Pion detection efficiency and rejection

The threshold velocity v for particles to produce Cerenkov light is given by the index of refraction of the radiator gas. The corresponding values are shown in the Table 2. A typical value is , corresponding to about 3 GeV/c pion momentum.

Unfortunately, sub-threshold pions can be also detected due to several mechanism :

• Slow particles can produce knock-on electrons ( - electrons) with energies above the threshold.
• Nuclear reactions in the detector materials (mirrors, gas, magnetic shields and support frames) and in the materials in front of it (in drift chambers region) may produce secondary particles with velocities above threshold. An important contribution is pion charge exchange with in the final state and subsequent conversion to - rays and electrons.
• In some gases, charged particles generate scintillation light, which can cause false Cerenkov signal.

The GEANT-based package appears adequate to accurately estimate the pion detection probability due to secondary particle production. It takes into account the important particle-nuclei interactions.

Fig. 20 represents the estimated detection efficiency for three initial momenta : 0.8, 1.8 and 2.7 GeV/c, as a function of the CC segment number.

Figure 20: detection efficiency.

One can see that

• The pion detection probability increases with momentum. The mean probability for 0.8 GeV/c pions is 0.3 1.0 % , and for 2.0 GeV/c it is 1.0 2.0 %
• This probability is larger for smaller Theta angles. This is likely a reflection of the higher material density at forward angles.

Full dots in the figure show the detection efficiency for with momenta 10 above the threshold. The efficiency is as low as . Quolitatively this is explained in the inserted graph in Fig. 20 - where the Cerenkov light intensity is plotted as a function of the momentum. Pions with momenta above threshold generate only about of the light intensity of electrons. The rejection can therefore be improved by setting higher detection threshold.

The detection probability was estimated for three different initial momenta - 2.7, 3.0 and 3.3 GeV/c for Freon-12 gas. (See fig. 21)

The threshold for Cerenkov light in this case is 3.0 GeV/c.

Figure 21: meson detection efficiency as a function of segment number.

One can see that the difference in and detection efficiencies is similar to the difference between and : only about of all segments are active in the case of . The detection probability for the small angle segments is larger then for most others, however the number of events in this region is very small.