The experimental aspects of measuring
B(KOPIO) are quite challenging. The KOPIO reaction (Eq. 1) is a
three-body decay where only one decay product, a neutral pi
meson, is observed. There are many competing decays that also
yield pi mesons but with branching ratios that are billions of
times larger. And observing a decay mode with a branching ratio
on the order of 3E-11 requires a prodigious number of K-mesons in
order to achieve the desired sensitivity. Thus, a detection
technique must be developed in conjunction with an intense
source of K mesons that has these key features:
1. Maximum redundancy for observing this kinematically unconstrained decay;
2. Optimized system for ensuring that the
observed neutral pi meson is the only observable particle
emanating from the decay; and
3. Multiple handles for identifying possible
small backgrounds that might simulate the KOPIO decay mode.
It is with these issues in mind that the
KOPIO experiment shown in Fig. 1 has been designed.
The concept for the KOPIO experiment is
presented schematically in Fig. 2. The beam and detectors employ
state-of-the-art advanced technologies in novel configurations.
Important elements of the system are based on established
measurement techniques and new aspects have been studied in beam
measurements and with prototypes and simulations.
Fig. 1. An engineering perspective of the KOPIO
experiment. The apparatus occupies approximately 15 m along the
beam line indicated by the dotted lines at left.
Fig. 2. Schematic of the KOPIO experiment. The time-bunched
neutral K meson beam enters from the left. K decays are imaged
using the detection of two photons which interact in the
preradiator and the calorimeter on the right. The decay region
is surrounded by the barrel veto detectors. The calorimeter
shown at the right covers an area of 25 m^2.
The 24 GeV primary proton beam from the AGS
is presented to a K meson production target in 200 ps wide
pulses at a rate of 25 MHz giving a micro-bunch time separation
of 40 ns. A 500 mSR solid angle neutral beam is extracted at 40
degrees to produce a “soft'' K meson spectrum peaked at 0.65 GeV/c;
K mesons in the range from about 0.4 GeV/c to 1.3 GeV/c are
used. The vertical acceptance of the beam (0.005 r) is kept much
smaller than the horizontal acceptance (0.1 r) so that effective
collimation can be obtained to severely limit beam halos and to
obtain another constraint on the decay vertex position.
Downstream of the final beam collimator is a 4 m long decay
region that is surrounded by the main detector. Approximately
16% of the K mesons decay yielding a decay rate of about 14 MHz.
The beam region is evacuated to a level of 1E-7 Torr to suppress
neutron-induced pi meson production. The decay region is
surrounded by an efficient Pb/scintillator photon veto detector
(“barrel veto''). In order to simplify triggering and off-line
analysis, only events with the signature of a single K meson
decay producing two photons occurring within the period between
micro-bunches are accepted.
KOPIO requires a low energy, time-structured
neutral K meson beam to allow determination of the incident K
meson momentum using the time-of-flight technique. This intense
beam, with its special characteristics, can be provided only by
the BNL AGS upgraded as described below in conjunction with this
CFI International Access fund application. Although the AGS is
already the highest intensity high energy proton accelerator, it
appears feasible using innovative new techniques and new
instrumentation to boost the beam intensity by 50% or more while
modifying the time structure of the beam to be ideal for KOPIO.
Utilizing a low momentum K meson beam
permits a detection system for the daughter particle, pi meson
which decays to two photons, that yields a fully constrained
reconstruction of the pi meson decay vertex, mass, energy, and
momentum in the K meson centre of mass system. This is
accomplished by measuring the position of interaction, angle,
and energy of each individual photon in a fine-grained
preradiator detector followed by an efficient calorimeter.
The preradiator detector is the
responsibility of the Canadian group. It is a new type of high
resolution, high efficiency gamma ray imaging device. In
applications detecting medium energy gamma rays (from particle
physics to medical imaging diagnostics) it is often desirable to
measure as many kinematic properties as possible, such as
energy, position and time of interaction, and angle, with high
resolution and high efficiency. However, in previous detectors,
it was generally necessary to choose at most two among high
efficiency, high energy resolution, and good position
resolution. Very few detector configurations can also measure
photon angles (e.g. pair spectrometers) but at the cost of low
efficiency and complexity. Some segmented crystal detector
configurations have been used to simultaneously obtain good
energy resolution, efficiency and modest position measurements
but these are extremely costly making them unsuitable for very
large area coverage.
The KOPIO experiment preradiator will be the
first large scale imaging detector for medium energy photons
that delivers all the desirable measurements with high
precision. Using an array of plastic scintillation counters and
dual coordinate wire chambers the preradiator will accurately
measure positions, energies and angles of photons in the range
of 50 to 500 MeV with high efficiency. The resolutions expected
and 25 mrad, for position, energy and angle,
respectively, based on KOPIO prototype and other measurements.
Efficiency of detection for photons in the preradiator is
expected to be 70%. This configuration of photon imaging
detectors has already been shown to have other potential
applications as will be discussed below.
The system for vetoing extra particles and
suppressing backgrounds is also well understood. These features,
which are similar to those employed successfully in the BNL
E787/E949 discovery of
(another important rare K meson decay
experiment performed by members of the KOPIO group), provide the
necessary redundancy and checks to ensure a successful
The goal of the KOPIO experiment is to
obtain about 50 events with a signal to background ratio of 2:1.
The present plan calls for running of the experiment to commence
in 2006 followed by three or more years of data acquisition. The
proposed measurement will yield a statistical uncertainty in the
measurement of the CP violation parameter of the SM of less than
10%. In addition to the KOPIO decay, many other radiative type K
decays of significant interest and numerous searches for non-SM
processes will also be accessed simultaneously. At the
conclusion of the KOPIO experiment, either the SM picture of CP
violation will be shown to be consistent with present knowledge
and the relevant parameters measured accurately, or a new
approach to understanding the world of fundamental particles
will have been shown to be required.
However, in order for the KOPIO experiment
to achieve its goals, it is necessary to upgrade the AGS
accelerator facility to provide significantly more intense beams
with the appropriate beam time structure. The methods,
equipment, and collaboration necessary to realize the increase
in AGS intensity by 50% or more and provide the micro-bunched
beam are described below in the section entitled Infrastructure
August 05, 2004
by Gary Schroeder.