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Register a new userPolarization transfer in the $^{16}$O$(p,p)$ reaction at forward angles and structure of the spin-dipole resonances
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Cross sections and polarization transfer observables in the $^{16}$O$(p,p)$ reactions at 392 MeV were measured at several angles between $theta_{lab}=$ 0$^circ$ and 14$^circ$. The non-spin-flip (${Delta}S=0$) and spin-flip (${Delta}S=1$) strengths in transitions to several discrete states and broad resonances in $^{16}$O were extracted using a model-independent method. The giant resonances in the energy region of $E_x=19-$27 MeV were found to be predominantly excited by ${Delta}L=1$ transitions. The strength distribution of spin-dipole transitions with ${Delta}S=1$ and ${Delta}L=1$ were deduced. The obtained distribution was compared with a recent shell model calculation. Experimental results are reasonably explained by distorted-wave impulse approximation calculations with the shell model wave functions.
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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.
The $gamma p rightarrow K^+Sigma^0$ differential cross section at extremely forward angles was measured at the BGOOD experiment. A three-quarter drop in strength over a narrow range in energy and a strong dependence on the polar angle of the $K^+$ in the centre-of-mass of the reaction is observed at a centre-of-mass energy of 1900,MeV. Residing close to multiple open and hidden strangeness thresholds, the structure appears consistent with meson-baryon threshold effects which may contribute to the reaction mechanism.
The cross section of the ${}^{16}{rm O}(p,p){}^{16}{rm O}(0^-,T=1)$ scattering was measured at a bombarding energy of 295 MeV in the momentum transfer range of $1.0 mathrm{fm^{-1}}$ $le$ $q_{rm c.m.}$ $le$ $2.1 mathrm{fm^{-1}}$. The isovector $0^-$ state at $E_x$ = 12.8 MeV is clearly separated from its neighboring states owing to the high energy resolution of about 30 keV. The cross section data were compared with distorted wave impulse approximation (DWIA) calculations employing shell-model wave functions. The observed cross sections around $q_{rm c.m.}$ $simeq$ $1.7 {rm fm^{-1}}$ are significantly larger than predicted by these calculations, suggesting pionic enhancement as a precursor of pion condensation in nuclei. The data are successfully reproduced by DWIA calculations using random phase approximation response functions including the $Delta$ isobar that predict pionic enhancement.
A windowless and self-supporting ice target is described. An ice sheet with a thickness of 29.7 mg/cm$^2$ cooled by liquid nitrogen was placed at the target position of a magnetic spectrometer and worked stably in the $^{16}$O$(p,p)$ experiment at $E_{p}=392$ MeV. Background-free spectra were obtained.
Differential cross sections and photon beam asymmetries for the gamma p -> pi+ n reaction have been measured for 0.6<cos(theta)<1 and Egamma=1.5-2.95 GeV at SPring-8/LEPS. The cross sections monotonically decrease as the photon beam energy increases for 0.6<cos(theta)<0.9. However, the energy dependence of the cross sections for 0.9<cos(theta)<1 and Egamma=1.5-2.2 GeV (W=1.9-2.2 GeV) is different, which may be due to a nucleon or Delta resonance. The present cross sections agree well with the previous cross sections measured by other groups and show forward peaking, suggesting significant t-channel contributions in this kinematical region. The asymmetries are found to be positive, which can be explained by rho-exchange in the t-channel. Large positive asymmetries in the small |t| region, where the rho-exchange contribution becomes small, could be explained by introducing pi-exchange interference with the s-channel.
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