67 research outputs found
Direct Production of Lightest Regge Resonances
We discuss direct production of Regge excitations in the collisions of
massless four-dimensional superstring states, focusing on the first excited
level of open strings ending on D-branes extending into higher dimensions. We
construct covariant vertex operators and identify ``universal'' Regge states
with the internal parts either trivial or determined by the world-sheet SCFT
describing superstrings propagating on an arbitrary Calabi-Yau manifold. We
evaluate the amplitudes involving one such massive state and up to three
massless ones and express them in the helicity basis. The most important
phenomenological applications of our results are in the context of low-mass
string (and large extra dimensions) scenarios in which excited string states
are expected to be produced at the LHC as soon as the string mass threshold is
reached in the center-of-mass energies of the colliding partons. In order to
facilitate the use of partonic cross sections, we evaluate them and tabulate
for all production processes: gluon fusion, quark absorbing a gluon,
quark-antiquark annihilation and quark-quark scattering.Comment: 43 pages, RevTeX 4.
Molecular Dynamics Study of Bamboo-like Carbon Nanotube Nucleation
MD simulations based on an empirical potential energy surface were used to
study the nucleation of bamboo-like carbon nanotubes (BCNTs). The simulations
reveal that inner walls of the bamboo structure start to nucleate at the
junction between the outer nanotube wall and the catalyst particle. In
agreement with experimental results, the simulations show that BCNTs nucleate
at higher dissolved carbon concentrations (i.e., feedstock pressures) than
those where non-bamboolike carbon nanotubes are nucleated
Time-Dependent Partition-Free Approach in Resonant Tunneling Systems
An extended Keldysh formalism, well suited to properly take into account the
initial correlations, is used in order to deal with the time-dependent current
response of a resonant tunneling system. We use a \textit{partition-free}
approach by Cini in which the whole system is in equilibrium before an external
bias is switched on. No fictitious partitions are used. Besides the
steady-state responses one can also calculate physical dynamical responses. In
the noninteracting case we clarify under what circumstances a steady-state
current develops and compare our result with the one obtained in the
partitioned scheme. We prove a Theorem of asymptotic Equivalence between the
two schemes for arbitrary time-dependent disturbances. We also show that the
steady-state current is independent of the history of the external perturbation
(Memory Loss Theorem). In the so called wide-band limit an analytic result for
the time-dependent current is obtained. In the interacting case we propose an
exact non-equilibrium Green function approach based on Time Dependent Density
Functional Theory. The equations are no more difficult than an ordinary Mean
Field treatment. We show how the scattering-state scheme by Lang follows from
our formulation. An exact formula for the steady-state current of an arbitrary
interacting resonant tunneling system is obtained. As an example the
time-dependent current response is calculated in the Random Phase
Approximation.Comment: final version, 18 pages, 9 figure
Spinor formalism for massive fields with half-integral spin
In this article, we extend the %Weyl-van der Waerden spinor technique for
calculating helicity amplitudes to general massive fields of half-integer
spins. We find that the little group generators can be represented as
first-order differential operators in the spinor formalism. We use the spinor
forms of the generators to get the explicit form of the massive fields of any
spin and any helicity. We also deal with the three-particle S-matrix by these
spinor form generators, and find that we are able to extend the explicit form
of the three-particle vertex obtained by Benincasa and Cachazo to the massive
case. We present the explicit expressions for the amplitudes with external
particles of the lowest helicities up to -3/2. Group theory, in the form of
raising operators of the little group, then dictates other amplitudes with
higher helicity in the same spin multiplets. The formalism allows, in
principle, to determine the electromagnetic form-factors of charged particles
of arbitrary helicities, without additional assumptions about the underlying
lagrangian. We find that restrictions which follow from gauge and Lorentz
invariance are nearly as restrictive as in the massless case.Comment: 21 pages, 1 figure
Protons in near earth orbit
The proton spectrum in the kinetic energy range 0.1 to 200 GeV was measured
by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at
an altitude of 380 km. Above the geomagnetic cutoff the observed spectrum is
parameterized by a power law. Below the geomagnetic cutoff a substantial second
spectrum was observed concentrated at equatorial latitudes with a flux ~ 70
m^-2 sec^-1 sr^-1. Most of these second spectrum protons follow a complicated
trajectory and originate from a restricted geographic region.Comment: 19 pages, Latex, 7 .eps figure
Search for antihelium in cosmic rays
The Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle
Discovery during flight STS-91 in a 51.7 degree orbit at altitudes between 320
and 390 km. A total of 2.86 * 10^6 helium nuclei were observed in the rigidity
range 1 to 140 GV. No antihelium nuclei were detected at any rigidity. An upper
limit on the flux ratio of antihelium to helium of < 1.1 * 10^-6 is obtained.Comment: 18 pages, Latex, 9 .eps figure
Search for gravitational waves from Scorpius X-1 in the second Advanced LIGO observing run with an improved hidden Markov model
We present results from a semicoherent search for continuous gravitational waves from the low-mass x-ray binary Scorpius X-1, using a hidden Markov model (HMM) to track spin wandering. This search improves on previous HMM-based searches of LIGO data by using an improved frequency domain matched filter, the J-statistic, and by analyzing data from Advanced LIGO's second observing run. In the frequency range searched, from 60 to 650 Hz, we find no evidence of gravitational radiation. At 194.6 Hz, the most sensitive search frequency, we report an upper limit on gravitational wave strain (at 95% confidence) of h095%=3.47×10-25 when marginalizing over source inclination angle. This is the most sensitive search for Scorpius X-1, to date, that is specifically designed to be robust in the presence of spin wandering. © 2019 American Physical Society
Erratum: "A Gravitational-wave Measurement of the Hubble Constant Following the Second Observing Run of Advanced LIGO and Virgo" (2021, ApJ, 909, 218)
[no abstract available
A neural-network-based method of model reduction for the dynamic simulation of MEMS
10.1088/0960-1317/11/3/311Journal of Micromechanics and Microengineering113226-233JMMI
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