Baylor’s Experimental High Energy Physics group among scientists with significant contributions to the successful maintenance and operation of the experiment measuring one of nature’s force-carrying particles
WACO, Texas (April 7, 2022) – After 10 years of careful analysis and scrutiny, scientists of the CDF collaboration at the U.S. Department of Energy's Fermi National Accelerator Laboratory – including Baylor University researchers with the Experimental High Energy Physics (HEP) group – announced today that they have achieved the most precise measurement to date of the mass of the W boson, one of nature's force-carrying particles.
Using data collected by the Collider Detector at Fermilab, or CDF, scientists have now determined the particle's mass with a precision of 0.01% – twice as precise as the previous best measurement. This precision is comparable to a kicker making a field goal from a distance of two and a half miles.
Baylor HEP researchers – including Jay R. Dittmann, Ph.D., professor of physics; Kenichi Hatakeyama, Ph.D., associate professor of physics; and Jon Wilson, Ph.D., postdoctoral research associate – all contributed significantly to the successful construction and operation of the CDF experiment that produced the data for the measurement. In particular, the Baylor physicists helped design, assemble and test the Level 1 tracking trigger system used to identify collision events for analysis.
The new precision measurement, published in the journal Science, allows scientists to test the Standard Model of particle physics, the theoretical framework that describes nature at its most fundamental level. The result: The new mass value shows tension with the value scientists obtain using experimental and theoretical inputs in the context of the Standard Model.
"This result required a tremendous and steadfast effort over the course of many years," Wilson said. "It involved new work to understand our particle detector at a level that we had never before achieved as well as significant advances in our theoretical understanding of how the W boson behaves. When we finished and looked at the final result, we saw that it was different from the predictions of the Standard Model of particle physics."
If confirmed, this measurement suggests the potential need for improvements to the Standard Model calculation or extensions to the model.
The creation and growth of the Baylor HEP research group, which has been part of the CDF experimental collaboration since 2003, has always been a big part of Baylor's push towards Research 1 status, a designation achieved by the University in December 2021. World-leading scientific results like this one are the fruit of Baylor's investment in basic research.
"With its gigantic particle accelerators and huge international collaborations, experimental high energy physics is truly big science," Dittmann said. "It's a privilege to team up with colleagues from around the world to make a real impact on our collective understanding of nature. I am grateful that we could help Baylor earn a place among the top universities in this exciting field."
The W boson is a messenger particle of the weak nuclear force. It is responsible for the nuclear processes that make the sun shine and particles decay. Using high-energy particle collisions produced by the Tevatron collider at Fermilab, the CDF collaboration collected huge amounts of data containing W bosons from 1985 to 2011.
"While the data were being analyzed, we carefully hid the measured value from ourselves to avoid introducing unconscious bias," Wilson said. "As part of the CDF leadership team, I and many other CDF members thoroughly reviewed all parts of the measurement to make sure everything was done correctly. We only uncovered the result after everyone was convinced that it was completely trustworthy."
The mass of a W boson is about 85 times the mass of a proton, or approximately 80,000 MeV/c2. CDF researchers have worked on achieving increasingly more precise measurements of the W boson mass for more than 20 years. The central value and uncertainty of their latest mass measurement is 80,433 ± 9 MeV/c2. This result uses the entire dataset collected from the Tevatron collider at Fermilab. It is based on the observation of 4.2 million W boson candidates, about four times the number used in the analysis the collaboration published in 2012.
"When I was reviewing the previous measurement, published in 2012, I thought it was already a state-of-the-art measurement," Hatakeyama said. "Analyzing four times more data certainly helps, but understanding the data collected over nine years with changing accelerator and particle detector conditions to achieve this precision requires tremendous tireless effort. This is one of the most significant scientific achievements from the Fermilab Tevatron collider."
The collaboration also compared their result to the best value expected for the W boson mass using the Standard Model, which is 80,357 ± 6 MeV/c2. This value is based on complex Standard Model calculations that intricately link the mass of the W boson to the measurements of the masses of two other particles: the top quark, discovered at the Tevatron collider at Fermilab in 1995, and the Higgs boson, discovered at the Large Hadron Collider at CERN in 2012.
The CDF collaboration comprises 400 scientists at 54 institutions in 23 countries. Fermilab is America's premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC, a joint partnership between the University of Chicago and the Universities Research Association Inc. Visit Fermilab's website at www.fnal.gov and follow us on Twitter at @Fermilab.
