Hunting Down the Higgs
American Independence Day is commonly associated with fireworks, barbecues, and colonial revolutionaries, but last summer physicists stole the show. On July 4, 2012, representatives for the Large Hadron Collider (LHC) announced the discovery of a new particle that by all accounts looks and behaves like the long-sought-after Higgs boson. It marks one of the most historic discoveries at CERN, the subatomic physics laboratory near Geneva, Switzerland where the LHC is located, and has cast ripples throughout the physics community.
If the new particle is confirmed as the Higgs, it will fill in the final piece of a half-century-long quest to confirm the Standard Model: the crowning glory of theoretical physics, and a tremendously successful theory for describing nature’s most fundamental interactions. Because of this, many physicists view the likely discovery of the Higgs boson as one the most important achievements not just in the history of physics, but in the greater history of humankind. It would be a testament to the power of science and logic to answer complex questions about the universe.
Mechanistically, the Higgs boson explains why the other fundamental particles can have mass, and as such the bombastically nicknamed “God particle” has occupied an unusually prominent space in the public imagination. The Higgs boson caught the attention of media outlets ranging from PhD Comics, to the Economist, to will.i.am’s twitter feed. The announcement was dubbed the “Breakthrough of the Year” by the journal Science, and it was re-imagined by Wired magazine as a Steve-Jobs-style product reveal. Nick Cave and the Bad Seeds even penned a song about the new particle.
Nowhere, however, was the excitement more acute than among the physicists directly involved in the discovery itself. UC Berkeley and Lawrence Berkeley National Laboratory (LBL) have been involved with particle physics research at CERN since before the LHC’s inception, and there was a large community of University physicists cheering on the announcement. Seth Zenz, a postdoc at Princeton who did his graduate work at UC Berkeley, was at CERN at the time of the announcement working with one of the two detectors involved in the discovery, the Compact Muon Solenoid (CMS). “There was a lot of excitement about the announcement really being public,” he said. “I was actually up all night waiting in line to make sure I had a place in the CERN auditorium. It was sort of like waiting in line for a rock concert you’re really excited about: you wish you weren’t dealing with it, you wish you weren’t worried people were going to get in front of you in line, but at the same you know you’re all there for something that’s very interesting.”
Back in Berkeley, graduate student Louise Skinnari (now a postdoc at Cornell) was working in parallel on the competing detector, ATLAS (A Toroidal LHC Apparatus). She recalled that a live video feed from the CERN auditorium to UC Berkeley drew a full crowd, even despite the time difference resulting in a midnight start time. “We had a gathering both of experimentalists working on ATLAS, and also theorists either from UC Berkeley or from LBL to view the seminars, and we had a party,” she said. “I left the physics department at something like 5:00 in the morning, and I wasn’t the last one to leave.”
Yet amidst the jubilance, the field is also slightly uneasy as it looks to the future. Theory has long pointed the way forward in particle physics, and collider experiments have followed in tow, confirming the existence of a list of new particles that all but had to exist. But the list comes to an abrupt end with the likely discovery of the Higgs. In the next couple of years, the LHC may turn up new anomalies, ushering in a new era in which experiments take the lead in deepening our knowledge of physics. Or nothing unexpected may turn up at all, leaving both theory and experiment at a frustrating impasse. At this point the future is anybody’s guess.
Rooted in Experiment
Modern particle physics was born at the hands of an experimentalist—UC Berkeley’s own Ernest O. Lawrence—with his invention of the LHC’s ancestor, the cyclotron. Lawrence arrived at UC Berkeley in the summer of 1928, leaving an assistant professorship at Yale to join the physics department at what was still a budding university. He was known as an extrovert, but was also something of a hermit, spending night after night poring over scientific publications and struggling, as many physicists were at the time, to figure out a better way to build upon Ernest Rutherford’s discovery of the atomic nucleus.
One of the more natural ways people had hoped to explore the atom at the time was by bombarding it with charged particles. Physicists knew that such charged particles could be accelerated between metal plates at different voltages, but in order to be useful as nuclear probes, the particles needed to be accelerated to high energies, on the order of millions of electron volts. To achieve this for electrons using brute force methods, experimenters needed a machine capable of generating a million volts, nearly an impossible task then and now. Three million volts across a distance of one meter is enough to seed a bolt of lightning.
During one particular evening in early 1929, Lawrence had an epiphany while reading through a largely ignored article written in German by a Norwegian researcher. The breakthrough came when Lawrence became inspired to think about accelerating particles in a series of smaller steps rather than all at once. To accomplish this, he proposed using alternating currents, resonance phenomena, and magnetic fields to force the particles around in a circle, where they could be accelerated again and again by the same plates.
In his book, The Making of the Atomic Bomb, the journalist Richard Rhodes writes that the discovery proceeded to consume the entirety of Lawrence’s time and energy. The following day he reportedly accosted one of his graduate students with a litany of questions about the mathematics of the idea for a new accelerator, and simultaneously abandoned the student to fend for himself regarding a previously assigned thesis project. “Oh, that,” Lawrence said. “Well, you know as much on that now as I do. Just go ahead on your own.”
In the early 1930s, Lawrence and his graduate students, notably M. Stanley Livingston and David Sloan, brought the cyclotron dream to reality and constructed a series of new “atom smashers” of increasing sizes and levels of sophistication. The earliest prototypes were small, and Lawrence himself would later joke at “the lack of good engineering design.” Beate Heinemann, who is a physics professor at UC Berkeley, staff scientist at LBL, and ATLAS deputy spokesperson, marvels at the simplicity of the early machines, particularly in comparison with the gargantuan proportions of modern descendents. “Of course,” she said, “the first cyclotrons could fit in the palm of your hand.”
Intriguingly, Lawrence built the new machines without regard to any particular theory of physics. The acceleration of charged particles to extremely high energies was reason enough to push him and his colleagues forward. “He didn’t actually have a real physics goal in mind. He simply had a new technology,” said Heinemann, “Of course the Nobel prize is given for science, but can sometimes be given purely for technical developments.” Indeed, the invention of the cyclotron won Lawrence the 1939 Nobel Prize in physics, UC Berkeley’s first Nobel Prize of any kind.
The early cyclotrons paved the way for a multitude of scientific breakthroughs to come, many of them driven by experimental capability. LBL cyclotrons have been involved in the discovery of no fewer than 14 elements, including plutonium, berkelium, and californium. During World War II a modified cyclotron called the calutron was used to enrich uranium for the Manhattan Project. Advances in cyclotron physics spurred on the related development of synchrotrons (another type of particle accelerator) like LBL’s Bevatron, where the anti-proton was discovered. Today LBL’s flagship facility is a synchrotron called the Advanced Light Source, which produces intense beams of ultraviolet and x-ray light that have facilitated the discovery of new states of matter, diagnosed air pollution sources in Mexico City, and enabled the imaging of more than 4,000 different types of protein structures.
Lawrence’s ideas reach their grandest scale in supercolliders, however, with none more impressive than the Large Hadron Collider. Today the LHC accelerates particles to higher energies than anything ever built before. By smashing these high-energy particles against each other, it can probe shorter length scales than science has ever been able to achieve. Using the energy from the collisions, it is even capable of creating whole new classes of particles. The ring built to guide proton collisions at the LHC has a 5.3-mile diameter that would stretch from UC Berkeley’s Campanile to Lake Merritt and, if unwound, would extend from downtown Oakland to Concord. When the system is brought up to full power in 2015, protons at the LHC will be accelerated to within one millionth of one percent short of the speed of light—generating collisions at 14 trillion electron volts—and the system will acquire data at the rate of 15 thousand terabytes per year. It is literally the world’s largest machine.
Theory’s “Unreasonable Effectiveness”
Impressive as the last century’s advances in experimental particle physics were, however, particle theory has come further still. When Rutherford discovered the atomic nucleus in 1909, the finding came as a complete surprise to the scientific community. This is in stark contrast to today: the 2012 Higgs candidate discovery has been predicted for nearly 50 years.
The idea for the Higgs particle originated in 1964, when a number of scientists, including the particle’s namesake—Peter Higgs—were poking and prodding the promising equations and theories of the day. A total of three groups of scientists made the discovery independently: Robert Brout and François Englert at the Free University of Brussels; Higgs, based at the University of Edinburgh; and Gerald Guralnik, C. Richard Hagen, and Tom Kibble at Imperial College in London. All three groups published their first papers within the space of a few months.
Five of the theory’s six founders are still living (Brout died in 2011), and the fact that the science has gravitated around the title “Higgs boson” is still a matter of contention in the particle physics community. (It also creates a thorny problem for the Nobel Committee, which only allows up to three people to share credit for any given discovery.) Many argue that Higgs’s contribution to the theory remains the most important because he was the first person to actually postulate new particles, but there were also some historical blunders in credit attribution. Steven Weinberg wrote a famous paper in 1967 calling attention to the new particle, but mistakenly cited Higgs first in his reference list despite the fact that Brout and Englert’s paper was published earlier.
Regardless, Higgs and his contemporaries made their discoveries by exploring the implications of the fact that in quantum mechanics every fundamental particle can be classified into one of two categories—as a boson or as a fermion. This classification dictates specific rules about how the particle will behave in the presence of other particles like it. In short, fermions like to have their own space, while bosons tend to clump together. It is a deceptively simple principle with far-reaching consequences. The fermionic nature of electrons undergirds the basic structure of the periodic table, and by extension all of chemistry. The bosonic nature of photons enables the existence of lasers.
In 1961 Jeffrey Goldstone took one aspect of the theory of bosons and fermions to a logical extreme and discovered that fundamental bosons have to be massless under a broad set of circumstances. Higgs and his contemporaries made their great contribution to science by finding a loophole in this theorem: certain types of fundamental bosons do acquire mass in one special case. Later on it would turn out that this scenario precisely matched the circumstances that exist in our current understanding of the universe. For instance, it would eventually be discovered that the W and Z bosons, which are responsible for radioactivity, both have mass.
At the time that Higgs was originally doing his work, though, none of this was yet known. The journal Physics Letters rejected his original paper after initial review, judging the work “of no obvious relevance to physics.” Undeterred, Higgs added two sentences to the manuscript postulating that his idea would also imply the existence of new type of boson (later to be named in his honor), and mailed the paper off to the competing Physical Review Letters. It was published in October of 1964, and remains today one of the most highly cited papers in the history of physics. (Physics Letters did not completely miss out on the discovery. They did publish a prior manuscript Higgs authored laying the groundwork for the October publication.)
Today, the Higgs boson is an integral part of the Standard Model, a theory that, despite its underwhelming name, is the world’s most complete and fundamental picture of testable physics. In the Standard Model, there are only 17 different particles, and all of the visible matter in the universe springs forth from these basic ingredients. The Higgs boson’s most notable role within the theory is that its existence explains why the fundamental particles in the Standard Model—bosons and fermions alike—can have mass, through a phenomenon called the Higgs mechanism.
The idea is that the entire universe is embedded within a mathematical construct called a quantum field (the Higgs field) and that particles traveling from one place to another in space acquire mass by interacting with this field. The physicist David Miller famously put this into layman’s terms for the British Parliament by describing the Higgs field as akin to a room full of politicians. If a random person walks through the room, no one pays any attention and he can pass from one corner of the room to another with relative ease. This person has no interaction with the rest of the Higgs field and therefore has no mass. In contrast, if the prime minister enters the room, politicians notice and flock about her in a big huddle. It takes a great deal of effort for the prime minister to get moving, and an equal amount to slow down once movement has been initiated. This would be the case for a very massive particle. Pushing the analogy further, a Higgs particle could be conjured up by starting a rumor. Politicians would huddle together as the rumor is being whispered about, and the huddle would take on the characteristics of a particle.
The ability of the Higgs to give other particles mass is undoubtedly why the “God particle” nickname has stuck, although the originally intended meaning of the nickname is not entirely clear. It first appeared in the title of Leon Lederman’s popular science book on the subject. Lederman has said, perhaps in jest, that it was a shortening of “goddamn particle,” so-named because experimentalists were finding the Higgs particle so difficult to find. There are several reasons for the difficulty in finding the Higgs. It carries no charge, so it does not easily scatter off of other particles. It turns out to be extremely heavy (as far as particles go), so only a massive particle accelerator like the LHC could generate collisions with sufficient energy to produce the Higgs. It is unstable, and decays into other types of particles almost immediately after being created.
Perhaps the most remarkable thing about the past year’s discovery is the predictive power of the original work. “It’s a huge triumph of theory,” said Heinemann. “And it’s really amazing that this project was carried out over multiple generations of people. To me, it’s just very fascinating what human minds can achieve.” She cited the discovery as an example of what Eugene Wigner called “the unreasonable effectiveness of mathematics” in the title of a famous 1960 essay. In it, Wigner argues that there is an uncanny ability of mathematics to explain and predict physical phenomena, as well as to raise deep and sometimes unsettling questions about the nature of truth. Ultimately, he deems it “a wonderful gift which we neither understand nor deserve.”
Mike Hance, an LBL postdoc working on ATLAS with Heinemann, was similarly fascinated by the prescience of the original theories by Higgs and his contemporaries. “We’re talking about a theory that’s 48 or 50 years old at this point,” he said. “We found exactly the particle that this group of guys predicted back in the 60s. It has all the same properties, and we found it in exactly the way that the people writing detector proposals in the 1980s predicted that we would find it. It’s really incredible that a theory could have survived this long and been this successful over that period of time.”
Looks like a Duck, Quarks like a Duck…
The search for the Higgs particle is not over yet. In the immediate future the LHC’s physicists are trying to discern whether the new particle is actually the Higgs (as defined by the Standard Model), or merely “Higgs-like.” Heinemann said that as of the July 4th announcement, the ATLAS and CMS experiments have officially announced that a new particle of some sort has been discovered with a mass of about 126 GeV. This is about 250 thousand times heavier than an electron, but slightly lighter than the heaviest fundamental particle, the top quark. The term discovery has a specific meaning in particle physics: there is a 5-sigma confidence that the 126 GeV signal is real. In other words, the odds are less than one in a million that the signals measured by the CMS and ATLAS detectors are caused by spurious noise.
Beyond this, it is also known that the new particle is a boson and that the particle has an even spin. (Spin, which resembles angular momentum, is a fundamental property of all the elementary particles.) There are no other elementary particles in nature with these characteristics, so the recent discovery is already unique. In order to fully be acceptable under the classification of the Standard Model’s version of the Higgs boson, measurements have to confirm a laundry list of characteristics. Particle physicists are feverishly working to sort these out. Heinemann said that two necessary Higgs characteristics—that the newly discovered particle have a spin of zero, and that its parity be even—have already been confirmed with about 90 percent confidence.
Fantasies and Nightmares
Exciting as the discovery of the new Higgs-like particle is, particle physicists are slightly anxious these days. In the years leading up to the LHC’s inauguration date in 2008, theorists and experimentalists alike often divided the prospects for the future of particle physics into one of three categories.
First, the LHC could find nothing, and thereby establish that the Higgs boson does not exist. “Had it not been there, then it would be clear that basically our entire framework for how particle physics works was inaccurate,” said Skinnari. “It would have been in one way a bit of a disaster, because it would mean there’s something very fundamental, which we just don’t understand at all. But at the same time it would have been extremely interesting.”
Second, the LHC could find the Higgs, which would confirm the basic premises of the Standard Model, but new and unforeseen discoveries would follow rapidly on the heels of the Higgs announcement. There could actually be more than one Higgs particle, for example, or other new and completely unforeseen particles could emerge from the machine. Particle physicists often term this the “dream” scenario, because it would light the way for a host of theoretical pursuits.
And finally, the LHC’s future could unfold into a sort of “nightmare” scenario, where the discovery of the Higgs boson is confirmed, and where the experimental Higgs perfectly matches the predictions of the Standard Model, but where nothing else is discovered. This would shore up the Standard Model, while simultaneously cutting off all hope for future insights left to be gained from collider physics. To be fair, confirming the Standard Model would be an enormous accomplishment in and of itself. Physicists are also keen to point out that particle physics would still survive as a broader field, and that the LHC still has a long life of measurements yet to be taken, perhaps even lasting 20 years or more. Still, if the Higgs turns out to look exactly as the Standard Model predicts, and no additional particles are discovered, it could ultimately sound the death knell for particle colliders. “At that point, you really have sort of an existential crisis for the field,” said Hance. “It’s hard to ignore the fact that the LHC costs somewhere between 5 and 10 billion dollars to build. Will we have a chance to invest that kind of money again in a [newer] facility, where we may not be able to find anything new if there’s nothing else out there? It would be a tough sell to Congress.”
The concern is not only about job security. “The Standard Model and Higgs boson give us certain inconsistencies in the scale at which things happen that we don’t quite understand,” said Zenz. One of these is an issue in particle physics called the hierarchy problem. It turns out that if you step back and examine the Standard Model from a broad perspective, there should be a certain natural mass scale for the Higgs particle, and this mass scale turns out to be 17 orders of magnitude larger than the experimentally constrained Higgs particle mass, a huge discrepancy.
Stepping back further, the Standard Model is also incomplete in the sense that, while it provides a nearly perfect model for the interactions between particles at small scales and high energies, it has absolutely nothing to say about the role of gravity. Cosmological experiments have also raised further questions about the most fundamental interactions between particles. Since 1933 scientists have seen a steady accumulation of evidence for mysterious “dark matter,” which causes galaxies to rotate faster than they should. More recently, an even more mysterious “dark energy” has been discovered, responsible for the accelerating expansion of the Universe. The Standard Model has been able to offer no help in understanding either problem, despite the fact that in combination, dark matter and dark energy comprise a whopping 95 percent of the Universe’s energy.
With the development of the LHC it was hoped that new insights might be gleaned toward finding a solution to any or all of these questions. “The risk is not that we’re going to run out of things we don’t understand,” said Zenz. “The risk is that we might run out of new ways to attack them.”
With the recent Higgs-like discovery, at least the first of these three scenarios has been ruled out. Zenz, Skinnari, Heinemann, Hance, and the other particle experimentalists and theorists working at the LHC are still waiting to find out whether the second or third scenario seems to be taking effect.
On the one hand, there have been two recent updates on the Higgs analysis in November and in March, and almost all of the Higgs properties thus far characterized are consistent with the Standard Model. “On some level I was a little bit disappointed in November with the updates,” said Skinnari. “In July there were some fluctuations where some rates were a bit too high, and CMS was close to not seeing anything at all in one channel [where events should have been measured], but then in November none of those early indications were enhanced in any way.”
On the other hand, there are still a great many opportunities to search for physics beyond the Standard Model left in the LHC’s lifespan. In 2015 the experiment is scheduled to start up again, with the energy of proton collisions increasing first to 13 TeV and eventually to 14 TeV, nearly twice the energy that had been explored previously. Beyond that, there are many engineering upgrades planned for 2018, which will accelerate the LHC’s ability to accumulate statistics. “The LHC has a lot of life in it left,” said Zenz. “And there are still reasons to believe we’ll see something.”
Heinemann said that some of the most exciting things particle physicists might see 10 years down the road at the LHC might include “more Higgs particles, new types of quarks, or other new forms of matter—which could give us dark matter candidates.”
“The most exciting that we could see when we turn on at 13 TeV is a lot of really crazy looking events that look like they are coming from the decays of, for instance, supersymmetric particles,” said Hance.
Zenz and Skinnari hoped for discoveries beyond what anyone can imagine. “I’m not desperate for any one particular framework of new physics to happen,” Skinnari said. “If anything, I would like to see just something that really doesn’t make sense, something that is not at all consistent with what we expect. Because that would be so much fun, having something completely unknown discovered and trying to figure out what it is.” Whatever the outcome, the years 2015 and 2016 will be “a very important time to determine whether there’s going to be a clear new physics world to explore, or whether the Standard Model is going to live through another decade of being the answer to particle association.”
This is the ultimate question on everyone’s mind, and for better or worse, the only way to come by a satisfactory answer is to keep on improving the experiment, collecting data, running analyses, and following trains of logic to their natural conclusions. Heinemann recalled an old saying in physics and philosophy: “It’s very hard to make predictions, especially about the future.” The aphorism certainly holds for the role of particle physics in science, as well as for the future of the LHC. Ultimately, the world of physics has weathered many storms such as these, and while the future is uncertain, physicists can always be counted on to continue probing the depths of the unknown universe.