Top quarks are produced in particle-antiparticle pairs () in proton-antiproton collisions. They are produced only rarely. We have found 43 events that are consistent with top production. In order to find these events we had to sift through 50 million events that were recorded on magnetic tape by the CDF experiment.
Top quarks decay to a W-boson and a b-quark. The W-boson decays to either leptons or quarks.
Events where both W-bosons decay to leptons (dilepton events) are the cleanest but are the rarest kind of top decay, only 5% of the events decay this way.
Events where one W-boson decays to leptons and the other decays to quarks (lepton + jets) are much more plentiful, about 30% of events decay this way. However the background from W-bosons that have jets associated with them is large. To reduce this background we look for the b-quarks that we expect to find in events. The b-quark lives for about seconds before decaying and it can travel up to several millimeters in this time. The decay vertex of the b-quark is displaced from the primary vertex of the event At the heart of CDF is a Silicon Vertex detector (SVX) that can measure these displaced vertices (SVX method).
Here are some plots that show how well we can identify b-quark decays in the SVX. Shown are various distributions for a data sample that is rich in b-quarks compared to what we expect for a b-quark Monte Carlo.
We can also look for additional leptons in the events that come from the decay of the b-quarks (SLT method). If a jet in an event contains either a displaced vertex or a lepton then we consider it to be b-tagged.
The jet multiplicity distribution of the events compared to the expected background for the SVX method
The jet multiplicity distribution of the events compared to the expected sum of background+top for the SVX method
Using these 3 techniques we find the following numbers of events.
There are 5 b-tags in the 6 dilepton events. This would be unlikely for background events but would be expected if the events contain top. There are also events in the lepton+jets sample that have 2 b-tags. This is also unlikely if the events were all background.
The combined probability is , which is equivalent to a deviation in a Gaussian distribution. Based on the excess number of SVX tagged events, we expect an excess of 7.8 SLT tags and 3.5 dilepton events from production, in good agreement with the observed numbers.
If the events are indeed the decay of a heavy object then we should be able to reconstruct a common mass value. We fit the events in the lepton+jets sample that have at least 4 jets to the hypothesis and obtain as mass of
The reconstructed mass of the lepton + 4 jets events containing identified b-quarks
The reconstructed mass of all the lepton + 4 jet events