The "parent" kaon: this is the "strange" particle (that's right, that's really what these peculiar elementary particle physicists call it!) which E865 is trying to study. We never see the kaon itself, since it "decays" or changes to other particles far upstream of our main detector.

How do we know that the kaon actually made a transformation to other particles? We use Einstein's E=Mc**2 idea. Turns out that if you know the energy and momentum of the "daughter" particles, and you add them all up (remember that momentum is a vector!) to get Etot and Momentumtot, then,
(Mc**2)**2 = (Etot)**2 - (Abs(Momentumtot)**2).

But to know the masses of the individual particles, we must "identify" them. We know that kaons can only "decay" (or transform or change) into one of: pions, electrons, muons, and neutrinos. (Pion mesons are a kind of hadron; The electron, muon, and neutrinos are all different kinds of leptons.)

Accelerator. The BNL Alternating Gradient Synchrocyclotron make particles for this experiment and a lot of others too.

Beamline. To make particles for a given experiment, protons are "kicked" out of the AGS into a "beamline", which is first a "target" (usually a small piece of metal, with lots of protons in it) followed by a set of magnets which bend and focus the particles, and "transport" them to our experiment, in much the way that light is "transported" by lenses and mirrors.

Decay Volume or Space. After the "beamline", there is a field free region through which the kaons travel. Some of them "decay" or transform themselves in this space into various possible "daughter sets" or "final states". Common ones are: K+ to muon neutrino (about 63% of the time); K to two pions (pi+ pi0, about 20%); K+ to pi0 positron neutrino, about 5% of the time (where a positron is like an electron but postively charged); and K+ to three pions (1-2% of the time). The decay E865 was searching for, was K+ to pi+ mu+ e-. This was not expected to be seen, because of an idea called lepton conservation. If we had seen it, it would have been very exciting. Before we started, the limit was about 2 such decays in 10,000 million K+ decays. So far we have pushed the limit down about a factor of 10, and are hoping for another factor of 3.

The first detector magnet: D5. In this big magnet, positive particles are bent left and negative ones are bent right. Neutral ones just go straight.

"Proportional wire chambers", P1, P2. These are planes of wires enclosed in a gas volume. They act a little like a Geiger counter: charged particles ionize gas in the wire chambers, and the ionized gas (and correspondingly freed electrons) travel to collection points. By seeing which collection points (which wires) actually give a signal, the path or "trajectory" or the particles can be reconstructed.

The second dector magnet: D6. In this big magnet, positive particles are bent right and negative ones are bent left. They end up nearly in the same direction they started in before entering D5. The experiment then detects positive particles on the right and negative particles on the left hand side of the apparatus.

"C1", the first Cerenkov counter. To learn more about Cerenkov radiation see the Cerenkov folder, or if you are online, try a search. I found: http://www.cakes.mcmail.com/cerenkov/cerenkov.htm. The Cerenkov counters in E865 were special in the low mass of their mirrors (made of polycarbonate aluminized using an electron beam to evaporate the aluminum and deposit it very uniformly, and covered with an overlayer of MgO2/MgO, which is resistant to oxidation and degradation in humid atmospheres.)

The Cerenkov counters were also special in the light weight of the windows, which were made of kevlar (the material used to make bullet proof vests), and sewn together rather than attached with heavy polyurethane layers, as had been the case for the experiment before E865.

The Cerenkov counters operated with one of two gases in the volumes: usually CH4 on the right, to be sure to catch any e+ (positrons) which might be masquerading as pions or muons. On the left, to be sure that no other particles masqueraded as electrons, we used hydrogen, which only electrons from our decays were going fast enough to make Cerenkov light in.

The Cerenkov counters work by reflecting light from the special aluminized mirrors, into a "collector" which "funnels" the light down to a small detector (a photomultiplier) at the end of the collector. A collector from C1 is included in the travelling exploratorium. In the exploratorium at iThemba labs, the mirrors supports, and some mirrors, from one quarter of C1 are included.

C2: The second Cerenkov counter, after D6, makes another check on the type of particle: is it going fast enough to give Cerenkov light? A collector from C2 in included in the travelling exploratorium. In the exploratorium at iThemba labs, the mirrors from one eigth of C2 are included.

P3 and P4: These proporational chambers measure the position and angle of charged particles after their bending in D6. Since the particles are bent according to their momentum, the combination of measurements in P1,P2 and P3,P4 allow a measurement of the momentum of the particles. Only events in which the daughter charged particles apparently intersect ("make a vertex") in the decay volume are included as events for E865 study.

Calorimeter. This calorimeter is lead, with scintillator fibers running lengthwise through the lead. The electrons, positrons, and photons have dramatic electromagnetic interactions in this calorimeter, losing most of their energy in it. Muons and pions, because of their higher mass than electrons and positrons usually leave only a small part of their energy ("minimum ionizing particle" characteristic deposition) in the calorimeter. Pions occasionally undergo nuclear interactions, about a third of the time, and don't usually lose much energy, even if they interact.

Muon detector. This is a lot of iron interspersed with tubes, again much like large Geiger counter tubes. Electrons have lost most of their energy before getting to this muon detector. Pions typically begin to lose energy in the calorimeter and quickly lose the rest of their energy in the first part of the muon detector. Muons just keep on going, and usually make it all the way to the end of the muon detector.

The muon: is bent in D5 and D6, and detected in P1, P2, P3, and P4, from which its momentum, and position and angles in the decay volume can be inferred. It makes nearly no Cerenkov light, deposits only a small amount of energy (from electromagnetic interactions, typical of a heavy "minimum ionizing particle") in the calorimeter and usually goes to the end of the muon detector.

The pion: is bent in D5 and D6, and detected in P1, P2, P3, and P4, from which its momentum, and position and angles in the decay volume can be inferred. It makes nearly no Cerenkov light, and typically interacts only electromagnetically in the calorimeter, giving a small amount of energy typical of a "minimum ionizing particle". Occasionally the pion interacts by nuclear interactions in the calorimeter but even when it does interact it typically deposits only a small amount of energy in the calorimeter. It usually stops in the first part of the muon detector.

The electron or positron: is bent in D5 and D6, and detected in P1, P2, P3, and P4, from which its momentum, and position and angles in the decay volume can be inferred. It makes quite a bit of Cerenkov light, deposits nearly all its energy in the calorimeter and gives almost no signal in the muon detector.