Search Results (15 total)

We measured the angular dependence of the three recoil-proton polarization components in two-body photodisintegration of the deuteron at a photon energy of 2 GeV. These new data provide a benchmark for calculations based on quantum chromodynamics. Two of the five existing models have made predictions of polarization observables. Both explain the longitudinal polarization transfer satisfactorily. Transverse polarizations are not well described, but suggest isovector dominance.

We report new measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets with ⟨θ_lab⟩≈6.0°. The ⁴He result is A_PV=(+6.40±0.23(stat)±0.12(syst))×10^-6. The hydrogen result is A_PV=(-1.58±0.12(stat)±0.04(syst))×10^-6. These results significantly improve constraints on the electric and magnetic strange form factors G_E^s and G_M^s. We extract G_E^s=0.002±0.014±0.007 at ⟨Q²⟩=0.077  GeV², and G_E^s+0.09G_M^s=0.007±0.011±0.006 at ⟨Q²⟩=0.109  GeV², providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ⁴He at an average scattering angle ⟨θ_lab⟩=5.7° and a four-momentum transfer Q²=0.091  GeV². From these data, for the first time, the strange electric form factor of the nucleon G_E^s can be isolated. The measured asymmetry of A_PV=(6.72±0.84_(stat)±0.21_(syst))×10^-6 yields a value of G_E^s=-0.038±0.042_(stat)±0.010_(syst), consistent with zero.

We present the first measurement of the Q² dependence of the neutron spin structure function g₂ⁿ at five kinematic points covering 0.57  (GeV/c)²≤Q²≤1.34  (GeV/c)² at x≃0.2. Though the naive quark-parton model predicts g₂=0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g₂ⁿ can be predicted using next-to-leading order fits to world data for g₁ⁿ. Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q², indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g₁ⁿ are consistent with next-to-leading order fits to world data.

We report on a precision measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_PV=[-131±14(stat)±10(syst)]×10^-9, leading to the determination of the weak mixing angle sin⁡²θ_W^eff=0.2397±0.0010(stat)±0.0008(syst), evaluated at Q²=0.026  GeV². Combining this result with the measurements of sin⁡²θ_W^eff at the Z⁰ pole, the running of the weak mixing angle is observed with over 6σ significance. The measurement sets constraints on new physics effects at the TeV scale.

We report on measurements of the neutron spin asymmetries A_1,2ⁿ and polarized structure functions g_1,2ⁿ at three kinematics in the deep inelastic region, with x=0.33, 0.47, and 0.60 and Q²=2.7, 3.5, and 4.8  (GeV∕c)², respectively. These measurements were performed using a 5.7  GeV longitudinally polarized electron beam and a polarized ³He target. The results for A₁ⁿ and g₁ⁿ at x=0.33 are consistent with previous world data and, at the two higher-x points, have improved the precision of the world data by about an order of magnitude. The new A₁ⁿ data show a zero crossing around x=0.47 and the value at x=0.60 is significantly positive. These results agree with a next-to-leading-order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (PQCD) assuming hadron helicity conservation. Results for A₂ⁿ and g₂ⁿ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d₂ⁿ was evaluated and the new result has improved the precision of this quantity by about a factor of 2. When combined with the world proton data, polarized quark distribution functions were extracted from the new g₁ⁿ∕F₁ⁿ values based on the quark-parton model. While results for Δu∕u agree well with predictions from various models, results for Δd∕d disagree with the leading-order PQCD prediction when hadron helicity conservation is imposed.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons. Significant contributions to this asymmetry could arise from the contributions of strange form factors in the nucleon. The measured asymmetry is A=−15.05±0.98(stat)±0.56(syst)  ppm at the kinematic point ⟨θ_lab⟩=12.3° and ⟨Q²⟩=0.477  (GeV∕c)². Based on these data as well as data on electromagnetic form factors, we extract the linear combination of strange form factors G_E^s+0.392G_M^s=0.014±0.020±0.010, where the first error arises from this experiment and the second arises from the electromagnetic form factor data. This paper provides a full description of the special experimental techniques employed for precisely measuring the small asymmetry, including the first use of a strained GaAs crystal and a laser-Compton polarimeter in a fixed target parity-violation experiment.

We report a measurement of the parity-violating asymmetry in fixed target electron-electron (Møller) scattering: A_PV=[-175±30(stat)±20(syst)]×10^-9. This first direct observation of parity nonconservation in Møller scattering leads to a measurement of the electron’s weak charge at low energy Q_W^e=-0.053±0.011. This is consistent with the standard model expectation at the current level of precision: sin⁡²θ_W(M_Z)_MS̅ =0.2293±0.0024(stat)±0.0016(syst)±0.0006(theory).

We have measured the neutron spin asymmetry A₁ⁿ with high precision at three kinematics in the deep inelastic region at x=0.33, 0.47, and 0.60, and Q²=2.7, 3.5, and 4.8   (GeV/c)², respectively. Our results unambiguously show, for the first time, that A₁ⁿ crosses zero around x=0.47 and becomes significantly positive at x=0.60. Combined with the world proton data, polarized quark distributions were extracted. Our results, in general, agree with relativistic constituent quark models and with perturbative quantum chromodynamics (PQCD) analyses based on the earlier data. However they deviate from PQCD predictions based on hadron helicity conservation.

The ratio of the electric and magnetic form factors of the proton G_Ep/G_Mp, which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e→p→ep→ reaction. The new data presented span the range 3.5<Q²<5.6 GeV² and are well described by a linear Q² fit. Also, the ratio sqrt[Q²] F_2p/F_1p reaches a constant value above Q²  =  2 GeV².

The QED radiative corrections to virtual Compton scattering (reaction ep→epγ) are calculated to first order in α_em≡e²/4π. A detailed study is presented for the one-loop virtual corrections and for the first order soft-photon emission contributions. Furthermore, a full numerical calculation is given for the radiative tail, corresponding to photon emission processes, where the photon energy is not very small compared with the lepton momenta. We compare our results with existing works on elastic electron-proton scattering, and show for the ep→epγ reaction how the observables are modified due to these first order QED radiative corrections. We show results for both unpolarized and polarized observables of virtual Compton scattering in the low energy region (where one is sensitive to the generalized polarizabilities of the nucleon), as well as for the deeply virtual Compton scattering.

Absolute differential cross sections for the reaction ep→epγ have been measured at a four-momentum transfer with virtuality Q²  =  0.33 GeV² and polarization ε  =  0.62 in the range 33.6 to 111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high-resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point [ 〈θ_lab〉  =  12.3° and 〈Q²〉  =  0.48 (GeV/c)²] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor G_E^s. The result, A  =  -14.5±2.2 ppm, is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies G_E^s+0.39G_M^s  =  0.023±0.034(stat)±0.022(syst)±0.026(δG_Eⁿ), where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor.

We have investigated the second-sound velocity u₂ in rotating superfluid helium by observing the resonant modes of a rotating cavity. We have discovered an unexpected decrease in u₂. This effect has been measured in different second-sound resonators, and as a function of the second-sound frequency (10³≤ω≤10⁵ rad/sec), the HeII temperature (1.3≤T≤2.1 °K), and the angular velocity of rotation (Ω≤8 rad/sec). We have found that the decease in u₂ is proportional to Ω and is inversely proportional to the second-sound frequency. We show that this decrease may be explained within the the two-fluid model of Landau and Khalatnikov if we suppose the existence of a thermodynamic coupling between the dissipative heat flux and the mutual-friction force. The principal cause of this coupling, we suggest, is entropy transport by the moving vortices. We discuss the origin of this entropy transport using the idea, suggested by Hall and Vinen, of the moving vortices dragging the normal fluid. The theoretical expression we obtain for the decrease of u₂ in rotating helium is in reasonable agreement with our experimental results. We propose that this effect can be considered as the analog in rotating HeII of the thermomagnetic effect in the mixed state of type-II superconductors.

It is shown that the Landau two-fluid equations may be thought of as the Hamilton equations of two pairs of conjugate variables. This result is used to study the vortex dynamics and to obtain a method for generalizing the Landau equations when the superfluid velocity is no longer irrotational. These generalized equations are also derived from a variational principle, using a slightly modified form of Lin's constraint. Lastly we analyze the consequences on the Josephson effect of the existence of two pairs of conjugate variables.