Can ATLAS avoid missing the long lived stau?

A heavy charged stable or semi-stable particle is predicted in many models of physics beyond the standard model. A case in point is Gauge Mediate Supersymmetry Breaking (GMSB) with high tanβ where the is the Next to Lightest Supersymmetric Particle (NLSP) and couples weakly to the gravitino. The signal we are looking for is a heavy long-lived charged particle with low beta. The momentum spectrum of those sleptons is model dependent. The ones with beta significantly lower than 1 can be identified as staus, and are the subject of this note. In ATLAS, in order to match event fragments from different sub detectors into events correctly, great emphasis is placed on beam crossing identification, BCID. This assumes that the particles that leave signals in the detector travel nearly at the speed of light (beta=1). In this note we will consider a particle with beta smaller than one. Such a particle may be completely lost during data collection, or might be marked with the wrong BCID. The concept of wrong BCID due to low particle speed was not considered in the ATLAS trigger and data acquisition design. ATLAS was definitely not designed to deal with it. Therefore, the primary interest of this document is in assuring that design issues will not cause ATLAS to miss a potential big discovery.