The mass of the W boson particle has been discovered by the Giant Hadron Collider to be precisely what the Customary Mannequin of particle physics predicts it to be, contradicting earlier outcomes from Fermilab that hinted at a unique mass and, subsequently, the potential for brand spanking new physics.
Whereas the invention additional cements the Customary Mannequin as our greatest depiction of the particle world, scientists had been hoping that their mannequin was truly fallacious, and that the discrepancy within the mass of the W boson may level the way in which to new theories which may clarify puzzles such because the id of darkish matter, which accounts for 85% of all of the matter in the universe but stays successfully invisible to us.
Bosons are elementary particles that carry the forces of nature. The sturdy power that binds quarks collectively inside protons and neutrons is carried by a boson referred to as the gluon, the electromagnetic power’s boson is the photon, and the weak power, which is chargeable for radioactive decay, has three bosons: W+, W– and the Z boson.
Measuring the lots of those particles is hard, as a result of they’ve an extremely fleeting existence earlier than they decay into different particles. So, with their finest efforts, physicists first create the bosons by colliding beams of protons touring at nearly the velocity of sunshine inside a particle accelerator. For instance, on the LHC, the protons collide with a complete vitality of 13 trillion electronvolts (eV). Upon collision, the protons are compelled to smash aside into different particles, a few of that are bosons (that is how the Higgs boson, which carries the Higgs subject that just about provides every part its mass, was found on the Giant Hadron Collider). The bosons themselves then additionally decay, and the easiest way to measure their mass is to mix the lots of all of the particles that the decaying bosons produce.
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Bosons decay into particles referred to as leptons (or antileptons), that are electrons, muons or tau particles (a lepton is outlined by a half-integer spin, so 1/2 or 3/2). The Z boson decays into two additional particles referred to as muons, that are comparatively simple to measure. That is, in actual fact, why the Z boson’s mass is well-known, with a price of 91,187.6 MeV and an error margin of ± 2.1 MeV (million eV).
The W+ and W– bosons, nonetheless, decay right into a lepton (or antilepton) plus a neutrino, and that’s the place the issue lies.
Neutrinos are very slight, elusive particles that may zip via detectors like ghosts. Trillions of neutrinos are even operating via your physique proper now, however you possibly can’t inform. That is why it takes a cubic kilometer of ice laced with photomultiplier tubes on the IceCube Neutrino Observatory on the South Pole to detect them. The Giant Hadron Collider can detect neutrinos as effectively, but it surely has solely acquired this functionality just lately through two detectors, FASER (the Ahead Search Experiment) and SND (Scattering and Neutrino Detector). The LHC introduced its first neutrino detections in August 2023.
The Customary Mannequin predicts that the mass of the W+ and W– bosons is 80,357 MeV, ± 6 MeV, based mostly on a concept that mixes the electromagnetic power with the weak power, referred to as “electroweak concept.” Nevertheless, in 2022, physicists who re-analyzed outdated information from 2011 (produced by Fermilab’s Tevatron particle accelerator in Illinois, USA) decided a W boson mass of 80,433 MeV, ± 9 MeV. This took the W boson mass out of the vary of the Customary Mannequin. If it was appropriate, then it implied new physics similar to “supersymmetry” (which posits that each particle within the Customary Mannequin has a further, rather more huge counterpart) and Quantum Loop Gravity (which describes how the material of the universe may be product of tiny quantum loops). In consequence, the physics world turned very excited by the chances.
Alas, it was to not be.
In 2023, the LHC’s ATLAS experiment measured the mass of the W boson as 80,360 MeV ± 16 MeV, which is certainly in step with the Customary Mannequin — however given Fermilab’s tantalizing findings, there was a priority that ATLAS had some unrecognized systematic error affecting its measurements.
Nevertheless, new measurements of the W boson’s mass have been made by the LHC’s Compact Muon Solenoid (CMS) experiment, and are additionally in keeping with the Customary Mannequin, producing a mass of 80,360.2 ± 9.9 MeV. This corresponds to simply 1.42 x 10^–25 kilograms.
“Principally, we used a 14,000-ton scale to measure the burden of a particle that has a mass of 1 x 10^–25kg, or about 80 instances the mass of a proton,” physicist Michalis Bachtis of the College of California, Los Angeles, mentioned in a assertion.
Many physicists had after all been hoping there would show to be a discrepancy within the mass of the W boson, as this might have opened the door for brand spanking new physics that might be required to elucidate that discrepancy mass. Taking supersymmetry for example, this idea may level the way in which in the direction of explaining darkish matter. A number one candidate for darkish matter proper now’s a kind of particle referred to as a WIMP, which stands for Weakly Interacting Large Particle — and a large, weakly interacting particle would match completely throughout the confines of supersymmetry. Alas, at the moment no supersymmetric companions to particles within the Customary Mannequin have but been discovered, and the idea of supersymmetry is much from confirmed.
“All people hoped we’d measure it away from the idea, igniting hopes for brand spanking new physics,” mentioned Bachtis. “By confirming that the mass of the W boson is in keeping with the idea, we have now to seek for new physics elsewhere, possibly by finding out the Higgs boson with excessive precision as effectively.”
Nonetheless, confirming the mass of the W boson does open the door to different issues. For instance, it is doable to make use of this mass measurement to raised decide the energy of the Higgs subject, or to raised perceive electroweak concept. These developments are choices due to the way in which the CMS measured the W boson mass: by calibrating the vitality of the emitted muons with margin of error of simply 0.01%, which is orders of magnitude extra exact than what had as soon as been thought doable.
“This new stage of precision will enable us to sort out important measurements, similar to these involving the W, Z and Higgs boson, with enhanced accuracy,” mentioned Ph.D. pupil Elisabetta Manca, who has been engaged on this undertaking with Bachtis for 8 years.
So, the Customary Mannequin wins once more — however with growing cosmological mysteries similar to darkish matter, darkish vitality and even the Hubble rigidity, one thing in our understanding of physics goes to have to interrupt in some unspecified time in the future to gentle the way in which ahead for the world of physics.
The findings are described on CERN’s CMS web site.