Search Results (11 total)

The beam-beam effects have been the dominating sources of beam loss and lifetime limitations in the Tevatron proton-antiproton collider [V. Shiltsev , Phys. Rev. ST Accel. Beams 8, 101001 (2005)]. Electron lenses were originally proposed for compensation of electromagnetic long-range and head-on beam-beam interactions of proton and antiproton beams [V. Shiltsev , Phys. Rev. ST Accel. Beams 2, 071001 (1999).]. Results of successful employment of two electron lenses built and installed in the Tevatron are reported by Shiltsev et al. [Phys. Rev. Lett. 99, 244801 (2007); New J. Phys. 10, 043042 (2008)] and by Zhang et al. [X.-L. Zhang , Phys. Rev. ST Accel. Beams 11, 051002 (2008)]. In this paper we present design features of the Tevatron electron lenses (TELs), discuss the generation of electron beams, describe different modes of operation, and outline the technical parameters of various subsystems.

High-precision measurements of the proton elastic form-factor ratio, μ_pG_E^p/G_M^p, have been made at four-momentum transfer, Q², values between 0.2 and 0.5  GeV². The new data, while consistent with previous results, clearly show a ratio less than unity and significant differences from the central values of several recent phenomenological fits. By combining the new form-factor ratio data with an existing cross-section measurement, one finds that in this Q² range the deviation from unity is primarily due to G_E^p being smaller than expected.

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 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 report on a precision measurement of the neutron spin structure function g₁ⁿ using deep inelastic scattering of polarized electrons by polarized ³He. For the kinematic range 0.014<x<0.7 and 1<Q²<17 (GeV/c)², we obtain ∫0.014^0.7g₁ⁿ(x) dx  =  -0.036±0.004(stat)±0.005(syst) at an average Q²  =  5 (GeV/c)². We find relatively large negative values for g₁ⁿ at low x. The results call into question the usual Regge theory method for extrapolating to x  =  0 to find the full neutron integral ∫1g₁ⁿ(x) dx, needed for testing the quark-parton model and QCD sum rules.

We present a theory that models the reflectance difference spectrum of bulk, spontaneously ordered Ga_0.5In_0.5P. Near the band gap E₀ this spectrum exhibits a sharp, negative feature at E₀ and a broad positive feature that peaks near E₀+Δ_S. The zero crossing between these two peaks occurs near E₀+Δ_C. For the sample studied in this paper, the spin-orbit splitting Δ_s and the crystal-field splitting Δ_C are 120 and 25 meV, respectively. Two previous calculations, which assume constant transition-matrix elements, were able to produce a negative peak at E₀, but not the positive feature. In this paper, the reflectance difference spectrum near the band gap is calculated using an 8-band k⋅p model and an explicit treatment of the momentum or k dependence of the transition-matrix elements. The new calculation produces both the negative peak at E₀ and the positive feature that peaks near E₀+Δ_S. The positive feature is attributed to the strong k dependence of the matrix element anisotropy. A strong coupling, enhanced by ordering, between three valence bands is essential. A problem associated with the analytical expression for the dielectric function ɛ used in previous calculations is discussed.

Using time-resolved small-signal exciton absorption bleaching at low temperature as a spectroscopic technique, the optical transition energies from all three valence bands in spontaneously ordered Ga_xIn_1-xP have been measured with high accuracy. The origin of the bleaching signal and the contributions of reflection, strain, and binding energy are discussed. With three measured energies from each sample, all parameters in the quasicubic perturbation model can be fitted. Good agreement is obtained with a spin-orbit-splitting parameter of 103 meV, nearly independent of the degree of ordering. The ratio of band-gap reduction to crystal-field-splitting parameter is found to be 2.7, slightly higher than previous works. This difference is attributed to a more accurate determination of light-hole-like band-gap energy.

We have measured the sub-barrier fusion cross sections for ¹⁶,17,18O on ¹⁶O and we present the data and the experimental method in detail. The data were analyzed with a one-dimensional potential barrier inversion model, a two-dimensional incoming wave boundary condition model, and a two-dimensional Wentzel-Kramers-Brillouin model. We find that a multidimensional model is necessary to describe the enhanced fusion cross sections in oxygen and the two-dimensional incoming wave boundary condition model suggests that a complete description of the fusion, inelastic, and elastic cross sections is possible.

We have measured the sub-barrier fusion cross sections for the ¹⁶O+ ¹⁶O and ¹⁶O+ ¹⁸O systems from E_lab=13.5–25 MeV. We find no significant enhancement of the ¹⁶O+ ¹⁸O cross section compared with the ¹⁶O+ ¹⁶O cross section; this is quite different from the sub-barrier fusion of the calcium isotopes which exhibit a strong isotope dependence.

The total cross section for ⁷Li(α,n)¹⁰B has been measured from threshold to E_α=5.67 MeV, and inverted by detailed balance to find the ¹⁰B(n,α₀) cross section from 0<E_n≤0.78 MeV. Parameters of two resonances, at E_n=241 and 493 keV, are determined from the data.