A Mini Time Projection Chamber for the PEN Experiment

The PEN experiment aims to measure the $\pi^+ \rightarrow e^+ \nu(\gamma )$ branching ratio with a relative uncertainty of $5\times 10^{-4}$ or smaller using a stopped beam approach. Achieving this precision requires strict control of many systematics, including pion decay vertex position and identification of decay-in-flight events. We have designed and implemented a miniature time projection chamber (50\,$\times$\,50\,$\times$\,50\,mm$^3$, with 4 anode readout wires) to track beam pions in order to control the aforementioned systematics. This paper discusses the performance of the MTPC during the fall 2009 data collection period at PSI.