1995 Top Quark Discovery


In 1995 two experiments at (D0 and CDF) announced the discovery of top quark, the heaviest fundamental particle in Nature ever detected. The recollections of NIU physicists are described here , and in a poster on the first top event. This page describes this discovery and some of the contributions made by NIU.

The world we live in contains a handful of elementary particles out of which everything, like atoms, is composed. The current understanding has twelve particles and their antiparticles. Particles called leptons are unaffected by the strong force under any circumstances; the most familiar is the electron. Particles called quarks do feel the strong nuclear force and it dominates their existence. Protons and neutrons are examples of particles made from bound quarks. This Table shows the relationship between the particles plus four others which are the carriers of the forces between particles:

The six quarks and six leptons are each grouped in three pairs, ordered by mass, by the weak nuclear force which causes transitions between pairs and groups. Some forms of radioactive decay are due to this. For leptons, the electron and its neutrino constitute the lowest mass pair. The three quark generations are the up and down quark (from which protons and neutrons are made), the strange and charm quarks, and the top and bottom quarks. As the second and third generation particles are heavier, the weak nuclear force causes these particles to decay to the lightest leptons and quarks. Thus normal atoms are made from only first generation particles with the heavier generations existing only briefly either as the results of high energy interactions, or in the very hot early universe.

The Experiment

A top quark is about 200 times heavier than a proton. They can be made by converting energy into mass using E=mc2. Currently only the accelerator at Fermilab has enough energy to produce top quarks, which are made in top-antitop pairs. A top quark quickly decays to a W and a b quark. The W then decays into either a muon, tau or electron plus neutrino or into two quarks. So a top quark pair produced by a proton-antiproton collision can be detected by observing its electron, muon, and quark decay products. In the 17 collisions used by D0 to discover the top quark, there were 10 electrons and 18 muons with 7 muons coming from W decays and 11 from b quark decays.

NIU's primary contribution to D0 was aspects of the muon detector. From 1983-85 the system was designed and ideas on the iron thickness and layout of the position measurement elements (which were wire chambers, essentially modern Geiger tubes) were adopted. One idea was to increase the number of measurements from the miniumum six in the initial design to ten. This addition turned out to be critical as the chambers were hampered by hydrocarbon deposits during operations in the high radiation environment. The redundancy allowed us to still identify muons with a very high efficiency. NIU staff and students worked on all aspects of the muon system, from helping to build wire chambers, testing electronics, and writing software. From 1985-97 NIU was responsible for muon identification which converted the information from our 10,000 wires of in the central muon system to points in space and identified muons. This can be seen in the event 417 and event 79 displays from two proton-antiproton collisions which contained top quarks. The muon detector is on the outside with hit wires marked by ``x" and identified muons marked by a line. The first event (417) has a high momentum muon probably from a W decay (there are no other particles close to it in the detector just before the muon system). The second event (79) has two low momentum muons (they have a large bend angle) plus other particles near the muons and so is probably from a b quark decay. It took over 400,000 lines of software to identify muons (plus calibrate, monitor and display the system). NIU students and staff wrote about 2/3 of the code for the central muon system with the rest being done by colleagues from Fermilab, Indiana and Florida State.

The Discovery of the Top Quark

D0 started collecting data in 1992. Priot to this we had written the initial software to identify particles and done preliminary studies of how to detect top quarks. In January 1993 the first top pair candidate ever observed by any experiment was collected by D0. This collision is event 417 in the event display above and contains a high momentum muon. But there are only 2 out of 4 wires with information in the muon detector closest to the center and these two hits appeared not to be on the muon ``line". What probably happened was that a large charge deposition occurred in that region just before the muon went through causing the high voltage to be temporarily corrupted (like a large spark). So there were two solutions: use the 2 points and the muon had a low momentum and wasn't from a top quark or recognize the points were corrupted. Code to spot this type of effect was just being developed and, depending on what version of the software we used, the muon could have either solution. The probability that it was either a low momentum muon or that the points were corrupt and it was high momentum were each estimated. The high momentum solution had a much larger probability (over 10,000 times larger) and was discussed on February 17, 1993 at a D0 top workshop held at NIU. We published this observation that year but did not claim to have discovered the top quark.

D0 and CDF continued to collect data. By early 1995 D0 had a sample of 17 collisions which we thought had top quarks. We estimated that only 4 of these came from non-top sources and so had enough confidence that the top quark had been observed.

For futher information contact Prof. David Hedin, Northern Illinois University, email: hedin@niu.edu

Last modified: 8 October 05