In the decades leading up to the European Organization for Nuclear Research's (CERN) July 4 announcement of the probable discovery of the Higgs boson, five Brown professors were hard at work theorizing the particle's existence and collecting and organizing the data that made possible this leap forward in physics.
The Standard Model of particle physics, which was developed in the early 1970s and has since been thoroughly tested, describes the way matter and forces interact around us. Over the past 40 years, experiments have slowly revealed the existence of key components of the theory, such as different quarks - the elementary particles that make up matter.
But since it was proposed, scientists' understanding of the Standard Model has been incomplete. For the theory to successfully describe the way the subatomic world works, another particle is needed - one that explains how elementary particles such as quarks and leptons acquire mass. That particle, the Higgs boson - commonly referred to as the "God particle" - was theorized in 1964, but until this summer, little experimental evidence supported its existence.
Proposing the boson
When the Higgs boson was first proposed, scientists were not searching for the origin of particle mass. Gerald Guralnik, professor of physics at Brown, co-authored one of the three papers that first proposed the particle's existence in 1964.
Guralnik said he began the work that led him to propose the existence of a new particle while he was completing his thesis as a doctoral student at Harvard. His adviser suggested he try to find solutions to quantum field theories - work that meant studying equations that explain the ways in which elementary particles behave. Guralnik said these equations, posited by theoretical physicist Yoichiro Nambu, were interesting because they had less symmetry than one would expect. Particles did not always behave the same way under conditions in which it seemed like they should.
The equations also required the existence of a zero-mass particle - a photon. But this was not required in the contemporary theories of electromagnetism, in which a zero-mass photon was not necessary. This discrepancy bothered Guralnik, though he was unable to find a proof that confirmed the photon in general electromagnetic theory had zero mass.
After he received his PhD, Guralnik continued working on his own equations to explain the non-symmetric behavior of particles at Imperial College London. In doing so, he made a crucial discovery - he came up with equations to describe the behavior of elementary particles that did not require a massless particle. Instead, they required a new particle with an undetermined mass - a particle that is now known as the Higgs boson.
"That was the key to the whole thing," Guralnik said.
He and his colleagues Carl Hagen and Tom Kibble carefully crafted a paper proposing their theory. Guralnik said he had concerns. "We had gone from madness to insanity," he said of their proposal. "It was considered borderline crackpot."
As they began talking about their theory to other scientists, "hardly anyone said this was a great idea," Guralnik said. "(The famous physicist Werner) Heisenberg told me that I didn't understand the laws of physics."
Despite the unpopularity of their theory, a few other scientists had a similar idea. Guralnik said after he and Hagen sent their paper off for review, they received another study in the mail by Peter Higgs. Higgs proposed a similar new particle, but his paper was done slightly differently and left out a few important parts of the explanation, so Guralnik said he did not take it seriously.
Nevertheless, when Higgs presented the idea at a conference that year, a reporter attached his name to the new particle, and it stuck.
"This theory that we did was a mathematical exercise," Guralnik said. "We would have never guessed we'd written down part of the theory of the universe. We were very lucky."
Searching for the particle
Guralnik said that after he wrote his 1964 paper proposing the existence of the particle, he turned his attention to other subfields of theoretical physics.
But it wasn't until the early 1980s, when researchers at CERN found experimental evidence for the unified field theory - which also relied on the Higgs boson's existence - that people began paying more attention to Guralnik's idea.
Scientists realized that the seemingly outrageous particle could make up the field through which matter travels to accumulate mass, filling an important hole in the theory. Some people describe the Higgs field as molasses-like - a permeating field that weighs down particles as they move through it. Others describe the Higgs field as a sea of paparazzi and say particles like quarks are like celebrities trying to get through.
Unfortunately, observing a Higgs boson to determine if these theories are correct is not exactly easy. First, researchers have to create the particle by colliding protons together at high energies, and then, since it lasts for a miniscule and unobservable amount of time, they have to study the behavior of the other particles that may be signs of the Higgs boson decaying.
Originally, such experiments took place at the Large Electron-Positron collider at CERN, near Geneva, Switzerland and an electron-positron collider at Cornell. But the LEP shut down in 2000 without finding anything conclusive.
At that time, the search for the Higgs moved to the Tevatron at the Fermi National Accelerator Laboratory in Illinois. But the Tevatron could not collide protons at nearly as high an energy level as the Large Hadron Collider that replaced the LEP at CERN in 2008.
Since its construction, the LHC has been the ma
in site of the search for the Higgs boson, with scientists on two experiments - ATLAS and CMS - working to detect evidence of the particle.
Four Brown professors are involved in the CMS experiment, including Professor of Physics Greg Landsberg, who has been its physics coordinator since the beginning of this year.
Landsberg said the high energy of the collider directly contributed to its ability to present such strong evidence for the existence of the Higgs this summer. In April 2011, the LHC surpassed the Tevatron to become the most powerful collider in the world, and this past April, scientists increased its maximum energy again, a step Landsberg said was necessary to find the data they needed. Now, the LHC operates with four times the amount of energy than the Tevatron did.
Seeing the signs
But even with a collider powerful enough to create a particle with the theorized mass of the Higgs boson, physicists still had to analyze incredible quantities of data to find evidence for its existence.
Professors of Physics David Cutts, Meenakshi Narain and Ulrich Heintz are involved in this process.
Cutts originally worked at the Fermilab but moved to CERN in 2011 to collect data in the control room. "I find it very exciting to see things happen with the data coming in, the types of events that are occurring," he said.
Cutts said he and his colleagues have to look at the data and select which events to record. Heintz works to analyze that recorded data. He said that if the Higgs boson exists, certain combinations of particles with certain amounts of energy should occur more frequently than would otherwise be expected.
For example, one way in which the Higgs may decay is by splitting into two photons. Occurrences in which two photons emerge from a proton collision are common, but if in fact they arose from a decaying Higgs, they would have a specific energy that relates to the proposed mass of the Higgs. Heintz and his colleagues work to see whether the data "bulges" around that energy level, he said.
Splitting into two photons is just one of the ways a Higgs boson may decay. There are four other key decay events, or decay channels, for which scientists working on the ATLAS and CMS experiments are looking. They also have theoretical predictions for the frequencies at which each of these channels will occur, which Heintz said also help to determine whether or not the Higgs exists.
A successful search
On July 4, researchers at CERN from both the ATLAS and CMS experiments independently announced that the evidence they had thus far observed aligned with the theoretical predictions of the Higgs boson.
Before making the highly anticipated and potentially revolutionary announcement, scientists intensely cross-checked their data to ensure that analyses were not biased by the theory.
"It was very climactic, but it was really the apex of a long process of internal scrutiny," Landsberg said.
"It was an unbelievable moment," Narain wrote in an email to The Herald.
Though they do not know for certain whether they have found the Higgs boson that completes the Standard Model, Landsberg said the announcement was still a milestone because they have definitely found a new particle.
The likelihood of this pattern of results arising without a new particle's existence are less than one in three million, Narain said in a University press release this summer.
"The fact that we found a new particle is fundamentally very important. There is a particle, which decays in a certain way and which is produced with a certain probability," Landsberg said. "It's the most important discovery of the century, if not the last 50 years of particle physics."
The particle could have slightly different properties from the one needed to complete the Standard Model, Landsberg said. Researchers will continue to analyze the decay channels to strengthen their understanding of what it is, he said.
By the time the collider shuts down for upgrades in December, researchers will be able to analyze six times as much data as was used in the discovery, Narain wrote.
If it is the Higgs boson, many questions still remain, such as why it has its particular mass.
Landsberg said he is often asked about the practical implications of the discovery, which he said "are not quite clear yet." But, he added, many other scientific discoveries - such as Rutherford's discovery of the atomic nucleus - did not have immediately obvious practical applications. "Yet just 30 years later we had nuclear power plants and nuclear bombs."
He called the findings to date the "first step on a long road to understanding the true nature of this particle."
Pushing the boundaries of physics
For other researchers, the potential discovery of the particle is just the beginning of a new era of particle physics.
Cutts, for instance, said his work on the CMS experiment is "not just a search for the Higgs - it's a study of top quarks, it's a study of the electroweak interactions, it's understanding something called quantum thermodynamics and the types of jets of particles that quarks materialize. There's a whole lot of fundamental physics that we probe."
The fact that scientists now have tools that can collide particles with enough
energy to possibly create a Higgs boson means that "there are very likely other particles that are in our reach to discover," Cutts said.
"We've finally gotten to an energy level where we can start seeing new things," he added. He said though the new particle is likely the Higgs that completes the Standard Model, the new things that they will likely soon discover will exist in the realm of physics beyond the boundaries of that model. "That's what I'm looking forward to," he said.
The biggest reward
But for all the physicists, the best part of the search for the Higgs was being part of such a collaborative effort.
"Just the thought that so many people could come together from so many places in the world," is what was most rewarding, Heintz said.
"These discoveries really take thousands of people working together," Landsberg said. "This is really a triumph for science, not a triumph for individuals - really a triumph for all of civilization."
Correction: Due to an editing error, an earlier version of the caption corresponding to this article incorrectly implied that the described research had led to proof of the existence of the Higgs boson. As the article describes, the research instead led to evidence that a new particle thus far consistent with the Higgs boson needed to complete the Standard Model exists. The Herald regrets the error.
An earlier version of this article stated that Gerald Guralnick, professor of physics at Brown, and his colleague Carl Hagen collaborated on a paper proposing the existence of what is now known as the Higgs boson. British scientist Tom Kibble also collaborated on the paper.