Physics prospects of the KTeV experiment at Fermilab Page: 4 of 6
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and, in addition, detecting the neutrino mode is
experimentally difficult. The K - 0r+1- lim-
its set s by the previous FNAL experiment, E7991,
dominate the world average. E7991 measured lim-
its for r0e+e- and lrOp+p- of 4.3 x 109 and
5.1 x 10-9, respectively. The limit set for rvi
was 5.8 x 10-5. At KTeV, we expect to reach
the single event sensitivity of 10-11 for the elec-
tron and muon modes, and 10-' for the neutrino
mode. If the branching ratios are at the predicted
level, KTeV will only set limits on these decays.
Thus for the next generation of experiment, the 27r
decays of neutral kaons provides the best system
with which to measure direct CP violation.
3 The Detector and its Performance
KTeV will employ the same technique to measure
E'/E as was developed during the latter part of data
taking by its experimental predecessor E731. 10
The technique involves simultaneous collection of
all four decay modes in the double ratio. This is
achieved by generating two nearly identical, par-
allel beams of K' and KS and configuring the
experiment to simultaneously record charged and
neutral 2a decays from the K7 and K' beams.
By simultaneously collecting data for the four de-
cay modes, systematic effects such as changes in
beam conditions, detector inefficiencies, and back-
grounds from other decay modes cancel to first
order in the double ratio.
Although the method to measure E'/e is the
same, the beam and the detector used to collect
the data are entirely new.6 The primary consider-
ations are that the detector have excellent electro-
magnetic calorimetry, precision tracking, a large
aperture, uniform field analysis magnet, hermetic
photon veto coverage, and a data aquisition sys-
tem which could handle a high trigger rate envi-
The KTeV beam has been designed to provide
intense beams of K2 with reduced backgrounds
from neutral and muon beam halo. Protons with
an intensity of 3.5 x 1012 protons per pulse are in-
cident with a 4.5mrad angle on a beryllium-oxide
target. The beam is collimated and sent through a
series of muon sweeping magnets, ultimately pro-
ducing two parallel neutral beams. Initial run-
ning indicates the neutron to kaon ratio is approx-
imately 1:1. The kaons decay in a vacuum vessel
which ends at a Kevlar window 160m downstream
from the target. Contained within the vessel are
the regenerator and 6 photon veto counters.
In the c'/E phase of running, a regenerator,
which is located 125m downstream of the target.
is placed in one of the K2 beams in order to coher-
ently regenerate the KS component of the beam.
The regenerator is comprised of 85 modules of
scintillator which are each instrumented with two
photomultiplier tubes. The total amount of ma-
terial is 1.8A1, resulting in -3% coherent regener-
ation for a 65 GeV kaon beam. The regenerator
position is alternated once a minute to reduce sys-
tematic effects from the beams and detector. The
regenerator is moved out of the beams for rare de-
The photon veto system consists of 10 coun-
ters: 6 located in the vacuum vessel, 3 in the
charged spectrometer system, and 1 just upstream
of the CsI calorimeter. By making the sys-
tem more hermetic and lowering the photon veto
threshold (100 MeV), the 37r0 background will be
lower than in E731.
Charged particle momenta are analyzed with
a large aperture analysis magnet and four drift
chambers, two located upstream and two down-
stream of the magnet. The KTeV drift chamber
frames are the same as were used in E731, but
the field wires have been restrung with aluminum
wires, thereby reducing the amount of scatter-
ing material. The chambers also have lower gas
gains and have been instrumented with new, low
noise preamplifiers. Initial calibration constants
for KTeV have yielded single hit resolutions of bet-
ter than 100pm. The analysis magnet is new and
has a larger aperture and more uniform field than
E731. The pt kick of the magnet has been in-
creased to 400 MeV/c for E'/E running. Helium
bags are located between the drift chambers to
reduce multiple scattering. The momentum reso-
lution is expected to be 0.35% for an average pion
track momentum of 35 GeV/c, a factor of two bet-
ter than the previous experiment.
The energy resolution for the calorimeter is
required to be 1% for 15 GeV photons. Pure
CsI was chosen because of its light output, speed,
and radiation hardness. The calorimeter is located
186m downstream from the target and comprises
3100 crystals of 50cm length (27Xo). The blocks
closer to the beams have a transverse cross section
of (2.5cm)2 while the outer blocks are (5.0cm)2.
The non-uniformity of scintillation light along the
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Whitmore, J. & Collaboration, KTeV. Physics prospects of the KTeV experiment at Fermilab, article, October 1, 1996; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc679318/m1/4/: accessed February 17, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.