Speaker
Description
The trace anomaly is a quantity of fundamental interest in field theories, which signals whether the underlying theory is conformal. In the context of neutron stars, we propose the trace anomaly for a new measure of the conformality as an alternative to the speed of sound; here we specifically consider the normalized trace anomaly, $1/3 - P/\varepsilon$, with $P$ and $\varepsilon$ being the pressure and energy density, respectively.
By combining the current theoretical and the observational insights, we discuss several interesting features of this quantity:
The trace anomaly inferred from the recent neutron star observations shows the vanishing behavior already around 5-6 times the saturation density. It means that the matter is approximately conformal, which leads to the concept of the strongly-interacting conformal matter inside neutron stars.
We point out a monotonic change in the trace anomaly can lead to a peak in the speed of sound. This is in consonance with the current understanding, and the underlying physics is a rapid liberation of the physical degrees of freedom due to strong interactions of baryons.
We finally conjecture that there is a bound imposed by the positivity of the trace anomaly on the equation of state, and discuss its observational consequences.
