36,016 research outputs found
The 750 GeV Diphoton Excess as a First Light on Supersymmetry Breaking
One of the most exciting explanations advanced for the recent diphoton excess
found by ATLAS and CMS is in terms of sgoldstino decays: a signal of low-energy
supersymmetry-breaking scenarios. The sgoldstino, a scalar, couples directly to
gluons and photons, with strength related to gaugino masses, that can be of the
right magnitude to explain the excess. However, fitting the suggested resonance
width, Gamma ~ 45 GeV, is not so easy. In this paper we explore efficient
possibilities to enhance the sgoldstino width, via the decay into two Higgses,
two Higgsinos and through mixing between the sgoldstino and the Higgs boson. In
addition, we present an alternative and more efficient mechanism to generate a
mass splitting between the scalar and pseudoscalar components of the
sgoldstino, which has been suggested as an interesting alternative explanation
to the apparent width of the resonance.Comment: 14 pages, 3 figure
High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought.
Hotter, longer, and more frequent global change-type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought-induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean-type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water-use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation-type conversion
The Bak-Sneppen Model on Scale-Free Networks
We investigate by numerical simulations and analytical calculations the
Bak-Sneppen model for biological evolution in scale-free networks. By using
large scale numerical simulations, we study the avalanche size distribution and
the activity time behavior at nodes with different connectivities. We argue the
absence of a critical barrier and its associated critical behavior for infinite
size systems. These findings are supported by a single site mean-field analytic
treatment of the model.Comment: 5 pages and 3 eps figures. Final version appeared in Europhys. Let
Quantum Entanglement in -Spherium
There are very few systems of interacting particles (with continuous
variables) for which the entanglement of the concomitant eigenfunctions can be
computed in an exact, analytical way. Here we present analytical calculations
of the amount of entanglement exhibited by -states of \emph{spherium}. This
is a system of two particles (electrons) interacting via a Coulomb potential
and confined to a -sphere (that is, to the surface of a -dimensional
ball). We investigate the dependence of entanglement on the radius of the
system, on the spatial dimensionality , and on energy. We find that
entanglement increases monotonically with , decreases with , and also
tends to increase with the energy of the eigenstates. These trends are
discussed and compared with those observed in other two-electron atomic-like
models where entanglement has been investigated.Comment: 14 pages, 6 figures. J. Phys. A (2015). Accepte
Exploring Io's atmospheric composition with APEX: first measurement of 34SO2 and tentative detection of KCl
The composition of Io's tenuous atmosphere is poorly constrained. Only the
major species SO2 and a handful of minor species have been positively
identified, but a variety of other molecular species should be present, based
on thermochemical equilibrium models of volcanic gas chemistry and the
composition of Io's environment. This paper focuses on the spectral search for
expected yet undetected molecular species (KCl, SiO, S2O) and isotopes (34SO2).
We analyze a disk-averaged spectrum of a potentially line-rich spectral window
around 345 GHz, obtained in 2010 at the APEX-12m antenna (Atacama Pathfinder
EXperiment). Using different models assuming either extended atmospheric
distributions or a purely volcanically-sustained atmosphere, we tentatively
measure the KCl relative abundance with respect to SO2 and derive a range of
4x10^{-4}-8x10^{-3}. We do not detect SiO or S2O and present new upper limits
on their abundances. We also present the first measurement of the 34S/32S
isotopic ratio in gas phase on Io, which appears to be twice as high as the
Earth and ISM reference values. Strong lines of SO2 and SO are also analyzed to
check for longitudinal variations of column density and relative abundance. Our
models show that, based on their predicted relative abundance with respect to
SO2 in volcanic plumes, both the tentative KCl detection and SiO upper limit
are compatible with a purely volcanic origin for these species.Comment: Accepted for publication in ApJ. 11 pages, 4 figure
Terahertz surface plasmon polariton propagation and focusing on periodically corrugated metal wires
In this letter we show how the dispersion relation of surface plasmon
polaritons (SPPs) propagating along a perfectly conducting wire can be tailored
by corrugating its surface with a periodic array of radial grooves. In this
way, highly localized SPPs can be sustained in the terahertz region of the
electromagnetic spectrum. Importantly, the propagation characteristics of these
spoof SPPs can be controlled by the surface geometry, opening the way to
important applications such as energy concentration on cylindrical wires and
superfocusing using conical structures.Comment: accepted at PRL, submitted 29th May 200
Duality between Topologically Massive and Self-Dual models
We show that, with the help of a general BRST symmetry, different theories in
3 dimensions can be connected through a fundamental topological field theory
related to the classical limit of the Chern-Simons model.Comment: 13 pages, LaTe
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