|Clockwise from left: CMS event display, bubble chamber photograph, cloud chamber photograph.|
If you’ve ever seen computer displays like the one above, or old bubble chamber photographs, or even tinkered with a homemade cloud chamber, then you’ve seen particle tracks. Tracking is an important tool for particle physics experiments because tracks show you the comings and goings of individual particles. When coupled with a magnetic field, they also tell you the momentum of each particle, since slow particles curve in the field while fast ones fly straight through. Irène Joliot-Curie, daughter of Marie Curie and an early adopter of tracking for her radioactivity research in the 1930s, called it “the most beautiful phenomenon in the world, apart from childbirth.”
Nevertheless, tracking has some limitations. For one thing, it only reveals charged particles: Neutral particles leave no tracks unless they decay into charged particles. Also, a particle must survive long enough to enter the tracking detector to make a track. For particles with yoctosecond lifetimes (septillionths of a second), this is an issue. In practice, only five types of particles are commonly observed in tracking chambers: electrons, muons, pions, kaons and protons. The rest are inferred from the pattern of these particles’ trajectories or are identified by other techniques, such as calorimetry.
A new particle might show up as a new kind of track. The energy of collisions in the LHC is high enough to create new particles, even as-yet undiscovered particles. If a new particle is neutral and short-lived, like the Higgs boson, then it must be reconstructed from its decay products: a Higgs particle decays into two Z bosons, each of which decays into two electrons or two muons—an example involving four tracks. If, on the other hand, the new particle is charged and lives for tens of nanoseconds or more, it would pass through the detector for all the world to see.
In a recent paper, CMS scientists released results from a search for weird tracks hidden among the downpour of normal tracks from familiar particles. This analysis relied on unusual tools—the time that it took individual particles to fly through the detector and the amount of energy they lost along the way—finer detail than is needed for most analyses.
In the end, all was found to be consistent with known physics. These results rule out a variety of exotic theories with higher sensitivity than ever before.
|The U.S. physicists pictured above performed a careful search for tracks from heavy, charged, long-lived and as-yet undiscovered particles.|