A new result from the LHCb experiment could be an early indicator of an inconsistency in the Standard Model.
The subatomic universe is an intricate mosaic of particles and forces. The Standard Model of particle physics is a time-tested instruction manual that precisely predicts how particles and forces behave. But it’s incomplete, ignoring phenomena such as gravity and dark matter.
Today the LHCb experiment at CERN European research center released a result that could be an early indication of new, undiscovered physics beyond the Standard Model.
However, more data is needed before LHCb scientists can definitively claim they’ve found a crack in the world’s most robust roadmap to the subatomic universe.
“In particle physics, you can’t just snap your fingers and claim a discovery,” says Marie-Hélène Schune, a researcher on the LHCb experiment from Le Centre National de la Recherche Scientifique in Orsay, France. “It’s not magic. It’s long, hard work and you must be obstinate when facing problems. We always question everything and never take anything for granted.”
The LHCb experiment records and analyzes the decay patterns of rare hadrons—particles made of quarks—that are produced in the Large Hadron Collider’s energetic proton-proton collisions. By comparing the experimental results to the Standard Model’s predictions, scientists can search for discrepancies. Significant deviations between the theory and experimental results could be an early indication of an undiscovered particle or force at play.
This new result looks at hadrons containing a bottom quark as they transform into hadrons containing a strange quark. This rare decay pattern can generate either two electrons or two muons as byproducts. Electrons and muons are different types or “flavors” of particles called leptons. The Standard Model predicts that the production of electrons and muons should be equally favorable—essentially a subatomic coin toss every time this transformation occurs.
“As far as the Standard Model is concerned, electrons, muons and tau leptons are completely interchangeable,” Schune says. “It’s completely blind to lepton flavors; only the large mass difference of the tau lepton plays a role in certain processes. This 50-50 prediction for muons and electrons is very precise.”
But instead of finding a 50-50 ratio between muons and electrons, the latest results from the LHCb experiment show that it’s more like 34 muons generated for every 66 electrons.
“If this initial result becomes stronger with more data, it could mean that there are other, invisible particles involved in this process that see flavor,” Schune says. “We’ll leave it up to the theorists’ imaginations to figure out what’s going on.”
However, just like any coin-toss, it’s difficult to know if this discrepancy is the result of an unknown favoritism or the consequence of chance. To delineate between these two possibilities, scientists wait until they hit a certain statistical threshold before claiming a discovery, often 5 sigma.
“Five sigma is a measurement of statistical deviation and means there is only a 1-in-3.5-million chance that the Standard Model is correct and our result is just an unlucky statistical fluke,” Schune says. “That’s a pretty good indication that it’s not chance, but rather the first sightings of a new subatomic process.”
Currently, this new result is at approximately 2.5 standard deviations, which means there is about a 1-in-125 possibility that there’s no new physics at play and the experimenters are just the unfortunate victims of statistical fluctuation.
This isn’t the first time that the LHCb experiment has seen unexpected behavior in related processes. Hassan Jawahery from the University of Maryland also works on the LHCb experiment and is studying another particle decay involving bottom quarks transforming into charm quarks. He and his colleagues are measuring the ratio of muons to tau leptons generated during this decay.
“Correcting for the large mass differences between muons and tau leptons, we’d expect to see about 25 taus produced for every 100 muons,” Jawahery says. “We measured a ratio of 34 taus for every 100 muons.”
On its own, this measurement is below the line of statistical significance needed to raise an eyebrow. However, two other experiments—the BaBar experiment at SLAC and the Belle experiment in Japan—also measured this process and saw something similar.
“We might be seeing the first hints of a new particle or force throwing its weight around during two independent subatomic processes,” Jawahery says. “It’s tantalizing, but as experimentalists we are still waiting for all these individual results to grow in significance before we get too excited.”
More data and improved experimental techniques will help the LHCb experiment and its counterparts narrow in on these processes and confirm if there really is something funny happening behind the scenes in the subatomic universe.
“Conceptually, these measurements are very simple,” Schune says. “But practically, they are very challenging to perform. These first results are all from data collected between 2011 and 2012 during Run 1 of the LHC. It will be intriguing to see if data from Run 2 shows the same thing.”
While driven by the desire pursue curiosity, fundamental investigations are the crucial first step to innovation.
When scientists announced their discovery of gravitational waves in 2016, it made headlines all over the world. The existence of these invisible ripples in space-time had finally been confirmed.
It was a momentous feat in basic research, the curiosity-driven search for fundamental knowledge about the universe and the elements within it. Basic (or “blue-sky”) research is distinct from applied research, which is targeted toward developing or advancing technologies to solve a specific problem or to create a new product.
But the two are deeply connected.
“Applied research is exploring the continents you know, whereas basic research is setting off in a ship and seeing where you get,” says Frank Wilczek, a theoretical physicist at MIT. “You might just have to return, or sink at sea, or you might discover a whole new continent. So it’s much more long-term, it’s riskier and it doesn’t always pay dividends.”
When it does, he says, it opens up entirely new possibilities available only to those who set sail into uncharted waters.
Most of physics—especially particle physics—falls under the umbrella of basic research. In particle physics “we’re asking some of the deepest questions that are accessible by observations about the nature of matter and energy—and ultimately about space and time also, because all of these things are tied together,” says Jim Gates, a theoretical physicist at the University of Maryland.
Physicists seek answers to questions about the early universe, the nature of dark energy, and theoretical phenomena, such as supersymmetry, string theory and extra dimensions.
Perhaps one of the most well-known basic researchers was the physicist who predicted the existence of gravitational waves: Albert Einstein.
Einstein devoted his life to elucidating elementary concepts such as the nature of gravity and the relationship between space and time. According to Wilczek, “it was clear that what drove what he did was not the desire to produce a product, or anything so worldly, but to resolve puzzles and perceived imperfections in our understanding.”
In addition to advancing our understanding of the world, Einstein’s work led to important technological developments. The Global Positioning System, for instance, would not have been possible without the theories of special and general relativity. A GPS receiver, like the one in your smart phone, determines its location based on timed signals it receives from the nearest four of a collection of GPS satellites orbiting Earth. Because the satellites are moving so quickly while also orbiting at a great distance from the gravitational pull of Earth, they experience time differently from the receiver on Earth’s surface. Thanks to Einstein’s theories, engineers can calculate and correct for this difference.
There’s a long history of serendipitous output from basic research. For example, in 1989 at CERN European research center, computer scientist Tim Berners-Lee was looking for a way to facilitate information-sharing between researchers. He invented the World Wide Web.
While investigating the properties of nuclei within a magnetic field at Columbia University in the 1930s, physicist Isidor Isaac Rabi discovered the basic principles of nuclear magnetic resonance. These principles eventually formed the basis of Magnetic Resonance Imaging, MRI.
It would be another 50 years before MRI machines were widely used—again with the help of basic research. MRI machines require big, superconducting magnets to function. Luckily, around the same time that Rabi’s discovery was being investigated for medical imaging, scientists and engineers at the US Department of Energy’s Fermi National Accelerator Laboratory began building the Tevatron particle accelerator to enable research into the fundamental nature of particles, a task that called for huge amounts of superconducting wire.
“We were the first large, demanding customer for superconducting cable,” says Chris Quigg, a theoretical physicist at Fermilab. “We were spending a lot of money to get the performance that we needed.” The Tevatron created a commercial market for superconducting wire, making it practical for companies to build MRI machines on a large scale for places like hospitals.
Doctors now use MRI to produce detailed images of the insides of the human body, helpful tools in diagnosing and treating a variety of medical complications, including cancer, heart problems, and diseases in organs such as the liver, pancreas and bowels.
Another tool of particle physics, the particle detector, has also been adopted for uses in various industries. In the 1980s, for example, particle physicists developed technology precise enough to detect a single photon. Today doctors use this same technology to detect tumors, heart disease and central nervous system disorders. They do this by conducting positron emission tomography scans, or PET scans. Before undergoing a PET scan, the patient is given a dye containing radioactive tracers, either through an injection or by ingesting or inhaling. The tracers emit antimatter particles, which interact with matter particles and release photons, which are picked up by the PET scanner to create a picture detailed enough to reveal problems at the cellular level.
As Gates says, “a lot of the devices and concepts that you see in science fiction stories will never come into existence unless we pursue the concept of basic research. You’re not going to be able to construct starships unless you do the research now in order to build these in the future.”
It’s unclear what applications could come of humanity’s new knowledge of the existence of gravitational waves.
It could be enough that we have learned something new about how our universe works. But if history gives us any indication, continued exploration will also provide additional benefits along the way.
Experiments at CERN investigate antiparticles.
What do shrimp, tennis balls and pulsars all have in common? They are all made from matter.
Admittedly, that answer is a cop-out, but it highlights a big, persistent quandary for scientists: Why is everything made from matter when there is a perfectly good substitute—antimatter?
The European laboratory CERN hosts several experiments to ascertain the properties of antimatter particles, which almost never survive in our matter-dominated world.
Particles (such as the proton and electron) have oppositely charged antimatter doppelgangers (such as the antiproton and antielectron). Because they are opposite but equal, a matter particle and its antimatter partner annihilate when they meet.
Antimatter wasn’t always rare. Theoretical and experimental research suggests that there was an equal amount of matter and antimatter right after the birth of our universe. But 13.8 billion years later, only matter-made structures remain in the visible universe.
Scientists have found small differences between the behavior of matter and antimatter particles, but not enough to explain the imbalance that led antimatter to disappear while matter perseveres. Experiments at CERN are working to solve that riddle using three different strategies.
Antimatter under the microscope
It’s well known that CERN is home to Large Hadron Collider, the world’s highest-energy particle accelerator. Less known is that CERN also hosts the world’s most powerful particle decelerator—a machine that slows down antiparticles to a near standstill.
The antiproton decelerator is fed by CERN’s accelerator complex. A beam of energetic protons is diverted from CERN’s Proton Synchrotron and into a metal wall, spawning a multitude of new particles, including some antiprotons. The antiprotons are focused into a particle beam and slowed by electric fields inside the antiproton decelerator. From here they are fed into various antimatter experiments, which trap the antiprotons inside powerful magnetic fields.
“All these experiments are trying to find differences between matter and antimatter that are not predicted by theory,” says Will Bertsche, a researcher at University of Manchester, who works in CERN’s antimatter factory. “We’re all trying to address the big question: Why is the universe made up of matter these days and not antimatter?”
By cooling and trapping antimatter, scientists can intimately examine its properties without worrying that their particles will spontaneously encounter a matter companion and disappear. Some of the traps can preserve antiprotons for more than a year. Scientists can also combine antiprotons with positrons (antielectrons) to make antihydrogen.
“Antihydrogen is fascinating because it lets us see how antimatter interacts with itself,” Bertsche says. “We’re getting a glimpse at how a mirror antimatter universe would behave.”
Scientists in CERN’s antimatter factory have measured the mass, charge, light spectrum, and magnetic properties of antiprotons and antihydrogen to high precision. They also look at how antihydrogen atoms are affected by gravity; that is, do the anti-atoms fall up or down? One experiment is even trying to make an assortment of matter-antimatter hybrids, such as a helium atom in which one of the electrons is replaced with an orbiting antiproton.
So far, all their measurements of trapped antimatter match the theory: Except for the opposite charge and spin, antimatter appears completely identical to matter. But these affirmative results don’t deter Bertsche from looking for antimatter surprises. There must be unpredicted disparities between these particle twins that can explain why matter won its battle with antimatter in the early universe.
“There’s something missing in this model,” Bertsche says. “And nobody is sure what that is.”
Antimatter in motion
The LHCb experiment wants to answer this same question, but they are looking at antimatter particles that are not trapped. Instead, LHCb scientists study how free-range antimatter particles behave as they travel and transform inside the detector.
“We’re recording how unstable matter and antimatter particles decay into showers of particles and the patterns they leave behind when they do,” says Sheldon Stone, a professor at Syracuse University working on the LHCb Experiment. “We can’t make these measurements if the particles aren’t moving.”
The particles-in-motion experiments have already observed some small differences between matter and antimatter particles. In 1964 scientists at Brookhaven National Laboratory noticed that neutral kaons (a particle containing a strange and down quark) decay into matter and antimatter particles at slightly different rates, an observation that won them the Nobel Prize in 1980.
The LHCb experiment continues this legacy, looking for even more discrepancies between the metamorphoses of matter and antimatter particles. They recently observed that the daughter particles of certain antimatter baryons (particles containing three quarks) have a slightly different spatial orientation than their matter contemporaries.
But even with the success of uncovering these discrepancies, scientists are still very far from understanding why antimatter all but disappeared.
“Theory tells us that we’re still off by nine orders of magnitude,” Stone says, “so we’re left asking, where is it? What is antimatter’s Achilles heel that precipitated its disappearance?”
Antimatter in space
Most antimatter experiments based at CERN produce antiparticles by accelerating and colliding protons. But one experiment is looking for feral antimatter freely roaming through outer space.
The Alpha Magnetic Spectrometer is an international experiment supported by the US Department of Energy and NASA. This particle detector was assembled at CERN and is now installed on the International Space Station, where it orbits Earth 400 kilometers above the surface. It records the momentum and trajectory of roughly a billion vagabond particles every month, including a million antimatter particles.
Nomadic antimatter nuclei could be lonely relics from the Big Bang or the rambling residue of nuclear fusion in antimatter stars.
But AMS searches for phenomena not explained by our current models of the cosmos. One of its missions is to look for antimatter that is so complex and robust, there is no way it could have been produced through normal particle collisions in space.
“Most scientists accept that antimatter disappeared from our universe because it is somehow less resilient than matter,” says Mike Capell, a researcher at MIT and a deputy spokesperson of the AMS experiment. “But we’re asking, what if all the antimatter never disappeared? What if it’s still out there?”
If an antimatter kingdom exists, astronomers expect that they would observe mass particle-annihilation fizzing and shimmering at its boundary with our matter-dominated space—which they don’t. Not yet, at least. Because our universe is so immense (and still expanding), researchers on AMS hypothesize that maybe these intersections are too dim or distant for our telescopes.
“We already have trouble seeing deep into our universe,” Capell says. “Because we’ve never seen a domain where matter meets antimatter, we don’t know what it would look like.”
AMS has been collecting data for six years. From about 100 billion cosmic rays, they’ve identified a few strange events with characteristics of antihelium. Because the sample is so tiny, it’s impossible to say whether these anomalous events are the first messengers from an antimatter galaxy or simply part of the chaotic background.
“It’s an exciting result,” Capell says. “However, we remain skeptical. We need data from many more cosmic rays before we can determine the identities of these anomalous particles.”
The group brought their on-site drawing practice to the particle physics laboratory.
In March, about 30 participants in the Chicago chapter of the artist network Urban Sketchers visited Fermi National Accelerator Laboratory, located in west Chicagoland, and sketched their hearts out. They drew buildings, interiors and scenes of nature from the laboratory environment, capturing the laboratory's most iconic building, Wilson Hall, along with restored prairie land and the popular bison herd on site.
Urban Sketchers holds monthly “sketch crawls,” as they’re called. Their mission is to “show the world, one drawing at a time.”
Sketcher Harold Goldfus drew scenes of art and architecture.
“I regard myself as primarily a figurative artist. At the Urban Sketchers Chicago outing, I expected to sketch figures at Fermilab with hints of the environment in the background,” Goldfus said. “Instead, I found myself taken with the architecture and aesthetics of the interior of Wilson Hall, and decided on a more unconventional approach.”
The sketch crawl was organized by Peggy Condon and Wes Douglas from Urban Sketchers Chicago along with Fermilab Art Gallery curator Georgia Schwender.
“I was very inspired by Fermilab’s strong commitment to the arts. I didn’t expect this for a world-renowned scientific research institution,” said sketcher Lynne Fairchild. “I really appreciated that they found so many ways to honor the arts and culture: the art gallery, lecture series, the awe-inspiring sculptures on the campus, and the design of Wilson Hall, especially the beauty of the atrium.”
Editor's note: Fermilab previously posted a version of this article.
We know which way the dark matter wind should blow. Now we just have to find it.
Picture yourself in a car, your hand surfing the breeze through the open window. Hold your palm perpendicular to the wind and you can feel its force. Now picture the car slowing, rolling up to a stop sign, and feel the force of the wind lessen until it—and the car—stop.
This wind isn’t due to the weather. It arises because of your motion relative to air molecules. Simple enough to understand and known to kids, dogs and road-trippers the world over.
This wind has an analogue in the rarefied world of particle astrophysics called the “dark matter wind,” and scientists are hoping it will someday become a valuable tool in their investigations into that elusive stuff that apparently makes up about 85 percent of the mass in the universe.
In the analogy above, the air molecules are dark matter particles called WIMPs, or weakly interacting massive particles. Our sun is the car, racing around the Milky Way at about 220 kilometers per second, with the Earth riding shotgun. Together, we move through a halo of dark matter that encompasses our galaxy. But our planet is a rowdy passenger; it moves from one side of the sun to the other in its orbit.
When you add the Earth’s velocity of 30 kilometers per second to the sun’s, as happens when both are traveling in the same direction (toward the constellation Cygnus), then the dark matter wind feels stronger. More WIMPs are moving through the planet than if it were at rest, resulting in greater number of detections by experiments. Subtract that velocity when the Earth is on the other side of its orbit, and the wind feels weaker, resulting in fewer detections.
Astrophysicists have been thinking about the dark matter wind for decades. Among the first, way back in 1986, were theorist David Spergel of Princeton and colleagues Katherine Freese of the University of Michigan and Andrzej K. Drukier (now in private industry, but still looking for WIMPs).
“We looked at how the Earth’s motion around the sun should cause the number of dark matter particles detected to vary on a regular basis by about 10 percent a year,” Spergel says.
At least that’s what should happen—if our galaxy really is embedded in a circular, basically homogeneous halo of dark matter, and if dark matter is really made up of WIMPs.
The Italian experiment DAMA/NaI and its upgrade DAMA/Libra claim to have been seeing this seasonal modulation for decades, a claim that has yet to be conclusively supported by any other experiments. CoGeNT, an experiment in the Soudan Underground Laboratory in South Dakota, seemed to back them up for a time, but now the signals are thought to be caused by other sources such as high-energy gamma rays hitting a layer of material just outside the germanium of the detector, resulting in a signal that looks much like a WIMP.
Actually confirming the existence of the dark matter wind is important for one simple reason: the pattern of modulation can’t be explained by anything but the presence of dark matter. It’s what’s called a “model-independent” phenomenon. No natural backgrounds—no cosmic rays, no solar neutrinos, no radioactive decays—would show a similar modulation. The dark matter wind could provide a way to continue exploring dark matter, even if the particles are light enough that experiments cannot distinguish them from almost massless particles called neutrinos, which are constantly streaming from the sun and other sources.
“It’s a big, big prize to go after,” says Jocelyn Monroe, a physics professor at Royal Holloway University of London, who currently works on two dark matter detection experiments, DEAP-3600 at SNOLAB, in Canada, and DMTPC. “If you could correlate detections with the direction in which the planet is moving you would have unambiguous proof” of dark matter.
At the same time Spergel and his colleagues were exploring the wind’s seasonal modulation, he also realized that this correlation could extend far beyond a twice-per-year variation in detection levels. The location of the Earth in its orbit would affect the direction in which nucleons, the particles that make up the nucleus of an atom, recoil when struck by WIMPs. A sensitive-enough detector should see not only the twice-yearly variations, but even daily variations, since the detector constantly changes its orientation to the dark matter wind as the Earth rotates.
“I had initially thought that it wasn’t worth writing up the paper because no experiment had the sensitivity to detect the recoil direction,” he says. “However, I realized that if I pointed out the effect, clever experimentalists would eventually figure out a way to detect it.”
Monroe, as the leader of the DMTPC collaboration, is a member of the clever experimentalist set. The DMTPC, or Dark Matter Time-Projection Chamber, is one of a small number of direct detection experiments that are designed to track the actual movements of recoiling atoms.
Instead of semiconductor crystals or liquefied noble gases, these experiments use low-pressure gases as their target material. DMTPC, for example, uses carbon tetrafluoride. If a WIMP hits a molecule of carbon tetrafluoride, the low pressure in the chamber means that molecule has room to move—up to about 2 millimeters.
“Making the detector is super hard,” Monroe says. “It has to map a 2-millimeter track in 3D.” Not to mention reducing the number of molecules in a detector chamber reduces the chances for a dark matter particle to hit one. According to Monroe, DMTPC will deal with that issue by fabricating an array of 1-cubic-meter-sized modules. The first module has already been constructed and a worldwide collaboration of scientists from five different directional dark matter experiments (including DMTPC) are working on the next step together: a much larger directional dark matter array called the CYGNUS (for CosmoloGY with NUclear recoilS) experiment.
When and if such directional dark matter detectors raise their metaphorical fingers to test the direction of the dark matter wind, Monroe says they’ll be able to see far more than just seasonal variations in detections. Scientists will be able to see variations in atomic recoils not on a seasonal basis, but on a daily basis. Monroe envisions a sort of dark matter telescope with which to study the structure of the halo in our little corner of the Milky Way.
There’s always a chance that this next generation of dark matter detectors, or the generation after, still won’t see anything.
Even that, Monroe says, is progress.
“If we’re still looking in 10 years we might be able to say it’s not WIMPs but something even more exotic As far as we can tell right now, dark matter has got to be something new out there.”
Artist Chris Henschke’s latest piece inspired by particle physics machinery is the closest he’s gotten to the real thing.
Artist Chris Henschke has spent more than a decade exploring the intersection of art and physics. His pieces bring invisible properties and theoretical concepts to light through still images, sound and video.
His latest piece, called “Song of the Phenomena,” gives new life to a retired piece of equipment once used by a long-time collaborator of Henschke, University of Melbourne and Australian Synchrotron physicist Mark Boland.
The story of “Song of the Phenomena” begins in the 1990s. In 1991, Henschke enrolled in the University of Melbourne to study science, but he turned to sound design instead. Boland entered the same university to study physics.
Personal computers were just entering the market. Sound designers and animators began coding basic programs, and Henschke joined in. “I was always interested in making sounds and music, interested in light and art and physics and nature and how it all combines—either in our heads or the devices that mediate between us and nature,” he says.
Boland completed his thesis in physics at the Australian Radiation Laboratory (now called the Australian Radiation Protection and Nuclear Safety Agency). He was testing a new type of electron detector in a linear accelerator, or linac. The linac used radio waves to guide electrons through a series of accelerator cavities, which imparted more and more energy to the particles as they moved through.
That particular linac spent more than 20 years with the Australian Radiation Protection and Nuclear Safety Agency, where medical physics professionals used it to accelerate electrons to different energies to create calibration standards for radiation oncology treatments. Once they no longer needed it, Boland’s former advisor contacted him to ask if he’d like the accelerator or any of its still-working parts. He said yes, though he was unsure what he would do with it.
An artist’s view
In 2007 Henschke came to the Australian Synchrotron as part of an artist-in-residence program. Boland was familiar with his artwork; he had seen Henschke’s first piece exploring particle physics in the pages of Symmetry. Boland grew up with an appreciation for art; he says his parents made sure of that by “dragging” him through many galleries in his youth.
When Henschke and Boland met, they got into an hours-long conversation about physics. “We hit it off, we resonated,” Boland says, “and we’ve been working together ever since.”
Since that first residency program, Henschke has spent significant time at the Australian Synchrotron facility and at CERN European research center and has taken shorter trips to the DESY German national research center.
His process of creating artwork echoes the scientific process and the setup of an experiment, Boland says. Henschke thinks through the role that each piece of the artwork plays. Everything is where it is for a reason.
“He’s a perfectionist, he doesn't settle for second best,” Boland says. “He has the same level of professionalism and tenacity as an artist as a physicist does. It’s as if there’s a five-sigma quality test on his work as well.”
Once accelerator, now art
Boland mentioned the linac he had to Henschke during a conversation in early 2016. “Chris ran with it,” Boland says. “He took it and made it into his installation.”
Henschke discovered the machine hums at 220 hertz—the musical note of A—as it produces its resonant waves. “In a sense, particle accelerators are gigantic, high-energy synthesizers because they are creating high-energy waves at very specific frequencies and amplitudes,” Henschke says.
Henschke explored different aspects of the machine, still unsure how each part would come together as a final piece of art. “I have to let it speak to me, I have to let it speak for itself,” he says.
Finally it dawned on him; the art could be an echo of the accelerator’s past.
The accelerator no longer accelerates electrons. Instead Henschke feeds it a steady supply of electrons and their antimatter partners, positrons. He does this by placing it beside a pile of bananas, which release the particles as their potassium decays. (Using decaying fruit was a nod to Dutch still-life vanitas paintings, Henschke says.)
Observers cannot see the electrons and positrons in the piece, but they can hear them. Henschke ensured this by adding a Geiger counter, which emits a chirp each time it detects a particle.
Visitors can also hear the accelerator itself. Henschke attached speakers and pumped up the sound of the machine’s natural hum with a stereo amp (a bit too much at first; they blew up an oscilloscope they were using to measure the frequency). He used an AM radio coil to amplify the sound of the accelerator’s electromagnetic field.
“Song of the Phenomena” plays upon resonance, amplification and decay, Henschke says. “It creates this tension between the constant hum of the device versus the unpredictability of the subatomic emission.”
The idea of playing with the analogy between the linac’s resonance and sound resonance is one that Australian Synchrotron Director Andrew Peele appreciates. “A lot of science communication is about how you find analogies that people can engage with, and this is a great example,” Peele says.
Henschke displayed “Song of the Phenomena” at the Royal Melbourne Institute of Technology Gallery from November 17, 2016, to February 18, 2017. Since then, the apparatus has returned to the Australian Synchrotron, where it sits in a vast, open room where some of the facility’s synchrotron beamline stations used to stand. Scientists meet nearby for a weekly social coffee break.
Henschke is currently writing his thesis for his PhD in experimental art (with Boland as his advisor). In his next project, he hopes to tackle the subject of quantum entanglement.
Particle physics is a dance between theory and experiment.
Meenakshi Narain, a professor of physics at Brown University, remembers working on the DZero experiment at Fermi National Accelerator Laboratory near Chicago in the winter of 1994. She would bring blankets up to her fifth-floor office to keep warm as she sat at her computer going through data in search of the then-undiscovered top quark.
For weeks, her group had been working on deciphering some extra background that originally had not been accounted for. Their conclusions contradicted the collaboration’s original assumptions.
Narain, who was a postdoctoral researcher at the time, talked to her advisor about sharing the group’s result. Her advisor told her that if she had followed the scientific method and was confident in her result, she should talk about it.
“I had a whole sequence of logic and explanation prepared,” Narain says. “When I presented it, I remember everybody was very supportive. I had expected some pushback or some criticism and nothing like that happened.”
This, she says, is the scientific process: A multitude of steps designed to help us explore the world we live in.
“In the end the process wins. It’s not about you or me, because we’re all going after the same thing. We want to discover that particle or phenomenon or whatever else is out there collaboratively. That’s the goal.”
Narain’s group’s analysis was essential to the collaboration’s understanding of a signal that turned out to be the elusive top quark.
The modern hypothesis
“The scientific method was not invented overnight,” says Joseph Incandela, vice chancellor for research at the University of California, Santa Barbara. “People used to think completely differently. They thought if it was beautiful it had to be true. It took many centuries for people to realize that this is how you must approach the acquisition of true knowledge that you can verify.”
For particle physicists, says Robert Cahn, a senior scientist at Lawrence Berkeley National Laboratory, the scientific method isn’t so much going from hypothesis to conclusion, but rather “an exploration in which we measure with as much precision as possible a variety of quantities that we hope will reveal something new.
“We build a big accelerator and we might have some ideas of what we might discover, but it’s not as if we say, ‘Here’s the hypothesis and we’re going to prove or disprove it. If there’s a scientific method, it’s something much broader than that.”
Scientific inquiry is more of a continuing conversation between theorists and experimentalists, says Chris Quigg, a distinguished scientist emeritus at Fermilab.
“Theorists in particular spend a lot of time telling stories, making up ideas or elaborating ideas about how something might happen,” he says. “There’s an evolution of our ideas as we engage in dialogue with experiments.”
An important part of the process, he adds, is that the scientists are trained never to believe their own stories until they have experimental support.
“We are often reluctant to take our ideas too seriously because we’re schooled to think about ideas as tentative,” Quigg says. “It’s a very good thing to be tentative and to have doubt. Otherwise you think you know all the answers, and you should be doing something else.”
It’s also good to be tentative because “sometimes we see something that looks tantalizingly like a great discovery, and then it turns out not to be,” Cahn says.
At the end of 2015, hints appeared in the data of the two general-purpose experiments at the Large Hadron Collider that scientists had stumbled upon a particle 750 times as massive as a proton. The hints prompted more than 500 scientific papers, each trying to tell the story behind the bump in the data.
“It’s true that if you simply want to minimize wasting your time, you will ignore all such hints until they [reach the traditional uncertainty threshold of] 5 sigma,” Quigg said. “But it’s also true that as long as they’re not totally flaky, as long as it looks possibly true, then it can be a mind-expanding exercise.”
In the case of the 750-GeV bump, Quigg says, you could tell a story in which such a thing might exist and wouldn’t contradict other things that we knew.
“It helps to take it from just an unconnected observation to something that’s linked to everything else,” Quigg says. “That’s really one of the beauties of scientific theories, and specifically the current state of particle physics. Every new observation is linked to everything else we know, including all the old observations. It’s important that we have enough of a network of observation and interpretation that any new thing has to make sense in the context of other things.”
After collecting more data, physicists eventually ruled out the hints, and the theorists moved on to other ideas.
The importance of uncertainty
But sometimes an idea makes it further than that. Much of the work scientists put into publishing a scientific result involves figuring out how well they know it: What’s the uncertainty and how do we quantify it?
“If there’s any hallmark to the scientific method in particle physics and in closely related fields like cosmology, it’s that our results always come with an error bar,” Cahn says. “A result that doesn’t have an uncertainty attached to it has no value.”
In a particle physics experiment, some uncertainty comes from background, like the data Narain’s group found that mimicked the kind of signal they were looking for from the top quark.
This is called systematic uncertainty, which is typically introduced by aspects of the experiment that cannot be completely known.
“When you build a detector, you must make sure that for whatever signal you’re going to see, there is not much possibility to confuse it with the background,” says Helio Takai, a physicist at Brookhaven National Laboratory. “All the elements and sensors and electronics are designed having that in mind. You have to use your previous knowledge from all the experiments that came before.”
Careful study of your systematic uncertainties is the best way to eliminate bias and get reliable results.
“If you underestimate your systematic uncertainty, then you can overestimate the significance of the signal,” Narain says. “But if you overestimate the systematic uncertainty, then you can kill your signal. So, you really are walking this fine line in understanding where the issues may be. There are various ways the data can fool you. Trying to be aware of those ways is an art in itself and it really defines the thinking process.”
Physicists also must think about statistical uncertainty which, unlike systematic uncertainty, is simply the consequence having a limited amount of data.
“For every measurement we do, there’s a possibility that the measurement is a wrong measurement just because of all the events that happen at random while we are doing the experiment,” Takai says. “In particle physics, you’re producing many particles, so a lot of these particles may conspire and make it appear like the event you’re looking for.”
You can think of it as putting your hand inside a bag of M&Ms, Takai says. If the first few M&Ms you picked were brown and you didn’t know there were other colors, you would think the entire bag was brown. It wouldn’t be until you finally pulled out a blue M&M that you realized that the bag had more than one color.
Particle physicists generally want their results to have a statistical significance corresponding to at least 5 sigma, a measure that means that there is only a 0.00003 percent chance of a statistical fluctuation giving an excess as big or bigger than the one observed.
The scientific method at work
One of the most stunning recent examples of the scientific method – careful consideration of statistical and systematic uncertainties coming together – was announced in 2012 at the moment the spokespersons for the ATLAS and CMS experiments at the LHC revealed the discovery of the Higgs boson.
More than half a century of theory and experimentation led up to that moment. Experiments from the 1950s on had accumulated a wealth of information on particle interactions, but the interactions were only partially understood and seemed to come from disconnected sources.
“But brilliant theoretical physicists found a way to make a single model that gave them a good description of all the known phenomena, says Incandela, who was spokesperson for the CMS experiment during the Higgs discovery. “It wasn’t guaranteed that the Higgs field existed. It was only guaranteed that this model works for everything we do and have already seen, and we needed to see if there really was a boson that we could find that could tell us in fact that that field is there.”
This led to a generation-long effort to build an accelerator that would reach the extremely high energies needed to produce the Higgs boson, a particle born of the Higgs field, and then two gigantic detectors that could detect the Higgs boson if it appeared.
Building two different detectors would allow scientists to double-check their work. If an identical signal appeared in two separate experiments run by two separate groups of physicists, chances were quite good that it was the real thing.
“So there you saw a really beautiful application of the scientific method where we confirmed something that was incredibly difficult to confirm, but we did it incredibly well with a lot of fail-safes and a lot of outstanding experimental approaches,” Incandela says. “The scientific method was already deeply engrained in everything we did to the greatest extreme. And so we knew when we saw these things that they were real, and we had to take them seriously.”
The scientific method is so engrained that scientists don’t often talk about it by name anymore, but implementing it “is what separates the great scientists from the average scientists from the poor scientists,” Incandela says. “It takes a lot of scrutiny and a deep understanding of what you’re doing.”
This month scientists embedded sophisticated new instruments in the heart of a Large Hadron Collider experiment.
Sometimes big questions require big tools. That’s why a global community of scientists designed and built gigantic detectors to monitor the high-energy particle collisions generated by CERN’s Large Hadron Collider in Geneva, Switzerland. From these collisions, scientists can retrace the footsteps of the Big Bang and search for new properties of nature.
The CMS experiment is one such detector. In 2012, it co-discovered the elusive Higgs boson with its sister experiment, ATLAS. Now, scientists want CMS to push beyond the known laws of physics and search for new phenomena that could help answer fundamental questions about our universe. But to do this, the CMS detector needed an upgrade.
“Just like any other electronic device, over time parts of our detector wear down,” says Steve Nahn, a researcher in the US Department of Energy’s Fermi National Accelerator Laboratory and the US project manager for the CMS detector upgrades. “We’ve been planning and designing this upgrade since shortly after our experiment first started collecting data in 2010.”
The CMS detector is built like a giant onion. It contains layers of instruments that track the trajectory, energy and momentum of particles produced in the LHC’s collisions. The vast majority of the sensors in the massive detector are packed into its center, within what is called the pixel detector. The CMS pixel detector uses sensors like those inside digital cameras but with a lightning fast shutter speed: In three dimensions, they take 40 million pictures every second.
For the last several years, scientists and engineers at Fermilab and 21 US universities have been assembling and testing a new pixel detector to replace the current one as part of the CMS upgrade, with funding provided by the Department of Energy Office of Science and National Science Foundation.
Maral Alyari of SUNY Buffalo and Stephanie Timpone of Fermilab measure the thermal properties of a forward pixel detector disk at Fermilab. Almost all of the construction and testing of the forward pixel detectors occurred in the United States before the components were shipped to CERN for installation inside the CMS detector.
Stephanie Timpone consults a cabling map while fellow engineers Greg Derylo and Otto Alvarez inspect a completed forward pixel disk. The cabling map guides their task of routing the the thin, flexible cables that connect the disk's 672 silicon sensors to electronics boards.
The CMS detector, located in a cavern 100 meters underground, is open for the pixel detector installation.
Postdoctoral researcher Stefanos Leontsinis and colleague Roland Horisberger work with a mock-up of one side of the barrel pixel detector next to the LHC’s beampipe.
Leontsinis watches the clearance as engineers slide the first part of the barrel pixel just millimeters from the LHC’s beampipe.
The first half-moon of the innermost part of the pixel detector, called the Barrel, or BmO, is inserted along the beam pipe.
The second half-moon of the innermost barrel pixel is inserted.
Scientists and engineers connect the cooling pipes of the forward pixel detector. The pixel detector is flushed with liquid carbon dioxide to keep the silicon sensors protected from the LHC’s high-energy collisions.
Leontsinis carefully removes cables to make room for the forward pixel. Even a single cable slightly out of place can make it impossible to cleanly insert the new forward pixel detector.
Scientist John Conway of the University of California, Davis prepares to remove the second half of the forward pixel detector from its container.
The pixel detector consists of three sections: the innermost barrel section and two end caps called the forward pixel detectors. The tiered and can-like structure gives scientists a near-complete sphere of coverage around the collision point. Because the three pixel detectors fit on the beam pipe like three bulky bracelets, engineers designed each component as two half-moons, which latch together to form a ring around the beam pipe during the insertion process.
Over time, scientists have increased the rate of particle collisions at the LHC. In 2016 alone, the LHC produced about as many collisions as it had in the three years of its first run together. To be able to differentiate between dozens of simultaneous collisions, CMS needed a brand new pixel detector.
The upgrade packs even more sensors into the heart of the CMS detector. It’s as if CMS graduated from a 66-megapixel camera to a 124-megapixel camera.
Each of the two forward pixel detectors is a mosaic of 672 silicon sensors, robust electronics and bundles of cables and optical fibers that feed electricity and instructions in and carry raw data out, according to Marco Verzocchi, a Fermilab researcher on the CMS experiment.
The multipart, 6.5-meter-long pixel detector is as delicate as raw spaghetti. Installing the new components into a gap the size of a manhole required more than just finesse. It required months of planning and extreme coordination.
“We practiced this installation on mock-ups of our detector many times,” says Greg Derylo, an engineer at Fermilab. “By the time we got to the actual installation, we knew exactly how we needed to slide this new component into the heart of CMS.”
The most difficult part was maneuvering the delicate components around the pre-existing structures inside the CMS experiment.
“In total, the full three-part pixel detector consists of six separate segments, which fit together like a three-dimensional cylindrical puzzle around the beam pipe,” says Stephanie Timpone, a Fermilab engineer. “Inserting the pieces in the right positions and right order without touching any of the pre-existing supports and protections was a well-choreographed dance.”
For engineers like Timpone and Derylo, installing the pixel detector was the last step of a six-year process. But for the scientists working on the CMS experiment, it was just the beginning.
“Now we have to make it work,” says Stefanos Leontsinis, a postdoctoral researcher at the University of Colorado, Boulder. “We’ll spend the next several weeks testing the components and preparing for the LHC restart.”