685 research outputs found
Effect of electric field of the electrosphere on photon emission from quark stars
We investigate the photon emission from the electrosphere of a quark star. It
is shown that at temperatures MeV the dominating mechanism is the
bremsstrahlung due to bending of electron trajectories in the mean Coulomb
field of the electrosphere. The radiated energy flux from this mechanism
exceeds considerably both the contribution from the bremsstrahlung due to
electron-electron interaction and the tunnel pair creation.Comment: 9 pages, 2 figure
The long-term stability of extrasolar system HD 37124. Numerical study of resonance effects
We describe numerical tools for the stability analysis of extrasolar
planetary systems. In particular, we consider the relative Poincare variables
and symplectic integration of the equations of motion. We apply the tangent map
to derive a numerically efficient algorithm of the fast indicator MEGNO (a
measure of the maximal Lyapunov exponent) that helps to distinguish chaotic and
regular configurations. The results concerning the three-planet extrasolar
system HD 37124 are presented and discussed. The best fit solutions found in
earlier works are studied more closely. The system involves Jovian planets with
similar masses. The orbits have moderate eccentricities, nevertheless the best
fit solutions are found in dynamically active region of the phase space. The
long term stability of the system is determined by a net of low-order two-body
and three-body mean motion resonances. In particular, the three-body resonances
may induce strong chaos that leads to self-destruction of the system after Myrs
of apparently stable and bounded evolution. In such a case, numerically
efficient dynamical maps are useful to resolve the fine structure of the phase
space and to identify the sources of unstable behavior.Comment: 11 pages (total), 8 figures. Accepted for publication in MNRAS. The
definitive version will be/is available at
http://www.blackwellpublishing.com. The astro-ph version is prepared with low
resolution figures. To obtain the manuscript with full-resolution figures,
please visit http://www.astri.uni.torun.pl/~chris/mnrasIII.ps.g
Production of Drell--Yan lepton pairs in hadron collisions: transverse-momentum resummation at next-to-next-to-leading logarithmic accuracy
We consider the transverse-momentum (q_T) distribution of Drell--Yan lepton
pairs produced in hadron collisions. At small values of q_T, we resum the
logarithmically-enhanced perturbative QCD contributions up to
next-to-next-to-leading logarithmic accuracy. At intermediate and large values
of q_T, we consistently combine resummation with the known next-to-leading
order perturbative result. All perturbative terms up to order alpha_S^2 are
included in our computation which, after integration over q_T, reproduces the
known next-to-next-to-leading order result for the Drell--Yan total cross
section. We show and discuss the reduction in the scale dependence of the
results with respect to lower-order calculations, estimating the corresponding
perturbative uncertainty. We present a preliminary comparison with Tevatron Run
II data.Comment: Additional details shown in Fig.3. Note added on the quantitative
effect of the coefficient A^{(3)
Structure Functions of Unstable Lithium Isotopes
We study both the spin-average and spin-dependent structure functions of the
lithium isotopes, Li, which could be measured at RIKEN and other
nuclear facilities in the future. It is found that the light-cone momentum
distribution of the valence neutron in the halo of Li is very sharp and
symmetric around y = 1, because of the weak binding. The EMC ratios for Li
isotopes are then calculated. We study the possibility of extracting the
neutron structure function from data for the nuclear structure functions of the
Li isotopes. Next we calculate the spin-dependent structure functions of
Li isotopes, which have spin of 3/2. The effect of the nuclear
binding and Fermi motion on the multipole spin structure function,
, is about 10% in the region x < 0.7, but it becomes quite
important at large x. The spin structure function of is also
investigated. Finally, we discuss the modification of the Gottfried and Bjorken
integrals in a nuclear medium and point out several candidates for a pair of
mirror nuclei to study the flavor-nonsinglet quark distributions in nuclei.Comment: 23 pages + 7 tables + 15 figure
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