Progress of the hypernuclear decay pion spectroscopy programat MAMI-C
S. Nagao1#, P. Achenbach2 , N. Arai1, C. Ayerbe Gayoso2, R. Böhm2, O. Borodina23,
D. Bosnar4, V. Bozkurt2 , L. Debenjak5, M. O. Distler2, A. Esser2, I. Friščič4, Y. Fujii1,M. Fujita1, T. Gogami1, M. Gómez Rodríguez2, S. Hirose1, H. Kanda1, M. Kaneta1, E. Kim2,J. Kusaka1,A. Margaryan6, H. Merkel2, U. Müller2, S. N. Nakamura1, J. Pochodzalla2,C. Rappold3,J. Reinhold7, T. R. Saito238, A. Sanchez Lorente8, S. Sánchez Majos2,B. S. Schlimme2,M. Schoth2, F. Schulz2, C. Sﬁenti2, S. Širca5, Y. Takahashi1, L. Tang9,
M. Tiel2, K. Tsukada1, D. Uchiyama1
for the A1 hypernuclear collaboration
1

Graduate School of Science, Tohoku University, 980-8578, Japan
Institut für Kernphysik, Johannes Gutenberg-Universität, 55099 Mainz, German

2

3

GSI Helmholts Centre for Heavy Ion Research, 64291 Darmstadt, Germany
4
Department of Physics, University of Zagreb, Croatia
5University of Ljubjana and Institut “Jozef Stefan”, 1000 Ljubljana, Slovenia
6Yerevan Physics Institute, 375036 Yerevan, Armenia
7

Department of Physics, Florida International University, Miami, FL 33174, USA
8Helmholtz Institute Mainz, 55099 Mainz, Germany
9
Department of Physics, Hampton University, Hampton, VA 23668, USA# a corresponding author: E-mail nagao@lambda.phys.tohoku.ac.jp

Investigations of Λ hypernuclei are important to understand the hyperon-nucleon
interaction, structures of neutron stars, and unique behaviors of hypernuclei. The binding
energy of Λ in the hypernuclei is one of the most fundamental and important information forΛ hypernuclei. Most of the absolute binding energies for the ground states in the light
hypernuclei were measured by the emulsion experiments in 1970’s [1-4]. The excitationenergies have been measured by counter experiments: e.g. missing mass spectroscopy andgamma ray spectroscopy. From the results of these experiments and theoretical predictions,the understandings of the ΛN interaction have progressed. However, the energy levels of light
hypernuclei such as A=4 iso-doublet hypernuclei have not yet fully understood: 4ΛH and 4ΛHe
[5-6]. A large charge symmetry breaking potential between Λ and nucleon (Λ-N CSB) wasintroduced to reproduce ground and excited states energies of 4ΛH and 4ΛHe simultaneously[7]. However, the origin of large ΛN CSB potential has not been investigated well. Moreprecise data for various hypernuclei are definitely necessary. Recently, a binding energy of7
ΛHe, which was the last missing piece of A=7 iso-triplet hypernuclei, was newly reported

from the JLab E01-011 experiment [8]. An excited energy of 4ΛHe will be measured with anaccuracy of a few keV in the J-PARC E13 [9]. Much more statistics for several kinds of Λhypernuclei can be expected by overall scan technique in J-PARC E07 [10].
We designed cutting-edge experimental technique measuring the absolute binding energyof the ground state for the light hypernuclei with high accuracy: called “hypernuclear decaypion spectroscopy”. In the hypernuclear decay pion spectroscopy, we measure the momentumof the two-body decayed pion from a hypernucleus stopped in the target. With this newtechnique we can obtain the binding energy with about 30 keV accuracy thanks to the smallenergy struggling effect in a very thin target (~20 mg/cm2) and the high yield of the highintensity electron beam at Mainz Microtron C (MAMI-C). We have measured the momenta ofthe decayed pions with “Spek-A” and “Spek-C” which have the relative momentum
resolution of 10-4 and the large solid angle of 28 msr. In addition, we tagged K+ to suppress
the non-strangeness background in spectrometer “Kaos”.
We have performed the first feasibility experiment in 2011 and upgraded experiment in2012. We have successfully identified the decayed pion from 4ΛH and deduced its binding
energy. We are planning to take much more data with lower background condition in 2014.
I will report the latest results of hypernuclear decay pion spectroscopy, and discuss aboutbinding energy of Λ in 4ΛH and its formation probability.
References[1] M.Juric et al., Nucl. Phys. B52 (1973) 1.
[2] T.Cantwell et al., Nucl. Phys. A236 (1974) 445.
[3] J.Pniewski et al., Nucl. Phys. A443 (1985) 685.
[4] P.Dluzewski et al., Nucl. Phys. A484 (1988) 520.
[5] E.Hiyama et al., Phys. Rev. C 65 (2001) 011301.
[6] A.Nogga et al., Phys. Rev. Lett. 88 (2002) 172501.
[7] A.R.Bodmer et al., Phys. Rev. C 31 (1985) 1400.
[8] S.N.Nakamura et al., Phys. Rev. Lett. 110 (2013) 012502.
[9] H.Tamura et al., J-PARC E13 proposal.
[10] K.Imai, K.Nakazawa, H.Tamura et al., J-PARC E07 proposal.