Charge symmetry breaking of a hypernucleus 4HeK. Hosomi1), H. Sako1), K. Imai1), S. Haseagawa1), S. Sato1), K. Tanida1), H. Sugimura1),
Y. Ichikawa 1), and H. Ekawa1)
1) : Hadron Nuclear Physics Gr., JAEA

It has been known for a long time that nuclei of equalmass and with inverted proton and neutron numbers(mirror nuclei) exhibit very similar properties. Thesesimilarities are due to a characteristic of the nuclear force
called charge symmetry. A question to answer is whether thissymmetry holds also for hupernuclei which include a  particle.

References[1] M. Bedjidian et al., Phys. Lett. 83B, 252 (1979).
[2] T.O. Yamamoto, H. Ekawa, H. Hasegawa, H. Hosomi, Y.
Ichikawa, H. Sako, S. Sato, H. Sugimura, K. Tanida, et al.
Phys. Rev. Lett. 115, 222501 (2015) .

Fig. 1: The experimental setup of E13 [2]. The K- beam is injected to
the 4He target. The incident K- and the produced - are identified withthe aerogel Cherenkov detectors BAC2 and SAC1. The gamma rays
emitted from the target are detected with the Hyperball-J Ge detectors.

Counts/8keV

In the past, an experiment to study 4He gamma rays
measured a mass difference between 1+ and 0+ states of
1.15±0.04 MeV. The mass difference is very similar to that for4
H of 1.09±0.02 MeV. This measurement indicated that charge
symmetry holds for these hypernuclei [1]. However, theseexperiments had rather poor energy resolution, signal-to-noise
ratio, and gamma-ray statistics.
In order to improve the data quality, we proposed a newexperiment E13, where we irradiate K- beams on a 4He target,produce 4He, and measure the gamma ray from its exited state
as shown in Fig. 1 [2]. The reaction is K- + 4He→4He* + ,44
He*→ He + . For extremely intense K beams at J-PARC(typically 3 x 105 / spill), we developed the gamma-ray detector
system Hyperball-J, shown in Fig. 1. We included a mechanicalcooling system of Ge crystals to reduce radiation damage due tohigh beam rates, and employed PbWO4 counters with fast
response time (~10 ns) to reject background, such as a Compton
scattering and high energy photons from 0 decay with highparticle rates. We also suppressed background reactions byidentifying the incident K- beam particle and the produced with aerogel Cherenkov counters and time-of-flight detectors
shown in Fig. 1.
In April 2015, as the first experiment after recovering fromthe radiation accident at J-PARC Hadron Experimental Facility,
we performed the E13 experiment for 5 days with 2.3x1010 Kbeams at 1.5 GeV/c. We succeeded to measure gamma-ray
energy spectra with a very high energy resolution of 5 keV(FWHM), i.e. a factor of 20 improvement compared to the pastexperiment. The measured gamma-ray energy spectrum after the(K-, -) event selection is shown in Fig. 2. We observed the peakcorresponding to the transition of 4He from the 1+ excited state
to the 0+ ground state. The energy is determined to be
1.406±0.002 (stat.) ±0.002 (syst.) MeV, which excludes theprevious experimental data of 1.15±0.04 MeV.
By comparing the resulting energy levels in 4He and 4H, asshown in Fig. 3, a very large charge symmetry breaking of 320
keV is confirmed. This result implies that there is a difference
between the n interaction and p interaction, depending
strongly on their spin states. The present work provides
important data to understand the strong interaction between
baryons, and has triggered new theoretical studies on thisproblem.

Fig. 2: Gamma-ray energy spectra from 4He requiring (K-, -)reactions [1].

Fig. 3: Energy levels of mirror nuclei 4H and 4He [2].