885 research outputs found
Nonperturbative and perturbative aspects of photo- and electroproduction of vector mesons
We discuss various aspects of vector meson production, first analysing the
interplay between perturbative and nonperturbative aspects of the QCD
calculation. Using a general method adapted to incorporate both perturbative
and nonpertubative aspects, we show that nonperturbative effects are important
for all experimentally available values of the photon virtuality Q2. We compare
the huge amount of experimental information now available with our theoretical
results obtained using a specific nonperturbative model without free
parameters, showing that quite simple features are able to explain the data.Comment: 19 page
Performance of a Neutron Polarimeter to Measure the Electric Form Factor of the Neutron
This research was sponsored by the National Science Foundation Grant NSF PHY-931478
On Aharonov-Casher bound states
In this work bound states for the Aharonov-Casher problem are considered.
According to Hagen's work on the exact equivalence between spin-1/2
Aharonov-Bohm and Aharonov-Casher effects, is known that the
term cannot be neglected in the
Hamiltonian if the spin of particle is considered. This term leads to the
existence of a singular potential at the origin. By modeling the problem by
boundary conditions at the origin which arises by the self-adjoint extension of
the Hamiltonian, we derive for the first time an expression for the bound state
energy of the Aharonov-Casher problem. As an application, we consider the
Aharonov-Casher plus a two-dimensional harmonic oscillator. We derive the
expression for the harmonic oscillator energies and compare it with the
expression obtained in the case without singularity. At the end, an approach
for determination of the self-adjoint extension parameter is given. In our
approach, the parameter is obtained essentially in terms of physics of the
problem.Comment: 11 pages, matches published versio
Energy and Flux Measurements of Ultra-High Energy Cosmic Rays Observed During the First ANITA Flight
The first flight of the Antarctic Impulsive Transient Antenna (ANITA)
experiment recorded 16 radio signals that were emitted by cosmic-ray induced
air showers. For 14 of these events, this radiation was reflected from the ice.
The dominant contribution to the radiation from the deflection of positrons and
electrons in the geomagnetic field, which is beamed in the direction of motion
of the air shower. This radiation is reflected from the ice and subsequently
detected by the ANITA experiment at a flight altitude of 36km. In this paper,
we estimate the energy of the 14 individual events and find that the mean
energy of the cosmic-ray sample is 2.9 EeV. By simulating the ANITA flight, we
calculate its exposure for ultra-high energy cosmic rays. We estimate for the
first time the cosmic-ray flux derived only from radio observations. In
addition, we find that the Monte Carlo simulation of the ANITA data set is in
agreement with the total number of observed events and with the properties of
those events.Comment: Added more explanation of the experimental setup and textual
improvement
Ultra-High Energy Neutrino Fluxes and Their Constraints
Applying our recently developed propagation code we review extragalactic
neutrino fluxes above 10^{14} eV in various scenarios and how they are
constrained by current data. We specifically identify scenarios in which the
cosmogenic neutrino flux, produced by pion production of ultra high energy
cosmic rays outside their sources, is considerably higher than the
"Waxman-Bahcall bound". This is easy to achieve for sources with hard injection
spectra and luminosities that were higher in the past. Such fluxes would
significantly increase the chances to detect ultra-high energy neutrinos with
experiments currently under construction or in the proposal stage.Comment: 11 pages, 15 figures, version published in Phys.Rev.
WD 0141−675: a case study on how to follow-up astrometric planet candidates around white dwarfs
This work combines spectroscopic and photometric data of the polluted white dwarf WD 0141−675, which has a now retracted astrometric super-Jupiter candidate, and investigates the most promising ways to confirm Gaia astrometric planetary candidates and obtain follow-up data. Obtaining precise radial velocity measurements for white dwarfs is challenging due to their intrinsic faint magnitudes, lack of spectral absorption lines, and broad spectral features. However, dedicated radial velocity campaigns are capable of confirming close-in giant exoplanets (a few MJup) around polluted white dwarfs, where additional metal lines aid radial velocity measurements. Infrared emission from these giant exoplanets is shown to be detectable with JWST Mid-Infrared Instrument (MIRI) and will provide constraints on the formation of the planet. Using the initial Gaia astrometric solution for WD 0141−675 as a case study, if there were a planet with a 33.65 d period or less with a nearly edge-on orbit, (1) ground-based radial velocity monitoring limits the mass to <15.4 MJup, and (2) space-based infrared photometry shows a lack of infrared excess and in a cloud-free planetary cooling scenario, a substellar companion would have to be <16 MJup and be older than 3.7 Gyr. These results demonstrate how radial velocities and infrared photometry can probe the mass of the objects producing some of the astrometric signals, and rule out parts of the brown dwarf and planet mass parameter space. Therefore, combining astrometric data with spectroscopic and photometric data is crucial to both confirm and characterize astrometric planet candidates around white dwarfs
The ALICE TPC, a large 3-dimensional tracking device with fast readout for ultra-high multiplicity events
The design, construction, and commissioning of the ALICE Time-Projection
Chamber (TPC) is described. It is the main device for pattern recognition,
tracking, and identification of charged particles in the ALICE experiment at
the CERN LHC. The TPC is cylindrical in shape with a volume close to 90 m^3 and
is operated in a 0.5 T solenoidal magnetic field parallel to its axis.
In this paper we describe in detail the design considerations for this
detector for operation in the extreme multiplicity environment of central
Pb--Pb collisions at LHC energy. The implementation of the resulting
requirements into hardware (field cage, read-out chambers, electronics),
infrastructure (gas and cooling system, laser-calibration system), and software
led to many technical innovations which are described along with a presentation
of all the major components of the detector, as currently realized. We also
report on the performance achieved after completion of the first round of
stand-alone calibration runs and demonstrate results close to those specified
in the TPC Technical Design Report.Comment: 55 pages, 82 figure
Photoproduction of mesons associated with a leading neutron
The photoproduction of mesons associated with a leading
neutron has been observed with the ZEUS detector in collisions at HERA
using an integrated luminosity of 80 pb. The neutron carries a large
fraction, {}, of the incoming proton beam energy and is detected at
very small production angles, { mrad}, an indication of
peripheral scattering. The meson is centrally produced with
pseudorapidity {
GeV}, which is large compared to the average transverse momentum of the neutron
of 0.22 GeV. The ratio of neutron-tagged to inclusive production is
in the photon-proton
center-of-mass energy range { GeV}. The data suggest that the
presence of a hard scale enhances the fraction of events with a leading neutron
in the final state.Comment: 28 pages, 4 figures, 2 table
Search for contact interactions, large extra dimensions and finite quark radius in ep collisions at HERA
A search for physics beyond the Standard Model has been performed with
high-Q^2 neutral current deep inelastic scattering events recorded with the
ZEUS detector at HERA. Two data sets, e^+ p \to e^+ X and e^- p \to e^- X, with
respective integrated luminosities of 112 pb^-1 and 16 pb^-1, were analyzed.
The data reach Q^2 values as high as 40000 GeV^2. No significant deviations
from Standard Model predictions were observed. Limits were derived on the
effective mass scale in eeqq contact interactions, the ratio of leptoquark mass
to the Yukawa coupling for heavy leptoquark models and the mass scale parameter
in models with large extra dimensions. The limit on the quark charge radius, in
the classical form factor approximation, is 0.85 10^-16 cm.Comment: 28 pages, 4 figures, accepted for publication in Physics Letters
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