QCD Phase Diagram for Large $N_f$ : Analysis from Contact Interaction Effective Potential

In this paper, we discuss the impact of a higher number of light quark flavors, $N_f$, on the QCD phase diagram under extreme conditions. Our formalism is based on the Schwinger-Dyson equation, employing a specific symmetry-preserving vector-vector flavor-dressed contact interaction model of quarks in Landau gauge, utilizing the rainbow-Ladder truncation. We derive expressions for the dressed quark mass $M_f$ and effective potential $\Omega^{f}$ at zero, at finite temperature $T$ and the quark chemical potential $\mu$. The transition between chiral symmetry breaking and restoration is triggered by the effective potential of the contact interaction, whereas the confinement and deconfinement transition is approximated from the confinement length scale $\tilde{\tau}_{ir}$. Our analysis reveals that at $(T = \mu = 0)$, increasing $N_f$ leads to the restoration of chiral symmetry and the deconfinement of quarks when $N_f$ reaches its critical value, $N^{c}_{f} \approx 8$. At this critical value, In the chiral limit ($m_f = 0$), the global minimum of the effective potential occurs at the point where the dressed quark mass approaches zero ($M_f \rightarrow 0$). However, when a bare quark mass of $m_f = 7$ MeV is introduced, the global minimum shifts slightly to a nonzero value, approaching $M_f \rightarrow m_f$. At finite $T$ and $\mu$, we illustrate the QCD phase diagram in the $(T^{\chi,C}_{c} -\mu)$ plane, for various numbers of light quark flavors, noting that both the critical temperature $T_c$ and the critical chemical potential $\mu_c$ for chiral symmetry restoration and deconfinement decrease as $N_f$ increases. Moreover, the critical endpoint $(T_{EP}, \mu_{EP})$ also shifts to lower values with increasing $N_f$. Our findings are consistent with other low-energy QCD approaches.