On the strength of the U A (1) anomaly at the chiral phase transition in N f = 2 QCD

Abstract

Abstract:We study the thermal transition of QCD with two degenerate light flavours by lattice simulations using O(a)-improved Wilson quarks. Temperature scans are performed at a fixed value of N t = (aT ) −1 = 16, where a is the lattice spacing and T the temperature, at three fixed zero-temperature pion masses between 200 MeV and 540 MeV. In this range we find that the transition is consistent with a broad crossover. As a probe of the restoration of chiral symmetry, we study the static screening spectrum. We observe a degeneracy between the transverse isovector vector and axial-vector channels starting from the transition temperature. Particularly striking is the strong reduction of the splitting between isovector scalar and pseudoscalar screening masses around the chiral phase transition by at least a factor of three compared to its value at zero temperature. In fact, the splitting is consistent with zero within our uncertainties. This disfavours a chiral phase transition in the O(4) universality class.

Patterns and partners for chiral symmetry restoration

“…III B, the r.h.s. of (22)- (27) would vanish at strict Oð4Þ restoration, which supports our results in the previous sections.…”

“…Nevertheless, the latter limit is meant to be reached at a much higher temperature than chiral restoration, as seen, for instance, in the degeneration of screening masses [35]. The four correlators (34) and (35) have been actively investigated in lattice and theoretical analyses to study partner degeneracy [5,[22][23][24][25]27,36,37].…”

“…of (27)-(31) includes the effect of the η 0 , which couples through AðxÞ to the Uð3Þ formulation of the chiral Lagrangian [12]. For instance, (27) can be expressed in terms of πηðη 0 Þ → πηðη 0 Þ andKK → πηðη 0 Þ processes, all in the a 0 ð980Þ channel. Note that the light chiral limitm → 0 þ on the r.h.s.…”

Chiral and U(1)A restoration for the scalar and pseudoscalar meson nonets

“…These simulations are performed in the chiral limit for two flavors. In addition, in the recent analysis [45], results compatible with Uð1Þ A being restored at the chiral transition are also reported for two flavors and massive quarks. The influence of Uð1Þ A restoration on the phase diagram, the tricritical point and the transition order has also been investigated in the lattice in [46,47], while degeneration of parity partners for nucleons in the lattice have been analyzed recently in [48].…”

Axial U(1) symmetry at high temperature in 2-flavor lattice QCD

“…(We consider the theory with two degenerate quark flavors.) Above the critical temperature, T > T c , while the (spontaneously broken) chiral symmetry is known to be restored, the restoration/violation of the U(1) A symmetry is a long standing problem, which has been studied using in analytic methods [1][2][3] and effective theories [4,5] as well as lattice QCD simulations at N f = 2 [6][7][8][9] and N f = 2 + 1 [10][11][12][13]. In Ref.…”