2,131 research outputs found
Observational Bounds on Modified Gravity Models
Modified gravity provides a possible explanation for the currently observed
cosmic accelaration. In this paper, we study general classes of modified
gravity models. The Einstein-Hilbert action is modified by using general
functions of the Ricci and the Gauss-Bonnet scalars, both in the metric and in
the Palatini formalisms. We do not use an explicit form for the functions, but
a general form with a valid Taylor expansion up to second order about redshift
zero in the Riemann-scalars. The coefficients of this expansion are then
reconstructed via the cosmic expansion history measured using current
cosmological observations. These are the quantities of interest for theoretical
considerations relating to ghosts and instabilities. We find that current data
provide interesting constraints on the coefficients. The next-generation dark
energy surveys should shrink the allowed parameter space for modifed gravity
models quite dramatically.Comment: 23 pages, 5 figures, uses RevTe
Concentration profiles for fine and coarse sediments suspended by waves over ripples: An analytical study with the 1-DV gradient diffusion model
Field and laboratory measurements of suspended sediments over wave ripples
show, for time-averaged concentration profiles in semi-log plots, a contrast
between upward convex profiles for fine sand and upward concave profiles for
coarse sand. Careful examination of experimental data for coarse sand shows a
near-bed upward convex profile beneath the main upward concave profile.
Available models fail to predict these two profiles for coarse sediments. The
1-DV gradient diffusion model predicts the main upward concave profile for
coarse sediments thanks to a suitable (y)-function (where is the
inverse of the turbulent Schmidt number and y is the distance from the bed). In
order to predict the near-bed upward convex profile, an additional parameter
{\alpha} is needed. This parameter could be related to settling velocity
( equal to inverse of dimensionless settling velocity) or to convective
sediment entrainment process. The profiles are interpreted by a relation
between second derivative of the logarithm of concentration and derivative of
the product between sediment diffusivity and
Standard Model Extension with Gravity and Gravitational Baryogenesis
The Standard Model Extension with the inclusion of gravity is studied in the
framework of the gravitational baryogenesis, a mechanism to generate the baryon
asymmetry based on the coupling between the Ricci scalar curvature and the
baryon current (\partial_\mu R)J^\mu. We show that, during the radiation era of
the expanding Universe, a non vanishing time derivative of the Ricci curvature
arises as a consequence of the coupling between the coefficients for the
Lorentz and CPT violation and Ricci's tensor. The order of magnitude for these
coefficients are derived from current bounds on baryon asymmetry.Comment: 5 pages, no figure
fault gouge graphitization as evidence of past seismic slip
One moderate- to large-magnitude earthquake (M > 6) nucleates in Earth's crust every three days n average, but the geological record of ancient fault slip at meters-per-second seismic velocities (as opposed to subseismic slow-slip creep) remains debated because of the lack of established fault-zone evidence of seismic slip. Here we show that the irreversible temperature-dependent transformation of carbonaceous material (CM, a constituent of many fault gouges) into graphite is a reliable tracer of seismic fault slip. We sheared CM-bearing fault rocks in the laboratory at just above subseismic and at seismic velocities under both water-rich and water-deficient conditions and modeled the temperature evolution with slip. By means of micro-Raman spectroscopy and focused-ion beam transmission electron microscopy, we detected graphite grains similar to those found in the principal slip zone of the A.D. 2008 Wenchuan (Mw 7.9) earthquake (southeast Tibet) only in experiments conducted at seismic velocities. The experimental evidence presented here suggests that high-temperature pulses associated with seismic slip induce graphitization of CM. Importantly, the occurrence of graphitized fault-zone CM may allow us to ascertain the seismogenic potential of faults in areas worldwide with incomplete historical earthquake catalogues
Canted-cosine-theta magnet (CCT)-A concept for high field accelerator magnets
Canted-Cosine-Theta (CCT) magnet is an accelerator magnet that superposes fields of nested and tilted solenoids that are oppositely canted. The current distribution of any canted layer generates a pure harmonic field as well as a solenoid field that can be cancelled with a similar but oppositely canted layer. The concept places windings within mandrel's ribs and spars that simultaneously intercept and guide Lorentz forces of each turn to prevent stress accumulation. With respect to other designs, the need for pre-stress in this concept is reduced by an order of magnitude making it highly compatible with the use of strain sensitive superconductors such as Nb3Sn or HTS. Intercepting large Lorentz forces is of particular interest in magnets with large bores and high field accelerator magnets like the one foreseen in the future high energy upgrade of the LHC. This paper describes the CCT concept and reports on the construction of CCT1 a "proof of principle" dipole
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Mechanical Performance of Short Models for MQXF, the Nb3Sn Low-β Quadrupole for the Hi-Lumi LHC
In the framework of the Hi-Lumi LHC Project, CERN and U.S. LARP are jointly developing MQXF, a 150-mm aperture high-field Nb3Sn quadrupole for the upgrade of the inner triplet of the low-beta interaction regions. The magnet is supported by a shell-based structure, providing the preload by means of bladder-key technology and differential thermal contraction of the various components. Two short models have been produced using the same cross section currently considered for the final magnet. The structures were preliminarily tested replacing the superconducting coils with blocks of aluminum. This procedure allows for model validation and calibration, and also to set performance goals for the real magnet. Strain gauges were used to monitor the behavior of the structure during assembly, cool down and also excitation in the case of the magnets. The various structures differ for the shell partitioning strategies adopted and for the presence of thick or thin laminations. This paper presents the results obtained and discusses the mechanical performance of all the short models produced up to now
Accelerating Universe and Cosmological Perturbation in the Ghost Condensate
In the simplest Higgs phase of gravity called ghost condensation, an
accelerating universe with a phantom era (w<-1) can be realized without ghost
or any other instabilities. In this paper we show how to reconstruct the
potential in the Higgs sector Lagrangian from a given cosmological history
(H(t), \rho(t)). This in principle allows us to constrain the potential by
geometrical information of the universe such as supernova distance-redshift
relation. We also derive the evolution equation for cosmological perturbations
in the Higgs phase of gravity by employing a systematic low energy expansion.
This formalism is expected to be useful to test the theory by dynamical
information of large scale structure in the universe such as cosmic microwave
background anisotropy, weak gravitational lensing and galaxy clustering.Comment: 30 pages; typos corrected; version accepted for publication in JCA
Insertion Magnets
Chapter 3 in High-Luminosity Large Hadron Collider (HL-LHC) : Preliminary
Design Report. The Large Hadron Collider (LHC) is one of the largest scientific
instruments ever built. Since opening up a new energy frontier for exploration
in 2010, it has gathered a global user community of about 7,000 scientists
working in fundamental particle physics and the physics of hadronic matter at
extreme temperature and density. To sustain and extend its discovery potential,
the LHC will need a major upgrade in the 2020s. This will increase its
luminosity (rate of collisions) by a factor of five beyond the original design
value and the integrated luminosity (total collisions created) by a factor ten.
The LHC is already a highly complex and exquisitely optimised machine so this
upgrade must be carefully conceived and will require about ten years to
implement. The new configuration, known as High Luminosity LHC (HL-LHC), will
rely on a number of key innovations that push accelerator technology beyond its
present limits. Among these are cutting-edge 11-12 tesla superconducting
magnets, compact superconducting cavities for beam rotation with ultra-precise
phase control, new technology and physical processes for beam collimation and
300 metre-long high-power superconducting links with negligible energy
dissipation. The present document describes the technologies and components
that will be used to realise the project and is intended to serve as the basis
for the detailed engineering design of HL-LHC.Comment: 19 pages, Chapter 3 in High-Luminosity Large Hadron Collider (HL-LHC)
: Preliminary Design Repor
Optimizing the use of pressurized bladders for the assembly of HL-LHC MQXFB magnets
The use of pressurized bladders for stress control of superconducting magnets
was firstly proposed at Lawrence Berkeley National Laboratory (LBNL) in the
early 2000s. Since then, the so-called bladders and keys procedure has become
one of the reference techniques for the assembly of high-field accelerator
magnets and demonstrators. Exploiting the advantages of this method is today of
critical importance for Nb3Sn-based accelerator magnets, whose production
requires the preservation of tight stress targets in the superconducting coils
to limit the effects of the strain sensitivity and brittleness of the
conductor. The present manuscript reports on the results of an experimental
campaign focused on the optimization of the bladders and keys assembly process
in the MQXFB quadrupoles. These 7.2 m long magnets shall be among the first
Nb3Sn cryomagnets to be installed in a particle accelerator as a part of the
High Luminosity upgrade of the LHC. One of the main practical implications of
the bladders technique, especially important when applied to long magnets like
MQXFB, is that to insert the loading keys, the opening of a certain clearance
in the support structure is required. The procedure used so far for MQXF
magnets involved an overstress in the coils during bladder inflation. The work
presented here shows that such an overshoot can be eliminated thanks to
additional bladders properly positioned in the structure. This optimized method
was validated in a short model magnet and in a full-length mechanical model,
becoming the new baseline for the series production at CERN. Furthermore, the
results are supported by numerical predictions using Finite Element models
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