Non-perturbative QCD inputs for axion phenomenology: status and perspectives

• Claudio Bonanno (U. Bern)

Room: Auditorium Salvini I will review the current state of affairs concerning non-perturbative QCD inputs for axion phenomenology. I will provide a systematic survey of existing lattice QCD results, and I will outline how they compare with up-to-date predictions coming from chiral effective theories and semiclassics. I will conclude delineating the future challenges that await this field in the near future, and what are the current perspectives to meet them.

Thermal Production of QCD Axions from the Early Universe

• Alessio Notari (Universitat de Barcelona)
• Alessio Notari (Istituto Nazionale di Fisica Nucleare)

Room: Auditorium Salvini A population of thermally produced QCD axions from the early universe is expected to exist, in addition to a cold dark matter population. I discuss axion production from scattering with pions, including momentum dependence. I will show that to exploit the reach of current and upcoming cosmological surveys, non-perturbative calculations around and above the QCD crossover are needed. I will also review production from other Standard Model particles (quarks, leptons).

QCD axion at the QCD scale?

• Luca Di Luzio (Istituto Nazionale di Fisica Nucleare)

Room: Auditorium Salvini In a recent paper, Murayama proposed a GeV-scale axion scenario in which the up-quark mass arises dynamically from the QCD chiral condensate, which at the same time spontaneously breaks a Peccei–Quinn symmetry. If correct, this striking idea would imply that the QCD axion has effectively already been discovered and is hidden among the known pseudoscalar resonances in the 1–2 GeV region. The proposal, however, faces a serious challenge: it predicts too large a mass splitting between neutral and charged pions. In this talk, I will discuss several attempts to overcome this problem. Despite some partial improvements, we find a structural obstruction: the new Peccei–Quinn spurion breaks the accidental isospin symmetry of the chiral Lagrangian, generating an enhanced higher-order operator. This in turn also leads to sizable distortions in pion-pion scattering. Although a successful resolution appears unlikely, a definitive conclusion would require lattice simulations of the resulting deformed QCD theory.

Discussion Session Room: Auditorium Salvini