  Correlations and fluctuations of physical quantities are known to play an
important role in phase transitions and critical phenomena. In recent years
some experimental attempts were made in the scope of the Beam Energy Scan
program to locate a possible critical point in the QCD phase diagram. In this
work we use the Nambu--Jona-Lasinio model to investigate the off-diagonal quark
susceptibility, which is related to the quark--anti-quark scaled correlation at
the mean field level. We show that this correlation has a significant peak near
the critical point and, therefore, may be a useful quantity to measure in
experiment. We further study the effects of a repulsive vector coupling, which
reduces the strength of the scaled correlation near the critical point.
