838 research outputs found
An Energy Feedback System for the MIT/Bates Linear Accelerator
We report the development and implementation of an energy feedback system for
the MIT/Bates Linear Accelerator Center. General requirements of the system are
described, as are the specific requirements, features, and components of the
system unique to its implementation at the Bates Laboratory. We demonstrate
that with the system in operation, energy fluctuations correlated with the 60
Hz line voltage and with drifts of thermal origin are reduced by an order of
magnitude
A High Power Hydrogen Target for Parity Violation Experiments
Parity-violating electron scattering measurements on hydrogen and deuterium,
such as those underway at the Bates and CEBAF laboratories, require
luminosities exceeding cms, resulting in large beam
power deposition into cryogenic liquid. Such targets must be able to absorb 500
watts or more with minimal change in target density. A 40~cm long liquid
hydrogen target, designed to absorb 500~watts of beam power without boiling,
has been developed for the SAMPLE experiment at Bates. In recent tests with
40~A of incident beam, no evidence was seen for density fluctuations in
the target, at a sensitivity level of better than 1\%. A summary of the target
design and operational experience will be presented.Comment: 13 pages, 9 postscript figure
The Strange Quark Contribution to the Proton's Magnetic Moment
We report a new determination of the strange quark contribution to the
proton's magnetic form factor at a four-momentum transfer Q2 = 0.1 (GeV/c)^2
from parity-violating e-p elastic scattering. The result uses a revised
analysis of data from the SAMPLE experiment which was carried out at the
MIT-Bates Laboratory. The data are combined with a calculation of the proton's
axial form factor GAe to determine the strange form factor GMs(Q2=0.1)=0.37 +-
0.20 +- 0.26 +- 0.07. The extrapolation of GMs to its Q2=0 limit and comparison
with calculations is also discussed.Comment: 6 pages, 1 figure, submitted to Phys. Lett.
Embedding Flipped SU(5) into SO(10)
We embed the flipped SU(5) models into the SO(10) models. After the SO(10)
gauge symmetry is broken down to the flipped SU(5) \times U(1)_X gauge
symmetry, we can split the five/one-plets and ten-plets in the spinor
\mathbf{16} and \mathbf{\bar{16}} Higgs fields via the stable sliding singlet
mechanism. As in the flipped SU(5) models, these ten-plet Higgs fields can
break the flipped SU(5) gauge symmetry down to the Standard Model gauge
symmetry. The doublet-triplet splitting problem can be solved naturally by the
missing partner mechanism, and the Higgsino-exchange mediated proton decay can
be suppressed elegantly. Moreover, we show that there exists one pair of the
light Higgs doublets for the electroweak gauge symmetry breaking. Because there
exist two pairs of additional vector-like particles with similar
intermediate-scale masses, the SU(5) and U(1)_X gauge couplings can be unified
at the GUT scale which is reasonably (about one or two orders) higher than the
SU(2)_L \times SU(3)_C unification scale. Furthermore, we briefly discuss the
simplest SO(10) model with flipped SU(5) embedding, and point out that it can
not work without fine-tuning.Comment: RevTex4, 28 pages, 3 figures, typos correcte
Dust Devil Tracks
Dust devils that leave dark- or light-toned tracks are common on Mars and they can also be found on the Earth’s surface. Dust devil tracks (hereinafter DDTs) are ephemeral surface features with mostly sub-annual lifetimes. Regarding their size, DDT widths can range between ∼1 m and ∼1 km, depending on the diameter of dust devil that created the track, and DDT lengths range from a few tens of meters to several kilometers, limited by the duration and horizontal ground speed of dust devils. DDTs can be classified into three main types based on their morphology and albedo in contrast to their surroundings; all are found on both planets: (a) dark continuous DDTs, (b) dark cycloidal DDTs, and (c) bright DDTs. Dark continuous DDTs are the most common type on Mars. They are characterized by their relatively homogenous and continuous low albedo surface tracks. Based on terrestrial and martian in situ studies, these DDTs most likely form when surficial dust layers are removed to expose larger-grained substrate material (coarse sands of ≥500 μm in diameter). The exposure of larger-grained materials changes the photometric properties of the surface; hence leading to lower albedo tracks because grain size is photometrically inversely proportional to the surface reflectance. However, although not observed so far, compositional differences (i.e., color differences) might also lead to albedo contrasts when dust is removed to expose substrate materials with mineralogical differences. For dark continuous DDTs, albedo drop measurements are around 2.5 % in the wavelength range of 550–850 nm on Mars and around 0.5 % in the wavelength range from 300–1100 nm on Earth. The removal of an equivalent layer thickness around 1 μm is sufficient for the formation of visible dark continuous DDTs on Mars and Earth. The next type of DDTs, dark cycloidal DDTs, are characterized by their low albedo pattern of overlapping scallops. Terrestrial in situ studies imply that they are formed when sand-sized material that is eroded from the outer vortex area of a dust devil is redeposited in annular patterns in the central vortex region. This type of DDT can also be found in on Mars in orbital image data, and although in situ studies are lacking, terrestrial analog studies, laboratory work, and numerical modeling suggest they have the same formation mechanism as those on Earth. Finally, bright DDTs are characterized by their continuous track pattern and high albedo compared to their undisturbed surroundings. They are found on both planets, but to date they have only been analyzed in situ on Earth. Here, the destruction of aggregates of dust, silt and sand by dust devils leads to smooth surfaces in contrast to the undisturbed rough surfaces surrounding the track. The resulting change in photometric properties occurs because the smoother surfaces have a higher reflectance compared to the surrounding rough surface, leading to bright DDTs. On Mars, the destruction of surficial dust-aggregates may also lead to bright DDTs. However, higher reflective surfaces may be produced by other formation mechanisms, such as dust compaction by passing dust devils, as this may also cause changes in photometric properties. On Mars, DDTs in general are found at all elevations and on a global scale, except on the permanent polar caps. DDT maximum areal densities occur during spring and summer in both hemispheres produced by an increase in dust devil activity caused by maximum insolation. Regionally, dust devil densities vary spatially likely controlled by changes in dust cover thicknesses and substrate materials. This variability makes it difficult to infer dust devil activity from DDT frequencies. Furthermore, only a fraction of dust devils leave tracks. However, DDTs can be used as proxies for dust devil lifetimes and wind directions and speeds, and they can also be used to predict lander or rover solar panel clearing events. Overall, the high DDT frequency in many areas on Mars leads to drastic albedo changes that affect large-scale weather patterns
Parity Violation in Elastic Electron-Proton Scattering and the Proton's Strange Magnetic Form Factor
We report a new measurement of the parity-violating asymmetry in elastic electron scattering from the proton at backward scattering angles. This asymmetry is sensitive to the strange magnetic form factor of the proton as well as electroweak axial radiative corrections. The new measurement of A = -4.92±0.61±0.73 ppm provides a significant constraint on these quantities. The implications for the strange magnetic form factor are discussed in the context of theoretical estimates for the axial corrections
in NonCommutative Standard Model
We study the top quark decay to b quark and W boson in the NonCommutative
Standard Model (NCSM). The lowest contribution to the decay comes from the
terms quadratic in the matrix describing the noncommutative (NC) effects while
the linear term is seen to identically vanish because of symmetry. The NC
effects are found to be significant only for low values of the NC
characteristic scale.Comment: 11 page Latex file containing 2 eps figures (redrawn). More
discussion included. To appear in PR
Parity-violating Electron Deuteron Scattering and the Proton's Neutral Weak Axial Vector Form Factor
We report on a new measurement of the parity-violating asymmetry in
quasielastic electron scattering from the deuteron at backward angles at Q2=
0.038 (GeV/c)2. This quantity provides a determination of the neutral weak
axial vector form factor of the nucleon, which can potentially receive large
electroweak corrections. The measured asymmetry A=-3.51 +/- 0.57(stat) +/-
0.58(sys)ppm is consistent with theoretical predictions. We also report on
updated results of the previous experiment at Q2=0.091 (GeV/c)2, which are also
consistent with theoretical predictions.Comment: 4 pages, 2 figures, submitted to Phys. Rev. Let
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