36 research outputs found
The Qweak experiment: First determination of the weak charge of the proton
The Qweak Collaboration has completed a challenging measurement
of the parity-violating asymmetry in elastic electron-proton (ep) scattering at the
Thomas Jefferson National Accelerator Facility (Jefferson Lab). The initial result
reported here is extracted from the commissioning part of the experiment, constituting about 4% of the full data set. The parity-violating asymmetry at a low
momentum transfer Q2 = 0.025 GeV2 is Aep = −279 ± 35 (stat) ± 31 (syst) ppb,
which is the smallest and most precise asymmetry ever measured in ep scattering.
This result allowed the first determination of the weak charge of the proton Qp
W
from a global fit of parity-violating elastic scattering (PVES) results from nuclear
targets, where earlier data at higher Q2 constrain uncertainties of hadronic structure. The value extracted from the global fit is Qp
W (PVES) = 0.064 ± 0.012, in
agreement with the standard model prediction Qp
W (SM) = 0.0710 ± 0.0007. The
neutral weak charges of up and down quarks are extracted from a combined fit of
the PVES results with a previous atomic parity violation (APV) measurement on
133Cs. The analysis of the full Qweak data is ongoing and expected to yield a value
for the weak charge to about 5% precision. Because of the suppression of Qp
W , such
a high-precision measurement will place significant constraints to models of physics
beyond the standard model
Reduction of coherent betatron oscillations in a muon g-2 storage ring experiment using RF fields
This work demonstrates that two systematic errors, coherent betatron
oscillations (CBO) and muon losses can be reduced through application of radio
frequency (RF) electric fields, which ultimately increases the sensitivity of
the muon experiments. As the ensemble of polarized muons goes around a
weak focusing storage ring, their spin precesses, and when they decay through
the weak interaction, , the decay
positrons are detected by electromagnetic calorimeters. In addition to the
expected exponential decay in the positron time spectrum, the weak decay
asymmetry causes a modulation in the number of positrons in a selected energy
range at the difference frequency between the spin and cyclotron frequencies,
. This frequency is directly proportional to the magnetic
anomaly , where is the g-factor of the muon, which is
slightly greater than 2. The detector acceptance depends on the radial position
of the muon decay, so the CBO of the muon bunch following injection into the
storage ring modulate the measured muon signal with the frequency
. In addition, the muon populations at the edge of the beam
hit the walls of the vacuum chamber before decaying, which also affects the
signal. Thus, reduction of CBO and unwanted muon loss increases the
measurement sensitivity. Numerical and experimental studies with RF electric
fields yield more than a magnitude reduction of the CBO, with muon losses
comparable to the conventional method.Comment: 14 pages, 25 figure
A New Charged Lepton Flavor Violation Program at Fermilab
The muon has played a central role in establishing the Standard Model of
particle physics, and continues to provide valuable information about the
nature of new physics. A new complex at Fermilab, the Advanced Muon Facility,
would provide the world's most intense positive and negative muon beams by
exploiting the full potential of PIP-II and the Booster upgrade. This facility
would enable a broad muon physics program, including studies of charged lepton
flavor violation, muonium-antimuonium transitions, a storage ring muon EDM
experiment, and muon spin rotation experiments. This document describes a
staged realization of this complex, together with a series of next-generation
experiments to search for charged lepton flavor violation.Comment: A Contributed Paper for Snowmass 202
Beam dynamics corrections to the Run-1 measurement of the muon anomalous magnetic moment at Fermilab
This paper presents the beam dynamics systematic corrections and their uncertainties for the Run-1 dataset of the Fermilab Muon g-2 Experiment. Two corrections to the measured muon precession frequency ωam are associated with well-known effects owing to the use of electrostatic quadrupole (ESQ) vertical focusing in the storage ring. An average vertically oriented motional magnetic field is felt by relativistic muons passing transversely through the radial electric field components created by the ESQ system. The correction depends on the stored momentum distribution and the tunes of the ring, which has relatively weak vertical focusing. Vertical betatron motions imply that the muons do not orbit the ring in a plane exactly orthogonal to the vertical magnetic field direction. A correction is necessary to account for an average pitch angle associated with their trajectories. A third small correction is necessary, because muons that escape the ring during the storage time are slightly biased in initial spin phase compared to the parent distribution. Finally, because two high-voltage resistors in the ESQ network had longer than designed RC time constants, the vertical and horizontal centroids and envelopes of the stored muon beam drifted slightly, but coherently, during each storage ring fill. This led to the discovery of an important phase-acceptance relationship that requires a correction. The sum of the corrections to ω_{a}^{m} is 0.50±0.09 ppm; the uncertainty is small compared to the 0.43 ppm statistical precision of ω_{a}^{m}
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
Magnetic-field measurement and analysis for the Muon g − 2 Experiment at Fermilab
The Fermi National Accelerator Laboratory (FNAL) Muon g - 2 Experiment has measured the anomalous precession frequency a_{μ}(g_{μ} - 2)/2 of the muon to a combined precision of 0.46 parts per million with data collected during its first physics run in 2018. This paper documents the measurement of the magnetic field in the muon storage ring. The magnetic field is monitored by systems and calibrated in terms of the equivalent proton spin precession frequency in a spherical water sample at 34.7C. The measured field is weighted by the muon distribution resulting in \tilde{ω}'_{p}, the denominator in the ratio \tilde{ω}_{a}/\tilde{ω}'_{p} that together with known fundamental constants yields aμ. The reported uncertainty on \tilde{ω}'_{p} for the Run-1 data set is 114 ppb consisting of uncertainty contributions from frequency extraction, calibration, mapping, tracking, and averaging of 56 ppb, and contributions from fast transient fields of 99 ppb