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K. Kleinknecht and U. Uwer
3π 0 background. In high-energy experiments, the background can then be reduced
to a level below one percent.
9.5.4.3 Detectors Measuring Charged and Neutral Decay Modes
Simultaneously
For the measurement of the parameter ε of direct CP violation, the ratio of the
decay rates of K L into charged (π + π − ) and neutral (π 0 π 0 ) two-pion states has to be
measured with great precision. For this purpose, experimentalists reduce systematic
normalization uncertainties by measuring charged and neutral decays (from K L and
K S mesons) simultaneously.
Four such experiments have been constructed for high-energy K meson beams,
two of them at CERN (NA31 and NA48) and two at Fermilab (E731 and kTeV).
One experiment has been designed to detect K mesons of a few hundred MeV/c
momentum arising from the annihilation of stopping antiprotons in a hydrogen
target at the Low Energy Antiproton Ring (LEAR) at CERN (CPLEAR).
9.5.4.4 NA31
This detector (Fig. 9.7) [171], situated in a K L or K S beam from the CERN SPS, was
based on calorimetry and was designed for good stability and high efficiency. The
K L and the K S beam were produced by a 450 GeV proton beam with a production
angle of 3.6 mrad.
A schematic illustration of the beam layout and the apparatus is shown in Fig. 9.7.
The principal features can be summarized as follows:
• to adjust the steeply falling vertex distribution of the K S decays to the almost flat
vertex distribution of the K L decays, the K S target is located on a train that can
be positioned at 41 stations in the decay volume;
• an anticounter with a 7 mm lead converter in the K S beam is used to veto decays
in the collimator, defines the upstream edge of the decay region, and provides for
the relative calibration of the 2π 0 and π + π − energy scales to a precision better
than ±10 −3 ;
• two wire chambers spaced 25 m apart, with ±0.5 mm resolution in each
projection, track charged pions;
• a liquid-argon/lead sandwich calorimeter with strip readout detects photons with
±0.5 mm position resolution and an energy resolution of
σ E /E = 10%/E ⊕ 7.5%/
√
E ⊕ 0.6% (E in GeV);
• an iron/scintillator sandwich calorimeter measures the energy of charged pions
with ±65%/
√
E (E in GeV) energy resolution.
The energies of the two pions and their opening angle are used to measure the
invariant mass of the charged pair.
K. Kleinknecht and U. Uwer
3π 0 background. In high-energy experiments, the background can then be reduced
to a level below one percent.
9.5.4.3 Detectors Measuring Charged and Neutral Decay Modes
Simultaneously
For the measurement of the parameter ε of direct CP violation, the ratio of the
decay rates of K L into charged (π + π − ) and neutral (π 0 π 0 ) two-pion states has to be
measured with great precision. For this purpose, experimentalists reduce systematic
normalization uncertainties by measuring charged and neutral decays (from K L and
K S mesons) simultaneously.
Four such experiments have been constructed for high-energy K meson beams,
two of them at CERN (NA31 and NA48) and two at Fermilab (E731 and kTeV).
One experiment has been designed to detect K mesons of a few hundred MeV/c
momentum arising from the annihilation of stopping antiprotons in a hydrogen
target at the Low Energy Antiproton Ring (LEAR) at CERN (CPLEAR).
9.5.4.4 NA31
This detector (Fig. 9.7) [171], situated in a K L or K S beam from the CERN SPS, was
based on calorimetry and was designed for good stability and high efficiency. The
K L and the K S beam were produced by a 450 GeV proton beam with a production
angle of 3.6 mrad.
A schematic illustration of the beam layout and the apparatus is shown in Fig. 9.7.
The principal features can be summarized as follows:
• to adjust the steeply falling vertex distribution of the K S decays to the almost flat
vertex distribution of the K L decays, the K S target is located on a train that can
be positioned at 41 stations in the decay volume;
• an anticounter with a 7 mm lead converter in the K S beam is used to veto decays
in the collimator, defines the upstream edge of the decay region, and provides for
the relative calibration of the 2π 0 and π + π − energy scales to a precision better
than ±10 −3 ;
• two wire chambers spaced 25 m apart, with ±0.5 mm resolution in each
projection, track charged pions;
• a liquid-argon/lead sandwich calorimeter with strip readout detects photons with
±0.5 mm position resolution and an energy resolution of
σ E /E = 10%/E ⊕ 7.5%/
√
E ⊕ 0.6% (E in GeV);
• an iron/scintillator sandwich calorimeter measures the energy of charged pions
with ±65%/
√
E (E in GeV) energy resolution.
The energies of the two pions and their opening angle are used to measure the
invariant mass of the charged pair.