Physics prospects of the KTeV experiment at Fermilab Page: 6 of 6

4 The E'/e Measurement
The previous generation of experiments measured
e'/e with a precision of (6 - 7) x 10-4. In the case
of the E731 experiment, the statistical error dom-
inated the measurement error. To reduce the sta-
tistical error, KTeV will increase the intensity of
the beam by a factor of 5, improve the data aqui-
sition live time by a factor of 2, and increasing the
running time for the experiment. The KTeV goal
is to collect 107 CP violating 27r decays, thereby
reducing the statistical error by a factor of 5.
At this level, reduction of systematic errors
will also be crucial. The dominant systematic er-
ror from the E731 measurement came from the
neutral energy scale. By choosing pure, radiation
hard CsI for the electromagnetic calorimeter, we
are assured of similar electron and photon show-
ering with little change in response over the entire
running period. This is critical, since we perform
the energy calibration for the calorimeter using
KL -+ ez ,F decays, but the physics mode of
interest is 270. The neutral energy scale is di-
rectly related to the decay vertex Z position. An
error in the scale effects the e'/e measurement be-
cause it influences the number of decays which are
accepted within a chosen Z region. The second
largest systematic error to the previous measure-
ment came from cross-over decays from the re-
generator beam which are misidentified as coming
from the K7 beam. Better collimation and a fully
active regenerator reduce the probability of these
type of events. The regenerator is instrumented to
be fully active, allowing rejection of backgrounds
produced from incoherent regeneration and inelas-
tic scattering in the regenerator. In addition, the
regenerator alternates between beams, thereby re-
ducing many systematic effects from the detector.
Backgrounds are addressed by reducing beam halo
and multiple scattering within the detector, im-
proving the e - w particle identification, and in-
creasing the photon veto coverage. "Accidental"
activity, i.e. activity within the detector which is
not correlated with the physics event of interest,
contributes background to the sample and is re-
duced with improvements in beam conditions and
improved timing resolution within the detector.
Because the Ki and KS vertex distributions are
extremely different, a potential source of system-
atic error comes from the uncertainty of the accep-
tance. The detector acceptance is controlled by

carefully monitoring the detector alignments and
defining apertures. In addition, large samples of
K - e 7+, v and 370 events will be collected to
map the detector geometry and to tune the detec-
tor geometries in Monte Carlo, which ultimately
is used in determining the acceptance.
5 Conclusions
KTeV has started taking hadron beam. Initial in-
dications are that the detector will meet or exceed
its design specifications. The detector is currently
configured for e'/e data taking. The KTeV run
plan is to switch between running the e'/e and
rare decay programs. We anticipate that in the
near future KTeV will be able to achieve the goals
of its physics program, reducing the error on e'/e
by almost an order of magnitude and significantly
improving on measurements of many rare decays.
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Whitmore, J. & Collaboration, KTeV. Physics prospects of the KTeV experiment at Fermilab, article, October 1, 1996; Batavia, Illinois. ( accessed April 25, 2019), University of North Texas Libraries, Digital Library,; crediting UNT Libraries Government Documents Department.

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