694 research outputs found
Tensor Meson Production in Proton-Proton Collisions from the Color Glass Condensate
We compute the inclusive cross-section of tensor mesons production in
proton-proton collisions at high-energy. We use an effective theory inspired
from the tensor meson dominance hypothesis that couples gluons to
mesons. We compute the differential cross-section in the
-factorization and in the Color Glass Condensate formalism in the
low density regime. We show that the two formalisms are equivalent for this
specific observable. Finally, we study the phenomenology of mesons by
comparing theoretical predictions of different parameterizations of the
unintegrated gluon distribution function. We find that -meson production
is another observable that can be used to put constraints on these
distributions.Comment: 26 pages, 3 figures, to be submitted in Phys. Rev.
Penser la relation d’aide et de soin à partir du handicap: enjeux et ambivalences.
International audienceCet article est l'éditorial d'un premier numéro spécial de la revue Alter sur la thématique "Care et handicap". Un deuxième numéro suivra. Ces deux numéros spéciaux de la revue Alter autour de la thématique « care et handicap » ont pour objectif de prolonger le débat et le dialogue initiés entre différents courants de pensée sur la nature de la relation d’aide, sur ses effets en termes de qualification et de définition des personnes (aidantes comme aidées), sur ses ambivalences et tensions inhérentes. Ils reviennent notamment sur le débat entre Disability Studies et Ethique du care
Resonantly enhanced pair production in a simple diatomic model
A new mechanism for the production of electron-positron pairs from the
interaction of a laser field and a fully stripped diatomic molecule in the
tunneling regime is presented. When the laser field is turned off, the Dirac
operator has resonances in both the positive and the negative energy continua
while bound states are in the mass gap. When this system is immersed in a
strong laser field, the resonances move in the complex energy plane: the
negative energy resonances are pushed to higher energies while the bound states
are Stark shifted. It is argued here that there is a pair production
enhancement at the crossing of resonances by looking at a simple 1-D model: the
nuclei are modeled simply by Dirac delta potential wells while the laser field
is assumed to be static and of finite spatial extent. The average rate for the
number of electron-positron pairs produced is evaluated and the results are
compared to the single nucleus and to the free cases. It is shown that
positrons are produced by the Resonantly Enhanced Pair Production (REPP)
mechanism, which is analogous to the resonantly enhanced ionization of
molecular physics. This phenomenon could be used to increase the number of
pairs produced at low field strength, allowing the study of the Dirac vacuum.Comment: 11 pages, 4 figure
Magnetic properties of HO2 thin films
We report on the magnetic and transport studies of hafnium oxide thin films
grown by pulsed-laser deposition on sapphire substrates under different oxygen
pressures, ranging from 10-7 to 10-1 mbar. Some physical properties of these
thin films appear to depend on the oxygen pressure during growth: the film
grown at low oxygen pressure (P ~= 10-7 mbar) has a metallic aspect and is
conducting, with a positive Hall signal, while those grown under higher oxygen
pressures (7 x 10-5 <= P <= 0.4 mbar) are insulating. However, no intrinsic
ferromagnetic signal could be attributed to the HfO2 films, irrespective of the
oxygen pressure during the deposition.Comment: 1
Landau-Zener-St\"uckelberg interferometry in pair production from counterpropagating lasers
The rate of electron-positron pair production in linearly polarized
counter-propagating lasers is evaluated from a recently discovered solution of
the time-dependent Dirac equation. The latter is solved in momentum space where
it is formally equivalent to the Schr\"odinger equation describing a strongly
driven two-level system. The solution is found from a simple transformation of
the Dirac equation and is given in compact form in terms of the
doubly-confluent Heun's function. By using the analogy with the two-level
system, it is shown that for high-intensity lasers, pair production occurs
through periodic non-adiabatic transitions when the adiabatic energy gap is
minimal. These transitions give rise to an intricate interference pattern in
the pair spectrum, reminiscent of the Landau-Zener-St\"uckelberg phenomenon in
molecular physics: the accumulated phase result in constructive or destructive
interference. The adiabatic-impulse model is used to study this phenomenon and
shows an excellent agreement with the exact result.Comment: 22 pages, 7 figure
Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process
UV ice photodesorption is an important non-thermal desorption pathway in many
interstellar environments that has been invoked to explain observations of cold
molecules in disks, clouds and cloud cores. Systematic laboratory studies of
the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have
been performed at the DESIRS vacuum UV beamline of the synchrotron facility
SOLEIL. The photodesorption spectral analysis demonstrates that the
photodesorption process is indirect, i.e. the desorption is induced by a photon
absorption in sub-surface molecular layers, while only surface molecules are
actually desorbing. The photodesorption spectra of CO and N2 in binary ices
therefore depend on the absorption spectra of the dominant species in the
subsurface ice layer, which implies that the photodesorption efficiency and
energy dependence are dramatically different for mixed and layered ices
compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO
will effectively quench CO photodesorption, while enhancing N2 photodesorption
by a factors of a few (compared to the pure ices) when the ice is exposed to a
typical dark cloud UV field, which may help to explain the different
distributions of CO and N2H+ in molecular cloud cores. This indirect
photodesorption mechanism may also explain observations of small amounts of
complex organics in cold interstellar environments.Comment: 21 pages 5 figure
Explicit volume-preserving numerical schemes for relativistic trajectories and spin dynamics
A class of explicit numerical schemes is developed to solve for the
relativistic dynamics and spin of particles in electromagnetic fields, using
the Lorentz-BMT equation formulated in the Clifford algebra representation of
Baylis. It is demonstrated that these numerical methods, reminiscent of the
leapfrog and Verlet methods, share a number of important properties: they are
energy-conserving, volume-conserving and second order convergent. These
properties are analysed empirically by benchmarking against known analytical
solutions in constant uniform electrodynamic fields. It is demonstrated that
the numerical error in a constant magnetic field remains bounded for long time
simulations in contrast to the Boris pusher, whose angular error increases
linearly with time. Finally, the intricate spin dynamics of a particle is
investigated in a plane wave field configuration.Comment: 15 pages, 9 figure
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