July 4, 2012 | by David Orenstein
Theorists first suggested the existence of the Higgs boson to explain why some particles have mass and others don’t — 48 years ago. Brown physicists, who for decades have been deeply involved in high-energy particle physics research, have played important roles in the massive worldwide hunt to find the Higgs. On Wednesday, they were all able to declare an apparent and historic victory, reflect on the search, and dream about what happens next.
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Early on July 4, 2012, physicists at the CERN particle accelerator lab in Geneva, Switzerland, announced that they had discovered a new particle that is consistent with the Higgs boson, an elusive but essential fundamental particle that physicists believe is responsible for endowing matter with mass. Brown University professors Greg Landsberg, Meenakshi Narain, Ulrich Heintz, and David Cutts played important roles in the historic and apparently successful search and one, Gerald Guralnik, was an originator of the theory that predicted the boson 48 years ago.
Landsberg, physics coordinator for the CMS experiment at CERN, celebrated the announcement, which physicists around the world hailed as an inspiring outcome of a monumental effort.
“This is a triumph of a major experimental work, which took 20 years of hard labor to accomplish and efforts of thousands of people,” he said. “This is easily the greatest discovery in particle physics in the last 30 or 40 years.”
Guralnik, the Chancellor’s Professor of Physics at Brown, worked with colleagues Carl Hagen and Tom Kibble to make sense of an upheaval in fundamental physics theory in the 1960s. In 1964, the trio posited a new explanation for certain fundamental forces and particles of the universe that became essential to later developing a “standard model” of physics. That standard model offers a unifying explanation of electromagnetism, the “weak” force that governs radioactive decay, and the “strong” force that holds atomic nuclei together. Also at that time, papers by Robert Brout and François Englert, and Peter Higgs, the long-sought particle’s namesake, independently produced similar predictions. The newly announced results from the CERN experiments yielded a particle that is consistent with the expectations.
“It is a wonderful feeling of great satisfaction and amazement,” Guralnik said. “We started out to solve an interesting and challenging abstract problem. We were surprised by the answer that turned up.”
The theories that emerged in 1964 offered the idea of a Higgs mechanism that pervades space and interacts, more or less strongly, with other particles to give them mass. Since then, thousands of physicists have dedicated decades to confirm the idea by finding the mechanism’s constituent particles called Higgs bosons and measuring their mass.
Now that the boson appears to have been found and measured — in a mass range between 125 and 126 billion electron-volts — the implications are immense and encouraging, said Brown physics professors involved in the experiments.
“Now the time has come that we will be able to determine the properties of the particle we observe, establish that it is the Higgs boson or something else, whether it agrees with the standard model predictions or whether there are some differences,” Narain said. “It will likely start an entirely new era of particle physics in which we finally understand how and why some particles have mass and others do not.”
In a statement on Brown’s Department of Physics website, Landsberg said, “This is our ‘raison d'etre’ — the very reason we spent the last 20 years building, commissioning, and running this experiment. The last two weeks have been a roller-coaster ride for us — the moments of ultimate, breathtaking thrill, when it feels like your heart leaps out of your body. The ride is now over and we are thrilled to share this excitement with the rest of the world today.”
For decades, scores of Brown University physics professors, postdocs, and students have been exploring at the highest energy frontiers of particle physics. That has included efforts to find the boson. Brown professor David Cutts was a founding member the D-Zero experiment at Fermilab near Chicago. The effort paved the way for the higher-energy experiments at CERN’s Large Hadron Collider (LHC), achieving important milestones including the 1996 discovery of the massive top quark. That helped to confirm the “standard model” of particle physics that also requires the existence of the Higgs boson.
“In the 21st century the frontier of the highest energy research has moved to CERN. We’re delighted to be part of that effort,” Cutts said. “We are all tremendously excited by this discovery of a new particle state, which seems to be the predicted object often called the Higgs boson. We have achieved a new understanding of nature at its most fundamental level.”
CMS, for Compact Muon Solenoid particle detector, is one of two huge experiments that sought the Higgs boson. The other is called ATLAS. Narain leads a group in the CMS experiment that is responsible for developing the techniques to find bottom quarks among the data that CMS records. This is crucial to look for one of the possible indirect signals of a Higgs boson emerging from a collision of protons and then decaying into smaller parts, for instance a bottom quark and an oppositely charged anti-bottom quark.
Narain explained how CMS distinguished the signal of the new particle from the massive amounts of complex data produced by the collisions of protons at near-light speed. Calculations show that the signal was very unlikely to be the product of some other interactions.
“Particle interactions are probabilistic processes,” she said. “The number and type of interactions that happen fluctuate around an expected value. We can compute the size of these fluctuations. So we take all the processes we know and estimate how likely the sum of them is to fluctuate to produce the signal we observe. With the current results this probability is very small. Both experiments reported a significance of five sigma. This means that they estimate this probability [of the results being spurious, rather than the new particle] to be as small as one in 3 million.”
Heintz had an inside look at the quality of the data and the analysis. His primary role in CMS was to help critique the analysis, acting as an internal peer reviewer. Heintz noted that today’s announcement was orders of magnitude more certain than the more tentative hints of the Higgs boson revealed by the lab at the end of last year.
“The results announced July 4 have been scrutinized within the collaboration for over a month now,” he said. “The results that CMS released in December 2011 had a probability of more than one in a thousand to be produced by a fluctuation in the background. We have made a lot of progress in the last six months.”
After 48 years, a new beginning
The announcement, while a cause for celebration for many physicists, hardly signals the end of their duties. The next steps include making even more precise measurement of the new particle and confirming that it is indeed the Higgs boson, the physicists said.
But much more than confirmation of the new discovery lies beyond.
“We have much ahead of us,” Cutts said. “The new accelerator and detector serve as the most powerful microscope ever built, allowing us to study the fundamental nature of the universe in greater detail than ever before. There are a vast array of studies, not only to understand the properties of this new state but to search for other states which could be related to other fundamental questions, such as ‘What is dark matter?’”
Meanwhile, Guralnik noted that a particularly grand goal of physics is yet to be obtained.
“The standard model does not contain gravitational interactions and these must be integrated into the standard model to give us a ‘theory of everything’,” he said. “The LHC allows the examination of regions of high energy that should help us understand the unification of all forces and it is very likely as information is collected our ideas will remain a core component.”
In other words, it’s time to start defining what the next 48 years of high energy physics will bring.