9 Symmetry Violations and Quark Flavour Physics
569
where the systematic uncertainty includes a part from acceptance calculations
(1.19 × 10 −4 ) and from the energy calibration (1.6 × 10 −4 ). The authors of [177]
deduce an upper limit e(ε /ε) < 17 × 10 −4 (95% C.L.), which is at variance with
the observation of the NA31 experiment.
9.5.6.4 The kTeV Experiment at Fermilab
The disagreement between the positive result of NA31 and the null result of E731
left an unsatisfactory state of affairs. For this reason, new experiments with a tenfold
increase in data-taking capacity and reduced systematic uncertainty were designed,
at both Fermilab and CERN.
The Fermilab experiment at the 800 GeV Tevatron, called kTeV, was described
in Sect. 9.5.4.6 [180]. The main improvements compared with E731 were:
– all four decay modes were measured concurrently;
– the electromagnetic calorimeter was made of CsI, with much improved energy
resolution;
– the regenerator at 123 m from the target was made of plastic scintillator, viewed
by photomultipliers such that inelastic regeneration could be detected by the
recoiling nucleus;
– the kaon momentum range from 40 to 160 GeV/c and the decay vertex region
from 110 m to 158 m from the target were the same for all decay modes.
As in the E731 experiment, a double beam of K L and regenerated K S entered
the decay volume. K 0 → π + π − decays were identified by their invariant mass.
Semileptonic K e3 events were reduced by a factor of 1000 by requiring the ratio
of the calorimetric energy E of a track to its momentum p to be less than 0.85.
K μ3 events were rejected by registering the muon penetrating the 4 m iron wall.
The invariant-ππ-mass shows a rms mass resolution of 1.6 MeV, and events in the
range 488–508 MeV were selected. Background from K S produced in incoherent
regeneration was suppressed mainly by vetoing events with a signal generated in the
active regenerator indicating the recoil of a nucleus in the scattering process. Further
reduction of this background was achieved by extrapolating the kaon direction back
to the regenerator exit face and calculating the transverse momentum of the kaon
relative to the line connecting this intercept with the target position. After a cut
against the backgrounds from semileptonic decays and from collimator scattering,
11.1 million and 19.29 million π + π − events remain in the vacuum beam and
regenerator beam samples, respectively.
The selection of 2π 0 events follows the lines described in Sect. 9.5.4.2. Events
with an invariant π 0 π 0 mass between 490 and 505 MeV were selected. Events in
which a kaon scatters in the collimator or the regenerator were reduced by a cut
in the ring number (RING), defined by the maximum deviation x coe or y coe (in
cm) of the center of energy of all showers from the center of the corresponding beam
569
where the systematic uncertainty includes a part from acceptance calculations
(1.19 × 10 −4 ) and from the energy calibration (1.6 × 10 −4 ). The authors of [177]
deduce an upper limit e(ε /ε) < 17 × 10 −4 (95% C.L.), which is at variance with
the observation of the NA31 experiment.
9.5.6.4 The kTeV Experiment at Fermilab
The disagreement between the positive result of NA31 and the null result of E731
left an unsatisfactory state of affairs. For this reason, new experiments with a tenfold
increase in data-taking capacity and reduced systematic uncertainty were designed,
at both Fermilab and CERN.
The Fermilab experiment at the 800 GeV Tevatron, called kTeV, was described
in Sect. 9.5.4.6 [180]. The main improvements compared with E731 were:
– all four decay modes were measured concurrently;
– the electromagnetic calorimeter was made of CsI, with much improved energy
resolution;
– the regenerator at 123 m from the target was made of plastic scintillator, viewed
by photomultipliers such that inelastic regeneration could be detected by the
recoiling nucleus;
– the kaon momentum range from 40 to 160 GeV/c and the decay vertex region
from 110 m to 158 m from the target were the same for all decay modes.
As in the E731 experiment, a double beam of K L and regenerated K S entered
the decay volume. K 0 → π + π − decays were identified by their invariant mass.
Semileptonic K e3 events were reduced by a factor of 1000 by requiring the ratio
of the calorimetric energy E of a track to its momentum p to be less than 0.85.
K μ3 events were rejected by registering the muon penetrating the 4 m iron wall.
The invariant-ππ-mass shows a rms mass resolution of 1.6 MeV, and events in the
range 488–508 MeV were selected. Background from K S produced in incoherent
regeneration was suppressed mainly by vetoing events with a signal generated in the
active regenerator indicating the recoil of a nucleus in the scattering process. Further
reduction of this background was achieved by extrapolating the kaon direction back
to the regenerator exit face and calculating the transverse momentum of the kaon
relative to the line connecting this intercept with the target position. After a cut
against the backgrounds from semileptonic decays and from collimator scattering,
11.1 million and 19.29 million π + π − events remain in the vacuum beam and
regenerator beam samples, respectively.
The selection of 2π 0 events follows the lines described in Sect. 9.5.4.2. Events
with an invariant π 0 π 0 mass between 490 and 505 MeV were selected. Events in
which a kaon scatters in the collimator or the regenerator were reduced by a cut
in the ring number (RING), defined by the maximum deviation x coe or y coe (in
cm) of the center of energy of all showers from the center of the corresponding beam