31,094 research outputs found
Exclusive electroproduction of J/psi mesons
A nonperturbative calculation of elastic electroproduction of the J/psi meson
is presented and compared to the experimental data.
Our model describes well the observed dependences of the cross sections on
the photon virtuality Q2 and on the energy, and the measured ratio R of
longitudinal to transverse cross sections.Comment: Five *.eps figure
Nonperturbative QCD treatment of photoproduction
We present a nonperturbative QCD calculation of elastic meson
production in photon-proton scattering at high energies. Using light cone wave
functions of the photon and vector mesons, and the framework of the model of
the stochastic QCD vacuum, we calculate the differential and integrated elastic
cross sections for \gamma p \goto J/\psi p . With an energy dependence
following the two-pomeron model we are able to give a consistent description of
the integrated cross sections and the differential cross sections at low
in the range from 20 GeV up to the highest HERA energies. We discuss different
approaches to introduce saturation and find no specific effects up to energies
presently available. We also calculate and compare to experiments the cross
section for photoproduction.Comment: 24 pages, 10 figures, 4 table
Ultra-Light Scalar Fields and the Growth of Structure in the Universe
Ultra-light scalar fields, with masses of between m=10^{-33} eV and
m=10^{-22} eV, can affect the growth of structure in the Universe. We identify
the different regimes in the evolution of ultra-light scalar fields, how they
affect the expansion rate of the universe and how they affect the growth rate
of cosmological perturbations. We find a number of interesting effects, discuss
how they might arise in realistic scenarios of the early universe and comment
on how they might be observed.Comment: 12 pages, 11 figure
Direct and indirect exciton mixing in a slightly asymmetric double quantum well
We studied, theoretically, the optical absorption spectra for a slightly
asymmetric double quantum well (DQW), in the presence of electric and magnetic
fields. Recent experimental results for a 10.18/3.82/9.61 nm GaAs Al(_{.33}
)Ga(_{.67})As DQW show clearly the different behavior in the luminescence peaks
for the indirect exciton (IX) and left direct exciton (DX) as a function of the
external electric field. We show that the presence of a peak near the (DX)
peak, attributed to an impurity bound left (DX) in the experimental results,
could be a consequence of the non-trivial mixing between excitonic states.Comment: 8 pages and 8 figure
Quintessence in a quandary: prior dependence in dark energy models
The archetypal theory of dark energy is quintessence: a minimally coupled
scalar field with a canonical kinetic energy and potential. By studying random
potentials we show that quintessence imposes a restricted set of priors on the
equation of state of dark energy. Focusing on the commonly-used
parametrisation, , we show that there is a natural
scale and direction in the plane that distinguishes quintessence
as a general framework. We calculate the expected information gain for a given
survey and show that, because of the non-trivial prior information, it is a
function of more than just the figure of merit. This allows us to make a
quantitative case for novel survey strategies. We show that the scale of the
prior sets target observational requirements for gaining significant
information. This corresponds to a figure of merit FOM, a
requirement that future galaxy redshift surveys will meet.Comment: 5 pages, 3 figures. For the busy reader, Fig. 1 is the money plot.
v2: Minor changes, matches published version. Code open source at
gitorious.org/random-quintessenc
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