In an extraordinary feat of precision physics, CMS measures the mass of the W boson, and finds it to be in good agreement with the prediction by the Standard Model of particle physics.
In the most precise measurement of its kind ever obtained at the LHC, the CMS collaboration at CERN reports the mass of the W boson to be 80360.2 ± 9.9 MeV. This puts the value of the mass in strong agreement with the Standard Model of particle physics, our current best theory governing the world of particle physics.
It is the first measurement with such precision in the challenging environment at the LHC, which is characterised by multiple simultaneous collisions. The decay of the W boson studied here emerges from just one of the collisions, while the rest generate a burst of additional particles, creating noise in the measurement. Honing in on the mass of the W boson is a cornerstone of the long-term CMS precision physics program.
Above: Experimental results for the mass of the W boson. The grey band represents the values in agreement with the Standard Model. The CMS result is highlighted in red, and agrees with the predicted mass with very high precision.
The W boson was discovered at CERN in 1983, but its mass has been particularly challenging to measure even 40 years later. The mass of the W boson is one of the key predictions of the fundamental parameters of the Standard Model, and therefore an incredibly important test of the model itself. This particular study started in 2016 and reaches an extraordinary precision, matching that of the CDF experiment’s recent result (shown above).
W bosons produced at the centre of the CMS detector decay almost instantly into a muon and a neutrino. Muons are detected by the CMS detector, but the elusive neutrinos escape detection. If both muons and neutrinos could be detected, the mass of the W boson would be measured directly from the energy and flight direction of the particles, as it is done for the Higgs boson, for instance. To tackle this challenge, researchers use the famous E=mc2 relationship between mass (m) and energy (E): the larger the mass, the larger the energy and momentum of the muons. Therefore, by studying the muon momentum, the team infers the value of the W boson’s mass with very high accuracy. After all, the ‘M’ in CMS stands for ‘Muon’ and refers to its ability to do exactly this: measure muons accurately and precisely.
A dedicated high-performance kinematics reconstruction algorithm was developed for this study. Its validation on a different particle, the Z boson, was a central element in the methodology. The Z boson has a very well-known mass and decays into two muons that are clearly detected by CMS. By pretending to only detect one muon, the Z can be treated as if it were decaying into a muon and a neutrino - exactly like the W boson. Then, the Z boson mass can be determined in the same way as for the W boson, and the results compared with the known mass of the Z. This is only possible because the reconstruction algorithm has been specifically designed to be independent of the Z boson, which has not been the case in previous measurements done by other experiments. A remarkable consistency is observed in these tests, excluding the presence of any source of experimental bias that could skew the measurement.
In addition to this, the team used the best-understood theoretical inputs and latest experimental techniques. Several auxiliary studies were performed to test these ingredients. In particular, an additional W boson mass measurement was performed in which the theoretical assumptions are relaxed and theoretical parameters are constrained by the data. While this implies slightly less precision on the W boson mass determination, the consistency of this cross-check measurement with the main one provides further confirmation of the robustness of the theoretical methodology employed.
Two years ago, the CDF collaboration at the Tevatron reported a new determination of the W boson mass with a sub-permille precision, better than all other measurements combined. The Tevatron was a proton-antiproton collider at Fermilab that operated between 1984 and 2011. The result deviates significantly from the Standard Model prediction and the average of the other measurements from CERN. This led many theorists to wonder about ways to supplement the Standard Model, advocating possible new physics effects to explain this discrepancy.
The much anticipated CMS result matches the remarkable precision of the CDF result while very strongly supporting the Standard Model value, giving the current model even more credence.
Above: A W boson candidate event seen in the CMS detector as part of the measurement of the mass of the W boson. In this event, a candidate W boson is produced, which decays to a muon and a neutrino (not seen). The muon is represented by the red line. The missing transverse momentum due to the neutrino is represented by the pink arrow. The display is interactive, and can also be viewed in a separate page here.
The Standard Model of particle physics is a very constrained theory: a few parameters impact several fundamental phenomena that are measured at colliders in various ways. By combining this indirect information in a global analysis (the ElectroWeak fit), one obtains an indirect determination of the W boson mass, to be compared to the direct measurement. This is one of the most crucial tests of the Standard Model: new phenomena – such as new particles, new forces, or new dimensions – that are not foreseen by the Standard Model, may alter this well-defined set of relations between different quantities.
These changes can be experimentally observed as a difference between the indirect determination of the W boson mass - the mass that would fit other observed phenomena - and its directly measured value, which would potentially yield evidence for new physics. This was the idea behind the Electro-Weak Precision Test program, carried out mostly at CERN and SLAC in the ’90s and early 2000s with the physics plan of the Large Electron Positron (LEP) collider and the SLAC linear collider (SLC), respectively. At CERN, four experiments (ALEPH, DELPHI, L3, and OPAL) were operated at LEP, hosted in the same tunnel where the Large Hadron Collider was built later on. At SLAC, the SLD detector operated at the SLC. These experiments were able to test the Standard Model to an astonishing precision.
Measuring the mass of the W boson with this level of precision is a remarkable feat for the CMS Collaboration and a testament to the capabilities of the detector and the ingenuity of the developed experimental techniques. CMS will continue to push the boundaries of precision measurements at the LHC and through the High Luminosity LHC.
Read more about these results:
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CMS Physics Analysis Summary (SMP-23-002): " Measurement of the W boson mass in proton-proton collisions at 13 TeV "
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Display of collision events: CERN CDS
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