Physicists involved in the CMS experiment have just released an extensive measurement of a particle that has been challenging to study but has intrigued the physics community for a long time.
Today, researchers working on the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) revealed a new measurement of the W boson’s mass. This comes after an unexpected finding from the Collider Detector at Fermilab (CDF) experiment in 2022. This is the first mass measurement of the W boson by the CMS team and introduces a novel technique, making it the most thorough analysis of the W boson’s mass to date. After nearly ten years of work, CMS has determined that the mass of the W boson aligns with theoretical predictions, finally clarifying a long-standing mystery. You can view the published paper.
The final analysis utilized a dataset of 300 million recorded events from the 2016 run of the LHC, as well as 4 billion simulated events. The team reconstructed and measured the mass from over 100 million W bosons, arriving at a value of 80,360.2 ± 9.9 megaelectron volts (MeV). This result aligns with the Standard Model’s predictions of 80,357 ± 6 MeV. They also conducted a separate analysis to verify their theoretical assumptions.
“The outcome from CMS is distinctive due to its precision and our approach to determining uncertainties,” stated Patty McBride, a prominent scientist at the U.S. Department of Energy’s Fermi National Research Laboratory and a former CMS spokesperson. “We’ve gained significant insights from the CDF and other experiments that investigated the W boson’s mass. We benefit from their work, which allows us to advance our study significantly.”
Since the W boson’s discovery in 1983, physicists from 10 different experiments have measured its mass.
The W boson is a crucial component of the Standard Model, the theoretical framework that explains the interactions of nature at its most elementary level. A precise understanding of the W boson’s mass helps scientists explore the relationships among particles and forces, including the strength of the Higgs field and the union of electromagnetism with the weak force, which is responsible for radioactive decay.
“The entire universe operates in a delicate equilibrium,” remarked Anadi Canepa, deputy spokesperson of the CMS experiment and a senior scientist at Fermilab. “If the W mass deviates from our expectations, it could indicate the presence of new particles or forces.”
The newly obtained measurement from CMS has an impressive precision of 0.01%. This level of detail is akin to measuring a 4-inch-long pencil with a precision ranging between 3.9996 and 4.0004 inches. However, unlike a pencil, the W boson is a fundamental particle without physical dimensions and possesses a mass lighter than a single silver atom.
“This measurement is extremely challenging,” Canepa emphasized. “We require multiple measurements from different experiments to confirm the value.”
What sets the CMS experiment apart from others that have measured this mass is its compact configuration, specialized sensors designed for fundamental particles known as muons, and a powerful solenoid magnet that alters the paths of charged particles as they navigate through the detector.
“We achieved this thanks to a combination of a bigger dataset, the experience we gained from previous studies on the W boson, and the latest theoretical advancements,” Bendavid explained. “This has allowed us to move beyond using the Z boson as a reference point.”
As part of this analysis, the team also evaluated 100 million tracks from the decays of well-known particles to recalibrate a major portion of the CMS detector, enhancing its precision significantly.
“This new level of accuracy will empower us to conduct crucial measurements involving the W, Z, and Higgs bosons with improved certainty,” Manca noted.
The analysis’s most challenging aspect was the time commitment, as it necessitated the creation of a novel analysis method and an in-depth understanding of the CMS detector.
“I began this research as a summer student, and now I’m in my third year as a postdoctoral researcher,” Manca shared. “It’s a marathon, not a sprint.”
The Compact Muon Solenoid (CMS) experiment is partially funded by the Department of Energy’s Office of Science and the National Science Foundation. It stands as one of two large general-purpose experiments at the Large Hadron Collider (LHC) located at CERN, the European Particle Physics Laboratory.
For more details: Measurement of the W boson mass in proton-proton collisions at √s= 13 TeV