8,420 research outputs found
Dark Left-Right Model: CDMS, LHC, etc
The Standard Model of particle interactions is extended to include fermion
doublets (n,e)_R transforming under the gauge group SU(2)_R such that n is a
Dirac scotino (dark-matter fermion), with odd R parity. Based on recent CDMS
data, it is shown how this new dark left-right model (DLRM2) favors a Z' gauge
boson at around 1 or 2 TeV and be observable at the LHC. The new W_R gauge
bosons may also contribute significantly to lepton-flavor-changing processes
such as mu to e gamma and mu-e conversion in a nucleus or muonic atom.Comment: 7 pages, 4 figures, typo corrected, talk at Dark Side of Universe
2010, Leon, Mexic
Optimal uptake kinetics: physiological acclimation explains the pattern of nitrate uptake by phytoplankton in the ocean
Phytoplankton supply the base of the marine food web and drive the biogeochemical cycles of carbon and nutrients. Over much of the ocean, their growth is limited by their uptake of nitrogen (as nitrate), which has most commonly been described by the hyperbolic Michaelis-Menten (MM) equation. However, the lack of a theory to explain variations in MM constants has hindered our ability to predict the response of marine ecosystems to changes in environmental conditions. The MM equation fits data from short-term experiments well, but does not agree with steady-state experiments over wide ranges of nutrient concentrations. In contrast, the recently developed optimal uptake kinetics (OU) does agree with the latter and can also describe the observed pattern of MM half-saturation constants from field. experiments. OU kinetics explains the observed pattern of N uptake as the result of a general physiological trade-off between nutrient uptake capacity and affinity. The existence of a general trade-off would imply a relatively high degree of predictability in the response of nutrient uptake to changing nutrient concentrations and thus provide a basis for predicting effects of climate change on marine ecosystems and biogeochemical cycles
c-axis Raman Scattering in MgB2: Observation of a Dirty-Limit Gap in the pi-bands
Raman scattering spectra from the ac-face of thick MgB2 single crystals were
measured in zz, xz and xx polarisations. In zz and xz polarisations a threshold
at around 29 cm^{-1} forms in the below Tc continuum but no pair-breaking peak
is seen, in contrast to the sharp pair-breaking peak at around 100 cm^{-1} seen
in xx polarisation. The zz and xz spectra are consistent with Raman scattering
from a dirty superconductor while the sharp peak in the xx spectra argues for a
clean system. Analysis of the spectra resolves this contradiction, placing the
larger and smaller gap magnitudes in the sigma and pi bands, and indicating
that relatively strong impurity scattering is restricted to the pi bands.Comment: Revised manuscript accepted for publication in Physical Review
Letter
Magnetic and superconducting properties on S-type single-crystal CeCuSi probed by Cu nuclear magnetic resonance and nuclear quadrupole resonance
We have performed Cu nuclear magnetic resonance/nuclear quadrupole
resonance measurements to investigate the magnetic and superconducting (SC)
properties on a "superconductivity dominant" (-type) single crystal of
CeCuSi. Although the development of antiferromagnetic (AFM)
fluctuations down to 1~K indicated that the AFM criticality was close, Korringa
behavior was observed below 0.8~K, and no magnetic anomaly was observed above
0.6 K. These behaviors were expected in -type
CeCuSi. The temperature dependence of the nuclear spin-lattice
relaxation rate at zero field was almost identical to that in the
previous polycrystalline samples down to 130~mK, but the temperature dependence
deviated downward below 120~mK. In fact, in the SC state could be
fitted with the two-gap -wave rather than the two-gap -wave
model down to 90~mK. Under magnetic fields, the spin susceptibility in both
directions clearly decreased below , indicative of the formation of
spin singlet pairing. The residual part of the spin susceptibility was
understood by the field-induced residual density of states evaluated from
, which was ascribed to the effect of the vortex cores. No magnetic
anomaly was observed above the upper critical field , but the
development of AFM fluctuations was observed, indicating that superconductivity
was realized in strong AFM fluctuations.Comment: 10 pages, 8 figure
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