Embedding the Standard Model symmetry group in higher-dimensional theories often results in additional gauge groups at low energy that have observable consequences at collider experiments. For this reason, simple extensions to the Standard Model group are always promising candidates for new physics independent of specific models. One such extension is an additional SU(N) with fundamental fermions similar to the quarks of QCD. If the interaction is QCD-like, generic searches for new massive quarks and stable massive particles are likely to have some sensitivity to this possibility.
However, if the confinement scale of the interaction is smaller than the mass of the fundamental fermions, QCD-like string breaking by spontaneous production of fermion-antifermion pairs as in the familiar jet fragmentation of QCD, is exponentially suppressed. In this case, the fermions, called "quirks", are bound in pairs by their "infracolor" strings. The large range of quirk masses and infracolor confinement scales allowed by previous measurements and cosmological constraints results in widely varied and rather bizarre phenomenologies that present unusual challenges for detection.
For very small confinement scales, the bound state can be macroscopic, with string lengths of several meters. Quirks in such a bound state leave non-helical trajectories in the detector that will be, at least, very challenging to identify and reconstruct. For very large confinement scales, the quirks undergo rapid re-annihilation, resulting in "hadronic fireball" that will be difficult to distinguish from QCD backgrounds. However, for a few orders of magnitude in the confinement scale, the quirk pair is stable but the string length is smaller than our detector resolution and a simple, but distinct signature emerges. In this "mesoscopic" case,