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K. Kleinknecht and U. Uwer
Two sets of information can be used to separate 2π and leptonic decays. First, the
invariant mass m ππ is required to be equal to m K within the experimental resolution.
Second, all experimenters use lepton identification.
The most frequently used methods for electron identification at intermediate
energies, around 10 GeV, are Cerenkov counters, and identification through comparison of the energy deposition in electromagnetic and hadronic showers. At high
energies, i.e. for electrons with energies between 10 GeV and 100 GeV, electron
identification in calorimetric detectors works on the principle that for a particle of
momentum p, the energy E deposited in a calorimeter by an electron (or photon) is
much higher than for a hadron of the same momentum.
For the identification of muons one uses their penetration through several (∼8)
interaction lengths of material in order to distinguish them from pions interacting in
this absorber.
Once the 2π decay mode has been identified, one has to know, in general,
whether the K S or K L from which the decay products originate has undergone
scattering on its way from its production to the decay point. In the case of a shortlived beam produced by protons interacting in a target near to the detector, this is
done by calculating the distance of the intercept of the reprojected kaon momentum
p K in the target plane from the target center, p t . Unscattered events cluster around
p t = 0. In the case of a long-lived beam, one uses the component of p K transverse to
the beam, p t , or the angle θ between the kaon direction p K and the beam direction
in order to separate transmitted and coherently regenerated (θ = 0 = p t ) kaons
from events due to kaons that have undergone scattering, or diffractive, or inelastic,
regeneration.
9.5.4.2 Neutral Decay Modes
The detection of the neutral decay mode K L → π 0 π 0 → 4γ is complicated by the
presence of the decay K L → 3π 0 → 6γ with a 21% branching ratio. This decay
can simulate 4γ events for kinematic reasons, e.g. if two γ rays are missed by the
detector. Very specific kinematic features of the 2π 0 decay were therefore used in
the early medium-energy experiments in order to obtain a clean K L → 2π 0 signal
[167–170, 172].
For kaon energies between 40 GeV and 200 GeV, totally absorbing electromagnetic calorimeters are used. These calorimeters consist of scintillating crystals,
Cerenkov lead glass counters, or liquid-noble-gas detectors with or without lead
radiators. Their longitudinal thickness is around 25 radiation lengths, and their
transverse segmentation corresponds to the transverse width of an electromagnetic
shower, given by the Moli` ere radius R M of the material. In this way, the energies E i
and the transverse positions (x i , y i ) of each of the four photon-induced showers are
measured in the calorimeter. This is the only information available for reconstruct-
K. Kleinknecht and U. Uwer
Two sets of information can be used to separate 2π and leptonic decays. First, the
invariant mass m ππ is required to be equal to m K within the experimental resolution.
Second, all experimenters use lepton identification.
The most frequently used methods for electron identification at intermediate
energies, around 10 GeV, are Cerenkov counters, and identification through comparison of the energy deposition in electromagnetic and hadronic showers. At high
energies, i.e. for electrons with energies between 10 GeV and 100 GeV, electron
identification in calorimetric detectors works on the principle that for a particle of
momentum p, the energy E deposited in a calorimeter by an electron (or photon) is
much higher than for a hadron of the same momentum.
For the identification of muons one uses their penetration through several (∼8)
interaction lengths of material in order to distinguish them from pions interacting in
this absorber.
Once the 2π decay mode has been identified, one has to know, in general,
whether the K S or K L from which the decay products originate has undergone
scattering on its way from its production to the decay point. In the case of a shortlived beam produced by protons interacting in a target near to the detector, this is
done by calculating the distance of the intercept of the reprojected kaon momentum
p K in the target plane from the target center, p t . Unscattered events cluster around
p t = 0. In the case of a long-lived beam, one uses the component of p K transverse to
the beam, p t , or the angle θ between the kaon direction p K and the beam direction
in order to separate transmitted and coherently regenerated (θ = 0 = p t ) kaons
from events due to kaons that have undergone scattering, or diffractive, or inelastic,
regeneration.
9.5.4.2 Neutral Decay Modes
The detection of the neutral decay mode K L → π 0 π 0 → 4γ is complicated by the
presence of the decay K L → 3π 0 → 6γ with a 21% branching ratio. This decay
can simulate 4γ events for kinematic reasons, e.g. if two γ rays are missed by the
detector. Very specific kinematic features of the 2π 0 decay were therefore used in
the early medium-energy experiments in order to obtain a clean K L → 2π 0 signal
[167–170, 172].
For kaon energies between 40 GeV and 200 GeV, totally absorbing electromagnetic calorimeters are used. These calorimeters consist of scintillating crystals,
Cerenkov lead glass counters, or liquid-noble-gas detectors with or without lead
radiators. Their longitudinal thickness is around 25 radiation lengths, and their
transverse segmentation corresponds to the transverse width of an electromagnetic
shower, given by the Moli` ere radius R M of the material. In this way, the energies E i
and the transverse positions (x i , y i ) of each of the four photon-induced showers are
measured in the calorimeter. This is the only information available for reconstruct-