Scientists at the D0 experiment discover new path to the top

On 8 December, scientists from the D0 experiment at Fermilab’s Tevatron announced the first evidence for top quarks produced singly, rather than in pairs. The top quark has played a prominent role in the physics programme at the Tevatron ever since it was discovered there nearly 12 years ago. Just before the discovery in 1995, D0 collaborators were already turning their attention to the electroweak production of single top quarks, with theorists suggesting that the cross-section should be large enough to observe in the Tevatron’s proton–antiproton collisions.

A top quark is expected to be produced by itself only once in every 2 × 10¹⁰ proton–antiproton collisions, through the electroweak processes shown in figure 1. Although the cross-section is not much smaller than for top-quark pair-production, the signature for single top production is easily mimicked by other background processes that occur at much higher rates.

To stand a chance of observing the electroweak process, D0 physicists had to develop sophisticated selection procedures, resulting in around 1400 candidates selected from the thousands of millions of events recorded over the past four years (corresponding to 1 fb^-1 of collision data). The team expected only about 60 true single top events among all these candidates, so had to exploit every piece of information to establish the presence of the events.

The researchers used three different techniques (boosted decision trees, matrix element-based likelihood discriminants and Bayesian neural networks) to combine many discriminating features in ways that enable single top quark events to be recognized. In this way they effectively reduced the multidimensional system to a single, powerful variable.

With agreement among the three measurements, the D0 team finds the cross-section for single top quark production to be 4.9 ±1.4 pb, consistent with the Standard Model prediction (D0 Collaboration 2006). They estimate the chance of measuring this value as the result of a background fluctuation at less than 1 in 2800 (3.4 σ). This result establishes the first evidence for single top quark production.

The analysis also constrains the magnitude of |V_tb|, an important parameter of the Standard Model’s Cabibbo–Kobayashi–Maskawa (CKM) matrix, which describes how quarks can change from one type to another. If the CKM matrix describes the intermixing of three generations of quarks – with top and bottom forming the third generation – the value of |V_tb| should be close to one. Any departure from this value could be a sign of new physics, be it a new family of quarks or some unforeseen physical process. The D0 result provides the first opportunity for a direct measurement of |V_tb| and constrains it to lie between 0.68 and 1 with a 95% probability, consistent with the presence of only three generations of quarks.

In addition to its inherent success, this analysis is an important milestone in the D0 Collaboration’s continued search for the Standard Model Higgs boson. Higgs production is predicted to occur at rates smaller than single top quark production in the presence of substantial “irreducible” backgrounds (including single top). In this regard, D0 is developing a refined ability to “reduce the irreducible”, exemplified by this analysis and the recent evidence for the associated production of W and Z bosons. These high-level analyses and the detailed understanding of the growing data-set are becoming the backbone of D0’s search for the Higgs boson.