The Search for Single Top Quark Production

Where in the World is Single Top?

''Where in the World is Carmen Sandiego?'' This is the name of a popular interactive video game where players become Interpol detectives and conduct a police investigation. Particle physicists are also detectives, and though we do not investigate crime scenes, we do investigate the fundamental properties of matter and the underlying laws of our universe. One of the most interesting cases for particle physicists today is the search for the creation of single top quarks at high energy particle colliders.

The top quark was discovered at the Fermilab Tevatron Collider in 1995. It is by far the heaviest known elementary particle observed in nature. Quarks are one of the fundamental building blocks of matter in the universe. They do not appear individually, but they combine to form particles called hadrons like the proton and neutron. Similarly, antiquarks combine to form antiprotons and antineutrons.

Besides quarks, the other fundamental building block of matter consists of leptons. The most commonly known lepton is the electron, which together with protons and neutrons form atoms and molecules, out of which everything around us is made. These two building blocks and their interactions form the ''standard model'' of particle physics.

The standard model has successfully predicted the existence of top quarks and their creation in particle-antiparticle pairs through the strong force at the Tevatron, also known as top quark pair production. Another prediction for the Tevatron is single top quark production or the creation of individual top quarks without the corresponding partner. Single top quark production has not yet been observed, but the search is an important test of the standard model and provides a laboratory to look for new physical forces and particles.

Unlike top quark pair production, single top quark production occurs through the weak force which is also responsible for the radioactive decay of particles like the neutron. The weak force not only produces the top quark, but it is also responsible for its decay into a bottom quark, a lepton, and a neutrino which can be measured by a detector.

The Tevatron collides protons and antiprotons with the highest energy in the world and produces over two million collisions or events per second. From this vast number of collisions, interesting and rare events are selected for further study and recorded with the DØ detector. The standard model predicts that on average, a single top quark is produced for every 330 million collisions.

The first step in the experimental search for single top quark production is to select events that contain a b quark, a lepton, and a neutrino from the 330 million collisions. This reduces the number to 700 events. But life as an experimental particle physicist isn't that easy because many other processes look like a single top quark event. These imitators are called background events.

At this stage, there are 50 times more background events than expected single top quark events or signal events. Because the background is so much larger than the expected signal, it would take over 20 years to collect enough data to definitively prove the existence of a signal using simple event counting techniques. Therefore, a more advanced analysis technique is needed that takes into account the full event information.

We use a technique derived from a field of machine learning called classification. We use a tool called a neural network to do this. This technique comes from models developed in the early 1960's where scientists tried to mimic the function of the human brain by emulating neurons. In our analysis, a neuron corresponds to a physical property of the event. Many neurons are combined to form a neural network. Simulated single top quark signal as well as background events are presented to the network such that the neurons learn the difference between the two. After the network has been trained in this way, it is used to classify the 600 interesting events as signal or pesky background.

Selected signal-like event as seen in the detector.

We use the result of this classification to look for evidence of single top quark production. We have searched through 10,000 billion collisions recorded with the DØ detector, corresponding to over 3 Terabytes of data. We have reached a sensitivity to single top quark production that is a factor two better than any previous search. We have not found significant evidence for single top quark production in our data. While this is not quite enough sensitivity to confirm or refute the standard model, we are nevertheless exploring a region sensitive to models of new physics beyond the standard model.

The story does not end here. The DØ detector has four times more data available for analysis. We are also improving the understanding of our detector and incorporating new analysis techniques. So, as a colleague of ours stated in a seminar, ''Where in the World is Single Top?''

The full article can be found here. A summary for high energy physicists can be found here. For more information on this analysis, please contact the primary authors: Aran Garcia-Bellido, Leonard Christofek, Ann Heinson, Supriya Jain, and Reinhard Schwienhorst.