symmetry

The world’s largest astronomical movie

The Large Synoptic Survey Telescope will track billions of objects for 10 years, creating unprecedented opportunities for studies of cosmic mysteries.

LSST Camera

When the Large Synoptic Survey Telescope begins to survey the night sky in the early 2020s, it’ll collect a treasure trove of data. The information will benefit a wide range of groundbreaking astronomical and astrophysical research, addressing topics such as dark matter, dark energy, the formation of galaxies and detailed studies of objects in our very own cosmic neighborhood, the Milky Way.

LSST’s centerpiece will be its 3.2-gigapixel camera, which is being assembled at the US Department of Energy’s SLAC National Accelerator Laboratory. Every few days, the largest digital camera ever built for astronomy will compile a complete image of the Southern sky. Moreover, it’ll do so over and over again for a period of 10 years. It’ll track the motions and changes of tens of billions of stars, galaxies and other objects in what will be the world’s largest stop-motion movie of the universe.

Fulfilling this extraordinary task requires extraordinary technology. The camera will be the size of a small SUV, weigh in at a whopping 3 tons, and use state-of-the-art optics, imaging technology and data management tools. But how exactly will it work?

LSST mirror animation infographic

Artwork by Sandbox Studio, Chicago with Ana Kova

Collecting ancient light

It all starts with choosing the right location for the telescope. Astronomers want the sharpest images of the dimmest objects for their analyses, and they also want to maximize their observation time. They need the nights to be dark and the air to be dry and stable. 

It turns out that the Atacama Desert, a plateau in the foothills of the Andes Mountains, scores very high for these criteria. That’s where LSST will be located—at nearly 8700 feet altitude on the Cerro Pachón ridge in Chile, 60 miles from the coastal town of La Serena.

The next challenge is that most objects LSST researchers want to study are so far away that their light has been traveling through space for millions to billions of years. It arrives on Earth merely as a faint glow, and astronomers need to collect as much of that glow as possible. For this purpose, LSST will have a large primary mirror with a diameter close to 28 feet. 

The mirror will be part of a sophisticated three-mirror system that will reflect and focus the cosmic light into the camera.  

The unique optical design is crucial for the telescope’s extraordinary field of view—a measure of the area of sky captured with every snapshot. At 9.6 square degrees, corresponding to 40 times the area of the full moon, the large field of view will allow astronomers to put together a complete map of the Southern night sky every few days.

After bouncing off the mirrors, the ancient cosmic light will enter the camera through a set of three large lenses. The largest one will have a diameter of more than 5 feet. 

Together with the mirrors, the lenses’ job is to focus the light as sharply as possible onto the focal plane—a grid of light-sensitive sensors at the back of the camera where the light from the sky will be detected.

A filter changer will insert filters in front of the third lens, allowing astronomers to take images with different kinds of cosmic light that range from the ultraviolet to the near-infrared. This flexibility enhances the range of possible observations with LSST. For example, with an infrared filter researchers can look right through dust and get a better view of objects obscured by it.  By comparing how bright an object is when seen through different filters, astronomers also learn how its emitted light varies with the wavelength, which reveals details about how the light is produced.

LSST Camera Focal Plane

Artwork by Sandbox Studio, Chicago with Ana Kova

An Extraordinary Imaging Device

The heart of LSST’s camera is its 25-inch-wide focal plane. That’s where the light of stars and galaxies will be turned into electrical signals, which will then be used to reconstruct images of the sky. The focal plane will hold 189 imaging sensors, called charge-coupled devices, that perform this transformation. 

Each CCD is 4096 pixels wide and long, and together they’ll add up to the camera’s 3.2 gigapixels. A “good” star will be the size of only a handful of pixels, whereas distant galaxies might appear as somewhat larger fuzzballs.

The focal plane will consist of 21 smaller square arrays, called rafts, with nine CCDs each. This modular structure will make it easier and less costly to replace imaging sensors if needed in the future. 

To the delight of astronomers interested in extremely dim objects, the camera will have a large aperture (f/1.2, for the photographers among us), meaning that it’ll let a lot of light onto the imaging sensors. However, the large aperture will also make the depth of field very shallow, which means that objects will become blurry very quickly if they are not precisely projected onto the focal plane. That’s why the focal plane will need to be extremely flat, demanding that individual CCDs don’t stick out or recess by more than 0.0004 inches.

To eliminate unwanted background signals, known as dark currents, the sensors will also need to be cooled to minus 150 degrees Fahrenheit. The temperature will need to be kept stable to half a degree. Because water vapor inside the camera housing would form ice on the sensors at this chilly temperature, the focal plane must also be kept in a vacuum.  

In addition to the 189 “science” sensors that will capture images of the sky, the focal plane will also have three specialty sensors in each of the four corners of the focal plane. Two so-called guiders will frequently monitor the position of a reference star and help LSST stay in sync with the Earth’s rotation. The third sensor, called a wavefront sensor, will be split into two halves that will be positioned six-hundredths of an inch above and below the focal plane. It’ll see objects as blurry “donuts” and provide information that will be used to adjust the telescope’s focus.

Cinematography of astronomical dimension

Once the camera has taken enough data from a patch in the sky, about every 36 seconds, the telescope will be repositioned to look at the next spot. A computer algorithm will determine the patches in the sky that will be surveyed by LSST on any given night.

While the telescope is moving, a shutter between the filter and the third lens camera will close to prevent more light from falling onto the imaging sensors. At the same time, the CCDs will be read out and their information digitized. 

The data will be sent into the processing and analysis pipeline that will handle LSST’s enormous flood of information (about 20 terabytes of data every single night). There, it will be turned into useable images. The system will also flag potential interesting events and send out alerts to astronomers within a minute. 

This way—patch by patch—a complete image of the entire Southern sky will be stitched together every few days. Then the imaging process will start over and repeat for the 10-year duration of the survey, ultimately creating the largest time-lapse movie of the universe ever made and providing researchers with unprecedented research opportunities.   

For more information on LSST, visit LSST’s website or SLAC’s LSST camera website.


LSST camera poster
Artwork by Sandbox Studio, Chicago with Ana Kova
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Right on target

These hardy physics components live at the center of particle production.

A row of slim silver objects sit in the center of a tube.

For some, a target is part of a game of darts. For others, it’s a retail chain. In particle physics, it’s the site of an intense, complex environment that plays a crucial role in generating the universe’s smallest components for scientists to study.

The target is an unsung player in particle physics experiments, often taking a back seat to scene-stealing light-speed particle beams and giant particle detectors. Yet many experiments wouldn’t exist without a target. And, make no mistake, a target that holds its own is a valuable player.

Scientists and engineers at Fermilab are currently investigating targets for the study of neutrinos—mysterious particles that could hold the key to the universe’s evolution.

Intense interactions

The typical particle physics experiment is set up in one of two ways. In the first, two energetic particle beams collide into each other, generating a shower of other particles for scientists to study.

In the second, the particle beam strikes a stationary, solid material—the target. In this fixed-target setup, the powerful meeting produces the particle shower.

As the crash pad for intense beams, a target requires a hardy constitution. It has to withstand repeated onslaughts of high-power beams and hold up under hot temperatures.

You might think that, as stalwart players in the play of particle production, targets would look like a fortress wall (or maybe you imagined dartboard). But targets take different shapes—long and thin, bulky and wide. They’re also made of different materials, depending on the kind of particle one wants to make. They can be made of metal, water or even specially designed nanofibers.

In a fixed-target experiment, the beam—say, a proton beam—races toward the target, striking it. Protons in the beam interact with the target material’s nuclei, and the resulting particles shoot away from the target in all directions. Magnets then funnel and corral some of these newly born particles to a detector, where scientists measure their fundamental properties.

The particle birthplace

The particles that emerge from the beam-target interaction depend in large part on the target material. Consider Fermilab neutrino experiments.

In these experiments, after the protons strike the target, some of the particles in the subsequent particle shower decay—or transform—into neutrinos.

The target has to be made of just the right stuff.

“Targets are crucial for particle physics research,” says Fermilab scientist Bob Zwaska. “They allow us to create all of these new particles, such as neutrinos, that we want to study.”

Graphite is a goldilocks material for neutrino targets. If kept at the right temperature while in the proton beam, the graphite generates particles of just the right energy to be able to decay into neutrinos.

For neutron targets, such as that at the Spallation Neutron Source at Oak Ridge National Laboratory, heavier metals such as mercury are used instead.

Maximum interaction is the goal of a target’s design. The target for Fermilab’s NOvA neutrino experiment, for example, is a straight row—about the length of your leg—of graphite fins that resemble tall dominoes. The proton beam barrels down its axis, and every encounter with a fin produces an interaction. The thin shape of the target ensures that few of the particles shooting off after collision are reabsorbed back into the target.

Robust targets

“As long as the scientists have the particles they need to study, they’re happy. But down the line, sometimes the targets become damaged,” says Fermilab engineer Patrick Hurh. In such cases, engineers have to turn down—or occasionally turn off—the beam power. “If the beam isn’t at full capacity or is turned off, we’re not producing as many particles as we can for science.”

The more protons that are packed into the beam, the more interactions they have with the target, and the more particles that are produced for research. So targets need to be in tip-top shape as much as possible. This usually means replacing targets as they wear down, but engineers are always exploring ways of improving target resistance, whether it’s through design or material.

Consider what targets are up against. It isn’t only high-energy collisions—the kinds of interactions that produce particles for study—that targets endure.

Lower-energy interactions can have long-term, negative impacts on a target, building up heat energy inside it. As the target material rises in temperature, it becomes more vulnerable to cracking. Expanding warm areas hammer against cool areas, creating waves of energy that destabilize its structure.

Some of the collisions in a high-energy beam can also create lightweight elements such as hydrogen or helium. These gases build up over time, creating bubbles and making the target less resistant to damage.

A proton from the beam can even knock off an entire atom, disrupting the target’s crystal structure and causing it to lose durability.

Clearly, being a target is no picnic, so scientists and engineers are always improving targets to better roll with a punch.

For example, graphite, used in Fermilab’s neutrino experiments, is resistant to thermal strain. And, since it is porous, built-up gases that might normally wedge themselves between atoms and disrupt their arrangement may instead migrate to open areas in the atomic structure. The graphite is able to remain stable and withstand the waves of energy from the proton beam.

Engineers also find ways to maintain a constant target temperature. They design it so that it’s easy to keep cool, integrating additional cooling instruments into the target design. For example, external water tubes help cool the target for Fermilab’s NOvA neutrino experiment.

Targets for intense neutrino beams

At Fermilab, scientists and engineers are also testing new designs for what will be the lab’s most powerful proton beam—the beam for the laboratory’s flagship Long-Baseline Neutrino Facility and Deep Underground Neutrino Experiment, known as LBNF/DUNE.

LBNF/DUNE is scheduled to begin operation in the 2020s. The experiment requires an intense beam of high-energy neutrinos—the most intense in the world. Only the most powerful proton beam can give rise to the quantities of neutrinos LBNF/DUNE needs.

Scientists are currently in the early testing stages for LBNF/DUNE targets, investigating materials that can withstand the high-power protons. Currently in the running are beryllium and graphite, which they’re stretching to their limits. Once they conclusively determine which material comes out on top, they’ll move to the design prototyping phase. So far, most of their tests are pointing to graphite as the best choice.

Targets will continue to evolve and adapt. LBNF/DUNE provides just one example of next-generation targets.

“Our research isn’t just guiding the design for LBNF/DUNE,” Hurh says. “It’s for the science itself. There will always be different and more powerful particle beams, and targets will evolve to meet the challenge.”

Editor's note: A version of this article was originally published by Fermilab.

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How to make a Higgs boson

It doesn’t seem like collisions of particles with no mass should be able to produce the “mass-giving” boson, the Higgs. But every other second at the LHC, they do.

scientists are checking out who they know can interact with the Higgs field, while searching for any potential mystery particles

Einstein’s most famous theory, often written as E=mc2, tells us that energy and matter are two sides of the same coin. 

The Large Hadron Collider uses this principle to convert the energy contained within ordinary particles into new particles that are difficult to find in nature—particles like the Higgs boson, which is so massive that it almost immediately decays into pairs of lighter, more stable particles.

But not just any collision can create a Higgs boson.

“The Higgs is not just created from a ‘poof’ of energy,” says Laura Dodd, a researcher at the University of Wisconsin, Madison. “Particles follow a strict set of laws that dictate how they can form, decay and interact.”

One of these laws states that Higgs bosons can be produced only by particles that interact with the Higgs field—in other words, particles with mass.

The Higgs field is like an invisible spider’s web that permeates all of space. As particles travel through it, some get tangled in the sticky tendrils, a process that makes them gain mass and slow down. But for other particles—such as photons and gluons—this web is completely transparent, and they glide through unhindered.

Given enough energy, the particles wrapped in the Higgs field can transfer their energy into it and kick out a Higgs boson. Because massless particles do not interact with the Higgs field, it would make sense to say that they can’t create a Higgs. But scientists at the LHC would beg to differ.

The LHC accelerates protons around its 17-mile circumference to just under the speed of light and then brings them into head-on collisions at four intersections along its ring. Protons are not fundamental particles, particles that cannot be broken down into any smaller constituent pieces. Rather they are made up of gluons and quarks. 

As two pepped-up protons pass through each other, it’s usually pairs of massless gluons that infuse invisible fields with their combined energy and excite other particles into existence—and that includes Higgs bosons. 

We know that particles follow strict rules about who can talk to whom.

How? Gluons have found a way to cheat.

“It would be impossible to generate Higgs bosons with gluons if the collisions in the LHC were a simple, one-step processes,” says Richard Ruiz, a theorist at Durham University’s Institute for Particle Physics Phenomenology.

Luckily, they aren’t.

Gluons can momentarily “launder” their energy to a virtual particle, which converts the gluon’s energy into mass. If two gluons produce a pair of virtual top quarks, the tops can recombine and  annihilate into a Higgs boson. 

To be clear, virtual particles are not stable particles at all, but rather irregular disturbances in quantum mechanical fields that exist in a half-baked state for an incredibly short period of time. If a real particle were a thriving business, then a virtual particle would be a shell company.

Theorists predict that about 90 percent of Higgs bosons are created through gluon fusion. The probability of two gluons colliding, creating a top quark-antitop pair and propitiously producing a Higgs is roughly one in 2 billion. However, because the LHC generates 10 million proton collisions every second, the odds are in scientists’ favor and the production rate for Higgs bosons is roughly one every two seconds.

Shortly after the Higgs discovery, scientists were mostly focused on what happens to Higgs bosons after they decay, according to Dodd.

“But now that we have more data and a better understanding of the Higgs, we’re starting to look closer at the collision byproducts to better understand how frequently the Higgs is produced through the different mechanisms,” she says.

The Standard Model of particle physics predicts that almost all Higgs bosons are produced through one of four possible processes. What scientists would love to see are Higgs bosons being created in a way that the Standard Model of particle physics does not predict, such as in the decay of a new particle. Breaking the known rules would show that there is more going on than physicists previously understood.

“We know that particles follow strict rules about who can talk to whom because we’ve seen this time and time again during our experiments,” Ruiz says. “So now the question is, what if there is a whole sector of undiscovered particles that cannot communicate with our standard particles but can interact with the Higgs boson?” 

Scientists are keeping an eye out for anything unexpected, such as an excess of certain particles radiating from a collision or decay paths that occur more or less frequently than scientists predicted. These indicators could point to undiscovered heavy particles morphing into Higgs bosons. 

At the same time, to find hints of unexpected ingredients in the chain reactions that sometimes make Higgs bosons, scientists must know very precisely what they should expect.

“We have fantastic mathematical models that predict all this, and we know what both sides of the equations are,” Ruiz says. “Now we need to experimentally test these predictions to see if everything adds up, and if not, figure out what those extra missing variables might be.”

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ADMX brings new excitement to dark matter search

Scientists on the Axion Dark Matter Experiment have demonstrated technology that could lead to the discovery of theoretical light dark matter particles called axions. 

Equipment and panels for the ADMX experiment fill the room. An ADMX experiment banner hangs above.

Forty years ago, scientists theorized a new kind of low-mass particle that could solve one of the enduring mysteries of nature: what dark matter is made of. Now a new chapter in the search for that particle, the axion, has begun.

This week, the Axion Dark Matter Experiment (ADMX) unveiled a new result (published in Physical Review Letters) that places it in a category of one: It is the world’s first and only experiment to have achieved the necessary sensitivity to “hear” the telltale signs of these theoretical particles. This technological breakthrough is the result of more than 30 years of research and development, with the latest piece of the puzzle coming in the form of a quantum-enabled device that allows ADMX to listen for axions more closely than any experiment ever built.  

ADMX is managed by the US Department of Energy’s Fermi National Accelerator Laboratory and located at the University of Washington. This new result, the first from the second-generation run of ADMX, sets limits on a small range of frequencies where axions may be hiding, and sets the stage for a wider search in the coming years.

“This result signals the start of the true hunt for axions,” says Fermilab’s Andrew Sonnenschein, the operations manager for ADMX. “If dark matter axions exist within the frequency band we will be probing for the next few years, then it’s only a matter of time before we find them.”

One theory suggests that galaxies are held together by a vast number of axions, low-mass particles that are almost invisible to detection as they stream through the cosmos. Efforts in the 1980s to find these particles, named by theorist Frank Wilczek, currently of the Massachusetts Institute of Technology, were unsuccessful, showing that their detection would be extremely challenging.

ADMX is an axion haloscope—essentially a large, low-noise, radio receiver, which scientists tune to different frequencies and listen to find the axion signal frequency. Axions almost never interact with matter, but with the aid of a strong magnetic field and a cold, dark, properly tuned, reflective box, ADMX can “hear” photons created when axions convert into electromagnetic waves inside the detector.

“If you think of an AM radio, it’s exactly like that,” says Gray Rybka, co-spokesperson for ADMX and assistant professor at the University of Washington. “We’ve built a radio that looks for a radio station, but we don't know its frequency. We turn the knob slowly while listening. Ideally we will hear a tone when the frequency is right.”

Listening for Dark Matter with ADMX

Video of Listening for Dark Matter with ADMX

This detection method, which might make the "invisible axion" visible, was invented by Pierre Sikivie of the University of Florida in 1983. Pioneering experiments and analyses by a collaboration of Fermilab, the University of Rochester and Brookhaven National Laboratory, as well as scientists at the University of Florida, demonstrated the practicality of the experiment. This led to the construction in the late 1990s of a large-scale detector at Lawrence Livermore National Laboratory that is the basis of the current ADMX.

It was only recently, however, that the ADMX team has been able to deploy superconducting quantum amplifiers to their full potential, enabling the experiment to reach unprecedented sensitivity. Previous runs of ADMX were stymied by background noise generated by thermal radiation and the machine’s own electronics.

Fixing thermal radiation noise is easy: A refrigeration system cools the detector down to 0.1 Kelvin (roughly -460 degrees Fahrenheit). But eliminating the noise from electronics proved more difficult. The first runs of ADMX used standard transistor amplifiers, but then ADMX scientists connected with John Clarke, a professor at the University of California Berkeley, who developed a quantum-limited amplifier for the experiment. This much quieter technology, combined with the refrigeration unit, reduces the noise by a significant enough level that the signal, should ADMX discover one, will come through loud and clear.

“The initial versions of this experiment, with transistor-based amplifiers, would have taken hundreds of years to scan the most likely range of axion masses. With the new superconducting detectors, we can search the same range on timescales of only a few years,” says Gianpaolo Carosi, co-spokesperson for ADMX and scientist at Lawrence Livermore National Laboratory.

“This result plants a flag,” says Leslie Rosenberg, professor at the University of Washington and chief scientist for ADMX. “It tells the world that we have the sensitivity, and have a very good shot at finding the axion. No new technology is needed. We don’t need a miracle anymore, we just need the time.”

ADMX will now test millions of frequencies at this level of sensitivity. If axions were found, it would be a major discovery that could explain not only dark matter, but other lingering mysteries of the universe. If ADMX does not find axions, that may force theorists to devise new solutions to those riddles.

“A discovery could come at any time over the next few years,” says scientist Aaron Chou of Fermilab. “It’s been a long road getting to this point, but we’re about to begin the most exciting time in this ongoing search for axions.”

Editor’s note: This article is based on a Fermilab press release.

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The big book of physics

Looking for the latest info on particle physics? There’s a book for that.

PDG Particle Physics Booklet for Symmetry

Want to know the latest research on the Higgs boson? Or the current findings on the search for dark energy?

You could search the internet, or even the latest scientific literature. Or you could find all your answers in one spot: The Review of Particle Physics, an 1800-page doorstopper compendium of measurements, tables and review articles that includes everything we currently know about the building blocks of the universe and the fundamental forces that govern it.  

In an era of overwhelming information, the 60-year-old publication serves as a continually updated, curated hierarchy of research results. “In a field as large as particle physics, it’s good to have a central place to find authoritative answers and information you might need,” says Juerg Beringer, group leader of the Particle Data Group, or PDG, at Lawrence Berkeley National Laboratory, which oversees the publication. In addition to research results, PDG also covers the tools of the HEP trade, such as detectors, accelerators, probability and statistics.

And though it’s the most-cited publication in particle physics, it’s not just for scientists. The book is distributed free of charge, around the world, to anyone.

“A fair fraction of our audience is students or the general public who are interested in learning about the field,” Beringer says. 

First, a wallet card reference 

The book’s beginnings were much humbler—in fact, it was originally designed to fit in your pocket. Physicists Murray Gell-Mann and Art Rosenfeld published a Particle Properties Table in the 1957 Annual Review of Nuclear Science, which then was produced as a wallet card showing easy-to-access experimental and theoretical information on the few particles known at that time.

By 1964, thanks to an explosion of experiments, the number of known particle measurements had grown so much that the wallet card became a small book (though wallet cards could still be requested—smaller ones to fit American wallets, and larger, more readable ones for European wallets).

“Enrico Fermi once said, ‘If I could remember the names of all these particles, I would have been a botanist,’” says Michael Barnett, former head of the Particle Data Group. “And now we have many, many more particles.”

The book has continued to grow since then—usually around 10 percent per year. The Particle Data Group, which updates the Review, is led by a small team but involves an international collaboration of 223 authors from 148 institutions in 24 countries. Every year, team members scan newly published scientific articles to determine what new information should be included, and how. It might be a new measurement of a particle, or a new review of a field, like inflation. Though there may be a discussion about which information to include and when, the process is generally conflict-free, and any quibbles are placed in the footnotes. 

In print, serendipity

The publication is updated yearly online, and a print version is updated every two years. The Review was put on the internet in 1995, and since then the website has had more than 130 million hits. But the printed publications remain popular: For recent editions, the group distributed 14,000 copies of the book and 32,000 copies of the booklet. 

They say you don’t feel like a particle physicist until you get your first booklet.

“Graduate students use it like a textbook,” Barnett says. “They write in the margins, bookmark pages, underline things they want to remember.” Having a physical book also encourages experiences searching on the internet can’t provide: serendipitous exploration of other topics in physics.

The smaller, spiral-bound booklet—which was originally also meant to fit into your pocket, but now at 348 pages will at least fit into your bag—is often used in classrooms as an introduction to the field. 

“They say you don’t feel like a particle physicist until you get your first booklet,” Barnett says. 

New information added to the book ebbs and flows with the startup and shutdown of high-energy physics facilities and experiments. In 2012 the group was about to send an edition to print when the Higgs boson discovery was announced. They stopped the presses, commissioned an addendum to the Higgs review article summarizing the discovery and were able to include it in the final manuscript before it went to press.

“That was very, very exciting for everyone,” Beringer says. 

A more searchable future

The latest edition includes more than 3000 new measurements from 721 papers and reviews on everything from the Higgs boson to Grand Unified Theories. But all the new information pushed the book’s size to 1800 pages, and it began to look more like a telephone book than a textbook.

“It became a health hazard to carry it around with you,” Beringer says. So the group took out the data tables and particle measurements; now those are just listed online. Beringer says online usage is becoming increasingly important, and they are working to make the information more easily searchable. 

Budgetary constraints and the general trend toward online publishing have made it increasingly difficult to offer a free printed version of the PDG book, Beringer says. So the group is considering alternatives such as offering print-on-demand for a fee. 

But the group hopes to find a way to continue printing the book for the 2018 edition, slated to publish this summer. A recent survey showed that 80 percent of respondents still want a printed booklet, and more than two-thirds want a printed book. The group also wants to continue to make the book accessible to readers who may not be able to afford the cost of printing and shipping. 

Printed information, it seems, still has its place in a digital world. In fact, Barnett recently received an email from someone asking for a new booklet because their copy had been stolen. 

“I’m happy to see that it’s still so valuable that someone will steal it,” he says. 

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Photon declared champion

After a week of appreciation for each of the four particle contenders, the photon emerged as the winner in the Subatomic Smackdown.

Illustration of four Rock 'Em Sock 'Em robots representing the four particles in the Subatomic Smackdown

This week, four particles finished what they'd started.

In February, the proton, photon, electron and neutron threw their hats into the ring for the Subatomic Smackdown. This month, they sparred at the open house for the National High Magnetic Field Laboratory in Florida. (The electron won.) They even held a press conference at a meeting of the American Physical Society in Los Angeles.

 On Monday, the countdown to the final event began. #TeamProton, #TeamPhoton, #TeamElectron and #TeamNeutron each had a day to pump up their particle.

Then today, a poll opened on Twitter to determine the winner. The electron and the photon quickly emerged as the favorites, but midway through the day, the proton came out of nowhere to overtake the electron. (This might have had something to do with the surprise restart of the Large Hadron Collider, which collides protons.) It wasn't enough to put the proton in the lead, though, and the photon powered through to victory.

It was a good, clean fight, and the fans showed us the worth of each of the particle participants.

Fermi National Accelerator Laboratory declared their support for protons, which will help them create the world's most intense beam of neutrinos—particles too aloof to participate in the Smackdown themselves—for the upcoming Deep Underground Neutrino Experiment. Protons also play a starring role at Brookhaven National Accelerator Laboratory, which collides them in the Relativistic Heavy Ion Collider, or RHIC. Clara Nellist explained that scientists use protons in the LHC because they have a magnetic charge, which scientists can use to steer them, and because they’re heavy enough not to lose most of their energy as they loop around the accelerator complex.

Amanda Solliday threw her support behind the photon for its achievements in X-ray science at SLAC National Accelerator Laboratory. The University of Wisconsin-Milwaukee also cheered on the photon, which it uses to answer fundamental bio-molecular questions in partnership with the BioXFEL Science and Technology Center. David J. Gillcrist pointed out that photons carry the electromagnetic force that allows electrons to interact. Stephanie Keys supported the photon for its achievements at the Canadian Light Source. Alan Fry gave a shout-out to photons for helping scientists study black holes. @SandHillScienceMill mentioned that photons will be essential to the upcoming LUX-ZEPLIN, or LZ, dark matter experiment. And Karl Gumerlock thanked photons for cohering to create frickin’ laser beams.

The Mag Lab championed the electron, which it uses to study everything from qubits to cancer. They pointed out the particle’s role in operating electronics, including the cell phones and computers readers used to vote in the Twitter poll. Kristen Coyne cheered the electron for being the particle behind superconductivity. Brookhaven lent support to #TeamElectron; they accelerate the particles to nearly the speed of light at their National Synchrotron Light Source II to “see” the atomic structures of proteins, battery materials and catalysts in action. Electrons also help them cool the particle beams at RHIC.

William Ratcliff pointed out that neutrons allow scientists to see through steel. Rob Dimeo touted the neutron’s ability to help scientists watch atoms and molecules move in materials and to see magnetism at the nanoscale. Oak Ridge National Laboratory supported the neutron for its role in a variety of areas of research, from studying 3D printing for rocket science; to searching for ways to build better vehicle armor and safer suspension bridges; to working to make better medicines by studying crystals grown in space; to gaining insights into aquatic biochemistry through the study of sturgeon ear bones.

The photon earned the most votes, but in the end, the real winner is science.

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The room where it happens

Symmetry goes inside the CERN Control Centre on restart day.

Accelerator operators and CERN's Director General stand together, watching the process of the LHC beam on a computer screen.

Mike Lamont, the deputy head of the Beams Department at CERN, home to the Large Hadron Collider, turns and looks toward the entrance of the accelerator control center. “There she is,” he says.

Fabiola Gianotti, the Director General of CERN, strolls towards the crowd of engineers, operators and physicist huddled around the wall of screens that form the control panel for LHC.

“Who is the operator on shift?” Gianotti asks, and a woman perched in front waves her over. Gianotti hands her a large Colomba Gocce di Gioccolato, a traditional Italian Easter cake.

For most of CERN, today is a holiday. But for the people who operate the LHC, it’s time to wake up the world’s most powerful particle accelerator after a regularly scheduled three-month shutdown for repairs. The last several weeks have been a gradual build-up to this moment.

“We’ve done 10,000 tests,” says Rende Steerenberg, the leader of the Operations Group, the team that makes sure CERN’s entire accelerator complex is running smoothly.

The morning started as most mornings do in the CERN Control Centre. The person responsible for the LHC’s pre-beam checkout gave a short presentation about everything that had happened during the previous 24 hours and gave an update on the plan for the day. The 10-minute presentation ended with a pithy statement in a big yellow text box: LHC ready for beam injection. Normally, only a handful of people show up to this daily briefing. But today, every seat at the table was filled.

The CCC is a large, open room with long, vertical windows looking toward the snow-capped Jura mountains. It’s divided into four circular islands, each responsible for one part of CERN’s accelerator complex. Today the LHC island is buzzing with life and energy.

Jörg Wenninger, who is responsible for the LHC operation, switches between English, French and German as he briefs various colleagues. “What’s the intensity for today?” asks one of the specialists for the radio-frequency cavities, the machinery that accelerates the proton beam. Each team settles in front of their control panels as the well-choreographed process begins.

The restart doesn’t happen all at once. Rather, engineers send single packets of protons partway into the LHC and intentionally absorb them with collimators before sending the next packets slightly further.

“We adjust the steering and make sure that there’s nothing blocking the proton’s path,” Steerenberg says.

Sector by sector, the trajectory of the proton beam is measured and fine-tuned. After about two hours, the beam makes it to Point 5—halfway around the accelerator. Then there’s an electrical trip near the beam dump. The relevant expert gets in a car and makes his way through the French countryside to check on the hardware, which is stored above ground near the access point.

In addition to holding the title for “World’s Single Largest Machine,” the LHC is also perhaps the most complicated contraption ever built. Every part needs to work flawlessly for the overall accelerator to function. The people responsible are no strangers to troubleshooting problems as they arise.

“It’s really variable,” Steerenberg says. “Sometimes we can run for many days without any faults, but other times it’s one after the other.”

Whenever there’s a problem, the LHC’s operators and experts tackle it with the efficiency of a well-oiled machine.

“At the very beginning, when the LHC first started up, we were more anxious,” Steerenberg says. “After seven years of operation, we’re all really used to it.”

The expert returns. “He’s back so we’ll restart in a few minutes,” Wenninger announces to the room.

At 12:17 p.m., the operators and machine experts laugh and cheer.

“C’est bon, ça circule!” applauds Frédérick Bordry, CERN’s Director for Accelerators and Technology, in French. The first proton beam has made it all the way around the LHC and is circulating clockwise at close to 11,000 times every second.

“On attend le beam deux,” he says.

Within 20 minutes, beam two is circulating in the opposite direction.

“Bravo, mesdames et messieurs!” Bordry cheers. “Fantastique.”

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Keeping the LHC cold

The LHC is one of the coldest places on the planet.

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Liquid helium is constantly pulsing through sophisticated plumbing that runs both inside and outside of the Large Hadron Collider. Thanks to this cryogenic cooling system, the LHC is colder than interstellar space. 

But why does it need to be kept at these intensely frigid temperatures?

“Because if not, the magnets would not work,” says Serge Claudet, the deputy head of CERN’s cryogenics group.

The cable that is coiled to make the LHC’s powerful electromagnets carries 11,800 amperes of current—roughly as much as a small bolt of lightning. The average toaster, for reference, uses only 9 amperes. 

For a cable the width of a finger to carry this much current and not burn up, it must be a superconductor. A superconductor is a type of material that carries an electrical current with zero electrical resistance. You see evidence of electrical resistance every time you turn on a light. If a lightbulb filament were made from a superconducting wire, it would give off no heat and no light—the electricity would pass straight through.

Most industrial superconductors gain the magical property of superconductivity only at extremely low temperatures—a few degrees above absolute zero. 

So perhaps surprisingly, the LHC lives in a pleasantly warm tunnel, about 80 degrees Fahrenheit. To insulate the superconducting magnets from this temperate climate, engineers nestled layers of insulation inside one another like a matryoshka doll, each colder than the last, protecting the magnetic core. 

On the very outside is a vacuum chamber, which acts like the walls of a thermos. On the very inside, the magnets are submerged in a static bath of 1.9-Kelvin superfluid liquid helium, which seeps into every nook and cranny of the LHC’s magnetic coils and supports.

If engineers had to worry only about protecting the LHC from the warmth of the tunnel, two feet of protection swollen with liquid helium might be enough. But their most formidable foe lies within.

“Most heating is internal,” says Gareth Jones, a CERN cryogenic operator. “It comes from the proton beam and the magnets.”

Heat is a measurement of how much particles jostle, and the 3.5 quintillion protons that stream through the heart of the LHC certainly create a stir. Every time a proton rounds a corner, it releases quick bursts of light, which are absorbed by the surrounding material and awaken sleeping molecules. 

Meanwhile, the loosely bound electrons of the copper-coated beampipe flow through the metal in pursuit of the positively charged proton beam, generating an electrical current. Some electrons will even leave their atomic confines and leap into the vacuum, only to crash and liberate even more electrons. These electrons move like water down a river gorge, bouncing off obstacles and swirling in eddies. All of this generates more and more heat, which threatens the sensitive conditions required to keep the magnets superconducting.

“If the magnets get above 2.17 Kelvin, they start to lose their superconducting properties,” says Guy Crockford, an LHC operator. “When this happens, what was originally just a little bit of internal heating quickly escalates into a lot of heat.”

To keep these magnets cool, engineers designed a complex cryogenic system that takes advantage of a very simple principle: When a liquid transforms into a gas, it absorbs heat. This is why we feel cold after a shower; it’s not because the water is cold, but because it carries away our heat as droplets evaporate off our skin. 

A long and thin pipe pierces the magnet support structure and delivers a stream of pressurized, ultra-cold liquid helium. As the liquid helium absorbs the excess heat, it evaporates and is quickly pumped out. 

Another cooling pipe runs through the inside of the beampipe and sops up energy right at the source. These internal capillaries are fed by a highway of five pipes running alongside the LHC: Two transport cold helium for injection; two carry warm helium back for re-cooling; and one is the main artery that helps maintain the pressure and temperature of the entire circuit. The LHC cycles about 16 liters of liquid helium every second to keep the entire system operational.

Despite all of these efforts, LHC magnets do sometimes heat up enough to lose their superconductivity in an event called a magnet quench. 

“It’s normally just one concentrated point that warms up, and it happens so fast,” Crockford says. 

Sensors detect the change in voltage and trigger a system that fires quench heater strips, which distribute the heat throughout the entire magnet and divert the electrical current away from the magnet. At the same time, the LHC beam is automatically rerouted into a concrete block called a beam dump, and the entire accelerator takes a pause for a few hours while the magnet recovers back to its super-cooled state.

“This has happened only about once every two years,” Crockford says. “We want to protect our magnets at all costs, and cryogenics is always on our mind.”

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