October 12, 2016
Harvard students join faculty at CERN in Europe to tackle physics’ mysteries
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Once you know enough math, Harvard Ph.D. student Tony Tong said, you get to know physics. And physics, he said, is simply amazing.
“[Physics] is always helpful to answer the question of ‘Why?’ Why the skies are blue, why the universe is so big, basic stuff,” Tong said. “I’m always curious about those questions and the solution is always so beautiful.”
Tong, it seems, had come to the right place. He was speaking on a warm July day in a small courtyard at the European Organization for Nuclear Research, known as CERN, the scientific campus on the outskirts of Geneva that is the world’s beating heart for high-energy particle physics.
LHC at CERN
Home of the world’s most powerful particle accelerator, the Large Hadron Collider (LHC), CERN made world headlines in 2012 when scientists announced the discovery of the Higgs boson, the final undiscovered particle in the theoretical framework of the universe called the Standard Model.
CERN CMS Higgs Event
CERN ATLAS Higgs Event
The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
The eyes of the scientific world remain focused on CERN today because the LHC is back in operation after a major upgrade that boosted its energy to 13 tera electron volts, allowing it to crash beams of protons into each other more powerfully than ever before. Now that the Standard Model is complete, scientists are looking for what’s still mysterious, sometimes called the “new physics” or “physics beyond the Standard Model.” Its form, presumably, would involve a particle born of these high-energy collisions, one that points the way to an even broader understanding of the universe, shedding light on such puzzling areas as dark matter, supersymmetry, dark energy, and even gravity, which has stubbornly refused to fit neatly into our understanding of the universe’s basic forces.
Standard model of Supersymmetry DESY
CERN fired up its first accelerator in 1957. Among its milestone discoveries are the elementary particles called W and Z bosons, antihydrogen — the antimatter version of the common element — and the creation of the World Wide Web to share massive amounts of information among scientists, scattered at institutions around the world.
The CERN campus, which straddles the Switzerland-France border amid breathtaking views of the distant Alps, produces more than just science, however. In ways technological, theoretical, educational, and inspirational, it also produces scientists.
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Inside the Antimatter Factory at CERN, the ATRAP antimatter experiment seeks to slow and trap antimatter for comparison with ordinary matter.
“Those four years at CERN doing research were a very important part of my training,” said Harvard Physics Department chair Masahiro Morii, who was a research scientist at CERN early in his career. “It taught me things that are a bit difficult to quantify, but changed my perspective very drastically on what it means to be a scientist, what it means to be a high-energy physicist.”
Year-round, the graduate students and postdoctoral fellows taking their initial career steps work among established scientists, learning and gaining experience difficult to get outside of CERN or a handful of other facilities around the world. Harvard’s Donner Professor of Science John Huth said what becomes apparent is science’s messiness.
“They see the process as it unfold with all its warts. Science is pretty messy when you get into the nitty-gritty,” Huth said. “It’s just an invaluable experience. Even if you become a scientist in a different discipline or you leave science entirely, understanding that intrinsic messiness is really important.”
In an environment focused on the practice of physics rather than the teaching of it, CERN puts the onus for learning onto the student, Morii said. Students build and test equipment, make sure what’s installed is running properly, and pluck the most meaningful pieces from the resulting data tsunami. They analyze it at all hours of the day and sometimes deep into the night, since there’s always someone awake and logged onto Skype to answer a question or share an insight.
“People are really passionate, so it doesn’t really feel like you’re up until 11 doing your job. Maybe you’re thinking about something on the train home and you wanted to look into it. It’s not regular hours, but I don’t think that deters anyone,” said Harvard physics Ph.D. student Julia Gonski. “People like the work and it’s fun. Twenty-four hours a day, you can get on Skype and someone you know is on Skype and working.”
While fellows and graduate students are at CERN year-round, each summer the campus’ population swells as undergraduates eager to take part in the world’s most famous science experiment step off the plane in Geneva.
At CERN, they become part of a unique city of physicists from around the world, with different educational and cultural backgrounds but the same passions and similar goals.
“It was this enormous scientific laboratory, with thousands of people working all hours of the night trying to understand the fundamentals of the universe, as corny as that is to say,” said Harvard postdoctoral fellow Alexander Tuna, who first came to CERN as a summer undergrad from Duke University in 2009. “It was really immersive and fun. There’s always someone around with an interesting insight or an answer to a question.”
The secrets of the universe
As a visitor approaches CERN, the giant brown orb of the multistory Globe of Science and Innovation comes into view.
The globe, looking like an enormous particle half-buried in the earth, serves as a CERN welcome center and is far more visually appealing than the main campus across the street. Protected by fences with access limited through guard stations, the campus’ narrow, twisting roadways wind between boxy, industrial-looking buildings numbered instead of named, as if creativity there is reserved for science instead of infrastructure. Even the cafeteria that serves as a central gathering spot is named simply “Restaurant 1.”
“It was different than I expected,” said Harvard junior Matthew Bledsoe. “I figured a place on the forefront of physics would look fresher and newer, new buildings and stuff. But [they are] 1950s and ’60s-era buildings, so the buildings are pretty old. It looks like a factory.”
Visitors quickly learn to look past the boxy exteriors to what’s inside. There they find thousands of people working on 18 experiments, seven associated with the LHC and the others with smaller accelerators and a decelerator, which is used for antimatter experiments like those run by Harvard Physics Professor Gerald Gabrielse’s ATRAP collaboration.
ATRAP, short for “antihydrogen trap,” relies on the LHC’s high energy to make protons collide with a target to create antiprotons. The experiment then cools and slows the antiprotons, and combines them with positrons, the antimatter equivalent of electrons, to create antihydrogen for study and comparison with ordinary hydrogen. Gabrielse, who pioneered antimatter experiments at CERN, said that for students who want to go into high-energy physics, getting a taste of the enormous collaborations that are behind such experiments is key.
“If you’re interested in making a career in doing those kinds of things [experimental particle physics], it’s extremely important to have this experience,” Gabrielse said.
The LHC, with its potential to pierce the veil between the known world of the Standard Model and the mysteries that the model does not address, takes center stage. Yet to visitors wandering the halls and sidewalks of CERN, the LHC is nowhere to be seen.
That’s because the LHC is buried 300 feet underground in a massive tunnel that runs 17 miles from Switzerland into France and back again. Its twin proton beams circle in opposite directions, crossing four times on their journey. At those crossings are four major particle detectors, one of which is ATLAS, a massive machine backed by a worldwide collaboration in which Harvard scientists play lead roles, and which was one of two experiments to detect the Higgs boson.
Outside the ATLAS control room at the LHC. Joe Sherman/Harvard Staff Photographer
“You can think of it (ATLAS) as a really large camera surrounding the collision point where protons collide,” Tuna said.
ATLAS, which stands for A Toroidal LHC Apparatus, is 180 feet long, 82 feet in diameter, and weighs 7,000 tons. When the proton beams collide, they scatter particles in all directions. ATLAS dutifully records these collisions, producing far more data than current computing technology can store, so filters are employed that screen out more mundane results and keep only the most promising for analysis.
The complex undertaking requires a collaboration that is as massive as the task the researchers have set for themselves. It includes about 3,000 physicists from 175 institutions in 38 countries.
“This is the center of particle physics right now,” said Harvard Ph.D. student Karri DiPetrillo. “As a scientist, you like asking nature questions and seeing what the answer is. Because we have thousands of people working on a single experiment, you know we’re asking some of the hardest questions in the universe. If it takes thousands of people to find the answer, you know that it’s a good question.”
For decades, physicists exploring the most basic particles that make up the universe were guided by the Standard Model, which held that everything is made of a limited number of quarks, leptons, and bosons. Over the years, one by one, experimental physicists, including Harvard faculty members, found the particles predicted by the theory: bottom quark, W boson, Z boson, top quark. In 2012, they found the Higgs boson, the last theorized particle.
When the huge hubbub over the Higgs discovery faded, particle physicists began to assess the field’s new reality. After decades in which theoretical physicists were leading, telling experimental physicists what new particle to look for, the roles are now reversed.
As reliable as the Standard Model has been, it doesn’t explain everything. And, while theoretical physicists have several ideas of where those mysteries might fit into current knowledge, no evidence exists to tip the scales toward one idea or another.
Even the Higgs boson still holds secrets, as detecting it didn’t completely explain it. Scientists who continue to probe the Higgs boson hope that the particle may yet reveal clues — inconsistencies from what is expected from the Standard Model — that will outline the broader path forward.
“There are really two paths. One path is to really push on what we understand about the Higgs boson because that has the strangest properties associated with it and if you push the theory at all the Higgs creates the most problems for it,” Huth said. “The other is the discovery region for something new, like dark matter.”
The undergraduate summer
A scientist’s path to CERN usually starts with a passion for physics. Graduate student Nathan Jones credits a family road trip to Colorado during which he read a library book about the universe. Undergrad Bledsoe was wowed by a trip to Fermilab outside Chicago as a high school freshman, while grad student Gonski traces it to the annoyance she felt when she learned her high school chemistry teacher had gotten the science wrong.
“I remember being in chemistry class in high school when they told us protons and neutrons are indivisible,” said Gonski, who learned otherwise from Stephen Hawking’s “A Brief History of Time.” “I was so offended … I remember being frustrated and asking my parents, ‘Did you guys know?’ At that point I wanted to see how far down we can go [in particle size].”
After that initial spark, students take classes and often work in a campus laboratory before heading overseas. Some undergraduates go to CERN through the Undergraduate Summer Research Experience program run by the University of Michigan for students across the country. Several Harvard students benefitted instead from the Weissman International Internship Program Grant, established in 1994 to provide faraway opportunities for them.
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A field of sunflowers stands at the roadside on the approach to CERN.
Once the funding is set, there’s nothing left but the plane ride and moving into their new digs. Undergraduates live in settings ranging from downtown Geneva to the French countryside. Last summer, three Harvard students — Ben Garber ’17, Gary Putnam ’17, and Bledsoe — rented an apartment over the border in France and commuted to work each day by bike, while Katie Fraser ’18 stayed closer, at CERN’s on-campus hostel.
Days consisted of morning lectures on topics relevant to their work. After those lectures — and the occasional pickup basketball game at lunchtime — they’d spend afternoons working on a project. Garber worked with Tuna and DiPetrillo on an analysis of Higgs boson decay (the particle itself exists for a tiny period of time) into two W bosons. Bledsoe worked on hardware, building and testing a circuit board to be used in the planned 2018 ATLAS upgrade, in the cavernous Building 188 under the tutelage of Theo Alexopoulos from the Technical University of Athens. Wherever they were, whether doing project tasks or having cafeteria conversations, the students were steeped in physics.
“It was a lot of fun, different than I expected. You learn stuff just by being there, pick up vocabulary in lunchtime conversations,” Fraser said. “It definitely solidified my desire to go into high-energy physics.”
Melissa Franklin, Mallinckrodt Professor of Physics, said lessons can be found behind almost every door at CERN.
“I was just amazed, it was unbelievable,” said Franklin, who first visited between her undergraduate and graduate years. “I went to every place I could on site and just knocked on doors and bugged people … You learn so much by osmosis. You have to learn to hang around and ask good questions.”
Jennifer Roloff, a Harvard physics Ph.D. student, first came to CERN in 2011 as an undergraduate and has been back every summer. Now she helps manage the University of Michigan summer undergraduate program, which gives her a broad view of the student experience.
“There are definitely some students who do miss home,” Roloff said. “For a lot of them it’s the first time out of the country [or] the first time long-term out of the country. For a lot of them, they realize this is not what they want to do. CERN is not for everyone. There are challenges and difficulties that are not in other physics.”
That understanding, Gabrielse said, is as important a lesson as finding your intellectual home.
“Some decide, based on it, to go into the field. Some decide not to,” Gabrielse said. “That guidance too is valuable.”
Yet being at CERN is not just about science. Students have their weekends free and can explore their new surroundings. Some hike the Alps or the closer Jura Mountains. Others walk the ancient streets of Geneva, visiting its lakefront, restaurants, museums, and other attractions. Putnam loved a park near the University of Geneva where people played on large chessboards with giant pieces. He also soaked up the area’s natural splendor.
“It’s so beautiful here,” Putnam said. “Sometimes I forget and do the normal thing of looking down and not paying attention, but being able to look up and see the mountains is really special.”
On call for a particle emergency
Life at CERN as graduate students is not quite so fancy-free. Visits are limited to summers early in graduate careers as they complete coursework, but once that’s done, they can come and stay to conduct dissertation research.
To keep the ATLAS collaboration running, graduate students are required to spend a year of research time doing work to benefit the experiment itself, to ensure that high-quality data is collected, for example, or that potentially significant collision events aren’t lost in the data.
“We have to make sure the data we’re receiving is like you expect it, ready for analysis,” DiPetrillo said. “[It’s taken] probably half of my time in the last year; the other half has been working on Standard Model measurement of the Z boson.”
A little physicist humor written on a CERN blackboard. Joe Sherman/Harvard Staff Photographer
Part of DiPetrillo’s duty is assisting in ATLAS’ day-to-day operation, working in the ATLAS control room — with its Mission Control feel, and dominated by a wall-sized screen — and monitoring one of several subsystems that make the whole operation work. Monitoring those subsystems makes ATLAS a 24/7 proposition.
In addition to working overnight in the control room, DiPetrillo is often on call to back up someone on site. While on call, she has to stay near her phone and within an hour’s drive of the facility in case something goes wrong. If that happens, she troubleshoots the problem with the person in the control room or pushes the problem up to someone more senior.
“You can think of ATLAS as always taking data so we always need people watching it, making sure ATLAS is working in a way that we want [it] to, that the detector is working … and that data looks the way we expect,” DiPetrillo said.
When not on call or manning the control room overnight, a graduate student’s life at CERN is full of meetings to share and hear the latest findings, and of hours poring over the latest data looking for the kind of statistical bump that might indicate a new particle — or a new something else.
The LHC’s recent upgrade has made scientists hopeful that a new particle will be discovered soon. But if not, another upgrade planned for about 2018 may do the trick. While the recent upgrade made the energy of the proton beam higher, the next one will increase luminosity, or the number of protons in the beam, multiplying the number of collisions at any given moment and improving the odds of detecting extremely rare events.
“We’re all here to … discover stuff, but it’s so difficult. It’s impossible to do as one person,” Gonski said. “I would love to be one person on the 500-person team to discover [supersymmetry’s] stop quark. It’d be great for physics if we all discovered this and for me to say I want to do this — be a tiny fraction of a large group effort.”
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