28 research outputs found
Precision Electron-Beam Polarimetry using Compton Scattering at 1 GeV
We report on the highest precision yet achieved in the measurement of the
polarization of a low energy, (1 GeV), electron beam, accomplished
using a new polarimeter based on electron-photon scattering, in Hall~C at
Jefferson Lab. A number of technical innovations were necessary, including a
novel method for precise control of the laser polarization in a cavity and a
novel diamond micro-strip detector which was able to capture most of the
spectrum of scattered electrons. The data analysis technique exploited track
finding, the high granularity of the detector and its large acceptance. The
polarization of the A, ~GeV electron beam was measured with a
statistical precision of ~1\% per hour and a systematic uncertainty of
0.59\%. This exceeds the level of precision required by the \qweak experiment,
a measurement of the vector weak charge of the proton. Proposed future
low-energy experiments require polarization uncertainty ~0.4\%, and this
result represents an important demonstration of that possibility. This
measurement is also the first use of diamond detectors for particle tracking in
an experiment.Comment: 9 pages, 7 figures, published in PR
Measurement of the Neutron Radius of 208Pb Through Parity-Violation in Electron Scattering
We report the first measurement of the parity-violating asymmetry A_PV in the
elastic scattering of polarized electrons from 208Pb. A_PV is sensitive to the
radius of the neutron distribution (Rn). The result A_PV = 0.656 \pm 0.060
(stat) \pm 0.014 (syst) ppm corresponds to a difference between the radii of
the neutron and proton distributions Rn - Rp = 0.33 +0.16 -0.18 fm and provides
the first electroweak observation of the neutron skin which is expected in a
heavy, neutron-rich nucleus.Comment: 6 pages, 1 figur
New Precision Limit on the Strange Vector Form Factors of the Proton
The parity-violating cross-section asymmetry in the elastic scattering of
polarized electrons from unpolarized protons has been measured at a
four-momentum transfer squared Q2 = 0.624 GeV and beam energy E =3.48 GeV to be
A_PV = -23.80 +/- 0.78 (stat) +/- 0.36 (syst) parts per million. This result is
consistent with zero contribution of strange quarks to the combination of
electric and magnetic form factors G_E^s + 0.517 G_M^s = 0.003 +/- 0.010 (stat)
+/- 0.004 (syst) +/- 0.009 (ff), where the third error is due to the limits of
precision on the electromagnetic form factors and radiative corrections. With
this measurement, the world data on strange contributions to nucleon form
factors are seen to be consistent with zero and not more than a few percent of
the proton form factors.Comment: 5 pages, 3 figure
Comparative yield loss estimates due to blast in some upland rice cultivars.
Leaf and panicle blast severities and grain yield of some upland rice cultivars were measured in three successive years in field plots unprotected or protected with fungicides. The variation in disease severities in different plots was used to establish relationships between severity of leaf and panicle blast and yield. Linear multiple regression equations for each cultivar by year were developed to estimate the yield decrease in different cultivars per unit increase in disease. Leaf blast severities at maximum tillering or booting stage and panicle blast 25 days after heading accounted for variation in grain yield in most of the cultivars. General equations combining five early and eight medium-duration rice cultivars were developed. The estimated percentage losses in grain yield due to blastwere 2.7 and 1.5 for one percent increase in blast in the early and medium-duration cultivars, respectively
Parity-Violating Inelastic Electron-Proton Scattering at Low Above the Resonance Region
We report the measurement of the parity-violating asymmetry for the inelastic
scattering of electrons from the proton, at GeV and GeV, above the resonance region. The result ~ppm agrees with theoretical
calculations, and helps to validate the modeling of the interference
structure functions and used in those
calculations, which are also used for determination of the two-boson exchange
box diagram () contribution to parity-violating elastic
scattering measurements. A positive parity-violating asymmetry for inclusive
production was observed, as well as positive beam-normal single-spin
asymmetry for scattered electrons and a negative beam-normal single-spin
asymmetry for inclusive production.Comment: 18 pages, 9 figures, version accepted in Physical Review
Q
The Qweak experiment, which took data at Jefferson Lab in the period 2010 - 2012, will precisely determine the weak charge of the proton by measuring the parity-violating asymmetry in elastic e-p scattering at 1.1 GeV using a longitudinally polarized electron beam and a liquid hydrogen target at a low momentum transfer of Q2 = 0.025 (GeV/c)2. The weak charge of the proton is predicted by the Standard Model and any significant deviation would indicate physics beyond the Standard Model. The technical challenges and experimental apparatus for measuring the weak charge of the proton will be discussed, as well as the method of extracting the weak charge of the proton. The results from a small subset of the data, that has been published, will also be presented. Furthermore an update will be given of the current status of the data analysis