233 Search Results

In this paper, we report the first measurement of the parity-violating elastic electron scattering asymmetry on ²⁷Al. The ²⁷Al elastic asymmetry is $$A_{\text{PV}}$$ = 2.16 ± 0.11(stat) ± 0.16(syst) ppm, and was measured at $$\langle Q^2\rangle$$ = 0.02357 ± 0.00010 GeV$^2$, $$\angleθ_{\text{lab}}$$ = 7.61° ± 0.02°, and $$\langle E_{\text{lab}}$$ = 1.157 GeV with the Q_weak apparatus at Jefferson Lab. Predictions using a simple Born approximation as well as more sophisticated distorted-wave calculations are in good agreement with this result. From this asymmetry the ²⁷Al neutron radius $$R_n$$ = 2.89 ± 0.12 fm was determined using a many-models correlation technique. Themore » corresponding neutron skin thickness $$R_n – R_p$$ = –0.04 ± 0.12 fm is small, as expected for a light nucleus with a neutron excess of only 1. This result thus serves as a successful benchmark for electroweak determinations of neutron radii on heavier nuclei. A tree-level approach was used to extract the ²⁷Al weak radius $$R_w$$ = 3.00 ± 0.15 fm, and the weak skin thickness $$R_{\text{wk}} – R_{\text{ch}}$$ = –0.04 ± 0.15 fm. The weak form factor at this $Q^2$ is $$F_{\text{wk}}$$ = 0.39 ± 0.04.« less

We report measurements of the parity-conserving beam-normal single-spin elastic scattering asymmetries B_n on ¹²C and ²⁷Al, obtained with an electron beam polarized transverse to its momentum direction. These measurements add an additional kinematic point to a series of previous measurements of B_n on ¹²C and provide a first measurement on ²⁷Al. The experiment utilized the Q_weak apparatus at Jefferson Lab with a beam energy of 1.158 GeV. The average lab scattering angle for both targets was 7.7°, and the average Q² for both targets was 0.02437 GeV² (Q = 0.1561 GeV). The asymmetries are B_n = -10.68 ± 0.90 (stat)more » ± 0.57 (syst) ppm ¹²C and B_n = -12.16 ± 0.58 (stat) ± 0.62 (syst) ppm for ²⁷Al. The results are consistent with theoretical predictions, and are compared to existing data. When scaled by Z/A, the Q dependence of all the far-forward angle (θ < 10°) data from ¹H to ²⁷Al can be described by the same slope out to Q ≈ 0.35 GeV. Larger-angle data from other experiments in the same Q range are consistent with a slope about twice as steep.« less

Missing mass spectroscopy with the $$(e,e^{\prime}K^{+})$$ reaction was performed at JLab Hall C for the neutron rich $$\Lambda$$ hypernucleus $$^{9}_{\Lambda}{\rm Li}$$. The ground state energy was obtained to be $$B_{\Lambda}^{\rm g.s.}=8.84\pm0.17^{\rm stat.}\pm0.15^{\rm sys.}~{\rm MeV}$$ by using shell model calculations of a cross section ratio and an energy separation of the spin doublet states ($$3/2^{+}_1$$ and $$5/2^{+}_1$$). In addition, peaks that are considered to be states of [$$^{8}{\rm Li}(3^{+})\otimes s_{\Lambda}=3/2^{+}_{2}, 1/2^{+}$$] and [$$^{8}{\rm Li}(1^{+})\otimes s_{\Lambda}=5/2^{+}_{2}, 7/2^{+}$$] were observed at $$E_{\Lambda}(\#2)=1.74\pm0.27^{\rm stat.}\pm0.11^{\rm sys.}~{\rm MeV}$$ and $$E_{\Lambda}(\#3)=3.30\pm0.24^{\rm stat.}\pm0.11^{\rm sys.}~{\rm MeV}$$, respectively. The $$E_{\Lambda}(\#3)$$ is larger than shell model predictions by a few hundredmore » keV, and the difference would indicate that a $${\rm ^{5}He}+t$$ structure is more developed for the $$3^{+}$$ state than those for the $$2^{+}$$ and $$1^{+}$$ states in a core nucleus $$^{8}{\rm Li}$$ as a cluster model calculation suggests.« less

A beam-normal single-spin asymmetry generated in the scattering of transversely polarized electrons from unpolarized nucleons is an observable related to the imaginary part of the two-photon exchange process. Here we report a 2% precision measurement of the beam-normal single-spin asymmetry in elastic electron-proton scattering with a mean scattering angle of ${\theta }_{\mathrm{lab}}=7.9°$ and a mean energy of 1.149 GeV. The asymmetry result is $B_n=–5.194\pm 0.067\left(\mathrm{stat}\right)\pm 0.082$more » (syst) ppm. This is the most precise measurement of this quantity available to date and therefore provides a stringent test of two-photon exchange models at far-forward scattering angles ( ${\theta }_{\mathrm{lab}}\to 0$) where they should be most reliable.« less

We report the measurement of the parity-violating asymmetry for the inelastic scattering of electrons from the proton, at $Q^2 = 0.082$ GeV$^2$ and $ W = 2.23$ GeV, above the resonance region. The result $$A_{\rm Inel} = - 13.5 \pm 2.0 ({\rm stat}) \pm 3.9 ({\rm syst})$$ ppm agrees with theoretical calculations, and helps to validate the modeling of the $$\gamma Z$$ interference structure functions $$F_1^{\gamma Z}$$ and $$F_2^{\gamma Z}$$ used in those calculations, which are also used for determination of the two-boson exchange box diagram ($$\Box_{\gamma Z}$$) contribution to parity-violating elastic scattering measurements. A positive parity-violating asymmetry for inclusivemore » $$\pi^-$$ production was observed, as well as positive beam-normal single-spin asymmetry for scattered electrons and a negative beam-normal single-spin asymmetry for inclusive $$\pi^-$$ production.« less

We present extractions of the nucleon non-singlet moments utilizing new precision data on the deuteron F₂ structure function at large Bjorken-x determined via the Rosenbluth separation technique at Jefferson Lab Experimental Hall C. These new data are combined with a complementary set of data on the proton previously measured in Hall C at similar kinematics and world data sets on the proton and deuteron at lower x measured at SLAC and CERN. The new Jefferson Lab data provide coverage of the upper third of the x range, crucial for precision determination of the higher moments. In contrast to previous extractions,more » these moments have been corrected for nuclear effects in the deuteron using a new global fit to the deuteron and proton data. The obtained experimental moments represent an order of magnitude improvement in precision over previous extractions using high x data. Furthermore, recent exciting developments in Lattice QCD calculations provide a first ever comparison of these new experimental results with calculations of moments carried out at the physical pion mass, as well as an innovative approach which first calculates the quark distributions directly before determining moments.« less

Abstract not provided.