The QPweak Main Detectors:
| The QPweak main electron Cherenkov detector consists of 8 quartz bars coupled to photomultiplier tubes, arranged in an octagonal pattern around the beam line. Each of the quartz bars is 2 meters long (glued together from two 1 meter long synthetic quartz pieces), 18 cm wide and 2 cm thick in the direction of the scattered electron beam. Cherenkov detectors emit light when particles move through them at high enough energies. | Attach:MainDetector_07.jpg Δ |
This light is emitted coherently, because the particle creating the light has a speed that is larger than the speed of light in the chosen material.
The purpose of the main electron detectors is to measure the number of electrons scattered at a specifically chosen angle and how that number changes when the electron helicity (i.e. spin) is reversed. The difference between these numbers is the primary observable of the QPweak experiment and is directly related to the weak charge of the proton, a fundamental parameter of the Standard Model ( c.f. Theory Section ).
The nice feature of quartz Cherenkov detectors is that they are relatively insensitive to the other particles that are present while running QPweak; those we don't want to see (mostly pions, photons and neutrons).
For more information see:
Article 1 and references within
Article 2 and references within
Or the ever popular wikipedia