5,154 research outputs found
Second order symmetry operators for the massive Dirac equation
Employing the covariant language of two-spinors, we find what conditions a curved four-dimensional Lorentzian spacetime must satisfy for existence of a second order symmetry operator for the massive Dirac equation. The conditions are formulated as existence of a set of Killing spinors satisfying a set of covariant linear differential equations. Using these Killing spinors, we then state the most general form of such an operator. Partial results for the zeroth and first order are presented and interpreted as well. Computer algebra tools from the Mathematica package suite xAct were used for the calculations
The effects of matter density uncertainties on neutrino oscillations in the Earth
We compare three different methods to evaluate uncertainties in the Earth's
matter density profile, which are relevant to long baseline experiments, such
as neutrino factories.Comment: 3 pages, 1 figure. Talk given at the NuFact'02 Workshop, London, 1-6
July, 200
The LHCb Timing and Fast Control system
In this paper we describe the LHCb Timing and Fast Control (TFC) system. It is different from that of the other LHC experiments in that it has to support two levels of high-rate triggers. Furthermore, emphasis has been put on partitioning and on locating the TFC mastership in one type of module: the Readout Supervisor. The Readout Supervisor handles all timing, trigger, and control command distribution. It generates auto-triggers as well as controls the trigger rates. Partitioning is handled by a programmable patch panel/switch introduced in the TTC distribution network between a pool of Readout Supervisors and the Front-End electronics. I
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Branched siloxanes and methods for synthesis
The present invention describes branched and functionalized siloxanes and methods for making such compounds. The compounds have a variety of uses. One preferred application is as novel planarizing material for lithography, in which case functionalized branched siloxane, such as an epoxy-modified branched siloxane is particularly useful.Board of Regents, University of Texas Syste
Vapor-solid-solid growth dynamics in GaAs nanowires
Semiconductor nanowires are promising material systems for coming of age
nanotechnology. The usage of the vapor solid solid (VSS) route, where the
catalyst used for promoting axial growth of nanowire is a solid, offers certain
advantages compared to the common vapor liquid solid (VLS) route (using liquid
catalyst). The VSS growth of group-IV elemental nanowires have been
investigated by other groups in situ during growth in a transmission electron
microscope (TEM). Though it is known that compound nanowire growth has
different dynamics compared to monoatomic semiconductors, the dynamics of VSS
growth of compound nanowires has not been understood. Here we investigate VSS
growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a
model system. The growth kinetics and dynamics at the wire-catalyst interface
by ledge-flow is studied and compared for liquid and solid catalysts at similar
growth conditions. Here the temperature and thermal history of the system is
manipulated to control the catalyst phase. In the first experiment discussed
here we reduce the growth temperature in steps to solidify the initially liquid
catalyst, and compare the dynamics between VLS and VSS growth observed at
slightly different temperatures. In the second experiment we exploit thermal
hysteresis of the system to obtain both VLS and VSS at the same temperature.
The VSS growth rate is comparable or slightly slower than VLS growth. Unlike in
the VLS case, during VSS growth we see several occasions where a new layer
starts before the previous layer is completely grown, i.e. multilayer growth.
Understanding the VSS growth mode enables better control of nanowire properties
by widening the range of usable nanowire growth parameters
Proposal to Search for Heavy Neutral Leptons at the SPS
A new fixed-target experiment at the CERN SPS accelerator is proposed that
will use decays of charm mesons to search for Heavy Neutral Leptons (HNLs),
which are right-handed partners of the Standard Model neutrinos. The existence
of such particles is strongly motivated by theory, as they can simultaneously
explain the baryon asymmetry of the Universe, account for the pattern of
neutrino masses and oscillations and provide a Dark Matter candidate.
Cosmological constraints on the properties of HNLs now indicate that the
majority of the interesting parameter space for such particles was beyond the
reach of the previous searches at the PS191, BEBC, CHARM, CCFR and NuTeV
experiments. For HNLs with mass below 2 GeV, the proposed experiment will
improve on the sensitivity of previous searches by four orders of magnitude and
will cover a major fraction of the parameter space favoured by theoretical
models.
The experiment requires a 400 GeV proton beam from the SPS with a total of
2x10^20 protons on target, achievable within five years of data taking. The
proposed detector will reconstruct exclusive HNL decays and measure the HNL
mass. The apparatus is based on existing technologies and consists of a target,
a hadron absorber, a muon shield, a decay volume and two magnetic
spectrometers, each of which has a 0.5 Tm magnet, a calorimeter and a muon
detector. The detector has a total length of about 100 m with a 5 m diameter.
The complete experimental set-up could be accommodated in CERN's North Area.
The discovery of a HNL would have a great impact on our understanding of
nature and open a new area for future research
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