The Fourth of July was a momentous day in the world of physics, and the University of Washington played a significant part in it.
Scientists around the world celebrated the detection of what appears to be the Higgs boson, an elusive subatomic particle whose existence is a major step in understanding the origins of the universe.
“For the UW group, it is particularly exciting because it comes after more than 20 years of dedicated effort,” Henry Lubatti, a UW physics professor, said via email from an international physics conference in Melbourne, Australia, that began with the Web-based announcement of the discovery.
The much-anticipated finding came from the European Center for Nuclear Research, or CERN, from its headquarters in Switzerland. CERN’s Large Hadron Collider began operating in 2008 with the expectation that it would detect the Higgs boson if such a particle existed.
In the very early hours of Wednesday morning, several UW physicists joined a crowd of about 175 people to watch the announcement at a restaurant near Seattle Center. Before the Web presentation began, the UW faculty members explained the purpose and significance of the research to the audience, as well as the UW role in conducting the research and building a key part of one of the CERN detectors.
Anna Goussiou, a UW physics professor who was at the Seattle event, said it was an emotional moment when she saw the first evidence for the Higgs boson flash on the screen during the webcast.
“I jumped up and screamed, ‘There it is!’ all teary-eyed. I’ve been looking for the Higgs non-stop since the year 2000 and this is for me the discovery of a lifetime,” she said.
Goussiou, along with her students and postdoctoral researchers, made significant contributions to the discovery as they searched collider data for signals of particle decays that could indicate the presence of the Higgs boson. Lubatti and UW physics Professor Gordon Watts searched the data for other signs that the Higgs might be present.
In the end, the data showed a particle with the expected properties of a Higgs boson had been detected, though it could take several more years of work to confirm whether the newly found particle is, in fact, a Higgs boson.
Watts said he felt a bit of anxiety and then a sense of relief at the Seattle restaurant early Wednesday as he watched the results from CERN display on the screen.
“I knew what Atlas results were, but if (the other detector) didnt see it also then it just didnt matter,” he said. “Id heard rumors, of course. But when that plot went up I froze. We had it! The crowd there collectively said ‘ooooh’ and then started clapping and that brought me back. I have to say that I felt much calmer after that – almost relief. It was out there, for everyone to see. No more secrets, no more worrying if we were right or not, just done.”
The Higgs boson is considered a key to the “standard model,” which physicists use to understand the nature of matter and how the universe is put together. In the standard model, elementary particles such as quarks and electrons acquire mass by interacting with the Higgs field, and the Higgs boson is an excitation of the Higgs field. Observing the Higgs boson helps to confirm that the field exists.
“The most important significance is that the Higgs discovery provides the missing link in the unification of two of the four fundamental forces in nature,” Goussiou said. “It brings us a lot closer to understanding the absolute symmetry we believe existed at the very early stage of the universe, right after the Big Bang.”
The Large Hadron Collider at CERN was designed and built specifically to make high-energy physics observations, and it has plenty of work left to do.
“We are looking to make sure the Higgs works exactly as we expect it to,” Watts said. “Any deviations will mean something interesting, and perhaps a clue to the pieces of the puzzle we are still missing. Longer term, and in parallel, we will need to understand how dark matter and dark energy fit into the puzzle.”
The collider works by sending nuclei of hydrogen atoms racing at nearly the speed of light in opposite directions through parallel underground tubes that form a circle about 16.5 miles in circumference along the Swiss-French border. Detectors are positioned to observe what happens when the nuclei collide. The results indicating the presence of the long-sought Higgs boson came from two different detectors, CMS and Atlas, the one in which UW has been significantly involved.
The Atlas detector contains numerous subsystems, two of which – the calorimeter and the muon spectrometer – were central to the Higgs boson discovery. The UW group played a key role in designing and building the muon spectrometer. The forward muon detector on Atlas contains more than 430 chambers filled with aluminum tubes ranging in length from about 5 feet to 10 feet. Each tube contains a gold-plated tungsten wire just half the width of a human hair strung through the center to detect what happens when subatomic particles collide at nearly the speed of light.
About 30,000 of the tubes were made at UW between 2000 and 2007, fitted into 80 chambers and shipped to Geneva. Another 60,000 tubes were made with UW methods and specifications at two other U.S. sites. In Geneva, the chambers were mounted into 32 sections shaped like giant pie wedges, which fit together into two rings at either end of the main Atlas detector.
The UW group’s muon detector development, led by Lubatti and Colin Daly in mechanical engineering, began in 1989 as part of a U.S. project called the Superconducting Super Collider that eventually was cancelled. The group joined the Atlas detector group in 1993 to work on the muon spectrometer.
The other institutions that worked on the manufacture of tubes for Atlas using techniques and specifications developed at UW are the University of Michigan; the University of California, Irvine; Brookhaven National Laboratory; and the Boston Muon Consortium, which includes Harvard University, the Massachusetts Institute of Technology; and Tufts, Boston and Brandeis universities.
Others in the UW Physics Department involved in building the muon system include faculty members Joseph Rothberg and Paul Mockett and staff members David Forbush and Matt Twomey. William Kuykendall, a laboratory engineer in mechanical engineering, also participated.