ATLAS / CERN cooperation
The Standard Model of Particle Physics has stood the test of time for many decades, and the discovery of the Higgs boson in 2012 provided the final observational piece of the puzzle. But that did not stop physicists from persistently looking for new physics beyond what the model predicts. In fact, we know that the model must be imperfect because it does not embody gravity or explain the presence of dark matter in the Universe. Nor can it explain the accelerating rate of expansion of the Universe, which many physicists attribute to dark energy.
The latest hint on how the Standard Model may need revision comes from a new accurate W boson measurement from Fermilab’s CDF II collaboration. This measurement yielded a statistically significant mass for the W boson than predicted by the Standard Model — of the order of seven standard deviations, according to a new collaborative paper published in the journal Science. It also conflicts with previous precision measurements of W boson mass.
“The surprisingly high W boson mass value reported by the CDF Partnership directly calls into question a fundamental element at the heart of the Standard Model, where both experimental observations and theoretical predictions are believed to be well established and well understood,” said Claudio Campagn. . University of California, Santa Barbara) and Martijn Mulders (CERN) wrote in an accompanying perspective. “The finding … offers an exciting new perspective on the current understanding of the most basic structures of matter and forces in the universe.”
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That being said, physicists have found themselves here in the past: they are tempted by hints of exciting new physics only to disprove their hopes as more evidence came. Extraordinary claims require great evidence, and this is certainly an excellent claim. “If it’s true, it’s important because the Standard Model would be wrong,” said Clifford Cheung, a physicist at Caltech, in Ars. “But clear disagreement in the experiments requires tremendous attention.”
Magnification / The standard model of elementary particles, including antibodies.
The Standard Model describes the basic building blocks of the Universe and how matter evolved. These blocks can be divided into two main breeds: fermions and bosons. Fermions make up all the matter in the Universe and include leptons and quarks. Leptons are particles that are not involved in the retention of the atomic nucleus, such as electrons and neutrinos. Their job is to help matter change through nuclear fission into other particles and chemical elements, using weak nuclear power. Quarks are the atomic nucleus.
Bosons are the bonds that connect the other particles to each other. The bosons pass from one particle to another and this creates forces. There are four “measurement bosons” related to strength. Gluon is associated with strong nuclear power: it “sticks” the nucleus of an atom together. The photon transmits the electromagnetic force, which creates light. W and Z bosons carry the weak nuclear force and cause different types of nuclear fission. And then there is the Higgs boson, a manifestation of the Higgs field. The Higgs field is an invisible entity that penetrates the Universe. The interactions between the Higgs field and the particles help to supply particles with mass, with the particles that interact more intensely having larger masses.
Advertisement Enlarge / Experimental measurements and theoretical predictions for boson mass W.
CDF / Fermilab cooperation
The W boson is considered a key building block of the Standard Model and improving its mass measurements helps physicists continue to improve and test the Standard Model. But it is a difficult measurement. As Ars Science author John Timmer stated in 2012:
[The W boson] was first identified in the 1980s at CERN’s SPS accelerator, which is now part of the LHC-powered accelerator chain. Since then, various accelerators have produced enough Ws to provide an estimate of its mass, all of which place it at just over 80GeV, in an error range of about 100MeV.
Since we could not directly detect W bosons with the material, the researchers had to add the mass and energy released when it decomposes. This includes the energy transferred by any photons, the mass and momentum of the particles, and estimates of any energy transferred by the fast-moving neutrinos, which pass through the detectors without a trace. Residual errors in mass estimation stem from uncertainties in these various processes.
The CDF II team examined 10 years of recorded data, amounting to approximately 4 million potential W boson events, and came up with a mass of 80.433GeV, ± 0.9.4. This is not in line with previous W boson mass measurements, including those made by CDF II in 2012 (80,387GeV, ± 0.02) and by ATLAS at CERN in 2018 (80,370GeV, ± 19).
“It simply came to our notice then. [new] “Physics is likely to be sought after by the LHC — but also at some intensity with previous measurements,” Caltech physicist Michele Papucci told Ars.
