Chemical potential (in)dependence of hadron scatterings in the hadronic phase of QCD-like theories and its applications

TL;DR

This paper develops a method to analyze hadron-hadron scattering at nonzero chemical potential in the hadronic phase, showing that scattering phase shifts are independent of chemical potential when accounting for effective mass modifications.

Contribution

The authors extend the HAL QCD method to nonzero chemical potential, demonstrating the μ-independence of scattering phase shifts and applying it to lattice QCD simulations.

Findings

Scattering phase shifts are μ-independent when effective mass dependence is considered.

Interaction potential depends on μ only through the effective mass.

Numerical analysis confirms μ-independence in lattice simulations.

Abstract

We formulate a method for calculating the hadron-hadron scattering amplitudes at nonzero chemical potential ($\mu$) in the hadronic phase at zero temperature, where the baryon number symmetry remains to be violated. Although it is widely believed that the physical quantities do not change even if we turn on a small $\mu$ at zero temperature, the shape of correlation functions for a single hadron depends on $\mu$. Then, the dispersion relation of the single hadron is modified to $E({\bf p},\mu) = \sqrt{{\bf p}^2+m^2}-\mu n_{O}$. Here, $m$ and $n_O$ denote the hadron mass at $\mu=0$ and the quantum number, respectively. From this relation, it is possible that the effective mass of the hadron depends on $\mu$. We extend the HAL QCD method at $\mu=0$ to the case of $\mu \ne 0$, which allows us to extract the scattering phase shifts via the interaction potential. We have found that the interaction potential can depend on $\mu$ only through the effective mass while the scattering phase shifts, obtained by solving the Schr\"{o}dinger equation with the interaction potential, are independent of $\mu$. We also numerically analyze the S-wave scatterings of two pions with isospin $I=2$ and two scalar diquarks within the framework of QC$_{2}$D at nonzero quark chemical potential. While the lattice is not exactly set to zero temperature, the $\mu$-independence can be observed. Furthermore, we improve the results for the S-wave scatterings of two hadrons obtained above by taking the $\mu$-independence for granted. Thanks to the asymmetric property of the correlation functions for diquarks at $\mu\neq0$, we can access a long-$\tau$ regime and can reduce the systematic error coming from inelastic contributions.