568
K. Kleinknecht and U. Uwer
leading to a value
e
ε
ε
= (23.0 ± 6.5) × 10
−4 ,
(9.167)
or 3.5 standard deviations from zero.
9.5.6.3 The Experiment E731 at Fermilab
In this experiment [177] charged and neutral decays were registered in separate runs
with a slightly different detector. On the other hand, in E731 K L and K S decays were
collected simultaneously using a split beam: in one half of the beam cross section,
K L mesons from the target were allowed to decay over a long decay region of 27 m
(charged decays) or 42 m (neutral decays), while in the other half, the K L beam hit
a block of B 4 C, whereby a beam of K S mesons was regenerated.
The vertex distribution of the events from regenerated K S was concentrated in
a small region behind the regenerator, positioned at 123 m from the target, owing
to the typical K S decay length of 5 m. On the other hand, the vertex distribution of
K L → π 0 π 0 decays extended from 110 to 152 m from the target. The detector
acceptances for K L and K S decays therefore were very different, and since the
decay volumes for K L → π + π − and K L → π 0 π 0 were different, this acceptance
correction does not cancel in the double ratio.
The π + π − decays were selected by requiring the invariant ππ mass to be near
the kaon mass and the transverse kaon momentum to satisfy p 2
t < 250 MeV
2 /c 2 .
The background from incoherent kaon regeneration amounted to (0.155 ± 0.014)%.
The extrapolated 3π 0 background under the peak is 1.78% and 0.049% in the
vacuum and regenerator beams, respectively. Neutral background from incoherent
scattering in the regenerator was subtracted by evaluating the distribution of the
transverse center of energy of each event around the center of each beam. After this
background subtraction, the event numbers in the vacuum beams were 327 × 10 3
(π + π − ) and 410 × 10 3 (π 0 π 0 ). In the regenerator beams, there were 1.06 × 10 6
π + π − events and 0.800×10 6 2π 0 events. The regeneration amplitude was assumed
to fall in accordance with a power of the kaon momentum, p α . In the fit, the
parameter α, the regeneration amplitude at 70 GeV/c momentum, and e(ε /ε)
were varied. The results were
α = −0.6025 ± 0.0065
(9.168)
and
ε
ε
= (7.4 ± 5.2 (stat) ± 2.9 (syst)) × 10
−4 ,
(9.169)
K. Kleinknecht and U. Uwer
leading to a value
e
ε
ε
= (23.0 ± 6.5) × 10
−4 ,
(9.167)
or 3.5 standard deviations from zero.
9.5.6.3 The Experiment E731 at Fermilab
In this experiment [177] charged and neutral decays were registered in separate runs
with a slightly different detector. On the other hand, in E731 K L and K S decays were
collected simultaneously using a split beam: in one half of the beam cross section,
K L mesons from the target were allowed to decay over a long decay region of 27 m
(charged decays) or 42 m (neutral decays), while in the other half, the K L beam hit
a block of B 4 C, whereby a beam of K S mesons was regenerated.
The vertex distribution of the events from regenerated K S was concentrated in
a small region behind the regenerator, positioned at 123 m from the target, owing
to the typical K S decay length of 5 m. On the other hand, the vertex distribution of
K L → π 0 π 0 decays extended from 110 to 152 m from the target. The detector
acceptances for K L and K S decays therefore were very different, and since the
decay volumes for K L → π + π − and K L → π 0 π 0 were different, this acceptance
correction does not cancel in the double ratio.
The π + π − decays were selected by requiring the invariant ππ mass to be near
the kaon mass and the transverse kaon momentum to satisfy p 2
t < 250 MeV
2 /c 2 .
The background from incoherent kaon regeneration amounted to (0.155 ± 0.014)%.
The extrapolated 3π 0 background under the peak is 1.78% and 0.049% in the
vacuum and regenerator beams, respectively. Neutral background from incoherent
scattering in the regenerator was subtracted by evaluating the distribution of the
transverse center of energy of each event around the center of each beam. After this
background subtraction, the event numbers in the vacuum beams were 327 × 10 3
(π + π − ) and 410 × 10 3 (π 0 π 0 ). In the regenerator beams, there were 1.06 × 10 6
π + π − events and 0.800×10 6 2π 0 events. The regeneration amplitude was assumed
to fall in accordance with a power of the kaon momentum, p α . In the fit, the
parameter α, the regeneration amplitude at 70 GeV/c momentum, and e(ε /ε)
were varied. The results were
α = −0.6025 ± 0.0065
(9.168)
and
ε
ε
= (7.4 ± 5.2 (stat) ± 2.9 (syst)) × 10
−4 ,
(9.169)