Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 49 (2016)

Candidate resonant tetraneutron state populated by the
4He(8 He,8 Be) reaction
K. Kisamori,¤1 S. Shimoura,¤2 H. Miya,¤1,2 M. Assie,¤3 H. Baba,¤1 T. Baba,¤4 D. Beaumel,¤1,3 M. Dozono,¤1T. Fujii,¤1,2 N. Fukuda,¤1 S. Go,¤1,2 F. Hammache,¤3 E. Ideguchi,¤4 N. Inabe,¤1 M. Itoh,¤6 D. Kameda,¤1
S. Kawase,¤2 T. Kawabata,¤4 M. Kobayashi,¤2 Y. Kondo,¤1,7 T. Kubo,¤1 Y. Kubota,¤1,2M. Kurata-Nishimura,¤1 C. S. Lee,¤1,2 Y. Maeda,¤8 H. Matsubara,¤1 S. Michimasa,¤2 K. Miki,¤5 T. Nishi,¤1,9S. Noji,¤10 S. Ota,¤2 S. Sakaguchi,¤1,11 H. Sakai,¤1 Y. Sasamoto,¤2 M. Sasano,¤1 H. Sato,¤1 Y. Shimizu,¤1A. Stolz,¤10 H. Suzuki,¤1 M. Takaki,¤2 H. Takeda,¤1 S. Takeuchi,¤1 A. Tamii,¤5 L.T. Tang,¤2 H. Tokieda,¤2M. Tsumura,¤4 T. Uesaka,¤1 K. Yako,¤2 Y. Yanagisawa,¤1 and R. Yokoyama¤2The possible existence of a tetraneutron systemforming a resonance state is still an open and fascinating question; however, theoretical and experimental studies suggest that the tetraneutron does not exist as a bound state1,2) . An experiment to search
for the tetraneutron state has been performed bymeasuring the double-charge exchange (DCX) reaction 4 He(8 He,8 Be) at 186 MeV/u at RIBF using theSHARAQ spectrometer. Utilizing the large positive Qvalue of the (8 He,8 Be) reaction, an almost recoil-lesscondition of the four-neutron system was achieved inorder to populate weakly interacting four-neutron systems efficiently. The detail of the experiment and dataanalysis have been described in previous reports3,4) .
We obtained the missing-mass spectrum of a tetraneutron system, as shown in Fig. 1 (a). To interpret this spectrum, we assumed two different states:(1) direct decay contributing to the continuum with afinal-state interaction between the two correlated neutron pairs; (2) a possible resonant state of the tetraneutron system near the threshold. To demonstrate
the significance of the yields near the threshold, we
fitted the experimental data with a trial function assuming the absence of the resonant state and estimatedthe goodness-of-fit with the likelihood ratio test (Fig. 1(b)). Because of the small contribution from the continuum and experimental background, the significancelevel of the peak is 4.9 with the analysis of the lookelsewhere effect.
In conclusion, the four events in the 0 < E4n <2 MeV region are candidates for the resonant stateof the tetraneutron system. The mean energy of theevents is evaluated to be 0.83±0.65 MeV with an additional systematic uncertainty of 1.25 MeV due touncertainty in the calibration of the missing-mass energy. These results suggest a possible resonant stateof the tetraneutron system, although the possibilityof a bound state is not experimentally excluded. The
upper limit of the width of the peak is estimated tobe 2.6 MeV (FWHM), which is mainly determined byexperimental missing-mass resolution. The rather narrow width may be understood by considering a small
phase space for the four-body decay.
The result indicating the resonant state may suggest
necessity of strong many-body forces, such as isospin T=3/2 three-body force and/or T = 2 four-body force,
which are incompatible with the present understandingof nuclear interactions5) . Our results should serve as abasis for further investigations.
12Counts/ 2 MeV

10continuum + backgroundbackground × 10(a)wave packet
just after reaction

8resonancedirect decay

642

si

0
7
65
43
2
10-20-10

0

10-100

10

2030

4050

60

2030

4050

60

E4n (MeV)

(b)

-20

E4n (MeV)

Fig. 1. (a) Missing-mass spectrum of the tetraneutron system. The solid (red) line represents the sum of the direct decay of the correlated two-neutron pairs and theestimated background. The dashed (blue) line represents the estimated background multiplied by a factorof 10. (b) Evaluation of the goodness of fit for each bin.

†Condensed from the article in Phys. Rev. Lett. 116, 052501(2016)∗1 RIKEN Nishina Center∗2 Center for Nuclear Study, the University of Tokyo∗3 IPN, Orsay∗4 Department of Physics, Kyoto University∗5 Research Center Nuclear Physics, Osaka University∗6 Cyclotron and Radioisotope Center, Tohoku University∗7 Department of Physics, Tokyo Institute of Technology∗8 Faculty of Engineering, University of Miyazaki∗9 Department of Physics, the University of Tokyo∗10 NSCL, Michigan State University∗11 Faculty of Science, Kyushu University

References
1) F. Marques et al.: Phys. Rev. C 65, 044006 (2002).
2) S. Pieper: Phys. Rev. Lett. 90, 252501 (2003).
3) K. Kisamori et. al.: Accelerator Progress Rep. Vol. 45(2012).
4) K. Kisamori et. al.: Accelerator Progress Rep. Vol. 46(2013).
5) R. Lazauskas et. al.: Phys. Rev. C 71, 044004 (2005).

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