Theoretical studies and experimental searches for a bound or resonant state in a system of four neutrons (tetraneutron) were started more than 50 years ago. The first experimental observation of the tetraneutron resonance was reported in 2016. I shall discuss the theoretical description of the low-energy tetraneutron resonance consistent with the experiment which however has a poor statistics and hence large error bars. This description is based on ab initio calculations within the no-core shell model extended to the description of four-body continuum within the democratic decay approximation.
Moscow State University, Institute for Nuclear Physics