Magnetic susceptibility of QCD matter and its decomposition from the lattice

Bali GS, Endrödi G, Piemonte S. Magnetic susceptibility of QCD matter and its decomposition from the lattice. Journal of High Energy Physics . 2020;2020(7): 183.We determine the magnetic susceptibility of thermal QCD matter by means of first principles lattice simulations using staggered quarks with physical masses. A novel method is employed that only requires simulations at zero background field, thereby circumventing problems related to magnetic flux quantization. After a careful continuum limit extrapolation, diamagnetic behavior (negative susceptibility) is found at low temperatures and strong paramagnetism (positive susceptibility) at high temperatures. We revisit the decomposition of the magnetic susceptibility into spin- and orbital angular momentum- related contributions. The spin term - related to the normalization of the photon lightcone distribution amplitude at zero temperature - is calculated non-perturbatively and extrapolated to the continuum limit. Having access to both the full magnetic susceptibility and the spin term, we calculate the orbital angular momentum contribution for the first time. The results reveal the opposite of what might be expected based on a free fermion picture. We provide a simple parametrization of the temperature- and magnetic field-dependence of the QCD equation of state that can be used in phenomenological studies

Progress in finite temperature lattice QCD

I review recent progress in finite temperature lattice calculations, including the determination of the transition temperature, equation of state, screening of static quarks and meson spectral functions.Comment: 8 pages, LaTeX, uses iopart.cls, invited talk presented at Strangeness in Quark Matter 2007 (SQM 2007), Levoca, Slovakia, June 24-29, 200

Hadronic vacuum polarization and muon from magnetic susceptibilities on the lattice

Bali GS, Endrödi G. Hadronic vacuum polarization and muon from magnetic susceptibilities on the lattice. Physical Review D. 2015;92(5): 054506.We present and test a new method to compute the hadronic vacuum polarization function in lattice simulations. This can then be used, e.g., to determine the leading hadronic contribution to the anomalous magnetic moment of the muon. The method is based on computing susceptibilities with respect to external electromagnetic plane wave fields and allows for a precision determination of both the connected and the disconnected contributions to the vacuum polarization. We demonstrate that the statistical errors obtained with our method are much smaller than those quoted in previous lattice studies, primarily due to a very effective suppression of the errors of the disconnected terms. These turn out to vanish within small errors, enabling us to quote an upper limit. We also comment on the accuracy of the vacuum polarization function determined from present experimental R-ratio data

QCD phase diagram for nonzero isospin-asymmetry

Brandt B B, Endrödi G, Schmalzbauer S. QCD phase diagram for nonzero isospin-asymmetry. Physical Review D. 2018;97(5): 054514.The QCD phase diagram is studied in the presence of an isospin asymmetry using continuum extrapolated staggered quarks with physical masses. In particular, we investigate the phase boundary between the normal and the pion condensation phases and the chiral/deconfinement transition. The simulations are performed with a small explicit breaking parameter in order to avoid the accumulation of zero modes and thereby stabilize the algorithm. The limit of vanishing explicit breaking is obtained by means of an extrapolation, which is facilitated by a novel improvement program employing the singular value representation of the Dirac operator. Our findings indicate that no pion condensation takes place above T≈160  MeV and also suggest that the deconfinement crossover continuously connects to the BEC-BCS crossover at high isospin asymmetries. The results may be directly compared to effective theories and model approaches to QCD