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Register a new userThe Charge Form Factor of the Neutron at Low Momentum Transfer from the $^{2}vec{rm H}(vec{rm e},{rm e}{rm n}){rm p}$ Reaction
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We report new measurements of the neutron charge form factor at low momentum transfer using quasielastic electrodisintegration of the deuteron. Longitudinally polarized electrons at an energy of 850 MeV were scattered from an isotopically pure, highly polarized deuterium gas target. The scattered electrons and coincident neutrons were measured by the Bates Large Acceptance Spectrometer Toroid (BLAST) detector. The neutron form factor ratio $G^{n}_{E}/G^{n}_{M}$ was extracted from the beam-target vector asymmetry $A_{ed}^{V}$ at four-momentum transfers $Q^{2}=0.14$, 0.20, 0.29 and 0.42 (GeV/c)$^{2}$.
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We report measurements of the cross section and a complete set of polarization transfer observables for the ${}^{16}{rm O}(vec{p},vec{n}){}^{16}{rm F}$ reaction at a bombarding energy of $T_p$ = 296 MeV and a reaction angle of $theta_{rm lab}$ = $0^{circ}$. The data are compared with distorted-wave impulse approximation calculations employing the large configuration-space shell-model (SM) wave functions. The well-known Gamow-Teller and spin-dipole (SD) states at excitation energies of $E_x$ $lesssim$ 8 MeV have been reasonably reproduced by the calculations except for the spin--parity $J^{pi}$ = $2^-$ state at $E_x$ = 5.86 MeV. The SD resonance at $E_x$ $simeq$ 9.5 MeV appears to have more $J^{pi}$ = $2^-$ strength than $J^{pi}$ = $1^-$ strength, consistent with the calculations. The data show significant strength in the spin-longitudinal polarized cross section $ID_L(0^{circ})$ at $E_x$ $simeq$ 15 MeV, which indicates existence of the $J^{pi}$ = $0^-$ SD resonance as predicted in the SM calculations.
Differential cross sections and complete sets of polarization observables are presented for the Gamow-Teller $^{12}{rm C}(vec{p},vec{n})^{12}{rm N}({rm g.s.},1^+)$ reaction at a bombarding energy of 296 MeV with momentum transfers $q$ of 0.1 to $2.2{rm fm}^{-1}$. The polarization transfer observables are used to deduce the spin-longitudinal cross section, $ID_q$, and spin-transverse cross sections, $ID_p$ and $ID_n$. The data are compared with calculations based on the distorted wave impulse approximation (DWIA) using shell-model wave functions. Significant differences between the experimental and theoretical results are observed for all three spin-dependent $ID_i$ at momentum transfers of $q gtrsim 0.5{rm fm}^{-1}$, suggesting the existence of nuclear correlations beyond the shell model. We also performed DWIA calculations employing random phase approximation (RPA) response functions and found that the observed discrepancy is partly resolved by the pionic and rho-mesonic correlation effects.
We present measurements of the electron helicity asymmetry in quasi-elastic proton knockout from $^{2}$H and $^{12}$C nuclei by polarized electrons. This asymmetry depends on the fifth structure function, is antisymmetric with respect to the scattering plane, and vanishes in the absence of final-state interactions, and thus it provides a sensitive tool for their study. Our kinematics cover the full range in off-coplanarity angle $phi_{pq}$, with a polar angle $theta_{pq}$ coverage up to about 8 degrees. The missing energy resolution enabled us to determine the asymmetries for knock-out resulting in different states of the residual $^{11}$B system. We find that the helicity asymmetry for $p$-shell knockout from $^{12}$C depends on the final state of the residual system and is relatively large (up to $approx 0.16$), especially at low missing momentum. It is considerably smaller (up to $approx 0.01$) for $s$-shell knockout from both $^{12}$C and $^2$H. The data for $^2$H are in very good agreement with theoretical calculations, while the predictions for $^{12}$C exhibit differences with respect to the data.
We present first measurements of the double ratio of the polarization transfer components $(P^{prime}_{!x} !/ P^{prime}_{!z} )_p/ (P^{prime}_{!x} !/ P^{prime}_{!z} )_s$ for knock-out protons from $s$ and $p$ shells in $^{12}{rm C}$ measured by the $^{12}{rm C}(vec{e},{e}vec{p},)$ reaction in quasi-elastic kinematics. The data are compared to theoretical predictions in relativistic distorted-wave impulse approximation. Our results show that differences between $s$- and $p$-shell protons, observed when compared at the same initial momentum (missing momentum) largely disappear when the comparison is done at the same proton virtuality. We observe no density-dependent medium modifications for protons from $s$ and $p$ shells with the same virtuality in spite of the large differences in the respective nuclear densities.
The $^{15}{rm N}(p,gamma)^{16}{rm O}$ reaction provides a path from the CN cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor for this reaction is dominated by resonant capture through two strong $J^{pi}=1^{-}$ resonances at $E_{R}= 312$ and 962 keV and direct capture to the ground state. Recently, a new measurement of the astrophysical factor for the $^{15}{rm N}(p,gamma)^{16}{rm O}$ reaction has been published [P. J. LeBlanc {it et al.}, Phys. Rev. {bf C 82}, 055804 (2010)]. The analysis has been done using the $R$-matrix approach with unconstrained variation of all parameters including the asymptotic normalization coefficient (ANC). The best fit has been obtained for the square of the ANC $C^{2}= 539.2$ fm${}^{-1}$, which exceeds the previously measured value by a factor of $approx 3$. Here we present a new $R$-matrix analysis of the Notre Dame-LUNA data with the fixed within the experimental uncertainties square of the ANC $C^{2}=200.34$ fm${}^{-1}$. Rather than varying the ANC we add the contribution from a background resonance that effectively takes into account contributions from higher levels. Altogether we present 8 fits, five unconstrained and three constrained. In all the fits the ANC is fixed at the previously determined experimental value $C^{2}=200.34$ fm${}^{-1}$. For the unconstrained fit with the boundary condition $B_{c}=S_{c}(E_{2})$, where $E_{2}$ is the energy of the second level, we get $S(0)=39.0 pm 1.1 $ keVb and normalized ${tilde chi}^{2}=1.84$, i.e. the result which is similar to [P. J. LeBlanc {it et al.}, Phys. Rev. {bf C 82}, 055804 (2010)]. From all our fits we get the range $33.1 leq S(0) leq 40.1$ keVb which overlaps with the result of [P. J. LeBlanc {it et al.}, Phys. Rev. {bf C 82}, 055804 (2010)]. We address also physical interpretation of the fitting parameters.
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