Refutation of the Standard Model is cancelled.

The Large Hadron Collider puts an end to the W-boson fever. “You’ll have to look elsewhere for new physics,” says one of the researchers.

The mass of the W boson particle discovered at the Large Hadron Collider is exactly corresponds Standard Model of particle physics, which contradicts previous results from Fermilab that hinted at a different mass and therefore the potential existence of new physics.

Although this discovery further solidified the Standard Model as the best view of the particle world, scientists hoped that their model was in fact flawed, and that the discrepancy in the mass of the W boson could point the way to new theories that could explain mysteries such as the essence of dark matter, which makes up 85% of all matter in the Universe, but remains virtually invisible to us.

Bosons are the fundamental particles that carry the forces of nature. The strong force, which binds quarks inside protons and neutrons, is carried by a boson called a gluon, the electromagnetic force boson is a photon, and the weak force, responsible for radioactive decay, is carried by three bosons: the W+, W- and Z-boson.

Measuring the mass of these particles is difficult because they exist for an incredibly short time before breaking apart into other particles. So, with their best efforts, physicists first create bosons by colliding beams of protons traveling at almost the speed of light inside a particle accelerator. For example, at the LHC, protons collide with a total energy of 13 trillion electronvolts (eV). When they collide, protons decay into other particles, some of which are bosons (this is how the Higgs boson, responsible for the Higgs field that gives particles mass, was discovered at the Large Hadron Collider). The bosons themselves then also decay, and the best way to measure their mass is to add up the masses of all the particles that are formed as a result of the boson decay.

Bosons decay into particles called leptons (or antileptons), which are electrons, muons, or tau particles (lepton is defined by half-integer spin, i.e. 1/2 or 3/2). The Z boson decays into two other particles called muons, which are relatively easy to measure. That is why the mass of the Z boson is well known: it is 91,187.6 MeV with an error of ± 2.1 MeV (million eV).

The W+ and W- bosons, however, decay into a lepton (or antilepton) plus a neutrino, and this is where the problem lies.

Neutrinos are very weak, elusive particles that zip through detectors like ghosts. Trillions of neutrinos are even rushing through your body right now, but you cannot feel it. That's why detecting them requires a cubic kilometer of ice, strewn with photomultiplier tubes, located at the IceCube neutrino observatory at the South Pole. The Large Hadron Collider can also detect neutrinos, but this ability has only recently appeared thanks to two detectors – FASER (Forward Search Experiment) and SND (Scattering and Neutrino Detector). The LHC announced its first neutrino detections in August 2023.

The Standard Model predicts the mass of the W+ and W- bosons to be 80,357 MeV, ±6 MeV, based on a theory that combines the electromagnetic force with the weak force, called the “electroweak theory.” However, in 2022, physicists reanalyzing old data from 2011 (obtained at the Tevatron particle accelerator at Fermilab, Illinois, USA) determined the mass of the W boson to be 80,433 MeV, ±9 MeV. This brought the mass of the W boson out of the range of the Standard Model. If this were true, it would mean the emergence of new physical phenomena such as “supersymmetry” (which suggests that every particle in the Standard Model has an additional, much more massive counterpart) and quantum loop gravity (which describes that the fabric of the Universe may be composed of tiny quantum loops). As a result, the world of physics became excited about the possibilities.

Alas, this was not destined to come true.

In 2023, the ATLAS experiment at the LHC measured the mass of the W boson to be 80,360 MeV ± 16 MeV, which is indeed consistent with the Standard Model, but given Fermilab's tantalizing findings, there has been concern that ATLAS has some unacknowledged systematic error affecting its measurements.

However, new measurements of the W boson mass from the CMS (Compact Muon Solenoid) experiment at the LHC are also consistent with the Standard Model and give a mass of 80,360.2 ± 9.9 MeV. This corresponds to only 1.42 x 10^-25 kilograms.

“Essentially, we used a 14,000-ton scale to measure the mass of a particle that has a mass of 1 x 10^-25 kg, which is about 80 times the mass of a proton,” physicist Michalis Bakhtis of the University of California, Los Angeles, said in a statement. -Angeles.

Many physicists, of course, hoped that a discrepancy in the mass of the W boson would be proven, since this would open the door to new physics that would explain this discrepancy in mass. Using supersymmetry as an example, this concept could point the way to explaining dark matter. The leading candidate for dark matter right now is a type of particle called WIMP, which stands for Weakly Interacting Massive Particle—a massive, weakly interacting particle that fits perfectly into the framework of supersymmetry. Alas, at present, supersymmetric partners of Standard Model particles have not yet been found, and the theory of supersymmetry is far from being proven.

“Everyone hoped that we would measure it in isolation from theory and thereby raise hopes for new physics,” says Bakhtis. “By confirming that the W boson mass is consistent with theory, we are forced to look elsewhere for new physics, perhaps studying the Higgs boson with high precision.”

However, confirming the mass of the W boson opens the door to other things. For example, one could use this mass measurement to better estimate the strength of the Higgs field or to better understand electroweak theory. These achievements are possible because of the way CMS measured the mass of the W boson: by calibrating the energy of the emitted muons to an error of just 0.01%, which is orders of magnitude more precise than previously thought.

“This new level of precision will allow us to make critical measurements such as W, Z and the Higgs boson with increased precision,” says graduate student Elisabetta Manca, who has been working on this project with Bakhtis for 8 years.

So, the Standard Model wins again – but with the rise of cosmological mysteries such as dark matter, dark energy and even the Hubble tension, something in our understanding of physics must break at some point to illuminate the way forward for the world of physics.