60 research outputs found
Flux profile scanners for scattered high-energy electrons
The paper describes the design and performance of flux integrating Cherenkov
scanners with air-core reflecting light guides used in a high-energy, high-flux
electron scattering experiment at the Stanford Linear Accelerator Center. The
scanners were highly radiation resistant and provided a good signal to
background ratio leading to very good spatial resolution of the scattered
electron flux profile scans.Comment: 22 pages, 17 figure
Parity violating pion electroproduction off the nucleon
Parity violating (PV) contributions due to interference between and
exchange are calculated for pion electroproduction off the nucleon. A
phenomenological model with effective Lagrangians is used to determine the
resulting asymmetry for the energy region between threshold and
resonance. The resonance is treated as a Rarita-Schwinger field with
phenomenological transition currents. The background contributions
are given by the usual Born terms using the pseudovector Lagrangian.
Numerical results for the asymmetry are presented.Comment: 17 pages, RevTeX, 6 figures (in separate file figs.uu), uses epsf,
accepted for publication in Z. Phys.
Parity Violating Measurements of Neutron Densities
Parity violating electron nucleus scattering is a clean and powerful tool for
measuring the spatial distributions of neutrons in nuclei with unprecedented
accuracy. Parity violation arises from the interference of electromagnetic and
weak neutral amplitudes, and the of the Standard Model couples primarily
to neutrons at low . The data can be interpreted with as much confidence
as electromagnetic scattering. After briefly reviewing the present theoretical
and experimental knowledge of neutron densities, we discuss possible parity
violation measurements, their theoretical interpretation, and applications. The
experiments are feasible at existing facilities. We show that theoretical
corrections are either small or well understood, which makes the interpretation
clean. The quantitative relationship to atomic parity nonconservation
observables is examined, and we show that the electron scattering asymmetries
can be directly applied to atomic PNC because the observables have
approximately the same dependence on nuclear shape.Comment: 38 pages, 7 ps figures, very minor changes, submitted to Phys. Rev.
Deuteron Electroweak Disintegration
We study the deuteron electrodisintegration with inclusion of the neutral
currents focusing on the helicity asymmetry of the exclusive cross section in
coplanar geometry. We stress that a measurement of this asymmetry in the quasi
elastic region is of interest for an experimental determination of the weak
form factors of the nucleon, allowing one to obtain the parity violating
electron neutron asymmetry. Numerically, we consider the reaction at low
momentum transfer and discuss the sensitivity of the helicity asymmetry to the
strangeness radius and magnetic moment. The problems coming from the finite
angular acceptance of the spectrometers are also considered.Comment: 30 pages, Latex, 7 eps figures, submitted to Phys.Rev.C e-mail:
[email protected] , [email protected]
The Weak Charge of the Proton and New Physics
We address the physics implications of a precision determination of the weak
charge of the proton, QWP, from a parity violating elastic electron proton
scattering experiment to be performed at the Jefferson Laboratory. We present
the Standard Model (SM) expression for QWP including one-loop radiative
corrections, and discuss in detail the theoretical uncertainties and missing
higher order QCD corrections. Owing to a fortuitous cancellation, the value of
QWP is suppressed in the SM, making it a unique place to look for physics
beyond the SM. Examples include extra neutral gauge bosons, supersymmetry, and
leptoquarks. We argue that a QWP measurement will provide an important
complement to both high energy collider experiments and other low energy
electroweak measurements. The anticipated experimental precision requires the
knowledge of the order alpha_s corrections to the pure electroweak box
contributions. We compute these contributions for QWP, as well as for the weak
charges of heavy elements as determined from atomic parity violation.Comment: 22 pages of LaTeX, 5 figure
Neutral weak currents in pion electroproduction on the nucleon
Parity violating asymmetry in inclusive scattering of longitudinally
polarized electrons by unpolarized protons with or meson
production, is calculated as a function of the momentum transfer squared
and the total energy of the -system. This asymmetry, which is
induced by the interference of the one-photon exchange amplitude with the
parity-odd part of the -exchange amplitude, is calculated for the
processes ( is a virtual photon and
a virtual Z-boson) considering the -contribution in the channel,
the standard Born contributions and vector meson ( and )
exchanges in the channel. Taking into account the known isotopic properties
of the hadron electromagnetic and neutral currents, we show that the P-odd term
is the sum of two contributions. The main term is model independent and it can
be calculated exactly in terms of fundamental constants. It is found to be
linear in . The second term is a relatively small correction which is
determined by the isoscalar component of the electromagnetic current. Near
threshold and in the -region, this isoscalar part is much smaller (in
absolute value) than the isovector one: its contribution to the asymmetry
depend on the polarization state (longitudinal or transverse) of the virtual
photon.Comment: 30 pages 9 figure
Measurement of tensor polarization in elastic electron-deuteron scattering in the momentum-transfer range 3.8≤q≤4.6 fm-1
The tensor polarization t20 of the recoil deuteron in elastic e-d scattering has been measured for three values of four-momentum transfer, q=3.78, 4.22, and 4.62 fm-1. The data have been used to locate the first node in the charge monopole form factor of the deuteron at q=4.39±0.16 fm-1. The results for t20 are in reasonable agreement with expectations based on the nucleon-meson description of nuclear dynamic
Tensor polarization in elastic electron-deuteron scattering in the momentum transfer range 3.8≤Q≤4.6 fm-1
The tensor polarization of the recoil deuteron in elastic electron-deuteron scattering has been measured at the Bates Linear Accelerator Center at three values of four-momentum transfer Q=3.78, 4.22, and 4.62 fm-1, corresponding to incident electron energies of 653, 755, and 853 MeV. The scattered electrons and the recoil deuterons were detected in coincidence. The recoil deuterons were transported to a liquid hydrogen target to undergo a second scattering. The angular distribution of the d→-p scattering was measured using a polarimeter. The polarimeter was calibrated in an auxiliary experiment using a polarized deuteron beam at the Laboratoire National Saturne. A Monte Carlo procedure was used to generate interpolated calibration data because the energy spread in the deuteron energies in the Bates experiment spanned the range of deuteron energies in the calibration experiment. The extracted values of t20 are compared to predictions of different theoretical models of the electromagnetic form factors of the deuteron: nonrelativistic and relativistic nucleon-meson dynamics, Skyrme model, quark models, and perturbative quantum chromodynamics. Along with the world data the structure functions A(Q) and B(Q) are used to separate the charge monopole and charge quadrupole form factors of the deuteron. A node in the charge monopole form factor is observed at Q=4.39±0.16 fm-1
Constraints on the Nucleon Strange Form Factors at Q^2 ~ 0.1 GeV^2
We report the most precise measurement to date of a parity-violating
asymmetry in elastic electron-proton scattering. The measurement was carried
out with a beam energy of 3.03 GeV and a scattering angle =6
degrees, with the result A_PV = -1.14 +/- 0.24 (stat) +/- 0.06 (syst) parts per
million. From this we extract, at Q^2 = 0.099 GeV^2, the strange form factor
combination G_E^s + 0.080 G_M^s = 0.030 +/- 0.025 (stat) +/- 0.006 (syst) +/-
0.012 (FF) where the first two errors are experimental and the last error is
due to the uncertainty in the neutron electromagnetic form factor. This result
significantly improves current knowledge of G_E^s and G_M^s at Q^2 ~0.1 GeV^2.
A consistent picture emerges when several measurements at about the same Q^2
value are combined: G_E^s is consistent with zero while G_M^s prefers positive
values though G_E^s=G_M^s=0 is compatible with the data at 95% C.L.Comment: minor wording changes for clarity, updated references, dropped one
figure to improve focu
Virtual Compton Scattering and Neutral Pion Electroproduction in the Resonance Region up to the Deep Inelastic Region at Backward Angles
We have made the first measurements of the virtual Compton scattering (VCS)
process via the H exclusive reaction in the nucleon resonance
region, at backward angles. Results are presented for the -dependence at
fixed GeV, and for the -dependence at fixed near 1.5 GeV.
The VCS data show resonant structures in the first and second resonance
regions. The observed -dependence is smooth. The measured ratio of
H to H cross sections emphasizes the different
sensitivity of these two reactions to the various nucleon resonances. Finally,
when compared to Real Compton Scattering (RCS) at high energy and large angles,
our VCS data at the highest (1.8-1.9 GeV) show a striking -
independence, which may suggest a transition to a perturbative scattering
mechanism at the quark level.Comment: 20 pages, 8 figures. To appear in Phys.Rev.
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