11,708 research outputs found
The annihilation of virtual photons into pseudoscalar mesons
We investigate the possibility to constrain the pion distribution amplitude
from the gamma* gamma* -> pi transition. For a surprisingly large range in the
two photon virtualities we find that the transition form factor is essentially
independent of the distribution amplitude. This in turn entails a
parameter-free prediction of QCD. The gamma* gamma* -> eta, eta' form factors
are also briefly discussed. We estimate that experimental studies might be
feasible at the existing e+ e- experiments BaBar, Belle, and CLEO.Comment: 22 pages latex, 9 figure
Two-Photon Annihilation into Baryon-Antibaryon Pairs
We study the handbag contribution to two-photon annihilation into
baryon-antibaryon pairs at large energy and momentum transfer. We derive
factorization of the process amplitude into a hard gamma gamma -> q qbar
subprocess and form factors describing the soft q qbar -> B Bbar transition,
assuming that the process is dominated by configurations where the (anti)quark
approximately carries the full momentum of the (anti)baryon. The form factors
represent moments of time-like generalized parton distributions, so-called B
Bbar distribution amplitudes. A characteristic feature of the handbag mechanism
is the absence of isospin-two components in the final state, which in
combination with flavor symmetry provides relations among the form factors for
the members of the lowest-lying baryon octet. Assuming dominance of the handbag
contribution, we can describe current experimental data with form factors of
plausible size, and predict the cross sections of presently unmeasured B Bbar
channels.Comment: 20 pages latex, 4 figures. v2: minor clarifications, references
update
Dissociation rates of J/psi's with comoving mesons - thermal vs. nonequilibrium scenario
We study J/psi dissociation processes in hadronic environments. The validity
of a thermal meson gas ansatz is tested by confronting it with an alternative,
nonequilibrium scenario. Heavy ion collisions are simulated in the framework of
the microscopic transport model UrQMD, taking into account the production of
charmonium states through hard parton-parton interactions and subsequent
rescattering with hadrons. The thermal gas and microscopic transport scenarios
are shown to be very dissimilar. Estimates of J/psi survival probabilities
based on thermal models of comover interactions in heavy ion collisions are
therefore not reliable.Comment: 12 pages, 6 figure
The last orbit of binary black holes
We have used our new technique for fully numerical evolutions of orbiting
black-hole binaries without excision to model the last orbit and merger of an
equal-mass black-hole system. We track the trajectories of the individual
apparent horizons and find that the binary completed approximately one and a
third orbits before forming a common horizon. Upon calculating the complete
gravitational radiation waveform, horizon mass, and spin, we find that the
binary radiated 3.2% of its mass and 24% of its angular momentum. The early
part of the waveform, after a relatively short initial burst of spurious
radiation, is oscillatory with increasing amplitude and frequency, as expected
from orbital motion. The waveform then transitions to a typical `plunge'
waveform; i.e. a rapid rise in amplitude followed by quasinormal ringing. The
plunge part of the waveform is remarkably similar to the waveform from the
previously studied `ISCO' configuration. We anticipate that the plunge
waveform, when starting from quasicircular orbits, has a generic shape that is
essentially independent of the initial separation of the binary.Comment: 5 pages, 5 figures, revtex
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
Accurate Evolutions of Orbiting Black-Hole Binaries Without Excision
We present a new algorithm for evolving orbiting black-hole binaries that
does not require excision or a corotating shift. Our algorithm is based on a
novel technique to handle the singular puncture conformal factor. This system,
based on the BSSN formulation of Einstein's equations, when used with a
`pre-collapsed' initial lapse, is non-singular at the start of the evolution,
and remains non-singular and stable provided that a good choice is made for the
gauge. As a test case, we use this technique to fully evolve orbiting
black-hole binaries from near the Innermost Stable Circular Orbit (ISCO)
regime. We show fourth order convergence of waveforms and compute the radiated
gravitational energy and angular momentum from the plunge. These results are in
good agreement with those predicted by the Lazarus approach.Comment: 4 pages, revtex4, 3 figs, references added, typos fixe
A two-dimensional Fermi liquid with attractive interactions
We realize and study an attractively interacting two-dimensional Fermi
liquid. Using momentum resolved photoemission spectroscopy, we measure the
self-energy, determine the contact parameter of the short-range interaction
potential, and find their dependence on the interaction strength. We
successfully compare the measurements to a theoretical analysis, properly
taking into account the finite temperature, harmonic trap, and the averaging
over several two-dimensional gases with different peak densities
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