The hunt for long-lived exotic beasts

The hunt for exotic massive long-lived particles is an important element in the ATLAS collaboration’s programme of searches. The signatures associated with such long-lived objects are particularly striking and experimentally challenging. At the LHC they could appear as slow-moving and highly ionizing objects that could slip into the next bunch-crossing, saturate the read-out electronics and confound the event reconstruction software. An alternative approach to the direct detection of moving long-lived particles is to search for those that stop in the detector and subsequently decay. This is the method used in a recent search by the ATLAS collaboration.

The new search looks for metastable R-hadrons that would be formed from gluinos and light quarks (ATLAS collaboration 2011a). If produced, some R-hadrons would stop in the dense calorimeter material – following electromagnetic and hadronic interactions. Within the scenario of split-supersymmetry, an R-hadron could decay to a final state of jets and a neutralino. During 2010, the experiment used jet triggers to record candidate decays in empty bunch crossings when no proton–proton collisions were intended. With the subsequent analysis, which required estimations of cosmic and beam-related backgrounds along with the uncertainties on R-hadron stopping rates, ATLAS has set upper limits on the pair-production cross-section for gluinos with lifetimes in the range 10^–5–10³ s. From this, the collaboration has obtained a lower mass limit for the gluino of around 340 GeV at the 95% CL (see figure). Although the search was inspired by split-supersymmetry, the results are generally applicable for any heavy object decaying to jets.

This complex work complements other, more conventional, searches for long-lived particles that interact or decay in the ATLAS detector. These results allow stringent limits to be set on topical models of new physics. Moreover, the collaboration is performing experimentally driven searches up to the limits of the detector’s capability to detect long-lived objects. For example, a search based on early collision data sought exotic particles with large electric charge (up to 17e).

With more data and a continually improving knowledge of the detector response, the ATLAS collaboration is aiming at a set of comprehensive searches for long-lived objects, which possess a range of colour, electric and magnetic charges, and appear as stable objects or decay to a variety of final states.