Search for 6ΛH hypernucleus by the (π-, K+) reaction at pπ-=1.2GeV/c

H. Sugimura1) for J-PARC E10 Collaboration
1) R.G for Hadron Physics, ASRC, JAEA

The study of neutron-rich hypernuclei is one of the mostimportant topics in the strangeness nuclear physics. The gluelike role of the Λ hyperon is expected to be critical in nuclei
beyond the neutron-drip line. The knowledge of the behavior ofhyperons in a neutron-excess environment will significantly
affect our understanding of neutron stars because the addition ofhyperons softens the Equation of State of matter at the core. To
study its effects, we selected the 6ΛH hypernucleus which
consists of one proton, four neutrons, and one Λ hyperon.
Theoretical calculations of binding energy of 6ΛH wereperformed with several models, and these mass differences weremore than 3MeV. Therefore, it is important to measure the Λbinding energy experimentally. On the other hand, theoretical
calculation of production cross section cannot be investigatedbecause production mechanisms of neutron-rich hypernuclei are
not well known. It would be a key to get information ofproduction mechanism if the cross section is measured precisely.
An experiment was proposed aiming at a precise
spectroscopic study of the light neutron-rich Λ hypernucleus
6 H via the 6Li(π-,K+) reaction at the π- beam momentum of 1.2
ΛGeV/c [1]. The (π-,K+) reaction is good tool to access theneutron-rich region because two protons are changed to one
neutron and one Λ hyperon. However, the production crosssection of the (π-,K+) reaction is considerably small, roughly
10-3 of that of the non-charge-exchange (π+,K+) reaction.
Therefore, we planned to use 107 pion beams to observe morethan 100 events of 6ΛH and to determine the binding energy in agood statistical accuracy. To accept such a high rate beam, theSilicon Strip Detector and Scintillating Fiber Tracker weredeveloped. They have sufficient tolerance to high rate beams up
to 108 Hz.
The experiment was performed at the K1.8 beam line of theJ-PARC Hadron Experimental Facility. The K1.8 beam line
spectrometer and the Superconducting Kaon Spectrometer(SKS) were used as shown in Fig.1. We used high intensity pionbeams of 1.2-1.4×107spill (2s spill). The total number of pion
irradiated on a 6Li target was 1.4×1012.
Mass scale was calibrated using p(π±,K+)Σ± reactions with anaccuracy of 1.26 MeV/c2. Missing-mass resolution was
estimated to be 3.2 MeV (FWHM) from the observed energy
spectrum of 12C(π+,K+)12ΛC reaction.
Figure 2 shows the missing-mass spectrum of the 6Li(π-,K+)
reaction [2]. The vertical axis shows the double differential
cross section in the laboratory frame. A magnified view aroundthe Λ bound region is shown in the inset. The arrow labeled as
4 H+2n shows the particle decay threshold. No significant peak
Λstructure was observed around the shown arrow.
An upper limit was estimated from the number of events
around the particle decay threshold. There were 3 events in thethreshold region within the missing-mass window. The upper
limit of the number of signal events is estimated to be 6.68
events at the 90% confidence level which corresponds to1.2nb/sr.
The present result suggests either there is no bound or nonarrow resonance state in 6ΛH or the production cross section ismuch less than we expected. Quantitative theoretical calculation

Fig.1 The setup of 6ΛH production experiment. The silicon strip
detector (SSD) and Scintillating Fiber Tracker are located upstream
and downstream of the target, respectively.

Fig.2 Missing-mass spectrum of the 6Li(π-,K+) reaction at themomentum of 1.2 GeV/c. A magnified view around the Λ boundregion is shown in the inset. The arrow labelled as 4ΛH+2n shows theparticle decay threshold.

of the production cross section which explains the present result
is necessary for further studies of the 6ΛH structure.
References[1] A. Sakaguchi et al., J-PARC Proposal E10.
[2] H. Sugimura et al., Phys. Lett. B729, 39 (2014).