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Renxin  Xu's focus is compressed baryonic matter, such as pulsars especially,  in astrophysics. Baryonic nuclei in the daily life are forbidden to fuse  by compression due to the Coulomb repulse; nevertheless, it is usually  unexpected in extraterrestrial extreme-environments: the gravity in a  core of massive evolved star is so strong that all the other forces  (including the Coulomb one) could be negligible. Compressed baryonic  matter is then produced after supernova, manifesting itself as pulsars  observed. The study of this compressed baryonic matter can not only be  meaningful in fundamental physics (e.g., the fundamental strong  interaction at low-energy scale, testing gravity theories, detecting  nano-Hertz background gravitational waves), but has also profound  implications in engineering applications (including time standard and  navigation), and additionally, is focused by international as well as  Chinese advanced facilities, either terrestrial or in space.

What's  the state of gravity-compressed matter produced during supernova?  Unfortunately/fortunately, this remains unknown, to be relevant to the  nature of non-perturbative QCD (quantum chromo-dynamics). Historically,  in 1930s, Lev Landau speculated that dense matter at supra-nuclear  density in stellar cores could be considered as gigantic nuclei (the  prototype of standard model of neutron star). A giant nucleus should  be neutron-rich (renamed neutron star thus) via neutronization. However,  Renxin Xu and his collaborators proposed a very different point of view: the  rump left behind after a supernova could be a strangeon star. Nucleon is  the constituent of a nucleus, while strangeon is named as the  constituent of a gigantic nucleus if three flavor symmetry (u, d and s)  restores there. They are both developing the strangeon star model and  expecting to test it by further observations.

Besides pulsar-like compact objects, strangeon matter could also be manifested in the form of cosmic rays and even dark matter. Strangeon nuggets, with kinematic energy order of EeV, could be produced in the ejecta of merging strangeon stars, mixing in the ingredients of ultra-high energy cosmic ray.  The nugget could possibly be left behind after the cosmic QCD-phase transition, as a candidate of dark matter.