455 research outputs found
Higgs production as a probe of anomalous top couplings
The LHC may be currently seeing the first hints of the Higgs boson. The
dominant production mode for the Higgs at the LHC involves a top-quark loop. An
accurate measurement of Higgs production cross-sections and decay widths can
thus be used to obtain limits on anomalous top couplings. We find that such an
exercise could potentially yield constraints that are stronger than those
derived from low-energy observables as well as direct bounds expected from the
top pair-production process.Comment: Version published in JHE
Water induced sediment levitation enhances downslope transport on Mars
On Mars, locally warm surface temperatures (~293 K) occur, leading to the possibility of (transient) liquid water on the surface. However, water exposed to the martian atmosphere will boil, and the sediment transport capacity of such unstable water is not well understood. Here, we present laboratory studies of a newly recognized transport mechanism: âlevitationâ of saturated sediment bodies on a cushion of vapor released by boiling. Sediment transport where this mechanism is active is about nine times greater than without this effect, reducing the amount of water required to transport comparable sediment volumes by nearly an order of magnitude. Our calculations show that the effect of levitation could persist up to ~48 times longer under reduced martian gravity. Sediment levitation must therefore be considered when evaluating the formation of recent and present-day martian mass wasting features, as much less water may be required to form such features than previously thought
Astronomical Spectroscopy
Spectroscopy is one of the most important tools that an astronomer has for
studying the universe. This chapter begins by discussing the basics, including
the different types of optical spectrographs, with extension to the ultraviolet
and the near-infrared. Emphasis is given to the fundamentals of how
spectrographs are used, and the trade-offs involved in designing an
observational experiment. It then covers observing and reduction techniques,
noting that some of the standard practices of flat-fielding often actually
degrade the quality of the data rather than improve it. Although the focus is
on point sources, spatially resolved spectroscopy of extended sources is also
briefly discussed. Discussion of differential extinction, the impact of
crowding, multi-object techniques, optimal extractions, flat-fielding
considerations, and determining radial velocities and velocity dispersions
provide the spectroscopist with the fundamentals needed to obtain the best
data. Finally the chapter combines the previous material by providing some
examples of real-life observing experiences with several typical instruments.Comment: An abridged version of a chapter to appear in Planets, Stars and
Stellar Systems, to be published in 2011 by Springer. Slightly revise
The flavor puzzle in multi-Higgs models
We reconsider the flavor problem in the models with two Higgs doublets. By
studying two generation toy models, we look for flavor basis independent
constraints on Yukawa couplings that will give us the mass hierarchy while
keeping all Yukawa couplings of the same order. We then generalize our findings
to the full three generation Standard Model. We find that we need two
constraints on the Yukawa couplings to generate the observed mass hierarchy,
and a slight tuning of Yukawa couplings of order 10%, much less than the
Standard Model. We briefly study how these constraints can be realized, and
show how flavor changing currents are under control for mixing in
the near-decoupling limit.Comment: 26 pages, typos are corrected, references are added, the final
versio
Modular and predictable assembly of porous organic molecular crystals
Nanoporous molecular frameworks are important in applications such as separation, storage and catalysis. Empirical rules exist for their assembly but it is still challenging to place and segregate functionality in three-dimensional porous solids in a predictable way. Indeed, recent studies of mixed crystalline frameworks suggest a preference for the statistical distribution of functionalities throughout the pores rather than, for example, the functional group localization found in the reactive sites of enzymes. This is a potential limitation for 'one-pot' chemical syntheses of porous frameworks from simple starting materials. An alternative strategy is to prepare porous solids from synthetically preorganized molecular pores. In principle, functional organic pore modules could be covalently prefabricated and then assembled to produce materials with specific properties. However, this vision of mix-and-match assembly is far from being realized, not least because of the challenge in reliably predicting three-dimensional structures for molecular crystals, which lack the strong directional bonding found in networks. Here we show that highly porous crystalline solids can be produced by mixing different organic cage modules that self-assemble by means of chiral recognition. The structures of the resulting materials can be predicted computationally, allowing in silico materials design strategies. The constituent pore modules are synthesized in high yields on gram scales in a one-step reaction. Assembly of the porous co-crystals is as simple as combining the modules in solution and removing the solvent. In some cases, the chiral recognition between modules can be exploited to produce porous organic nanoparticles. We show that the method is valid for four different cage modules and can in principle be generalized in a computationally predictable manner based on a lock-and-key assembly between modules
Physics Opportunities of e+e- Linear Colliders
We describe the anticipated experimental program of an e+e- linear collider
in the energy range 500 GeV -- 1.5 TeV. We begin with a description of current
collider designs and the expected experimental environment. We then discuss
precision studies of the W boson and top quark. Finally, we review the range of
models proposed to explain the physics of electroweak symmetry breaking and
show, for each case, the central role that the linear collider experiments will
play in elucidating this physics. (to appear in Annual Reviews of Nuclear and
Particle Science)Comment: 93 pages, latex + 23 figures; typos corrections + 1 reference adde
A Profile Likelihood Analysis of the Constrained MSSM with Genetic Algorithms
The Constrained Minimal Supersymmetric Standard Model (CMSSM) is one of the
simplest and most widely-studied supersymmetric extensions to the standard
model of particle physics. Nevertheless, current data do not sufficiently
constrain the model parameters in a way completely independent of priors,
statistical measures and scanning techniques. We present a new technique for
scanning supersymmetric parameter spaces, optimised for frequentist profile
likelihood analyses and based on Genetic Algorithms. We apply this technique to
the CMSSM, taking into account existing collider and cosmological data in our
global fit. We compare our method to the MultiNest algorithm, an efficient
Bayesian technique, paying particular attention to the best-fit points and
implications for particle masses at the LHC and dark matter searches. Our
global best-fit point lies in the focus point region. We find many
high-likelihood points in both the stau co-annihilation and focus point
regions, including a previously neglected section of the co-annihilation region
at large m_0. We show that there are many high-likelihood points in the CMSSM
parameter space commonly missed by existing scanning techniques, especially at
high masses. This has a significant influence on the derived confidence regions
for parameters and observables, and can dramatically change the entire
statistical inference of such scans.Comment: 47 pages, 8 figures; Fig. 8, Table 7 and more discussions added to
Sec. 3.4.2 in response to referee's comments; accepted for publication in
JHE
Calculating the energy spectra of magnetic molecules: application of real- and spin-space symmetries
The determination of the energy spectra of small spin systems as for instance
given by magnetic molecules is a demanding numerical problem. In this work we
review numerical approaches to diagonalize the Heisenberg Hamiltonian that
employ symmetries; in particular we focus on the spin-rotational symmetry SU(2)
in combination with point-group symmetries. With these methods one is able to
block-diagonalize the Hamiltonian and thus to treat spin systems of
unprecedented size. In addition it provides a spectroscopic labeling by
irreducible representations that is helpful when interpreting transitions
induced by Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance
(NMR) or Inelastic Neutron Scattering (INS). It is our aim to provide the
reader with detailed knowledge on how to set up such a diagonalization scheme.Comment: 29 pages, many figure
An Exploration of the Relations between External Representations and Working Memory
It is commonly hypothesized that external representations serve as memory aids and improve task performance by means of expanding the limited capacity of working memory. However, very few studies have directly examined this memory aid hypothesis. By systematically manipulating how information is available externally versus internally in a sequential number comparison task, three experiments were designed to investigate the relation between external representations and working memory. The experimental results show that when the task requires information from both external representations and working memory, it is the interaction of information from the two sources that determines task performance. In particular, when information from the two sources does not match well, external representations hinder instead of enhance task performance. The study highlights the important role the coordination among different representations plays in distributed cognition. The general relations between external representations and working memory are discussed
Charged-Higgs phenomenology in the Aligned two-Higgs-doublet model
The alignment in flavour space of the Yukawa matrices of a general
two-Higgs-doublet model results in the absence of tree-level flavour-changing
neutral currents. In addition to the usual fermion masses and mixings, the
aligned Yukawa structure only contains three complex parameters, which are
potential new sources of CP violation. For particular values of these three
parameters all known specific implementations of the model based on discrete
Z_2 symmetries are recovered. One of the most distinctive features of the
two-Higgs-doublet model is the presence of a charged scalar. In this work, we
discuss its main phenomenological consequences in flavour-changing processes at
low energies and derive the corresponding constraints on the parameters of the
aligned two-Higgs-doublet model.Comment: 46 pages, 19 figures. Version accepted for publication in JHEP.
References added. Discussion slightly extended. Conclusions unchange
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