In 2017, protons at the Large Hadron Collider collided with an energy of 5.02 TeV. The CMS detector recorded these collisions, and among billions of events, physicists hunted for a rare occurrence: the production of a single top quark without its antiparticle. Usually, top quarks appear in pairs — quark and antiquark together, like two sides of the same coin. But sometimes nature makes an exception and produces only one side.
To catch such an event, scientists looked for events with a single electron or muon and several jets of particles, one of which revealed its origin from a b-quark. With 302 inverse picobarns of accumulated luminosity (think of it as the volume of collected data), they were able to measure the probability of the process. It turned out that a single top quark is produced with a cross section of 17.6 picobarns, while a single top antiquark is produced with only 6.6 picobarns. The difference is nearly threefold: nature clearly prefers quarks over antiquarks, a long-known asymmetry tied to the workings of the weak interaction.
From these numbers, physicists extracted a quantity that shows how readily the top quark participates in weak decays. The result is 0.92 with an uncertainty of about 0.1. This agrees beautifully with predictions from the Standard Model, calculated to high precision. So far, everything is going according to plan, but more precise measurements at other energies could reveal discrepancies that point to new particles or forces.
The solo dance of the top quark at 5.02 TeV is now recorded for the first time. Next steps include more data and different collision energies to check whether this dance ever strays from the rhythm prescribed by theory.
as written by the authors arXiv:2603.13592
The single top quark $$t$$-channel production cross section is measured in proton-proton collisions at the CERN LHC at $$\sqrt{s}$$ = 5.02 TeV, using data recorded with the CMS detector in 2017, corresponding to an integrated luminosity of 302 pb$$^{-1}$$, and resulting in the first CMS measurement of the process at that energy. Events with one electron or muon and two or more jets, among which at least one is identified as originating from a b quark fragmentation, are analyzed. The combined cross section of single top quark (tq) and single top antiquark ($$\mathrm{\bar{t}q}$$) production is $$\sigma_{\mathrm{tq+\bar{t}q}}$$ = 25.4$$^{+3.6}_{-3.5}$$ (stat) $$^{+4.2}_{-3.9}$$ (syst) $$\pm$$ 0.5 (lumi) pb. The individual cross sections are measured to be $$\sigma_{\mathrm{tq}}$$ = 17.6$$^{+2.8}_{-2.7}$$ (stat) $$^{+2.6}_{-2.4}$$ (syst) $$\pm$$ 0.3 (lumi) pb and $$\sigma_{\mathrm{\bar{t}q}}$$ = 6.6$$^{+2.4}_{-1.6}$$ (stat) $$^{+2.1}_{-2.5}$$ (syst) $$\pm$$ 0.1 (lumi) pb. Their ratio is measured to be $$\mathcal{R}_{\mathrm{t-ch}}$$ = 2.7$$^{+1.5}_{-0.8}$$ (stat) $$^{+1.3}_{-0.3}$$(syst). The absolute value of the Cabibbo$$-$$Kobayashi$$-$$Maskawa matrix element is found to be $$\lvert f_{\mathrm{LV}}V_\mathrm{tb}\rvert$$ = 0.92 $$\pm$$ 0.09 (exp) $$\pm$$ 0.01 (thy). The measurements are in good agreement with the standard model predictions at next-to-next-to-leading order accuracy in quantum chromodynamics.
The top quark is the heaviest known elementary particle: it is heavier than a gold atom, even though it has no size.