The saga of the tetraneutron

In light of a new experiment for the double-charge-exchange reaction $^{4}$He($^{8}$He,$^{8}$Be) at 186 MeV/u [1], which claims an identification of a resonant tetraneutron state we review results of experimental search and theoretical studies within methods of few body physics for a tetraneutron. Particularly, we discuss searches of the tetraneutron using the pion double charge exchange reaction $^{4}$He($\pi^{-}$,$\pi ^{+})^{4}$n, heavy-ion transfer reactions such as $^{7}$Li($^{11}$B,$^{14}$O)$^{4}$n and $^{7}$Li($^{7}$Li,$^{10}$C)$^{4}$n, as well as search of the tetraneutron and multineutrons in fission reactions. We present and review theoretical analyses of the tetraneutron in the framework of various theoretical approaches such as variational methods, the method of Faddeev-Yakubovsky equations, and the method of hyperspherical harmonics within the existing modern two- and three-nucleon interactions. Today there is no unambiguous answer for the existence of the trineutron as a bound or resonance state. This issue will be addressed. [1] K. Kisamori, et al., Phys Rev. Lett. 116, 052501 (2016).