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The CluTim algorithm: an improvement on the impact parameter estimates

A Drift Chamber is a detector used in high energy physics experiments for determining charged particles trajectories. The signal pulses from all the wires are then collected and the particle trajectory is tracked assuming that the distances of closest approach (the impact parameter) between the particle trajectory and the wires coincide with the distance between the cluster ions generated by the particle and the wire closer to it. The widespread use of helium based gas mixtures, which produces a low ionization clusters density (12 cluster/cm in a 90/10 helium/iso-butane mixture), introduces a sensible bias in the impact parameter assumption, particularly for short impact parameters and small cell drift chambers. Recently, an alternative track reconstruction (Cluster Counting/Timing) technique has been proposed, which consists in measuring the arrival times on the wires of each individual ionization cluster and combining these times to get a bias free estimate of the impact parameter. However, in order to efficiently exploiting the cluster timing technique, it is necessary to have read-out interfaces capable of processing a large quantity of high speed signals. We describe the design of a read-out board capable of acquiring the information coming from a fast digitization of the signals generated in a drift chamber and the algorithm for identifying the individual ionization pulse peaks and recording their time and amplitude.