1,010 research outputs found
The Q_{1,2}-Q_7 interference contributions to b -> s gamma at O(alpha_s^2) for the physical value of m_c
The B -> X_s gamma branching ratio is currently measured with around 5%
accuracy. Further improvement is expected from Belle II. To match such a
precision on the theoretical side, evaluation of O(alpha_s^2) corrections to
the partonic decay b -> X_s^part gamma are necessary, which includes the b -> s
gamma, b -> s g gamma, b -> s g g gamma, b -> s qbar q gamma decay channels.
Here, we evaluate the unrenormalized contribution to b -> s gamma that stems
from the interference of the photonic dipole operator Q_7 and the
current-current operators Q_1 and Q_2. Our results, obtained in the cut
propagator approach at the 4-loop level, agree with those found in parallel by
Fael et al. who have applied the amplitude approach at the 3-loop level.
Partial results for the same quantities recently determined by Greub et al.
agree with our findings, too.Comment: 9 pages, 1 figur
Particle-Like Description in Quintessential Cosmology
Assuming equation of state for quintessential matter: , we
analyse dynamical behaviour of the scale factor in FRW cosmologies. It is shown
that its dynamics is formally equivalent to that of a classical particle under
the action of 1D potential . It is shown that Hamiltonian method can be
easily implemented to obtain a classification of all cosmological solutions in
the phase space as well as in the configurational space. Examples taken from
modern cosmology illustrate the effectiveness of the presented approach.
Advantages of representing dynamics as a 1D Hamiltonian flow, in the analysis
of acceleration and horizon problems, are presented. The inverse problem of
reconstructing the Hamiltonian dynamics (i.e. potential function) from the
luminosity distance function for supernovae is also considered.Comment: 35 pages, 26 figures, RevTeX4, some applications of our treatment to
investigation of quintessence models were adde
Constraints on alternative models to dark energy
The recent observations of type Ia supernovae strongly support that the
universe is accelerating now and decelerated in the recent past. This may be
the evidence of the breakdown of the standard Friemann equation. We consider a
general modified Friedmann equation. Three different models are analyzed in
detail. The current supernovae data and the Wilkinson microwave anisotropy
probe data are used to constrain these models. A detailed analysis of the
transition from the deceleration phase to the acceleration phase is also
performed.Comment: 10 pages, 1 figure, revtex
Dilatonic ghost condensate as dark energy
We explore a dark energy model with a ghost scalar field in the context of
the runaway dilaton scenario in low-energy effective string theory. We address
the problem of vacuum stability by implementing higher-order derivative terms
and show that a cosmologically viable model of ``phantomized'' dark energy can
be constructed without violating the stability of quantum fluctuations. We also
analytically derive the condition under which cosmological scaling solutions
exist starting from a general Lagrangian including the phantom type scalar
field. We apply this method to the case where the dilaton is coupled to
non-relativistic dark matter and find that the system tends to become quantum
mechanically unstable when a constant coupling is always present. Nevertheless,
it is possible to obtain a viable cosmological solution in which the energy
density of the dilaton eventually approaches the present value of dark energy
provided that the coupling rapidly grows during the transition to the scalar
field dominated era.Comment: 26 pages, 6 figure
Mechanical model of the ultra-fast underwater trap of Utricularia
The underwater traps of the carnivorous plants of the Utricularia species
catch their preys through the repetition of an "active slow deflation / passive
fast suction" sequence. In this paper, we propose a mechanical model that
describes both phases and strongly supports the hypothesis that the trap door
acts as a flexible valve that buckles under the combined effects of pressure
forces and the mechanical stimulation of trigger hairs, and not as a panel
articulated on hinges. This model combines two different approaches, namely (i)
the description of thin membranes as triangle meshes with strain and curvature
energy, and (ii) the molecular dynamics approach, which consists in computing
the time evolution of the position of each vertex of the mesh according to
Langevin equations. The only free parameter in the expression of the elastic
energy is the Young's modulus E of the membranes. The values for this parameter
are unequivocally obtained by requiring that the trap model fires, like real
traps, when the pressure difference between the outside and the inside of the
trap reaches about 15 kPa. Among other results, our simulations show that, for
a pressure difference slightly larger than the critical one, the door buckles,
slides on the threshold and finally swings wide open, in excellent agreement
with the sequence observed in high-speed videos.Comment: Accepted for publication in Physical Review
Effect of synthesis conditions on formation pathways of metal organic framework (MOF-5) Crystals
Metal Organic Frameworks (MOFs) represent a class of nanoporous crystalline materials with far reaching potential in gas storage, catalysis, and medical devices. We investigated the effects of synthesis process parameters on production of MOF-5 from terephthalic acid and zinc nitrate in diethylformamide. Under favorable synthesis conditions, we systematically mapped a solid formation diagram in terms of time and temperature for both stirred and unstirred conditions. The synthesis of MOF-5 has been previously reported as a straightforward reaction progressing from precursor compounds in solution directly to the final MOF-5 solid phase product. However, we show that the solid phase formation process is far more complex, invariably transferring through metastable intermediate crystalline phases before the final MOF-5 phase is reached, providing new insights into the formation pathways of MOFs. We also identify process parameters suitable for scale-up and continuous manufacturing of high purity MOF-5
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