The Collider Detector at Fermilab. Image: Fermilab
Scientists have discovered a potential challenge to established physics embedded in the mass of the W boson, a particle that carries a fundamental force of nature, which has now been measured with unprecedented precision as part of a decade-long international effort.The W boson is the messenger particle of the weak nuclear force, also known as the weak interaction, a phenomenon that exists inside atoms. It constitutes one of four basic forces in the universe, along with gravity, electromagnetism, and the strong nuclear force.
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The particle’s mass has been probed by many experiments, but a new measurement by the Collider Detector at Fermilab (CDF) collaboration, a group of some 400 scientists who worked with the Tevatron particle collider at the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Illinois, has achieved twice the precision of its predecessors. These experiments can shed light on unresolved questions in science, such as the nature of dark matter, an unidentified substance that makes up most of the matter in the universe. Most importantly, they act as important tests of the Standard Model of particle physics, a well-corroborated theoretical framework of the forces that govern reality.That’s why the new CDF measurement is such a bombshell: It confirms that the mass of the W boson, which is “one of the most important parameters in particle physics,” is far heavier than predicted by the Standard Model, exposing a “significant tension” between models and experiments that may ultimately point to new physics, according to a study published on Thursday in Science. “We were very pleasantly surprised,” said Ashutosh Kotwal, the lead author of the new study and the Fritz London Professor of Physics at Duke University, in an email. “We were so focused on the precision and robustness of our analysis that the value itself was more like a wonderful shock.”
“This result could not have been achieved without the beautifully built and operated CDF experiment, whose superb performance enabled such a detailed and robust analysis of the data,” he continued. “It is also a testament to all the skilled scientists who worked behind the scenes on the Tevatron collider to deliver the massive amount of data. I feel an enormous sense of gratitude to my CDF collaborators and Tevatron colleagues for providing this opportunity and I am very pleased that these data contain such valuable information.” Kotwal and his colleagues pinned down the mass of the W boson to a value of 80,433.5, which is measured in megaelectronvolts divided by the speed of light squared. The new mass is a full seven standard deviations away from what is predicted by the Standard Model, suggesting that we are missing a major piece of the puzzle with regards to the forces that shape the universe. If future experiments corroborate the CDF’s measurement, it’s possible that scientists will need to develop novel physics to account for the discrepancy. “If confirmed, the difference between the measured value and the Standard Model calculation of the W boson mass would have to be due to a new mechanism in nature, a new fundamental principle we do not know about,” Kotwal said. “This could manifest as a new particle or subatomic interaction and be discovered in running and future experiments.”
“The Standard Model is known to be incomplete because it does not explain the dark matter in the universe, nor the excess of matter over antimatter,” he continued. “Our measurement is in direct contention with the Standard Model, which to date has been the most successful quantum theory of matter and forces.” The new measurement is based on a sample of 4 million W bosons generated in the aftermath of collisions between protons and their antimatter counterparts, antiprotons, during an experiment that ran at the Tevatron collider from 2002 to 2011. The results are “more precise than all previous measurements of [the W boson’s mass] combined,” according to the study, thanks to a host of updates introduced by the CDF collaboration, including “new information about the colliding proton's structure that the particle physics community has collected over the last decade,” Kotwal noted.“Importantly, our analysis procedures demonstrate a number of very precise checks of internal consistency, which no other analysis has demonstrated at this level,” he continued. “The combination of a four times larger dataset, more insightful methods and ideas of using our data, and new information about the proton structure allowed us to improve the precision of this measurement substantially.”But though the CDF collaboration has set a high benchmark for studying this important particle’s mass, the findings will have to be corroborated by other experiments before any sweeping conclusions about the efficacy of the Standard Model can be made. The Tevatron Collider is now retired, but the Large Hadron Collider is also probing the mass of the W boson. In addition, the development of new equipment, such as an electron-positron collider, could further refine the mass of the particle while also shedding light on the nature of dark matter, strange theories like supersymmetry, and the properties of the influential Higgs boson particle.“The theoretical physics community will be taking a close look at the calculations,” Kotwal said. “They will be exploring extensions of the Standard Model that could bring the theory in line with our measurement. These ideas could motivate a new round of experiments that will be sensitive to the new physics.”