10,699 research outputs found
On Higgs-exchange DIS, physical evolution kernels and fourth-order splitting functions at large x
We present the coefficient functions for deep-inelastic scattering (DIS) via
the exchange of a scalar phi directly coupling only to gluons, such as the
Higgs boson in the limit of a very heavy top quark and n_f effectively massless
light flavours, to the third order in perturbative QCD. The two-loop results
are employed to construct the next-to-next-to-leading order physical evolution
kernels for the system (F_2 F_phi) of flavour-singlet structure functions. The
practical relevance of these kernels as an alternative to MSbar factorization
is bedevilled by artificial double logarithms at small values of the scaling
variable x, where the large top-mass limit ceases to be appropriate. However,
they show an only single-logarithmic enhancement at large x. Conjecturing that
this feature persists to the next order also in the present singlet case, the
three-loop coefficient functions facilitate exact predictions (backed up by
their particular colour structure) of the double-logarithmic contributions to
the fourth-order singlet splitting functions, i.e., of the terms (1-x)^a
ln^k(1-x) with k = 4, 5, 6 and k = 3, 4, 5, respectively, for the off-diagonal
and diagonal quantities to all powers a in 1-x.Comment: 75 pages, LaTeX, 13 figures (.eps). FORM and Fortran files of main
results appended to sourc
Screening and Anti-Screening Effects in J/psi Production on Nuclei
The nuclear effects in J/psi hadro- and electroproduction on nuclei are
considered in framework of reggeon approach. It is shown that screening regime
which holds for electroproduction at x_F > 0.7 and for hadroproduction at x_F >
-(0.3-0.4) is changed with anti-screening regime for smaller x_F values.Comment: 6 pages, 6 figures. Small changes in wordin
Chromospheric CaII Emission in Nearby F, G, K, and M stars
We present chromospheric CaII activity measurements, rotation periods and
ages for ~1200 F-, G-, K-, and M- type main-sequence stars from ~18,000
archival spectra taken at Keck and Lick Observatories as a part of the
California and Carnegie Planet Search Project. We have calibrated our
chromospheric S values against the Mount Wilson chromospheric activity data.
From these measurements we have calculated median activity levels and derived
R'HK, stellar ages, and rotation periods for 1228 stars, ~1000 of which have no
previously published S values. We also present precise time series of activity
measurements for these stars.Comment: 62 pages, 7 figures, 1 table. Second (extremely long) table is
available at http://astro.berkeley.edu/~jtwright/CaIIdata/tab1.tex Accepted
by ApJ
Exclusive charm production in pbar p collisions at s^1/2 <15 GeV
We discuss the open charm production in peripheral reactions
and , where
and stand for and
, respectively, at GeV, which corresponds to the
energy range of FAIR. Our consideration is based on the topological
decomposition of the planar quark and diquark diagrams which allows to estimate
consistently meson and baryon exchange trajectories and energy scale parameters
as well. The spin dependence is determined by the effective interaction of
lowest exchanged resonance. Unknown parameters are fixed by an independent
analysis of open strangeness production in and reactions and of SU(4) symmetry. We present the corresponding cross
sections and longitudinal double-spin asymmetries for exclusive binary
reactions with open charm mesons and baryons in the final state. The
polarization observables have a non-trivial and dependence which is
sensitive to details of the open charm production mechanism.Comment: 26 pages, 18 figure
New Models of General Relativistic Static Thick Disks
New families of exact general relativistic thick disks are constructed using
the ``displace, cut, fill and reflect'' method. A class of functions used to
``fill'' the disks is derived imposing conditions on the first and second
derivatives to generate physically acceptable disks. The analysis of the
function's curvature further restrict the ranges of the free parameters that
allow phisically acceptable disks. Then this class of functions together with
the Schwarzschild metric is employed to construct thick disks in isotropic,
Weyl and Schwarzschild canonical coordinates. In these last coordinates an
additional function must be added to one of the metric coefficients to generate
exact disks. Disks in isotropic and Weyl coordinates satisfy all energy
conditions, but those in Schwarzschild canonical coordinates do not satisfy the
dominant energy condition.Comment: 27 pages, 14 figure
Planetary Companions Around Two Solar Type Stars: HD 195019 and HD 217107
We have enlarged the sample of stars in the planet search at Lick
Observatory. Doppler measurements of 82 new stars observed at Lick Observatory,
with additional velocities from Keck Observatory, have revealed two new planet
candidates.
The G3V/IV star, HD 195019, exhibits Keplerian velocity variations with a
period of 18.27 d, an orbital eccentricity of 0.03 +/- 0.03, and M sin i = 3.51
M_Jup. Based on a measurement of Ca II H&K emission, this star is
chromospherically inactive. We estimate the metallicity of HD 195019 to be
approximately solar from ubvy photometry.
The second planet candidate was detected around HD 217107, a G7V star. This
star exhibits a 7.12 d Keplerian period with eccentricity 0.14 +/- 0.05 and M
sin i = 1.27 M_Jup. HD 217107 is also chromospherically inactive. The
photometric metallicity is found to be [Fe/H] = +0.29 +/- 0.1 dex. Given the
relatively short orbital period, the absence of tidal spin-up of HD 217107
provides a theoretical constraint on the upper limit of the companion mass of <
11 M_Jup.Comment: 15 pages, plus 6 figures. To appear in Jan 1999 PAS
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
Ten Low Mass Companions from the Keck Precision Velocity Survey
Ten new low mass companions have emerged from the Keck precision Doppler
velocity survey, with minimum (msini) masses ranging from 0.8 mjup to 0.34
msun. Five of these are planet candidates with msini < 12 mjup, two are brown
dwarf candidates with msini ~30 mjup, and three are low mass stellar
companions. Hipparcos astrometry reveals the orbital inclinations and masses
for three of the (more massive) companions, and it provides upper limits to the
masses for the rest. A new class of extrasolar planet is emerging,
characterized by nearly circular orbits and orbital radii greater than 1 AU.
The planet HD 4208b appears to be a member of this new class. The mass
distribution of extrasolar planets continues to exhibit a rapid rise from 10
mjup toward the lowest detectable masses near 1 msat.Comment: 26 pages, TeX, plus 13 postscript figure
Produtividade de milho e feijão em sistema integração lavoura-pecuária.
O objetivo deste trabalho foi avaliar a produtividade de grãos de milho e feijão em sistema integrado com pastagem anual de inverno. O sistema de integração lavoura-pecuária, constituído por pastagens anuais de inverno e culturas graníferas de verão, como o milho e o feijão, é uma opção para otimizar o uso de áreas agrícolas, sem comprometer a produtividade de grãos das culturas estivais
Free-jet acoustic investigation of high-radius-ratio coannular plug nozzles
The experimental and analytical results of a scale model simulated flight acoustic exploratory investigation of high radius ratio coannular plug nozzles with inverted velocity and temperature profiles are summarized. Six coannular plug nozzle configurations and a baseline convergent conical nozzle were tested for simulated flight acoustic evaluation. The nozzles were tested over a range of test conditions that are typical of a Variable Cycle Engine for application to advanced high speed aircraft. It was found that in simulate flight, the high radius ratio coannular plug nozzles maintain their jet noise and shock noise reduction features previously observed in static testing. The presence of nozzle bypass struts will not significantly affect the acousticn noise reduction features of a General Electric type nozzle design. A unique coannular plug nozzle flight acoustic spectral prediction method was identified and found to predict the measured results quite well. Special laser velocimeter and acoustic measurements were performed which have given new insights into the jet and shock noise reduction mechanisms of coannular plug nozzles with regard to identifying further benificial research efforts
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