High-precision measurement of the W boson mass with the CMS experiment
Article Date: 8 April 2026
Article URL: https://www.nature.com/articles/s41586-026-10168-5
Article Title: High-precision measurement of the W boson mass with the CMS experiment
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Summary
The CMS Collaboration reports a new, very precise direct measurement of the W boson mass using proton–proton collision data at √s = 13 TeV collected in 2016. The analysis uses 117 million W→μν decays (the largest sample ever used for an mW measurement) and a highly granular three-dimensional template fit in muon transverse momentum, pseudorapidity and charge. Advanced theory predictions (SCETLIB matched to NNLO calculations) and an extensive nuisance-parameter profiling strategy are used to constrain production modelling in situ rather than relying on Z-boson tuning. New, very tight muon momentum calibrations based on J/ψ (validated with ϒ(1S) and Z) and improved recoil and efficiency corrections are central to the result.
Key Points
- Measured W mass: mW = 80,360.2 ± 2.4 (stat) ± 9.6 (syst) = 80,360.2 ± 9.9 MeV.
- Dataset: 117 million W→μν events from 2016 pp collisions at 13 TeV (integrated luminosity 16.8 fb−1).
- Analysis method: binned maximum-likelihood fit to a 3D distribution (pTμ, ημ, qμ) using templates and in‑fit profiling of experimental and theory nuisance parameters.
- Main systematic uncertainties: muon momentum calibration (4.8 MeV) and parton distribution functions (PDFs, 4.4 MeV). Other theory uncertainties (pT(V) modelling) are constrained in situ using the W data and validated with Z measurements.
- The result agrees with the global electroweak fit (mW ≈ 80,353 ± 6 MeV) and with most previous measurements, but disagrees with the CDF II 2022 result (80,433.5 ± 9.4 MeV), so it strengthens the existing tension in the field.
- Robust cross-checks: W-like Z measurement, direct Z mass extraction, an alternative ‘helicity fit’ that relaxes production-model assumptions, and many stability tests (pileup, time periods, pT and mT variations).
- Additional measurement: mass difference mW+ − mW− = 57.0 ± 30.3 MeV (compatible with zero at ~1.9σ within uncertainties).
Author style
This is a punchy, high-impact precision result. CMS has pushed experimental calibration, data-driven constraints and modern resummed QCD predictions to produce an mW measurement with precision comparable to the most precise single result to date. If you follow precision tests of the Standard Model or the ongoing W-mass controversy, the technical details matter — the paper explains why PDF and muon-scale control dominate and how they were addressed.
Context and relevance
Precision on mW is a direct probe of the electroweak sector and a sensitive indirect search for beyond‑Standard‑Model physics. The world average before this result was mW = 80,369.2 ± 13.3 MeV; the global electroweak fit predicts mW = 80,353 ± 6 MeV. The CDF II 2022 measurement sits significantly higher and has produced substantial debate. CMS’s new, independent, high-precision result sits near the EW-fit value and away from the CDF value, so it is an important contribution to resolving that puzzle. The analysis also demonstrates techniques — J/ψ-based muon-scale calibration, in‑fit profiling of theory nuisance parameters, and SCET-based pT resummation — that will matter for future precision collider measurements.
Why should I read this?
Short version: if you care about whether the Standard Model still hangs together, or about tiny tensions that could be the first hint of new physics, this paper is worth your time. CMS has almost matched the best single‑experiment precision and used clever in‑data theory checks, so the result shifts the balance of evidence in the W‑mass debate. Skim the key figures and the uncertainty breakdown if you want the headlines; dive into muon calibration, PDF treatment and the TNP (theory nuisance parameter) approach if you want the guts.