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184 • Gallas et al.’s (1995) experiment was designed to look for neutral weakly interacting massive particles (WIMPs). The use of “Search for” in the title usually, but not always, indicates that the experiment found no evidence for the existence of the sought after phenomenon.1 The experimenters noted that the time-of-flight technique to search for such particles had been initially suggested by Schrock (1978). Schrock had remarked that the then current work on the unified theory of electroweak interactions had suggested that a stable, neutral, massive lepton might exist. He further noted that The test which I propose utilizes a beam dump followed by a massive electronic target calorimeter with muon and, ideally, also electron spectrometers. At present it is, we believe, the most, and probably the only, practical way to search for (effectively stable) E0’s [the neutral particles]. The crucial aspect of the experiment is the use of timing to an accuracy of ~1 nsec in order to discriminate between the arrival of massless and massive neutral leptons and to select the latter. . . . The proposed test makes crucial use of the rf [radiofrequency] structure of a pulse of protons from the accelerator.2 At Fermilab, a pulse consists of a large number of individual bunches of protons, each of width ~1 nsec, separated by 18.8 nsec. This rf structure is preserved by neutrinos from the dump. (Schrock 1978, 1688) Schrock assumed a flight path of 1 kilometer, a total uncertainty in the timing of 1.5 nanoseconds, and a neutrino energy of 20 GeV. He estimated that the experiment would be capable of discovering a neutral lepton with mass greater than 0.6 GeV.3 The heavier particles would have a longer time of flight than the almost massless neutrinos.4 This was the experiment performed by the Fermilab, Michigan State, MIT, and Florida (FMMF) collaboration (Gallas et al. 1995).5 The neutrino beam was created when a primary beam of 800 GeV protons struck a tarCHAPTER 16 “Search for Neutral Weakly Interacting Massive Particles in the Fermilab Tevatron Wideband Neutrino Beam” Search for Weakly Interacting Massive Particles • 185 get. The collisions produced pions as well as other particles. The pion decays into a muon and a neutrino, and the muon decays into an electron and two neutrinos. Neither of these decays could produce the heavy neutral lepton Schrock had suggested, because the mass of both the pion and the muon is too small, but that heavy neutral particle might be produced directly in the proton interactions or by the decay of other heavier particles . The heavy neutral particles would then be detected in the apparatus used to detect neutrinos. The protons were in bunches with an approximately Gaussian distribution, each with a standard deviation of less than 1 nanosecond and separated by 18.83 nanoseconds. The charged secondaries [produced by these collisions] and their light decay products were highly relativistic, so they maintained the time structure of the proton beam as they traversed the focusing quadrupole triplet magnet train, the decay volume, and the shielding before reaching the FMMF detector.6 The hadron and muon shield, located 542 m downstream of the neutrino target [this allowed sufficient time for a reasonable number of the unstable charged secondaries to decay, producing neutrinos], consisted of an aluminum beam dump followed by a berm containing dirt with steel and lead inserts. (Gallas et al. 1995, 7) The beam dump and the berm eliminated virtually all of the charged particles in the beam, leaving only neutrinos or other weakly interacting neutral particles. The detector consisting of a target calorimeter, in which the neutral particles interacted, and a muon spectrometer is shown schematically in figure 16.1.7 A veto wall to reject charged particles was placed in front of the target/calorimeter. The calorimeter consisted of alternating planes of target material, flash chambers, and proportional tube planes: “The muon spectrometer measured the momentum and polarity of any high-energy muons that exited the downstream end of the calorimeter. Information from the flash chambers, proportional tube planes, and spectrometer was used to locate the event vertex, measure the event energy, find the position of the end of electromagnetic or hadronic showers, track muons and measure their momentum, and identify the final state event topology” (7). A key element of the apparatus was the 16 scintillation counters that measured the time of flight (TOF) by recording the event time relative to the rf clock of the accelerator. Because they...

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