General Physics Colloquium, Professor Michalis Bachtis, UCLA

Title:  How heavy is the W boson?  Measuring the mass of about 80 times the mass of a proton with a 14,000 ton scale

Host:  Jennet Dickinson

Abstract:  Precision measurements of the fundamental parameters of nature could reveal deviations that arise from new physics phenomena. The W boson is the mediator of the weak force responsible for radioactivity and has a mass of about 80 times the mass of the proton. While the W boson mass is a free parameter of the theory of particle physics known as the Standard Model, its measured interactions with other particles constrain the mass value to a narrow range. A W mass measurement outside this range would hint at new physics. The CDF experiment at Tevatron published the most precise measurement of the W boson mass in 2022 that shows a discrepancy of about seven standard deviations from the theory, making a top priority of the field of collider physics to validate this measurement. Based on the complexity of the detectors and the collision environment at the Large Hadron Collider, a precise measurement of the W boson mass was considered science fiction. After several years of work on detector calibration and particle reconstruction leading to a level of understanding never reached before, the CMS experiment has just completed a measurement of the W boson mass. The result along with the challenges encountered will be discussed along with a possible interpretation towards future directions for collider physics.