54,199 research outputs found
Detectors and Concepts for sub-100 ps timing with gaseous detectors
We give a short compendium of the main ongoing detectors and concepts capable
of performing accurate sub-100 ps timing at high particle fluxes and on large
areas, through technologies based on gaseous media. We briefly discuss the
state-of-the-art, technological limitations and prospects, and a new bizarre
idea
Minimal string-scale unification of gauge couplings
We look for the minimal particle content which is necessary to add to the
standard model in order to have a complete unification of gauge couplings and
gravity at the weakly coupled heterotic string scale. Using the current
precision electroweak data, we find that the presence of a vector-like fermion
at an intermediate scale and a non-standard hypercharge normalization are in
general sufficient to achieve this goal at two-loop level. If one requires the
extra matter scale to be below the TeV scale, then it is found that the
addition of three vector-like fermion doublets with a mass around 700 GeV
yields a perfect string-scale unification, provided that the affine levels are
kY=13/3, k2=1 and k3=2, as in the SU(5) X SU(5) string-GUT. Furthermore, if
supersymmetry is broken at the unification scale, the Higgs mass is predicted
in the range 125 GeV - 170 GeV, depending on the precise values of the top
quark mass and tan(beta) parameter.Comment: 12 pages, 2 figures, references updated and comments added, final
version to appear in Phys. Lett.
Equivalence of the sine-Gordon and massive Thirring models at finite temperature
Using the path-integral approach, the quantum massive Thirring and
sine-Gordon models are proven to be equivalent at finite temperature. This
result is an extension of Coleman's proof of the equivalence between both
theories at zero temperature. The usual identifications among the parameters of
these models also remain valid at .Comment: 9 pages, standard LaTe
Spinning Relativistic Particle in an External Electromagnetic Field
The Hamiltonian formulation of the motion of a spinning relativistic particle
in an external electromagnetic field is considered. The approach is based on
the introduction of new coordinates and their conjugated momenta to describe
the spin degrees of freedom together with an appropriate set of constraints in
the Dirac formulation. For particles with gyromagnetic ratio , the
equations of motion do not predict any deviation from the standard Lorentz
force, while for an additional force, which corresponds to the
magnetic dipole force, is obtained.Comment: Latex file, 11 page
Two and three electrons in a quantum dot: 1/|J| - expansion
We consider systems of two and three electrons in a two-dimensional parabolic
quantum dot. A magnetic field is applied perpendicularly to the electron plane
of motion. We show that the energy levels corresponding to states with high
angular momentum, J, and a low number of vibrational quanta may be
systematically computed as power series in 1/|J|. These states are relevant in
the high-B limit.Comment: LaTeX, 15 pages,6 postscript figure
Collisionless reconnection: The sub-microscale mechanism of magnetic field line interaction
Magnetic field lines are quantum objects carrying one quantum
of magnetic flux and have finite radius . Here
we argue that they possess a very specific dynamical interaction. Parallel
field lines reject each other. When confined to a certain area they form
two-dimensional lattices of hexagonal structure. We estimate the filling factor
of such an area. Antiparallel field lines, on the other hand, attract each
other. We identify the physical mechanism as being due to the action of the
gauge potential field which we determine quantum mechanically for two parallel
and two antiparallel field lines. The distortion of the quantum electrodynamic
vacuum causes a cloud of virtual pairs. We calculate the virtual pair
production rate from quantum electrodynamics and estimate the virtual pair
cloud density, pair current and Lorentz force density acting on the field lines
via the pair cloud. These properties of field line dynamics become important in
collisionless reconnection, consistently explaining why and how reconnection
can spontaneously set on in the field-free centre of a current sheet below the
electron-inertial scale.Comment: 13 journal pages, 6 figures, submitted to Ann. Geophy
Neural-network selection of high-redshift radio quasars, and the luminosity function at z~4
We obtain a sample of 87 radio-loud QSOs in the redshift range 3.6<z<4.4 by
cross-correlating sources in the FIRST radio survey S{1.4GHz} > 1 mJy with
star-like objects having r <20.2 in SDSS Data Release 7. Of these 87 QSOs, 80
are spectroscopically classified in previous work (mainly SDSS), and form the
training set for a search for additional such sources. We apply our selection
to 2,916 FIRST-DR7 pairs and find 15 likely candidates. Seven of these are
confirmed as high-redshift quasars, bringing the total to 87. The candidates
were selected using a neural-network, which yields 97% completeness (fraction
of actual high-z QSOs selected as such) and an efficiency (fraction of
candidates which are high-z QSOs) in the range of 47 to 60%. We use this sample
to estimate the binned optical luminosity function of radio-loud QSOs at , and also the LF of the total QSO population and its comoving density. Our
results suggest that the radio-loud fraction (RLF) at high z is similar to that
at low-z and that other authors may be underestimating the fraction at high-z.
Finally, we determine the slope of the optical luminosity function and obtain
results consistent with previous studies of radio-loud QSOs and of the whole
population of QSOs. The evolution of the luminosity function with redshift was
for many years interpreted as a flattening of the bright end slope, but has
recently been re-interpreted as strong evolution of the break luminosity for
high-z QSOs, and our results, for the radio-loud population, are consistent
with this.Comment: 20 pages. Accepted for publication in MNRAS on 3 March 201
Texture Zeros and Weak Basis Transformations
We investigate the physical meaning of some of the "texture zeros" which
appear in most of the Ansatze on quark masses and mixings. It is shown that
starting from arbitrary quark mass matrices and making a suitable weak basis
transformation one can obtain some of these sets of zeros which therefore have
no physical content. We then analyse the physical implications of a
four-texture zero Ansatz which is in agreement with all present experimental
data.Comment: 11 pages, typeset using revte
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