Hunting for the platypus particle

Leptoquarks would be produced in pairs, and each would decay into a lepton (such as an electron) and a quark (which becomes a jet). This is one of the leptoquark-like events found in the CMS data set. There are too few like this to rule out standard physics explanations.

Last week’s Physics in a Nutshell described a hypothetical particle called a leptoquark. The leptoquark would have features of the far more familiar leptons and quarks, the way a platypus has features of ducks and beavers. If leptoquarks do exist, they would reveal a deep connection between these two fundamental classes of particles.

Physicists have been searching for leptoquarks for years, but have never found one. If they do exist, then they must have a higher mass than previous experiments were able to reach. Leptoquarks could also allow ordinary matter to spontaneously decay, something that has never been observed. If leptoquarks have a high mass, then fluctuations in ordinary matter would rarely reach it and decays would be too infrequent to have been noticed. Both of these considerations point to a high energy scale, so it’s worth looking for leptoquarks at the LHC, the highest-energy collider in the world.

CMS scientists searched through all of the data collected in 2011, which corresponds to about 500 trillion proton-proton collisions. They were looking for events in which a leptoquark and an anti-leptoquark were produced by the energy of the collision, each decaying into a lepton and a quark (or their antimatter equivalents). Some leptons, like the electron, leave a clean track through the CMS detector, while others, like the neutrino, are invisible and have to be inferred from an imbalance in the debris. A quark always produces a spray of particles known as a jet.

The search turned up a handful of events with these characteristics, but no more than would be expected from known physics processes. Therefore, this result set the most stringent limits yet on the mass of leptoquarks. CMS scientists are already hard at work examining the 2012 data, in which protons collide with a higher energy and therefore are capable of producing more massive leptoquarks, should they exist.

Why scour a mountain of data to search for a particle that might not exist? To paraphrase George Mallory, “because it could be there.”

—Jim Pivarski

The U.S. physicists pictured above made major contributions to this search for leptoquarks.
A track-based level 1 trigger for the high-luminosity environment at the LHC is a substantial technical and scientific challenge. These researchers from Cornell University have investigated 3D integrated circuit technologies using simulation, chip testing and university-based nanofabrication that may lead to these technologies being part of the CMS tracker upgrade.