The Z boson’s heavy cousin

Just like General Mills mascots Sprout and the Jolly Green Giant are small and large versions of one another, the hypothetical Z’ boson is a more massive cousin of the familiar Z boson. No evidence has been found that supports the existence of a Z’ boson.

Since its discovery in 1983, the Z boson has been a workhorse for particle physics. Z bosons are the particles that mediate the weak nuclear force, and they can decay into any of the known quarks and leptons except for the top quark. This versatility is one of the reasons that experiments using CERN’s LEP accelerator, active from 1989 to 2000, were so prolific. By making Z bosons, it was possible to investigate an impressive spectrum of different phenomena.

One phenomenon seen in quarks and leptons is particle generations. Particle generations are groups of particles, with each group having strikingly similar properties. The first generation consists of up and down quarks, electrons and electron neutrinos. From these fundamental particles, you can create all of the familiar matter of the cosmos. There are two additional generations, one that consists of the charm and strange quarks, muons and muon neutrinos, and another that consists of top and bottom quarks, taus and tau neutrinos. The second and third generations are very similar to the first, but their members are unstable. Nobody really understands why there is more than one generation of quarks and leptons.

Scientists would like to know whether the multiple generations seen in quarks and leptons also apply to force-mediating bosons. Thus far, each of the force-carrying particles of the Standard Model (W and Z bosons, photons and gluons) seems to be unique. That is, there is no evidence to hint at generations in the force-mediating bosons. On the other hand, there are several speculative theories that predict the existence of heavier cousins of the W and Z bosons. The unimaginative names of these hypothetical particles are the W’ (W prime) and Z’ (Z prime) boson.

CMS scientists have searched for a heavy Z’ boson that decays into a top quark-antiquark pair. This particular decay mode is interesting because producing top quarks via ordinary Standard Model physics is rather rare, making it easier to isolate and identify collisions in which a Z’ might have been made. While the decay of Z’ bosons into top quarks is the only decay channel considered, top quarks decay in a plethora of ways. Accordingly, this particular search actually required three unique analyses (analyses one, two and three), including an analysis that has already been featured in Fermilab Today.

CMS scientists observed no evidence for the existence of Z’ bosons. The Standard Model has resisted yet another attempt to topple it from its pedestal.

These physicists contributed to this analysis.
Charged particle reconstruction is an essential ingredient in any physics analysis. Tracks are used to find vertices, to tag b-jets, to reconstruct electrons and muons and to seed the particle flow algorithm. During 2012, these U.S. physicists made crucial contributions to improve the existing algorithms and monitor the tracking performance.