Top Top Z

The author has been waiting a long time to use this joke.

If you listen in on particle physics conversations, you’ll hear a lot of alphabet soup, such as “b to s gamma,” “Z to tau tau” and “q q-bar to X.” Reactions among particles provide a view to the underlying physics: You can learn how particles are related by how willing they are to collide and how often they decay a particular way.

I’ve long held a secret hope that someone would one day discover the “Z Z top event” (two Z bosons and a top quark), but this combination is just too rare. CMS has recently announced the next best thing: top top Z.

That is, in a large collection of proton-proton collisions, CMS scientists found that some of them produced a pair of top quarks and a Z boson. This shows that top quarks can interact with Z bosons, just like all other quarks.

This is not an assertion to be taken lightly. Top quarks are different from all other quarks, primarily because of their exceptionally high mass — 35 times heavier than the second heaviest. This difference in mass is responsible for many of the unique properties of the top quark. For instance, Z bosons decay into quark-antiquark pairs for every type of quark except top. Z bosons cannot decay into top quarks because they are heavier than the Z boson itself. Thus, it is well known that Z bosons interact with all other types of quarks, but only now do we learn that they interact with top quarks as well.

This analysis is challenging because the detector signature for top top Z resembles many other types of events, collectively called backgrounds. In particular, top top W has many features in common with top top Z because W and Z are both weak force bosons with similar masses. This analysis improves upon a previous one in that it distinguishes the W from the Z, measuring their rates separately (though top top W by itself cannot be clearly distinguished from its backgrounds).

Another challenge is that these event types, top top W and top top Z, are both exceedingly rare: a hundred times less common than Higgs production. This analysis is therefore one example of progress beyond the Higgs.

Jim Pivarski

These U.S. scientists contributed to this analysis.
These physicists used an X-ray source to test modules of the CMS forward pixel detectors. This method allows careful study of the detector components and validation of the electronics’ design before they are put into the CMS detector.