A search for excited leptons is conducted much the way one might study these two objects. Kick the rock and the rock is unchanged. Kick the nest and hornets come out, which reveals that the nest has an inner structure. By “kicking” leptons and seeing if we can get them to emit photons or Z bosons, scientists are studying the question of whether leptons are built of smaller objects still.
On the other hand, history tells us to be cautious. Molecules gave way to atoms, which gave way in turn to protons, neutrons and electrons. While electrons have maintained their designation as structureless objects, the protons and neutrons no longer hold that mantle, with the quarks replacing them. So why couldn’t there be objects smaller than quarks and leptons?
Well, in principle, there could. However, despite years of effort, no evidence for these smaller building blocks have been found. On the other hand, the LHC is designed to probe energy regimes that have never before been studied. And since higher-energy beams means we can study smaller objects, scientists continually revisit this question.
In today’s analysis, CMS physicists restricted their investigations into finding out whether leptons (specifically electrons and muons) are made of even more fundamental constituents. To understand how they did that, imagine walking along and seeing a football-sized gray mass on the ground. Let’s use the figure above to illustrate two possible scenarios. One example might be a rock, which is a solid object with no inner structure. (Yes, I’m ignoring atoms. Work with me here.) Another example might be a ground-based hornets’ nest. Now a hornet nest definitely has structure, from the combs within it to the hornets themselves.
These U.S. CMS scientists from Kansas State University made important contributions to this analysis.
Now let’s kick these two examples. In the case of the structureless rock, nothing much happens. However, in the case of the nest, at least one hornet is certain to come out. (Probably more, but, again, work with me.) The fact that the nest emits something is a sign that it contains smaller constituents.
The CMS analysis is somewhat analogous. One tries to create leptons or leptons with smaller building blocks inside them. This second class is called excited leptons. The object emitted by an excited lepton might be a photon or a Z boson. So scientists looked for events in which a lepton and photon or lepton and Z boson were emitted. If we saw more of those than we expected, we’d have evidence that excited leptons exist and that leptons are governed by a set of building blocks and internal forces.
No evidence for excited leptons was found, which allowed the analyzers to rule out any building blocks larger than about 1/2,000 that of a proton. Although there are no building blocks larger than that, smaller ones could exist. CMS is extending this analysis using this year’s data taken at a collision energy of 13 trillion electronvolts. This added energy will allow researchers to find objects as small as 1/3,200 that of a proton.