KEK Annual Report 2005

Experiments to search for deeply-bound kaonic
nuclear states E471/E549
M. Iwasaki for E471/E549 collaboration
RIKEN, JapanThe low energy KN interaction studied at KEK-PS E228 by means of energy shift
and the level broadening of the kaonic hydrogen atom [1], which shows strong upward shift of the atomic 1s level due to the KN interaction. This experiment solves
long standing problem, so called “kaonic hydrogen puzzle”, and it open the theoretical interpretation that the Λ(1405) could be a meson-baryon bound state instead of
conventional three-quark sub-threshold resonance below K − p. By assuming that theΛ(1405) to be a K − p bound state, Akaishi and Yamazaki predicted meta-stable boundstate of kaon in several light nuclei [2]. They predicted that the deeply bound kaonicstate forms in extremely high nucleon density due to the strong attractive nature of
the KN interaction in isospin T = 0 channel below threshold. The experimental study
shows that one can seach such state by observing (K − 4 He)atomic →< K− ppn >T=0 +nreaction most easily.
Recently, we have been working on experiments to search for such a extremely exotic
state extensively by KEK-PS E471 and E549 (dedicated), and E570 (parasitic). In
these experiments, we had applied stopped K − 4 He reaction to detect mono-energetic
nucleon production by means of time-of-ﬂight (TOF) method. The ﬁrst result fromE471 is published in reference [3], which shows mono-energetic peak formation in theproton spectrum at around 3115 MeV in the missing energy. Observed peak is out ofour expectation in many ways. If we assume that the observed peak to be a boundstate, then the kaon binding energy is more than twice deeper than predicted andthe isospin of the peak should be one instead of zero. It is also true that the E471
experimental apparatus is dedicated for neutron spectroscopy and not for proton. Infact, there is no tracking device for the proton so that the obtained proton spectrum
is not inclusive spectrum but requires one additional charged particle to pin down thekaon reaction point as a vertex of the TOF start.
The neutron spectrum, we have obtained in E471, also shows very interesting structure at the similar missing mass as proton’s one [4]. However, the statistics of theobserved structure is not strong enough to conclude in a deﬁnitive way.
Therefore, to study more detail of the proton spectrum and also to have enoughstatistics for neutron spectrum, we proposed upgraded experiment E549.

Fig. 1 shows our experimental setup for E549/E570. The biggest diﬀerence from E471
experimental setup is the newly installed proton tracking devices (two-layers of TOF1

Figure 1: Upgraded experimental setup for E549/E570
counter walls, and a four-layered drift chamber in both left- and right- arms dedicatedfor proton ). To improve statistics of the neutron spectrum, neutron counter walls arealso upgraded from four to 7 layers with better neutron shielding around K5 beam-line,
which enables us to reduce the threshold level of the neutron counter so as to havebetter eﬃciency. In the present setting we are able to detect neutron with 2.5 times
better eﬃciency than before.
As described, the nucleon spectroscopy from the kaon stopped in the liquid helium
target was extended to E570 to have enough statistics for the neutron spectrum as aparasitic mode to the experiment until the end of year 2006, which is described in theseparate report. Presently, we are extensively analysis on the whole data set. Becauseof the limitation of the human resource for the analysis, we are proceeding he analysis
in two independent analysis teams for the data no to have biased result, we are notyet reached to be able to discuss both neutron and proton data.

References[1] M. Iwasaki et al., Phys. Rev. Lett. 78 (1997) 3067.
[2] Y. Akaishi and T. Yamazaki, Phys. Rev. C65 (2002) 044005.
[3] T. Suzuki et. al., Phys. Lett. B597(2004) 263.
[4] M. Iwasaki et al., arXiv:nucl-ex/0310018.
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