Measurement of hadron masses in 2-color finite density QCD

TL;DR

This study uses lattice simulations to explore how hadron masses change in 2-color QCD at finite density, revealing phase-dependent spectral ordering and degeneracies linked to symmetry breaking.

Contribution

It provides the first detailed analysis of hadron spectra across hadronic and superfluid phases in 2-color QCD at finite density, highlighting spectral degeneracies and symmetry effects.

Findings

Spectral ordering flips at the phase transition point.

Degeneracy of meson, diquark, and antidiquark masses below the transition.

Effective masses of pion increase linearly, rho meson decreases with increasing chemical potential.

Abstract

We investigate hadron spectra in 2-color QCD using lattice simulation with $N_{f}=2$ at low temperature and finite density in which there appears not only the hadronic phase but also the superfluid phase. We first calculate the pion and rho meson spectrum, which is well-known from previous works. The spectral ordering of these mesons flips around the quark chemical potential $\mu=m^{0}_{\pi}/2$ ($m^{0}_{\pi}$: the pion mass at $\mu=0$), where the phase transition between the hadronic and superfluid phases occurs. For $\mu \gtrsim m^{0}_{\pi}/2$, the effective mass for the pion linearly increases while the one for the rho meson monotonically decreases. Furthermore, we measure hadron spectra with the isospin $I=0$ and the angular momentum $J^{P}=0^{\pm}$. The effective masses for the meson, diquark, and antidiquark with the same quantum number become degenerate just below $\mu = m^{0}_{\pi}/2$, and the three hadrons have the same mass in the superfluid phase. It suggests that mixing occurs between spectra associating with mesons and baryons due to the $U(1)_{B}$ symmetry breaking. This phenomenon can be explained in the linear sigma model with the approximate $SU(4)$ Pauli-Gursey symmetry.