Apply Gauge/Gravity Duality To Study QCD Phase Diagram

Quantum chromodynamics(QCD)is the fundamental theory describing QCDmatte interactions.In medium/high energy nuclear physics,determining the location of the critical endpoint is crucial for understanding the phase structure of QCD species.However,due to the strong coupling property of QCD at low energy scales,perturbation methods are not applicable.Recent explorations of lattice QCD(LQCD)and low-energy effective theories suggest the existence of a critical endpoint(CEP)between the hadronic phase and the quark-gluon plasma(QGP).The CEP indicates that the phase transition between the two phases is first-order when the chemical potential is higher than the critical value,while a smooth transition,known as a "Crossover" occurs when the chemical potential is lower.Although LQCD can provide equations of state at zero chemical potential,obtaining equations of state at finite chemical potential is challenging due to the sign problem.In 1997,Maldacena proposed the Anti-de Sitter/Conformal Field Theory(Ad S/CFT)dual conjecture,which has become a powerful tool for studying strongly coupled systems.This study utilizes the gauge/gravity duality to investigate QCD phase transitions.We fix the model parameters by comparing the model results with the latest data of lattice QCD,e.g.,the equations of state,baryon number susceptibility,etc.Further,our model predicts the equations of state at finite chemical potential,which agrees with the recent LQCD simulation.We offer a reliable phase diagram of QCD under finite temperature and finite baryon number chemical potentials,including the precise location of CEP and the first-order transition line between the QGP and Hadron gas.Furthermore,we apply holographic probe action to study quark and gluon condensation,which agrees with LQCD data.Moreover,we explore the possibility of the first-order phase transition in the model-predicted phase diagram as the first-order transition in the early universe and calculate its gravitational wave energy spectrum.Our results indicate that IPTA and SKA can observe it,and even NANOGrav under some extreme conditions.