Hadron Properties And QCD Phase Structure In Strong Magnetic Field From Lattice QCD

The properties of strongly interacting matter in the external magnetic field have attracted many studies in recent years.Due to the existence of the magnetic field,many interesting phenomena appeared in the QCD phase structure e.g.(inverse)magnetic catalysis and the reduction of pseudo critical temperature.To study the QCD phase structure and its chiral properties,we performed lattice simulations in(2+1)-flavor QCD using highly improved staggered action at zero temperature on 323×96 lattices.Simulations at finite temperature are performed with the same spatial lattice size Nσ=32 and temporal lattice size NT=6,8,10,12,14,16,24 corresponding to temperature T∈[17,281]MeV.In the simulations lattice spacing a was fixed to 0.117 fm with the pion mass tuned to around 220 MeV,and the magnetic field strength ranges from 0 to～2.5GeV2 except T=0 case.At zero temperature,magnetic field strength eB∈[0,3.35]GeV2.At zero temperature,we present lattice QCD results for masses and magnetic polarizabilities of light and strange pseudoscalar mesons,chiral condensates,decay constants of neutral pion and kaon in the presence of background magnetic fields with eB ranging up to around 3.35 GeV2(～70 Mπ2)in the vacuum.We find that the masses of neutral pseudoscalar mesons monotonously decrease as the magnetic field strength grows and then saturate at a nonzero value,while there exists a nonmonotonous behavior of charged pion and kaon masses in the magnetic field.We observe a qB scaling of the up and down quark flavor components of neutral pion mass,neutral pion decay constant as well as the quark chiral condensates at 0.05(?)eB(?)3.35 GeV2.We show that the correction to the Gell-Mann-Oakes-Renner relation involving the neutral pion is less than 6%and the correction for the relation involving neutral kaon is less than 30%at eB(?)3.35 GeV2.At finite temperature,we point out that chiral condensates at nonzero temperature and magnetic fields are in strict connection to the space-time integral of corresponding two-point neutral meson correlation functions in the pseudoscalar channel via the Ward-Takahashi identity.Screening masses of neutral pseudoscalar mesons,which are defined as the exponential decay of the corresponding spatial correlation functions in the long distance,thus are intrinsically connected to(inverse)magnetic catalysis of chiral condensates.We find that screening lengths,i.e.inverses of screening masses of π0,K0 and ηss0,turn out to have the similar complex eB and T dependences of the corresponding chiral condensates.Although the transition temperature is found to always decrease as eB grows,we show that the suppression due to magnetic fields becomes less significant for hadron screening length and chiral condensates with heavier quarks involved,and ceases to occur for ηss0 and strange quark chiral condensate.The complex eB and T dependences of both screening masses and chiral condensates,reflecting the crossover nature of the QCD transition,are attributed to the competition between sea and valence quark effects.These findings could be useful to guide low-energy models and effective theories of QCD.