Observation of a new production mechanism of the heaviest elementary particle
Discovered at the Tevatron proton-antiproton collider in 1995, the top quark is the heaviest elementary particle. Studying its production and subsequent decays allows us to probe subtle effects and perhaps discover the existence of new forces or heavier particles that could interact with the top quark and modify its predicted behavior. In high energy hadron colliders, top quarks are predominantly produced in pairs of particle+antiparticle via the strong interaction (where gluons are exchanged), such as: pp → g → tt. Top quarks can also be produced singly, together with a bottom quark (b), or its antipartilce (b), or with other lighter quarks (q), via the electroweak interaction (exchanging a W boson): pp → W → tb, also called "s-channel" production, and pp → tbq, or "t channel". These different production modes can be separated by the different particles reconstructed in the final state and their different kinematics. The observation of the inclusive s+t channel production was only achieved in 2009 independently by the D0 and CDF experiments at Fermilab's Tevatron accelerator in Batavia, IL: 14 years after the top quark discovery! The D0 collaboration was first to observe the dominant t channel alone in 2011, after the Tevatron was closed and CERN's Large Hadron Collider (LHC) became the highest energy collider. But the interest to isolate and measure independently the s-channel has grown because it turns out that this particular production mechanism is quite difficult to measure at the LHC. Additionally, we expect that possible new boson particles would enhance only the s channel production, and possible new forces would only affect the t channel: it is therefore important to precisely measure both channels separately to test and possibly discriminate the presence of new particles of forces beyond those predicted in the Standard Model.
Rochester graduate student Yun-Tse Tsai led the analysis of the full Tevatron data collected at the D0 detector to measure precisely the s and t channels as her thesis project. Together with her advisor, Prof. Aran Garcia-Bellido, the Rochester team used a powerful technique to try to identify around 83 s-channel signal events in around 1300 observed events, and around 320 t-channel events from 9500 selected events. The Rochester researchers developed a new method to simultaneously measure the two single top-quark production channels with high precision, and thereby obtained a measurement without assuming the Standard Model prediction for either. They found an excess of events above the background-only prediction and reported the first evidence for the s-channel single top-quark production with a significance of 3.7 standard deviations (the result of this work was published in Phys. Lett B 726, 656 in 2013).