A joint Fermilab/SLAC publication

Speedy single top sighting at the LHC

03/15/11
CMS scientists observed an excess in their data consistent with the production of single top quarks after combining just a couple of variables. Image courtesy of the CMS Collaboration.

CMS scientists observed an excess in their data consistent with the production of single top quarks after combining just a couple of variables. Image courtesy of the CMS Collaboration.

For the first time, scientists at the Large Hadron Collider have spotted single top quark production.

It took Fermilab scientists at the Tevatron more than 13 years to observe the particle in its solo state after they discovered it paired with its antiparticle in 1995. The CMS experiment teased signs of the single top out of 36 inverse picobarns of collision data, roughly what Tevatron experiments collect over three days.

“We were surprised we saw it so soon,” said CMS physicist Philip Coleman Harris, who announced the accomplishment on Monday at the Rencontres de Moriond conference. The ATLAS experiment also made public impressive limits on the single top quark.

Scientists took the news as one of many signs that experiments at the LHC are prepared to make great strides in the coming years.

Energy makes all the difference. Rare single top quarks appear buried in background particles in collisions at the Tevatron, which have a center-of-mass energy of about 2 TeV. The LHC’s center-of-mass energy in 2010 was more than three times that high, at 7 TeV. At that energy, the production gap between the number of single top quarks and the number of background particles widens significantly, making them much easier to pick from the pack.

CMS scientists also found evidence of the single top quark through a multivariate analysis using 37 variables. Image courtesy of the CMS Collaboration.

CMS scientists also found evidence of the single top quark through a multivariate analysis using 37 variables. Image courtesy of the CMS Collaboration.

At the Tevatron, the CDF and DZero experiments shook single top quarks from their data through a multivariate process, in which scientists combine data about dozens of variables in an algorithm to infer new information. At the LHC, CMS needed to look at only two variables. This simpler search gives scientists a new understanding of what they’ve found, said CMS Physics Coordinator Gigi Rolandi.

“When you have fewer variables, you understand the search better,” he said. “You can see if it shows the right behavior. A multivariate search is less intuitive.”

The CMS collaboration also found the single top quark using a multivariate search. But it’s the additional success of the simpler search that gives scientists the most hope for future hunting for new physics.

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