January 10, 2002
Background on the Search for Rare Kaon Decays
Experiment 787 (E787) at Brookhaven National Laboratory’s Alternating Gradient Synchrotron (AGS) is a collaboration involving physicists from Brookhaven Lab, Princeton University, the Canadian laboratory TRIUMF, the University of Alberta, the University of British Columbia, the Japanese laboratory KEK, Fukui University, and Osaka University. Using the intense Low Energy Separated Beam available only at Brookhaven’s AGS, these physicists are looking for what are known as rare kaon decays.
Kaons are subatomic particles made up of a pair of even smaller, elementary particles called quarks, specifically one quark and one anti quark. Because kaons are inherently unstable, they spontaneously break down, or decay, into two or more particles — and, in the process, temporarily transform into other intermediate particles that emit still more particles.
Kaons do not decay in only one way. Moreover, the frequency of any decay mode depends upon, among other things, the masses of the intermediate particles invoked along the way. Some routes are common, while others are strictly forbidden according to physics theory. Certain decay pathways, however, while not taken too frequently, do sometimes occur. Hence, when a kaon decays along a rare or forbidden route, it is called a rare kaon decay.
Since 1988, E787 has been searching for one such suppressed but theoretically allowed decay that has never been detected experimentally: a positively charged kaon decaying into a positively charged pion, a neutrino, and an antineutrino. The interest in this decay centers on the fact that, when the antiquark within the kaon decomposes, infrequently it is momentarily transformed into an anti top quark — the antiparticle of the top quark.
If E787 spots this decay and it occurs significantly more often than predicted by the modern physics theory of elementary particles known as the Standard Mode, then E787 will have discovered new physics. If, however, the frequency of this type of rare kaon decay matches what has been predicted, then the experiment will have made an important confirmation of the Standard Model.
Detecting only a pion
Detecting a positive pion — only a positive pion and nothing else — has been the goal of the E787 experiment. Though a neutrino and antineutrino are also produced by this decay, they interact too weakly to be detected. Since a kaon often decays into a charged pion plus a neutral pion and since a neutral pion immediately decays into two photons, the experiment must be able to detect the two photons as well — to rule out their presence when claiming to see only a positive pion.
Unlike other rare kaon decay detectors at the AGS in which the kaons decay in flight, the E787 detector stops its kaons in a scintillating fiber target before they decay. The particles resulting from the kaon decays then travel through what is called a drift chamber, which measures momentum. To determine the trajectories of charged particles, the time that it takes the electrons that these particles knock off gas molecules to reach the known locations of the wires within the chamber is measured.
A huge solenoid magnet surrounds the entire detector. It bends charged
particles traversing the chamber in characteristic fashion, depending upon
their charge and momentum. Around the drift chamber is an array of
scintillation counters. To measure energy and range, these devices count
the flashes of light produced in a fluorescent material by the ionizing
radiation associated with the particles passing through. By correlating
these two measurements with the momentum measured in the drift chamber,
the particles can be identified.
Surrounding the counter array and forming end caps near the drift chamber are detectors that are sensitive to photon interactions. These are used to eliminate from the experiment’s consideration any of the more frequent kaon decays that produce photon interactions. The detector uses cesium iodide crystals. To allow more complete vetoing of background processes that produce photons, a supplementary veto array surrounds the entire range stack.