But somewhere above the control room, some 100 kilometers over Lake Michigan, a particle reaction is taking place. A very energetic proton coming from another galaxy ends its hundred-million-years journey as it enters the Earth's upper atmosphere, hits the nucleus of a Nitrogen atom, and creates a shower of secondary particles. Most of these are pions and kaons that only manage to travel a few hundred meters before they decay into electrons, muons, and neutrinos.
Electrons are quickly absorbed as they get slowed down by photon bremsstrahlung, and they too fail to make it to the Earth's surface; neutrinos instead punch through the whole planet and leave on the opposite side. One very energetic muon, however, heads straight toward B0. It penetrates the ground above the CDF collision hall, only losing few tens of GeV as it punches through earth and concrete, and finally makes into the CDF electromagnetic calorimeter, where it suddenly decays into an electron and two neutrinos. The neutrinos leave the scene unnoticed, but the electron immediately interacts with a lead nucleus, generating an electromagnetic shower, and recordable flashes of light in the scintillator sheets.
Just as this odd chain of events is unfolding, a proton and an antiproton bunch are heading against each other in the center of the detector. A proton and an antiproton hit each other head-on just at a time when one red down-quark in the proton is taking, for a unmeasurably small instant, a large fraction of the total energy of its parent. The red down quark is on a collision course with an anti-up quark which is also endowed with a large energy. The anti-up quark is anti-blue, so in total the quark-antiquark pair has a net amount of colour charge; but just before the two bodies get close enough to interact, the antiquark chances to emit an energetic gluon.
The gluon carries away the anti-blueness of the anti-up quark, transmuting it into anti-redness. This allows the quark-antiquark pair to produce a weak interaction: hadronic matter and antimatter disappear together as dry Martini and Gin in a carefully prepared drink, colour and anticolour vanish, and the negative unit of electric charge is transferred to a fresh new W boson.
Besides the energetic gluon, all that remains as a trace of the annihilation in the point where the pair vanished is a W endowed with very large energy. The boson soon gets rid of some of it, by emitting a energetic photon; then it travels from the quarks' annihilation point only a tiny fraction of an attometer, a fantastically short distance, before disintegrating: and it does so yielding an electron-antineutrino pair.
As soon as the remains of the original proton and antiproton realize they have been stripped of one quark and have become coloured they break apart, creating two streams of low-energy hadrons that leave the scene in opposite directions, mostly along the beam pipe. The energetic gluon extends the colour string that still connects it to the antiproton remnants until this breaks, yielding two charged pions; but unusually, one of the two pions is of extremely low energy, and ends up spiralling within the beam pipe. The leading pion instead is quite energetic, and it heads straight into the plug calorimeter, after leaving some trace of its passage in three of the four layers of the silicon vertex detector.
As it reaches the calorimeter, the pion withstands a peculiar interaction with the dense matter: it plunges into a neutron in a lead nucleus, and it transfers its up quark to the neutron, receiving a down quark in exchange. The reaction has the result that the charged pion turns into a neutral one, while the neutron turns into a proton ! While the former lead atom, for an instant turned into bismuth, breaks apart into ligther nuclear fragments, the neutral pion only manages to travel a millimeter or so before it decays into two photons . The latter pair in turn produce an electromagnetic cascade, each converting into electron-positron pairs as they come close to other lead nuclei.
The charged pion has managed a trick dreaded by experimentalists: one that deserves a name by itself, "charge exchange". What is observable of the energetic pion in the detector is just a stiff charged track, associated to an electromagnetic shower in the calorimeter, one quite similar to what would be expected by an energetic electron. What is worse, pions are almost never produced as isolated particles in a hadronic collision: the fragmentation of quarks and gluons usually produce them in the company of other light hadrons, and the chance that one single particle is the visible product of the fragmentation is in the one-in-ten-thousand ballpark. Hence, if an experimentalist observes an isolated electromagnetic deposit with a corresponding charged track pointing at it, it will be hard to convince her that what created the signals was a pion; an electron due to the decay of a W or Z boson will usually be the most likely explanation, despite the relative rarity of vector boson production !
Let us now return to the other three energetic particles produced by the unusual reaction: the electron, the antineutrino, and the photon. They move out of the intereaction point roughly orthogonally with respect to the beam pipe, and head toward the inner layers of the tracking system. The neutrino traverses unhindered the layers of sensitive material, oblivious of the dense hadronic matter it zips through; the electron leaves a stream of ionization in the gas of the tracking chamber, and passes through the strong electromagnetic field of lead atoms in the calorimeter, gradually losing its energy by emitting photons. The electron finally gets captured by one of the lead atoms, while the secondary photons produce a cascade of electron-positron pairs and additional photons, whose end result is the generation of a few flashes of ultraviolet light in the plastic scintillator hit by the shower.
As for the energetic photon emitted by the hard subprocess together with the W boson, it meets a similar end: it zips unhindered through the tracking chamber, but as soon as it enters the calorimeter it passes by other lead atoms, turning into an electron-positron pair; the two hit other lead atoms, undergo bremsstrahlung creating further photons, and these in turn create other electron-positron pairs: the end result is a shower of secondaries. As electrons and positrons traverse the scintillating material, they create flashes of ultraviolet light. From the amount of released light, experimentalists will be able to infer the energy of the primary incident particle.
I believe it is useful to take stock. What remains of the hard collision, as recorded by the CDF detector, is the signal of one real electron and an energetic photon, plus a further spurious electron signal originated by the leading charged pion into which the initial-state gluon fragmented. In addition to that, the W decay neutrino has left the detector with a significant amount of momentum, so the particle momenta corresponding to the observed energy releases do not add up to zero in the transverse plane: there is significant missing transverse energy that hints at the neutrino escape.
But wait, this is not all: there is also the electromagnetic shower left at exactly the same time by the cosmic-ray muon. That energy deposit perfectly mimics the signature of a second energetic photon. All in all, what experimentalists have in their hands is a spectacularly improbable event: one which appears to feature two electrons, two photons, and significant missing transverse energy. It is going to be dubbed "e-e-gamma-gamma-met event", but a better description would be "an event from another world".
A "lego" plot of the event, where each vertical bar identifies a localized energy deposit in the CDF calorimeter, cut and unrolled along the azimuthal angle, is shown below. The event generated a huge interest as it was generally considered the first manifestation of Supersymmetry. Later studies and the investigation of much larger data samples have concluded that the event could not be ascribed to new physics but rather to some odd coincidence of detector effects and rare, but known, standard model processes.
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