Top pair production cross section in the lepton+jets channel using b-tagging


Introduction

Particle physics is the study of the fundamental constituents of matter and their interactions. During the 20th century remarkable progress was made in the understanding of physics at this microscopic scale.

All known elementary particles and the theories describing the interaction between them were summarized into a coherent framework called the Standard Model. The particles are divided into fermions, which are the building blocks of matter, and bosons which are the force carriers that allow the particles to interact with each other. The fermions are in turn divided into three generations of leptons and quarks.

The Standard Model of particle physics has been tested to extraordinary precision, and so far it has withstood all tests. However we know that it cannot be a complete theory of everything as it fails to incorporate gravity. It also has no explanation for the dark matter present in our universe. Therefore it is important to continue to probe the boundaries of the Standard Model.

The top quark

The heaviest elementary particle known today, the top quark, weighs as much as 175 protons or the equivalent of a gold atom. To produce such massive particles requires enormous energies. The top quark was discovered in 1995 by the and CDF experiments at the Tevatron accelerator, in head-on collisions of protons and anti-protons travelling very close to the speed of light. The Tevatron accelerator, located at Fermi National Accelerator Laboratory ( Fermilab) outside Chicago, is still the only accelerator built with sufficient energy to produce top quarks.

The production cross section

Top quarks are predominantly produced in pairs (one top and one anti-top quark together). This analysis aims at determining how likely this is to happen (physicists call this probablility of particles being created the production cross section and it is measured in the unit of picobarns, abbreviated pb). According to theory, top quarks are produced only in 1 out of about 10 billion collisions at the Tevatron energy (a collision is often referred to as an event). Particularly interesting would be to measure a production rate which is incompatible with the prediction from theory, as this would imply new and unknown physics.

To measure the production cross section we first count the number of event with the correct signature. We then subtract the number of events which has the same signature but are coming from other sources than top quarks (such events are called background events). If we select more events in our data than we predict from background sources, we interpret the excess as coming from the production of top quarks. The magnitude of the excess determines the cross section.

The event signature

The top quark has an extremely short lifetime, and decays almost immediately into lighter so called bottom quarks and particles called W bosons. These particles are also very unstable and will in turn decay into lighter particles. This means we cannot directly see top quarks. Instead we look for events which has the correct signature, compatible with coming from the decay of two heavy top quarks.

The specific signature we looked for consisted of an electron or a muon with an accompaning neutrino from the decay of one W boson, two particle jets coming from the decay of the two bottom quarks and an additional two particle jets coming from the decay of the other W boson into two quarks. It is called the lepton+jets final state, since what we expect to see in the detector is a lepton (electron or muon) and four particle jets from the four quarks. The electron and the particle jets are detected by the calorimeter, the muon is detected in the muon detector. The neutrino, which doesn't interact with the matter in the detector, escapes and will leave a trace of unbalanced energy which will point in its direction.


Finally we require that at least one of the particle jets in the event is identified as coming from a bottom quark. The bottom quark will produce a particle which travels a few millimeters before it decays into lighter particles. If we find such a decay point inside a jet, we identify this jet as coming from a bottom quark. This is a powerful tool to discard a lot of the background events, since they very seldom have jets coming from bottom quarks.

Result

Shown in the figures below is a comparison of the number of events predicted (the histogram) and observed (the points). The left figure shows events with one jet identified as coming from a bottom quark, and the right plot shows events with two jets identified as coming from bottom quarks. Most of the top quark events have three or four jets, whereas events with one or two jets mainly come from background sources. The first two jet multiplicity bins are therefore not used in the calculation of the cross section. They do however confirm that we can predict the correct amount of background events.



In the last two jet multiplicity bins the observed number of events far exceeds the background prediction. The excess results in a cross section of:


This is in good agreement with what is expected from the Standard Model, which is 6.8 ▒ 0.4 pb.

Outlook

Using the selection for top quark events like the one described above, properties of the top quark - such as its mass - can also be studied. Other decay modes of the top quarks are also being checked for agreement with the Standard Model. By refining our analysis methods and using the continuosly accumulating data, we are moving towards doing precision measurements of top quark physics. By the year 2009 we expect to have a data sample roughly 20 times larger than the one used for this analysis. This will allow us to continue to verify the predictions of the Standard Model or find something that deviates from the expected behaviour and thus evidence for new and exciting physics.

The full article can be found here. For more information on this analysis, please contact the primary authors C. Clement, R. Demina, T. Golling, A. Juste, A. Khanov, S. Lager, F. Rizatdinova, E. Shabalina and J. Strandberg.