Mythology is replete with creatures that are exotic blends of more familiar animals, for example gryphons, mermaids and centaurs. Finding ordinary animals is commonplace, but discovering one of these blended ones would be a true triumph of science.
There are similarities in particle physics. For instance, the Standard Model contains the very familiar quarks and leptons. These two classes of particles have very different properties. Quarks feel all of the known subatomic forces and are found in the center of atoms. Leptons feel only two of the three known subatomic forces (they do not react via the strong nuclear force), and the most familiar lepton, the electron, orbits far from the atomic nucleus. Further, a single quark cannot convert into a single lepton, and vice versa. These really are quite different beasties.
However, the goal of particle physics is unification. We hope one day to generate a single, overlapping theory that contains but one type of particle and one type of force. We are very far from that goal and will need to somehow account for the existence of the very different quarks and leptons.
One possibility is that a quark and lepton can fuse to make a hybrid particle called a leptoquark. Leptoquarks would contain all the properties of quarks and leptons and would be a step on the path to building a unified theory.
Leptoquarks are speculative particles, and they pop up in many proposed theories. And, like any good researchers, CMS scientists studied their data to see if they could find evidence that supported the particle’s existence. After considerable effort, the CMS experiment submitted for publication not one, but two papers reporting on a leptoquark search. One paper looked for leptoquarks produced individually, while the other looked for leptoquarks produced in pairs.
No evidence was observed for the existence of leptoquarks, which means either that the idea is wrong or that the measurement didn’t have enough energy to make them. These two papers were reported using LHC data recorded in 2012 at an energy of 8 trillion electronvolts. CMS is recording data now at a much higher energy, and researchers are refining their analyses to dig into this new possible treasure trove. The hunt for leptoquarks isn’t over yet.