leptoquark
/ LEP-toh-kwork /
In the Standard Model, quarks and leptons are two separate castes that never directly turn into one another: a quark stays a quark, an electron stays a lepton, and the line between them is never crossed. A leptoquark is a hypothetical new particle that would straddle that line — a single particle carrying both the quark's color charge and a lepton's identity, able to absorb a quark and spit out a lepton, or vice versa. It is, in effect, a doorway between the two families nature seems to keep so strictly apart.
Such a particle is a natural prediction of theories that unify quarks and leptons, including many grand unified theories, since once the two families share a deeper home, a particle linking them is almost inevitable. A leptoquark would be heavy and carry fractional electric charge like a quark; in a collider it could be produced and then decay into a quark plus a lepton, for instance a jet of hadrons recoiling against an electron or a tau. Its mass and how strongly it couples to each generation are free parameters that experiment must pin down.
Leptoquarks have drawn fresh attention because of the 'flavor anomalies' — a handful of measurements of how often certain mesons decay to electrons versus muons that drifted away from Standard Model predictions, and which a leptoquark could elegantly explain. The candid caveat is that as data accumulated, some of the most striking of those anomalies faded back toward the Standard Model, so the case has weakened. Direct collider searches have also found no leptoquark and set lower limits on its mass. It remains a watched, but unconfirmed, possibility.
A leptoquark heavy enough to escape detection so far could still leave fingerprints: it might subtly tilt how often a B meson decays to a muon versus an electron, which is exactly the kind of 'flavor anomaly' that briefly made leptoquarks fashionable.
A leptoquark would turn a quark into a lepton — a bridge between two families.
Several of the flavor anomalies that excited interest in leptoquarks have since shifted back toward the Standard Model as more data arrived; a leptoquark remains an unconfirmed hypothesis, not a discovery.