Experimental progress

We have already achieved many of the necessary goals necessary for a successful completion of this experiment:

• DAQ: We have tested the full complement of 10 of the PSI/Berkeley designed VME-64 based TDC400 modules which provide deadtime free readout of both the TPC and the present wire chambers. These modules work smoothly and provide concurrent data collection and readout. We have also tested data compression of the original TDC400 events to reduce data flow to tape.

• TPC: We have tested a full size prototype of the TPC with electron tracking by MWPC's over a limited solid angle. With these chambers we were able to track the muons completely in 3D and match them with their decay electrons.

• Front-end electronics: The fast positive ion motion in our hydrogen TPC creates significant pedestal shifts after the large muon track signals. To counter this and restore the pedestal below trigger threshold immediately, we have added fast baseline restoration circuitry to our front-end electronics. We can now track electrons efficiently right after the muon tracks.

• Gas purity: Our collaboration has developed sophisticated gas chromatographic techniques to analyze sub-ppm quantities of various impurities (N$_2$, O$_2$, H$_2$O, etc) in our gas. We have also assembled the apparatus to produce high-purity protium gas by electrolysis from deuterium-depleted water (c$_d\leq$ 1 ppm).

• Impurity detection: Besides chemical analysis of our gas, we have developed several techniques by which the TPC signals can be used to directly monitor very low impurity levels. The primary means are: detection of capture events at the muon stop point, observation of $\mu d$ diffusion and $p\mu d$ fusion products.

• Background reduction: Using global pile-up rejection, we have demonstrated reduction of our accidental background to the $10^{-5}$ level by tracking muons in the TPC and electrons in the MWPC's to a common vertex. This is already more than enough for a $10^{-5}$ lifetime measurement.

• Background reduction with local pile-up rejection: We have demonstrated that a strong reduction of our background signals can also be achieved using local pile-up conditions. An accidental level of $1.5\cdot 10^{-4}$ was observed, sufficient for reaching fitting precisions of $10^{-5}$. This method of analysis will allow running with larger beam rates at correspondingly lower losses from pile-up rejection.