E. Korkmaz

First Determination of the Weak Charge of the Proton

The Q(weak) experiment has measured the parity-violating asymmetry in ep elastic scattering at Q(2)=0.025(GeV/c)(2), employing 145 μA of 89% longitudinally polarized electrons on a 34.4 cm long liquid hydrogen target at Jefferson Lab. The results of the experiments commissioning run, constituting approximately 4% of the data collected in the experiment, are reported here. From these initial results, the measured asymmetry is A(ep)=-279±35 (stat) ± 31 (syst) ppb, which is the smallest and most precise asymmetry ever measured in ep scattering. The small Q(2) of this experiment has made possible the first determination of the weak charge of the proton Q(W)(p) by incorporating earlier parity-violating electron scattering (PVES) data at higher Q(2) to constrain hadronic corrections. The value of Q(W)(p) obtained in this way is Q(W)(p)(PVES)=0.064±0.012, which is in good agreement with the standard model prediction of Q(W)(p)(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 neutrons weak charge to be Q(W)(n)(PVES+APV)=-0.975±0.010.

Transverse beam spin asymmetries in forward-angle elastic electron-proton scattering

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q2=0.15, 0.25 (GeV/c)2. The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A(n) provides a direct probe of the imaginary component of the 2gamma exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

Precision measurement of the weak charge of the proton

Large experimental programmes in the fields of nuclear and particle physics search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson completed the set of particles predicted by the standard model, which currently provides the best description of fundamental particles and forces. However, this theory’s limitations include a failure to predict fundamental parameters, such as the mass of the Higgs boson, and the inability to account for dark matter and energy, gravity, and the matter–antimatter asymmetry in the Universe, among other phenomena. These limitations have inspired searches for physics beyond the standard model in the post-Higgs era through the direct production of additional particles at high-energy accelerators, which have so far been unsuccessful. Examples include searches for supersymmetric particles, which connect bosons (integer-spin particles) with fermions (half-integer-spin particles), and for leptoquarks, which mix the fundamental quarks with leptons. Alternatively, indirect searches using precise measurements of well predicted standard-model observables allow highly targeted alternative tests for physics beyond the standard model because they can reach mass and energy scales beyond those directly accessible by today’s high-energy accelerators. Such an indirect search aims to determine the weak charge of the proton, which defines the strength of the proton’s interaction with other particles via the well known neutral electroweak force. Because parity symmetry (invariance under the spatial inversion (x, y, z)u2009→u2009(−x, −y, −z)) is violated only in the weak interaction, it provides a tool with which to isolate the weak interaction and thus to measure the proton’s weak charge1. Here we report the value 0.0719u2009±u20090.0045, where the uncertainty is one standard deviation, derived from our measured parity-violating asymmetry in the scattering of polarized electrons on protons, which is −226.5u2009±u20099.3 parts per billion (the uncertainty is one standard deviation). Our value for the proton’s weak charge is in excellent agreement with the standard model2 and sets multi-teraelectronvolt-scale constraints on any semi-leptonic parity-violating physics not described within the standard model. Our results show that precision parity-violating measurements enable searches for physics beyond the standard model that can compete with direct searches at high-energy accelerators and, together with astronomical observations, can provide fertile approaches to probing higher mass scales. Measurement of the asymmetry in the parity-violating scattering of polarized electrons on protons gives the weak charge of the proton as 0.0719u2009±u20090.0045, in agreement with the standard model.Large experimental programmes in the fields of nuclear and particle physics search for evidence of physics beyond that explained by current theories. The observation of the Higgs boson completed the set of particles predicted by the standard model, which currently provides the best description of fundamental particles and forces. However, this theory’s limitations include a failure to predict fundamental parameters, such as the mass of the Higgs boson, and the inability to account for dark matter and energy, gravity, and the matter–antimatter asymmetry in the Universe, among other phenomena. These limitations have inspired searches for physics beyond the standard model in the post-Higgs era through the direct production of additional particles at high-energy accelerators, which have so far been unsuccessful. Examples include searches for supersymmetric particles, which connect bosons (integer-spin particles) with fermions (half-integer-spin particles), and for leptoquarks, which mix the fundamental quarks with leptons. Alternatively, indirect searches using precise measurements of well predicted standard-model observables allow highly targeted alternative tests for physics beyond the standard model because they can reach mass and energy scales beyond those directly accessible by today’s high-energy accelerators. Such an indirect search aims to determine the weak charge of the proton, which defines the strength of the proton’s interaction with other particles via the well known neutral electroweak force. Because parity symmetry (invariance under the spatial inversion (x, y, z)u2009→u2009(−x, −y, −z)) is violated only in the weak interaction, it provides a tool with which to isolate the weak interaction and thus to measure the proton’s weak charge1. Here we report the value 0.0719u2009±u20090.0045, where the uncertainty is one standard deviation, derived from our measured parity-violating asymmetry in the scattering of polarized electrons on protons, which is −226.5u2009±u20099.3 parts per billion (the uncertainty is one standard deviation). Our value for the proton’s weak charge is in excellent agreement with the standard model2 and sets multi-teraelectronvolt-scale constraints on any semi-leptonic parity-violating physics not described within the standard model. Our results show that precision parity-violating measurements enable searches for physics beyond the standard model that can compete with direct searches at high-energy accelerators and, together with astronomical observations, can provide fertile approaches to probing higher mass scales.Measurement of the asymmetry in the parity-violating scattering of polarized electrons on protons gives the weak charge of the proton as 0.0719u2009±u20090.0045, in agreement with the standard model.

Drift chambers for a precision measurement of the Michel parameters in muon decay

Planar drift chambers will be used in a solenoidal magnetic field to measure the Michel parameters in muon decay. The chamber manufacture procedure uses flat glass tables and precise ceramic glass spacers to provide precise anode– cathode spacing. Results of bench and beam tests of the prototype chambers are discussed. # 2001 Elsevier Science B.V. All rights reserved.

Segmented detector for recoil neutrons in the p(γ, n)π+ reaction

Abstract A segmented neutron detector has been constructed and used for recoil neutron (6–13xa0MeV) measurements of the reaction γp→nπ+ very close to threshold. BC-505 liquid scintillator was used to allow pulse shape discrimination between neutrons and photons. A measurement of the absolute efficiency of the detector was performed using stopped pions in the reaction π−p→nγ. Results of the efficiency calibration are compared to a Monte Carlo simulation.

Early Results from the Qweak Experiment

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 scattering to provide the first determination of the proton’s weak charge . The experiment employed a 180 μ A longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° θ 2 = 0.025 GeV 2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin 2 ( θ w ) at the level of 0.3%, and to help constrain the vector weak quark charges C 1 u and C 1 d . The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH 2 target. The new constraints on C 1 u and C 1 d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron.

Apparatus for a measurement of charge symmetry breaking in np→dπ0☆

Abstract TRIUMF experiment E704 is a study of Charge Symmetry Breaking in the reaction np→dπ 0 . The observable is the forward–backward asymmetry in the centre-of-mass differential cross-section, predicted to be −0.0035 at 5 MeV above threshold. As a check on the understanding of certain systematic effects the reaction pp→dπ + , which cannot have an asymmetry, is measured also. Similarly, neutron–proton elastic scattering is used to verify spectrometer acceptance. We describe the beams, target, and detectors developed for this experiment.

Early Results from the Q{sub weak} Experiment

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 scattering to provide the first determination of the proton’s weak charge . The experiment employed a 180 μ A longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° θ 2 = 0.025 GeV 2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin 2 ( θ w ) at the level of 0.3%, and to help constrain the vector weak quark charges C 1 u and C 1 d . The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH 2 target. The new constraints on C 1 u and C 1 d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron.

Early results from the

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 scattering to provide the first determination of the proton’s weak charge . The experiment employed a 180 μ A longitudinally polarized 1.16 GeV electron beam on a 35 cm long liquid hydrogen target. Scattered electrons in the angular range 6° θ 2 = 0.025 GeV 2 were detected in eight Cerenkov detectors arrayed symmetrically around the beam axis. The goals of the experiment were to provide a measure of to 4.2% (combined statisstatistical and systematic error), which implies a measure of sin 2 ( θ w ) at the level of 0.3%, and to help constrain the vector weak quark charges C 1 u and C 1 d . The experimental method is described, with particular focus on the challenges associated with the world’s highest power LH 2 target. The new constraints on C 1 u and C 1 d provided by the subset of the experiment’s data analyzed to date will also be shown, together with the extracted weak charge of the neutron.