QCD knows new quarks

Abstract

We find that a big gap between indicators for the breaking strengths of the global chiral SU(2) and U(1) axial symmetries in QCD of the Standard Model (SM) can be interpreted as a new fine-tuning problem. This may thus imply calling for a class of Beyond the SM, which turns out to favor having a new chiral symmetry and the associated massless new quark insensitive to the chiral SU(2) symmetry for the lightest up and down quarks, so that the fine-tuning is relaxed. Our statistical estimate shows that QCD of the SM is by more than 300 standard deviations off the desired parameter space, which is free from the fine-tuning, and the significance will be greater as the lattice measurements on the QCD hadron observables get more accurate. As one viable candidate, we introduce a dark QCD model with massless new quarks, which can survive current experimental, cosmological, and astrophysical limits, and also leave various phenomenological and cosmological consequences, to be probed in the future. This is a new indication from QCD, which gives a new avenue to deeper understand QCD, and provides a new guideline to consider going beyond the SM.

22 pages, 5 figures; analysis updated, several discussions and references added; claims further refined

References in corpus (5)

• Revised constraints and Belle II sensitivity for visible and invisible axion-like particles
• Masses and decay constants of the $\eta$ and ${\eta }^{\prime }$ mesons from lattice QCD
• The Chiral Phase Transition in three-flavor QCD from Lattice QCD
• New interpretation of chiral phase transition: Violation of trilemma in QCD
• Dynamical solution of the strong CP problem within QCD ?