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The KOPIO Experiment

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 are


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 measurement.

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 (TO COME).




Last updated August 05, 2004 by Gary Schroeder.