High-precision measurements of $\pi p$ elastic differential cross sections in the second resonance region

Cross sections for ${\pi }^{\pm }p$ elastic scattering have been measured to high precision by the EPECUR Collaboration for beam momenta between 800 and 1240 MeV/$c$ using the ITEP proton synchrotron. The data precision allows comparisons of the existing partial-wave analyses on a level not possible previously. These comparisons imply that over the covered energy range, the Carnegie-Mellon-Berkeley analysis is significantly more predictive when compared to versions of the Karlsruhe-Helsinki analyses.

Figure 1

Experimental setup (top view). 1FCH1-4 and 2FCH1-4 are 1-mm pitch proportional chambers; DC1-DC8, drift chambers; ${\mathrm{LH}}_2$, liquid hydrogen target; S1, S2, and A1, trigger scintillation counters.

Figure 2

Two-dimensional distribution over the difference between calculated c.m. scattering angles for pion and proton, assuming that the pion goes to the left arm, $\Delta \theta$ (abscissa) and the scattering angle $\theta$ (ordinate) (a) and its slice at $\theta =84$ degree (b).

Figure 3

Differential cross sections for selected angles in the c.m. frame, $\theta$, for ${\pi }^{-}p\to {\pi }^{-}p$ [top panels: (a) $\theta ={40}^{\circ }$, (b) $\theta ={100}^{\circ }$, and (c) $\theta ={110}^{\circ }$] and ${\pi }^{+}p\to {\pi }^{+}p$ [bottom panels: (d) $\theta ={70}^{\circ }$, (e) $\theta ={80}^{\circ }$, and (f) $\theta ={120}^{\circ }$]. New EPECUR data (statistical errors only) are plotted as blue filled circles with previous measurements presented as black open circles. The data from earlier experiments (statistical errors only) are within bins of $\Delta \theta =\pm 1^{\circ }$ [8]. An existing GW DAC fit, WI08 [4], is plotted with a red double solid curve while the older KH80 [1], KA84 [2], and CMB [3] fits are plotted as blue dash-dotted, green short dashed, and black dashed curves, respectively. New EPECUR data are not included in any presented fits.

Figure 4

Comparison of ${\chi }^2$/data distributions covered $W=1450$–1750 MeV, (a) ${\pi }^{-}p\to {\pi }^{-}p$ and (b) ${\pi }^{+}p\to {\pi }^{+}p$. An existing GW DAC fit, WI08 [4], is plotted with red double filled circles while the older KH80 [1], KA84 [2], and CMB [3] fits are plotted as blue filled triangles, green crosses, and black stars, respectively. The new SAID WI14 solution (including new EPECUR data) is shown by magenta open squares. Horizontal lines show average value of ${\chi }^2$/data for associated solution. The curves are added to guide the eye. On the right plot, vertical arrows indicate resonance energies $W_R$ and horizontal bars show full $\left(\Gamma \right)$ and partial $\left({\Gamma }_{\pi N}\right)$ widths $\left(I=3/2\right)$ associated with the SAID $\pi N$ solution WI08 [4].