Connecting the dots

Learning to connect the dots is one of the first artistic tasks a child accomplishes. In today’s article, we learn how physicists accomplish the same task for a much more complicated puzzle.

When two protons collide in the center of the CMS detector, the collision energy can create hundreds of electrically charged particles. These particles roar through the apparatus, crossing individual detector elements. Each particle marks the location of its passage, leaving a string of dots that can be seen on a computer screen.

One of the trickiest jobs in particle physics is to teach a computer how to connect the dots and reconstruct the tracks of all of the particles that exited the collision. That’s correct: The child’s simple pastime of connect-the-dots can consume the efforts of many of the finest minds in an experiment like CMS. The difficulty stems from the fact that there are hundreds of tracks and that, in a bit of an inconvenient oversight, nobody bothered to put numbers beside the dots to tell the computer which to connect.

Reconstructing tracks is one of the first tasks that an experiment must accomplish in order to begin to analyze the data. Before the tracks are identified, the data is a mess of little dots. Once the tracks are determined, scientists can begin to sort out the physical process that occurred by figuring out that this particle went this way while another particle went that.

In addition to reconstructing the tracks of particles, scientists also reconstruct the origin of the particles. This is the location at which the collision between two protons occurred. Until you know the origin and trajectory of the particles, you can’t even begin to understand what sort of collision was recorded.

CMS scientists have worked long and hard to develop the algorithms to accomplish these challenging tasks. In a recent paper, they described the result of their efforts. Particles leaving the collision at angles near 90 degrees measured from the beam can be reconstructed about 94 percent of the time. For the special case of isolated muons, the reconstruction probability rises to 100 percent. The location of the origin of the collision can be localized with a precision about 0.01 millimeters, or about half the size of the finest human hair. These algorithms are fast and flexible, and scientists continue to improve on them in anticipation of the resumption of operations in early 2015.

Don Lincoln

These U.S. scientists contributed to this analysis.
The upgraded CMS tracking detector is based on the vision of our late colleague Simon Kwan.