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Fluctuations and correlations of hot QCD matter in an external magnetic field

Effects of an external magnetic field on the fluctuations of quark number, fluctuations and correlations of conserved charges, including baryon number, electric charge and strangeness, are studied in the 2+1 flavor Polyakov--Nambu--Jona-Lasinio model. We find that magnetic field increases fluctuations and correlations in the regime of chiral crossover. It makes the transition of quadratic fluctuations more abrupt, and the peak structure of quartic fluctuations more pronounced. Our calculations indicate that $χ_{4}^{B}/χ_{2}^{B}$, $χ_{4}^{Q}/χ_{2}^{Q}$, and $χ_{11}^{BQ}$ are very sensitive to the external magnetic field and maybe can be used for probes for the strong magnetic field produced in the early stage of noncentral collisions.