240 research outputs found
CsI(Tl) for WIMP dark matter searches
We report a study of CsI(Tl) scintillator to assess its applicability in
experiments to search for dark matter particles. Measurements of the mean
scintillation pulse shapes due to nuclear and electron recoils have been
performed. We find that, as with NaI(Tl), pulse shape analysis can be used to
discriminate between electron and nuclear recoils down to 4 keV. However, the
discrimination factor is typically (10-15)% better than in NaI(Tl) above 4 keV.
The quenching factor for caesium and iodine recoils was measured and found to
increase from 11% to ~17% with decreasing recoil energy from 60 to 12 keV.
Based on these results, the potential sensitivity of CsI(Tl) to dark matter
particles in the form of neutralinos was calculated. We find an improvement
over NaI(Tl) for the spin independent WIMP-nucleon interactions up to a factor
of 5 assuming comparable electron background levels in the two scintillators.Comment: 16 pages, 8 figures, to be published in Nucl. Instrum. and Meth. in
Phys. Res.
Adaptive Mesh Refinement for Characteristic Grids
I consider techniques for Berger-Oliger adaptive mesh refinement (AMR) when
numerically solving partial differential equations with wave-like solutions,
using characteristic (double-null) grids. Such AMR algorithms are naturally
recursive, and the best-known past Berger-Oliger characteristic AMR algorithm,
that of Pretorius & Lehner (J. Comp. Phys. 198 (2004), 10), recurses on
individual "diamond" characteristic grid cells. This leads to the use of
fine-grained memory management, with individual grid cells kept in
2-dimensional linked lists at each refinement level. This complicates the
implementation and adds overhead in both space and time.
Here I describe a Berger-Oliger characteristic AMR algorithm which instead
recurses on null \emph{slices}. This algorithm is very similar to the usual
Cauchy Berger-Oliger algorithm, and uses relatively coarse-grained memory
management, allowing entire null slices to be stored in contiguous arrays in
memory. The algorithm is very efficient in both space and time.
I describe discretizations yielding both 2nd and 4th order global accuracy.
My code implementing the algorithm described here is included in the electronic
supplementary materials accompanying this paper, and is freely available to
other researchers under the terms of the GNU general public license.Comment: 37 pages, 15 figures (40 eps figure files, 8 of them color; all are
viewable ok in black-and-white), 1 mpeg movie, uses Springer-Verlag svjour3
document class, includes C++ source code. Changes from v1: revised in
response to referee comments: many references added, new figure added to
better explain the algorithm, other small changes, C++ code updated to latest
versio
Measurements of Scintillation Efficiency and Pulse-Shape for Low Energy Recoils in Liquid Xenon
Results of observations of low energy nuclear and electron recoil events in
liquid xenon scintillator detectors are given. The relative scintillation
efficiency for nuclear recoils is 0.22 +/- 0.01 in the recoil energy range 40
keV - 70 keV. Under the assumption of a single dominant decay component to the
scintillation pulse-shape the log-normal mean parameter T0 of the maximum
likelihood estimator of the decay time constant for 6 keV < Eee < 30 keV
nuclear recoil events is equal to 21.0 ns +/- 0.5 ns. It is observed that for
electron recoils T0 rises slowly with energy, having a value ~ 30 ns at Eee ~
15 keV. Electron and nuclear recoil pulse-shapes are found to be well fitted by
single exponential functions although some evidence is found for a double
exponential form for the nuclear recoil pulse-shape.Comment: 11 pages, including 5 encapsulated postscript figure
The MACHO Project 2nd Year LMC Microlensing Results and Dark Matter Implications
The MACHO Project is searching for galactic dark matter in the form of
massive compact halo objects (Machos). Millions of stars in the Large
Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge are
photometrically monitored in an attempt to detect rare gravitational
microlensing events caused by otherwise invisible Machos. Analysis of two years
of photometry on 8.5 million stars in the LMC reveals 8 candidate microlensing
events, far more than the event expected from lensing by low-mass stars
in known galactic populations. From these eight events we estimate the optical
depth towards the LMC from events with 2 < \that < 200 days to be
\tau_2^{200} \approx 2.9 ^{+1.4}_{-0.9} \ten{-7}. This exceeds the optical
depth of 0.5\ten{-7} expected from known stars and is to be compared with an
optical depth of 4.7\ten{-7} predicted for a ``standard'' halo composed
entirely of Machos. The total mass in this lensing population is \approx
2^{+1.2}_{-0.7} \ten{11} \msun (within 50 kpc from the Galactic center). Event
timescales yield a most probable Macho mass of 0.5^{+0.3}_{-0.2}\msun,
although this value is quite model dependent.Comment: 10 pages, 6 epsf figures and style file included, 451k, also at
http://wwwmacho.mcmaster.ca/Pubs/Pubs.html; To appear in the Proceedings of
"Sources and Detection of Dark Matter in the Universe", Santa Monica, CA,
Feb., 199
A Binary Lensing Event Toward the LMC: Observations and Dark Matter Implications
The MACHO collaboration has recently analyzed 2.1 years of photometric data
for about 8.5 million stars in the Large Magellanic Cloud (LMC). This analysis
has revealed 8 candidate microlensing events and a total microlensing optical
depth of . This significantly
exceeds the number of events (1.1) and the microlensing optical depth predicted
from known stellar populations: , but it is
consistent with models in which about half of the standard dark halo mass is
composed of Machos of mass \sim 0.5 \msun. One of these 8 events appears to
be a binary lensing event with a caustic crossing that is partially resolved
which allows us to estimate the distance to the lenses. If the source star is
not a short period binary star, then we show that the lens system is very
likely to reside in the LMC. However, if we assume that the optical depth for
LMC-LMC lensing is large enough to account for our entire lensing signal, then
the binary event does not appear to be consistent with lensing of a single LMC
source star by a binary residing in the LMC. Thus, while the binary lens may
indeed reside in the LMC, there is no indication that most of the lenses reside
in the LMC.Comment: 5 pages, 3 postscript figures included; To appear in the Proceedings
of the Dark Matter '96 Conference held in Santa Monica, CA, Feb., 199
Energy Flow in the Hadronic Final State of Diffractive and Non-Diffractive Deep-Inelastic Scattering at HERA
An investigation of the hadronic final state in diffractive and
non--diffractive deep--inelastic electron--proton scattering at HERA is
presented, where diffractive data are selected experimentally by demanding a
large gap in pseudo --rapidity around the proton remnant direction. The
transverse energy flow in the hadronic final state is evaluated using a set of
estimators which quantify topological properties. Using available Monte Carlo
QCD calculations, it is demonstrated that the final state in diffractive DIS
exhibits the features expected if the interaction is interpreted as the
scattering of an electron off a current quark with associated effects of
perturbative QCD. A model in which deep--inelastic diffraction is taken to be
the exchange of a pomeron with partonic structure is found to reproduce the
measurements well. Models for deep--inelastic scattering, in which a
sizeable diffractive contribution is present because of non--perturbative
effects in the production of the hadronic final state, reproduce the general
tendencies of the data but in all give a worse description.Comment: 22 pages, latex, 6 Figures appended as uuencoded fil
A Search for Selectrons and Squarks at HERA
Data from electron-proton collisions at a center-of-mass energy of 300 GeV
are used for a search for selectrons and squarks within the framework of the
minimal supersymmetric model. The decays of selectrons and squarks into the
lightest supersymmetric particle lead to final states with an electron and
hadrons accompanied by large missing energy and transverse momentum. No signal
is found and new bounds on the existence of these particles are derived. At 95%
confidence level the excluded region extends to 65 GeV for selectron and squark
masses, and to 40 GeV for the mass of the lightest supersymmetric particle.Comment: 13 pages, latex, 6 Figure
Baryons: What, When and Where?
We review the current state of empirical knowledge of the total budget of
baryonic matter in the Universe as observed since the epoch of reionization.
Our summary examines on three milestone redshifts since the reionization of H
in the IGM, z = 3, 1, and 0, with emphasis on the endpoints. We review the
observational techniques used to discover and characterize the phases of
baryons. In the spirit of the meeting, the level is aimed at a diverse and
non-expert audience and additional attention is given to describe how space
missions expected to launch within the next decade will impact this scientific
field.Comment: Proceedings Review for "Astrophysics in the Next Decade: JWST and
Concurrent Facilities", ed. X. Tielens, 38 pages, 10 color figures. Revised
to address comments from the communit
Evolution of Robustness and Plasticity under Environmental Fluctuation: Formulation in Terms of Phenotypic Variances
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