more involved than measuring, for example, the momentum. Therefore, the experimental uncertainty is much larger, and a cut on the velocity will not reject all background muons.

There are two distinct ways of measuring the velocity of muon-like particles, the ionization energy loss (${\mathrm{d}}E/{\mathrm{d}}x$) inside the tracker and the time-of-flight (ToF) measurement, which measures the time between the bunch crossing and the passing through the muon chambers. The former measurement only provides information up to $\beta \lesssim 0.9$ while the latter is mainly limited from below—at the design luminosity of the LHC, particles with velocities less than about $0.6$ cannot be assigned to the correct bunch crossing anymore.

Although the precision of each velocity measurement is not overwhelming, a combination of both measurements provides a highly efficient background rejection. This is due to the fact that for stau signal events these two measurements are clearly correlated while for background events no correlation is present [@PoS2008LHC012]. According to [@CMS-PAS-EXO-08-003] a cut using both measurements, $\beta_{{\mathrm{d}}E/{\mathrm{d}}x}$, $\beta_{\rm ToF}<0.8$, leads to a background rejection factor of about $10^{-7}$ for single stau candidates.

Thus, if the probability of a mis-identification of two muons within one event is not correlated, a background rejection factor of 