On Skyrmion semiclassical quantization in the presence of an isospin chemical potential

Cohen, Thomas D.; Ponciano, Juan A.; Scoccola, Norberto N.

On Skyrmion semiclassical quantization in the presence of an isospin chemical potential

Authors

Cohen, Thomas D.

Ponciano, Juan A.

Scoccola, Norberto N.

Description

The semiclassical description of Skyrmions at small isospin chemical potential $μ_I$ is carefully analyzed. We show that when the calculation of the energy of a nucleon is performed using the straightforward generalization of the vacuum sector techniques ($μ_I=0$), together with the "natural" assumption $μ_I = {\cal O} (N_c^0)$, the proton and neutron masses are nonlinear in $μ_I$ in the regime $|μ_I| < m_π$. Although these nonlinearities turn out to be numerically quite small, such a result fails to strictly agree with the very robust prediction that for those values of $\mui$ the energy excitations above the vacuum are linear in $μ_I$. The resolution of this paradox is achieved by studying the realization of the large $N_c$ limit of $QCD$ in the Skyrme model at finite $\mui$. This is done in a simplified context devoid of the technical complications present in the Skyrme model but which fully displays the general scaling behavior with $N_c$. The analysis shows that the paradoxical result appears as a symptom of using the semi-classical approach beyond its regime of validity and that, at a formal level, the standard methods for dealing with the Skyrme model are only strictly justified for states of high isospin $I \sim N_c$.
12 pages, 4 figures