Ⅱ-4. Hadron Physics

RIKEN Accel. Prog. Rep. 52 (2019)

Spectroscopy of pionic atoms in 122 Sn(d, 3 He) reaction and angulardependence of the formation cross sections†T. Nishi,∗1,∗2 K. Itahashi,∗2 G. P. A. Berg,∗3 H. Fujioka,∗4 N. Fukuda,∗2 N. Fukunishi,∗2 H. Geissel,∗5R. S. Hayano,∗1 S. Hirenzaki,∗6 K. Ichikawa,∗1 N. Ikeno,∗7 N. Inabe,∗2 S. Itoh,∗1 M. Iwasaki,∗2 D. Kameda,∗2S. Kawase,∗8 T. Kubo,∗2 K. Kusaka,∗2 H. Matsubara,∗8 S. Michimasa,∗8 K. Miki,∗1 G. Mishima,∗1 H. Miya,∗2H. Nagahiro,∗6 M. Nakamura,∗2 S. Noji,∗1 K. Okochi,∗1 S. Ota,∗8 N. Sakamoto,∗2 K. Suzuki,∗9 H. Takeda,∗2Y. K. Tanaka,∗1 K. Todoroki,∗1 K. Tsukada,∗2 T. Uesaka,∗2 Y. N. Watanabe,∗1 H. Weick,∗5 H. Yamakami,∗4and K. Yoshida∗2(piAF Collaboration)
The central peak is assigned mainly to formation ofthe 2p pionic state. The right most peak in the largermass side is contributed from higher pionic orbitals.
The pionic states in 121 Sn are observed for the firsttime. We made an elaborate fitting of the observed
spectrum and deduced the 1s and 2p binding energies
and the 1s width. The deduced binding energies andthe width agree with theoretical calculations.
In the measurement, a large angular acceptance of
BigRIPS provided a very unique opportunity. As
shown in Fig. 2 of Ref. 2), we observe reaction-angle dependence of the pionic atom formation spectra for the
first time. The dependence of both 1s and 2p states aredetermined experimentally. Comparing the measured
dependence with a theoretical prediction,3) they arequalitatively agreeing well, and the observed dependence directly confirms the assignment of the angularmomenta of the structures in the spectrum.
Quantitatively comparing the measured absoluteformation cross sections with the theoretical predictions, we find that the measured 2p cross section fairly
well agrees with the theory and that the 1s cross section is smaller by a factor of about 20%. Since a ratioof the measured cross sections of the 1s to 2p state is
attributed with relatively small errors, the deviationfrom the theory invokes important questions in understanding of the pionic atom formation reactions.
In conclusion, we conducted first pionic atom spectroscopy at RIBF and measured pionic 121 Sn as peakstructures in the missing-mass spectrum for the firsttime. Reaction angle dependence of the pionic 1s and
2p formation cross sections are observed and comparedwith theoretical calculations. The measured dependence qualitatively agree with the theory confirming
the correct assignment of the observed structures inthe spectrum. Measured absolute 2p cross section wellagrees with the theory but the theory overestimatesthe 1s cross section by a factor of about 5. The measurement firmly proves potential of the RIBF for futurehigh precision spectroscopy of pionic atoms.

Spectroscopy of deeply bound pionic atoms has been
contributing to understanding of non-perturbative lowenergy region of the QCD.1) We have started an experimental spectroscopy of pionic atoms at RIBF andconducted our first experiment in 2010 by measuring122
Sn(d, 3 He) reactions near the π − emission threshold for about 15h. We employed deuteron beam withthe energy of 498.9 ± 0.2 MeV and a typical intensity of 2 × 1011 /s accelerated by SRC. Such a highintensity deuteron beam may serve as an irreplaceablebasis for high statistical and systematic precision in
the spectroscopy and let us achieve opportunities toreach understanding of the the low-energy QCD with
unprecedented accuracy.
During the experiment, the deuteron beam impinged
on a thin 122 Sn target located at a nominal target position of BigRIPS. We used BigRIPS as a spectrometer
to momentum-analyze the emitted 3 He and measured
the Q-value of the reaction in the missing-mass measurement. We reconstructed the tracks of the 3 He by aset of tracking detectors placed near the F5 dispersive
focal plane. We installed a set of scintillation counters
at F5 and F7 focal planes and identified the particles.
After applying detailed optical aberration corrections,
we achieved excitation spectra of the target Sn nucleus with the best resolution of 0.42 MeV (FWHM).
The absolute excitation energy is calibrated by usinga polyethylene target to observe two-body reactions ofH(d, 3 He)π 0 .
In the measured excitation spectrum of the target Snnucleus, we observe three prominent peak structuresnear the π − emission threshold as depicted in Fig. 1of our original paper Ref. 2). The left most peak inthe smaller-mass side is assigned to the 1s pionic state
mainly coupling with a neutron hole state of (3s1/2 )−1n .
†
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9Condensed from the article in Phys. Rev. Lett. 125, 152505(2018)
University of Tokyo
RIKEN Nishina Center
University of Notre Dame
Kyoto University
GSI Helmholtzzentrum für Schwerionenforschung GmbH
Nara Women’s UniversityTottori University
CNS, University of Tokyo
Stefan Meyer Institute for Subatomic Physics

References
1) K. Suzuki et al., Phys. Rev. Lett. 92, 072302 (2004).
2) T. Nishi et al., Phys. Rev. Lett. 120, 152505 (2018).
3) N. Ikeno et al., PTEP 2015, 033D01 (2015).

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