum, the HTL mass parameters, and the strong coupling
constant. After choosing a prescription for the mass parameters, the three-loop
results for the pressure and trace anomaly are found to be in very good
agreement with recent lattice data down to , which are
temperatures accessible by current and forthcoming heavy-ion collision
experiments.Comment: 27 pages, 11 figures; corresponds with published version in JHE
Maternal Recreational Exercise during Pregnancy in relation to Children's BMI at 7 Years of Age
Exposures during fetal life may have long-term health consequences including risk of childhood overweight. We investigated the associations between maternal recreational exercise during early and late pregnancy and the children's body mass index (BMI) and risk of overweight at 7 years. Data on 40,280 mother-child pairs from the Danish National Birth Cohort was used. Self-reported information about exercise was obtained from telephone interviews around gestational weeks 16 and 30. Children's weight and height were reported in a 7-year follow-up and used to calculate BMI and overweight status. Data was analyzed using multiple linear and logistic regression models. Recreational exercise across pregnancy was inversely related to children's BMI and risk of overweight, but all associations were mainly explained by smoking habits, socioeconomic status, and maternal pre-pregnancy BMI. Additionally, we did not find exercise intensity or changes in exercise habits in pregnancy related to the children's BMI or risk of overweight
Three-loop Phi-derivable Approximation in QED
In this paper we examine Phi-derivable approximations in QED. General
theorems tell us that the gauge dependence of the n-loop Phi-derivable
approximation shows up at order g^(2n) where g is the coupling constant. We
consider the gauge dependence of the two-loop Phi-derivable approximation to
the Debye mass and show that it is of order e^4 as expected. We solve the
three-loop Phi-derivable approximation in QED by expanding sum-integrals in
powers of e^2 and m/T, where m is the Debye mass which satisfies a variational
gap equation. The results for the pressure and the Debye mass are accurate to
order e^5.Comment: 10 pages, 5 figures. v2: typos corrected and references adde
The time delay of the quadruple quasar RX J0911.4+0551
We present optical lightcurves of the gravitationally lensed components A
(=A1+A2+A3) and B of the quadruple quasar RX J0911.4+0551 (z = 2.80). The
observations were primarily obtained at the Nordic Optical Telescope between
1997 March and 2001 April and consist of 74 I-band data points for each
component. The data allow the measurement of a time delay of 146 +- 8 days (2
sigma) between A and B, with B as the leading component. This value is
significantly shorter than that predicted from simple models and indicates a
very large external shear. Mass models including the main lens galaxy and the
surrounding massive cluster of galaxies at z = 0.77, responsible for the
external shear, yield H_0 = 71 +- 4 (random, 2 sigma) +- 8 (systematic)
km/s/Mpc. The systematic model uncertainty is governed by the surface-mass
density (convergence) at the location of the multiple images.Comment: 12 pages, 3 figures, ApJL, in press (June 20, 2002
Electronic band structure and carrier effective mass in calcium aluminates
First-principles electronic band structure investigations of five compounds
of the CaO-Al2O3 family, 3CaO.Al2O3, 12CaO.7Al2O3, CaO.Al2O3, CaO.2Al2O3 and
CaO.6Al2O3, as well as CaO and alpha-, theta- and kappa-Al2O3 are performed. We
find that the conduction band in the complex oxides is formed from the oxygen
antibonding p-states and, although the band gap in Al2O3 is almost twice larger
than in CaO, the s-states of both cations. Such a hybrid nature of the
conduction band leads to isotropic electron effective masses which are nearly
the same for all compounds investigated. This insensitivity of the effective
mass to variations in the composition and structure suggests that upon a proper
degenerate doping, both amorphous and crystalline phases of the materials will
possess mobile extra electrons
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