2,825 research outputs found
The dependence of the hard diffractive photoproduction of vector meson or photon and the range of pQCD validity
We consider two coupled problems.
We study the dependence on photon virtuality for the semihard
quasi--elastic photoproduction of neutral vector mesons on a quark, gluon or
real photon (at
GeV)). To this end we calculate the corresponding amplitudes (in an
analytical form) in the lowest nontrivial approximation of perturbative QCD. It
is shown that the amplitude for the production of light meson varies very
rapidly with the photon virtuality near .
We estimate the bound of the pQCD validity region for such processes. For the
real incident photon the obtained bound for the meson production is very
high. This bound decreases fast with the increase of , and we expect that
the virtual photoproduction at HERA gives opportunity to test the pQCD results.
The signature of this region is discussed. For the hard Compton effect the pQCD
should work good at not too high , and this effect seems measurable
at HERA.Comment: ReVTeX, 36 pages, 5 Postscript figures, uses epsf.st
High Energy Photon-Photon Collisions at a Linear Collider
High intensity back-scattered laser beams will allow the efficient conversion
of a substantial fraction of the incident lepton energy into high energy
photons, thus significantly extending the physics capabilities of an
electron-electron or electron-positron linear collider. The annihilation of two
photons produces C=+ final states in virtually all angular momentum states. The
annihilation of polarized photons into the Higgs boson determines its
fundamental two-photon coupling as well as determining its parity. Other novel
two-photon processes include the two-photon production of charged lepton pairs,
vector boson pairs, as well as supersymmetric squark and slepton pairs and
Higgstrahlung. The one-loop box diagram leads to the production of pairs of
neutral particles. High energy photon-photon collisions can also provide a
remarkably background-free laboratory for studying possibly anomalous
collisions and annihilation. In the case of QCD, each photon can materialize as
a quark anti-quark pair which interact via multiple gluon exchange. The
diffractive channels in photon-photon collisions allow a novel look at the QCD
pomeron and odderon. Odderon exchange can be identified by looking at the heavy
quark asymmetry. In the case of electron-photon collisions, one can measure the
photon structure functions and its various components. Exclusive hadron
production processes in photon-photon collisions test QCD at the amplitude
level and measure the hadron distribution amplitudes which control exclusive
semi-leptonic and two-body hadronic B-decays.Comment: Invited talk, presented at the 5th International Workshop On
Electron-Electron Interactions At TeV Energies, Santa Cruz, California, 12-14
December 200
Soliton states in mesoscopic two-band-superconducting cylinders
In the framework of the Ginzburg-Landau approach, we present a
self-consistent theory of specific soliton states in mesoscopic (thin-walled)
two-band-superconducting cylinders in external parallel magnetic fields. Such
states arise in the presence of "Josephson-type" interband coupling, when phase
winding numbers are different for each component of the superconducting order
parameter. We evaluate the Gibbs free energy of the sysyem up to second-order
terms in a certain dimensionless parameter
, where
and are the magnetic and kinetic
inductance, respectively. We derive the complete set of exact soliton
solutions. These solutions are thoroughly analyzed from the viewpoint of both
local and global (thermodynamic) stability. In particular, we show that
rotational-symmetry-breaking caused by the formation of solitons gives rise to
a zero-frequency rotational mode. Although soliton states prove to be
thermodynamically metastable, the minimal energy gap between the lowest-lying
single-soliton states and thermodynamically stable zero-soliton states can be
much smaller than the magnetic Gibbs free energy of the latter states, provided
that intraband "penetration depths" differ substantially and interband coupling
is weak. The results of our investigation may apply to a wide class of
mesoscopic doubly-connected structures exhibiting two-band superconductivity.Comment: 15 pages, 3 figure
TeV-scale electron Compton scattering in the Randall-Sundrum scenario
The spin-2 graviton excitations in the Randall-Sundrum gravity model provides
a t-channel contribution to electron Compton scattering which competes
favourably with the standard QED contributions. The phenomenological
implications of these contributions to the unpolarized and polarized
cross-sections are evaluated.Comment: 11 pages, 5 figure
Tailoring and enhancing spontaneous two-photon emission processes using resonant plasmonic nanostructures
The rate of spontaneous emission is known to depend on the environment of a
light source, and the enhancement of one-photon emission in a resonant cavity
is known as the Purcell effect. Here we develop a theory of spontaneous
two-photon emission for a general electromagnetic environment including
inhomogeneous dispersive and absorptive media. This theory is used to evaluate
the two-photon Purcell enhancement in the vicinity of metallic nanoparticles
and it is demonstrated that the surface plasmon resonances supported by these
particles can enhance the emission rate by more than two orders of magnitude.
The control over two-photon Purcell enhancement given by tailored
nanostructured environments could provide an emitter with any desired spectral
response and may serve as an ultimate route for designing light sources with
novel properties
Description of paramagnetic--spin glass transition in Edwards-Anderson model in terms of critical dynamics
Possibility of description of the glass transition in terms of critical
dynamics considering a hierarchy of the intermodal relaxation time is shown.
The generalized Vogel-Fulcher law for the system relaxation time is derived in
terms of this approach. It is shown that the system satisfies the
fluctuating--dissipative theorem in case of the absence of the intermodal
relaxation time hierarchy.Comment: 10 pages, 6 figure
Self-consistent nonlinear kinetic simulations of the anomalous Doppler instability of suprathermal electrons in plasmas
Suprathermal tails in the distributions of electron velocities parallel to the magnetic field are found in many areas of plasma physics, from magnetic confinement fusion to solar system plasmas. Parallel electron kinetic energy can be transferred into plasma waves and perpendicular gyration energy of particles through the anomalous Doppler instability (ADI), provided that energetic electrons with parallel velocities v ≥ (ω + Ωce )/k are present; here Ωce denotes electron cyclotron frequency, ω the wave angular frequency and k the component of wavenumber parallel to the magnetic field. This phenomenon is widely observed in tokamak plasmas. Here we present the first fully self-consistent relativistic particle-in-cell simulations of the ADI, spanning the linear and nonlinear regimes of the ADI. We test the robustness of the analytical theory in the linear regime and follow the ADI through to the steady state. By directly evaluating the parallel and perpendicular dynamical contributions to j · E in the simulations, we follow the energy transfer between
the excited waves and the bulk and tail electron populations for the first time. We find that the ratio Ωce /(ωpe + Ωce ) of energy transfer between parallel and perpendicular, obtained from linear analysis, does not apply when damping is fully included, when we find it to be ωpe /(ωpe + Ωce ); here ωpe denotes the electron plasma frequency. We also find that the ADI can arise beyond the previously expected range of plasma parameters, in particular when Ωce > ωpe . The simulations also exhibit a spectral feature which may
correspond to observations of suprathermal narrowband emission at ωpe detected from low density tokamak plasmas
Stoner gap in the superconducting ferromagnet UGe2
We report the temperature () dependence of ferromagnetic Bragg peak
intensities and dc magnetization of the superconducting ferromagnet UGe2 under
pressure (). We have found that the low- behavior of the uniform
magnetization can be explained by a conventional Stoner model. A functional
analysis of the data produces the following results: The ferromagnetic state
below a critical pressure can be understood as the perfectly polarized state,
in which heavy quasiparticles occupy only majority spin bands. A Stoner gap
decreases monotonically with increasing pressure and increases
linearly with magnetic field. We show that the present analysis based on the
Stoner model is justified by a consistency check, i.e., comparison of density
of states at the Fermi energy deduced from the analysis with observed
electronic specific heat coeffieients. We also argue the influence of the
ferromagnetism on the superconductivity.Comment: 5 pages, 4 figures. to be published in Phys. Rev.
Using Scalars to Probe Theories of Low Scale Quantum Gravity
Arkani-Hamed, Dimopoulos and Dvali have recently suggested that gravity may
become strong at energies near 1 TeV which would remove the hierarchy problem.
Such a scenario can be tested at present and future colliders since the
exchange of towers of Kaluza-Klein gravitons leads to a set of new dimension-8
operators that can play important phenomenological roles. In this paper we
examine how the production of pairs of scalars at , and
hadron colliders can be used to further probe the effects of graviton tower
exchange. In particular we examine the tree-level production of pairs of
identical Higgs fields which occurs only at the loop level in both the Standard
Model and its extension to the Minimal Supersymmetric Standard Model. Cross
sections for such processes are found to be potentially large at the LHC and
the next generation of linear colliders. For the case the role
of polarization in improving sensitivity to graviton exchange is emphasized.Comment: 32 pages, 12 figures, latex, remarks added to tex
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