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This paper reports a set of recent measurements and searches from the LHCb experiment that probe both the Standard Model and possible new particles. Using proton–proton collision data collected in different years, LHCb measured properties of the Z and W bosons and of the top quark in the detector’s forward region. The collaboration also searched for two types of hypothetical “mediators” that could connect ordinary matter to a dark sector: axion‑like particles (ALPs) and heavy neutral leptons (HNLs). These searches found no signals and set new exclusion limits.
What the researchers did: LHCb used several datasets. A Z‑boson mass was measured from Z→µ+µ− decays in 2016 data with 1.7 inverse femtobarns, giving mZ = 91,185.7 ± 8.3 (stat) ± 3.9 (syst) MeV. A model‑independent proof‑of‑concept W‑boson mass determination was performed using 100 inverse picobarns at 5.02 TeV, reporting mW = 80,369 ± 130 (exp) ± 33 (theory) MeV; the paper notes this does not supersede the earlier 13 TeV result but demonstrates a novel method. Production cross sections for W bosons were measured at 13 TeV using 5.1 inverse femtobarns, and top‑quark production was measured for the first time in the forward region using 5.4 inverse femtobarns of 2016–2018 data.
How the measurements work at a high level: LHCb’s detector covers particles produced at small angles to the beam, the so‑called forward region. That acceptance gives access to partons (the quarks and gluons inside the proton) at unusually low or high momentum fraction, which helps constrain parton distribution functions (PDFs) that feed into many theoretical predictions. The Z mass was extracted from the two muon final state after careful momentum calibration using known resonances. The W mass study used differential muon distributions and a strategy that separates detector effects from theoretical signal modelling so the signal model can be updated independently. The top analysis identified b‑quark jets with a new deep‑learning tagger that improves b‑ and c‑tagging efficiencies by 11–53% compared to older methods.
Why it matters: Precision measurements of W, Z and top properties test the internal consistency of the Standard Model and provide inputs that reduce uncertainties in other searches. The forward measurements from LHCb are complementary to central detectors and can tighten constraints on PDFs. The searches for ALPs and HNLs test well‑motivated ideas for dark‑sector mediators. For ALPs decaying to two photons, LHCb sets the best limits on the symmetry‑breaking scale f_a for ALP masses between about 4.9 and 10 GeV and reports the experiment’s first search in a fully neutral final state. For HNLs produced in B meson decays and decaying to µπ, the collaboration sets new upper limits on the mixing parameter |U_{µN}|^2 across HNL masses from 1.5 to 5.5 GeV, improving on Run‑1 limits by roughly an order of magnitude.
Important caveats and uncertainties: Some results are expressly proof‑of‑concept and limited by the amount of data or by theoretical modelling. The model‑independent W mass approach is demonstrated on a small 100 pb−1 dataset and the authors note that combinations of different W mass measurements are limited by dependence on theoretical signal models. All direct searches reported no candidates, so they produce exclusion limits rather than detections. The paper also indicates that larger datasets from LHC Run‑3 and upgraded tracking techniques (including long‑lived “T‑track” signatures) will be needed to extend sensitivity and to study additional HNL decay modes. The excerpt may be truncated and contains the main results and limitations presented by the collaboration.