) ) reaction

Double Gamow–Teller giant resonance (DGTGR) is the simplest two-phonon modes occurring in the spin-isospin channel, which is accompanied by the flip of both of the spin and isospin twice, without change of the orbital angular momentum. This mode was first proposed by Auerbach in 1989[1] but not yet observed despite of many experimental challenges[2, 3, 4]. The search for the DGTGR attracts particular attention because of links to the neutrino physics; the transition strength and the centroid energy of the DGTGR are suggested to be strongly correlated with the nuclear matrix element for a neutrino-less double beta decay[5].

We are searching for the DGTGR with the double-charge exchange reaction (¹²C, ¹²Be(0⁺₂)) at several hundred MeV/nucleon. We use this reaction for following reasons;  First, this reaction is expected to have relatively large cross section because the transition from ¹²C(0⁺_g.s.) to ¹²Be(0⁺₂) through the intermediate state of ¹²B(1⁺) is strong[6]. Second, Be(0⁺₂) decays into the ground state by emitting an electron-positron pair. Detecting the delayed-γ ray from the ¹²Be(0⁺₂) serves to tag the events of the double spin-flip mode.

A pilot experiment using the (¹²C, ¹²Be(0⁺₂)) reaction was performed at RCNP in 2014. In this experiment, the detection of γrays emitted from ¹²Be(0⁺₂) states was hampered by strong background coming from the stopper. Therefore, we performed a background-free experiment at RIBF in 2021 and the S/N ratio was improved 5 times as much as that of experiment at RCNP.

In this talk, the overview of the experiment mainly at RIBF and the preliminary result will be presented.

[1] N. Auerbach, L. Zamick, and D. Zheng, Annals of Phys. 192, 77 (1989).

[2] S. Mordechai et al., Phys. Rev. Lett. 60, 408 (1988).

[3] J. Blomgren et al., Phys. Lett. B 362, 34 (1995).

[4] K. Takahisa, AIP Conference Proceedings 915, 815 (2007).

[5] N. Shimizu, J. Menéndez, and K. Yako, Phys. Rev. Lett. 120, 142502 (2018).

[6] H. Matsubara et al., Few-Body Systems 54, 1433 (2013).