570
K. Kleinknecht and U. Uwer
spot at the CsI position, to which the event was assigned by use of the x-position of
the center of energy:
RING = 4 × Max((x
2
coe , ,y
2
coe ) .
(9.170)
The signal was selected by the cut RING < 110 cm 2 . The largest background
comes from regenerator scattering in the regenerator beam, 1.13%, adding up to
a total of 1.235% in that beam. Also, in the vacuum beam, the events scattered
in the nearby regenerator make the largest contribution to the background, 0.25%,
which is 0.48% in total. After all cuts and background subtraction, the remaining
signal consists of 3.3 million and 5.55 million events in the vacuum and regenerator
beams, respectively. The 3.3 million K L → π 0 π 0 events are the limiting factor in
the statistical uncertainty in the double ratio.
Since the vertex distributions of K L decays (flat) and K S decays (concentrated
behind the regenerator) are very different, the raw double ratio has to be corrected
by the double ratio of acceptances (Fig. 9.16). The quality of the Monte Carlo simulations for the acceptances was checked by reproducing the z vertex distributions
of the vacuum beam data for different decay modes (Fig. 9.17). In general the
agreement is good, except that the π + π − data show a slope of (−0.70 ± 0.30) ×
10 −4 /m.
The result for e(ε /ε) is [180]
ε
ε
= (20.71 ± 1.48 (stat) ± 2.39 (syst)) × 10
−4
= (20.7 ± 2.8) × 10
−4 .
(9.171)
The systematic uncertainty for the neutral decays is mainly due to background, CsI
energy calibration and acceptance corrections; for the charged decays, it is mainly
due to uncertainties in the acceptance and trigger efficiency.
The acceptance correction that has to be applied is about 5 × 10 −2 for R, or
∼ 80 × 10 −4 for e(ε /ε), four times larger than the signal.
9.5.6.5 The NA48 Experiment
When the NA31 observation of a nonvanishing e(ε /ε) was not confirmed by the
result from the E731 experiment, the CERN-based collaboration set out to construct
a new, improved detector with the goal of achieving a precision measurement of
e(ε /ε) with a total uncertainty of 0.2 × 10 −4 .
The new experiment was designed
– to measure all four decay modes concurrently by using two incident proton
beams;
– to improve on neutral-background rejection by developing a liquid-krypton
electromagnetic calorimeter with substantially better energy resolution;
K. Kleinknecht and U. Uwer
spot at the CsI position, to which the event was assigned by use of the x-position of
the center of energy:
RING = 4 × Max((x
2
coe , ,y
2
coe ) .
(9.170)
The signal was selected by the cut RING < 110 cm 2 . The largest background
comes from regenerator scattering in the regenerator beam, 1.13%, adding up to
a total of 1.235% in that beam. Also, in the vacuum beam, the events scattered
in the nearby regenerator make the largest contribution to the background, 0.25%,
which is 0.48% in total. After all cuts and background subtraction, the remaining
signal consists of 3.3 million and 5.55 million events in the vacuum and regenerator
beams, respectively. The 3.3 million K L → π 0 π 0 events are the limiting factor in
the statistical uncertainty in the double ratio.
Since the vertex distributions of K L decays (flat) and K S decays (concentrated
behind the regenerator) are very different, the raw double ratio has to be corrected
by the double ratio of acceptances (Fig. 9.16). The quality of the Monte Carlo simulations for the acceptances was checked by reproducing the z vertex distributions
of the vacuum beam data for different decay modes (Fig. 9.17). In general the
agreement is good, except that the π + π − data show a slope of (−0.70 ± 0.30) ×
10 −4 /m.
The result for e(ε /ε) is [180]
ε
ε
= (20.71 ± 1.48 (stat) ± 2.39 (syst)) × 10
−4
= (20.7 ± 2.8) × 10
−4 .
(9.171)
The systematic uncertainty for the neutral decays is mainly due to background, CsI
energy calibration and acceptance corrections; for the charged decays, it is mainly
due to uncertainties in the acceptance and trigger efficiency.
The acceptance correction that has to be applied is about 5 × 10 −2 for R, or
∼ 80 × 10 −4 for e(ε /ε), four times larger than the signal.
9.5.6.5 The NA48 Experiment
When the NA31 observation of a nonvanishing e(ε /ε) was not confirmed by the
result from the E731 experiment, the CERN-based collaboration set out to construct
a new, improved detector with the goal of achieving a precision measurement of
e(ε /ε) with a total uncertainty of 0.2 × 10 −4 .
The new experiment was designed
– to measure all four decay modes concurrently by using two incident proton
beams;
– to improve on neutral-background rejection by developing a liquid-krypton
electromagnetic calorimeter with substantially better energy resolution;