#### IMSc Webinar

#### From hadrons to quarks in neutron stars

#### Toru Kojo

##### CCNU

*Neutron stars are cosmic laboratories to study the properties of dense QCD matter at almost zero temperature. The neutron star mass-radius relation has the one-to-one correspondence to the QCD equation of state and gives strong constraints on our understanding on the properties of the matter. I review recent developments in neutron star observations and combine them together with the constraints from nuclear laboratory experiments as well as low- and high-density limits of QCD calculations. I will argue what the current constraints tell about the QCD phase structure and effective degrees of freedom in cold matter. I will discuss that many neutron stars are in the domain beyond purely hadronic regime so that the understanding based on quark degrees of freedom will be essential. It will be argued that the easiest way to be compatible with the existing constraints is to assume the crossover transition from hadronic to quark matter and to describe quark matter based on the picture used in the hadron spectroscopy. In this context the concepts of quark-hadron continuity and quarkyonic matter scenarios will be discussed.*

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