Spectroscopy of pionic atoms in $\mathbf{{}^{122}{\textbf Sn}({\textit d},{}^3{\textbf He})}$ reaction and angular dependence of the formation cross sections

TL;DR

This study investigates pionic atom states in tin nuclei via the (d,3He) reaction, observing the 2p state for the first time and analyzing the angular dependence of formation cross sections with high precision.

Contribution

First observation of the 2p pionic state in tin nuclei and detailed measurement of its angular-dependent formation cross sections.

Findings

The 2p state was observed for the first time in Sn nuclei.

The spectrum at finite angles was measured for the first time.

Discrepancy found between measured and calculated cross sections for the 1s state.

Abstract

We observed the atomic $1s$ and $2p$ states of $\pi^-$ bound to ${}^{121}{\rm Sn}$ nuclei as distinct peak structures in the missing mass spectra of the ${}^{122}{\rm Sn}(d,{}^3{\rm He})$ nuclear reaction. A very intense deuteron beam and a spectrometer with a large angular acceptance let us achieve potential of discovery, which includes capability of determining the angle-dependent cross sections with high statistics. The $2p$ state in a Sn nucleus was observed for the first time. The binding energies and widths of the pionic states are determined and found to be consistent with previous experimental results of other Sn isotopes. The spectrum is measured at finite reaction angles for the first time. The formation cross sections at the reaction angles between 0 and $2^\circ$ are determined. The observed reaction-angle dependence of each state is reproduced by theoretical calculations. However, the quantitative comparison with our high-precision data reveals a significant discrepancy between the measured and calculated formation cross sections of the pionic $1s$ state.