The CCRS experiment at the CERN-ISR

A very interesting thing happened in the second round of ISR experiments, two first round experiments decided to combine their detectors and join forces. The Saclay–Strasbourg experiment (R102) had proposed a second spectrometer arm (Arm 2) to study what was produced opposite in azimuth to balance the transverse momentum of the high pT hadrons [281]. Then, together with the CCR experiment (R103), it was proposed [282] to add the CCR lead glass counters (PbGl) behind the Arm 2 to enable improved detection of single electrons and gamma rays at high pT as well as to continue the search for e+e- pairs. This became R105, the CERN–Columbia–Rockefeller–Saclay experiment, CCRS (Figure 7.1). This detector turned out to be very powerful and was rewarded with one major discovery, one near-miss and several excellent measurements.

The key features of this detector were the following:

1. (i)≥105 charged hadron rejection from electron identification in the Cerenkov counter combined with matching the momentum and energy of an electron candidate in the magnetic spectrometer and the PbGl;

2. (ii) minimum of material in the aperture to avoid external conversions;

3. (iii) zero magnetic field on the axis to avoid de-correlating conversion pairs;

4. (iv) rejection of conversions in the vacuum pipe (and small opening angle internal conversions) by requiring single ionization in a hodoscope of scintillation counters H′ close to the vacuum pipe, preceded by a thin track chamber to avoid conversions in the H′ counters;

5. (v) precision measurement of π0 and η, the predominant background source;

6. (vi) precision background determination in the direct single e± signal channel by adding an external converter, to distinguish direct single e± from e± from photon conversion.