  In this paper, we use the two-flavor Nambu-Jona-Lasinio (NJL) model with the
proper time regularization to study the finite-volume effects of QCD chiral
phase transition.
  Within a cubic volume of finite size $L$, we choose the stationary wave
condition (SWC) as the real physical spatial boundary conditions of quark
fields and compare our results with that by means of commonly used
(anti-)period boundary condition (APBC or PBC). It is found that the results by
means of SWC are obviously different to the results from the APBC or PBC.
Although the three boundary conditions give the same chiral crossover
transition curve in the infinite volume limit, the limit size $L_0$ (when
$L\geq L_{0}$, the chiral quark condensate $-\left\langle { \bar \psi \psi}
\right\rangle_L$ is indistinguishable from that at $L=\infty$) using SWC is
$L_0\approx 500$ fm which is much larger than the results obtained using APBC
or PBC. More importantly, $L_0\approx 500$ fm is also much large than the
typical size of the quark-gluon plasma produced by the relativistic heavy ion
collisions. This means that the finite volume effects play a very important
role in Relativistic Heavy Ion Collisions. In addition, we also found that when
$L\leq 2$ fm, even at zero temperature the chiral symmetry is effectively
restored. Furthermore, to quantitatively reflect the finite volume effects on
the QCD chiral phase transition, we introduce a new vacuum susceptibility,
$\chi_{1/L}(T)=-\frac{\partial \left\langle { \bar \psi \psi}
\right\rangle}{\partial (1/L)}$. With this new vacuum susceptibility, it is
very interesting to find $\chi_{1/L}(T=0)=\chi_{1/L}(T=1/L)$ for SWC.

High Energy Physics - Phenomenology