In the CMS detector at the Large Hadron Collider, protons accelerated to nearly the speed of light collided. Among billions of such collisions, physicists searched for a rare signature: the production of a single heavy particle that immediately decays into a W boson and a b quark. The W boson lives for a tiny fraction of a second and decays into an electron or muon, while the b quark turns into a jet of particles, like the shards of a broken cup scattering across a table.
Such a signature could reveal a 'vector-like quark'—a hypothetical heavy relative of the ordinary quarks that make up protons and neutrons. If it existed, its appearance would stand out against the background of ordinary processes. But after analyzing data accumulated over several years of collider operation, no excess of events was found. It's like listening to hundreds of hours of ocean noise and not hearing a single foreign sound.
Such constraints are like narrowing the search area on a map: the more precisely we know where a particle is not, the fewer places remain to look. Perhaps the next generation of experiments or more refined analysis methods will finally hear that sound in the noise.
as written by the authors arXiv:2604.17564
A search is performed for the single production of a heavy vector-like quark (VLQ), decaying into a W boson and a b quark. The analysis uses proton-proton collision data collected by the CMS experiment at the CERN LHC at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 138 fb$$^{-1}$$. The search targets events with leptonic W boson decays. The event signature consists of one electron or muon, large transverse momentum imbalance, at least one jet consistent with coming from the fragmentation of a b quark and having large transverse momentum, and at least one jet in the forward region of the detector. No significant excess over the standard model prediction is observed. Upper limits are set at the 95% confidence level on the production cross section of a VLQ and its coupling $$\kappa_\mathrm{W}$$ to the standard model sector. For a VLQ decaying exclusively into Wb, the upper limit on $$\kappa_\mathrm{W}$$ depends on the VLQ mass and reaches values as low as 0.086 for masses around 1.4 TeV. For $$\kappa_\mathrm{W}$$ = 0.2 the lower limit on the VLQ mass is 2.4 TeV. These are the most stringent limits to date on the single production of VLQs decaying into Wb.
Vector-like quarks, if found, would be the first known particles to acquire mass differently from all others — not through the Higgs mechanism.
In Liu Cixin's novel 'The Three-Body Problem,' physicists encounter particles that violate familiar laws — searching for exotic quarks is somewhat like trying to catch such an anomaly in a real experiment.