Measurement of the Top Quark Mass Using Dilepton Events
The DØ Collaboration

August 19, 1997

The current theory of the nature of the universe (the "Standard Model") describes the forces acting on two types of particles, called "leptons" and "quarks". The most familiar lepton is the electron, while two varieties of quarks (called "up" and "down") combine to form neutrons and protons. In addition, there are heavier, unstable versions of the quarks and leptons which can be created by colliding energetic particles. At Fermilab, we collide beams of protons and antiprotons in the hope of creating previously unseen particles.

Before the most recent run of the experiment, we had already found five different quarks: up, down, charm, strange, and bottom are their names. The Standard Model predicted the existence of a sixth, called the top quark. In fact, the theory told us almost everything about the top quark: how it would be produced and how it would decay into lighter particles. For example, we knew that top quarks would be most often produced two at a time. In addition, the top has several possible ways of decaying, including some in which the end result contains electrons or muons (a muon is a lepton; it's just a heavier version of an electron). These particles are easy for us to see, and they are not produced in the run-of-the-mill proton-antiproton collision. So, those top quark decays that contain electrons or muons are of special interest to us. About 30% of the time, the pair of top quarks produced in a collision will yield a single electron or muon (we call such cases "single-lepton" events). Even rarer are the cases in which two electrons or muons are produced ("dilepton" events): these occur only about 5% of the time.

So, thanks to the theory, we knew quite a bit about the top quark before we ever saw one. Based on this information, we had a good idea of what to look for. After looking at the results of two years' worth of proton-antiproton collisions, it was clear that both our experiment and the CDF experiment had in fact seen top quarks . After another two years, we had collected a sample of about thirty single-lepton and five dilepton events.

With this sample, we measured the one property of the top quark that the theory didn't predict -- its mass. To do this, we first used our collection of single-lepton events and "reconstructed" the mass based on the decay products observed. (The process is somewhat like estimating the size of a hailstone based on the dent left on your car.) As a result, we found that the top quark has a mass of 173.3 GeV/c2 (by comparison, a proton has a mass of about 1 GeV/c2), and our uncertainty on that measurement was 8.4 GeV/c2.

We can reduce this uncertainty a little bit by also considering the five dilepton events. However, there's an even better reason to look at them. Recall that the top quark was predicted by the Standard Model. Now, the Standard Model is a good theory, and it can explain everything we've seen in the lab so far, but that doesn't mean it's the right theory. There are other possible theories, some of which predict different types of particles. These new particles may just be too heavy for us to have seen them. It might even be the case that some of the events which we think are due to top quark production are really the result of producing these new particles. So, if we were to measure the dilepton events and find the mass is inconsistent with 173.3 GeV/c2, we would have a strong indication that we were seeing more than just the top quark.

As it turns out, the top quark mass one obtains from the dilepton events comes out to 168.4 GeV/c2, with an uncertainty of 12.8 GeV/c2. This is in good agreement with the single-lepton number, so we have no reason to believe that anything other than the top quark is appearing in our data sample.

As a final step, we combine the information we've gained from the single-lepton and dilepton events, and obtain our best measurement of the top quark mass: 172.0 GeV/c2, with and uncertainty of 7.5 GeV/c2.

A copy of our paper "Measurement of the Top Quark Mass Using Dilepton Events ", submitted for publication in Physical Review Letters, is available as a preprint from hep-ex/9706014.

Both the Fermilab particle accelerator and the DØ and CDF detectors are currently being upgraded. When this is done (in 1999) we'll start collecting top quark events again, and end up with at least twenty times as many as we have now. Using this larger sample, we'll be able to explore the properties of the top quark even more accurately than we have so far.

For further information contact Dr. Erich Varnes, email:

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Last modified: 19 August 97