Acceptance and efficiency calculation

To calculate realistic single track efficiencies, real data events are used as 'background' to embed simulated particles into. This procedure is rather (computing-)time-consuming because it requires full reconstruction of the entire events for every simulated track (or group of tracks; a few tracks can be embedded in each event). The simulation is generally run using pions, to serve multiple analyses at the same time. The macro accept_embed.C is used to calculate tracking efficiency from ROOT micro-DSTs produced from embedding data.

An important acceptance effect for kaons is the in-flight decay. This generates a kink in the track, which is detected in the tracking software if it occurs inside the TPC, leading to a truncation of the track. If the decay occurs before the main TPC, the measured track inside the MTPC is a muon or a pion, which have larger $dE/dx$ than kaons and therefore to not contribute to the extracted yield. These effects are corrected for using a simple simulation with GNA49 without embedding. The correction factors are calculated using accepted_mconly.C . A separate program, calc_mchits , is used to calculate the number of simulated hits in the fiducial volumes of the TPC. The default simulation generates points outside the fiducial, because $E \times B$-distortions may move points from outside the fiducial to the inside.

The final correction is the product of the efficiency correction from embedding and the acceptance/decay correction from simulation.