Single Top Quarks: How They Are Caught and Counted express
Inside protons accelerated to nearly the speed of light, a particle heavier than a gold atom is sometimes born — the top quark. It lives for tiny fractions of a second and decays, leaving a trace in the ATLAS detector. Physicists collected such traces for four years to understand how often the top quark and its mirror twin, the antiquark, appear.
It turned out that theory predicts these events fairly well, but not perfectly. The discrepancies are especially noticeable when the quark is emitted with a large transverse momentum, as if a ball, after being hit, flew a bit farther than calculations expected. This could simply be a measurement uncertainty, or it could be a hint of unknown forces.
To check whether new physics is hiding here, scientists used a special method: they looked for deviations in a single parameter of the theory. This parameter describes how four quarks interact simultaneously, bypassing ordinary forces. If it turned out to be nonzero, that would be a discovery. So far, the data say: the parameter is very small, but measurements continue.
In the end, the picture is almost clear: the Standard Model holds, but the edges of its predictions tremble slightly. Perhaps something new is hiding in these tremors, or perhaps we just need to measure more precisely.
Original abstract
as written by the authors arXiv:2601.04938
The production of single top quarks and top antiquarks via the $$t$$-channel exchange of a virtual $$W$$ boson is measured in proton-proton collisions at a centre-of-mass energy of 13 TeV at the Large Hadron Collider. The full Run 2 data sample recorded with the ATLAS detector in the years 2015-2018 is used, corresponding to an integrated luminosity of 140 fb$$^{-1}$$. The absolute and normalised production cross-sections are measured differentially as a function of the transverse momentum and absolute rapidity of the top quark and top antiquark. In addition, the ratio of top quark to top antiquark production cross-sections is measured. The measured distributions are compared with next-to-leading-order quantum chromodynamics predictions obtained with different combinations of matrix-element generators, parton-shower programs and proton parton distribution functions, as well as to next-to-next-to-leading-order calculations. Overall, good agreement is observed between the measurements and the theoretical predictions. For most measured distributions, the sensitivity to differences between the predictions is limited by the systematic uncertainties in the measurement. The measured differential distributions are also interpreted in an effective field theory approach to constrain the Wilson-Coefficient $$C_{Qq}^{3,1}$$ associated with a four-quark operator. The interpretation accounts for the effect of the selection efficiency, which is altered significantly by non-zero contributions from $$C_{Qq}^{3,1}$$.
The top quark is so heavy that producing it requires energy hundreds of times the proton's mass — which is why it can only be created in the most powerful accelerators.