  We investigate the phase structure of strongly interacting matter and baryon
number fluctuations in the Polyakov loop improved Nambu--Jona-Lasinio (PNJL)
model. The calculation shows that both the chiral and deconfinement
transitions, as well as their coincidence and separation determine the basic
QCD phase structure. The contour maps and the three-dimensional diagrams of the
net-baryon kurtosis and skewness present well the trace of QCD phase structure.
Comparing with the experimental data, we find that the existence of a critical
end point (CEP) of chiral transition is crucial to explain the non-monotonic
energy dependence and the large deviation from Poisson baseline of net-proton
kurtosis. In particular, the relation between the chiral and deconfinement
transitions in the crossover region is also reflected by the baryon number
fluctuations. This study shows that the measurements of higher moments of
multiplicity distributions of conserved charges are powerful to investigate the
criticality and even the chiral and deconfinement transitions in the crossover
region.
