Fermilab's TeVatron just released the best mass measurement of the W-boson, ever. Here's what doesn't add up.

The Standard Model, our most successful theory of elementary particles of all-time, has some very deep and intricate relationships between the properties of the different particles baked into it. Based on the measured properties of the other particles, the rest mass energy of the W-boson ought to be 80.35 GeV, but the latest results from the CDF collaboration reveal a value of 80.43 GeV, at a remarkable 7-sigma significance. This marks the first experimental particle physics result that disagrees with the Standard Model at such high significance. If there's no mistake, it could be our first clue to what lies beyond the known frontiers of physics.

In the entire history of science, no theory has been more successful, in terms of predictions matching the results of experiments and observations, than the Standard Model of particle physics. Describing all of the known elementary particles as well as three of the fundamental forces relating them ⁠— electromagnetism, the strong nuclear force, and the weak nuclear force ⁠— we’ve never once conducted an experiment whose results contradicted this theory’s predictions. Particle accelerators from Brookhaven to SLAC to LEP to HERA to Fermilab to the Large Hadron Collider have tried again and again, but have never once found a robust anomaly that’s held up to further scrutiny.

And yet, in a new paper published in the April 8, 2022 issue of Science, the Collider Detector at Fermilab (CDF) experimental collaboration just released their latest results, which offer the most precise measurements of the mass of one of those fundamental particles, the W-boson, ever. Although the Standard Model predicts its rest mass energy, exquisitely, to be 80.36 giga-electron-volts (GeV), the CDF collaboration instead found 80.43 GeV, with an uncertainty of just 0.0094 GeV attached to it. This represents a 7-sigma discrepancy from the Standard Model’s predictions: the most robust experimental anomaly ever seen. Here’s the science behind this incredible result, and what it means for the Universe.

The Standard Model is, in a nutshell, our modern theory of particle physics. It includes:

six flavors of quark with three colors each, along with their anti-quark counterparts,
three types of charged leptons and three types of neutral, left-handed leptons (the neutrinos), along with their anti-lepton counterparts,
the photon, which is the massless boson that mediates the electromagnetic force,
the eight gluons, which are the eight massless bosons that mediate the strong nuclear force,
the three weak bosons — the W+, the W-, and the Z — which have large masses and mediate the weak nuclear force,
and the Higgs boson, which is a scalar particles that couples to, and gives mass to, all particles that have a non-zero mass.
The Standard Model itself details the relationships between these various particles, such as what couples to and interacts with which other particles. However, there are some properties that can only be determined from measuring them, such as the masses of the individual fundamental particles.

One very important property that the Standard Model doesn’t give you wiggle-room for, however, is how the particles affect one another. If the top quark was much more massive than it is, for example, it would increase the mass of the proton, because the particles inside the proton couple to particles which also couple to the top quark. As a result, if you can measure the masses of all-but-one of the Standard Model particles, the rest of the Standard Model will tell you what that last particle’s mass ought to be.