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 February 24, 2014

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). 

Results

 The figure above shows the two-dimensional probability contour of the s and t production cross sections, as well as the predictions from different exotic theoretical models. The results are consistent with the Standard Model, and provide some constraints on models of new physics.

In the past few months, the CDF experiment at the Tevatron performed a similar analysis with its own data and found very similar results to D0's. The Rochester group and other scientists from both experiments have worked to combine the two measurements and obtain enough statistical significance to claim the first observation of the s-channel top quark production. The combined production cross section is measured to be: 1.29 ± 0.25 pb, in agreement with the Standard Model prediction of 1.05 ± 0.06 pb. The probability of observing a statistical fluctuation of the background to a cross section of the observed size or larger is 1.8×10-10, corresponding to a significance of 6.3 standard deviations for the presence of s-channel contribution to the production of single-top quarks. This new combination result has been submitted to Physical Review Letters. The Fermilab press release can be found here. This measurement will be one of the Tevatron's legacies and shows how the Tevatron can complement the physics reach of the LHC, given that some analyses are actually more difficult to perform at the higher energy of the LHC. This result completes the list of possible production modes of top quarks and imposes constraints on possible theories beyond the Standard Model.

The LHC will continue to provide useful insights from high precision measurements of the tt and t channels, and will almost double its collision energy in the Spring of 2015. Indeed, the LHC experiments have already discovered a new production mechanism (the associated Wt production, which was inaccessible at the Tevatron), but there are, alas, no signs of new particles or forces yet. 

Combination 

The summary of all different analysis and their combination.

Rochester researchers 

Prof. Garcia-Bellido and Dr. Tsai, after her PhD thesis defense (Dr. Tsai is currently a postdoc at the SLAC National Accelerator Laboratory in Menlo Park, CA).