The parity violation A4 experiment at forward and backward angles

The A4 Collaboration at the MAMI accelerator in Mainz measures the parity-violating asymmetry in the cross-section of elastic scattering of longitudinally polarized electrons off unpolarized protons from which the contribution of strange sea quarks to the vector form factors of the proton can be derived. Measurements at forward angles 30^° ⩽ θ ⩽ 40^° and two different momentum transfers Q² of 0.23 (GeV/c)² and 0.11 (GeV/c)² have been performed in the past. Measurements at backward angles 140^° ⩽ θ ⩽ 150^° are underway for a separate determination of the strange electric and the strange magnetic form factor at Q ² = 0.23(GeV/c)² .     

Strangeness contributions to nucleon form factors

We review a recent theoretical determination of the strange quark content of the electromagnetic form factors of the nucleon. These are compared with a global analysis of current experimental measurements in parity-violating electron scattering.     

Strange quarks in the nucleon sea

The HAPPEX Collaboration measured parity-violating electron scattering from ⁴He$(e, e)$and H(e, e) in 2004 and 2005 for Q ²≤0.11GeV^2. Results for the strange-quark contributions to the electromagnetic form factors of the nucleon from the 2004 data will be reviewed. Preliminary results from the 2005 data, which have significantly greater statistical precision, are G _E ^s = 0.004±0.014_stat±0.013_syst for Q ² = 0.0772GeV^2 from the helium data and G _E ^s +0.088G _M ^s = 0.004±0.011_stat±0.005_syst±0.004_FF for Q ² = 0.1089GeV^2 from the hydrogen data.     

New results from the HAPPEX Experiments at Q<Superscript>2</Superscript> = 0.1GeV/c<Superscript>2</Superscript>

The nucleon's strange electric and magnetic form factors G _E ^s and G _M ^s can be probed via parity-violating electron scattering. The HAPPEX Collaboration has made new measurements of the parity-violating asymmetry A _PV in elastic scattering of 3GeV electrons off hydrogen and ⁴He targets with 〈θ_lab〉 ≈ 6.0^° . For ⁴He the preliminary result is A _PV = (+ 6.43±0.23(stat)±0.22(syst))×10^-6 . For hydrogen the preliminary result is A _PV = (- 1.60±0.12(stat)±0.05(syst))×10^-6 . From these values we extract G ^s _E = 0.004±0.014±0.013 at 〈Q ²〉 = 0.077 GeV/c^2 , and G ^s _E +0.09G ^s _M = 0.004±0.011±0.005 at 〈Q ²〉 = 0.109 GeV/c^2 , both consistent with zero, providing stringent new limits on the role of strange quarks in the vector structure of the nucleon.     

The E158 experiment

We have carried out a precision measurement of the parity-violating asymmetry A _PV in the scattering of longitudinally polarized electrons off electrons in a liquid-hydrogen target. The measurement was performed with the 50GeV beam line at SLAC. The final result with the full data set collected in three production runs is A _PV = - 131±14 (stat) ±10 (syst) parts per billion. The result leads to new limits on possible contact interactions at the TeV scale. We discuss future prospects for more precise measurements.     

Results from the forward G0 experiment

The G0 experiment is dedicated to the determination of the strange quark contribution to the electric and magnetic nucleon form factors for a large range of momentum transfers between 0.1 to 1(GeV/c)² . This information is provided by the asymmetries of cross-sections measured with longitudinally polarized electrons in elastic electron-proton scattering and quasi-elastic electron-deuteron scattering. A set of measurements at two different Q² will allow the complete separation of the electric and magnetic weak, as well as axial nucleon form factors. This report summarizes the physics case, gives details about the dedicated set-up used, and shows the results of the combination of the strange quark contribution in the electric and magnetic form factors of the protons. The experiment was performed at the Jefferson Laboratory, during years 2003 and 2004, and will be completed after backward-angle measurements in 2006, 2007.     

Measurement of the electric form factor of the neutron at Q2 = 0.3-0.8 (GeV/ c) 2 ⋆

The electric form factor of the neutron, G_E,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q ² = 0.3, 0.6 and 0.8 (GeV/c)². Corrections for nuclear binding effects have been applied.This revised version was published online in March 2005. In the previous version, the email address of one author was inadvertently assigned to multiple authors.     

Nucleon elastic form factors

The nucleon form factors are still the subject of active investigation even after an experimental effort spanning 50 years. This is because they are of critical importance to our understanding of the electromagnetic properties of nuclei and provide a unique testing ground for QCD motivated models of nucleon structure. Progress in polarized beams, polarized targets and recoil polarimetry have allowed an important and precise set of data to be collected over the last decade. I will review the experimental status of elastic electron scattering from the nucleon along with an outlook for future progress.     

Flavor content of nucleon form factors in a VMD approach

The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 Collaborations shows a good overall agreement. It is shown that the strangeness contribution to the electric and magnetic form factors is of the order of a few percent. In particular, the strange quark contribution to the charge radius is small 〈r ² _s〉_E = 0.005 fm^2 and to the magnetic moment it is positive μ_s = 0.315 μ_N .     

P, T violating magneto-electro-optics

We study linear and bilinear magneto-electro-optical effects due to the propagation of light in centro-symmetric media in the presence of P, T violating interactions and external transverse and longitudinal electric and/or magnetic fields. We show that new magneto-electric optical effects appear. In particular, we show the existence of a Jones birefringence proportional to the square of the transverse field amplitude. All these effects are an unambiguous signature of the P, T violation, and a search for such new phenomena could also provide novel limits on electric dipole moment (EDM) of matter.     

Improved radiative corrections for (e, e'p) experiments: Beyond the peaking approximation and implications of the soft-photon approximation

Analyzing (e, e'p) experimental data involves corrections for radiative effects which change the interaction kinematics and which have to be carefully considered in order to obtain the desired accuracy. Missing momentum and energy due to bremsstrahlung have so far often been incorporated into the simulations and the experimental analyses using the peaking approximation. It assumes that all bremsstrahlung is emitted in the direction of the radiating particle. In this article we introduce a full angular Monte Carlo simulation method which overcomes this approximation. As a test, the angular distribution of the bremsstrahlung photons is reconstructed from H(e, e'p) data. Its width is found to be underestimated by the peaking approximation and described much better by the approach developed in this work. The impact of the soft-photon approximation on the photon angular distribution is found to be minor as compared to the impact of the peaking approximation.     

Future directions in parity violation

I discuss the prospects for future studies of parity-violating (PV) interactions at low energies and the insights they might provide about open questions in the standard model as well as physics that lies beyond it. I cover four types of parity-violating observables: PV electron scattering; PV hadronic interactions; PV correlations in weak decays and searches for the permanent electric dipole moments of quantum systems.     

Controlling helicity-correlated beam asymmetries in a polarized electron source

The control of helicity-correlated changes in the electron beam is a critical issue for the next generation of parity-violating electron scattering measurements. The underlying causes and methods for controlling these changes are reviewed with reference to recent operational experience at Jefferson Lab.     

Sensitivity of the pQCD deuteron structure to the nucleon form factors

We discuss the applicability of pQCD to the elastic scattering of electrons on protons and deuterons. We analyze the Q²-dependence of the reduced deuteron form factor, taking into account the recent data on the electric proton form factor and we find that the value of the QCD-scale parameter Λ differs essentially from the value Λ = 0.1 GeV, previously found using the dipole parametrization of the electromagnetic nucleon form factors G_E and G_M. Moreover, the predicted scaling behavior of the reduced deuteron form factor cannot be recovered in the Dirac and Pauli representations for the nucleon electromagnetic form factors. Received: 14 October 2002 / Accepted: 12 November 2002 / Published online: 11 March 2003 RID="a" ID="a"Permanent address: National Science Center KFTI, 310108 Kharkov, Ukraine. RID="b" ID="b"e-mail: etomasi@cea.fr Communicated by V.V. Anisovich     

An experiment for the measurement of the bound- β-decay of the free neutron

The hyperfine-state population of hydrogen after the bound-β-decay of the neutron directly yields the neutrino left-handedness or a possible right-handed admixture and possible small scalar and tensor contributions to the weak force. Using the through-going beam tube of a high-flux reactor, a background free hydrogen rate of ca. 3s^-1 can be obtained. The detection of the neutral hydrogen atoms and the analysis of the hyperfine states is accomplished by Lamb shift source type quenching and subsequent ionization. The constraints on the neutrino helicity and the scalar and tensor coupling constants of the weak interaction can be improved by a factor of ten.     

Implications of the JLab proton polarization data for the behavior of strange nucleon form factors

A special eight-resonance unitary and analytic model of nucleon electromagnetic structure is used to analyze first the classical proton form factor data obtained by the Rosenbluth technique, and then also the contradictory JLab proton polarization data on the ratio μ_p G _Ep(Q ²)/G _Mp(Q ²) , with the aim to investigate the implications of the latter for the behavior of strange nucleon form factors.     

Inelastic final-state interactions in B → VV →π K processes

We study the final-state interactions in B →π K decays through B → VV →π K processes where the inelastic rescattering occurs via single pion exchange. The next-to-leading order low-energy effective Hamiltonian and Bauer-Stech-Wirbel (BSW) model are used to evaluate the weak transition matrix elements and final-state interactions. We found that the final-state interaction effects in B →ρ K^*→π K processes are significant. The Fleischer-Mannel relation for the Cabibbo-Kobayashi-Maskawa (CKM) angle γ can be significantly modified. Received: 13 August 1998     

Asymptotic behavior of nucleon electromagnetic form factors in time-like region

We study the asymptotic behavior of the ratio of Pauli and Dirac electromagnetic nucleon form factors, F₂/F₁, in time-like region, for different parametrizations built for the space-like region. We investigate how fast the ratio F₂/F₁ approaches the asymptotic limits according to the Phragmèn-Lindel?f theorem. We show that the QCD-inspired logarithmic behavior of this ratio results in very far asymptotics, experimentally unachievable. This is also confirmed by the normal component of the nucleon polarization, P_y, in e⁺ + e^-↦N + ˉ (in collisions of unpolarized leptons), which is a very interesting observable, with respect to this theorem. Finally we observe that the 1/Q parametrization of F₂/F₁ contradicts this theorem.