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Higher W boson mass hints at chinks in Standard Model’s armor

Illustration of a candidate event for a W boson decaying into one muon and one neutrino from proton-proton collisions, recorded by the Large Hadron Collider's ATLAS detector in 2018.
Enlarge / Illustration of a candidate event for a W boson decaying into one muon and one neutrino from proton-proton collisions, recorded by the Large Hadron Collider’s ATLAS detector in 2018.

ATLAS Collaboration/CERN

The Standard Model of Particle Physics has withstood rigorous test after test over many decades, and the discovery of the Higgs boson in 2012 provided the last observational piece of the puzzle. But that hasn’t kept physicists from doggedly searching for new physics beyond what the model predicts. In fact, we know the model must be incomplete because it doesn’t incorporate gravity or account for the presence of dark matter in the Universe. Nor can it account for the accelerating rate of expansion of the Universe, which many physicists attribute to dark energy.

The latest hint as to how the Standard Model might need revising comes from a new precise measurement of the W boson by Fermilab’s CDF II collaboration. That measurement yielded a statistically significant higher mass for the W boson than predicted by the Standard Model—on the order of seven standard deviations, according to the collaboration’s new paper published in the journal Science. It also conflicts with prior precision measurements of the W boson’s mass.

“The surprisingly high value of the W boson mass reported by the CDF Collaboration directly challenges a fundamental element at the heart of the Standard Model, where both experimental observables and theoretical predictions were thought to have been firmly established and well understood,” Claudio Campagnari (University of California, Santa Barbara) and Martijn Mulders (CERN) wrote in an accompanying perspective. “The finding … offers an exciting new perspective on the present understanding of the most basic structures of matter and forces in the universe.”

That said, physicists have been here before: tantalized by hints of exciting new physics only to have their hopes dashed as more evidence came in. Extraordinary claims require extraordinary evidence, and this is definitely an extraordinary claim. “If true, it is important because the Standard Model would be wrong,” Clifford Cheung, a physicist at Caltech, told Ars. “But the clear disagreement in experiments warrants immense caution.”

The standard model of elementary particles, including anti-particles.
Enlarge / The standard model of elementary particles, including anti-particles.

The Standard Model describes the basic building blocks of the Universe and how matter evolved. Those blocks can be divided into two basic clans: fermions and bosons. Fermions make up all the matter in the Universe and include leptons and quarks. Leptons are particles that are not involved with holding the atomic nucleus together, such as electrons and neutrinos. Their job is to help matter change through nuclear decay into other particles and chemical elements, using the weak nuclear force. Quarks make up the atomic nucleus.

Bosons are the ties that bind the other particles together. Bosons pass from one particle to another, and this gives rise to forces. There are four force-related “gauge bosons.” The gluon is associated with the strong nuclear force: it “glues” an atom’s nucleus together. The photon carries the electromagnetic force, which gives rise to light. The W and Z bosons carry the weak nuclear force and give rise to different types of nuclear decay. And then there is the Higgs boson, a manifestation of the Higgs field. The Higgs field is an invisible entity that pervades the Universe. Interactions between the Higgs field and particles help provide particles with mass, with particles that interact more strongly having larger masses.

Experimental measurements and theoretical predictions for the W boson mass.
Enlarge / Experimental measurements and theoretical predictions for the W boson mass.

CDF Collaboration/Fermilab

The W boson is considered a key building block of the Standard Model, and improving the measurements of its mass helps physicists continue to refine and test the Standard Model. But it’s a tricky measurement to make. As Ars Science Editor John Timmer reported in 2012:

[The W boson] was first detected back in the 1980s at CERN’s SPS accelerator, which now forms part of the accelerator chain that feeds the LHC. Since then, various accelerators have produced enough Ws to provide an estimate of its mass, with all of them placing it at just above 80GeV, at an error range of about 100MeV….

Since we can’t directly detect W bosons with the hardware, researchers have had to add up the mass and energy that are liberated when it decays. This includes the energy carried by any photons, the mass and momentum of particles, and estimates of any energy carried away by fast-moving neutrinos, which pass through the detectors without a trace. The residual errors in the mass estimate come from the uncertainties in these various processes.

The CDF II team combed through 10 years of recorded data, amounting to about 4 million candidate W boson events, and came up with a mass of 80.433GeV, ±0.9.4. That’s inconsistent with previous measurements of the W boson’s mass, including those made by CDF II in 2012 (80.387GeV, ±0.02) and by ATLAS at CERN in 2018 (80.370GeV, ±19).

“It’s a puzzling result, as it is not only in significant tension with the Standard Model—which by itself would not be a bad thing as it would point to [new] physics likely searchable by the LHC—but also somewhat in tension with previous measurements,” Caltech physicist Michele Papucci told Ars.

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