Let’s talk top

The LHC can be considered a factory for creating top quarks. Since the top quark is so heavy, high precision studies of its properties are thought to be a promising way to search for new high energy phenomena.

In March 1995, the discovery of the top quark was announced at the Fermilab Tevatron. With fewer than one hundred collisions between them, the DZero and CDF experiment were able to estimate the mass of the top quark and the production probability, but the estimates were relatively imprecise. Of course, discovery was only the beginning of the story. In the 15 years that followed, about 200 times as much beam was delivered to each of the experiments, resulting in a substantial improvement in the number of top quarks produced. By the time the Tevatron stopped running in the fall of 2011, both CDF and DZero produced about 75,000 events with top quarks.

The LHC has been running for a much shorter time, with nearly all of the beam delivered in 2011. The LHC has been colliding beams with 3.5 times as much energy as the Tevatron. The beams were also brighter, meaning that there were more collisions per second. The higher collision energy makes it easier to produce pairs of top quarks. About 770,000 top quark events have already occurred in the CMS detector, over 10 times what was observed in the Tevatron experiments. In the upcoming year, it is expected that the LHC will deliver perhaps three to five times more data than has been seen so far. In a couple of years, when the LHC is running under design conditions, one pair of top quarks will be created every second. The LHC really is a high-precision top quark factory.

CMS has recently published the results of an extensive study of top quark production. While this measurement has only explored less than one percent of the data CMS has recorded so far, the precision of the study is already impressive. The analysis was also cross-checked with many different techniques and strategies, varying to optimize the extraction of the measurements.

The CMS experimenters are carefully reviewing the larger data set, extending the studies. As the number of recorded events becomes larger, the collaboration’s detailed understanding of detector performance starts to limit the precision of their measurements. Ongoing studies are focused on achieving the required performance by the detector and this will lead to a substantial improvement in the measurements. These improvements are critical as top quarks stop being discovery material and become ordinary; merely backgrounds or decay products for searches for even rarer phenomena.

—Don Lincoln

These physicists worked on this complex analysis. Mikhail Makouski, Kansas State University (not pictured), also contributed.
This group of physicists has worked on algorithms to find bottom quarks and applied them to identify events in which top quarks are produced.