Neutron-proton scattering in the context of the $d^{*}$(2380) resonance

New data on quasifree polarized neutron-proton scattering in the region of the recently observed $d^{*}$ resonance structure are obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, checks of the quasifree character of the scattering process, on all obtained $A_y$ angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in ${}^3{D}_3-{}^3{G}_3$ coupled partial waves at ($2380\pm 10-i40\pm 5$) MeV—in accordance with the $d^{*}$ dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross-section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at $T_d=2.27$ GeV is discussed.

Figure 1

Angular distributions of vector [$A_y$; (a)] and tensor [$A_{yy}$; (b)] analyzing powers in $dp$ scattering at $T_d=2.27$ GeV. Filled circles denote results from this work averaged over beam polarization “up” and “down” runs, whereas open symbols denote results previously obtained at Argonne National Laboratory at $T_d=2.0$ GeV [15] (open circles) and COSY-ANKE at $T_d=2.27$ GeV [17] (open squares). Error bars denote statistical uncertainties.

Figure 2

(a) Vector analyzing power $A_y$ in $pp$ scattering at $T_p=1.135$ GeV. Filled circles denote results from this work, whereas open symbols denote results previously obtained with EDDA at $T_p=1.132$ GeV [18]. The solid line represents the SAID SP07 phase-shift solution [19]. (b) Variation of the proton polarization over the measured energy interval plotted as the ratio to the SAID SP07 phase-shift solution. The datum at 2.377 GeV derives from (a) and denotes the average over the full energy range covered in this experiment.

Figure 3

Distribution of spectator proton momenta (in the deuteron rest frame) in the $dp\to pn+p_{\mathrm{spectator}}$ reaction within the acceptance of the WASA detector. Data are given by filled circles. The solid line shows the expected distribution for the quasifree process based on the CD Bonn potential [20] deuteron wave function. For data analysis, only events with $p_{\mathrm{spectator}}<$ 0.16 GeV/$c$ were used.

Figure 4

Angular distributions of the $np$ analyzing power without consideration of the spectator momentum, i.e., without classifying the collected $pn$ scattering events according to their effective total center-of-mass energy $\sqrt{s}$. That way the data set corresponds to the weighted average over the measured interval representing effectively the range $\sqrt{s}=2.377\pm 0.03$ GeV (corresponding to $1.075\mathrm{GeV}\le T_n\le 1.195$ GeV). Open circles and filled squares denote $A_y$ extraction by the combination “up” with “unpolarized” and “down” with “unpolarized,” respectively. Filled circles give the statistically weighted average over both methods. Error bars denote statistical uncertainties. The solid line represents the SAID SP07 phase-shift solution [19], whereas the dashed (dotted) line gives the result of the new weighted (unweighted) SAID partial-wave solution (see text).

Figure 5

$A_y$ angular distributions for $\sqrt{s}=2.360$, 2.367, 2.374, 2.380, 2.387, 2.392, 2.403, and 2.440 GeV, corresponding to $T_n=1.095$, 1.108, 1.125, 1.139, 1.156, 1.171, 1.197, and 1.27 GeV. Filled symbols denote results from this work taking into account the spectator four-momentum information. Error bars on filled symbols include both statistical and systematic uncertainties. Open symbols denote previous work [8, 9, 10, 11]. Solid lines represent the SAID SP07 phase-shift solution [19], whereas dashed (dotted) lines give the result of the new weighted (unweighted) SAID partial-wave solution (see text).

Figure 6

Energy dependence of the $np$ analyzing power at ${\Theta }_n^{\mathrm{c}.\mathrm{m}.}={70}^{\circ }$. Filled symbols denote the results of this work; open symbols, those from previous work [8, 9, 10, 22, 23, 24, 25, 26, 27]. For the meaning of the curves see Fig. 4. The vertical arrow and horizontal bar indicate the pole and width of the resonance.

Figure 7

Argand diagrams of ${}^3{D}_3$ (a) and ${}^3{G}_3$ (b) partial waves as well as of their coupling amplitude ${\epsilon }_3$ (c) in the new partial-wave solution. Values are plotted as small filled triangles in 10-MeV steps and as small squares in 50-MeV steps, together with the corresponding total energy $\sqrt{s}$. The thick filled circle gives the energy position of the resonance pole. The dotted curve in (a) is a circle fitted to the loop; its diameter equals the branching ratio $B_{\mathrm{el}}$.

Figure 8

Speed plot of the ${}^3{D}_3$ partial wave. The dotted vertical line indicates the position of the maximum. The vertical arrow and horizontal bar indicate the pole and width of the resonance as averaged over the values obtained in contour and speed plots (see text).

Figure 9

Total $pn$ cross-section (a) and total isoscalar nucleon-nucleon cross-section (b) dependence of the incident neutron (nucleon) energy $T_n$. Data are from Lisowski et al. [33] (open squares), Devlin et al. [34] (open triangles), and Sharov et al. [35] (open circles). Horizontal bars indicate the energy resolution of the incident neutrons. Solid and dashed curves, representing the SP07 and the new solution, respectively, are averaged over the experimental energy resolution of Ref. [34]. The dotted line gives the new solution without averaging over the experimental energy resolution. The vertical arrow and horizontal bar indicate the pole and width of the resonance.

Figure 10

Energy dependence of the transversal (a) and longitudinal (b) total $np$ cross-section differences. Symbols denote data from Refs. [9, 35, 36, 37, 38, 39, 40]. For the meaning of the curves see Fig. 4. The vertical arrow and horizontal bar indicate the pole and width of the resonance.

Figure 11

Angular distribution of the differential cross section $d\sigma /d\Omega$ at $T_n=1.135$ GeV corresponding to the resonance energy $\sqrt{s}=2.38$ GeV. For the meaning of the curves see the caption to Fig. 4. Plotted data are from Ref. [41] ($T_n=1.135$ GeV) and Ref. [42] ($T_n=1.118$ GeV).