It's often said in science that extraordinary claims require extraordinary evidence. Recent measurements of the mass of the elementary particle known as the W boson provide a useful case study as to why. Last year, Fermilab physicists caused a stir when they reported a W boson mass measurement that deviated rather significantly from theoretical predictions of the so-called Standard Model of Particle Physics—a tantalizing hint of new physics. Others advised caution, since the measurement contradicted prior measurements.
That caution appears to have been warranted. The ATLAS collaboration at CERN's Large Hadron Collider (LHC) has announced a new, improved analysis of their own W boson data and found the measured value for its mass was still consistent with Standard Model. Caveat: It's a preliminary result. But it lessens the likelihood of Fermilab's 2022 measurement being correct.
"The W mass measurement is among the most challenging precision measurements performed at hadron colliders," said ATLAS spokesperson Andreas Hoecker. "It requires extremely accurate calibration of the measured particle energies and momenta, and a careful assessment and excellent control of modeling uncertainties. This updated result from ATLAS provides a stringent test, and confirms the consistency of our theoretical understanding of electroweak interactions.”
As we've reported previously, 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.
The Standard Model 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 W boson—which turned 40 this year—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. We can't directly detect W bosons, so researchers have had to add up the mass and energy released 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.
For their 2022 measurement, Fermilab's 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, ±0.094. 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, ±0.019).
If it ever turns out to be correct, this higher mass would be evidence of possible as-yet-undiscovered particles affecting the W boson in some way. The most obvious candidates would be the exotic particles predicted by supersymmetry theory (SUSY), which calls for supersymmetric partners of all existing known particles in the Standard Model. The catch is that no particle accelerator to date, including the LHC, has yet uncovered any hint of SUSY particles in the data.
The 2022 measurement was roughly twice as precise (117 parts per million) as the previous best one, but the possibility of an unknown error creeping in couldn't be discounted. Additional independent measurements are needed to confirm the finding one way or the other, and this latest ATLAS measurement adds to all the evidence supporting the Standard Model's predictions for the W boson.
The ATLAS team essentially reanalyzed its 2011 data sample of W bosons on which their 2018 measurement result was based, using improved data-fitting techniques to determine the mass that do a better job taking into account how the proton's momentum is shared among its constituent quarks and gluons. The team also performed dedicated proton-proton runs to verify the W boson production process, reducing the systemic uncertainty there.
ATLAS ended up with a W boson mass of 80360 MeV with an uncertainty of 16 MeV (megelectron volts). That's 10 MeV lower than the previous ATLAS result and 16 percent more precise. This is still not the final word on the matter. The measurement will undergo rigorous review, and other physics experiments will continue to make their own improved measurements. There are also several proposed electron-positron colliders under consideration, which could be well-suited for an even more improved measurement of the W boson's mass.
