Evidence for a First Order, Finite Temperature Phase Transition in 8 Flavor QCD

Abstract

As part of our ongoing investigations of QCD with many flavors of quarks, here we report on studies of the finite temperature phase transition for eight-flavor QCD with the DBW2 gauge action and na\"ive staggered fermions. We find a clear first order phase transition between the chirally asymmetric phase at zero temperature and the chirally symmetric phase at finite temperature, signaled by a two-state signal for $\langle\bar{\psi}\psi\rangle$ at a non-zero temperature. We see this signal at a gauge coupling of $\beta=0.54$, where, to set the scale, the zero temperature value for $f_\pi$, in the chiral limit, is 0.06661(92). This strong, first-order signal is seen for two different values of the quark mass, $m_q=0.007$ and 0.0195, at $N_\tau=8$ and 6 respectively. Using $f_\pi(m_q)$ as the scale, the critical temperature is measured to be $T_c/f_\pi=1.638(93)$ at $m_\pi/f_\pi=3.329(30)$ for $m_q=0.007$, and $T_c/f_\pi=1.779(27)$ at $m_\pi/f_\pi=4.093(15)$ for $m_q=0.0195$. At a weaker coupling $\beta=0.56$, where at zero temperature and in the chiral limit we find $f_\pi=0.0312(10)$, the first order signal becomes numerically invisible to us for the $N_\tau \leq 14$ lattices we have investigated so far.