The quarks and leptons of the Standard Model are assumed to be point-like. Note that this doesn’t require that they have no size; it merely means that if we make the simplifying assumption that they have zero size, then we can make predictions that are in good agreement with measurements.
The history of fundamental physics is littered with the corpses of physics models of particles once thought to have zero size, from atoms to the protons and neutrons in atomic nuclei. Each of them eventually was shown to have a measurable size and consequently to be made of yet smaller particles.
For about 50 years, scientists have been poking at the quarks and leptons, trying to see if they are made of something smaller. However, the simple fact is that the success of the Standard Model has actually stymied our progress. Physicists have theorized on what the building blocks that make up quarks and leptons might be, but because the data agrees so well with the point-like hypothesis, there is no universally agreed-upon theory describing the building blocks at the next-lower level. So physicists simply look at the quarks and leptons at higher and higher energy, which is equivalent to using a more and more powerful microscope. If we start to observe that there is a size at which the particles no longer act like point particles, we’ll have seen our first glimpse at the next-lower set of building blocks.
In order to understand how scientists do this, let’s think about planets. Newton’s law of gravity treats all objects—even planets—as point particles. If a comet swings by the Earth at a large distance and you use Newton’s equations to describe the comet’s path, you can reduce the Earth to a point, essentially replacing the physical Earth with a mathematical abstraction of the Earth that has all the Earth’s mass in a particle with zero size. The Newtonian predictions for the path of the comet in both scenarios will be identical.
However, the zero-size planet model falls apart if the path of the comet gets closer than about 4,000 miles from the center of the Earth. That’s because this is the size of the Earth, and the comet will crash into it.
This is an easy-to-visualize situation, but think about what it means. At a certain size (greater than 4,000 miles), Newton’s theory of gravity governs the motion of the comet. Below that size (under 4,000 miles), a different physical theory governs what happens to the comet. To more precisely describe what happens to the comet, then, you need to take into account the strength of the rock that makes up the Earth. Gravity is no longer the only governing theory.
Similarly, scientists look at quarks and leptons at higher and higher energies to see if they continue to act like point particles of the Standard Model or if it begins to look as if some new sort of physical phenomenon is starting to become important.
CMS physicists looked for events in which the collision of two protons resulted in very high-energy pairs of muons. The evidence is entirely consistent with a point-like quark and muons down to size scales about 1/10,000 that of a proton. The point-particle model of the muon continues to hold.
|These U.S. physicists contributed to this analysis.|