Astronomers recently discovered a stronger-than-expected glow of gamma rays at the center of the Andromeda galaxy, the nearest major galaxy to the Milky Way. The signal has fueled hopes that scientists are zeroing in on a sign of dark matter, which is five times more prevalent than normal matter but has never been detected directly. 

Researchers believe that gamma rays—a very energetic form of light—could be produced when hypothetical dark matter particles decay or collide and destroy each other. However, dark matter isn’t the only possible source of the gamma rays. A number of other cosmic processes are known to produce them. 

So what do Andromeda’s gamma rays really tell us about dark matter? To find out, Symmetry’s Manuel Gnida talked with Eric Charles and Mattia Di Mauro, two members of the Fermi-LAT collaboration—an international team of researchers that found the Andromeda gamma-ray signal using the Large Area Telescope, a sensitive “eye” for dark matter on NASA’s Fermi Gamma-ray Space Telescope. 

Both researchers are based at the Kavli Institute for Particle Astrophysics and Cosmology, a joint institute of Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory. The LAT was conceived of and assembled at SLAC, which also hosts its operations center.

Have you discovered dark matter?

What’s so unusual about the results?

Why do galaxies glow in gamma rays?

What does all this have to do with dark matter?

Is dark matter the most likely interpretation?

What would it take to know for certain?

Tens of thousands of accelerators exist around the world, producing powerful particle beams for the benefit of medical diagnostics, cancer therapy, industrial manufacturing, material analysis, national security, and nuclear as well as fundamental particle physics. Particle beams can also be used to produce powerful beams of X-rays. 

Many of these particle accelerators rely on artfully crafted components called cavities. 

The world’s longest linear accelerator (also known as a linac) sits at the Department of Energy’s SLAC National Accelerator Laboratory. It stretches two miles and accelerates bunches of electrons to very high energies. 

The SLAC linac has undergone changes in its 50 years of operation that illustrate the evolution of the science of accelerator cavities. That evolution continues and will determine what the linac does next.

Robust copper

An accelerator cavity is a mostly closed, hollow chamber with an opening on each side for particles to pass through. As a particle moves through the cavity, it picks up energy from an electromagnetic field stored inside. Many cavities can be lined up like beads on a string to generate higher and higher particle energies. 

When SLAC’s linac first started operations, each of its cavities was made exclusively from copper. Each tube-like cavity consisted of a 1-inch-long, 4-inch-wide cylinder with disks on either side. Technicians brazed together more than 80,000 cavities to form a straight particle racetrack.  

Scientists generate radiofrequency waves in an apparatus called a klystron that distributes them to the cavities. Each SLAC klystron serves a 10-foot section of the beam line. The arrival of the electron bunch inside the cavity is timed to match the peak in the accelerating electric field. When a particle arrives inside the cavity at the same time as the peak in the electric field, then that bunch is optimally accelerated. 

“Particles only gain energy if the variable electric field precisely matches the particle motion along the length of the accelerator,” says Sami Tantawi, an accelerator physicist at Stanford University and SLAC. “The copper must be very clean and the shape and size of each cavity must be machined very carefully for this to happen.”

In its original form, SLAC’s linac boosted electrons and their antimatter siblings, positrons, to an energy of 50 billion electronvolts. Researchers used these beams of accelerated particles to study the inner structure of the proton, which led to the discovery of fundamental particles known as quarks.

Today almost all accelerators in the world—including smaller systems for medical and industrial applications—are made of copper. Copper is a good electric conductor, which is important because the radiofrequency waves build up an accelerating field by creating electric currents in the cavity walls. Copper can be machined very smoothly and is cheaper than other options, such as silver.  

“Copper accelerators are very robust systems that produce high acceleration gradients of tens of millions of electronvolts per meter, which makes them very attractive for many applications,” says SLAC accelerator scientist Chris Adolphsen. 

Today, one-third of SLAC’s original copper linac is used to accelerate electrons for the Linac Coherent Light Source, a facility that turns energy from the electron beam into what is currently the world’s brightest X-ray laser light.

Researchers continue to push the technology to higher and higher gradients—that is, larger and larger amounts of acceleration over a given distance. 

“Using sophisticated computer programs on powerful supercomputers, we were able to develop new cavity geometries that support almost 10 times larger gradients,” Tantawi says. “Mixing small amounts of silver into the copper further pushes the technology toward its natural limits.” Cooling the copper to very low temperatures helps as well. Tests at 45 Kelvin—negative 384 degrees Fahrenheit—have shown to increase acceleration gradients 20-fold compared to SLAC’s old linac. 

Copper accelerators have their limitations, though. SLAC’s historic linac produces 120 bunches of particles per second, and recent developments have led to copper structures capable of firing 80 times faster. But for applications that need much higher rates, Adolphsen says, “copper cavities don’t work because they would melt.”

Chill niobium

For this reason, crews at SLAC are in the process of replacing one-third of the original copper linac with cavities made of niobium. 

Niobium can support very large bunch rates, as long as it is cooled. At very low temperatures, it is what’s known as a superconductor.

“Below the critical temperature of 9.2 Kelvin, the cavity walls conduct electricity without losses, and electromagnetic waves can travel up and down the cavity many, many times, like a pendulum that goes on swinging for a very long time,” says Anna Grassellino, an accelerator scientist at Fermi National Accelerator Laboratory. “That’s why niobium cavities can store electromagnetic energy very efficiently and can operate continuously.” 

You can find superconducting niobium cavities in modern particle accelerators such as the Large Hadron Collider at CERN and the CEBAF accelerator at Thomas Jefferson National Accelerator Facility. The European X-ray Free-Electron Laser in Germany, the European Spallation Source at CERN, and the Facility for Rare Isotope Beams at Michigan State University are all being built using niobium technology. Niobium cavities also appear in designs for the next-generation International Linear Collider. 

At SLAC, the niobium cavities will support LCLS-II, an X-ray laser that will produce up to a million ultrabright light flashes per second. The accelerator will have 280 cavities, each about three feet long with a 3-inch opening for the electron beam to fly through. Sets of eight cavities will be strung together into cryomodules that keep the cavities at a chilly 2 Kelvin, which is colder than interstellar space.

Each niobium cavity is made by fusing together two halves stamped from a sheet of pure metal. The cavities are then cleaned very thoroughly because even the tiniest impurities would degrade their performance.

The shape of the cavities is reminiscent of a stack of shiny donuts. This is to maximize the cavity volume for energy storage and to minimize its surface area to cut down on energy dissipation. The exact size and shape also depends on the type of accelerated particle.

“We’ve come a long way since the first development of superconducting cavities decades ago,” Grassellino says. “Today’s niobium cavities produce acceleration gradients of up to about 50 million electronvolts per meter, and R&D work at Fermilab and elsewhere is further pushing the limits.”

Hot plasma

Over the past few years, SLAC accelerator scientists have been working on a way to push the limits of particle acceleration even further: accelerating particles using bubbles of ionized gas called plasma. 

Plasma wakefield acceleration is capable of creating acceleration gradients that are up to 1000 times larger than those of copper and niobium cavities, promising to drastically shrink the size of particle accelerators and make them much more powerful.

“These plasma bubbles have certain properties that are very similar to conventional metal cavities,” says SLAC accelerator physicist Mark Hogan. “But because they don’t have a solid surface, they can support extremely high acceleration gradients without breaking down.”

Hogan’s team at SLAC and collaborators from the University of California, Los Angeles, have been developing their plasma acceleration method at the Facility for Advanced Accelerator Experimental Tests, using an oven of hot lithium gas for the plasma and an electron beam from SLAC’s copper linac.

Researchers create bubbles by sending either intense laser light or a high-energy beam of charged particles through plasma. They then send beams of particles through the bubbles to be accelerated.

When, for example, an electron bunch enters a plasma, its negative charge expels plasma electrons from its flight path, creating a football-shaped cavity filled with positively charged lithium ions. The expelled electrons form a negatively charged sheath around the cavity.

This plasma bubble, which is only a few hundred microns in size, travels at nearly the speed of light and is very short-lived. On the inside, it has an extremely strong electric field. A second electron bunch enters that field and experiences a tremendous energy gain. Recent data show possible energy boosts of billions of electronvolts in a plasma column of just a little over a meter.

“In addition to much higher acceleration gradients, the plasma technique has another advantage,” says UCLA researcher Chris Clayton. “Copper and niobium cavities don’t keep particle beams tightly bundled and require the use of focusing magnets along the accelerator. Plasma cavities, on the other hand, also focus the beam.”

Much more R&D work is needed before plasma wakefield accelerator technology can be turned into real applications. But it could represent the future of particle acceleration at SLAC and of accelerator science as a whole.

Much of the matter in the universe is made up of tiny particles called quarks. Normally it’s impossible to see a quark on its own because they are always bound tightly together in groups. Quarks only separate in extreme conditions, such as immediately after the Big Bang or in the center of stars or during high-energy particle collisions generated in particle colliders.

Scientists at Louisiana Tech University are working on a study of quarks and the force that binds them by analyzing data from the ATLAS experiment at the LHC. Their measurements could tell us more about the conditions of the early universe and could even hint at new, undiscovered principles of physics.

The particles that stick quarks together are aptly named “gluons.” Gluons carry the strong force, one of four fundamental forces in the universe that govern how particles interact and behave. The strong force binds quarks into particles such as protons, neutrons and atomic nuclei.

As its name suggests, the strong force is the strongest—it’s 100 times stronger than the electromagnetic force (which binds electrons into atoms), 10,000 times stronger than the weak force (which governs radioactive decay), and a hundred million million million million million million (10³⁹) times stronger than gravity (which attracts you to the Earth and the Earth to the sun).

But this ratio shifts when the particles are pumped full of energy. Just as real glue loses its stickiness when overheated, the strong force carried by gluons becomes weaker at higher energies.

“Particles play by an evolving set of rules,” says Markus Wobisch from Louisiana Tech University. “The strength of the forces and their influence within the subatomic world changes as the particles’ energies increase. This is a fundamental parameter in our understanding of matter, yet has not been fully investigated by scientists at high energies.”

Characterizing the cohesiveness of the strong force is one of the key ingredients to understanding the formation of particles after the Big Bang and could even provide hints of new physics, such as hidden extra dimensions.

“Extra dimensions could help explain why the fundamental forces vary dramatically in strength,” says Lee Sawyer, a professor at Louisiana Tech University. “For instance, some of the fundamental forces could only appear weak because they live in hidden extra dimensions and we can’t measure their full strength. If the strong force is weaker or stronger than expected at high energies, this tells us that there’s something missing from our basic model of the universe.”

By studying the high-energy collisions produced by the LHC, the research team at Louisiana Tech University is characterizing how the strong force pulls energetic quarks into encumbered particles. The challenge they face is that quarks are rambunctious and caper around inside the particle detectors. This subatomic soirée involves hundreds of particles, often arising from about 20 proton-proton collisions happening simultaneously. It leaves a messy signal, which scientists must then reconstruct and categorize.

Wobisch and his colleagues innovated a new method to study these rowdy groups of quarks called jets. By measuring the angles and orientations of the jets, he and his colleagues are learning important new information about what transpired during the collisions—more than what they can deduce by simple counting the jets.

The average number of jets produced by proton-proton collisions directly corresponds to the strength of the strong force in the LHC’s energetic environment.

“If the strong force is stronger than predicted, then we should see an increase in the number of proton-protons collisions that generate three jets. But if the strong force is actually weaker than predicted, then we’d expect to see relatively more collisions that produce only two jets. The ratio between these two possible outcomes is the key to understanding the strong force.”

After turning on the LHC, scientists doubled their energy reach and have now determined the strength of the strong force up to 1.5 trillion electronvolts, which is roughly the average energy of every particle in the universe just after the Big Bang. Wobisch and his team are hoping to double this number again with more data.

“So far, all our measurements confirm our predictions,” Wobisch says. “More data will help us look at the strong force at even higher energies, giving us a glimpse as to how the first particles formed and the microscopic structure of space-time.”

Nearly a mile below the surface of Lead, South Dakota, scientists are preparing for a physics experiment that will probe one of the deepest questions of the universe: Why is there more matter than antimatter?

To search for that answer, the Deep Underground Neutrino Experiment, or DUNE, will look at minuscule particles called neutrinos. A beam of neutrinos will travel 800 miles through the Earth from Fermi National Accelerator Laboratory to the Sanford Underground Research Facility, headed for massive underground detectors that can record traces of the elusive particles.

Because neutrinos interact with matter so rarely and so weakly, DUNE scientists need a lot of material to create a big enough target for the particles to run into. The most widely available (and cost effective) inert substance that can do the job is argon, a colorless, odorless element that makes up about 1 percent of the atmosphere.

The researchers also need to place the detector full of argon far below Earth’s surface, where it will be protected from cosmic rays and other interference.

“We have to transfer almost 70,000 tons of liquid argon underground,” says David Montanari, a Fermilab engineer in charge of the experiment’s cryogenics. “And at this point we have two options: We can either transfer it as a liquid or we can transfer it as a gas.”

Either way, this move will be easier said than done.

Liquid or gas?

The argon will arrive at the lab in liquid form, carried inside of 20-ton tanker trucks. Montanari says the collaboration initially assumed that it would be easier to transport the argon down in its liquid form—until they ran into several speed bumps. 

Transporting liquid vertically is very different from transporting it horizontally for one important reason: pressure. The bottom of a mile-tall pipe full of liquid argon would have a pressure of about 3000 pounds per square inch—equivalent to 200 times the pressure at sea level. According to Montanari, to keep these dangerous pressures from occurring, multiple de-pressurizing stations would have to be installed throughout the pipe. 

Even with these depressurizing stations, safety would still be a concern. While argon is non-toxic, if released into the air it can reduce access to oxygen, much like carbon monoxide does in a fire. In the event of a leak, pressurized liquid argon would spill out and could potentially break its vacuum-sealed pipe, expanding rapidly to fill the mine as a gas. One liter of liquid argon would become about 800 liters of argon gas, or four bathtubs’ worth. 

Even without a leak, perhaps the most important challenge in transporting liquid argon is preventing it from evaporating into a gas along the way, according to Montanari. 

To remain a liquid, argon is kept below a brisk temperature of minus 180 degrees Celsius (minus 300 degrees Fahrenheit).

“You need a vacuum-insulated pipe that is a mile long inside a mine shaft,” Montanari says. “Not exactly the most comfortable place to install a vacuum-insulated pipe.”

To avoid these problems, the cryogenics team made the decision to send the argon down as gas instead. 

Routing the pipes containing liquid argon through a large bath of water will warm it up enough to turn it into gas, which will be able to travel down through a standard pipe. Re-condensers located underground act as massive air conditioners will then cool the gas until becomes a liquid again.

“The big advantage is we no longer have vacuum insulated pipe,” Montanari says. “It is just straight piece of pipe.”

Argon gas poses much less of a safety hazard because it is about 1000 times less dense than liquid argon. High pressures would be unlikely to build up and necessitate depressurizing stations, and if a leak occurred, it would not expand as much and cause the same kind of oxygen deficiency. 

The process of filling the detectors with argon will take place in four stages that will take almost two years, Montanari says. This is due to the amount of available cooling power for re-condensing the argon underground. There is also a limit to the amount of argon produced in the US every year, of which only so much can be acquired by the collaboration and transported to the site at a time.

Argon for answers

Once filled, the liquid argon detectors will pick up light and electrons produced by neutrino interactions.

Part of what makes neutrinos so fascinating to physicists is their habit of oscillating from one flavor—electron, muon or tau—to another. The parameters that govern this “flavor change” are tied directly to some of the most fundamental questions in physics, including why there is more matter than antimatter. With careful observation of neutrino oscillations, scientists in the DUNE collaboration hope to unravel these mysteries in the coming years.  

“At the time of the Big Bang, in theory, there should have been equal amounts of matter and antimatter in the universe,” says Eric James, DUNE’s technical coordinator. That matter and antimatter should have annihilated, leaving behind an empty universe. “But we became a matter-dominated universe.” 

James and other DUNE scientists will be looking to neutrinos for the mechanism behind this matter favoritism. Although the fruits of this labor won’t appear for several years, scientists are looking forward to being able to make use of the massive detectors, which are hundreds of times larger than current detectors that hold only a few hundred tons of liquid argon. 

Currently, DUNE scientists and engineers are working at CERN to construct Proto-DUNE, a miniature replica of the DUNE detector filled with only 300 tons of liquid argon that can be used to test the design and components. 

“Size is really important here,” James says. “A lot of what we’re doing now is figuring out how to take those original technologies which have already being developed... and taking it to this next level with bigger and bigger detectors.”

“Dear Ms. Higgins,” began the email to an employee of Fermi National Accelerator Laboratory. “My name is Micky Dolenz. I am in the entertainment business and probably best known for starring in a ’60s TV show called The Monkees. I have also been a big fan of particle physics for many decades.”

The message, which laboratory archivist Valerie Higgins received in November 2016, was legit. And it turns out Dolenz wasn’t kidding about his love of physics. Dolenz visited Fermilab on February 10 and impressed and amazed the scientists he met with his knowledge of (and genuine affection for) the science of quarks, leptons and bosons. Dolenz was, by all accounts, just as excited to meet with Fermilab scientists as they were to meet with him.

“He was so enthusiastic about the lab,” Higgins says. “It was such a treat to see someone of his stature and popularity be so interested and knowledgeable about our kind of physics.”

Previously unbeknownst to most of the lab’s employees, Dolenz’s association with Fermilab actually stretches back more than 40 years. The last time Dolenz visited Fermilab, the year was 1970. The Monkees TV show had wound down, and Dolenz, then 25, was starring in a play called Remains to Be Seen at the Pheasant Run Playhouse in nearby St. Charles, Illinois. Fermilab wasn’t even called Fermilab yet—it still went by the name National Accelerator Laboratory.

Dolenz says he remembers his first visit well. At the time, the lab consisted of a few trailers and bungalows—Fermilab’s now-iconic high-rise building, Wilson Hall, would not be completed until 1973. Dolenz had lunch with several of the scientists then toured the construction site for the Main Ring, the future home of Fermilab’s first superconducting accelerator, the Tevatron.

Dolenz captured some of his visit on 16mm film, footage he says he still has in storage. Dolenz called his previous tour of Fermilab “wonderful” and “a dream come true.”

Dolenz credits a junior high science teacher with sparking his interest in physics. He spent much of his childhood in Los Angeles building oscilloscopes and transceivers for ham radios and other gadgets. “I was always curious, always building stuff,” he says. “While the other kids were reading Superman comics, I was reading Science News. I loved it all, particularly particle physics and quantum physics.” 

Dolenz was in training to be an architect, but at age 20, the Monkees audition offered him the opportunity to catapult to worldwide fame as a TV star and musician instead. (“I’m not an idiot,” he says of accepting the role.) Still, he maintained his interest in science—his first email address, created in the 1990s, was “Higgs137,” referencing both the then-undiscovered Higgs boson and the measure of the fine structure constant.

That interest in science has remained strong, Fermilab physicists noted during the February tour. Dolenz toured the underground cavern that houses detectors for the MINOS, NOvA and MINERvA neutrino experiments, the Muon g-2 experiment hall (where scientists played the theme from The Monkees when he walked in), and the DZero detector in the long-since completed Main Ring. He also spent time in three control rooms.

In every location, he impressed the scientists he met with his understanding of physics and his full-on joy at seeing science in action.

“Who knew he is a life-long physics aficionado?” says scientist Adam Lyon, who gave Dolenz his Tevatron tour. “I had a great time talking with him.”

Dolenz says he sees plenty of connection between his twin interests of physics and music, noting that Einstein played the violin; Richard Feynman played bongos; and Queen guitarist Brian May is an astrophysicist on several experimental collaborations.

“According to theory the universe is constantly vibrating, down to even the smallest particles,” Dolenz says. “We talked a lot about vibrations in the ’60s, and Eastern philosophy has been talking about the vibration of the universe for thousands of years. Music is vibration and meter and frequency. There’s a lot of overlap.”

Dolenz enjoyed his time at Fermilab so much that he hung out at the lab’s on-site pub until late in the evening, chatting with scientists. And according to Higgins, who spent the most time with him, he’s hoping to return very soon.

“He’s still looking for the footage he shot in 1970, and plans to donate that to the archive,” she says. “But I told him he’s welcome here anytime.”

According to the known laws of physics, the universe we see today should be dark, empty and quiet. There should be no stars, no planets, no galaxies and no life—just energy and simple particles diffusing further and further into an expanding universe.

And yet, here we are.

Cosmologists calculate that roughly 13.8 billion years ago, our universe was a hunk of thick, hot energy with no boundaries and its own rules. But then, in less than a microsecond, it matured, and the fundamental laws and properties of matter arose from the pandemonium. How did our elegant and intricate universe emerge? 

The three conditions

The question “How is it here?” alludes to a conundrum that arose during the development of quantum mechanics. 

In 1928 Paul Dirac combined quantum theory and special relativity to predict the energy of an electron moving near the speed of light. But his equations produced two equally favorable answers: one positive and one negative. Because energy itself cannot be negative, Dirac mused that perhaps the two answers represented the particle’s two possible electric charges. The idea of oppositely charged matter-antimatter pairs was born.

Meanwhile, about six minutes away from Dirac’s office in Cambridge, physicist Patrick Blackett was studying the patterns etched in cloud chambers by cosmic rays. In 1933 he detected 14 tracks that showed a single particle of light colliding with an air molecule and bursting into two new particles. The spiral tracks of these new particles were mirror images of each other, indicating that they were oppositely charged. This was one of the first observations of what Dirac had predicted five years earlier—the birth of an electron-positron pair.

Today it’s well known that matter and antimatter are the ultimate wonder twins. They’re spontaneously born from raw energy as a team of two and vanish in a silent poof of energy when they merge and annihilate. This appearing-disappearing act spawned one of the most fundamental mysteries in the universe: What is engraved in the laws of nature that saved us from the broth of appearing and annihilating particles of matter and antimatter?

“We know this cosmic asymmetry must exist because here we are,” says Jessie Shelton, a theorist at the University of Illinois. “It’s a puzzling imbalance because theory requires three conditions—which all have to be true at once—to create this cosmic preference for matter.”

In the 1960s physicist Andrei Sakharov proposed this set of three conditions that could explain the appearance of our matter-dominated universe. Scientists continue to look for evidence of these conditions today.

1. Breaking the tether

The first problem is that matter and antimatter always seem to be born together. Just as Blackett observed in the cloud chambers, uncharged energy transforms into evenly balanced matter-antimatter pairs. Charge is always conserved through any transition. For there to be an imbalance in the amounts of matter and antimatter, there needs to be a process that creates more of one than the other.

“Sakharov’s first criterion essentially says that there must be some new process that converts antimatter into matter, or vice versa,” says Andrew Long, a postdoctoral researcher in cosmology at the University of Chicago. “This is one of the things experimentalists are looking for in the lab.”

In the 1980s, scientists searched for evidence of Sakharov’s first condition by looking for signs of a proton decaying into a positron and two photons. They have yet to find evidence of this modern alchemy, but they continue to search. 

“We think that the early universe could have contained a heavy neutral particle that sometimes decayed into matter and sometimes decayed into antimatter, but not necessarily into both at the same time,” Long says.

2. Picking a favorite

Matter and antimatter cannot co-habitate; they always annihilate when they come into contact. But the creation of just a little more matter than antimatter after the Big Bang—about one part in 10 billion—would leave behind the ingredients needed to build the entire visible universe.

How could this come about? Sakharov’s second criterion dictates that the matter-only process outlined in his first criterion must be more efficient than the opposing antimatter process. And specifically, “we need to see a favoritism for the right kinds of matter to agree with astronomical observations,” Shelton says.

Observations of light left over from the early universe and measurements of the first lightweight elements produced after the Big Bang show that the discrepancy must exist in a class of particles called baryons: protons, antiprotons and other particles constructed from quarks.

“These are snapshots of the early universe,” Shelton says. “From these snapshots, we can derive the density and temperature of the early universe and calculate the slight difference between the number of baryons and antibaryons.”

But this slight difference presents a problem. While there are some tiny discrepancies between the behavior of particles and their antiparticle counterparts, these idiosyncrasies are still consistent with the Standard Model and are not enough to explain the origin of the cosmic imbalance nor the universe’s tenderness towards matter.

3. Taking a one-way street

In particle physics, any process that runs forward can just as easily run in reverse. A pair of photons can merge and morph into a particle and antiparticle pair. And just as easily, the particle and antiparticle pair can recombine into a pair of photons. This process happens all around us, continually. But because it is cyclical, there is no net gain or loss for a type of matter.

If this were always true, our young universe could have been locked in an infinite loop of creation and destruction. Without something slamming the brakes on these cycles at least for a moment, matter could not have evolved into the complex structures we see today.

“For every stitch that’s knit, there a simultaneous tug on the thread,” Long says. “We need a way to force the reaction to move forward and not simultaneously run in reverse at the same rate.”

Many cosmologists suspect that the gradual expansion and cooling of the universe was enough to lock matter into being, like a supersaturated sweet tea whose sugar crystals drop to the bottom of the glass as it cools (or in the “freezing” interpretation, like a sweet tea that instantly freezes into ice, locking sugar crystals in place without giving them a chance to dissolve).

Other cosmologists think that the plasma of the early universe may have contained bubbles that helped separate matter and antimatter (and then served as incubators for particles to acquire mass).

Several experiments at CERN are looking for evidence that the universe meets Sakharov’s three conditions. For instance, several precision experiments at CERN’s Antimatter Factory are looking for minuscule differences between the intrinsic characteristics of protons and antiprotons. The LHCb experiment at the Large Hadron Collider is examining the decay patterns of unstable matter and antimatter particles.

Shelton and Long both hope that more research from experiments at the LHC will be the key to building a more complete picture of our early universe.

LHC experiments could discover that the Higgs field served as the lock that halted the early universe’s perpetually evolving and devolving particle soup—especially if the field contained bubbles that froze faster than others, providing cosmic petri dishes in which matter and antimatter could evolve differently, Long says. “More measurements of the Higgs boson and the fundamental properties of matter and antimatter will help us develop better theories and a better understanding of what and where we come from.”

What exactly transpired during the birth of our universe may always remain a bit of an enigma, but we continue to seek new pieces of this formidable puzzle.

Modern cosmology experiments—such as the BICEP instruments and the Keck Array in Antarctica—rely on superconducting photon detectors to capture signals from the early universe.

These detectors, called transition edge sensors, are kept at temperatures near absolute zero, at only tenths of a Kelvin. At this temperature, the “transition” between superconducting and normal states, the sensors function like an extremely sensitive thermometer. They are able to detect heat from cosmic microwave background radiation, the glow emitted after the Big Bang, which is only slightly warmer at around 3 Kelvin.

Scientists also have been experimenting with these same detectors to catch a different form of light, says Dan Swetz, scientist at the National Institute of Standards and Technology. These sensors also happen to work quite well as extremely sensitive X-ray detectors.

NIST scientists, including Swetz, design and build the thin, superconducting sensors and turn them into pixelated arrays smaller than a penny. They construct an entire X-ray spectrometer system around those arrays, including a cryocooler, a refrigerator that can keep the detectors near absolute zero temperatures.

Over the past several years, these X-ray spectrometers built at the NIST Boulder MicroFabrication Facility have been installed at three synchrotrons at US Department of Energy national laboratories: the National Synchrotron Light Source at Brookhaven National Laboratory, the Advanced Photon Source at Argonne National Laboratory and most recently at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory.

Organizing the transition edge sensors into arrays made a more powerful detector. The prototype sensor—built in 1995—consisted of only one pixel. These early detectors had poor resolution, says physicist Kent Irwin of Stanford University and SLAC. He built the original single-pixel transition edge sensor as a postdoc.

Like a camera, the detector can capture greater detail the more pixels it has.

“It’s only now that we’re hitting hundreds of pixels that it’s really getting useful,” Irwin says. “As you keep increasing the pixel count, the science you can do just keeps multiplying. And you start to do things you didn’t even conceive of being possible before.”

Each of the 240 pixels is designed to catch a single photon at a time. These detectors are efficient, says Irwin, collecting photons that may be missed with more conventional detectors.

Spectroscopy experiments at synchrotrons examine subtle features of matter using X-rays. In these types of experiments, an X-ray beam is directed at a sample. Energy from the X-rays temporarily excites the electrons in the sample, and when the electrons return to their lower energy state, they release photons. The photons’ energy is distinctive for a given chemical element and contains detailed information about the electronic structure.

As the transition edge sensor captures these photons, every individual pixel on the detector functions as a high-energy-resolution spectrometer, able to determine the energy of each photon collected.

The researchers combine data from all the pixels and make note of the pattern of detected photons across the entire array and each of their energies. This energy spectrum reveals information about the molecule of interest.

These spectrometers are 100 times more sensitive than standard spectrometers, says Dennis Nordlund, SLAC scientist and leader of the transition edge sensor project at SSRL. This allows a look at biological and chemical details at extremely low concentrations using soft (low-energy) X-rays.

“These technology advances mean there are many things we can do now with spectroscopy that were previously out of reach,” Nordlund says. “With this type of sensitivity, this is when it gets really interesting for chemistry.”

Nordlund and his colleagues—Sangjun Lee, a SLAC postdoctoral research fellow, and Jamie Titus, a Stanford University doctoral student (pictured above at SSRL, from right: Nordlund, Titus and Lee)—have already used the transition-edge-sensor spectrometer at SSRL to probe for nitrogen impurities in nanodiamonds and graphene, as well as closely examine the metal centers of proteins and bioenzymes, such as hemoglobin and photosystem II. The project at SLAC was developed with 
support by the Department of Energy’s Laboratory Directed Research and Development.

The early experiments at Brookhaven looked at bonding and the chemical structure of nitrogen-bearing explosives. With the spectrometer at Argonne, a research team recently took scattering measurements on high-temperature superconducting materials.

“The instruments are very similar from a technical standpoint—same number of sensors, similar resolution and performance,” Swetz says. “But it’s interesting, the labs are all doing different science with the same basic equipment.”

At NIST, Swetz says they’re working to pair these detectors with less intense light sources, which could enable researchers to do X-ray experiments in their personal labs.

There are plans to build transition-edge-sensor spectrometers that will work in the higher energy hard X-ray region, which scientists at Argonne are working on for the next upgrade of Advanced Photon Source.

To complement this, the SLAC and NIST collaboration is engineering spectrometers that will handle the high repetition rate of X-ray laser pulses such as LCLS-II, the next generation of the free-electron X-ray laser at SLAC. This will require faster readout systems. The goal is to create a transition-edge-sensor array with as many as 10,000 pixels that can capture more than 10,000 pulses per second.

Irwin points out that the technology developed for synchrotrons, LCLS-II and future cosmic-microwave-background experiments provides shared benefit.

“The information really keeps bouncing back and forth between X-ray science and cosmology,” Irwin says.

It’s not as flashy as Scooby Doo’s Mystery Machine, but scientists at Virginia Tech hope that their new vehicle will help solve mysteries about a ghost-like phenomena: neutrinos.

The Mobile Neutrino Lab is a trailer built to contain and transport a 176-pound neutrino detector named MiniCHANDLER (Carbon Hydrogen AntiNeutrino Detector with a Lithium Enhanced Raghavan-optical-lattice). When it begins operations in mid-April, MiniCHANDLER will make history as the first mobile neutrino detector in the US.

“Our main purpose is just to see neutrinos and measure the signal to noise ratio,” says Jon Link, a member of the experiment and a professor of physics at Virginia Tech’s Center for Neutrino Physics. “We just want to prove the detector works.”

Neutrinos are fundamental particles with no electric charge, a property that makes them difficult to detect. These elusive particles have confounded scientists on several fronts for more than 60 years. MiniCHANDLER is specifically designed to detect neutrinos' antimatter counterparts, antineutrinos, produced in nuclear reactors, which are prolific sources of the tiny particles.

Fission at the core of a nuclear reactor splits uranium atoms, whose products themselves undergo a process that emits an electron and electron antineutrino. Other, larger detectors such as Daya Bay have capitalized on this abundance to measure neutrino properties.

MiniCHANDLER will serve as a prototype for future mobile neutrino experiments up to 1 ton in size.

Link and his colleagues hope MiniCHANDLER and its future counterparts will find answers to questions about sterile neutrinos, an undiscovered, theoretical kind of neutrino and a candidate for dark matter. The detector could also have applications for national security by serving as a way to keep tabs on material inside of nuclear reactors.

MiniCHANDLER echoes a similar mobile detector concept from a few years ago. In 2014, a Japanese team published results from another mobile neutrino detector, but their data did not meet the threshold for statistical significance. Detector operations were halted after all reactors in Japan were shut down for safety inspections.

“We can monitor the status from outside of the reactor buildings thanks to [a] neutrino’s strong penetration power,” Shugo Oguri, a scientist who worked on the Japanese team, wrote in an email.

Link and his colleagues believe their design is an improvement, and the hope is that MiniCHANDLER will be able to better reject background events and successfully detect neutrinos.

Neutrinos, where are you?

To detect neutrinos, which are abundant but interact very rarely with matter, physicists typically use huge structures such as Super-Kamiokande, a neutrino detector in Japan that contains 50,000 tons of ultra-pure water. Experiments are also often placed far underground to block out signals from other particles that are prevalent on Earth’s surface.

With its small size and aboveground location, MiniCHANDLER subverts both of these norms.

The detector uses solid scintillator technology, which will allow it to record about 100 antineutrino interactions per day. This interaction rate is less than the rate at large detectors, but MiniCHANDLER makes up for this with its precise tracking of antineutrinos.

Small plastic cubes pinpoint where in MiniCHANDLER an antineutrino interacts by detecting light from the interaction. However, the same kind of light signal can also come from other passing particles like cosmic rays. To distinguish between the antineutrino and the riffraff, Link and his colleagues look for multiple signals to confirm the presence of an antineutrino.

Those signs come from a process called inverse beta decay. Inverse beta decay occurs when an antineutrino collides with a proton, producing light (the first event) and also kicking a neutron out of the nucleus of the atom. These emitted neutrons are slower than the light and are picked up as a secondary signal to confirm the antineutrino interaction.

“[MiniCHANDLER] is going to sit on the surface; it's not shielded well at all. So it's going to have a lot of background,” Link says. “Inverse beta decay gives you a way of rejecting the background by identifying the two-part event.”

Monitoring the reactors

Scientists could find use for a mobile neutrino detector beyond studying reactor neutrinos. They could also use the detector to measure properties of the nuclear reactor itself.

A mobile neutrino detector could be used to determine whether a reactor is in use, Oguri says. “Detection unambiguously means the reactors are in operation—nobody can cheat the status.”

The detector could also be used to determine whether material from a reactor has been repurposed to produce nuclear weapons. Plutonium, an element used in the process of making weapons-grade nuclear material, produces 60 percent fewer detectable neutrinos than uranium, the primary component in a reactor core.

“We could potentially tell whether or not the reactor core has the right amount of plutonium in it,” Link says.

Using a neutrino detector would be a non-invasive way to track the material; other methods of testing nuclear reactors can be time-consuming and disruptive to the reactor’s processes.

But for now, Link just wants MiniCHANDLER to achieve a simple—yet groundbreaking—goal: Get the mobile neutrino lab running.