The Mu2e tracker simulation is not “parameterized”. It starts with G4 energy deposits and creates hits using 1^st principle models of low-level physics (ionization, charge drift, dispersion, …) and electronics effects (gain, shaping, noise, …) directly. As such, efficiency and resolution are emergent properties, not input. The only inputs are basic values such as drift velocity, absolute gain, shaping time, threshold noise, etc. Though slower than a parameterized simulation, this approach allows us to correctly model pileup. Because the input values are lab-measured quantities (i.e. noise, gain, …) we can also compare measured performance of resolution and efficiency with simulation. Details of the tracker simulation are described in document 4241. The simulation is outlined in a few figures.

Electronics waveforms at the digitizer from simulated Conversion Electron events. The green dot indicates where the signal passed over threshold (including threshold noise). The signal (~100 mV on average) is ~5× the threshold (20 ± 4 mV nominal), which roughly agrees with SPICE models.

Drift resolution (position perpendicular to the wire). The core resolution is better than what we observe in the lab currently. Effects due to missing clusters and the bias near the wire are clearly visible.

Momentum resolution of selected reconstructed Conversion Electron tracks. The resolution is dominated by multiple scattering in the straw walls, but tails in the hit resolution do affect the momentum resolution tails. Track selections involve only measurable quantities (number of hits, fit χ², …). The momentum resolution depends sensitively on algorithms such as left-right ambiguity resolving, noise hit filtering, etc. The estimated momentum resolution performance meets Mu2e requirements, with a core resolution (110 KeV/c) much less than the energy straggling (~300 KeV/c) in the target, and backgrounds due to high-side tails that are small (20%) compared to our irreducible physics background (Decay In Orbit electrons).