A Revolution in the Standard Model of Particle Physics? The

 "Overweight" W Gauge Boson from CDF II

Lei Wu

The Standard Model (SM) of particle physics is one of the most fundamental theories of physics. It describes the fundamental particles that make up our world and the three fundamental interactions between them: the electromagnetic force, the weak force, and the strong force. In the Standard Model, all interactions are mediated by exchanging gauge bosons.

Discovered in 1983, the W boson is an electrically charged fundamental particle in the SM. Together with its neutral partner, the Z boson, they mediate the weak force, one of the Universe's four fundamental forces, which governs certain types of radioactive decay and plays an important role in the nuclear reactions that power the Sun. However, the W boson mass is "notoriously" difficult to measure, because the W boson decays into an invisible neutrino plus a charged lepton in collider experiments.

After over ten years of careful work, the Collider Detector at Fermilab (CDF) experimental collaboration at the Fermi National Accelerator Laboratory announced that they have achieved the most precise measurement to date of the W boson mass in the world, Mw=80,433.5 +/- 9.4 MeV/c². The experimental results were published as a cover article in the April 7 issue of Science [1].

The new CDF result shows a seven-sigma deviation from the prediction of the Standard Model Mw= 80,357 +/- 6 MeV/c2, as shown in Fig. 1. A seven-sigma discrepancy is significantly higher than the five-sigma level that physicists normally claim as a definitive discovery. However, due to its inconsistency with other experimental measurements, including those from ATLAS and LHCb, the CDF result is in the process of being further investigated.

Fig. 1. Tension with the value expected based on the SM. The horizontal bars denote the uncertainty of the various measurements [1]

Furthermore, the mass of the W boson can also be determined by internal symmetries and other SM parameters. Changing the W boson mass would affect the theoretical predictions, such as top quark mass and Z boson mass, from the consistency test of the SM. Therefore, a global electroweak precision fit is now essential to uncover any secrets that may be behind the new data.

If the new CDF measurement holds, it may lead to a revolution in particle physics, and require new physics beyond the SM. One possible explanation is related to the "God Particle" Higgs boson, which was discovered at the Large Hadron Collider in 2012 [2,3]. If the Higgs sector is larger than the SM (i.e., if there are multi-Higgs bosons), it would bring new contributions to the W boson mass. Another possibility is that the new CDF result might indicate the existence of a supersymmetry theory, which is a space-time symmetry between two basic classes of particles: bosons and fermions. The supersymmetric particles would correct the W boson via the quantum effect, making the W boson heavier.

With the Sagan standard in mind, we still need to wait for confirmation from other existing experiments or future high energy lepton colliders [4,5] before we claim this is a major discovery.

[1] https://www.science.org/doi/10.1126/science.abk1781

[2] https://doi.org/10.1016/j.physletb.2012.08.020

[3] https://doi.org/10.1016/j.physletb.2012.08.021

[4] AAPPS Bulletin. Aug2020, Vol. 30 Issue 4, p56-78. 23p.

[5] AAPPS Bulletin. Dec2017, Vol. 27 Issue 6, p21-25. 5p.