2,384 research outputs found
Cosmological Adaptive Mesh Refinement
We describe a grid-based numerical method for 3D hydrodynamic cosmological
simulations which is adaptive in space and time and combines the best features
of higher order--accurate Godunov schemes for Eulerian hydrodynamics with
adaptive particle--mesh methods for collisionless particles. The basis for our
method is the structured adaptive mesh refinement (AMR) algorithm of Berger &
Collela (1989), which we have extended to cosmological hydro + N-body
simulations. The resulting multiscale hybrid method is a powerful alternative
to particle-based methods in current use. The choices we have made in
constructing this algorithm are discussed, and its performance on the Zeldovich
pancake test problem is given. We present a sample application of our method to
the problem of first structure formation. We have achieved a spatial dynamic
range in a 3D multispecies gas + dark matter
calculation, which is sufficient to resolve the formation of primordial
protostellar cloud cores starting from linear matter fluctuations in an
expanding FRW universe.Comment: 14 pages, 3 figures (incl. one large color PS) to appear in
"Numerical Astrophysics 1998", eds. S. Miyama & K. Tomisaka, Tokyo, March
10-13, 199
R-parity violating resonant stop production at the Large Hadron Collider
We have investigated the resonant production of a stop at the Large Hadron
Collider, driven by baryon number violating interactions in supersymmetry. We
work in the framework of minimal supergravity models with the lightest
neutralino being the lightest supersymmetric particle which decays within the
detector. We look at various dilepton and trilepton final states, with or
without b-tags. A detailed background simulation is performed, and all possible
decay modes of the lighter stop are taken into account. We find that higher
stop masses are sometimes easier to probe, through the decay of the stop into
the third or fourth neutralino and their subsequent cascades. We also comment
on the detectability of such signals during the 7 TeV run, where, as expected,
only relatively light stops can be probed. Our conclusion is that the resonant
process may be probed, at both 10 and 14 TeV, with the R-parity violating
coupling {\lambda}"_{312} as low as 0.05, for a stop mass of about 1 TeV. The
possibility of distinguishing between resonant stop production and
pair-production is also discussed.Comment: 20 pages, 4 figures, 6 tables; Version accepted by JHE
Evidence for reversible control of magnetization in a ferromagnetic material via spin-orbit magnetic field
Conventional computer electronics creates a dichotomy between how information
is processed and how it is stored. Silicon chips process information by
controlling the flow of charge through a network of logic gates. This
information is then stored, most commonly, by encoding it in the orientation of
magnetic domains of a computer hard disk. The key obstacle to a more intimate
integration of magnetic materials into devices and circuit processing
information is a lack of efficient means to control their magnetization. This
is usually achieved with an external magnetic field or by the injection of
spin-polarized currents. The latter can be significantly enhanced in materials
whose ferromagnetic properties are mediated by charge carriers. Among these
materials, conductors lacking spatial inversion symmetry couple charge currents
to spin by intrinsic spin-orbit (SO) interactions, inducing nonequilibrium spin
polarization tunable by local electric fields. Here we show that magnetization
of a ferromagnet can be reversibly manipulated by the SO-induced polarization
of carrier spins generated by unpolarized currents. Specifically, we
demonstrate domain rotation and hysteretic switching of magnetization between
two orthogonal easy axes in a model ferromagnetic semiconductor.Comment: 10 pages including supplemental materia
Higgs boson enhancement effects on squark-pair production at the LHC
We study the Higgs boson effects on third-generation squark-pair production
in proton-proton collision at the CERN Large Hadron Collider (LHC), including
\Stop \Stop^*, \Stop\Sbot^*, and \Sbot \Sbot^*. We found that substantial
enhancement can be obtained through s-channel exchanges of Higgs bosons at
large , at which the enhancement mainly comes from , , and initial states. We compute the complete set of electroweak
(EW) contributions to all production channels. This completes previous
computations in the literature. We found that the EW contributions can be
significant and can reach up to 25% in more general scenarios and at the
resonance of the heavy Higgs boson. The size of Higgs enhancement is comparable
or even higher than the PDF uncertainties and so must be included in any
reliable analysis. A full analytical computation of all the EW contributions is
presented.Comment: 23 pages, 7 figures, 1 tabl
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Can preferential credit programs speed up the adoption of low-carbon agricultural systems in Mato Grosso, Brazil? Results from bioeconomic microsimulation
The need to balance agricultural production and environmental protection shifted the focus of Brazilian
land-use policy toward sustainable agriculture. In 2010,
Brazil established preferential credit lines to finance
investments into low-carbon integrated agricultural systems
of crop, livestock and forestry. This article presents a
simulation-based empirical assessment of integrated system
adoption in the state of Mato Grosso, where highly
mechanized soybean–cotton and soybean–maize doublecrop
systems currently prevail. We employ bioeconomic modeling to explicitly capture the heterogeneity of farm level costs and benefits of adoption. By parameterizing and validating our simulations with both empirical and experimental data, we evaluate the effectiveness of the ABC Integration credit through indicators such as land-use change, adoption rates and budgetary costs of credit provision. Alternative scenarios reveal that specific credit conditions might speed up the diffusion of low-carbon agricultural systems in Mato Grosso
Constraints on supersymmetry with light third family from LHC data
We present a re-interpretation of the recent ATLAS limits on supersymmetry in
channels with jets (with and without b-tags) and missing energy, in the context
of light third family squarks, while the first two squark families are
inaccessible at the 7 TeV run of the Large Hadron Collider (LHC). In contrast
to interpretations in terms of the high-scale based constrained minimal
supersymmetric standard model (CMSSM), we primarily use the low-scale
parametrisation of the phenomenological MSSM (pMSSM), and translate the limits
in terms of physical masses of the third family squarks. Side by side, we also
investigate the limits in terms of high-scale scalar non-universality, both
with and without low-mass sleptons. Our conclusion is that the limits based on
0-lepton channels are not altered by the mass-scale of sleptons, and can be
considered more or less model-independent.Comment: 20 pages, 8 figures, 2 tables. Version published in JHE
Giant QCD K-factors beyond NLO
Hadronic observables in Z+jet events can be subject to large NLO corrections
at TeV scales, with K-factors that even reach values of order 50 in some cases.
We develop a method, LoopSim, by which approximate NNLO predictions can be
obtained for such observables, supplementing NLO Z+jet and NLO Z+2-jet results
with a unitarity-based approximation for missing higher loop terms. We first
test the method against known NNLO results for Drell-Yan lepton pt spectra. We
then show our approximate NNLO results for the Z+jet observables. Finally we
examine whether the LoopSim method can provide useful information even in cases
without giant K-factors, with results for observables in dijet events that can
be compared to early LHC data.Comment: 38 pages, 13 figures; v2 includes additional reference
Multiple Parton Interactions in Z+jets production at the LHC. A comparison of factorized and non--factorized double parton distribution functions
We examine the contribution of Multiple Parton Interactions to Z+n-jets
production at the LHC, n=2,3,4, where the Z boson is assumed to decay
leptonically. We compare the results obtained with the correlated GS09 double
parton distribution function with those obtained with two instances of fully
factorized single parton distribution functions: MSTW2008LO and CTEQ6LO. It
appears quite feasible to measure the MPI contribution to Z+2/3/4 jets already
in the first phase of the LHC with a total luminosity of one inverse femtobarn
at 7 TeV. If as expected the trigger threshold for single photons is around 80
GeV, Z+2-jets production may well turn out to be more easily observable than
the gamma+3-jets channel. The MPI cross section is dominated by relatively soft
events with two jets balancing in transverse momentum.Comment: 15 pages, 3 plot
Next-to-leading order QCD corrections to Higgs boson production in association with a photon via weak-boson fusion at the LHC
Higgs boson production in association with a hard central photon and two
forward tagging jets is expected to provide valuable information on Higgs boson
couplings in a range where it is difficult to disentangle weak-boson fusion
processes from large QCD backgrounds. We present next-to-leading order QCD
corrections to Higgs production in association with a photon via weak-boson
fusion at a hadron collider in the form of a flexible parton-level Monte Carlo
program. The QCD corrections to integrated cross sections are found to be small
for experimentally relevant selection cuts, while the shape of kinematic
distributions can be distorted by up to 20% in some regions of phase space.
Residual scale uncertainties at next-to-leading order are at the few-percent
level.Comment: 17 pages, 7 figures, 1 tabl
Goldstone Bosons in Effective Theories with Spontaneously Broken Flavour Symmetry
The Flavour Symmetry of the Standard Model (SM) gauge sector is broken by the
fermion Yukawa couplings. Promoting the Yukawa matrices to scalar spurion
fields, one can break the flavour symmetry spontaneously by giving appropriate
vacuum expectation values (VEVs) to the spurion fields, and one encounters
Goldstone modes for every broken flavour symmetry generator. In this paper, we
point out various aspects related to the possible dynamical interpretation of
the Goldstone bosons: (i) In an effective-theory framework with local flavour
symmetry, the Goldstone fields represent the longitudinal modes for massive
gauge bosons. The spectrum of the latter follows the sequence of
flavour-symmetry breaking related to the hierarchies in Yukawa couplings and
flavour mixing angles. (ii) Gauge anomalies can be consistently treated by
adding higher-dimensional operators. (iii) Leaving the U(1) factors of the
flavour symmetry group as global symmetries, the respective Goldstone modes
behave as axions which can be used to resolve the strong CP problem by a
modified Peccei-Quinn mechanism. (iv) The dynamical picture of flavour symmetry
breaking implies new sources of flavour-changing neutral currents, which arise
from integrating out heavy scalar spurion fields and heavy gauge bosons. The
coefficients of the effective operators follow the minimal-flavour violation
principle.Comment: 27 pages, abstract and introduction extended, more detailed
discussion of heavy gauge boson spectrum and auxiliary heavy fermions,
outline restructured. Matches version to be published in JHE
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