40,622 research outputs found
Mixing of two-level unstable systems
Unstable particles can be consistently described in the framework of quantum
field theory. Starting from the full S-matrix amplitudes of B^+ --> (2 pi, 3
pi) l nu decays as examples in the energy region where the rho-omega resonances
are dominating, we propose a prescription for the mixing of two quasi
`physical' unstable states that differs from the one obtained from the
diagonalization of the M -i*Gamma/2 non-hermitian hamiltonian. We discuss some
important consequences for CP violation in the K_L - K_S system.Comment: 7 pages, Latex. A factor 1/2 removed from r.h.s. of Eqs. (12)-(15).
Conclusions unchange
Bilayer graphene: gap tunability and edge properties
Bilayer graphene -- two coupled single graphene layers stacked as in graphite
-- provides the only known semiconductor with a gap that can be tuned
externally through electric field effect. Here we use a tight binding approach
to study how the gap changes with the applied electric field. Within a parallel
plate capacitor model and taking into account screening of the external field,
we describe real back gated and/or chemically doped bilayer devices. We show
that a gap between zero and midinfrared energies can be induced and externally
tuned in these devices, making bilayer graphene very appealing from the point
of view of applications. However, applications to nanotechnology require
careful treatment of the effect of sample boundaries. This being particularly
true in graphene, where the presence of edge states at zero energy -- the Fermi
level of the undoped system -- has been extensively reported. Here we show that
also bilayer graphene supports surface states localized at zigzag edges. The
presence of two layers, however, allows for a new type of edge state which
shows an enhanced penetration into the bulk and gives rise to band crossing
phenomenon inside the gap of the biased bilayer system.Comment: 8 pages, 3 fugures, Proceedings of the International Conference on
Theoretical Physics: Dubna-Nano200
High Resolution Ionization of Ultracold Neutral Plasmas
Collective effects, such as waves and instabilities, are integral to our
understanding of most plasma phenomena. We have been able to study these in
ultracold neutral plasmas by shaping the initial density distribution through
spatial modulation of the ionizing laser intensity. We describe a relay imaging
system for the photoionization beam that allows us to create higher resolution
features and its application to extend the observation of ion acoustic waves to
shorter wavelengths. We also describe the formation of sculpted density
profiles to create fast expansion of plasma into vacuum and streaming plasmas
Further remarks on isospin breaking in charmless semileptonic B decays
We consider the isospin breaking corrections to charmless semileptonic decays
of B mesons. Both, the recently measured branching ratios of exclusive decays
by the CLEO Collaboration and the end-point reion of the inclusive lepton
spectrum in form factor models, can be affected by these corrections. Isospin
corrections can affect the determination of |V_ub| from exclusive semileptonic
B decays at a level comparable to present statistical uncertainties.Comment: Latex, 7 pages, 1 .ps figure, to appear in Phys. Rev.
Tensor mesons produced in tau lepton decays
Light tensor mesons (T = a_2, f_2 and K_2^*) can be produced in decays of tau
leptons. In this paper we compute the branching ratios of tau --> T pi nu
decays by assuming the dominance of intermediate virtual states to model the
form factors involved in the relevant hadronic matrix element. The exclusive
f_2(1270) pi^- decay mode turns out to have the largest branching ratio, of
O(10^-4) . Our results indicate that the contributions of tensor meson
intermediate states to the three-pseudoscalar channels of tau decays are rather
small.Comment: 10 pages, 1 figure. Version accepted for publication in PRD, some
typos are corrected and comments are added in section 4. Conclusions remain
unchange
Detection of Keplerian dynamics in a disk around the post-AGB star AC Her
So far, only one rotating disk has been clearly identified and studied in AGB
or post-AGB objects (in the Red Rectangle), by means of observations with high
spectral and spatial resolution. However, disks are thought to play a key role
in the late stellar evolution and are suspected to surround many evolved stars.
We aim to extend our knowledge on these structures.
We present interferometric observations of CO J=2-1 emission from the nebula
surrounding the post-AGB star AC Her, a source belonging to a class of objects
that share properties with the Red Rectangle and show hints of Keplerian disks.
We clearly detect the Keplerian dynamics of a second disk orbiting an evolved
star. Its main properties (size, temperature, central mass) are derived from
direct interpretation of the data and model fitting. With this we confirm that
there are disks orbiting the stars of this relatively wide class of post-AGB
objectsComment: 4 pages, 3 figure
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