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, a pair of slow-moving, charged quirks that together are electrically neutral leaves a trail high specific ionization (consistent with a low-momentum particle) along a straight path (consistent with a high-momentum particle) in the detector.
While the background for this signature is expected to be low, there are a number of caveats in detecting quirks in the ATLAS detector in addition to theoretical uncertainties in quirk dynamics. In particular, the quirks move slowly enough that the hits they make in the detector can be out-of-time with respect to particles traveling at the speed of light, a problem that is obviously less severe in the inner layers of the detector. Meanwhile, the most critical inner layers of the detector, the ATLAS pixels, have limited dynamic range for large dE/dx.
We have developed a standalone Monte Carlo simulation of quirk dynamics to better determine the signature and the sensitivity of the ATLAS detector for quirks. This is now moving towards implementation of quirks in Geant4, which will be required in order to fully understand both our signal and the issues that limit our rejection of backgrounds from random tracks.