W. T. H. van Oers

TRIUMF, Vancouver, British Columbia, Canada

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Publications (178)306.19 Total impact

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    ABSTRACT: The physics case and an experimental overview of the MOLLER (Measurement Of a Lepton Lepton Electroweak Reaction) experiment at the 12 GeV upgraded Jefferson Lab are presented. A highlight of the Fundamental Symmetries subfield of the 2007 NSAC Long Range Plan was the SLAC E158 measurement of the parity-violating asymmetry $A_{PV}$ in polarized electron-electron (M{\o}ller) scattering. The proposed MOLLER experiment will improve on this result by a factor of five, yielding the most precise measurement of the weak mixing angle at low or high energy anticipated over the next decade. This new result would be sensitive to the interference of the electromagnetic amplitude with new neutral current amplitudes as weak as $\sim 10^{-3}\cdot G_F$ from as yet undiscovered dynamics beyond the Standard Model. The resulting discovery reach is unmatched by any proposed experiment measuring a flavor- and CP-conserving process over the next decade, and yields a unique window to new physics at MeV and multi-TeV scales, complementary to direct searches at high energy colliders such as the Large Hadron Collider (LHC). The experiment takes advantage of the unique opportunity provided by the upgraded electron beam energy, luminosity, and stability at Jefferson Laboratory and the extensive experience accumulated in the community after a round of recent successfully completed parity-violating electron scattering experiments
    11/2014;
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    ABSTRACT: The Jefferson Lab Q_weak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise ${\vec{e}}$p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180 microA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16 GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Moller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4 cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8 degrees and 11.6 degrees were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cerenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7 GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q^2 = 0.025 GeV^2 was determined using dedicated low-current (~100 pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.
    09/2014;
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    ABSTRACT: A subset of results from the recently completed Jefferson Lab Qweak experiment are reported. This experiment, sensitive to physics beyond the Standard Model, exploits the small parity-violating asymmetry in elastic ep scattering to provide the first determination of the protons weak charge Qweak(p). The experiment employed a 180 uA longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons corresponding to Q2 of 0.025 GeV2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of Qweak(p) to 4.2 percent (combined statistical and systematic error), which implies a measure of sin2(thetaw) at the level of 0.3 percent, and to help constrain the vector weak quark charges C1u and C1d. The experimental method is described, with particular focus on the challenges associated with the worlds highest power LH2 target. The new constraints on C1u and C1d provided by the subset of the experiments data analyzed to date will also be shown, together with the extracted weak charge of the neutron.
    The European Physical Journal Conferences 11/2013; 66.
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    ABSTRACT: We discuss a new neutron EDM measurement of 10-28 e cm. For the improved UCN density, we will apply a new spallation UCN source of superfluid He. For magnetometry, 129Xe nuclear spins are injected into a EDM cell, to supress GPE. Performance of the prototype KEK-RCNP UCN source, and the obtained Ramsey resonance spectra are presented.
    10/2013;
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    ABSTRACT: The Qweak experiment has measured the parity-violating asymmetry in polarized e-p elastic scattering at Q^2 = 0.025(GeV/c)^2, employing 145 microamps of 89% longitudinally polarized electrons on a 34.4cm long liquid hydrogen target at Jefferson Lab. The results of the experiment's commissioning run are reported here, constituting approximately 4% of the data collected in the experiment. From these initial results the measured asymmetry is Aep = -279 +- 35 (statistics) +- 31 (systematics) ppb, which is the smallest and most precise asymmetry ever measured in polarized e-p scattering. The small Q^2 of this experiment has made possible the first determination of the weak charge of the proton, QpW, by incorporating earlier parity-violating electron scattering (PVES) data at higher Q^2 to constrain hadronic corrections. The value of QpW obtained in this way is QpW(PVES) = 0.064 +- 0.012, in good agreement with the Standard Model prediction of QpW(SM) = 0.0710 +- 0.0007. When this result is further combined with the Cs atomic parity violation (APV) measurement, significant constraints on the weak charges of the up and down quarks can also be extracted. That PVES+APV analysis reveals the neutron's weak charge to be QnW(PVES+APV) = -0.975 +- 0.010.
    07/2013;
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    ABSTRACT: The parity-violating asymmetry arising from inelastic electron-nucleon scattering at backward angle (~95 degrees) near the Delta(1232) resonance has been measured using a hydrogen target. From this asymmetry, we extracted the axial transition form factor G^A_{N\Delta}, a function of the axial Adler form factors C^A_i. Though G^A_{N\Delta} has been previously studied using charged current reactions, this is the first measurement of the weak neutral current excitation of the Delta using a proton target. For Q^2 = 0.34 (GeV/c)^2 and W = 1.18 GeV, the asymmetry was measured to be -33.4 \pm (5.3)_{stat} \pm (5.1)_{sys} ppm. The value of G^A_{N\Delta} determined from the hydrogen asymmetry was -0.05 \pm (0.35)_{stat} \pm (0.34)_{sys} \pm (0.06)_{theory}. These findings agree within errors with theoretical predictions for both the total asymmetry and the form factor. In addition to the hydrogen measurement, the asymmetry was measured at the same kinematics using a deuterium target. The asymmetry for deuterium was determined to be -43.6 \pm (14.6)_{stat} \pm (6.2)_{sys} ppm.
    12/2012;
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    ABSTRACT: The parity-violating (PV) asymmetry of inclusive π- production in electron scattering from a liquid deuterium target was measured at backward angles. The measurement was conducted as a part of the G0 experiment, at a beam energy of 360 MeV. The physics process dominating pion production for these kinematics is quasifree photoproduction off the neutron via the Δ0 resonance. In the context of heavy-baryon chiral perturbation theory, this asymmetry is related to a low-energy constant d(Δ)- that characterizes the parity-violating γNΔ coupling. Zhu et al. calculated d(Δ)- in a model benchmarked by the large asymmetries seen in hyperon weak radiative decays, and predicted potentially large asymmetries for this process, ranging from A(γ)-=-5.2 to +5.2  ppm. The measurement performed in this work leads to A(γ)-=-0.36±1.06±0.37±0.03  ppm (where sources of statistical, systematic and theoretical uncertainties are included), which would disfavor enchancements considered by Zhu et al. proportional to V(ud)/V(us). The measurement is part of a program of inelastic scattering measurements that were conducted by the G0 experiment, seeking to determine the N-Δ axial transition form factors using PV electron scattering.
    Physical Review Letters 03/2012; 108(12):122002. · 7.73 Impact Factor
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    ABSTRACT: We propose a new precision measurement of parity-violating electron scattering on the proton at very low Q^2 and forward angles to challenge predictions of the Standard Model and search for new physics. A unique opportunity exists to carry out the first precision measurement of the proton's weak charge, $Q_W =1 - 4\sin^2\theta_W$. A 2200 hour measurement of the parity violating asymmetry in elastic ep scattering at Q^2=0.03 (GeV/c)^2 employing 180 $\mu$A of 85% polarized beam on a 35 cm liquid Hydrogen target will determine the proton's weak charge with approximately 4% combined statistical and systematic errors. The Standard Model makes a firm prediction of $Q_W$, based on the running of the weak mixing angle from the Z0 pole down to low energies, corresponding to a 10 sigma effect in this experiment.
    02/2012;
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    ABSTRACT: The SPIN@FERMI collaboration has updated its 1991-95 Reports on the acceleration of polarized protons in Fermilab's Main Injector, which was commissioned by Fermilab. This Updated Report summarizes some updated Physics Goals for a 120-150 GeV/c polarized proton beam. It also contains an updated discussion of the Modifications and Hardware needed for a polarized beam in the Main Injector, along with an updated Schedule and Budget.
    10/2011;
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    ABSTRACT: We have measured the beam-normal single-spin asymmetries in elastic scattering of transversely polarized electrons from the proton, and performed the first measurement in quasielastic scattering on the deuteron, at backward angles (lab scattering angle of 108°) for Q² = 0.22 GeV²/c² and 0.63 GeV²/c² at beam energies of 362 and 687 MeV, respectively. The asymmetry arises due to the imaginary part of the interference of the two-photon exchange amplitude with that of single-photon exchange. Results for the proton are consistent with a model calculation which includes inelastic intermediate hadronic (πN) states. An estimate of the beam-normal single-spin asymmetry for the scattering from the neutron is made using a quasistatic deuterium approximation, and is also in agreement with theory.
    Physical Review Letters 07/2011; 107(2):022501. · 7.73 Impact Factor
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    Julie Roche, Willem T H van Oers, Ross D Young
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    ABSTRACT: Jefferson Lab has now demonstrated ablility to test the fundamental symmetries of nature, and thereby probe for new physics beyond the Standard Model. Here we review the tremendous advances in precision parity-violation measurements with CEBAF that enable searches for new physics. This has been demonstrated with a determination of the weak charge of the proton, which is found to be in agreement with the prediction of the standard electroweak theory, and at a precision that rules out relevant new physics to the TeV scale. We also review the planned future experiments which aim to further test the electroweak theory at Jefferson Lab, including a further improvement on the proton weak charge, an ultra-precise Møller measurement, and a probe of the axial quark charges in PVDIS.
    Journal of Physics Conference Series 06/2011; 299(1):012012.
  • Willem T H van Oers
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    ABSTRACT: The Thomas Jefferson National Accelerator Laboratory has the demonstrated ability to test the fundamental symmetries of nature to very great precision and thereby probe for new physics beyond the Standard Model (SM). In the following objectives and descriptions will be given of three Jefferson Laboratory (JLab) experiments: Qweak, MOLLER, and PVDIS. The Qweak experiment is to measure the weak charge of the proton (via the vector coupling of the Z0 boson to the proton). The MOLLER experiment is to measure the weak charge of the electron. The PV-DIS experiment will measure combinations of vector couplings and axial-vector couplings to the quarks of the nucleons. These three experiments follow from the advances made in precision parity-violating electron scattering measurements at the CEBAF of JLab. The Standard Model makes accurate predictions of the 'running' of the electroweak mixing angle or sin2(θw) from the Z0 pole down to low energies and therefore of the weak charges of the proton and electron. The Qweak experiment will make the first precision determination of the weak charge of the proton, Qpw = 1 – 4 sin2 (θw), from a measurement of the parity-violating asymmetry in the elastic scattering of longitudinally polarized electrons from the protons in a liquid hydrogen target at very low momentum transfer. The projected result will determine the proton's weak charge with a 4.1% total error and consequently sin2 (θw) with a 0.3% error. The Qweak experiment is at present three months into its commissioning run. The MOLLER experiment is to measure the parity-violating asymmetry in the scattering of 11 GeV longitudinally polarized electrons from the atomic electrons in a liquid hydrogen target. The longitudinal analyzing power Az is predicted to be 35.6 ppb at the kinematics of the experiment and is to be determined with a precision of 0.73 ppb, which would make the MOLLER experiment the most precise parity-violation experiment ever undertaken. The result would yield a measurement of the weak charge of the electron to 2.3% at an average Q2 value of 0.0056 (GeV/c)2 and in turn a determination of the electroweak mixing angle sin2(θw) with an uncertainty of ±0.00026 (stat) ±0.00013 (syst), comparable to the accuracy of the two best determinations at the Z0 pole. The PVDIS experiment is to measure the parity-violating asymmetry in deep inelastic scattering of longitudinally polarized electrons from an unpolarized deuterium target. The longitudinal analyzing power can be expressed in terms of the quark distribution functions of the deuterium target and the couplings C1q (axial electron x vector quark) and C2q (vector electron x axial quark), which in the Standard Model can be expressed in terms of sin2(θw).
    Journal of Physics Conference Series 05/2011; 295(1):012028.
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    ABSTRACT: In the G0 experiment, performed at Jefferson Lab, the parity-violating elastic scattering of electrons from protons and quasi-elastic scattering from deuterons is measured in order to determine the neutral weak currents of the nucleon. Asymmetries as small as 1 part per million in the scattering of a polarized electron beam are determined using a dedicated apparatus. It consists of specialized beam-monitoring and control systems, a cryogenic hydrogen (or deuterium) target, and a superconducting, toroidal magnetic spectrometer equipped with plastic scintillation and aerogel Cerenkov detectors, as well as fast readout electronics for the measurement of individual events. The overall design and performance of this experimental system is discussed.
    Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment - NUCL INSTRUM METH PHYS RES A. 03/2011; 646(1).
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    ABSTRACT: Proton total reaction cross sections (σR) have been measured for 51V, 54Fe, 56Fe, 57Fe, and 59Co at seven incident proton energies between 20.8 and 47.8 MeV at the University of Manitoba's Cyclotron Laboratory. Our results are compared with an optical-model analysis using global parameters. In general, the predicted total reaction cross sections agree quite well with the measured values. Nuclear transparency calculations have been made for these nuclei using all existing data between 8 and 100 MeV, and nuclei are found to be most reactive between 10 and 25 MeV, with nuclei becoming increasingly transparent as the incident proton energy increases above about 25 MeV. Using previously measured data from Si to 68Zn, along with the current measurements, we observe that when nuclear size effects are removed, the reaction cross section shows departures from average behavior near shell closings corresponding to N or Z values of 20 and 28.
    Canadian Journal of Physics 02/2011; 64(6):685-691. · 0.90 Impact Factor
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    ABSTRACT: Using a standard beam attenuation technique, proton total-reaction cross sections (σR) were measured for seven even isotopes of tin (112,114,116,118,120,122,124Sn) from 22 to 48 MeV with a typical accuracy of 2–3%. Two parameterizations of the data are presented, one based on an expression for the nuclear transparency, and the other on a semiempirical relation that has recently been developed.
    Canadian Journal of Physics 02/2011; 73:512-518. · 0.90 Impact Factor
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    ABSTRACT: A study of the 40Ca(p,2p)39K reaction has been made for two states of 39K, the 1d3/2−1(g.s.) and 2s1/2−1(2.53 MeV). Angular correlations at symmetric, coplanar angles between 30 and 105° have been measured. The data are compared with a distorted wave t-matrix calculation which utilizes a nonlocal bound state wave function. A further comparison is made with the angular correlation for unequal energies of the final state protons.
    Canadian Journal of Physics 02/2011; 51(9):1012-1016. · 0.90 Impact Factor
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    ABSTRACT: Recently experimental evidence has been presented for the existence of broad T = 1/2 resonances in the three-nucleon system. As a continuation of earlier studies the 6Li(p,α) reactions were investigated using 45.0 MeV incident protons. 3He and alpha-particle spectra were measured at extreme forward angles using a ΔE–E–VETO detector telescope in conjunction with an analog particle identifier. Excitation energies up to 30 MeV in 3He could be examined. The spectra do not show any structure that can be related to the resonances observed previously. However, the spectra exhibit evidence for quasi-free scattering of protons from the α cluster in 6Li. In a complementary study the 3He(p,2p) reactions were investigated at an incident energy of 45.0 MeV. Data were taken with coplanar, symmetric geometry at 10 pairs of angles between 25° and 60° and also with coplanar, asymmetric geometry at the following pairs of angles: θ1 = 40°; θ2 = 30°, 37.4°, 40°, 45°, 50°, 55°, 60°, and 70°. Pronounced kinematic bands due to the 3He(p,2p)d and 3He(p,2p)d* reactions were observed. A comparison of the continua of coincident proton–proton events with the predictions of four-body phase-space calculations shows good agreement. The four-body continua do not show any anomalies which can be interpreted as due to broad resonances in 3He.
    Canadian Journal of Physics 02/2011; 50(12):1295-1299. · 0.90 Impact Factor
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    ABSTRACT: We have measured parity violating asymmetries in elastic electron-proton and quasi-elastic electrondeuteron scattering at backward electron angle. These measurements have been done at two momentum transfers : Q2 = 0.22 and 0.63 (GeV/c)2 . Together with our previous forward angle measurement [1], we can extract strange quark contributions to the electromagnetic form factors of the nucleon, as well as nucleon axial form factor coming from the neutral weak interaction. The results indicate a strange quark magnetic contribution close to zero at these Q2 , and a possible non zero strange quark electric contribution for the high Q2 . The first Q2 behavior measurement of the nucleon axial form factor in elastic electron scattering shows a good agreement with radiative corrections calculated at Q2 = 0 and with a dipole form using the axial mass determined in neutrino scattering.
    EPJ Web of Conferences. 01/2010;
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    ABSTRACT: We have measured parity-violating asymmetries in elastic electron-proton and quasielastic electron-deuteron scattering at Q2=0.22 and 0.63 GeV2. They are sensitive to strange quark contributions to currents in the nucleon and the nucleon axial-vector current. The results indicate strange quark contributions of approximately < 10% of the charge and magnetic nucleon form factors at these four-momentum transfers. We also present the first measurement of anapole moment effects in the axial-vector current at these four-momentum transfers.
    Physical Review Letters 01/2010; 104(1):012001. · 7.73 Impact Factor
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    Willem T.H. van Oers
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    ABSTRACT: The weak interaction is manifested in parity-violating observables. With the weak interaction extremely well known parity-violating measurements in hadronic systems can be used to deduce strong interaction effects in those systems. Parity-violating analyzing powers in electron-proton scattering have led to determining the strange quark contributions to the charge and magnetization distributions of the nucleon. Parity-violating electron-proton and electron-electron scattering can also be performed to test the predictions of the Standard Model in the ‘running’ of the electroweak mixing angle or sin2θW.
    Nuclear Physics A 01/2010; · 2.50 Impact Factor

Publication Stats

1k Citations
306.19 Total Impact Points

Institutions

  • 1984–2013
    • TRIUMF
      Vancouver, British Columbia, Canada
    • Oregon State University
      • Department of Physics
      Corvallis, OR, United States
  • 2010–2012
    • University of Zagreb
      • Department of Physics (FKIT)
      Zagreb, Grad Zagreb, Croatia
  • 2011
    • Carnegie Mellon University
      • Department of Physics
      Pittsburgh, PA, United States
  • 1973–2011
    • University of Manitoba
      • Department of Physics and Astronomy
      Winnipeg, Manitoba, Canada
    • University of Birmingham
      Birmingham, England, United Kingdom
  • 2005–2007
    • College of William and Mary
      • Department of Physics
      Williamsburg, VA, United States
  • 1996
    • Russian Academy of Sciences
      Moskva, Moscow, Russia
  • 1993
    • University of Alberta
      • Department of Physics
      Edmonton, Alberta, Canada
  • 1975–1985
    • California State University, Los Angeles
      • Department of Physics and Astronomy
      Los Angeles, CA, United States
  • 1973–1976
    • University of California, Los Angeles
      Los Angeles, California, United States