AW, AZ and a higher quark

The world of elementary particles has something in common with ours: there are great inequalities in the properties of particles, as in the properties of human beings. The heaviest particle, the top quark, with its estimated mass of 172 GeV is five orders of magnitude heavier than the most common constituents of matter, the up and down quarks and the electron. Along with the top quark are three other heavy particles that we currently call elementary – perhaps just a label we need to use because we haven’t been able to break them further into smaller pieces: the Higgs boson (125 GeV), the Z boson ( 91 GeV) and the W boson (80 GeV).
These “elite” particles are very interesting to study, because their phenomenology is quite varied and rich. This is why at the Large Hadron Collider, CERN’s wonderful particle destroyer, we have been collecting images of their creation and decay since 2009. Now, getting a picture of each of them is already quite a feat: the most common of the four, the W boson, is produced only once in a million particle collisions, and the shyest, the Higgs boson, only once in three billion. And getting a pair of them in the same picture (with the exception of top-antitop quark pairs, which naturally get along well with each other) is even more difficult.

So you can imagine how difficult and rare it can be to get a trio of them in the same shot: it is no less a feat for a photographer who can imagine Bernard Arnault, Elon Musk and Jeff Bezos drinking together. The analog of this is what the CMS experiment was able to achieve recently, by identifying collisions that produced a top quark, a Z boson and a W boson in the same physical reaction.

The graph above is a technical representation of the reaction by which the heavyweight trio is produced by a proton-proton collision of the LHC. Time flows from left to right, so that the lines describe the history of the particles as they interact (where they join) and propagate through space (the vertical direction).

Below is a computer graphics reconstruction of the identified particles produced in one of the collisions seen by CMS, which are selected as “candidates” of the tWZ production process. If someone sold you an image as a genuine production of those three particles, you should report it to forensics (what? Isn’t there such a thing? Well, there should be one, if you ask me), as in the quantum world there can’t be there is no certainty about the identity of the phenomena. However, we can confidently state that the event belongs to a set whose elements have a significant possibility of being due to the reported process. Boring, yes, but accurate.

In the figure, the red line is a muon, produced by the decay of a W boson; the two green lines are the electrons produced by the decay of a Z boson; the two yellow cones represent hadronic jets produced by the decay of another W boson; and the orange cone is a jet of quarks b. The decay of the top quark produced the second W boson, so it could be argued that the picture actually depicts not three, but four heavy particles!

By collecting and studying events like the one pictured above, CMS researchers can study the properties of weak interactions in detail and try to see if everything is in order with theoretical predictions. The rationale is that when you study phenomena that theory predicts to be extremely rare, your chances of spotting the contribution of some very weak additional physical process become much greater. Now, we know that if there are new physical processes (new forces of nature, for example) they must very weakly influence the phenomenology of elementary particles as we know it: therefore, it makes sense to magnify very rare processes.

So far, the Standard Model has continued to deny us the satisfaction of finding something it doesn’t predict or include. But keep searching, and perhaps one day we will be able to better understand the most complex and wonderful puzzle that Nature has set us to solve.

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