Searching for a fourth generation

This analysis searched for another generation of quarks even heavier than the top quark discovered at Fermilab in 1995.

There are many fascinating measurements that have been made at particle colliders over the years, but perhaps my favorite was done at the LEP accelerator at CERN. After varying the beam energy, physicists made a plot showing the production of Z bosons in comparison to theoretical curves for the existence of two, three and four neutrinos. The data essentially demonstrated that the number of different types of neutrinos into which the Z boson could decay is identically three.

Physicists already know at least three types of neutrinos exist and that each neutrino is associated with its own specific lepton and two quarks. Each grouping of these particles is called a generation, with generation one consisting of the electron neutrino, the electron and the up and down quarks. Generation two includes the muon neutrino, muon and charm and strange quarks. Generation three consists of the tau neutrino, tau and top and bottom quarks. The precise value of three neutrino types suggested that there are exactly three different generations.

However, if we dig a little deeper, we find that the data actually only demonstrated that there were three light neutrinos into which the Z boson could decay. Additional heavy neutrinos were possible. Given that we see such a huge range of masses in the various quarks (for instance the top quark is about 100,000 times heavier than the up quark) it is at least conceivable that there could be a fourth generation with a heavy neutrino.

Naturally, the only way to be sure is to look for heavier particles of the fourth generation. These hypothetical particles don’t (yet) have the quirky names of the known quarks and are simply called t’ and b’ as analogs of the top and bottom quarks. CMS physicists looked for a b’ quark. Following the patterns observed in the lighter generations, we think that the b’ quark will decay into a top quark and a W boson. The top quark decays in the ordinary way into a bottom quark and another W boson. Because b’ quarks and antimatter quarks are produced at the same time, this doubles the particle count. Physicists then looked for events containing two bottom quarks and four W bosons.

Using the 2010 dataset (which is only a tiny fraction of the data expected to be collected in 2011) the analysis showed no evidence for the existence of a fourth generation, but it demonstrated CMS’ ability to quickly search for very complicated experimental signatures.

— Don Lincoln

These physicists performed this challenging analysis.
One strength of large modern collaborations is the intense review that each analysis undergoes before being submitted to a journal. This analysis was led by an excellent team of American and Taiwanese physicists. When the analyzers have done as complete a job as they know how, the analysis is then reviewed by an Analysis Review Committee. After ARC review, the analysis is further reviewed by some of the several thousand CMS collaborators and the physics coordination leadership. Only after the analysis passes all of this intense scrutiny is it sent to a journal for consideration. These physicists form the ARC for this analysis.