Pilar Coloma (left) and Seyda Ipek write calculations from floor to ceiling as they try to find solutions to lingering questions about our current models of the universe. Photo: Rashmi Shivni, OC
Some of the ideas you’ve probably had about theoretical physicists are true.
They toil away at complicated equations. The amount of time they spend on their computers rivals that of millennials on their hand-held devices. And almost nothing of what they turn up will ever be understood by most of us.
The statements are true, but as you might expect, the resulting portrait of ivory tower isolation misses the mark.
The theorist’s task is to explain why we see what we see and predict what we might expect to see, and such pronouncements can’t be made from the proverbial armchair. Theorists work with experimentalists, their counterparts in the proverbial field, as a vital part of the feedback loop of scientific investigation.
“Sometimes I bounce ideas off experimentalists and learn from what they have seen in their results,” said Fermilab theorist Pilar Coloma, who studies neutrino physics. “Or they may find something profound in theory models that they want to test. My job is all about pushing the knowledge forward so other people can use it.”
Predictive power
Theorists in particle physics — the Higgses and Hawkings of the world — push knowledge by making predictions about particle interactions. Starting from the framework known as the Standard Model, they calculate, say, the likelihood of numerous outcomes from the interaction of two electrons, like a blackjack player scanning through the possibilities for the dealer’s next draw.
Experimentalists can then seek out the predicted phenomena, rooting around in the data for a never-before-seen phenomenon.
Theorists’ predictions keep experimentalists from having to shoot in the dark. Like an experienced paleontologist, the theorist can tell the experimentalist where to dig to find something new.
“We simulate many fake events,” Coloma said. “The simulated data determines the prospects for an experiment or puts a bound on a new physics model.”
The Higgs boson provides one example. By 2011, a year before CERN’s ATLAS and CMS experiments announced they’d discovered the Higgs boson, theorists had put forth nearly 100 different proposals by as many different methods for the particle’s mass. Many of the predictions were indeed in the neighborhood of the mass as measured by the two experiments.
And like the paleontologist presented with a new artifact, the theorist also offers explanations for unexplained sightings in experimentalists’ data. She might compare the particle signatures in the detector against her many fake events. Or given an intriguing measurement, she might fold it into the next iteration of calculations. If experimentalists see a particle made of a quark combination not yet on the books, theorists would respond by explaining the underlying mechanism or, if there isn’t one yet, work it out.
“Experimentalists give you information. ‘We think this particle is of this type. Do you know of any Standard Model particle that fits?’” said Seyda Ipek, a theorist studying the matter-antimatter imbalance in the universe. “At first it might not be obvious, because when you add something new, you change the other observations you know are in the Standard Model, and that puts a constraint on your models.”
And since the grand aim of particle physics theory is to be able to explain all of nature, the calculation developed to explain a new phenomenon must be extendible to a general principle.
“Unless you have a very good prediction from theory, you can’t convert that experimental measurement into a parameter that appears in the underlying theory of the Standard Model,” said Fermilab theorist John Campbell, who works on precision theoretical predictions for the ATLAS and CMS experiments at the Large Hadron Collider.
Calculating moves
The theorist’s calculation starts with the prospect of a new measurement or a hole in a theory.
“You look at the interesting things that an experiment is going to measure or that you have a chance of measuring,” Campbell said. “If the data agrees with theory everywhere, there’s not much room for new physics. So you look for small deviations that might be a sign of something. You’re really trying to dream up a new set of interactions that might explain why the data doesn’t agree somewhere.”
In its raw form, particle physics data is the amount and location of the energy a particle deposits in a particle detector. The more sensitive the detector, the more accurate the experimentalists’ measurement, and the more precise the corresponding calculation needs to be.
Fermilab theorists John Campbell (left) and Ye Li work on a calculation that describes the interactions you might expect to see in the complicated environment of the LHC. Photo: Rashmi Shivni
The CMS detector at the Large Hadron Collider, for example, allows scientists to measure some probabilities of particle interactions to within a few percent. And that’s after taking into account that it takes one million or even one billion proton-proton collisions to produce just one interesting interaction that CMS would like to measure.
“When you’re making the measurement that accurately, it demands a prediction at a very high level,” Campbell said. “If you’re looking for something unexpected, then you need to know the expected part in quite a lot of detail.”
A paleontologist recognizes the vertebra of a brachiosaurus, and the theoretical particle physicist knows what the production of a pair of top quarks looks like in the detector. A departure from the known picture triggers him to take action.
“So then you embark on this calculation,” Campbell said.
Embark, indeed. These calculations are not pencil-and-paper assignments. A single calculation predicting the details of a particle interaction, for example, can be a prodigious effort that takes months or years.
So-called loop corrections are one example: Theorists home in on what happens during a particle event by adding detail — a correction — to an approximate picture.
Consider two electrons that approach each other, exchange a photon and diverge. Zooming in further, you predict that the photon emits and reabsorbs yet another pair of particles before it itself is reabsorbed by the electron pair. And perhaps you predict that, at the same time, one of the electrons emits and reabsorbs another photon all on its own.
Each additional quantum-scale effect, or loop, in the big-picture interaction is like pennies on the dollar, changing the accounting of the total transaction — the precision of a particle mass calculation or of the interaction strength between two particles.
With each additional loop, the task of performing the calculation becomes that much more formidable. (“Loop” reflects how the effects are represented pictorially in Feynman diagrams — details in the approximate picture of the interaction.) Theorists were computing one-loop corrections for the production of a Higgs boson arising from two protons until 1991. It took another 10 years to complete the two-loop corrections for the process. And it wasn’t until this year, 2016, that they finished computing the three-loop corrections. Precise measurements at the Large Hadron Collider would (and do) require precise predictions to determine the kind of Higgs boson that scientists would see, demanding the decades-long investment.
“Doing these calculations is not straightforward, or we would have done them a long time ago,” Campbell said.
Once the theorist completes a calculation, they might publish a paper or otherwise make their code broadly available. From there, experimentalists can use the code to simulate how it will look in the detector. Farms of computers map out millions of fake events that take into account the new predictions provided courtesy of the theorist.
“Without a network of computers available, our studies can’t be done in a reasonable time,” Coloma said. “A single computer can not analyze millions of data points, just as a human being could never take on such a task.”
If the simulation shows that, for example, a particle might decay in more ways than what the experiment has seen, the theorist could suggest that experimentalists expand their search.
“We’ve pushed experiments to look in different channels,” Ipek said. “They could look into decays of particles into two-body states, but why not also 10-body states?”
Theorists also work with an experiment, or multiple experiments, to put their calculations to best use. Armed with code, experimentalists can change a parameter or two to guide them in their search for new physics. What happens, for example, if the Higgs boson interacts a little more strongly with the top quark than we expect? How would that change what we see in our detectors?
“That’s a question they can ask and then answer,” Campbell said. “Anyone can come up with a new theory. It is best to try to provide a concrete plan that they can follow.”
Outlandish theories and concrete plans
Concrete plans ensure a fruitful relationship between experiment and theory. The wilder, unconventional theories scientists dream up take the field into exciting, uncharted territory, but that isn’t to say that they don’t also have their utility.
Theorists who specialize in physics beyond the Standard Model, for example, generate thousands of theories worldwide for new physics – new phenomena seen as new energy deposits in the detector where you don’t expect to see them.
“Even if things don’t end up existing, it encourages the experiment to look at its data in different ways,” Campbell said. An experiment could take so much data that you might worry that some fun effect is hiding, never to be seen. Having truckloads of theories helps mitigate against that. “You’re trying to come up with as many outlandish ideas as you can in the hope that you cover as many of those possibilities as you can.”
Theorists bridge the gap between the pure mathematics that describes nature and the data through which nature manifests.
“The field itself is challenging, but theory takes us to new places and helps us imagine new phenomena,” Ipek said.” We collectively work toward understanding every detail of our universe and that’s what ultimately matters most.”
Experiments at the LHC are once again recording collisions at extraordinary energies
Collisions recorded on May 7, 2016, by the CMS detector on the Large Hadron Collider. After a winter break, the LHC is now taking data again at extraordinary energies. Image: CERN
Editor’s note: The following news release about the restart of the Large Hadron Collider is being issued by the U.S. Department of Energy’s Fermi National Accelerator Laboratory on behalf of the U.S. scientists working on the LHC. Fermilab serves as the U.S. hub for the CMS experiment at the LHC and the roughly 1,000 U.S. scientists who work on that experiment, including about 100 Fermilab employees. Fermilab is a Tier 1 computing center for LHC data and hosts a Remote Operations Center to process and analyze that data. Read more information about Fermilab’s role in the CMS experiment and the LHC. Fermilab scientists are available for interviews upon request, including Joel Butler, recently elected next spokesperson of the CMS experiment.
After months of winter hibernation, the Large Hadron Collider is once again smashing protons and taking data. The LHC will run around the clock for the next six months and produce roughly 2 quadrillion high-quality proton collisions, six times more than in 2015 and just shy of the total number of collisions recorded during the nearly three years of the collider’s first run.
“2015 was a recommissioning year. 2016 will be a year of full data production during which we will focus on delivering the maximum number of data to the experiments,” said Fabiola Gianotti, CERN director general.
The LHC is the world’s most powerful particle accelerator. Its collisions produce subatomic fireballs of energy, which morph into the fundamental building blocks of matter. The four particle detectors located on the LHC’s ring allow scientists to record and study the properties of these building blocks and look for new fundamental particles and forces.
“We’re proud to support more than a thousand U.S. scientists and engineers who play integral parts in operating the detectors, analyzing the data and developing tools and technologies to upgrade the LHC’s performance in this international endeavor,” said Jim Siegrist, associate director of science for high-energy physics in the U.S. Department of Energy’s Office of Science. “The LHC is the only place in the world where this kind of research can be performed, and we are a fully committed partner on the LHC experiments and the future development of the collider itself.”
Between 2010 and 2013 the LHC produced proton-proton collisions with 8 teraelectronvolts of energy. In the spring of 2015, after a two-year shutdown, LHC operators ramped up the collision energy to 13 TeV. This increase in energy enables scientists to explore a new realm of physics that was previously inaccessible. Run II collisions also produce Higgs bosons — the groundbreaking particle discovered in LHC Run I — 25 percent faster than Run I collisions and increase the chances of finding new massive particles by more than 40 percent.
Almost everything we know about matter is summed up in the Standard Model of particle physics, an elegant map of the subatomic world. During the first run of the LHC, scientists on the ATLAS and CMS experiments discovered the Higgs boson, the cornerstone of the Standard Model that helps explain the origins of mass. The LHCb experiment also discovered never-before-seen five-quark particles, and the ALICE experiment studied the near-perfect liquid that existed immediately after the Big Bang. All these observations are in line with the predictions of the Standard Model.
“So far the Standard Model seems to explain matter, but we know there has to be something beyond the Standard Model,” said Denise Caldwell, director of the Physics Division of the National Science Foundation. “This potential new physics can only be uncovered with more data that will come with the next LHC run.”
For example, the Standard Model contains no explanation of gravity, which is one of the four fundamental forces in the universe. It also does not explain astronomical observations of dark matter, a type of matter that interacts with our visible universe only through gravity, nor does it explain why matter prevailed over antimatter during the formation of the early universe. The small mass of the Higgs boson also suggests that matter is fundamentally unstable.
The new LHC data will help scientists verify the Standard Model’s predictions and push beyond its boundaries. Many predicted and theoretical subatomic processes are so rare that scientists need billions of collisions to find just a small handful of events that are clean and scientifically interesting. Scientists also need an enormous amount of data to precisely measure well-known Standard Model processes. Any significant deviations from the Standard Model’s predictions could be the first step towards new physics.
The United States is the largest national contributor to both the ATLAS and CMS experiments, with 45 U.S. universities and laboratories working on ATLAS and 49 working on CMS.
CERN, the European Organization for Nuclear Research, is the world’s leading laboratory for particle physics. It has its headquarters in Geneva. At present, its member states are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, the Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Romania is a candidate for accession. Cyprus and Serbia are associate members in the pre-stage to membership. Turkey and Pakistan are associate members. India, Japan, the Russian Federation, the United States of America, Turkey, the European Union, JINR and UNESCO have observer status.
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.
The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.
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On April 27 and 28, Fermilab hosted the Neutrino – Latin America Workshop for visiting scientists. The workshop showcased Latin American collaboration with the laboratory throughout the years, and scientists discussed research opportunities both here at Fermilab and at institutions in Central and South America.
“Our intention was to increase the awareness of the DUNE scientific and technical program and to highlight the many areas where Latin American scientists and engineers can make important contributions within DUNE and the broader Fermilab neutrino program, including the short-baseline neutrino experiments,” said Mark Thomson, a co-spokesperson for DUNE and lead organizer of the workshop. Latin America has a rich history in particle physics, and this workshop highlighted the projects that resulted from the longstanding relationship Fermilab has with these nations, he said.
The past
As early as the 1930s, physicists at institutions in Argentina, Brazil and Mexico were studying cosmic rays and theoretical particle physics. Several of these nations expanded their focus in particle physics, but at the time, programs were few, and funding was minimal. In the early 1980s, Leon Lederman, Fermilab’s second director, realized the potential benefits of a relationship between Fermilab and our neighbors to the south after attending several symposia hosted in Latin America. By 1984, Lederman sponsored four Brazilian physicists — the first Latin American scientists to come to Fermilab — to participate on a fixed-target experiment.
Fermilab Neutrino Division Head Regina Rameika, one of the workshop’s first speakers, gives an overview of the lab’s neutrino program and discusses the importance of building international relationships for particle physics. Photo: Reidar Hahn
“Lederman took a bold step with inviting us to join the experimental high-energy physics program at Fermilab,” said Carlos Escobar, a guest scientist in Fermilab’s Neutrino Division and one of the first four Brazilian physicists to join the lab. “We had physicists working in theory for high-energy physics groups in our home institutions but no experimental groups using accelerators in particle physics. We were the pioneering Latin Americans here at Fermilab.”
Shortly after this group joined the lab, they began to reach out to their students and colleagues at home to train them for future projects. More and more Latin American students and scientists from multiple countries gained opportunities to learn and work at Fermilab, and they eventually became a valuable group to the laboratory’s growing neutrino program.
The present
The first day of the April workshop included presentations and discussions about past neutrino experiments, right up to current projects. Latin America has collaborated with Fermilab on several projects over the years, including MINOS and MINOS+, MiniBooNE, LArIAT and NOvA.
According to Julian Felix, a professor of physics at the University of Guanajuato in Mexico, Latin Americans made up nearly a quarter of the MINERvA collaboration. The proposed CAPTAIN MINERvA experiment, which would be an expansion of its predecessor with a focus in neutrino-argon interaction studies, would continue the tradition of collaboration with Latin American institutions.
“The test beam for the MINERvA experiment was done by several Latin American students, and all of the students made a big difference in this work,” he said in his presentation. “This experiment had the largest contributions from Latin America of any experiment at Fermilab, and my students and I gained a lot of experience from it.”
Ricardo Gomes of the Federal University of Goias in Brazil worked on remote operations for MINOS and NOvA. His group specializes in simulation programs, such as CORSIKA, which has potential use for DUNE. Photo: Reidar Hahn
Today, the in-progress Short-Baseline Neutrino program, with three supporting experiments hunting for a fourth type of neutrino, currently has 55 collaborating institutions from eight countries. Three institutions are from Brazil and one is from Puerto Rico. Workshop participants in the SBN program took the opportunity to invite their Latin American colleagues to join SBN.
“We want to reach out to this specific region of the world because they are valuable and bring a variety of ideas and opinions to our work,” said Regina Rameika, head of Fermilab’s Neutrino Division. “We especially want students to participate so they can gain experience. New programs like SBN are great because students can start at the beginning of the project and see it progress.”
Several workshop participants said that, while experience abroad is valuable, it’s just as important that highly trained professionals who studied at Latin American universities and institutions make their skills and talents available in their home nations.
“Our governments and countries are willing to fund physicists and engineers,” Felix said. “We need skilled professionals in industry in our home countries.”
Rameika said Fermilab is a good training ground for Latin American students, who participate in a particle physics experiment before they head back to their nations to inspire others to become scientists.
“Fermilab could be a part of this loop in which we bring students here, offer them profound experience in their field and then send them back to build stronger programs back home,” she said.
At the workshop, physicists on new projects in Latin America informed and invited Fermilab scientists to participate. ANDES, for example, is an underground laboratory located on the borders between Argentina, Brazil and Chile. It will be one of the first multidisciplinary underground facilities in the Southern Hemisphere. And, CONNIE a neutrino-nucleus interaction experiment led by Fermilab scientist Juan Estrada and located in a nuclear power plant near Rio de Janeiro, Brazil, will produce data to help answer questions about the Standard Model and even test safety applications in nuclear facilities.
The future
On the second day of the workshop, the spotlight was on future opportunities and upcoming experiments.
Celio Moura, a physicist from the Federal University of ABC in Brazil, participated in Fermilab’s liquid-argon detection research. Moura said all of the neutrino experiments are linked: “When you learn the tools to do one experiment, you can go on to do many more.” Photo: Reidar Hahn
Fermilab’s future flagship experiment DUNE is among the world’s largest neutrino experiments, currently with 850 collaborators from 149 institutions in 29 countries. Presenters discussed opportunities in software, scientific computing, theory and accelerator engineering for DUNE. The research scope includes supernova neutrinos, neutrino oscillation, proton decay and the universe’s matter-antimatter imbalance.
Several Latin American institutions have developed simulation technologies capable of handling the amount of data DUNE would produce. This is one of the many key areas in which Latin American collaboration is vital to the lab.
“DUNE is an incredibly exciting international partnership and will be the next big thing in particle physics,” Thomson said. “We hope to build on the existing Latin American participation in DUNE and the rest of the Fermilab neutrino program. Latin American scientists bring great expertise, and DUNE is an opportunity to form scientific partnerships in the next major international neutrino experiment. There are many benefits, including providing training for the next generation of Latin American physicists.”
CERN’s ProtoDUNE, the large-scale DUNE prototypes, also has opportunities for Latin American scientists and engineers in Switzerland. CERN enjoys strong Latin American participation, with approximately 300 physicists from 12 nations, according to Gustavo Otero y Garzon, a assistant professor at the University of Buenos Aires in Argentina and a physicist on CERN’s ATLAS experiment.
In 2015, Fermilab had a total of 162 visiting Latin American scientists and students from eight countries contributing to several neutrino experiments.
“Latin America is developing and growing its economy, and this is a perfect opportunity for us to engage their local industry and institutions to develop new technologies and share their expertise,” Escobar said. “These nations share a passion for science with us and will be effective partners for us at the frontier of particle physics.”
Fermilab broke ground on the Short-Baseline Neutrino Detector building on April 27. From left: Josh Kenney, FESS; Steve Dixon, AD; David Schmitz, University of Chicago; Ting Miao, ND; Ornella Palamara, ND; Peter Wilson, ND; Catherine James, ND. Photo: Reidar Hahn
On April 27, Fermilab broke ground on the building that will house the future Short-Baseline Near Detector.
The particle detector, SBND, is one of three that, together, scientists will use to search for the sterile neutrino, a hypothesized particle whose existence, if confirmed, could not only help us better understand the types of neutrino we already know about, but also provide clues about how the universe formed.
Members of the Fermilab Neutrino and Particle Physics divisions, working together with international collaborators, are currently refining the design of the detector itself. It will take about eight months to complete the SBND building.
The three detectors make up the laboratory’s Short-Baseline Neutrino Program, which will use a powerful neutrino beam generated by the Fermilab accelerator complex. The beam will pass first through SBND and then through the MicroBooNE detector, which is already installed and taking data, having observed its first neutrino interactions in October. Finally, the beam will travel through ICARUS, the largest of the three detectors. ICARUS, which was used in a previous experiment at the Italian Gran Sasso laboratory, is currently at the CERN laboratory in Switzerland receiving upgrades before its big move to Fermilab in 2017.
“The entire Short-Baseline Neutrino Program is looking for oscillations, or the transformations, of muon neutrinos into electron neutrinos,” said Peter Wilson, SBN program coordinator. “Sterile neutrinos might have a role in this oscillation process.”
The beam coming out of the accelerator comprises primarily muon neutrinos; the detectors will measure their transformation into electron neutrinos.
All three detectors have specific functions in detecting the transformation. As the detector closest to the beam source, SBND will take an initial measurement of the beam’s composition – how much the beam contains each of the different neutrino types.
“The intermediary and far detectors are used to search for sterile neutrinos in two different ways,” said Ornella Palamara, co-spokesperson for the SBND experiment. “Either there’s an appearance of an excess of electron neutrinos or there’s a disappearance of the number of muon neutrinos compared to the number we start with.”
If there are more electron neutrinos than predicted, then muon neutrinos may have oscillated first into sterile neutrinos and then to electron neutrinos. If the data show a smaller number of muon neutrinos than predicted, the muon neutrinos may have transformed only into sterile neutrinos, which cannot be seen in the far detectors.
Scientists first picked up on experimental hints of a sterile neutrino at Los Alamos National Laboratory’s LSND experiment in 1995. When the Fermilab experiment MiniBooNE followed up, scientists could not confirm the sterile neutrino’s existence, but neither could they rule it out.
“That’s the power of this program,” Palamara said. “We’re building off previous measurements, but we have more sensitive tools to measure the neutrinos.”
Part of the sensitivity of SBND lies in its liquid-argon time projection chamber, the active part of the detector, which will contain 112 tons of liquid argon. Neutrinos will interact with the nuclei of the argon atoms, and scientists on SBND will study the resulting particles to better understand the neutrinos that caused the interaction. Their findings will likely have application in future accelerator-based neutrino programs, such as the international Deep Underground Neutrino Experiment hosted by Fermilab.
The Short-Baseline Neutrino Program will begin taking data in 2018.
“The SBND groundbreaking is a noteworthy milestone, but it’s part of a much larger program,” Wilson said. “Many people are working on it, and everyone is excited to get the chance to understand new physics.”