Specific ionization in a drift chamber filled with helium-isobutane

This thesis discusses the measurement of the dE/ dx resolution of the BaBar drift chamber. The study was performed on the data obtained with a small scale prototype drift chamber (built at LAPP, Annecy, France) during test beam runs at CERN (PS) and Zurich (PSI). Drift chamber simulations were performed in order to study the expected performance. Good tracking, necessary in the analysis of the PS runs but unnecessary in the PSI runs, was obtained by performing a chi2 minimization of the space residuals, yielding a time to distance function. An average tracking resolution of ≈350mum was achieved. The resolution result for seven energy loss samplings per event was scaled up to 40 samples per event which will be available in the BaBar chamber. The expected dE/ dx resolution of the BaBar chamber, as obtained from the PS runs, is: (7.3 +/- 0.1 +/- 0.5)%. The result from the PSI runs is: (6.9 +/- 0.4 +/- 0.5)%