Fri·Sep 26, 2014Friday·September 26, 2014

1. 01*

   Strong coupling effects in near-barrier heavy-ion elastic scattering

   N. Keeley🇫🇷 · K.W. Kemper🇺🇸 · K. Rusek🇫🇷

   Accurate elastic scattering angular distribution data measured at bombarding energies just above the Coulomb barrier have shapes that can markedly differ from or be the same as the expected classical Fresnel scattering pattern depending on the structure of the projectile, the target or both. Examples are given such as 18O + 184W and 16O + 148,152Sm where the expected rise above Rutherford scattering due to Coulomb-nuclear interference is damped by coupling to the target excited states, and the extreme case of 11Li scattering, where coupling to the 9Li + n + n continuum leads to an elastic scattering shape that cannot be reproduced by any standard optical model parameter set. The recent availability of high quality 6He, 11Li and 11Be data provides further examples of the influence that coupling effects can have on elastic scattering. Conditions for strong projectile-target coupling effects are presented with special emphasis on the importance of the beam-target charge combination being large enough to bring about the strong coupling effects. Several measurements are proposed that can lead to further understanding of strong coupling effects by both inelastic excitation and nucleon transfer on near-barrier elastic scattering. A final note on the anomalous nature of 8B elastic scattering is presented as it possesses a more or less normal Fresnel scattering shape whereas one would a priori not expect this due to the very low breakup threshold of 8B. The special nature of 11Li is presented as it is predicted that no matter how far above the Coulomb barrier the elastic scattering is measured, its shape will not appear as Fresnel like whereas the elastic scattering of all other loosely bound nuclei studied to date should eventually do so as the incident energy is increased, making both 8B and 11Li truly "exotic".

   nucl-exnucl-thEPJA(2014)·40 citations
2. 02*

   The Q_weak Experimental Apparatus

   Qweak Collaboration:T. Allison🇺🇸 · M. Anderson🇨🇦 · D. Androic🇭🇷 · D.S. Armstrong🇺🇸 · A. Asaturyan🇦🇲 · T.D. Averett🇺🇸 · R. Averill🇺🇸 · J. Balewski🇺🇸 · J. Beaufait🇺🇸 · R.S. Beminiwattha🇺🇸 · J. Benesch🇺🇸 · F. Benmokhtar🇺🇸 and 116 other authors

   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.

   ↳ physics.ins-detnucl-exphysics.acc-phNucl.Instrum.Meth.A(2015)·61 citations
3. 03*

   Nucleon resonances in the ${\pi }^{-}p\to K^0\Lambda$ reaction near threshold

   Cheng-Zu Wu🇨🇳 · Qi-Fang Lü🇨🇳 · Ju-Jun Xie🇨🇳 · Xu-Rong Chen🇨🇳

   We investigate the two-body reaction ${\pi }^{-}p\to K^0\Lambda$ within the effective Lagrangian approach and the isobar model. In addition to the "background" contributions from $t$-channel $K^{\ast }$ exchange, $u$-channel $\Sigma (1192)$ and ${\Sigma }^{\ast }(1385)$ exchanges, and $s$-channel nucleon pole terms, the contributions from the nucleon resonances $N^{\ast }(1535)$, $N^{\ast }(1650)$, and $N^{\ast }(1720)$ are investigated. It is shown that the inclusion of these nucleon resonances contributions leads to a good description of the experimental total and differential cross sections data at low energy region. The $s$-channel $N^{\ast }(1535)$, $N^{\ast }(1650)$, and $N^{\ast }(1720)$ resonances and the $t$-channel $K^{\ast }$ exchange give the dominant contributions below $W=1.76$ GeV, while the $u$-channel $\Sigma (1192)$ and ${\Sigma }^{\ast }(1385)$ exchanges give the minor contributions.

   ↳ nucl-thhep-phnucl-exCommun.Theor.Phys.(2015)·10 citations
4. 04*

   Calculation of neutron-${}^3$He scattering up to 30 MeV

   A. Deltuva🇵🇹 · A. C. Fonseca🇵🇹

   Microscopic calculations of four-body collisions become very challenging in the energy regime above the threshold for four free particles. The neutron-${}^3$He scattering is an example of such process with elastic, rearrangement, and breakup channels. We aim to calculate observables for elastic and inelastic neutron-${}^3$He reactions up to 30 MeV neutron energy using realistic nuclear force models. We solve the Alt, Grassberger, and Sandhas (AGS) equations for the four-nucleon transition operators in the momentum-space framework. The complex-energy method with special integration weights is applied to deal with the complicated singularities in the kernel of AGS equations. We obtain fully converged results for the differential cross section and neutron analyzing power in the neutron-${}^3$He elastic scattering as well as the total cross sections for inelastic reactions. Several realistic potentials are used, including the one with an explicit $\Delta$ isobar excitation. There is reasonable agreement between the theoretical predictions and experimental data for the neutron-${}^3$He scattering in the considered energy regime. The most remarkable disagreements are seen around the minimum of the differential cross section and the extrema of the neutron analyzing power. The breakup cross section increases with energy exceeding rearrangement channels above 23 MeV.

   ↳ nucl-thcond-mat.quant-gasnucl-exPRC(2014)·27 citations

* Reconstructed cohort: no mailing for this day survives in the archive. Papers are grouped by their submission times and arXiv's announcement cut-off, assuming announcement without delay; positions follow identifier order. Validated at ~91% exact-day agreement against the archived era.