The mass of the W boson particle was found by Large Hadron Collider to be exactly what it is Standard Model Particle physicists expect this to be the case, which contradicts previous results from Fermilab that suggested a different mass, and thus the possibility of new physics.
While this discovery bolsters the Standard Model as the best picture of the particle world, scientists had hoped that their model was actually wrong, and that the discrepancy in the W boson's mass could point the way to new theories that might explain mysteries such as the identity of particles. Dark matterwhich represents 85% of all matter in universe But it remains virtually invisible to us.
Bosons They are fundamental particles that carry Forces of nature. the strong force which connects Quarks together inside Protons and Neutrons It is carried by a boson called GluonThe electromagnetic force boson is the photon, and the weak force, which is responsible for radioactive decay, has three bosons: W+, W–, and the Z boson.
Measuring the masses of these particles is difficult, because they have an incredibly fleeting existence before they decay into other particles. So, with their best efforts, physicists first create bosons by colliding beams of protons traveling at nearly the speed of light. speed of light Inside a particle accelerator. For example, at the LHC, protons collide with a total energy of 13 trillion electron volts (eV). When they collide, the protons are forced to break apart into other particles, some of which are bosons (and thus particles are formed). Higgs boson(The Higgs field, which holds almost everything together and gives everything its mass, was discovered at the Large Hadron Collider.) Then the bosons themselves decay too, and the best way to measure their mass is to add up the masses of all the particles produced by the decaying bosons.
Related to: How the successor to the Large Hadron Collider will search for the dark universe
Bosons decay into particles called leptons (or antileptons), which are Electronsmuons or tau particles (the lepton is defined by a 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. This is why the mass of the Z boson is well known, at 91187.6 MeV with a margin of error of ±2.1 MeV.
However, the W+ and W– bosons decay into a lepton (or antilepton) as well as NeutrinoAnd here lies the problem.
Neutrinos are extremely tiny particles that are difficult to detect and can pass through detectors like ghosts. There are trillions of neutrinos coursing through your body right now, but you can’t tell. That’s why it takes a cubic kilometer of ice mixed with photomultiplier tubes at the IceCube neutrino observatory at the South Pole to detect them. The Large Hadron Collider can also detect neutrinos, but it only recently gained that capability with two detectors, FASER (Forward Search Experiment) and SND (Scattering and Neutrino Detector). The LHC is set to announce its first neutrino detections in August 2023.
The Standard Model predicts that the mass of the W+ and W– bosons is 80.357 MeV, ± 6 MeV, based on a theory that combines the electromagnetic and weak forces, called the “electroweak theory.” However, in 2022, physicists who reanalyzed old data from 2011 (produced by Fermilab’s Tevatron particle accelerator in Illinois, USA) determined the mass of the W boson to be 80.433 MeV, ± 9 MeV. This took the mass of the W boson out of the Standard Model. If correct, it would imply new physics such as “supersymmetry” (which assumes that every particle in the Standard Model has an additional, more massive counterpart) and loop quantum gravity (which describes how the fabric of the universe could be made up of tiny quantum loops). As a result, the physics world is very excited about these possibilities.
Unfortunately, that was not the case.
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 the interesting results from Fermilab, there was concern that ATLAS might have some unrecognized systematic error affecting its measurements.
However, new measurements of the W boson mass have been made by the Compact Muon Solenoid (CMS) experiment at the LHC, which are also consistent with the Standard Model, yielding a mass of 80360.2 ± 9.9 MeV. This corresponds to only 1.42 × 10^–25 kilograms.
“Essentially, we used a 14,000-ton scale to measure the weight of a particle with a mass of 1 × 10^–25 kilograms, or about 80 times the mass of a proton,” said physicist Michalis Bakhtis of the University of California, Los Angeles, in a study published in the journal Nature Community. statement.
Many physicists have of course hoped that a W boson mass discrepancy would be proven, as this would open the door to new physics that would be needed to explain this mass discrepancy. Taking supersymmetry as an example, this concept could point the way toward an explanation for dark matter. The leading candidate for dark matter at the moment is a type of particle called a WIMP, short for Weakly Interacting Massive Particle – a weakly interacting massive particle that would fit perfectly within the bounds of supersymmetry. Unfortunately, no supersymmetric partners have yet been found for particles in the Standard Model, and supersymmetry is far from proven.
“Everyone was hoping that we could measure it outside of theory, which would spark hopes of new physics,” says Bakhtis. “By confirming that the W boson mass is consistent with theory, we have to look for new physics elsewhere, perhaps by studying the Higgs boson at high precision as well.”
However, confirming the mass of the W boson opens the door to other things. For example, it is possible to use this mass measurement to better judge the strength of the Higgs field, or to better understand electroweak theory. These developments are possible because of the way CMS measured the mass of the W boson: by calibrating the energy of the emitted muons with a margin of error of just 0.01%, which is only 0.01% of the total mass of the W boson. Requests to immensity More accurate than previously thought possible.
“This new level of precision will allow us to handle critical measurements, such as those involving the W, Z and Higgs bosons, with increasing precision,” said PhD student Elisabetta Manca, who has been working on the project with Bakhtis for eight years.
So, the Standard Model wins again – but with cosmic mysteries like dark matter growing, Dark energy Even the Hubble tension, something in our understanding of physics will have to break at some point in order to light the way forward for the world of physics.
The results are described on the CERN website. CMS site.
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