Title:First precision spectroscopy of pionic atoms at RIBF
Abstract:
We report preliminary results of our recent experiment
for spectroscopy of the pionic 121Sn atom
at the RI beam factory, RIKEN.
We have measured a spectrum of the 122Sn(d,3He) reaction
near the pion-emission threshold. Presently,
a detailed analysis is on-going and
the preliminary spectrum shows a distinct structure of peaks
in the bound-state region,
which is a signature of the first observation of pionic 121Sn atoms.
We observed not only the 1s state but also the 2s and 2p states.
Precision spectroscopy of pionic atoms provides information on the
strong interaction between pion and nucleus [1,2],
leading to the evaluation of the magnitude of the in-medium
quark condensate [2-4].
For a better determination of the isovector pion-nucleus scattering
lengths, spectroscopy of deeply-bound pionic states
like 1s or 2p states in relatively heavy atoms is important.
Systematic studies of such states will provide unique opportunities to
understand the chiral symmetry and the vacuum structures.
Our work has the goal to allow systematic spectroscopy of pionic
atoms with unprecedented resolution using the world-highest
intensity of heavy-ion beams.
The initial experimental spectra of pionic 121Sn atom already
show very interesting features.
We have succeeded in the first observation of the angular dependence
of the pionic-atom formation cross section
owing to the large angular acceptance of the spectrometer BigRIPS.
Details of the experimental procedures and preliminary results
will be reported.
[1] K. Itahashi et al., Phys. Rev. C62, 025202 (2000).
[2] K. Suzuki et al., Phys. Rev. Lett. 92, 072302 (2004).
[3] E.E. Kolomeitsev, N. Kaiser and W. Weise,
Phys. Rev. Lett. 90, 092501 (2003).
[4] D. Jido, T. Hatsuda and T. Kunihiro, Phys. Lett. B 670 (2008) 109.
