People sometimes tell me they don’t like science. It’s a dry collection of obscure facts, they might say, and just isn’t exciting. Those folks might have a different opinion if they had been sitting with me just before dawn one night this past March.
I was at the CERN laboratory, located on the border of France and Switzerland, watching the world’s newest and most powerful subatomic particle accelerator (often called an “atom smasher”) come to life. This scientific marvel is called the Large Hadron Collider or LHC.
As the first hints of the sun appeared behind the Alps, just beginning to chase away the black of night, I was glued to the computer monitors. The accelerator technicians, engineers and scientists had, for the first time, placed two beams in the machine and were attempting to raise the energy far above anything ever achieved before.
Slowly, over the course of 75 minutes, the energy was raised, first crossing the familiar border long dominated by the Fermi National Accelerator Laboratory’s accelerator and then pushing upward to the goal, three-and-a-half times what had been considered the frontier for the last quarter century. It was analogous to a jet airplane coming online in an era of propeller-driven ones. The researchers were jittery, and it wasn’t just because of the multiple espressos that seemed to be the drink of the night.
There were many reasons to be nervous. A similar attempt 18 months earlier had resulted in an electrical arc that had initiated a cascade of failures, resulting in tremendous damage. Tens of thousands of work hours and many millions of Swiss francs had gone into the repairs. Another incident might have caused the project to be canceled. Although the accelerator scientists were confident they had not overlooked anything, it was still a nerve-racking moment.
Putting their jitters aside, the experts followed meticulous protocol, feeding thousands of amps of electrical current into more than a thousand huge magnets. The amount of energy they were trying to control so precisely is the same amount that could melt about 10 tons of pure gold. This amount of energy could severely damage the equipment if it ever got out of control.
At 5:20 in the morning, they did it: two stable counter-rotating beams of protons, circulating at the expected energy. A new era had dawned.
A media circus still lay two weeks in the future, when the two beams would be intentionally collided in front of the world’s press. But on that day in March, we knew that the accelerator would function and an entire new frontier of knowledge lay before us, just waiting for scientists to explore.
The Large Hadron Collider now has been operating for half a year, colliding beams of protons at energies far higher than had previously been possible. The purpose of colliding these beams is to heat matter to temperatures last seen only in the early universe, just scant moments after the instant of creation. The scientific goal of the LHC research program is no less than to understand the nature and the origin of the universe itself.
Several members of Notre Dame’s Department of Physics are playing a crucial role at this frontier of knowledge. These ND professors and researchers study collisions that take under a trillionth of a trillionth of a second to occur. The collisions result in the most concentrated energy mankind has ever created. Indeed, the last time this energy density was common in our universe was less than a trillionth of a second after the universe began.
Over the last century, scientists have used great telescopes to look deep into the heavens. From studies of this sort, physicists believe that the cosmos was formed about 14 billion years ago in a cataclysmic explosion called the Big Bang. In 1925, the Belgian theoretical physicist and Catholic priest Georges Lemaître, at the Catholic University of Leuven, showed how Albert Einstein’s then-new General Theory of Relativity suggested that all of the matter of the universe was once concentrated in a single spot. This “primeval atom” expanded to the glorious cosmos we see today.
During the first instants of creation, this tiny universe was mind-bogglingly hot, with small particles flying about with tremendous energies. These particles crashed into one another with abandon, changing energy into matter and back again according to the rules laid out by Einstein more than a century ago.
It is only in the last hundred years or so that scientists have been able to recreate these early conditions. From the first tiny particle accelerators that can easily fit in the palm of your hand, our technical mastery has grown, resulting in the huge particle accelerator in Europe which can study the behavior of matter as it once existed in the early universe.
The Large Hadron Collider is big. Really big. Imagine a pipe a couple of inches in diameter and 17 miles long. Now bend the pipe into a huge circle and bury it in a tunnel 300 feet underground. Add 10,000 magnets, some of them 50 feet long and weighing 35 tons, and you’ve got the basic idea.
The performance numbers of the LHC are amazing. While this particle accelerator is accelerating beams of protons to energies three-and-a-half times higher than those achieved by the Fermilab Tevatron, the previous record holder, in a couple of years the LHC’s beam energies should rise to a full seven times greater than the Tevatron.
In addition, the beams will be very bright, causing many more collisions per second. The LHC’s design is for a beam brightness that is about a hundred times higher than the Tevatron’s. An envisioned future upgrade would increase the beam brightness to be 10 times higher than that. Just like the Tevatron before it, the LHC is expected to dominate the frontier of high energy research for 20 years.
While the accelerator is truly a marvelous bit of engineering, in many respects it’s just a tool. It can accelerate two beams of protons to nearly the speed of light and collide them at will. However, in order to learn about the science of creation, you need to record the collisions. For that, you need a particle detector. It is here that ND physicists join the story.
The LHC hosts four huge particle detectors. As with any interesting engineering problem, each of them reflects different design choices, with different capabilities. The experiments are known by acronyms: ALICE, ATLAS, CMS and LHCb. The CMS or Compact Muon Solenoid is the focus of research by ND physicists.
The basic numbers of the CMS detector are staggering. It is 50 feet high, 50 feet wide and 70 feet long. It weighs 12,500 tons. Inside, a full 100,000,000 distinct, individual components are arranged to record every detail of the collisions. It can inspect 800,000,000 collisions each second and automatically select the hundred or so that might hide a discovery.
Conceptually, one should think of it as a series of nested cylinders, soup cans of different sizes, nestled inside one another like a series of Russian matryoshka dolls. Nearly 3,000 physicists from four continents, 38 countries and more than 180 institutions were required to design, build and operate it. Toss in a similar number of engineers, technicians and computer professionals, all working for over a decade, and you get some idea of the scope of the effort.
The innermost detector carefully measures the trajectory of the hundreds of particles that come out of a typical collision. This layer is followed by equipment that measures the energy of these particles.
One such device, the electromagnetic calorimeter, or ECAL, is designed to carefully measure the energy of electrons and photons that are created in the particle collision. It is made of about 80,000 blocks of high-quality lead glass, similar to the crystal decanter your mother might have received for a wedding present. Imagine 90 tons of uncut Waterford crystal, glittering like diamonds.
Professors Colin Jessop and Nancy Marinelli have played a pivotal role in bringing the ECAL to life. With the help of other ND researchers and graduate students, they have worked on the software that will read out the data from this amazing bit of equipment. By writing programs that look at the integrity of the early data and report anything peculiar, the team has managed to significantly shorten the time it has taken to ensure the recorded data is of good quality.
Another critical task in which ND physicists participate is shaking down the trigger system. The most interesting phenomena that physicists are looking for are extremely rare. When the LHC is operating at full capacity, it is expected to generate 800,000,000 collisions per second. Since CMS acts essentially like a camera and can only record about 100 “photographs” each second, it is imperative that the experimenters be picky and that only the most interesting collisions are recorded. The trigger is a combination of electronic circuitry and fast programs that tell the equipment which collisions should be recorded for further study. It’s a daunting challenge, but the early indications are that the trigger is working marvelously.
“When I joined in 2004, the group was still small, because it was too early for Ph.D. students to join CMS,” says Jessop, “But now we have 18 people working on the experiment, and they’re making a big impact.” Graduate student Jamie Antonelli seconds that sentiment. “Our trigger work on the ECAL will ensure that we will record the most interesting collisions.”
This interest in the CMS ECAL has a scientific purpose. Scientists have spent the last century trying to uncover the ultimate building blocks of the universe. We now know that matter is made of atoms and that atoms themselves are made of protons, neutrons and electrons. In the 1960s, it became clear that protons and neutrons were composed in turn by particles called quarks.
Research performed over the last 40 years has discovered six types of quarks, with peculiar names: down, up, strange, charm, bottom and top. The heaviest quark (the top quark) was discovered in 1995 at Fermilab just outside Chicago by a team that included myself and other Notre Dame physicists.
We now know quite a bit about these quarks, but one mystery has perplexed physicists for several decades. Indeed to solve this mystery is the main reason the LHC was built. The quarks have very different masses — the ultra-heavy top quark is 100,000 times heavier than its cousin, the up quark. We do not yet understand why the masses of the quarks should vary so much.
In 1964, the Scottish physicist Peter Higgs took some of the ideas that were floating around at the time, added a few of his own, and made a proposal. Perhaps the reason particles have a mass is because there is an as-yet-undiscovered energy field in the universe. This energy field would interact with particles differently. Much like a barracuda can slip swiftly through water, while a sumo wrestler can only slog along, the ultra-light electron would barely interact with the Higgs field and the super-heavy top quark would interact a lot.
Forty-six years later, Higgs’ hypothesis has not been verified, but it’s not from lack of trying. His theory predicts a new particle, now called the Higgs boson. Over the last decades, many experiments at Fermilab, also with ND faculty involvement, have focused on trying to verify Higgs’ theory, to no avail. A lot of indirect evidence supports Higgs’ idea, but it will remain just a favored hypothesis until real evidence is uncovered.
So how can the Higgs field be discovered at the LHC? Among dozens of possibilities, a favored one looks at what happens when a Higgs boson converts into two photons, which is the decay mode predicted by Higgs’ theory. These photons are called gamma rays — essentially ordinary light on steroids. The ECAL, on which Notre Dame physicists work, is designed specifically to search for these decay products.
If physicists find particle collisions in which two high-energy photons are created, the scientists believe that eventually the Higgs boson will be observed, if it exists at all. “Searching for the two-photon decay of the Higgs boson is exceedingly tricky,” says graduate student Doug Berry, “but the ECAL was designed for precisely this task. If it’s there, we’ll find it.”
This research is literally a journey into the unknown. We don’t know the answers. The ideas that seem attractive now may be correct. Or the universe could be laughing at our hubris, and our research will reveal something nobody has ever considered. Indeed, the prospect of being the first to understand a fundamental truth about our cosmos is a driving factor for many a scientist.
The discovery of the Higgs boson is not the only goal of the LHC experiments. During the first several years of operations, it is expected that physicists will make thousands of measurements using this data. With so many possible things to study, one must be careful to select topics that will make a big impact on our understanding of the universe. One important topic is to carefully study the heaviest particle thus far discovered, the top quark.
For those who missed it in physics class, here is a quick quark primer. The natural home for quarks is ensconced in the center of atoms. The familiar proton and neutron are composed of two of the six known quarks, the up quark and the down quark. In fact, with enough up and down quarks, and with electrons to complete the atoms, you could make all of the matter in the entire universe.
While the up and down quark pair is found in ordinary matter, two other pairs of quarks, called “charm and strange” and “top and bottom,” exist. (The names are only an example of scientists being overly cute.) The only way we can make the other quarks at will is in particle accelerators. However, in the early universe, when the energies were very high, these other quarks also were present in abundance. Thus we need to study these quarks if we are to ever understand the history of our universe. Just as we must unearth extinct forms of life to appreciate the rich diversity of living things on our planet, these four other quarks can be thought of as fossils that can be briefly brought to life and studied in modern scientific equipment — a Jurassic Park of the subatomic realm.
To measure the properties of the top quark is an excellent research choice. First, the top quark has already been discovered, so it is guaranteed that we will make a measurement and not just search for something that turns out to not exist. Second, because the top quark is the heaviest of all known particles, it would interact more strongly with the Higgs boson than the others do. Thus top quark measurements are expected to guide physicists in their search for this still-undiscovered particle.
Finally, because the top quark is so heavy, a tremendous amount of energy is required to make it. It is in the highest energy collisions that we should expect to see new physical phenomenon, and thus choosing to study the top quark puts Notre Dame physicists in the center of the action.
Kevin Lannon, an ND assistant professor of physics, is an acknowledged world expert on the top quark. He and his research aides are already digging through the data, looking for the first evidence of top quarks at the LHC. As the LHC beams become more intense, the collider will become a top quark factory, churning out prodigious numbers of them.
“The top quark is an ideal laboratory in which to search for new physics,” says Lannon. “It was only discovered 15 years ago, and it certainly hasn’t yet revealed all its secrets.”
In order to search for top quarks, physicists must utilize another energy measuring device, the hadronic calorimeter or HCAL. Hadrons are particles that contain quarks within them, and the most familiar hadrons are the proton and neutron. Hadrons interact with matter differently than electrons and photons, so a second detector technology must be utilized. An important component of the HCAL was manufactured on the ND campus and shipped to Europe for installation. The fact that the equipment was built in South Bend gave invaluable research experience to on-campus undergraduates.
The projects listed here do not exhaust all of ND’s efforts to make the LHC experimental program a success. For instance, Professor Mike Hildreth has taken a leadership role in writing computer programs that simulate the CMS detector’s response. Simulation is an important tool to gain insight into how well the equipment is working. “I like working on simulation because the work can be done anywhere, even here in South Bend,” says Hildreth. “It’s also something that even undergraduate students can do with only a modest amount of training.”
The physics professors can set the research direction, but they also teach and serve on many committees. Much of the detailed research is done by the graduate students and postdoctoral researchers under the professors’ supervision and guidance. At any one time, ND has about eight people stationed at CERN. By living at the laboratory, the students know everything that is going on.
“It’s a great opportunity to live at CERN in the early days of the LHC program and help the experiment get started,” says graduate student Ted Kolberg. “It’s very exciting to be here at such a critical time, and we’re all looking forward to what’s coming up.”
ND’s LHC program recently received national affirmation. In a time of tight national budgets, its grant from the National Science Foundation was increased by 20 percent. And in a national competition, Professor Lannon received an Early Career Grant, given to only a select few junior faculty each year. “The research group has demonstrated its significant impact to the big funding agencies,” says physics department chair Mitch Wayne.
With the start of operations in March 2010, the LHC has given humankind a new tool to study the world, an entirely new window from which to peer at the moment of creation itself. The higher energy will allow us to look for things we could never see before, just like a much taller ladder lets you pick fruit from a taller tree. The last time we were in the position of exploring an entirely new energy regime was a quarter century ago. These are exciting times, and the Notre Dame physics family will be there, blazing a new trail into the quantum frontier.
Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory and an adjunct associate professor at ND. He is the author of two books about particle physics for the public, including The Quantum Frontier: The Large Hadron Collider. Follow his writing on cutting-edge science topics at facebook.com/pages/Don-Lincoln/10095813788.