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 gigantic nucleus should be neutron-rich (renamed neutron star thus) via neutronization. However, Renxin Xu and his collaborators proposed a very different 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.
徐仁新关注天文观测发现的压缩重子物质，特别是脉冲星。日常生活中的原子核是重子的典型代表，它们之间的Coulomb排斥有效地阻止其通过挤压物质而聚合起来。然而天体极端环境时常出人意料：大质量恒星演化至晚期时，其核心引力如此之强以至于其他任何力（当然包括Coulomb排斥）都难以媲美——压缩重子物质就这样在超新星爆发过程中诞生，并表现为观测到的脉冲星。对这类压缩重子物质的研究将不仅加深对强作用低能行为的认识， 而且有助于检验引力理论、探测低频引力波、建立精确的时间标准和导航体系，还是国际或国内在建或拟建大型天文望远镜的核心课题。
超新星产生的致密物质处于何种状态？不幸也是幸运的是，因跟量子色动力学的非微扰性质有关，这一问题至今没有定论。历史上Lev Landau曾推测恒星核心存在着巨大的原子核（可以看作“中子星模型”的原型），其密度接近甚至超过原子核密度。通过中子化过程，巨原子核一定是丰中子的（因而称之为“中子星”）。然而徐仁新及其合作者给出截然不同的观点：超新星遗留残骸不是中子星而是“奇子星”。众所周知，一般原子核由核子组成；如果三味对称性得到恢复的话，则巨原子核会由奇子组成。未来他们将进一步完善奇子星模型，并期待更多观测检验。
