3,097 research outputs found
Characteristics of coastline changes in mainland China since the early 1940s
Based on multi-temporal topographic maps, remote sensing images and field surveys covering the entire coastal zone of mainland China, the coastlines of six periods since the early 1940s were extracted. Coastline changes over the last 70 years were then analyzed in terms of coastline structure, coastline fractals, coastline change rates, land-sea patterns, and bay areas. The results showed that mainland coastline structure changed dramatically, and due to the significant coastline artificialization mainly driven by sea reclamation and coastal engineering, the remaining natural coastline merely accounts for less than one third at present. Coastline fractal dimension represented an overall spatial pattern of "north < entirety < south"; however, the discrepancy between the north and south coast was apparently narrowed due to dramatic coastline artificialization of northern China which in turn altered the whole pattern. Patterns and processes of land-sea interchange along the mainland coast were complex and varied spatially and temporally, with over 68% advancing toward sea and 22% retreating toward land. The net growth of land area was nearly 14.2 x10(3) km(2) with an average growth rate of 202.82 km(2) a(-1); and coast retreat was characterized by area decrease of 93 bays with a magnitude of 10.1 x10(3) km(2) and an average shrinking rate up to 18.19% or an average shrinking speed up to 144.20 km(2) a(-1), among which the total area of Bohai shrunk by 7.06%, with an average annual loss amounting to 82 km(2). The dramatic coastline changes along mainland China have brought about kinds of challenges to the coastal environment, therefore the integrated management, effective environment protection and sustainable utilization of coastlines is urgent
Gas accretion as the origin of chemical abundance gradients in distant galaxies
It has recently been suggested that galaxies in the early Universe can grow
through the accretion of cold gas, and that this may have been the main driver
of star formation and stellar mass growth. Because the cold gas is essentially
primordial, it has a very low abundance of elements heavier than helium
(metallicity). As it is funneled to the centre of a galaxy, it will lead the
central gas having an overall lower metallicity than gas further from the
centre, because the gas further out has been enriched by supernovae and stellar
winds, and not diluted by the primordial gas. Here we report chemical
abundances across three rotationally-supported star-forming galaxies at z~3,
only 2 Gyr after the Big Bang. We find an 'inverse' gradient, with the central,
star forming regions having a lower metallicity than less active ones, opposite
to what is seen in local galaxies. We conclude that the central gas has been
diluted by the accretion of primordial gas, as predicted by 'cold flow' models.Comment: To Appear in Nature Oct 14, 2010; Supplementary Information included
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Viability of MSSM scenarios at very large tan(beta)
We investigate the MSSM with very large tan(beta) > 50, where the fermion
masses are strongly affected by loop-induced couplings to the "wrong" Higgs,
imposing perturbative Yukawa couplings and constraints from flavour physics.
Performing a low-energy scan of the MSSM with flavour-blind soft terms, we find
that the branching ratio of B->tau nu and the anomalous magnetic moment of the
muon are the strongest constraints at very large tan(beta) and identify the
viable regions in parameter space. Furthermore we determine the scale at which
the perturbativity of the Yukawa sector breaks down, depending on the
low-energy MSSM parameters. Next, we analyse the very large tan(beta) regime of
General Gauge Mediation (GGM) with a low mediation scale. We investigate the
requirements on the parameter space and discuss the implied flavour
phenomenology. We point out that the possibility of a vanishing Bmu term at a
mediation scale M = 100 TeV is challenged by the experimental data on B->tau nu
and the anomalous magnetic moment of the muon.Comment: 29 pages, 7 figures. v2: discussion in sections 1 and 4 expanded,
conclusions unchanged. Matches version published in JHE
Measuring V_ub and probing SUSY with double ratios of purely leptonic decays of B and D mesons
The experimental prospects for precise measurements of the leptonic decays
B_u -> tau nu / mu nu, B_s -> mu+ mu-, D -> mu nu and D_s -> mu nu / tau nu are
very promising. Double ratios involving four of these decays can be defined in
which the dependence on the values of the decay constants is essentially
eliminated, thus enabling complementary measurements of the CKM matrix element
V_ub with a small theoretical error. We quantify the experimental error in a
possible future measurement of |V_ub| using this approach, and show that it is
competitive with the anticipated precision from the conventional approaches.
Moreover, it is shown that such double ratios can be more effective than the
individual leptonic decays as a probe of the parameter space of supersymmetric
models. We emphasize that the double ratios have the advantage of using |V_ub|
as an input parameter (for which there is experimental information), while the
individual decays have an uncertainty from the decay constants (e.g. f_B_s),
and hence a reliance on theoretical techniques such as lattice QCD.Comment: 21 pages, 4 figure
Temperature Dependence of Photoelectrical Properties of Single Selenium Nanowires
Influence of temperature on photoconductivity of single Se nanowires has been studied. Time response of photocurrent at both room temperature and low temperature suggests that the trap states play an important role in the photoelectrical process. Further investigations about light intensity dependence on photocurrent at different temperatures reveal that the trap states significantly affect the carrier generation and recombination. This work may be valuable for improving the device optoelectronic performances by understanding the photoelectrical properties
On the nature of the fourth generation neutrino and its implications
We consider the neutrino sector of a Standard Model with four generations.
While the three light neutrinos can obtain their masses from a variety of
mechanisms with or without new neutral fermions, fourth-generation neutrinos
need at least one new relatively light right-handed neutrino. If lepton number
is not conserved this neutrino must have a Majorana mass term whose size
depends on the underlying mechanism for lepton number violation. Majorana
masses for the fourth generation neutrinos induce relative large two-loop
contributions to the light neutrino masses which could be even larger than the
cosmological bounds. This sets strong limits on the mass parameters and mixings
of the fourth generation neutrinos.Comment: To be published. Few typos corrected, references update
Flavour physics constraints in the BMSSM
We study the implications of the presence of the two leading-order,
non-renormalizable operators in the Higgs sector of the MSSM to flavour physics
observables. We identify the constraints of flavour physics on the parameters
of the BMSSM when we: a) focus on a region of parameters for which electroweak
baryogenesis is feasible, b) use a CMSSM-like parametrization, and c) consider
the case of a generic NUHM-type model. We find significant differences as
compared to the standard MSSM case.Comment: 22 pages, 7 figure
The decay Bs -> mu+ mu-: updated SUSY constraints and prospects
We perform a study of the impact of the recently released limits on BR(Bs ->
mu+ mu-) by LHCb and CMS on several SUSY models. We show that the obtained
constraints can be superior to those which are derived from direct searches for
SUSY particles in some scenarios, and the use of a double ratio of purely
leptonic decays involving Bs -> mu+ mu- can further strengthen such
constraints. We also discuss the experimental sensitivity and prospects for
observation of Bs -> mu+ mu- during the sqrt(s)=7 TeV run of the LHC, and its
potential implications.Comment: 30 pages, 21 figures. v2: Improved discussion of constraints from B
-> tau nu, references adde
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