Axial U(1) symmetry and mesonic correlators at high temperature in $N_f=2$ lattice QCD

Abstract

We investigate the high-temperature phase of QCD using lattice QCD simulations with N f = 2 dynamical Möbius domain-wall fermions. On generated configurations, we study the axial U(1) symmetry, overlap-Dirac spectra, screening masses from mesonic correlators, and topological susceptibility. We find that some of the observables are quite sensitive to lattice artifacts due to a small violation of the chiral symmetry. For those observables, we reweight the Möbius domainwall fermion determinant by that of the overlap fermion. We also check the volume dependence of observables. Our data near the chiral limit indicates a strong suppression of the axial U(1) anomaly at temperatures ≥ 220 MeV.

Gliding Down the QCD Transition Line, from Nf = 2 till the Onset of Conformality

“…Beyond two flavours, the issue of the anomaly becomes more subtle: the definition of a proper order parameter for axial symmetry is entangled with different susceptibilities associated with different flavours [16]. Some studies indicate restoration above T c [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], others find hints of a near-coincidence of the two transitions [5,29]. Our recent study [32], which will be reviewed in detail in Section 4, attempts at quantifying the limit of the scaling window and finds compatibility with 3D O(4), thus implicitly suggesting a separation between the two transitions.…”

“…One final comment concerns the U(1) A symmetry: given its prominent role, it is natural to resort to its analysis to try to shed more light on the symmetry pattern. However, again, the problem remains open: the current understanding is that it seems to be effectively restored above T c [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], but there is no consensus on the restoration temperature. For instance, Ref.…”

Gliding down the QCD transition line, from $N_f=2$ till the onset of conformality

“…Beyond two flavors, the issue of the anomaly becomes more subtle: the definition of a proper order parameter for axial symmetry is entangled with different susceptibilities associated with different flavors [16]. Some studies indicate restoration above T c [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], others find hints of a near-coincidence of the two transitions [5,29]. Our recent study [32], which will be reviewed in detail in Section 4, attempts at quantifying the limit of the scaling window and finds compatibility with 3D O(4), thus implicitly suggesting a separation between the two transitions.…”

“…One final comment concerns the U (1) A symmetry: given its prominent role, it is natural to resort to its analysis to try to shed more light on the symmetry pattern. But, again, the problem remains open: the current understanding is that it seems to be effectively restored above T c [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31], but there is no consensus on the restoration temperature. For instance, Ref.…”

QCD transition at the physical point, and its scaling window from twisted mass Wilson fermions

“…[7,31,32]. Concerning axial symmetry, the current understanding is that it seems to be effectively restored above T c [22,27,[33][34][35][36][37][38][39][40][41][42][43][44][45], but there is no consensus on the restoration temperature. On the analytic side, a 4D analysis [46] reported scaling only for very light pion masses, m π < 1 MeV, and an apparent scaling for larger masses.…”