symmetry

The biggest little detectors

The ProtoDUNE detectors for the Deep Underground Neutrino Experiment are behemoths in their own right.

A person stands inside the ProtoDUNE detector, dwarfed by the empty space. The walls, floor, and ceiling are golden.

In one sense, the two ProtoDUNE detectors are small. As prototypes of the much larger planned Deep Underground Neutrino Experiment, they are only representative slices, each measuring about 1 percent of the size of the final detector. But in all other ways, the ProtoDUNE detectors are simply massive.

Once they are complete later this year, these two test detectors will be larger than any detector ever built that uses liquid argon, its active material. The international project involves dozens of experimental groups coordinating around the world. And most critically, the ProtoDUNE detectors, which are being installed and tested at the European particle physics laboratory CERN, are the rehearsal spaces in which physicists, engineers and technicians will hammer out nearly every engineering problem confronting DUNE, the biggest international science project ever conducted in the United States.

Gigantic detector, tiny neutrino

DUNE’s mission, when it comes online in the mid-2020s, will be to pin down the nature of the neutrino, the most ubiquitous particle of matter in the universe. Despite neutrinos’ omnipresence—they fill the universe, and trillions of them stream through us every second—they are a pain in the neck to capture. Neutrinos are vanishingly small, fleeting particles that, unlike other members of the subatomic realm, are heedless of the matter through which they fly, never stopping to interact.

Well, almost never.

Once in a while, scientists can catch one. And when they do, it might tell them a bit about the origins of the universe and why matter predominates over antimatter—and thus how we came to be here at all.

A global community of more than 1000 scientists from 31 countries are building DUNE, a megascience experiment hosted by the Department of Energy’s Fermi National Accelerator Laboratory. The researchers’ plan is to observe neutrinos using two detectors separated by 1300 kilometers—one at Fermilab outside Chicago and a second one a mile underground in South Dakota at the Sanford Underground Research Facility. Having one at each end enables scientists to see how neutrinos transform as they travel over a long distance.

The DUNE collaboration is going all-in on the bigger-is-better strategy; after all, the bigger the detector, the more likely scientists are to snag a neutrino. The detector located in South Dakota, called the DUNE far detector, will hold 70,000 metric tons (equivalent to about 525,000 bathtubs) of liquid argon to serve as the neutrino fishing net. It comprises four large modules. Each will stand four stories high and, not including the structures that house the utilities, occupy a footprint roughly equal to a soccer field.

In short, DUNE is giant.

Small Particles, Big Science: The International LBNF/DUNE Project

Video of Small Particles, Big Science: The International LBNF/DUNE Project

Lots of room in ProtoDUNE

The ProtoDUNE detectors are small only when compared to the giant DUNE detector. If each of the four DUNE modules is a 20-room building, then each ProtoDUNE detector is one room.

But one room large enough to envelop a small house.

As one repeatable unit of the ultimate detector, the ProtoDUNE detectors are necessarily big. Each is an enormous cube—about two stories high and about as wide—and contains about 800 metric tons of liquid argon.

Why two prototypes? Researchers are investigating two ways to use argon and so are constructing two slightly different but equally sized test beds. The single-phase ProtoDUNE uses only liquid argon, while the dual-phase ProtoDUNE uses argon as both a liquid and a gas.

“They’re the largest liquid-argon particle detectors that have ever been built,” says Ed Blucher, DUNE co-spokesperson and a physicist at the University of Chicago.

As DUNE’s test bed, the ProtoDUNE detectors also have to offer researchers a realistic picture of how the liquid-argon detection technology will work in DUNE, so the instrumentation inside the detectors is also at full, giant scale.

“If you’re going to build a huge underground detector and invest all of this time and all of these resources into it, that prototype has to work properly and be well-understood,” says Bob Paulos, director of the University of Wisconsin–Madison Physical Sciences Lab and a DUNE engineer. “You need to understand all the engineering problems before you proceed to build literally hundreds of these components and try to transport them all underground.”

A person on a scissor lift stands next to the tall, red cube that is ProtoDUNE.

A crucial step for ProtoDUNE was welding together the cryostat, or cold vessel, that will house the detector components and liquid argon.

Photo by CERN

Partners in ProtoDUNE

ProtoDUNE is a rehearsal for DUNE not only in its technical orchestration but also in the coordination of human activity.

When scientists were planning their next-generation neutrino experiment around 2013, they realized that it could succeed only by bringing the international scientific community together to build the project. They also saw that even the prototyping would require an effort of global proportions—both geographically and professionally. As a result, DUNE and ProtoDUNE actively invite students, early-career scientists and senior researchers from all around the world to contribute.

“The scale of ProtoDUNE, a global collaboration at CERN for a US-based megaproject, is a paradigm change in the way neutrino science is done,” says Christos Touramanis, a physicist at the University of Liverpool and one of the co-coordinators of the single-phase detector. For both DUNE and ProtoDUNE, funding comes from partners around the world, including the Department of Energy's Office of Science and CERN.

The successful execution of ProtoDUNE’s assembly and testing by international groups requires a unity of purpose from parties that could hardly be farther apart, geographically speaking.

Scientists say the effort is going smoothly.

“I’ve been doing neutrino physics and detector technology for the last 20 or 25 years. I’ve never seen such an effort go up so nicely and quickly. It’s astonishing,” says Fermilab scientist Flavio Cavanna, who co-coordinates the single-phase ProtoDUNE project. “We have a great collaboration, great atmosphere, great willingness to make it. Everybody is doing his or her best to contribute to the success of this big project. I used to say that ProtoDUNE was mission impossible, because—in the short time we were given to make the two detectors, it looked that way in the beginning. But looking at where we are now, and all the progress made so far, it starts turning out to be mission possible.”

An individual in purple gloves examines the wires of the APA. A colleague, also in purple gloves, watches a computer monitor.

The anode plane array (APA) is prepped for shipment at Daresbury Laboratory in the UK.
 

Christos Touramanis

Inside the liquid-argon test bed

So how do neutrino liquid-argon detectors work? Most of the space inside serves as the arena of particle interaction, where neutrinos can smash into an argon atom and create secondary particles. Surrounding this interaction space is the instrumentation that records these rare collisions, like a camera committing the scene to film. DUNE collaborators are developing and constructing the recording instruments that will capture the evidence of these interactions.

One signal is ionization charge: A neutrino interaction generates other particles that propagate through the detector’s vast argon pool, kicking electrons—called ionization electrons—off atoms as they go. The second signal is light.

Animation: Neutrino Detection in Liquid-Argon Time Projection Chamber

Video of Animation: Neutrino Detection in Liquid-Argon Time Projection Chamber

The first signal emerges as a streak of ionization electrons.

To record the signal, scientists will use something called an anode plane array, or APA. An APA is a screen created using 24 kilometers of precisely tensioned, closely spaced, continuously wound wire. This wire screen is positively charged, so it attracts the negatively charged electrons.

Much the way a wave front approaches the beach’s shore, the particle track—a string of the ionization electrons—will head toward the positively charged wires inside the ProtoDUNE detectors. The wires will send information about the track to computers, which will record its properties and thus information about the original neutrino interaction.

A group in the University of Wisconsin–Madison Physical Sciences Lab led by Paulos designed the single-phase ProtoDUNE wire arrays. The Wisconsin group, Daresbury Laboratory in the UK and several UK universities are building APAs for the same detector. The first APA from Wisconsin arrived at CERN last year; the first from Daresbury Lab arrived earlier this week.

“These are complicated to build,” Paulos says, noting that it currently takes about three months to build just one. “Building these 6-meter-tall anode planes with continuously wound wire—that’s something that hasn't been done before.”

ProtoDUNE Anode Plane Assembly

Video of ProtoDUNE Anode Plane Assembly

The anode planes attract the electrons. Pushing away the electrons will be a complementary set of panels, called the cathode plane. Together, the anode and cathode planes behave like battery terminals, with one repelling electron tracks and the other drawing them in. A group at CERN designed and is building the cathode plane.

The dual-phase detector will operate on the same principle but with a different configuration of wire arrays. A special layer of electronics near the cathode will allow for the amplification of faint electron tracks in a layer of gaseous argon. Groups at institutions in France, Germany and Switzerland are designing those instruments. Once complete, they will also send their arrays to be tested at CERN.

Then there’s the business of observing light.

The flash of light is the result of a release of energy from the electron in the process of getting bumped from an argon atom. The appearance of light is like the signal to start a stopwatch; it marks the moment the neutrino interaction in a detector takes place. This enables scientists to reconstruct in three dimensions the picture of the interaction and resulting particles.

On the other side of the equator, a group at the University of Campinas in Brazil is coordinating the installation of instruments that will capture the flashes of light resulting from particle interactions in the single-phase ProtoDUNE detector.

Two of the designs for the single-phase prototype—one by Indiana University, the other by Fermilab and MIT—are of a type called guiding bars. These long, narrow strips work like fiber optic cables: they capture the light, convert it into light in the visible spectrum and finally guide it to an external sensor.

A third design, called ARAPUCA, was developed by three Brazilian universities and Fermilab and is being partially produced at Colorado State University. Named for the Guaraní word for a bird trap, the efficient ARAPUCA design will be able to “trap” even very low light signals and transmit them to its sensors.

Rows of chips are laid out in a grid that is four chips tall and three wide. More grids trail off in the background.

The ARAPUCA array, designed by three Brazilian universities and Fermilab, was partially produced at Colorado State University.

D. Warner, Colorado State University

“The ARAPUCA technology is totally new,” says University of Campinas scientist Ettore Segreto, who is co-coordinating the installation of the light detection systems in the single-phase prototype. “We might be able to get more information from the light detection—for example, greater energy resolution.”

Groups from France, Spain and the Swiss Federal Institute of Technology are developing the light detection system for the dual-phase prototype, which will comprise 36 photomultiplier tubes, or PMTs, situated near the cathode plane. A PMT works by picking up the light from the particle interaction and converting it into electrons, multiplying their number and so amplifying the signal’s strength as the electrons travel down the tube.

With two tricked-out detectors, the DUNE collaboration can test their picture-taking capabilities and prepare DUNE to capture in exquisite detail the fleeting interactions of neutrinos.

Bringing instruments into harmony

But even if they’re instrumented to the nines inside, two isolated prototypes do not a proper test bed make. Both ProtoDUNE detectors must be hooked up to computing systems so particle interaction signals can be converted into data. Each detector must be contained in a cryostat, which functions like a thermos, for the argon to be cold enough to maintain a liquid state. And the detectors must be fed particles in the first place.

CERN is addressing these key areas by providing particle beam, innovative cryogenics and computing infrastructures, and connecting the prototype detectors with the DUNE experimental environment.

DUNE’s neutrinos will be provided by the Long-Baseline Neutrino Facility, or LBNF, which held an underground groundbreaking for the start of its construction in July. LBNF, led by Fermilab, will provide the construction, beamline and cryogenics for the mammoth DUNE detector, as well as Fermilab’s chain of particle accelerators, which will provide the world’s most intense neutrino beam to the experiment.

CERN is helping simulate that environment as closely as possible with the scaled-down ProtoDUNE detectors, furnishing them with particle beams so researchers can characterize how the detectors respond. Under the leadership of scientist Marzio Nessi, last year the CERN group built a new facility for the test beds, where CERN is now constructing two new particle beamlines that extend the lab’s existing network.

A narrow, wrapped object, the APA, is lifted by a crane. It's destination, the red ProtoDUNE cryostat, sits in the background.

The recently arrived anode plane array (hanging on the left) is moved by a crane to its new home in the ProtoDUNE cryostat.

Photo by CERN

In addition, CERN built the ProtoDUNE cryostats—the largest ever constructed for a particle physics experiment—which also will serve as prototypes for those used in DUNE. Scientists will be able to gather and interpret the data generated from the detectors with a CERN computing farm and software and hardware from several UK universities.

“The very process of building these prototype detectors provides a stress test for building them in DUNE,” Blucher says.

CERN’s beam schedule sets the schedule for testing. In December, the European laboratory will temporarily shut off beam to its experiments for upgrades to the Large Hadron Collider. DUNE scientists aim to position the ProtoDUNE detectors in the CERN beam before then, testing the new technologies pioneered as part of the experiment.

“ProtoDUNE is a necessary and fundamental step towards LBNF/DUNE,” Nessi says. “Most of the engineering will be defined there and it is the place to learn and solve problems. The success of the LBNF/DUNE project depends on it.”

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Voyage into the dark sector

A hidden world of particles awaits.

Higgs dirigible entering “dark sector”

We don’t need extra dimensions or parallel universes to have an alternate reality superimposed right on top of our own. Invisible matter is everywhere. 

For example, take neutrinos generated by the sun, says Jessie Shelton, a theorist at the University of Illinois at Urbana-Champaign who works on dark sector physics. “We are constantly bombarded with neutrinos, but they pass right through us. They share the same space as our atoms but almost never interact.”

As far as scientists can tell, neutrinos are solitary particles. But what if there is a whole world of particles that interact with one another but not with ordinary atoms? This is the idea behind the dark sector: a theoretical world of matter existing alongside our own but invisible to the detectors we use to study the particles we know.

“Dark sectors are, by their very definition, built out of particles that don't interact strongly with the Standard Model,” Shelton says.

The Standard Model is a physicist’s field guide to the 17 particles and forces that make up all visible matter. It explains how atoms can form and why the sun shines. But it cannot explain gravity, the cosmic imbalance of matter and antimatter, or the disparate strengths of nature's four forces.

On its own, an invisible world of dark sector particles cannot solve all these problems. But it certainly helps.

Animated illustration of the dark sector

Artwork by Sandbox Studio, Chicago with Ana Kova

The main selling point for the dark sector is that the theories comprehensively confront the problem of dark matter. Dark matter is a term physicists coined to explain bizarre gravitational effects they observe in the cosmos. Distant starlight appears to bend around invisible objects as it traverses the cosmos, and galaxies spin as if they had five times more mass than their visible matter can explain. Even the ancient light preserved in cosmic microwave background seems to suggest that there is an invisible scaffolding on which galaxies are formed.

Some theories suggest that dark matter is simple cosmic debris that adds mass—but little else—to the complexity of our cosmos. But after decades of searching, physicists have yet to find dark matter in a laboratory experiment. Maybe the reason scientists haven’t been able to detect it is that they’ve been underestimating it.

“There is no particular reason to expect that whatever is going on in the dark sector has to be as simple as our most minimal models,” Shelton says. “After all, we know that our visible world has a lot of rich physics: Photons, electrons, protons, nuclei and neutrinos are all critically important for understanding the cosmology of how we got here. The dark sector could be a busy place as well.”

According to Shelton, dark matter could be the only surviving particle out of a similarly complicated set of dark particles.

“It could even be something like the proton, a bound state of particles interacting via a very strong dark force. Or it could even be something like a hydrogen atom, a bound state of particles interacting via a weaker dark force,” she says.

Even if terrestrial experiments cannot see these stable dark matter particles directly, they might be sensitive to other kinds of dark particles, such as dark photons or short-lived dark particles that interact strongly with the Higgs boson.

“The Higgs is one of the easiest ways for the Standard Model particles to talk to the dark sector,” Shelton says.

As far as scientists know, the Higgs boson is not picky. It may very well interact will all sorts of massive particles, including those invisible to ordinary atoms. If the Higgs boson interacts with massive dark sector particles, scientists should find that its properties deviate slightly from the Standard Model’s predictions. Scientists at the Large Hadron Collider are precisely measuring the properties of the Higgs boson to search for unexpected quirks that could open a gateway to new physics.

At the same time, scientists are also using the LHC to search for dark sector particles directly. One theory is that at extremely high temperatures, dark matter and ordinary matter are not so different and can transform into one another through a dark force. In the hot and dense early universe, this would have been quite common.

“But as the universe expanded and cooled, this interaction froze out, leaving some relic dark matter behind,” Shelton says.

The energetic particle collisions generated by the LHC imitate the conditions that existed in the early universe and could unlock dark sector particles. If scientists are lucky, they might even catch dark sector particles metamorphosing into ordinary matter, an event that could materialize in the experimental data as particle tracks that suddenly appear from no apparent source. 

But there are also several feasible scenarios in which any interactions between the dark sector and our Standard Model particles are so tiny that they are out of reach of modern experiments, according to Shelton.

"These ‘nightmare’ scenarios are completely logical possibilities, and in this case, we will have to think very carefully about astrophysical and cosmological ways to look for the footprints of dark particle physics,” she says.

Even if the dark sector is inaccessible to particle detectors, dark matter will always be visible through the gravitational fingerprint it leaves on the cosmos.

“Gravity tells us a lot about how much dark matter is in the universe and the kinds of particle interactions dark sector particles can and cannot have,” Shelton says. “For instance, more sensitive gravitational-wave experiments will give us the possibility to look back in time and see what our universe looked like at extremely high energies, and could maybe reveal more about this invisible matter living in our cosmos.”

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Rivers in the sky

Local communities named newly discovered stellar streams for bodies of water close to home.

River of stars in the sky flows into waterfall and river

Most of the time, the Dark Energy Camera in Chile stares out into the deepest regions of space, measuring light from distant galaxies. But this gigantic eye sometimes discovers things closer to home—like the 11 newly found stellar streams that the Dark Energy Survey announced today. For a few lucky groups in Chile and Australia, this meant an extraordinary opportunity: getting to name an object in space.

“The people were very enthusiastic,” says Kyler Kuehn, a scientist with the Dark Energy Survey who coordinated the outreach effort in Australia. “I don’t know if they are aware how rarely people get to name things that are newly discovered in space—or anywhere, for that matter—but I was pretty excited about it."

Stellar streams are ribbons of stars orbiting a galaxy (in this case, our own Milky Way). These faint filaments are the remnants of dwarf galaxies or star clusters that have been ripped apart by the gravity of their monster neighbor. Unlike some celestial objects that have very specific naming conventions according to the International Astronomical Union, stellar streams have a bit of flexibility.

Previously discovered stellar streams were often named after constellations in the sky near their location—but with many streams often appearing close to one another and other objects such as dwarf galaxies using the same convention, things became messy. Carl Grillmair, a CalTech astronomer studying stellar streams, proposed using the names of rivers in Greek mythology, like the River Styx. From there, naming expanded into real-world rivers.

DES decided to go the terrestrial route. One set of stellar streams, located in the sky near the Indus constellation, received names of Indian rivers: Indus, Jhelum, Chenab and Ravi. The collaboration decided to name the other two groups of streams after native words relating to water or rivers in Chile, where the Dark Energy Camera is located at the Cerro Tololo Inter-American Observatory, and Australia, where the Anglo-Australian Telescope is often used to follow up on those DECam discoveries.

Students at Colegio Antonio Varas in Vicuña helped name the new stellar streams
Photo by Yeimy Vargas, Colegio Antonio Varas

In Chile, DES worked with students in the nearby town of Vicuña. High school students Dánae Rojas and Emerson Carvajal researched words from the native Quechua and Aymara cultures that were related to water, then presented several options to about 90 kindergarten and first-grade students. Their final selections were the Aymara name Aliqa Una, meaning Quiet Water, and two Quechua names, Palca, meaning Crossing Rivers, and Willka Yaku, or Sacred Water. Two Spanish names for local rivers near Vicuña, Elqui and Turbio, rounded out the set.

“It was absolutely wonderful to get the community involved in this process,” says Alfredo Zenteno, a DES scientist who, along with Kathy Vivas, led the outreach effort in Chile. “It is a way to make these new discoveries, which were made with a telescope in the region, close to them. For us, the astronomers, it is a way to thank the region that hosts the telescope and allows us to investigate the sky,”

In Australia, Kuehn worked with an Aboriginal storyteller and tribal elders to pick culturally sensitive and appropriate names in native languages.

“I wanted to honor the long history of aboriginal Australians doing astronomy,” Kuehn says. “Today's Aboriginal populations are the caretakers of some of the oldest continuous cultures on the planet, and their collective knowledge—including astronomical observations—date back tens of thousands of years.”

With a list of half a dozen names, Kuehn presented to a group of about 100 raucous adults at the Sydney Royal Botanic Gardens and 40 polite preschoolers, asking them to cheer to select their favorites. The Australian-named stellar streams are Wambelong, meaning Crazy Water in the Gamilaraay language, and Turranburra, the Dharug name for the Lane Cove River that runs near the headquarters of the Australian Astronomical Observatory. Scientists hope the names build connections between the nations that host the observatories and the discoveries they make about the universe that hosts us all. 

“It was wonderful to see the community have a chance to write in the sky,” Zenteno says.

Editor's note: You can learn more about the stellar streams and the accompanying release of three years of data from the Dark Energy Survey's lead lab, Fermilab.

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Not an ugly sweater party

University College London scientists make physics festive with sweaters and songs at their annual holiday gathering.

A scientific sweater

Every year, postdocs in high-energy physics at University College London are asked to give a short, light-hearted talk about their research for the holidays.

Louie Corpe, a UCL scientist on the ATLAS experiment at the Large Hadron Collider, says he had heard about some “fairly elaborate” presentations from previous years, including one given in the form of a Christmas carol.

“I’m a little competitive by nature,” he wrote in an email. “That’s where the idea of the Xmas jumper presentation came about.”

He converted two plots and a Feynman diagram into cross-stitch patterns for his talk on the topic of “exotic searches for long-lived particles,” which he gave wearing a sweater embroidered with an ATLAS event display—the handiwork of his fiancée, Emma.

A detailed look at the sweater
A detail of the sweater

“In particular, we are looking for displaced jets which decay in the ATLAS HCAL,” he wrote. “The jumper I was wearing described the topology we were looking for… The results of the analysis are not public yet, but I doubt anyone would be able to extract any useful information from my cross-stitched plot.”

Although he scored compliments for his outfit on social media, Corpe was not the winner of this year’s event. That honor went to postdoc Cheryl Patrick, who wrote and performed a five-song musical about her neutrinoless double beta decay experiment, SuperNEMO, with her PhD students singing back-up.

SuperNEMO The Musical

Video of SuperNEMO The Musical
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The 12 Days of Physicsmas

Add some science to your holiday carols.

Illustration of a snowman next to the words

There are plenty of songs about snow, decking the halls and holiday cheer—but where are the festive songs of science? For those singers who prefer curling up by the Bunsen burner (or a fiery ball of quark-gluon plasma) instead of the fireplace, Symmetry presents a new carol for your repertoire: “The 12 Days of Physicsmas.”

The 12 Days of Physicsmas

Video of The 12 Days of Physicsmas

Lyrics to sing along:

On the 12th day of Physicsmas

My true love sent to me:

Twelve theorists thinking,

Eleven students coding,

Ten protons smashing,

Nine muons spinning,

Eight gluons gluing,

Seven beamlines beaming,

Six quarks combining,

Five sigma results,

Four Nobel Prizes,

Three neutrinos,

Two neutron stars,

And a grand unified theory.

 

Happy holidays from the Symmetry team!

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Machine evolution

Planning the next big science machine requires consideration of both the current landscape and the distant future.

The Beam Transport Hall allows the electron beam from SLAC's linear accelerator to continue down through the Research Yard

Around the world, there’s an ecosystem of large particle accelerators where physicists gather to study the most intricate details of matter. 

These accelerators are engineering marvels. From planning to construction to operation to retirement, their lifespans stretch across decades. 

But to get the most out of their investments of talent and funding, laboratories planning such huge projects have to think even longer-term: What could these projects become in their next lives? 

The following examples show how some of the world’s big physics machines have evolved to stay at the forefront of science and technology.

Same tunnel, new collisions

Before CERN research center in Geneva, Switzerland, had its Large Hadron Collider, it had the Large Electron-Positron Collider. LEP was the largest electron-positron collider ever built, occupying a nearly 17-mile circular tunnel dug beneath the border of Switzerland and France. The tunnel took three years to completely excavate and build. 

The first particle beam traveled around the LEP circular collider in 1989. Long before then, the international group of CERN physicists and engineers were already thinking about what CERN’s next machine could be. 

“People were saying, ‘Well, if we do build LEP, then we should make it compatible with the [then-proposed] Large Hadron Collider,’” says James Gillies, a senior communications advisor and member of the strategic planning and evaluation unit at CERN. “If you want to have a future facility, you often have to engage the people who just finished designing one machine to start thinking about the next one.”

LEP’s designers chose an energy for the collider that would mass-produce Z bosons, fundamental particles discovered by earlier experiments at CERN. The LHC would be a step up from LEP, reaching higher energies that scientists hoped could produce the Higgs boson. In the 1960s, theorists proposed the Higgs as a way to explain the origin of the mass of elementary particles. And the new machine to look for it could be built in the same 17-mile tunnel excavated for LEP.  

Engineers began working on the LHC while LEP was still running. The new machine required enlargements to underground areas—it needed bigger detectors and new experimental halls. 

“That was challenging because these caverns are huge. As they were being excavated, the pressure on the LEP tunnel was reduced and the LEP beamline needed realignment,” Gillies says. “So you constantly had to realign the collider for experiments as you were digging.” 

After LEP reached its highest energy in 2000, it was switched off. The tunnel remained the same, says Gillies, but there were many other changes. Only one of the LEP detectors, DELPHI, remains underground at CERN as a visitors’ point. 

In 2012, LHC scientists announced the discovery of the long-sought Higgs boson. The LHC is planned to continue running until at least 2035, gradually increasing the intensity of its particle collisions. The research and development into the accelerator’s successor is already happening. The possibilities include a higher energy LHC, a compact linear collider or an even larger circular collider.

Large Electron-Positron Collider

Large Electron-Positron Collider

Location: CERN—Geneva, Switzerland

First beam: 1989

Link to LEP Timeline: Timeline

Courtesy of CERN
Large Hadron Collider

Large Hadron Collider

Location: CERN—Geneva, Switzerland

First beam: 2008

Link to LHC Timeline: Timeline 

Courtesy of CERN

High-powered science

Decades before the LHC came into existence, a suburb of Chicago was home to the most powerful collider in the world: the Tevatron. A series of accelerators at Fermi National Accelerator Laboratory boosted protons and antiprotons to nearly the speed of light. In the final, 4-mile Tevatron ring, the particles reached record energy levels, and more than 1000 superconducting magnets steered them into collisions. Physicists used the Tevatron to make the first direct measurement of the tau neutrino and to discover the top quark, the last observed lepton and quark, respectively, in the Standard Model.

The Tevatron shut down in 2011 after the LHC came up to speed, but the rest of Fermilab’s accelerator infrastructure was still hard at work powering research in particle physics—particularly on the abundant, mysterious and difficult-to-detect neutrino. 

Starting in 1999, a brand-new, 2-mile circular accelerator called the Main Injector was added to the Fermilab complex to increase the number of Tevatron particle collisions tenfold. It was joined in its tunnel by the Recycler, a permanent magnet ring that stored and cooled antiprotons. 

But before the Main Injector was even completed, scientists had identified a second purpose: producing powerful beams of neutrinos for experiments in Illinois and 500 miles away in Minnesota. By 2005, the proton beam circulating in the Main Injector was doing double duty: sending ever-more-intense beams to the Tevatron collider and smashing into a target to produce neutrinos. Following the shutdown of the Tevatron, the Recycler itself was recycled to increase the proton beam power for neutrino research.

“I’m still amazed at how we are able to use the Recycler. It can be difficult to transition if a machine wasn’t originally built for that purpose,” says Ioanis Kourbanis, the head of the Main Injector department at Fermilab. 

Fermilab’s high-energy neutrino beam is already the most intense in the world, but the laboratory plans to enhance it with future improvements to the Main Injector and the Recycler, and to build a brand-new neutrino beamline. 

Neutrinos almost never interact with matter, so they can pass straight through the Earth on their way to detectors onsite and others several hundred miles away. Scientists hope to learn more about neutrinos and their possible role in shaping our early universe.

The new beamline will be part of the Long-Baseline Neutrino Facility, which will send neutrinos 800 miles underground to the massive, mile-deep detectors of the Deep Underground Neutrino Experiment. Scientists from around the world will use the DUNE data to answer questions about neutrinos, thanks to the repurposed pieces of the Fermilab accelerator complex.

Tevatron

Tevatron

Location: Fermilab—Batavia, Illinois

First beam: 1983

Link to Tevatron Timeline: Timeline 

Courtesy of Fermilab
NuMI Horn

Neutrinos at the Main Injector (NuMI) beam

Location: Fermilab—Batavia, Illinois

First beam: 2004

Link to Fermilab Timeline: Timeline

Courtesy of Fermilab

A monster accelerator

When physicists first came up with the idea to build a two-mile linear accelerator at what is now called SLAC National Accelerator Laboratory, managed by Stanford University, they called it "Project M” for “Monster.” Engineers began building it from hand-drawn designs. Once completed, the machine was able to accelerate electrons to near the speed of light, producing its first particle beam in May 1966. 

The accelerator’s scientific purpose has gone through several iterations of particle physics experiments over the decades, from fixed-target experiments to the Stanford Linear Collider (the only electron-positron linear collider ever built) to an injector for a circular collider, the Positron-Electron Project. 

These experiments led to the discovery that protons are made of quarks, the first evidence that the charm quark existed (through observations of the J/psi particle, co-discovered with researchers at MIT) and the discovery of the tau lepton. 

In 2009, the lab used the accelerator as the backbone for a different type of science machine—an X-ray free-electron laser, the Linac Coherent Light Source. 

“Looking around, SLAC was the only place in the world with a linear accelerator capable of driving a free-electron laser,” says Claudio Pellegrini, a distinguished professor emeritus of physics at the University of California, Los Angeles and a visiting scientist and consulting professor at SLAC. Pellegrini first proposed the idea to transform SLAC’s linear accelerator.

The new machine, a DOE Office of Science user facility, would be the world’s first laser of its kind that could produce extremely bright hard X-rays, the high-energy X-rays that let scientists take snapshots of atoms and molecules. 

“Much of the physics and many of the tools learned and developed during the operation of the Stanford Linear Collider were directly applicable to the free electron laser,” says Lia Merminga, head of the accelerator directorate at SLAC. “This was a big factor in the LCLS being commissioned in record time. Without the Stanford Linear Collider experience, this significant body of work would have to be reinvented and reproduced almost from scratch.”

Little about the accelerator itself needed to change. But to create a free-electron laser, scientists needed to design a new part: an electron gun, a device that generates electrons to be injected into the accelerator. A collaboration of several national labs and UCLA created a new type of electron gun for LCLS, while other national labs helped build undulators, a series of magnets that would wiggle the electrons to create X-rays.

LCLS used only the last third of SLAC’s original linear accelerator. In part of the remaining section, scientists are developing plasma wakefield and other new particle acceleration techniques.

For the X-ray laser’s next iteration, LCLS-II, scientists are aiming for an even brighter laser that will fire 1 million pulses per second, allowing them to observe rare and exceptionally transient events. 

To do this, they will need to replace the original copper structures with superconducting technology. The technology is derived from designs for a large International Linear Collider proposed to be built in Japan.

“I’m in awe of the foresight of the original builders of SLAC’s linear accelerator,” Merminga adds. “We’ve been able to do so much with this machine, and the end is not yet in sight.” 

Fixed target and collider experiments

Fixed target and collider experiments

Location: SLAC—Menlo Park, California

First beam: 1966

Link to SLAC Timeline: Timeline 

Courtesy of SLAC
Linac Coherent Light Source

Linac Coherent Light Source

Location: SLAC—Menlo Park, California

First beam: 2009

Link to SLAC Timeline: Timeline 

Courtesy of SLAC
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A trip into totality

This summer, physics students were offered a unique opportunity to study cosmic rays during the total solar eclipse.

August’s Great American Eclipse

August’s Great American Eclipse brought at least a partial eclipse to most of the United States, and 14 states experienced totality, a phenomenon that occurs when the sun is completely eclipsed by the moon. Eight Illinois high school students and five of their teachers traveled into the zone of totality to witness the two to three breathtaking minutes when the moon completely blocked the sun.

However, unlike most sky-watchers on August 21, these students did more than just marvel at the eclipse: They studied it, hoping to learn something about the effects of the sun going dark. Their mission? To measure whether the eclipse changed the number of detected cosmic rays—particles from space that rain down on Earth—which could tell us something about where these cosmic rays come from.

“This was a real scientific question high school students have the opportunity to answer,” says Nate Unterman, an emeritus teacher at Glenbrook North High School. “The students came up with a very elegant, scientific hypothesis: The cosmic ray flux will change during an eclipse.”

Unterman and another Glenbrook North teacher, Tony Valsamis, came up with the idea to study cosmic rays during the eclipse at an American Association of Physics Teachers conference, and they knew where to look for researchers: The school’s cosmic ray club.

Students in the school’s cosmic ray club had already been studying the behavior of cosmic rays, which reach Earth’s surface as muons—particles that are similar to electrons—using small-scale detectors provided by QuarkNet, a program designed to give students and teachers opportunities to get involved with high-energy physics research.

Unterman and Valsamis recognized these same detectors could be used during the eclipse to see whether the number of muons reaching Earth would change—something no study has measured from the ground.

“I got a call from Mr. Unterman while he was at the AAPT conference telling me about this idea to study the eclipse,” says Clarissa Carr, a Glenbrook North student and participant in the cosmic ray club. “I was immediately on board.”

The path to totality 

Four days before the eclipse, the research team, which consisted of students and teachers from Glenbrook North and Ida Crown Jewish Academy, made a five-hour trek from the Chicago area to Jefferson College in Hillsboro, Missouri.

“I drove a school bus with three students in it,” Valsamis says. “The rest of it was full of detectors, mounts and electronics.”

Glenbrook North High School students standing beside bus

David Wang, Jacob Miller, Masha Matten, Clarissa Carr, Tamar Dallal, Allen Sears, Jacob Rosenberg and Ezra Schur pose in front of the bus used to transport equipment. 

Courtesy of Glenbrook North High School
Students unloading bus

Students unload equipment from the bus to set up the experiment. 

Courtesy of Glenbrook North High School
Students assembling equipment

Students assemble the equipment.

Courtesy of Glenbrook North High School
Student looking through telescope

Tony Valsamis sets up a camera to capture photos of the eclipse. 

Courtesy of Glenbrook North High School
August’s great american total eclipse

A photo of the eclipse captured by Tony Valsamis. 

Courtesy of Glenbrook North High School
Physicist and student looking at a laptop computer

Mark Adams, QuarkNet’s cosmic ray studies coordinator, and student Clarissa Carr monitor data collection. 

Courtesy of Glenbrook North High School
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Immediately after arriving at Jefferson College, which would serve as home base for the research team, the students hurried about unloading equipment, setting up detectors on their mounts and connecting wires. Setup took a whole day and then some, partially because of a faulty detector.

“When one of our detectors had a faulty power cable, we all had to gather around the detector and take it apart,” says Carr, who was responsible for log-keeping during the experiment as well as setup. “We managed to put it back together and get it working—it was memorable but stressful!”

After setup, the detectors could begin collecting baseline data to be compared with data from the eclipse. The researchers had nothing left to do but wait for totality. To pass the time, students visited a local farmer’s market, played volleyball and theorized about what the eclipse might be like.

“We were all hypothesizing about what we would see during the eclipse,” says Jacob Rosenberg, a Glenbrook North student. “None of us had a clue what to expect, but we were all excited.”

The big moment  

When the day of the eclipse finally arrived, crowds of people joined the research team at Jefferson College, eager to experience the United States’ first total solar eclipse in decades. As excitement filled the air, the research team made last minute adjustments to their detectors, making sure everything would be in working order during the short window of totality. With detectors pointed at the sky and eclipse glasses at the ready, the team was prepared. 

In the minutes leading up to totality, spectators at Jefferson College peered up through their glasses, waiting until the moon completely covered the sun. 

“The total solar eclipse was incredible to look at,” Rosenberg recalls. “There was a 360-degree sunset, and we could hear the noises of nature change as people ‘ooh'ed and ‘ahh'ed.”

 Valsamis came equipped to capture photos of the eclipse, amassing over 700 pictures. 

“None of my photos mimic the experience or explain how beautiful it was,” Valsamis says. “It was like the best picture but better, and being surrounded by enthusiastic people was infectious.”

The aftermath 

In the months since it happened, the eclipse may have become a passing memory to most, but it’s stayed at the forefront of the research team’s mind. Students from Ida Crown and Glenbrook North meet at least once a month to collaborate on data analysis.

“Students had a unique opportunity to do this research almost on their own," Valsamis says. “It was incredible to see the students learn to collaborate.”

While not all the data has been analyzed yet—and some potentially interesting data points have required more intense analysis—students have already benefited from the experience of conducting research.

“I’ve learned from this experiment the importance of being knowledgeable about what you’re doing, but being open to learning more,” Carr says. “I’ve also learned a lot about teamwork and community-building.”

In 2018, Carr and Rosenberg will present some of the results from the solar eclipse study at the annual American Association of Physics Teachers conference. Both students are excited about the opportunity—although understandably a little nervous.

“It’s a little intimidating to present in front of so many smart people, but I’m not too worried," Rosenberg says. “I remind myself that anyone, no matter age or experience, can always contribute to research and learning more about the universe.”

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Physics books of 2017

Gravitational waves take the top spot in Symmetry writer Mike Perricone’s yearly round-up of popular science books related to physics and astrophysics.

Illustration of book shelf with book spines from the 2017 book review

In 2017, we were treated to books about gravitational waves; unsung women critical to modern astronomy; the neutrino detector at the South Pole; and astrophysics both fast and slow.

Book cover for Ripples in Spacetime: Einstein, Gravitational Waves, and the Future of Astronomy, by Govert Schilling

Ripples in Spacetime: Einstein, Gravitational Waves, and the Future of Astronomy, by Govert Schilling

Einstein’s final prediction took the longest to confirm: Gravitational waves were finally detected in September of 2015, a century after the publication of his paper on general relativity. The discovery brought with it the 2017 Nobel Prize in Physics, shared by Rainer Weiss, Barry Barish and Kip Thorne. Govert Schilling, a science writer based in the Netherlands, places the discovery in the historical context of a 40-year search. Schilling is a captivating story-teller who creates a one-on-one conversation with his readers.

Book cover for The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars, by Dava Sobel

The Glass Universe: How the Ladies of the Harvard Observatory Took the Measure of the Stars, by Dava Sobel 

Dava Sobel (Longitude, The Planets, Galileo’s Daughter) shines a light on the irreplaceable contributions of the women “computers” at the Harvard Observatory. In the late 19th and early 20th centuries, these women exhaustively cataloged millions of stars from glass photographic plates (hence, The Glass Universe). One of them, Henrietta Swan Leavitt, concluded that the brightest variable stars had the longest periods, establishing a measuring standard across space still used today.

Book cover for Astrophysics for People in a Hurry, by Neil DeGrasse Tyson

Astrophysics for People in a Hurry, by Neil deGrasse Tyson

As soon as it was published, Astrophysics for People in a Hurry hit No. 1 on the New York Times best-seller list. In a TV interview, author Neil deGrasse Tyson characterized the reception as “an affirmation that people are interested in science.” Learn the laws of the universe with an attitude: As Tyson says, “Yes, Einstein was a badass.”

Universal: A Guide to the Cosmos, by Brian Cox and Jeff Forshaw

Universal: A Guide to the Cosmos, by Brian Cox and Jeff Forshaw

Cosmology and astrophysics for those who are not in a hurry—and who enjoy a challenge. This beautiful book excels on three levels: the striking graphics, the accessible introductions escalating into detailed discussions, and the accompanying case studies exhibiting the scientific method (such as “What is Light?”). Co-authors Brian Cox and Jeff Forshaw (Why Does E=mc2?) are physics professors at the University of Manchester; Cox is also Royal Society Professor for Public Engagement in Science.

Book cover for A Big Bang in a Little Room: The Quest to Create New Universes, by Zeeya Merali

A Big Bang in a Little Room: The Quest to Create New Universes, by Zeeya Merali

Creating a new universe at a particle accelerator might sound like science fiction, or just plain preposterous—until author Zeeya Merali places the idea in the context of other feats of modern cosmology. With a PhD in theoretical physics and cosmology from Brown University, Merali takes on the topic with knowledge and humor in conversation with leaders at the intersection of cosmology and particle physics.

Book cover for Gravity: A Very Short Introduction, by Timothy Clifton

Three titles from the invaluable Oxford University Press A Very Short Introduction series:

Gravity: A Very Short Introduction, by Timothy Clifton

Timothy Clifton, a gravitational specialist at Queen Mary University of London, starts with the everyday experiences of gravity and advances to its effects on the universe and scientists’ efforts to link it with quantum mechanics. He also discusses the impact of the discovery of gravitational waves.

Book cover for Telescopes: A Very Short Introduction, by Geoffrey Cottrell

Telescopes: A Very Short Introduction, by Geoffrey Cottrell

Geoffrey Cottrell, an astrophysicist at Oxford University, explores the principles, history and major discoveries of different types of telescopes: simple optical, radio, X-ray, gamma ray and space-based. He also looks to the next generation of telescopes, such as the ALMA radio telescope array in the Atacama desert of Chile.

Book cover for The Jazz of Physics: The Secret Link Between Music and the Structure of the Universe, by Stephon Alexander

The Jazz of Physics: The Secret Link Between Music and the Structure of the Universe, by Stephon Alexander

Pythagoras, Kepler, Newton and Einstein all pondered the link between music and physics. The great saxophonist John Coltrane incorporated physics and geometry into his work. “In our attempts to reveal new vistas in our understanding, we often must embrace an irrational, illogical process, sometimes fraught with mistakes and improvisational thinking,” writes physicist and jazz saxophonist Stephon Alexander.

Book cover for The Telescope in the Ice: Inventing a New Astronomy at the South Pole, by Mark Bowen

The Telescope in the Ice: Inventing a New Astronomy at the South Pole, by Mark Bowen 

Tracking a unique particle takes a unique particle detector. Meet IceCube: a cubic kilometer of “diamond-clear” ice more than a mile below the surface at the South Pole. The world’s largest particle detector, IceCube recorded the first extra-terrestrial high-energy neutrinos in 2010. Mark Bowen (Censoring Science, Thin Ice) narrates the story of the people and science behind the pursuit of the inscrutable particle. Bowen, a “recovering physicist,” journeyed to the Amundsen South Pole research station as a part of his research for the book.

Book cover for Magnitude: The Scale of the Universe, by Megan Watzke and Kimberly Arcand

Magnitude: The Scale of the Universe, by Megan Watzke and Kimberly Arcand

How big is big? How small is small? Kimberly Arcand and Megan Watzky, colleagues at NASA’s Chandra X-Ray Observatory, take an illustrated journey from subatomic particles to the most massive galaxies in the universe, from the speed of grass growing to the speed of light. They explore mass, time and temperature; speed and acceleration; and energy, pressure and sound. Watzke and Arcand’s other collaborations include Light: The Visible Spectrum and Beyond and Coloring the Universe: An Insider’s Look at Making Spectacular Images of Space.

Book cover for Mass : The Quest to Understand Matter From Greek Atoms to Quantum Fields, by Jim Baggott

Mass : The Quest to Understand Matter From Greek Atoms to Quantum Fields, by Jim Baggott

Even in the aftermath of uncovering the Higgs particle in 2012, Jim Baggott (The Quantum Story: A History in 40 Moments, others) points to our incomplete understanding of matter. The foundations of the universe, he says, are “built of ghosts and phantoms of a peculiar quantum kind.” Each chapter concludes with “Five things we learned,” such as Einstein’s dictum, via John A. Wheeler: “Matter tells space-time how to curve; space-time tells matter how to move.” Mass is worth some extra effort to keep up.

Book cover for The Quantum Labyrinth: How Richard Feynman and John Wheeler Revolutionized Time and Reality, by Paul Halpern

The Quantum Labyrinth: How Richard Feynman and John Wheeler Revolutionized Time and Reality, by Paul Halpern

Bongo-playing Richard Feynman and buttoned-down John A. Wheeler began their unlikely connection in 1939 when Feynman was Wheeler’s teaching assistant at Princeton. Wheeler’s ideas about the universe read almost like science fiction: black holes, worm holes and portals to the future and the past. Feynman won the Nobel Prize for his work in quantum electrodynamics. He depicted quantum reality as a function of alternative possibilities. Paul Halpern (Einstein’s Dice and Schrödinger’s Cat, Edge of the Universe) shows how these two formed their own alternate reality.

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