Searching for the next generation

While three generations of quarks have been observed, it is possible that more are to be found. If so, it will take the high-energy collisions of the LHC to find them.

The ordinary meaning of a generation is an age group in a family. For instance, a child, parent and grandparent come from three different generations. However, in particle physics, the meaning is different. The Standard Model tells us that matter is made of subatomic particles called leptons and quarks. The up and down quarks, combined with the electron and electron neutrino, make up ordinary matter. We say that these particles form a generation. However, physicists have discovered two additional generations of matter. These additional generations are carbon copies of the original one, but with particles that are heavier and unstable. Restricting ourselves to the quarks, the second generation contains the charm and strange quarks, while the third generation consists of the top and bottom quarks.

Measurements at CERN’s former Large Electron-Positron collider suggested that there might be exactly three generations, no more and no less. This is a very peculiar state of affairs. Why three? Why not two? Or four? Or more?

To be more specific, what they really showed was that there were three light neutrinos. Since each generation seems to include a light neutrino, that might mean there are just three generations. That gives the universe a little wiggle room. Maybe there is a fourth neutrino that happens to be heavy. If that were true, then a fourth generation would be possible. Given that the higher-numbered generations are heavier versions of the lower-numbered ones, it’s even a reasonable supposition.

One way to be sure is to look directly for particles of a fourth generation. The CMS experiment looked for a fourth-generation version of the top quark, which is provisionally (and unimaginatively) called t′ (t-prime). The fourth-generation cousin of the bottom quark is called the b′. Following suggestions from earlier precision measurements, the experiment assumed that the t′ and b′ quarks were about the same mass. If this is so, then the t′ will decay into a W boson and the familiar third-generation bottom quark. Under these assumptions, CMS showed that either t′ quarks don’t exist or that they are more than 550 times heavier than a proton. With the additional data expected to be recorded in 2012, CMS will repeat the analysis, looking for even heavier quarks.

Don Lincoln

These physicists contributed to this analysis.
These are the U.S. members of the Computing Operations team that take care of all central workflows, like the production of simulated data sets and data re-reconstruction, and also keep the transfers and the Tier-0 running perfectly.