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The CPLEAR experiment uses the antiproton beam of the LEAR facility - Low-Energy Antiproton Ring which operated at CERN from 1982 to 1996 - to produce neutral kaons through proton-antiproton annihilation in order to study CP, T and CPT violation in the neutral kaon system. According to the theory of the Big Bang, matter and antimatter would have existed in the same amount at the beginning of the Universe. If this was true, particles and antiparticles would have annihilated each other, creating photons, and thus the Universe would have been only compounded by light (one particle of matter for 10¹⁸ photons). However, we know today that only matter has remained (one billion times more particles than expected). What happened then, for the antimatter to disappear in favor of matter ? The idea is that a particle can transform into its antiparticle, and vice-versa, but that this process is not symmetric. The study of the ratio of neutral kaon and neutral anti-kaons production is thus an efficient tool to understand what happened in the early Universe that promoted the production of matter.

Low Energy Antiproton Ring experimental area.

The experiment

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. Its main aim was to study the symmetry properties of weak interaction by the study of CPT violation in the neutral kaon system, which offers a unique laboratory to test discrete symmetries. In addition, CPLEAR performed measurements about quantum coherence of wave functions, Bose-Einstein correlations in multi-pion states, regeneration of the short-lived kaon component in matter, the Einstein-Rosen-Podolsky paradox using entangled neutral-kaon pair states and the equivalence principle of general relativity.

Measurement of the time-reversal non-invariance

Kaons have the particularity not to conserve strangeness under weak interactions, meaning that a
K⁰
can transform into a
K⁰
and vice-versa. Time-reversal invariance would imply that all details of one of these transformations could be deductible from the other one, i.e. the probability for a kaon to decay 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.

The detector

Facility description

A scheme of the CPLEAR detector.

The high-speed detector of CPLEAR allowed 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.

To study the asymmetries between
K⁰
and
K⁰
decay rates in the various final states f (f = π⁺π^-, π⁰π⁰, π⁺π^-π⁰ , π⁰π⁰π⁺, πlν ), the CPLEAR collaboration used the fact that the strangeness of kaons is tagged by the charge of the accompanying kaon after the decay. This method gives indications on the evolution in time and properties of the neutral kaon decays. The neutral kaons are initially produced in the annihilation channels

• 
  p
  p → π⁺
  K⁻
  
  K⁰
  
• 
  p
  p → π^-
  K⁺
  
  K⁰
  

which happen when the 10⁶ anti-protons per second beam coming from the LEAR facility is stopped by a highly-pressurised 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 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.

In order to minimize the neutral kaon regeneration effect, which would modify the evolution time of
K⁰
and
K⁰
, liquid hydrogen is used in the detector to curtail the amount of matter in the decay volume.

Layout of the detector

The CPLEAR detector

The detector had to fulfill the specific requirements of the experiment and thus had to be able to :

• select the previous annihilation reaction among the very large number of multi-pions annihilation channels and an efficient kaon identification
• the capacity to distinguish between the different neutral-kaon decay channels
• measure the decay proper time
• acquire a large quantity of statistics, and for this it had to have both a high rate capability and a large geometrical coverage

Cylindrical tracking detectors were used to determine the charge signs, momenta and positions of the charged particles. They were followed by the particle identification detector (PID) which role was to identify the charged kaon. It was compounded by a 7Cherenkov 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.