QCD knows new quarks

TL;DR

The paper identifies a fine-tuning problem in QCD related to symmetry breaking and proposes a new dark QCD model with massless quarks as a potential solution, offering new insights into beyond Standard Model physics.

Contribution

It introduces a novel interpretation of symmetry breaking in QCD as a fine-tuning problem and proposes a dark QCD model with massless quarks to address this issue.

Findings

QCD parameters are over 300 standard deviations off the fine-tuned region

A new dark QCD model with massless quarks is consistent with current limits

Potential phenomenological and cosmological implications are discussed

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.