56 research outputs found
Measurement of T\u3csub\u3ec\u3c/sub\u3e suppression in tungsten using magnetic impurities
We have measured the effects of dilute magnetic-atom doping on the superconducting transition temperature of tungsten thin films. Our âTc tuningâ technique is accurate, precise, and simple. Experiments were performed using dc-magnetron-sputtered tungsten films with undoped values of Tc in the range of 70â150 mK. The magnetic-atom doping was achieved using ion implantation. Specific Tc suppressions of between 5% and 65% were targeted and observed in this study. The transition width of each undoped sample was â1 mK and the transition widths remained sharp after implantation with 56Fe+ ions. Our data are in good agreement with predictions of a linear dependence of Tc suppression with increasing magnetic-atom concentration, in the small concentration limit. At higher concentrations, antiferromagnetic coupling between the magnetic dopant atoms becomes important and the Tc-suppression effect is diminished. We use our Tc data to calculate the AbrikosovâGorâkov (AG) and RudermanâKittelâKasuyaâYosida (RKKY) spinâflip relaxation parameters ÏAG and ÏRKKY. We conclude with a brief discussion of applications of the Tc-tuning technique, and present our plans for future studies in this area
Nucleon scattering with higgsino and wino cold dark matter
Neutralinos that are mostly wino or higgsino are shown to be compatible with
the recent DAMA annual modulation signal. The nucleon scattering rates for
these dark matter candidates are typically an order of magnitude above the
oft-considered bino. Although thermal evolution of higgsino and wino number
densities in the early universe implies that they are not viable dark matter
candidates, non-thermal sources, such as from gravitino or moduli decay in
anomaly mediated supersymmetry breaking, suggest that they can be the dominant
source of cold dark matter. Their stealthiness at high energy colliders gives
even more impetus to analyze nucleon scattering detection methods. We also
present calculations for their predicted scattering rate with Germanium
detectors, which have yet to see evidence of WIMP scattering.Comment: 16 pages, LaTex, 4 figures, uses feynMF, minor changes made for PRD
publicatio
Stable Neutral Fermi Ball
Fermi Ball is a kind of nontopological soliton with fermions trapped in its
domain wall, and is suggested to arises from the spontaneous symmetry breaking
of the approximate symmetry in the early universe. We find that the
neutral thin-wall Fermi Ball is stable in the limited region of the scalar
self-coupling constant and the Yukawa coupling constant . We find
that the Fermi Ball is stabilized due to the curvature effect of the domain
wall caused by the fermion sector. We also discuss whether such stable Fermi
Balls may contribute to the cold dark matter.Comment: 18 pages in RevTeX, 5 figure
Generalized Analysis of Weakly-Interacting Massive Particle Searches
We perform a generalized analysis of data from WIMP search experiments for
point-like WIMPs of arbitrary spin and general Lorenz-invariant WIMP-nucleus
interaction. We show that in the non-relativistic limit only spin-independent
(SI) and spin-dependent (SD) WIMP-nucleon interactions survive, which can be
parameterized by only five independent parameters. We explore this
five-dimensional parameter space to determine whether the annual modulation
observed in the DAMA experiment can be consistent with all other experiments.
The pure SI interaction is ruled out except for very small region of parameter
space with the WIMP mass close to 50 GeV and the ratio of the WIMP-neutron to
WIMP-proton SI couplings . For the predominantly SD
interaction, we find an upper limit to the WIMP mass of about 18 GeV, which can
only be weakened if the constraint stemming from null searches for energetic
neutrinos from WIMP annihilation the Sun is evaded. None of the regions of the
parameter space that can reconcile all WIMP search results can be easily
accommodated in the minimal supersymmetric extension of the standard model.Comment: 27 pages, 3 figure
Inelastic Dark Matter
Many observations suggest that much of the matter of the universe is
non-baryonic. Recently, the DAMA NaI dark matter direct detection experiment
reported an annual modulation in their event rate consistent with a WIMP relic.
However, the Cryogenic Dark Matter Search (CDMS) Ge experiment excludes most of
the region preferred by DAMA. We demonstrate that if the dark matter can only
scatter by making a transition to a slightly heavier state (Delta m ~ 100kev),
the experiments are no longer in conflict. Moreover, differences in the energy
spectrum of nuclear recoil events could distinguish such a scenario from the
standard WIMP scenario. Finally, we discuss the sneutrino as a candidate for
inelastic dark matter in supersymmetric theories.Comment: 20 pages, 6 figure
Predicting Neutron Production from Cosmic-ray Muons
Fast neutrons from cosmic-ray muons are an important background to
underground low energy experiments. The estimate of such background is often
hampered by the difficulty of measuring and calculating neutron production with
sufficient accuracy. Indeed substantial disagreement exists between the
different analytical calculations performed so far, while data reported by
different experiments is not always consistent. We discuss a new unified
approach to estimate the neutron yield, the energy spectrum, the multiplicity
and the angular distribution from cosmic muons using the Monte Carlo simulation
package FLUKA and show that it gives a good description of most of the existing
measurements once the appropriate corrections have been applied.Comment: 8 pages, 7 figure
WIMP Annual Modulation with Opposite Phase in Late-Infall Halo Models
We show that in the late-infall model of our galactic halo by P. Sikivie the
expected phase of the annual modulation of a WIMP halo signal in direct
detection experiments is opposite to the one usually expected. If a
non-virialized halo component due to the infall of (collisionless) dark matter
particles cannot be rejected, an annual modulation in a dark matter signal
should be looked for by experimenters without fixing the phase a-priori.
Moreover, WIMP streams coming to Earth from directions above and below the
galactic plane should be expected, with a characteristic pattern of arrival
directions.Comment: 15 pages, 5 figure
Neutron production by cosmic-ray muons at shallow depth
The yield of neutrons produced by cosmic ray muons at a shallow depth of 32
meters of water equivalent has been measured. The Palo Verde neutrino detector,
containing 11.3 tons of Gd loaded liquid scintillator and 3.5 tons of acrylic
served as a target. The rate of one and two neutron captures was determined.
Modeling the neutron capture efficiency allowed us to deduce the total yield of
neutrons neutrons per muon
and g/cm. This yield is consistent with previous measurements at similar
depths.Comment: 12 pages, 3 figure
The phase-space structure of a dark-matter halo: Implications for dark-matter direct detection experiments
We study the phase-space structure of a dark-matter halo formed in a high
resolution simulation of a Lambda CDM cosmology. Our goal is to quantify how
much substructure is left over from the inhomogeneous growth of the halo, and
how it may affect the signal in experiments aimed at detecting the dark matter
particles directly. If we focus on the equivalent of ``Solar vicinity'', we
find that the dark-matter is smoothly distributed in space. The probability of
detecting particles bound within dense lumps of individual mass less than 10^7
M_\sun h^{-1} is small, less than 10^{-2}. The velocity ellipsoid in the Solar
neighbourhood deviates only slightly from a multivariate Gaussian, and can be
thought of as a superposition of thousands of kinematically cold streams. The
motions of the most energetic particles are, however, strongly clumped and
highly anisotropic. We conclude that experiments may safely assume a smooth
multivariate Gaussian distribution to represent the kinematics of dark-matter
particles in the Solar neighbourhood. Experiments sensitive to the direction of
motion of the incident particles could exploit the expected anisotropy to learn
about the recent merging history of our Galaxy.Comment: 13 pages, 13 figures, Phys. Rev. D in press. Postscript version with
high resolution figures available from
http://www.mpa-garching.mpg.de/~ahelmi/research/lcdm_dm.html; some changes in
the text; constraints on the effect of bound dark-matter lumps revised;
remaining conclusions unchange
Neutrino Telescopes' Sensitivity to Dark Matter
The nature of the dark matter of the Universe is yet unknown and most likely
is connected with new physics. The search for its composition is under way
through direct and indirect detection. Fundamental physical aspects such as
energy threshold, geometry and location are taken into account to investigate
proposed neutrino telescopes of km^3 volume sensitivities to dark matter. These
sensitivities are just sufficient to test a few WIMP scenarios. Telescopes of
km^3 volume, such as IceCube, can definitely discover or exclude superheavy (M
> 10^10 GeV) Strong Interacting Massive Particles (Simpzillas). Smaller
neutrino telescopes such as ANTARES, AMANDA-II and NESTOR can probe a large
region of the Simpzilla parameter space.Comment: 28 pages, 9 figure
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