CPLEAR is a collaboration of about 100 scientists, coming from 17 institutions from 9 different countries. Accepted in 1985, the experiment took data from 1990 until 1996.
Facility description The CPLEAR detector was able to determine the locations, the momenta and the charges of the
tracks at the production of the neutral kaon and at its decay, thus visualizing the complete event.
Strangeness is not conserved under weak interactions, meaning that under weak interactions a can transform into a and vice versa. To study the
asymmetries between and decay rates in the various final states f (f = π+π−, π0π0, π+π−π0, π0π0π+, π
lν), the CPLEAR collaboration used the fact that the strangeness of kaons is tagged by the charge of the accompanying kaon.
Time-reversal invariance would imply that all details of one of the transformations could be deducible from the other one, i.e. the
probability for a kaon to oscillate into an anti-kaon would be equal to the one for the reverse process. The measurement of these probabilities required the knowledge of the
strangeness of a kaon at two different times of its life. Since the strangeness of the kaon is given by the
charge of the accompanying kaon, and thus be known for each
event, it was observed that this symmetry was not respected, thereby proving the
T violation in neutral kaon systems under weak interaction. which happen when the 106 anti-protons per second beam coming from the LEAR facility is stopped by a highly pressurized
hydrogen gas target. The low
momentum of the antiprotons and the high
pressure allowed to keep the size of the stopping region small in the
detector. Since the proton-antiproton reaction happens at rest, the particles are produced
isotropically, and as a consequence, the detector has to have a near-4π symmetry. The whole detector was embedded in a 3.6 m long and 2 m diameter warm solenoidal
magnet providing a 0.44 T uniform
magnetic field. The antiprotons were stopped using a pressurized hydrogen gas target. A hydrogen gas target was used instead of liquid hydrogen to minimize the amount of matter in the decay volume. The target initially had a radius of 7 cm and subjected to a pressure of 16 bar. Changed in 1994, its radius became equal to 1.1 cm, under a 27 bar pressure.
Layout of the detector The detector had to fulfill the specific requirements of the experiment and thus had to be able to: • do an efficient kaon identification • select the annihilation channels mentioned in
Facility description among the very large number of multi-pions annihilation channels • distinguish between the different neutral-kaon decay channels • measure the decay proper time • acquire a large number of statistics, and for this, it had to have both a high rate capability and a large geometrical coverage Cylindrical tracking detectors together with a solenoid field were used to determine the charge signs, momenta and positions of the charged particles. They were followed by the particle identification detector (PID) whose role was to identify the charged kaon. It was compounded by a
Cherenkov detector, which carried out the kaon-pion separation; and
scintillators, measuring the energy loss and the
time of flight of the charged particles. It was also used for the
electron-pion separation. The detection of photons produced in π0 decays was performed by ECAL, an outermost lead/gas sampling calorimeter, complementary to the PID by separating pions and electrons at higher momenta. Finally, hardwired processors (HWK) were used to analyze and select the events in a few microseconds, deleting the unwanted ones, by providing a full event reconstruction with sufficient precision. == References ==