643 research outputs found
The Ground Sloth, Megalonyx, from Pleistocene Deposits of the Old Crow Basin, Yukon, Canada
The bear-sized ground sloth Megalonyx, endemic to North America, was widespread during the Pleistocene, reaching as far north as Alaska, Yukon, and Northwest Territories. Twenty-two specimens collected from 10 localities in the Old Crow Basin, northern Yukon, include several bones that can be referred to Jefferson's ground sloth (Megalonyx jeffersonii) on the basis of the distinctive morphology of the upper and lower caniniform teeth. All of the Yukon specimens are relatively small, suggesting a geological age earlier than Wisconsinan - probably Sangamonian. The Alaskan, Yukon, and Northwest Territories records imply that the species occupied a broad east-west range in northwestern North America during a warm phase of the late Pleistocene. Le paresseux marcheur Megalonyx, de la taille d'un ours, qui était endémique à l'Amérique du Nord, occupait au pléistocène une vaste aire s'étendant au Nord jusqu'en Alaska, au Yukon et aux Territoires du Nord-Ouest. Vingt-deux spécimens recueillis à 10 emplacements dans le bassin de Old Crow (partie nord du Yukon), comprennent plusieurs os que l'on peut attribuer au paresseux marcheur de Jefferson (Megalonyx jeffersonii) si l'on se fie à la morphologie particulière des dents supérieures et inférieures en forme de canines. Tous les spécimens du Yukon ont une taille relativement petite, ce qui suggère qu'ils datent d'un âge géologique antérieur au wisconsinien - probablement le sangamonien. Les relevés faits en Alaska, au Yukon et dans les Territoires du Nord-Ouest laissent entendre que l'espèce occupait une vaste aire s'étendant d'est en ouest dans la partie nord-occidentale de l'Amérique du Nord durant une phase de réchauffement du pléistocène tardif.
Electroweak Symmetry Breaking induced by Dark Matter
The mechanism behind Electroweak Symmetry Breaking (EWSB) and the nature of
dark matter (DM) are currently among the most important issues in high energy
physics. Since a natural dark matter candidate is a weakly interacting massive
particle or WIMP, with mass around the electroweak scale, it is clearly of
interest to investigate the possibility that DM and EWSB are closely related.
In the context of a very simple extension of the Standard Model, the Inert
Doublet Model, we show that dark matter could play a crucial role in the
breaking of the electroweak symmetry. In this model, dark matter is the
lightest component of an inert scalar doublet. The coupling of the latter with
the Standard Model Higgs doublet breaks the electroweak symmetry at one-loop,
"a la Coleman-Weinberg". The abundance of dark matter, the breaking of the
electroweak symmetry and the constraints from electroweak precision
measurements can all be accommodated by imposing an (exact or approximate)
custodial symmetry.Comment: 4 pages, no figure, one tabl
Footprints preserve terminal Pleistocene hunt? Human-sloth interactions in North America
Predator-prey interactions revealed by vertebrate trace fossils are extremely rare. We present footprint evidence
from White Sands National Monument in New Mexico for the association of sloth and human trackways.
Geologically, the sloth and human trackways were made contemporaneously, and the sloth trackways show
evidence of evasion and defensive behavior when associated with human tracks. Behavioral inferences from
these trackways indicate prey selection and suggest that humans were harassing, stalking, and/or hunting
the now-extinct giant ground sloth in the terminal Pleistocene
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A 15-T Pulsed Solenoid for a High-Power Target Experiment
The MERIT experiment, which ran at CERN in 2007, is a proof-of-principle test for a target system that converts a 4-MW proton beam into a high-intensity muon beam for either a neutrino factory complex or a muon collider. The target system is based on a free mercury jet that intercepts an intense proton beam inside a 15-T solenoidal magnetic field. Here, we describe the design and performance of the 15-T, liquid-nitrogen-precooled, copper solenoid magnet
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Experimental studies of targets and collimators for high intensity beams
First Measurement of Z/gamma* Production in Compton Scattering of Quasi-real Photons
We report the first observation of Z/gamma* production in Compton scattering
of quasi-real photons. This is a subprocess of the reaction e+e- to
e+e-Z/gamma*, where one of the final state electrons is undetected.
Approximately 55 pb-1 of data collected in the year 1997 at an e+e-
centre-of-mass energy of 183 GeV with the OPAL detector at LEP have been
analysed. The Z/gamma* from Compton scattering has been detected in the
hadronic decay channel. Within well defined kinematic bounds, we measure the
product of cross-section and Z/gamma* branching ratio to hadrons to be
(0.9+-0.3+-0.1) pb for events with a hadronic mass larger than 60 GeV,
dominated by (e)eZ production. In the hadronic mass region between 5 GeV and 60
GeV, dominated by (e)egamma* production, this product is found to be
(4.1+-1.6+-0.6) pb. Our results agree with the predictions of two Monte Carlo
event generators, grc4f and PYTHIA.Comment: 18 pages, LaTeX, 5 eps figures included, submitted to Physics Letters
Single Spin Asymmetry in Polarized Proton-Proton Elastic Scattering at GeV
We report a high precision measurement of the transverse single spin
asymmetry at the center of mass energy GeV in elastic
proton-proton scattering by the STAR experiment at RHIC. The was measured
in the four-momentum transfer squared range \GeVcSq, the region of a significant interference between the
electromagnetic and hadronic scattering amplitudes. The measured values of
and its -dependence are consistent with a vanishing hadronic spin-flip
amplitude, thus providing strong constraints on the ratio of the single
spin-flip to the non-flip amplitudes. Since the hadronic amplitude is dominated
by the Pomeron amplitude at this , we conclude that this measurement
addresses the question about the presence of a hadronic spin flip due to the
Pomeron exchange in polarized proton-proton elastic scattering.Comment: 12 pages, 6 figure
Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV
We report a measurement of the longitudinal double-spin asymmetry A_LL and
the differential cross section for inclusive Pi0 production at midrapidity in
polarized proton collisions at sqrt(s) = 200 GeV. The cross section was
measured over a transverse momentum range of 1 < p_T < 17 GeV/c and found to be
in good agreement with a next-to-leading order perturbative QCD calculation.
The longitudinal double-spin asymmetry was measured in the range of 3.7 < p_T <
11 GeV/c and excludes a maximal positive gluon polarization in the proton. The
mean transverse momentum fraction of Pi0's in their parent jets was found to be
around 0.7 for electromagnetically triggered events.Comment: 6 pages, 3 figures, submitted to Phys. Rev. D (RC
High non-photonic electron production in + collisions at = 200 GeV
We present the measurement of non-photonic electron production at high
transverse momentum ( 2.5 GeV/) in + collisions at
= 200 GeV using data recorded during 2005 and 2008 by the STAR
experiment at the Relativistic Heavy Ion Collider (RHIC). The measured
cross-sections from the two runs are consistent with each other despite a large
difference in photonic background levels due to different detector
configurations. We compare the measured non-photonic electron cross-sections
with previously published RHIC data and pQCD calculations. Using the relative
contributions of B and D mesons to non-photonic electrons, we determine the
integrated cross sections of electrons () at 3 GeV/10 GeV/ from bottom and charm meson decays to be = 4.0({\rm
stat.})({\rm syst.}) nb and =
6.2({\rm stat.})({\rm syst.}) nb, respectively.Comment: 17 pages, 17 figure
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