On the strength of the $U_A(1)$ anomaly at the chiral phase transition in $N_f=2$ QCD

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.

46 pages, 16 figure, 12 tables; v2: typos corrected; enhanced explanations and discussions; included study of systematic effects for the extraction of screening masses; conclusions unchanged; new version to match published version

References in corpus (12)

• The order of the quantum chromodynamics transition predicted by the standard model of particle physics
• Electrical conductivity and charge diffusion in thermal QCD from the lattice
• Local coherence and deflation of the low quark modes in lattice QCD
• Meson screening masses from lattice QCD with two light and the strange quark
• Deflation acceleration of lattice QCD simulations
• Non-perturbative improvement of quark mass renormalization in two-flavour lattice QCD
• A relation between screening masses and real-time rates
• Chiral dynamics in the low-temperature phase of QCD
• The equation of state of quark-gluon matter from lattice QCD with two flavors of twisted-mass Wilson fermions
• Effects of the anomaly on the two-flavor QCD chiral phase transition
• Meson Screening Masses in the Interacting QCD Plasma
• Vector screening masses in the quark-gluon plasma and their physical significance

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• Eigenvalue spectra of QCD and the fate of $U_A(1)$ breaking towards the chiral limit
• Modeling Finite-Volume Effects and Chiral Symmetry Breaking in Two-Flavor QCD Thermodynamics
• Chiral condensates and screening masses of neutral pseudoscalar mesons in thermomagnetic QCD medium
• QCD phase transitions in the light quark chiral limit
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• Second order chiral phase transition in three flavor quantum chromodynamics?
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• Testing thermal photon and dilepton rates
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• New interpretation of chiral phase transition: Violation of trilemma in QCD
• Thermal behavior of effective U_A(1) anomaly couplings in reflection of higher topological sectors
• Light quarks at finite temperature: chiral restoration and the fate of the $U(1{)}_A$ symmetry
• The pion-kaon scattering amplitude and the $K^0(700)$ and $K^{\ast }(892)$ resonances at finite temperature
• New aspect of chiral and axial breaking in QCD
• The role of strangeness in chiral and $U(1{)}_A$ restoration
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• Understanding the approach to thermalization from the eigenspectrum of non-Abelian gauge theories
• Photon emissivity of the quark-gluon plasma: a lattice QCD analysis of the transverse channel
• QCD at high temperature and density: selected highlights
• Universal microscopic spectrum of the unquenched QCD Dirac operator at finite temperature
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