Theory
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 small compared to 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 stable "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.
Phenomenlogy
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 detector resolutions: approximately 100 microns. These "mesoscopic" quirk pairs therefore interact like a single particle in the detector, resulting in a simple but distinct signature; a trail high specific ionization (consistent with a very low-momentum particle) along a straight path (consistent with a high-momentum particle).
Strategy
The initial objective is to develop a search for these "mesoscopic quirks" using the ATLAS detector with the goal of obtaining first results using data collected through the end of 2011. The strategy is to identify a simple, relatively loose identification technique that provides adequate sensitivity while minimizing the dependence on specific model assumptions and uncertainties in the complicated dynamics of the quirk pairs. In time, we hope to expand the search signature to include macroscopic quirks, building on or experience with this simplest case.
While the background for this signature is expected to be low, there are a number of caveats in detecting mesoscopic quirk pairs in the ATLAS detector in addition to theoretical uncertainties in quirk pair dynamics. First, a quirk pair must be produced in association with a photon or jet or the tightly bound pair will be lost down the beampipe undetected. So, we must require a recoiling jet in the event, which also provides a missing energy trigger since the quirk pair will not generally be reconstructed by the trigger. Second, 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. So, the reconstruction technique focuses on the elements of the inner detector: the pixels, SCT and TRT tracking detectors. Meanwhile, the most critical inner layers of the detector, the ATLAS pixels, have limited dynamic range for large dE/dx, so hits from very slow moving quirks can simply be lost.
In order to fully understand and overcome these challenges, a full Geant4 simulation of quirks will be required to assess the signal sensitivity as a function of quirk masses and confinement scales and to develop signal-free samples of data that are needed to model the expected backgrounds.
Status
Because understanding the nature of the signal is critical defining both the search strategy and the required background samples, the top priority has been the development of a robust simulation of quirk dynamics that is as complete as possible. Towards this end, we have developed a standalone Monte Carlo simulation of quirk dynamics to better define the ATLAS-specific signature for and sensitivity to mesoscopic quirks. Initial results of this work indicate that we will double to mass reach of a previous search at the Tevatron with 1 fb-1 of collected data. However, a full Geant-based simulation is required to develop specific cuts for signal selection and allow the development of an unbiased background sample.
Plans
The task of implementing the simulation of quirk dynamics in Geant is just beginning and involves fundamental changes to the way the Geant propagates particles through the detector volume.
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.