Who ordered a too heavy W boson?

Editor’s note: this press release was originally published by Brookhaven National Laboratory.

Physicists studying ghost-like particles called neutrinos from the international MicroBooNE collaboration have reported a first-of-its-kind measurement: a comprehensive set of the energy-dependent neutrino-argon interaction cross sections. This measurement marks an important step towards achieving the scientific goals of next-generation of neutrino experiments — namely, the Deep Underground Neutrino Experiment (DUNE).

Neutrinos are tiny subatomic particles that are both famously elusive and tremendously abundant. While they endlessly bombard every inch of Earth’s surface at nearly the speed of light, neutrinos can travel through a lightyear’s worth of lead without ever disturbing a single atom. Understanding these mysterious particles could unlock some of the biggest secrets of the universe.

The MicroBooNE experiment, located at the U.S. Department of Energy’s (DOE) Fermi National Accelerator Laboratory, has been collecting data on neutrinos since 2015, partially as a testbed for DUNE, which is currently under construction. To identify elusive neutrinos, both experiments use a low-noise liquid-argon time projection chamber (LArTPC) — a sophisticated detector that captures neutrino signals as the particles pass through frigid liquid argon kept at negative 303 degrees Fahrenheit. MicroBooNE physicists have been refining LArTPC techniques for large-scale detectors at DUNE.

Now, a team effort led by scientists at DOE’s Brookhaven National Laboratory, in collaboration with researchers from Yale University and Louisiana State University, has further refined those techniques by measuring the neutrino-argon cross section. Their work published today in Physical Review Letters.

“The neutrino-argon cross section represents how argon nuclei respond to an incident neutrino, such as those in the neutrino beam produced by MicroBooNE or DUNE,” said Brookhaven Lab physicist Xin Qian, leader of Brookhaven’s MicroBooNE physics group. “Our ultimate goal is to study the properties of neutrinos, but first we need to better understand how neutrinos interact with the material in a detector, such as argon atoms.”

One of the most important neutrino properties that DUNE will investigate is how the particles oscillate between three distinct “flavors”: muon neutrino, tau neutrino, and electron neutrino. Scientists know that these oscillations depend on neutrinos’ energy, among other parameters, but that energy is very challenging to estimate. Not only are neutrino interactions extremely complex in nature, but there is also a large energy spread within every neutrino beam. Determining the detailed energy-dependent cross sections provides physicists with an essential piece of information to study neutrino oscillations.

“Once we know the cross section, we can reverse the calculation to determine the average neutrino energy, flavor, and oscillation properties from a large number of interactions,” said Brookhaven Lab postdoc Wenqiang Gu, who led the physics analysis.

To accomplish this, the team developed a new technique to extract the detailed energy-dependent cross section.

“Previous techniques measured the cross section as a function of variables that are easily reconstructed,” said London Cooper-Troendle, a graduate student from Yale University who is stationed at Brookhaven Lab through DOE’s Graduate Student Research Program. “For example, if you are studying a muon neutrino, you generally see a charged muon coming out of the particle interaction, and this charged muon has well-defined properties like its angle and energy. So, one can measure the cross section as a function of the muon angle or energy. But without a model that can accurately account for “missing energy,” a term we use to describe additional energy in the neutrino interactions that can’t be attributed to the reconstructed variables, this technique would require experiments to act conservatively.”

The research team led by Brookhaven sought to validate the neutrino energy reconstruction process with unprecedented precision, improving theoretical modeling of neutrino interactions as needed for DUNE. To do so, the team applied their expertise and lessons learned from previous work on the MicroBooNE experiment, such as their efforts in reconstructing interactions with different neutrino flavors.

“We added a new constraint to significantly improve the mathematical modeling of neutrino energy reconstruction,” said Louisiana State University assistant professor Hanyu Wei, previously a Goldhaber fellow at Brookhaven.

The team validated this newly constrained model against experimental data to produce the first detailed energy-dependent neutrino-argon cross section measurement.

“The neutrino-argon cross section results from this analysis are able to distinguish between different theoretical models for the first time,” Gu said.

While physicists expect DUNE to produce enhanced measurements of the cross section, the methods developed by the MicroBooNE collaboration provide a foundation for future analyses.  The current cross section measurement is already set to guide additional developments on theoretical models.

In the meantime, the MicroBooNE team will focus on further enhancing its measurement of the cross section. The current measurement was done in one dimension, but future research will tackle the value in multiple dimensions — that is, as a function of multiple variables — and explore more avenues of underlying physics.

This work was supported by the DOE Office of Science.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov

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Editor’s note: Members of the CDF collaboration will give a scientific presentation with details about their measurement on Friday, April 8, at 4 p.m. CDT. Click here for more information and registration.

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 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. It corresponds to measuring the weight of an 800-pound gorilla to 1.5 ounces.

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.

“The number of improvements and extra checking that went into our result is enormous,” said Ashutosh V. Kotwal of Duke University, who led this analysis and is one of the 400 scientists in the CDF collaboration. “We took into account our improved understanding of our particle detector as well as advances in the theoretical and experimental understanding of the W boson’s interactions with other particles. When we finally unveiled the result, we found that it differed from the Standard Model prediction.”

If confirmed, this measurement suggests the potential need for improvements to the Standard Model calculation or extensions to the model.

Scientists have now determined the mass of the W boson with a precision of 0.01%. This is twice as precise as the previous best measurement and shows tension with the Standard Model.

The new value is in agreement with many previous W boson mass measurements, but there are also some disagreements. Future measurements will be needed to shed more light on the result.

“While this is an intriguing result, the measurement needs to be confirmed by another experiment before it can be interpreted fully,” said Fermilab Deputy Director Joe Lykken.

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.

CDF physicist Chris Hays of the University of Oxford said, “The CDF measurement was performed over the course of many years, with the measured value hidden from the analyzers until the procedures were fully scrutinized. When we uncovered the value, it was a surprise.”

The mass of a W boson is about 80 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.

“Many collider experiments have produced measurements of the W boson mass over the last 40 years,” said CDF co-spokesperson Giorgio Chiarelli, Italian National Institute for Nuclear Physics (INFN-Pisa). “These are challenging, complicated measurements, and they have achieved ever more precision. It took us many years to go through all the details and the needed checks. It is our most robust measurement to date, and the discrepancy between the measured and expected values persists.”

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.

CDF co-spokesperson David Toback, Texas A&M University, stated the result is an important contribution to testing the accuracy of the Standard Model. “It’s now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery,” he added. “If the difference between the experimental and expected value is due to some kind of new particle or subatomic interaction, which is one of the possibilities, there’s a good chance it’s something that could be discovered in future experiments.”

To obtain a copy of the paper, please contact scipak@aaas.org.

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.

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Georgian Technical University and the U.S. Department of Energy’s Fermi National Accelerator Laboratory have signed an agreement to collaborate on research in support of one of the largest particle physics experiments in the world: the international Deep Underground Neutrino Experiment, hosted by Fermilab. More than 1,400 scientists and engineers from over 35 countries collaborate on DUNE to discover the role neutrinos play in the universe.

“The seed for the collaboration with Georgian Technical University started two and a half years ago when we visited Tbilisi,“ said Stefan Söldner-Rembold, who served as the DUNE co-spokesperson for the last four years. “We were really impressed by the facilities and the strength of the team at the university. We are looking forward to working with the team at GTU to build this world-leading neutrino experiment.”

DUNE scientists are pursuing three major science goals: determine whether neutrinos could be the reason the universe is made of matter; look for undiscovered subatomic phenomena that could help realize Einstein’s dream of the unification of forces; and watch for neutrinos emerging from an exploding star, perhaps witnessing the birth of a neutron star or a black hole.

DUNE will advance these science goals using the world’s most intense neutrino beam, produced by the particle accelerators at Fermilab in Illinois. The neutrino beam will travel 1,300 kilometers straight through earth from Fermilab to the Sanford Underground Research Facility in South Dakota. Particle detectors based on state-of-the-art technologies will probe the neutrino beam at both Fermilab and SURF. They will make precision measurements of particle interactions as the neutrinos travel through the detectors.

Georgian Technical University is active in particle physics experiments around the world, collaborating on DUNE as well as the ATLAS and CMS experiments at the European particle physics laboratory CERN and the COMET experiment at the Japanese laboratory KEK. Georgian Technical University will make various contributions to DUNE, including building hardware for the neutrino detector at Fermilab.

“We are working on the construction of particle detector components for DUNE, and we have plans to expand our group with more students and postdocs. We will be very active members in the collaboration,” said Georgian Technical University Professor Zviadi Tsamalaidze.

“The agreement signed between Fermilab and Georgian Technical University is of key importance for Georgian science and students, for our university and for the country as a whole. I’d like to express my deepest gratitude to the government of the United States of America and the American people for their support, which paves the way for more Georgian researchers and students to reach the latest advances in science and provides a unique opportunity to become active participants in high-level scientific research,” said David Gurgenidze, rector of the Georgian Technical University.

Fermi National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visitscience.energy.gov.

The U.S. Department of Energy’s Fermi National Accelerator Laboratory is pleased to invite the public back to the Batavia, Illinois, site, beginning March 28. With the arrival of spring, the lab grounds will again be open to the public for outdoor activities, such as biking, hiking, running and viewing the bison herd. There will be only limited access to indoor areas at this time, as most public events and lectures will continue to be held virtually. All events are listed on Fermilab’s calendar of events.

As we reopen to the public, we have new visiting hours and access requirements in place, outlined below. We value our strong relationship with our community and look forward to welcoming everyone back with these new requirements in place.

Hours and access

• Two site entrances are open to the public: Pine Street and Batavia Road. The Wilson Road entrance is closed to the public.
  

  Fermilab will reopen to the public and visitors on March 28. The outside recreation areas will be open daily from dawn to dusk including the areas to view the bison herd. Photo: Ryan Postel, Fermilab

• Hours for outdoor visitors are dawn to dusk every day.
• The Lederman Science Center (LSC) is open from 8 a.m.-5 p.m. Monday through Friday and 9 a.m.-3 p.m. Saturday; the center is closed Sundays and Fermilab holidays. The LSC restrooms and drinking water will be available for the public on these days and times.
• Wilson Hall is currently closed to the public. It will be available for scheduled public tours, which will be posted at ed.fnal.gov.

Outdoor activities

• The public is invited to Fermilab to visit its bison herd and interpretive trails, and to enjoy walking, hiking, bicycling, running, rollerblading, skateboarding, roller-skating, cross-country skiing, snowshoeing, bird watching, photography and painting. The public must remain in designated public areas at all times and comply with all site rules and allowed uses in a safe manner, including complying with all traffic laws and driving rules as well as site signage.
• Prohibited public use of the Fermilab site includes operations of drones. Fishing, ice skating and dog-walking/dog run are currently not allowed. All outdoor activities must take place on designated site paths, trails or roads.

ID requirements

• All visitors 18 and older, including those on foot or on bicycle, as well as all adult passengers in vehicles, will be asked to show a government-issued photo ID to access the site. Those who have not yet obtained a REAL ID-compliant ID (REAL ID drivers’ license or passport) are encouraged to obtain one. Only REAL ID-compliant IDs will be accepted after May 3, 2023.
• After IDs are checked at the entrance gate, public visitors will receive a red sticker that must be visible all times when on the Fermilab site.
• Minors (children under age 18) must be accompanied by an adult.

COVID-19 protocols

• We recommend that you bring a face mask with you when visiting the lab. Visit https://www.fnal.gov/pub/visiting/hours/ to learn the current site safety requirements.
• At this time during a Low Community Level, proof of vaccination or a negative COVID-19 test are not required for public visitors entering indoor areas, and masks generally are not required except for certain designated spaces.
• If Community Levels rise to Medium or High, attesting for proof of vaccination or a negative COVID-19 test within the past 72 hours will be required to access indoor areas. Please see the Fermilab visitors site for guidance and updates including masking.

Fermilab is pleased to welcome the public once again to enjoy leisure activities on site. The public can provide feedback at Fermilab@fnal.gov. Additional information and details regarding Fermilab site access hours and requirements can be found on the Fermilab visitors site. Fermilab’s schedule and list of education and public engagement offerings can be found on the calendar for upcoming public events.

Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics 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. Visit Fermilab’s website at https://www.fnal.gov and follow us on Twitter @Fermilab.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.