Understanding the Search for the Higgs Boson: A Primer for Civilians
The fundamental sub-atomic particles.
For nearly four decades high-energy particle physicists have been in pursuit of an answer to the question “What is it that gives mass to the fundamental sub-atomic particles—the quarks and leptons—that make up all matter in the known universe?”
On July 5 the media universe was ablaze with the news that the scientific community was significantly closer to the answer—long-thought to be the particle known as the Higgs boson. That elusive particle was dubbed the “God Particle” by Nobel Prize winning physicist Leon M. Lederman in his 1993 book The God Particle: If the Universe is the Answer, What is the Question? The God Particle moniker was actually a term favored by Lederman’s publishers and popularized by the media; scientists generally dislike the nickname.
The faculty members of the physics department’s HEP group. Left to right: Rick Cavanaugh, Mark Adams, Cecilia Gerber, Nikos Varelas. Photo by Joshua Clark
Four faculty members from the Department of Physics have been part of the search for the Higgs—one of the longest and most expensive searches in the history of science. Professors Mark Adams, Nikos Varelas and Cecilia Gerber and Assistant Professor Rick Cavanaugh, as well as four researchers and six graduate students, comprise the department’s High Energy Physics (HEP) group.
Press releases from CERN, the multi-billion dollar facility near Geneva on the Swiss-French Border, and the Compact Muon Solenoid (CMS) Collaboration with which the LAS scientists are affiliated, explained that data recorded up to June 2012 indicates the existence of a particle that may be the Higgs—or might be another never-before-glimpsed particle. The results are being called “preliminary” but “dramatic” by CMS spokesperson Joe Incandela who stated that a 5 sigma signal at a mass of approximately 125 GeV indicates a boson. “…and it’s the heaviest boson ever found,” said Incadela. Further announcements are anticipated before the end of 2012.
“More data are needed to study the properties of this new particle so we can say for sure that this is the long-sought Higgs boson as predicted by the Standard Model of particle physics and not a Higgs-like particle predicted from a new ‘super theory’,” noted Varelas, who was in CERN in June to work on finalizing the CMS results and the internal approvals for making the results public.
Varelas (light green shirt, second row center) joins colleagues in a standing ovation. Photo courtesy of Fermilab Visual Media Services
Varelas was at Fermi National Accelerator Laboratory in Batavia on July 4, attending the live-feed CERN seminar and press conference. “This is indeed an incredible milestone for particle physics, perhaps one of the most important discoveries since the discovery of quarks in the late 1960s. The atmosphere at CERN and Fermilab was electric—with a standing ovation when the significance of the signals was presented,” he said. “I strongly believe that there will be more breathtaking discoveries. Perhaps we will find out that the particle we just discovered is not the Standard Model Higgs boson, but a twin sibling with slightly different properties. This will be even more exciting news since it will be an indication of new physics beyond the Standard Model. I am thrilled to have contributed to this discovery.”
“The quest forever has been to understand what we’re made of, how we came to be,” commented Mark Adams, the senior member of UIC’s HEP group. “What high-energy particle physicists are attempting is an understanding of the fundamental forces of nature and physical reality.”
But what exactly is the Higgs—or a boson for that matter—and how did science get to this exciting crossroads?
Image of a 2012 CMS event at CERN. Photo courtesy of CERN
The Higgs mechanism was mathematically theorized in the mid-1960s by a number of physicists including Peter Higgs, after whom the Higgs boson and the Higgs field are named. The theory allows for a mechanism by which the quarks and leptons—the mass-carrying particles classified as fermions—get their mass via interaction with the Higgs field. The field is composed of countless Higgs bosons—bosons being the other type of sub-atomic particle. (There are four known bosons: the photon, the gluon, the W and the Z—collectively called the gauge bosons; the graviton is another boson that is theorized.)
Theoretically, the clustering effect of the Higgs around a fermion particle creates a drag, or inertia effect, giving it mass; scientists hope that observation of the Higgs will not only answer the question of how particles acquire mass, but why different particles have different masses. Proof of the Higgs will fill in an important missing piece in the Standard Model of Physics, which was developed in the 1970s and is the most reliable and well-tested theory by which to describe the interactions of particles.
Construction of the DZero detector. Photo by Reidar Hahn
The quest to prove or disprove the existence of the Higgs boson—along with exploring other aspects of the sub-atomic world—has occupied the imaginations and careers of high-energy particle physicists for some four decades. The first generation of modern Higgs experiments took place at the Tevatron proton-antiproton collider at Fermilab, 35 miles west of the UIC campus. For more than 25 years beginning in 1983 the Tevatron was the most powerful particle accelerator in the world.
1991 aerial view of CERN, on the Swiss-French border.
Since March 30, 2010 the hunt for the Higgs has been focused at the Large Hadron Collider—or LHC—at CERN. The LHC is located in a 17-mile-circumference tunnel 328 feet below ground. “Our very persistent Higgs efforts began in the late 1980s on a fixed-target experiment at Fermilab, moved to searches at the Tevatron’s DZero detector, and will culminate at CERN,” explained Adams.
The members of the UIC HEP group work in concert with thousands of scientists from dozens of nations at the CMS experiment observing trillions of collisions per day between protons traveling at nearly the speed of light in a near-perfect vacuum. CMS is one of four major and two multi-purpose detector experiments at CERN’s LHC. CMS and its counterpart ATLAS, which also confirmed and announced the latest data, are the two largest experiments at the LHC that use general-purpose detectors to independently examine the debris of high-energy collisions. Having several independent experiments focusing on various aspects of the collisions allows for cross-confirmation of observations and discoveries.
Construction of the LHC and the CMS detector began in 1998 following a design phase that started in the mid-1990s. “UIC faculty have been CMS collaborators since 1995 and we are very excited that 2012 looks like it is going to be the definitive year,” said Adams. “Over the decades we’ve searched in many channels and are contributing to other searches in high energy physics, even if our work ultimately disproves the existence of the Higgs.”
The CMS cavern 2005. Photo by Maximilien Brice
April 2012 marked the beginning of the current experimental cycle. The LHC runs 24/7 and teams of scientists work in shifts around the clock from spring to late fall. “The data collection happens through fast-computer systems and data analysis is a huge part of the workload,” explained Varelas, who led the Higgs search at Fermilab’s DZero experiment for two years.
“When the protons collide they create secondary particles which give traces into the CMS detector system. Sifting through the copious proton-proton interactions that happen at a rate of multi-million per second, we need to look for patterns to identify interesting events that are related to the Higgs or other processes,” he continued. “Viable events are sorted down to about 350 events per second and written to disk for further high-level analysis.” With the use of powerful Internet hubs to store and transmit data, analysis is done not only at CERN, but at remote locations around the globe, including Fermilab.
The CMS detector 2009. Photo by Maximilien Brice
Scientists cannot actually see the Higgs boson itself, but rather observe traces of the particles left behind as it decays. The events they feverishly analyze continue to narrow the range of possible mass of the Higgs, through which it can be identified.” The Higgs boson was proposed in the year I was born and the quest for its existence is closing in after 48 years,” said Varelas. “This is an exhilarating year for science!”
“There are several big unanswered questions that motivate me,” said Cavanaugh, who has a joint appointment at UIC and Fermilab and is currently focusing on fundamental symmetries and dark matter. “One mystery is that nature seems to come in triplicates at the fundamental-particle level for matter. We have no idea why and understanding the origin of mass and the Higgs might help us understand this carbon-copy ritual nature seems to have. Every little piece of the puzzle will help us understand the big questions.”
While the need to know and understand the universe is the motivating force for scholars engaged in pure science, there are everyday benefits that flow from the work. “Particle physics is a successor of J.J. Thomson, who discovered the electron. Because he needed to understand the electron, we have electricity. Because Niels Bohr cared about the hydrogen atom, we have quantum mechanics which gave us the transistor and everything that has to do with micro-electronics,” said Gerber, a member of the Fermilab team that discovered the single-top quark production.
“We have the Internet because of the military but because of particle physics we have web browsers, medical imaging technology and cancer therapies as well as numerous industrial and manufacturing applications,” she continued. “Imagine if, because of economic hardship or dark-ages attitudes, we would have said at the beginning of the 1900s: ‘We cannot afford research!’ ‘Who cares what is inside of an atom?’ We would be living like our great-grandparents.”
“You can go back to Einstein,” added Varelas. “No one believed him about the theory of relativity. It was considered crazy with no practical application. But GPS technology is a direct descendant of that theoretical work.”
In his remarks regarding the latest announcement from CERN, U.S. Secretary of Energy Steven Chu expressed his support for these ideas: “today’s announcement on the latest results of this search shows the benefits of sustained investments in basic science by governments around the world.”
“The indirect benefits of pure science to inspiration and innovation, and ultimately to the economy, are huge but there’s also another aspect that is really wonderful,” said Cavanaugh. “It could easily be that before the Nobel Prize in Physics is awarded for the discovery of the Higgs, that CERN gets a different prize—the Nobel Peace Prize. This project brings together an incredible number of different countries and different cultures and puts them in an environment where they forget these differences and work together like a family. It is a very real benefit to society.”
In the final analysis, it is the pure quest for understanding that shapes what the LAS physicists most value. “We are at a very special moment in the history of the universe,” said Cavanaugh. “We live in an epoch where we can not only ask questions related to the extremes of our existence, but can even begin to answer some of the questions and understand the physical universe—from the microcosm of quarks to the macrocosm of the largest-scale structures of the universe—how the interactions between the unimaginably tiny and the overwhelmingly large work.”
Professor Varelas recommends two videos by Fermilab scientist Don Lincoln: What is a Higgs Boson? and Higgs Boson: How do you search for it?