324 research outputs found
Simulations of the Atomic Beam Transport in an Atomic Beam Source under the Influence of Spin-Selective Sextupole Magnets
Polarized internal gas targets are important tools in spin-physics experiments at par-
ticle accelerators. For many experiments it is imperative to get the highest possible
target density. Research in recent decades, however, led only to marginal increase of
the intensity of atomic beam sources (ABS), which constitute the main factor in in-
creasing the target density. The main problem, hindering further improvement, is the
complexity of the processes in the ABS, which prevents a complete description of all ef-
fects. A particularly big challenge is the description of the change in flow regime from
laminar to molecular during the formation of the atomic beam by the nozzle. Now,
with ever increasing computer power, it becomes possible to simulate all the processes
in a complete ABS and to use algorithms to optimize the device.
The present thesis summarizes the important effects in polarized internal gas targets
and describes the implementation of a new computer program, which was interfaced
with an optimization algorithm. The program is based on OpenFOAM 1.7.1 and uses
the Direct Simulation Monte Carlo (DSMC) method to simulate the gas flow. Besides a
generic interface for magnetic fields which act on the magnetic moments of the atoms,
many important effects like recombination and spin-exchange collisions are included
in the program. Furthermore, a generic framework for optimization is implemented.
It can be used to optimize very different problems with different optimization algo-
rithms. Currently, Adaptive Simulated Annealing (ASA) is implemented together with
the capability to handle the optimization of an ABS. All parts of the program were
tested and results are provided. Although the execution time of a simulation run is
still very long, the new program might become a valuable tool for simulating and op-
timizing polarized internal gas targets. This will lead to better statistics in experiments
and will enable certain new experiments
Unbiased cut selection for optimal upper limits in neutrino detectors: the model rejection potential technique
We present a method for optimising experimental cuts in order to place the
strongest constraints (upper limits) on theoretical signal models.
The method relies only on signal and background expectations derived from
Monte-Carlo simulations, so no bias is introduced by looking at actual data,
for instance by setting a limit based on expected signal above the ``last
remaining data event.'' After discussing the concept of the ``average upper
limit,'' based on the expectation from an ensemble of repeated experiments with
no true signal, we show how the best model rejection potential is achieved by
optimising the cuts to minimise the ratio of this ``average upper limit'' to
the expected signal from the model. As an example, we use this technique to
determine the limit sensitivity of kilometre scale neutrino detectors to
extra-terrestrial neutrino fluxes from a variety of models, e.g. active
galaxies and gamma-ray bursts. We suggest that these model rejection potential
optimised limits be used as a standard method of comparing the sensitivity of
proposed neutrino detectors.Comment: 18 pages, 7 figures, submitted to Astroparticle Physic
Atmospheric lepton fluxes at very high energy
The observation of astrophysical neutrinos requires a detailed understanding
of the atmospheric neutrino background. Since neutrinos are produced in meson
decays together with a charged lepton, important constraints on this background
can be obtained from the measurement of the atmospheric muon flux. Muons,
however, can also be produced as mu+ mu- pairs by purely electromagnetic
processes. We use the Z-moment method to study and compare the contributions to
the atmospheric muon and neutrino fluxes from different sources (pi/K decay,
charmed and unflavored hadron decay, and photon conversion into a muon pair).
We pay special attention to the contribution from unflavored mesons (eta, eta',
rho0, omega and phi). These mesons are abundant in air showers, their lifetimes
are much shorter than those of charged pions or kaons, and they have decay
branching ratios of order 10^-4 into final states containing a muon pair. We
show that they may be the dominant source of muons at E_mu >10^3 TeV.Comment: Pdflatex, 28 pages, 6 figure
Infrared gluons, intrinsic transverse momentum and rising total cross-sections
We discuss the infrared limit for soft gluon kt-resummation and relate it to
physical observables such as the intrinsic transverse momentum and the high
energy limit of total cross-sections.Comment: 8 pages, 6 figures, Presented at Hadron Structure '09, Tatranska
Strba, September 2009, Slovacchia, to be published in the Conference
Proceeding
Measuring the prompt atmospheric neutrino flux with down-going muons in neutrino telescopes
In the TeV energy region and above, the uncertainty in the level of prompt
atmospheric neutrinos would limit the search for diffuse astrophysical
neutrinos. We suggest that neutrino telescopes may provide an empirical
determination of the flux of prompt atmospheric electron and muon neutrinos by
measuring the flux of prompt down-going muons. Our suggestion is based on the
consideration that prompt neutrino and prompt muon fluxes at sea level are
almost identical.Comment: 4 pages, 3 figure
Shadowing of Ultrahigh Energy Neutrinos
The rise with energy of the neutrino--nucleon cross section implies that at
energies above few TeV the Earth is becoming opaque to cosmic neutrinos. The
neutrinos interact with the nucleons through the weak charged current,
resulting into absorption, and the weak neutral current, which provides a
redistribution of the neutrino energy. We Mellin transform the neutrino
transport equation and find its exact solution in the moment space. A simple
analytical formula is provided, which describes accurately the neutrino
spectrum, after the neutrinos have traversed the Earth. The effect of the weak
neutral current is most prominent for an initial flat neutrino spectrum and we
find that at low energies (around 1 TeV) the neutrino intensity is even
enhanced.Comment: gziped, tar file of LaTeX paper plus 2 postscript figures, 13 page
NEUTRINOS FROM PRIMORDIAL BLACK HOLES
The emission of particles from black holes created in the early Universe has
detectable astrophysical consequences. The most stringent bound on their
abundance has been obtained from the absence of a detectable diffuse flux of
100 MeV photons. Further scrutiny of these bounds is of interest as they, for
instance, rule out primordial black holes as a dark matter candidate. We here
point out that these bounds can, in principle, be improved by studying the
diffuse cosmic neutrino flux. Measurements of near-vertical atmospheric
neutrino fluxes in a region of low geomagnetic latitude can provide a
competitive bound. The most favorable energy to detect a possible diffuse flux
of primordial black hole origin is found to be a few MeV. We also show that
measurements of the diffuse flux is the most promising to improve
the existing bounds deduced from gamma-ray measurements. Neutrinos from
individual black hole explosions can be detected in the GeV-TeV energy region.
We find that the kilometer-scale detectors, recently proposed, are able to
establish competitive bounds.Comment: 19 pages plus 9 uuencoded and compressed postscript figure
Cosmic Rays from Decaying Vortons
The flux of high energy cosmic rays coming from the decay of vortons is
estimated. If the abundance of loops corresponding to a superconductivity scale
coincident with that of the string formation is corrected to be compatible with
the critical density of universe, it is found that the emission of one carrier
per vorton may produce a flux of one cosmic ray event per of detector
and per year.Comment: 8 pages, Latex, no figure
Soft Gluon kt-Resummation and the Froissart bound
We study soft gluon kt-resummation and the relevance of zero momentum gluons
for the energy dependence of total hadronic cross-sections. We discuss a model
in which consistency of the energy dependence of the cross-section with the
limitation of the Froissart bound, is directly related to the behaviour of the
strong coupling constant in the infrared region. Our predictions for the
asymptotic behaviour are shown to be related to the ansatz that the infrared
behaviour of the QCD strong coupling constant follows an inverse power law.Comment: To be published in Physics Letters B. This version is 25% shorter
than the previous one, as requested by the restrictions on the number of
pages for this Journal. Some equations have been skipped, some text has been
summarized. The earliest version may be useful for a better understanding of
some of the materia
Prompt atmospheric neutrinos and muons: dependence on the gluon distribution function
We compute the next-to-leading order QCD predictions for the vertical flux of
atmospheric muons and neutrinos from decays of charmed particles, for different
PDF's (MRS-R1, MRS-R2, CTEQ-4M and MRST) and different extrapolations of these
at small partonic momentum fraction x. We find that the predicted fluxes vary
up to almost two orders of magnitude at the largest energies studied, depending
on the chosen extrapolation of the PDF's. We show that the spectral index of
the atmospheric leptonic fluxes depends linearly on the slope of the gluon
distribution function at very small x. This suggests the possibility of
obtaining some bounds on this slope in ``neutrino telescopes'', at values of x
not reachable at colliders, provided the spectral index of atmospheric leptonic
fluxes could be determined.Comment: 20 pages including 8 figure
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