セミナー 2021年

Tuesday, January 12 2021, 11:00-12:00

Yu Zhang, Central China Normal University

Revisiting Correlated Dirac eigenvalues and axial anomaly in chiral symmetric QCD

Online (Zoom), indico page, slides (kek.jp only)
How axial anomaly manifests itself at the chiral phase transition temperature affects the nature of chiral phase transition and it is a fundamental question in QCD. We performed a first continuum and chiral extrapolation of two UA(1)UA(1) measures in (2+1)-flavor lattice QCD at T=205T=205 MeV. By using the proposed novel relations between the quark mass derivatives of Dirac eigenvalue spectrum ρρ and correlation functions among eigenvalues we find that the microscopic origin of the current manifested axial anomaly is the peak structure developed in ρρ with its height proportional to quark mass squared. These strongly suggest the chiral phase transition belongs to 3-d O(4) universality class.

Tuesday, January 5 2021, 16:00-17:00

Csaba Csaki, Cornell University

Magnetic scattering: pairwise little group and pairwise helicity

Online (Zoom) https://kds.kek.jp/event/36580/
I discuss how to construct a Lorentz-invariant S-matrix for the scattering of electrically and magnetically charged particles. A key ingredient is a revision of our fundamental understanding of multi-particle representations of the Poincare group. Surprisingly, the asymptotic states for electric-magnetic scattering transform with an additional little group phase, associated with pairs of electrically and magnetically charged particles. I will discuss the general construction of such states. The resulting “pairwise helicity” is identified with the quantized “cross product” of charges e1 g2- e2 g1 for every charge-monopole pair, and represents the extra angular momentum stored in the asymptotic electromagnetic field. We define a new kind of pairwise spinor-helicity variable, which serves as an additional building block for electric-magnetic scattering amplitudes. We then construct the most general 3-point S-matrix elements, as well as the full partial wave decomposition for the 2 -> 2 fermion-monopole S-matrix. In particular, we derive the famous helicity flip in the lowest partial wave as a simple consequence of a generalized spin-helicity selection rule, as well as the full angular dependence for the higher partial waves. Our construction provides a significant new achievement for the on-shell program, succeeding where the Lagrangian description has so far failed.

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