In 1820, Hans Christian Oersted gave a demonstration on electricity to a class of advanced students at the University of Copenhagen in Denmark. Using an early battery prototype, he looked to see what effect an electric current would have on a compass, and since he hadn’t had time to test his experiment beforehand, the outcome was just as unknown to him as it was to his students. When he completed the circuit by attaching a single wire to both ends of the battery, the resulting current caused the needle of the compass to line up with the wire, showing that electricity and magnetism were two facets of the same phenomenon.
In generating an electric current, Oersted had created a temporary magnet — an electromagnet. Physicists continued to develop electromagnets for their experiments, and today, they are everywhere: in MRI scanners, loudspeakers, transformers, electric motors — and particle accelerators.
Accelerator magnets bend and shape beams of subatomic particles as they shoot at velocities close to the speed of light. Experts design magnets so they can wield the beam in just the right way to yield the physics they’re after.
Accelerator magnets – how do they work?
The movement of charged particles – such as protons and electrons – creates a magnetic field. By the same token, magnetic fields influence the movement of charged particles. That’s the relationship Oersted helped uncover 200 years ago and later scientists would come to define: Electricity and magnetism are two sides of the same coin.
It’s a phenomenon that humanity has exploited to world-changing effect. The electrical grid that powers the device you’re using to read this arose from an understanding of the magnetism-electricity relationship.
Particle physicists have harnessed electromagnetism to explore the origins of our universe by controlling particle beams in accelerators, smashing them into a target and producing even more particles for scientists to study.
By passing an electrical current through a coiled wire, accelerator experts produce a temporary magnet with a north and south pole. These coiled wires form the poles of the electromagnets used in accelerators. They can be arranged not only into two-pole electromagnets, but magnets with four, six or even more poles.
Make no mistake: These are not like your household magnets. Accelerator magnets can be as long as a pickup truck — sometimes longer — and can weigh tons. It usually takes months to construct each one.
Regardless of the materials used to make them, accelerator magnets can be classified according to their number of poles. Most come in one of four types: Dipole magnets bend the beam, quadrupoles focus the beam, sextupoles correct the imperfect focusing of quadrupoles, and octupoles can help increase the stability of stored particle beams. In accelerator lingo, these are the different magnetic “multipoles” that scientists use to manipulate beams in these engines of discovery.
Accelerator magnets can be as long as a pickup truck — sometimes longer — and can weigh tons. It usually takes months to construct each one. They bend and focus particle beams, correct imperfect focusing and even increase beam stability. Photo: Reidar Hahn, Fermilab
Dipoles – it’s not easy steering beams
Dipoles are most often made of two separate coiled wires with their north and south poles facing each other. When current flows through the coils, a unidirectional magnetic field forms in the gap between the poles.
“Accelerator scientists and engineers can use that field to bend charged-particle beams along a curve,” said Jonathan Jarvis, an associate scientist at Fermilab. “Put simply, dipoles are our main way of getting beams where they need to go.”
If you happened to be riding on an proton heading straight for a magnetic field pointing downward, you and your proton would move to the left at an amount proportional to the magnet’s field strength. The stronger the magnetic field, the stronger the leftward pull you and your proton would feel. For vertical magnetic fields, the path you would trace out is a horizontal circular arc.
Dipole magnets are usually used to bend particle beams. In a circular accelerator, for example, multiple dipole magnets are lined up along the beam path. The particle beam moves through one after another, getting nudged in one direction with each pass so that it follows the curve.
Fast-acting dipoles can also be used to “kick” particle beams into or out of a circular accelerator’s main beam.
As a positively charged particle travels into the page and passes through the dipole magnet, it is deflected to the left at an angle proportional to the amount of force applied by the magnet. Image: Jerald Pinson
Quadrupoles – staying focused
Magnets applying a unidirectional force work well for bending particle beams in a particular direction, but they aren’t able to maintain a beam’s shape.
“If we leave the beam to its own devices in dipoles, it will come apart,” Jarvis said. “Just like a collection of gas molecules, a beam of particles has a temperature, and that random energy will cause the particles to naturally drift apart in an accelerator. If the beam particles aren’t brought back together, then they will slam into the walls of the vacuum pipes where they are circulating.”
So scientists use quadrupole magnets to refocus the wayward particles and bring them back into the fold.
As the name implies, quadrupoles have four alternating poles. They produce a special magnetic field that can bring particles back together, similar to how lenses can bend rays of light to a point.
A single quadrupole focuses a beam in one plane. For example, a quadrupole can squeeze the sides of the beam inward as it races through an accelerator, but — similar to the way a lump of Play-Doh responds when you squish its sides together — the beam will defocus in the other direction.
The solution is to string multiple quadrupoles together with alternating orientations. The beam passes through one and is squeezed in the horizontal direction. Then it passes through the next and is squeezed in the vertical direction. With each successive pinch, it becomes focused.
The net effect is a stable beam of particles rattling back and forth as they whip around the accelerator.
By the same token, quadrupoles can also defocus beams. As particles travel through an accelerator, there are times when it’s better for the beam to be a little less tightly packed, decreasing the likelihood that the particles will interfere with each other. As beams pass through quadrupoles of weaker magnetic strength, they are allowed to spread out first in the up-down direction, then in the left-right direction and so on until they’re suitably defocused.
Quadrupoles have four magnetic poles. In a particle accelerator, the poles push particles together if they deviate too far from the centralized beam. Quadrupoles focus in only one plane, so to squeeze an accelerator beam from both sides, these magnets are usually stacked one after the other, each rotated by 90 degrees relative to the previous one. In this way, the beam particles are pushed together in both directions as they travel through successive magnets. Image: Jerald Pinson
Sextupoles – color correction
Just as dipole magnets can bend a beam but aren’t able to keep it focused, quadrupoles can focus particles, but not all to the same location.
The particles that make up a beam have slightly different energies.
“Unfortunately, quadrupoles don’t behave exactly the same for all beam energies,” Jarvis said. “A higher-energy particle is less affected by a quadrupole’s magnetic field than a lower-energy particle.”
The result is that high- and low-energy particles are focused at different points along the beam’s path. This is similar to the way water droplets bend different colors of light to produce a stunning rainbow.
In quadrupoles, this ‘chromatic aberration’ produces differences in how rapidly the particles are bouncing back and forth in the accelerator, a phenomenon known to accelerator scientists as chromaticity.
“In many cases, to see the physics we want, we have to correct the chromaticity, and we do this using sextupoles,” Jarvis said.
When properly placed in the accelerator, these six-poled magnets force higher-energy particles back into alignment with the rest of the beam.
Quadrupole magnets are not able to focus particles with varying energies to a particular point, so scientists use sextupole magnets to correct for this chromatic aberration. Image: Jerald Pinson
Octupoles – mixing it up
We’ve all had that moment: you’re walking down a hallway when someone rounds a corner and ends up directly in your path. You both maneuver one way, then another, then back again in an attempt to avoid colliding, an encounter that can seem to last for ages. The reason it’s so hard to get past the other person is the result of your similar rates of movement. If one person moved more slowly, or simply stayed the course, then this behavior would get suppressed.
Particle beams can exhibit similar sorts of collective behavior if they all oscillate at the same frequency.
To stabilize the situation, eight-poled magnets, called octupoles, can be used to mix up the particles’ frequencies. Scientists call the resulting stabilization ‘Landau damping,’ and it provides a particle beam with a bit of natural immunity against some unstable behaviors.
Unfortunately, the increased stability and enhanced focus imparted by higher order multipole magnets comes at a cost.
“These magnets can produce harmful resonances and reduce the overall range of positions and energies that the stored particles are allowed to have,” Jarvis said. “If particles find themselves outside this range of the so-called ‘dynamic aperture’, then they will be lost from the accelerator.”
Integrable optics and beyond
Scientists at accelerator facilities around the world are working to generate more productive particle beams in their pursuit of the physics that underpins the universe.
One way they do this is by increasing the beam’s intensity — the number of particles they pack into a beam. But there’s a catch: As intensity increases, the way beams behave can become much more complex, straining the limits of how well traditional magnets can confine them.
To pave the way for the next generation of particle physics, accelerator scientists at Fermilab are considering fundamentally new types of magnets, ones that that can handle ever increasing beam intensities.
“These nonlinear magnets are effectively special combinations of many multipoles, and they have the potential to dramatically improve beam stability without making the trade-offs inherent with simple octupoles,” said Jarvis.
As scientists continue to push the boundaries of magnet technology, we will be able to peer deeper into the subatomic world – discovering exotic particles that exist in only the most extreme conditions, observing the mysterious transformation of neutrinos and the decay of muons, and ultimately come to a better understanding of how the universe began.
It’s surprising to think that the humble magnet is our gateway to some of the universe’s deepest mysteries, but then again, that’s the power of attraction.
This work is supported by the DOE Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.
In a multiyear effort involving three national laboratories from across the United States, researchers have successfully built and tested a powerful new magnet based on an advanced superconducting material. The eight-ton device — about as long as a semi-truck trailer — set a record for the highest field strength ever recorded for an accelerator focusing magnet and raises the standard for magnets operating in high-energy particle colliders.
The Department of Energy’s Fermilab, Brookhaven National Laboratory and Lawrence Berkeley National Laboratory designed, built and tested the new magnet, one of 16 they will provide for operation in the High-Luminosity Large Hadron Collider at CERN laboratory in Europe. The 16 magnets, along with another eight produced by CERN, serve as “optics” for charged particles: They will focus beams of protons into a tiny, infinitesimal spot as they approach collision inside two different particle detectors.
The ingredient that sets these U.S.-produced magnets apart is niobium-tin – a superconducting material that produces strong magnetic fields. These will be the first niobium-tin quadrupole magnets ever to operate in a particle accelerator.
“This accomplishment is a major milestone for the High-Luminosity LHC project, which relies heavily on the success of the niobium-tin superconducting magnet technology,” said Lucio Rossi, project leader of the High-Luminosity LHC project.
The ingredient that sets these U.S.-produced magnets apart is niobium-tin – a superconducting material that produces strong magnetic fields.
Like the current Large Hadron Collider, its high-luminosity successor will smash together beams of protons cruising around the 17-mile ring at close to the speed of light. The High Luminosity LHC will pack an additional punch: It will provide 10 times the collisions that are possible at the current LHC. With more collisions come more opportunities to discover new physics.
And the machine’s new focusing magnets will help it achieve that leap in delivered luminosity.
“We’ve demonstrated that this first quadrupole magnet behaves successfully and according to design, based on the multiyear development effort made possible by DOE investments in this new technology,” said Fermilab scientist Giorgio Apollinari, head of the U.S. Accelerator Upgrade Project, which leads the U.S.-based focusing-magnet project.
“It’s a very cutting-edge magnet, really on the edge of magnet technology,” said Brookhaven National Laboratory scientist Kathleen Amm, the Brookhaven representative for the Accelerator Upgrade Project.
What makes it successful is its impressive ability to focus.
This new magnet reached the highest field strength ever recorded for an accelerator focusing magnet. Designed and built by Fermilab, Brookhaven National Laboratory and Lawrence Berkeley National Laboratory, it will be the first niobium-tin quadrupole magnet ever to operate in a particle accelerator — in this case, the future High-Luminosity Large Hadron Collider at CERN. Photo: Dan Cheng, Lawrence Berkeley National Laboratory
Focus, magnets, focus
In circular colliders, two beams of particles race around the ring in opposite directions. An instant before they reach the collision point, each beam passes through a series of magnets that focus the particle beams into a tiny, infinitesimal spot, much the way lenses focus light rays to a point. Now packed as tightly with particles as the magnets can get them — smash! — the beams collide.
The scientific fruitfulness of that smash depends on how dense the beam is. The more particles that are crowded into the collision point, the greater the chance of particle collisions.
You get those tightly packed beams by sharpening the magnet’s focus. One way to do that is to widen the lens. Consider light:
“If you try to focus the light from the sun using a magnifying glass at a small point, you want to have a more ‘powerful’ magnifying glass,” said Ian Pong, Berkeley Lab scientist and one of the control account managers.
A larger magnifying glass focuses more of the sun’s rays than a smaller one. However, the light rays at the outer rim of the lens have to be bent more sharply in order to approach the same focal point.
Or consider a group of archers shooting arrows at an apple: More arrows will stick if the archers shoot from above, below and either side of the apple than if they are stationed at one post, firing from the same position.
The analog of the magnifying glass size and the archer array is the magnet’s aperture — the opening of the passageway the beam takes as it barrels through the magnet’s interior. If the particle beam is allowed to start wide before being focused, more particles will arrive at the intended focal point — the center of the particle detector.
The U.S. team widened the LHC focusing magnet’s aperture to 150 millimeters, more than double the current aperture of 70 millimeters.
But of course, a wider aperture isn’t enough. There is still the matter of actually focusing the beam, which means forcing a dramatic change in the beam’s size, from wide to narrow, by the time the beam reaches the collision point. And that requires an exceptionally strong magnet.
“The magnet has to squeeze the beam more powerfully than the LHC’s present magnets in order to create the luminosity needed for the HL-LHC,” Apollinari said.
“The magnet has to squeeze the beam more powerfully than the LHC’s present magnets in order to create the luminosity needed for the HL-LHC,” Apollinari said.
To meet the demand, scientists designed and constructed a muscular focusing magnet, calculating that, at the required aperture, it would have to generate a field exceeding 11.4 teslas. This is up from the current 7.5-tesla field generated by the niobium-titanium-based LHC quadrupole magnets. (For accelerator experts: The HL-LHC integrated luminosity goal is 3,000 inverse femtobarns.)
In January, the three-lab team’s first HL-LHC focusing magnet delivered above the goal performance, achieving an 11.5-tesla field and running continuously at this strength for five straight hours, just as it would operate when the High-Luminosity LHC starts up in 2027.
“These magnets are the currently highest-field focusing magnets in accelerators as they exist today,” Amm said. “We’re really pushing to higher fields, which allows us to get to higher luminosities.”
The new focusing magnet was a triumph, thanks to niobium-tin.
Niobium-tin for the win
The focusing magnets in the current LHC are made with niobium-titanium, whose intrinsic performance limit is generally recognized to have been reached at 8 to 9 teslas in accelerator applications.
The HL-LHC will need magnets with around 12 teslas, about 250,000 times stronger than the Earth’s magnetic field at its surface.
“So what do you do? You need to go to a different conductor,” Apollinari said.
Accelerator magnet experts have been experimenting with niobium-tin for decades. Electrical current coursing through a niobium-tin superconductor can generate magnetic fields of 12 teslas and higher — but only if the niobium and tin, once mixed and heat-treated to become superconductive, can stay intact.
“Once they’re reacted, it becomes a beautiful superconductor that can carry a lot of current, but then it also becomes brittle,” Apollinari said.
Famously brittle.
“If you bend it too much, even a little bit, once it’s a reacted material, it sounds like corn flakes,” Amm said. “You actually hear it break.”
Over the years, scientists and engineers have figured out how to produce niobium-tin superconductor in a form that is useful. Guaranteeing that it would hold up as the star of an HL-LHC focusing magnet was another challenge altogether.
Berkeley, Brookhaven and Fermilab experts made it happen. Their assembly process is a delicate, involved operation balancing niobium-tin’s fragility against the massive changes in temperature and pressure it undergoes as it becomes the primary player in a future collider magnet.
The process starts with wires containing niobium filaments surrounding a tin core, provided by an outside manufacturer. The wires are then fabricated into cables at Berkeley in just the right way. The teams at Brookhaven and Fermilab then wind these cables into coils, careful to avoid deforming them excessively. They heat the coils in a furnace in three temperature stages, a treatment that takes more than a week. During heat treatment the tin reacts with the filaments to form the brittle niobium-tin.
Having been reacted in the furnace, the niobium-tin is now at its most fragile, so it is handled with care as the team cures it, embedding it in a resin to become a solid, strong coil.
That coil is now ready to serve as one of the focusing magnet’s four poles. The process takes several months for each pole before the full magnet can be assembled.
“Because these coils are very powerful when they are energized, there is a lot of force trying to push the magnet apart,” Pong said. “Even if the magnet is not deforming, at the conductor level there will be a strain, to which niobium-tin’s performance is very sensitive. The management of the stress is very, very important for these high-field magnets.”
Heat treating the magnet coils — one of the intermediate steps in the magnet’s assembly — is also a subtle science. Each of the four coils of an HL-LHC focusing magnet weighs about one ton and has to be heat-treated evenly — inside and out.
“You have to control the temperature well. Otherwise the reaction will not give us the best performance. It’s a bit like cooking,” Pong said.
“You have to control the temperature well. Otherwise the reaction will not give us the best performance,” Pong said. “It’s a bit like cooking. It’s not just to achieve the temperature in one part of the coil but in the entire coil, end to end, top to bottom, the whole thing.”
And the four coils have to be aligned precisely with one another.
“You need very high field precision, so we have to have very high precision in how they align these to get good magnetic-field uniformity, a good quadrupole field,” Amm said.
The fine engineering that goes into the U.S. HL-LHC magnets has sharpened over decades, with a payoff that is energizing the particle accelerator community.
“This will be the first use of niobium-tin in accelerator focusing magnets, so it will be pretty exciting to see such a complex and sophisticated technology get implemented into a real machine,” Amm said.
“We were always carrying the weight of responsibility, the hope in the last 10, 20 years — and if you want to go further, 30, 40 years — focusing on these magnets, on conductor development, all the work,” Pong said. “Finally, we are coming to it, and we really want to make sure it is a lasting success.”
The magnet gets ready for a test at Brookhaven National Laboratory. Photo: Brookhaven National Laboratory
The many moving parts of an accelerator collaboration
Ensuring lasting success has as much to do with operational choreography as it does with exquisite engineering. Conducting logistics that span years and a continent requires painstaking coordination.
“Planning and scheduling are very important, and they’re quite challenging,” Pong said. “For example, transportation communication: We have to make sure that things are well-protected. Otherwise these expensive items can be damaged, so we have to foresee issues and prevent them. Delays also have an impact on the whole project, so we have to ensure components are shipped to destination in a timely schedule.”
Amm, Apollinari, Pong and Rossi acknowledge that the three-U.S.-lab team and CERN have met the challenges capably, operating as a well-oiled machine.
“The technologies developed at Fermilab, Brookhaven and Berkeley helped make the original LHC a success. And now again, these technologies out of the U.S. are really helping CERN be successful. It’s a dream team, and it’s an honor to be a part of it,” Amm said.
“This full-length, accelerator-ready magnet performance record is a real textbook case for international collaboration in the accelerator domain: Since the very beginning, the U.S. labs and CERN teamed up and managed to have a common and very synergic R&D, particularly for the quadrupole magnet that is the cornerstone of the upgrade,” Rossi said. “This has resulted in substantial savings and improved output.”
From now until about 2025, the U.S. labs will continue to build the large, hulking tubes, starting with fine strands of niobium and tin. They plan to begin delivering in 2022 the first of 16 magnets, plus four spares, to CERN. Installation will take place over the three years following.
“This success in the U.S. is a very good omen for the test of the CERN quadrupole magnet, a twin companion of the U.S. quadrupole. It also nicely complements the successful test in July 2019 at CERN of the 11.2-tesla dipole, which will be the first high field niobium-tin magnet to be installed for HL-LHC, in the upcoming months,” Rossi said.
“Our magnets are massive superconducting devices, focusing tiny invisible particle beams that are flying close to the speed of light through the bore. It’s quite magical,” Pong said.
“People say that ‘touchdown’ is a very beautiful word to describe the landing of an airplane, because you have a huge metal object weighing hundreds of tons, descending from the sky, touching a concrete runway very gently,” Pong said. “These magnets are not too different from that. Our magnets are massive superconducting devices, focusing tiny invisible particle beams that are flying close to the speed of light through the bore. It’s quite magical.”
The magic starts in 2027, when the High-Luminosity LHC comes online.
“We are doing today the work that future young researchers will use in 10 or 20 years from now to push the frontier of human knowledge, just like it happened when I was a young researcher here at Fermilab, using the Tevatron,” Apollinari said. “It’s a generational passing of the baton. We need to make the machines for the future generations, and with this technology, obviously what we can enable for the future generation is a lot.”
Learn more about the High-Luminosity LHC in Symmetry and in an 11-minute Fermilab YouTube video.
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC, a joint partnership between the University of Chicago and the Universities Research Association, Inc. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @Fermilab.
This accelerator magnet work is supported by the Department of Energy Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.
Photons are the fundamental particles of light. They illuminate our world, letting us see the universe we live in. But light has failed to show us an extraordinary 85% of the matter in the universe, called dark matter. Scientists hope that an as yet unseen cousin of the photon, called a dark photon, will provide a clue about the nature of this mysterious dark matter.
A dark photon may sound like a contradiction in terms, but physicists use it to describe the hypothesized, photon-like particles that simply pass through ordinary matter. These invisible particles are part of the hypothetical dark sector — less ominously known as the hidden sector — of quantum fields and particles.
Scientists Anna Grassellino, Roni Harnik and Alexander Romanenko lead the Dark SRF experiment at the U.S. Department of Energy’s Fermilab. To search for the elusive particles, the team has repurposed technology developed for particle accelerators. The researchers hope to generate dark photons and then spot signs of them traveling through solid metal.
The theorized dark photon has the same properties as a photon, but a different mass — that is, if it has any mass. (Photons are massless.) Also, dark photons and photons are inextricably linked: One type can morph into the other, and dark photons interact with matter only through this transforming act.
Fundamental particles are already known to come in varied copies of each other. For example, the familiar electron has two similar, heavier cousins — the muon and tau. That pattern is an important motivation in looking for dark photons, explained Harnik, the main theorist on the project.
“The fact that the muon exists as a copy of the electron makes one wonder whether nature tends to have several copied of each particle. If so, perhaps there is a similar replication of the photon,” Harnik said. “That’s the dark photon.”
Although theorists can propose the dark photon, theory alone cannot predict its mass or its interaction probability. Experimenters and observers have been able to eliminate broad swaths of possible properties. But the search in unexplored territory is on.
Grassellino, who is the Fermilab deputy chief technology officer and led the organization of the experiment, explained that theorists are pushing experimentalists “in all possible directions to fill this map and say, ‘Oh, nothing here, nothing here, nothing here. Where else should we look?’”
During experimental runs of Dark SRF, two cavities (shown here) are bathed in liquid helium to keep them cold. One is held perfectly still, tuned to a specific frequency and kept as empty of photons as possible. The other cavity, which is intended to generate dark photons, has a special setup to adjust it to match the frequency of the first cavity in real time. The experiment aims to catch photons produced in the adjustable cavity as they make their ghostly journey, through metal walls, into the empty cavity. Photo: Reidar Hahn, Fermilab
A technological case of adaptive reuse
The Dark SRF experiment searches for dark photons in a region that researchers have yet to probe. The region might be said to be low-hanging fruit for Fermilab, but it would be more accurate to say that Fermilab had already designed and built a tall enough ladder to easily harvest new dark photon fruit.
This metaphorical ladder is the superconducting accelerator cavity. Cavities are hollow, metal, resonating structures in particle accelerators that push particles to near the speed of light. Over the last decade or so, Fermilab has made sweeping strides in increasing their efficiency, getting particles to higher energies over shorter distances.
Romanenko was the first to realize that Fermilab’s cavities are seemingly tailor-made for a different use and could be instrumental in the search for dark photons.
“At Fermilab, we have unique technologies that we’re pushing to unprecedented levels of sensitivity or efficiency,” Grassellino said. “We recognize that we need to also make the effort to either think of what we can do with it for unique experiments like Dark SRF or make them available to the other experimenters.”
In Dark SRF, the superconducting cavity is designed so that photons of a specific microwave energy oscillate together, bouncing back and forth inside it about a 100 billion times before being lost. The cavity maintains the microwaves in much the way a bell or tuning fork maintains sound vibrations.
When kept filled, a cavity can contain around 10 septillion photons — a 1 followed by 25 zeroes. That’s about the number grains of sand in all the deserts and beaches in one thousand Earths (based on a high estimate of how sandy Earth is). The cavity’s astronomically high photon capacity makes it perfect for coaxing hypothetical dark photons out of their hiding place: Each of those regular photons has some chance of being converted into its dark counterpart — an alluring bet that dark photons will be seen, if they exist.
“If a dark photon indeed exists, the filled superconducting cavity acts as a transmitting antenna of dark photons,” Harnik said.
In addition to their excellence in storing photons, the cavities can also keep out stray light, creating a perfect place to hunt for photons arriving unexpectedly. In Dark SRF, a second empty cavity would pick up a dark-photon signal that originated from the 10 septillion photons vibrating inside the first.
“The beauty of this experiment is it’s so simple, given that we have all this technology in hand,” Romanenko said. “We’re starting to push these cavities into the quantum regime, pairing a cavity bursting with photons with another almost completely devoid of them and being able to detect a single one.”
A dark photon’s journey
If dark photons exist, it is their ability to travel through walls that the Dark SRF team will use to identify them. The Dark SRF experiment brings significantly improved technology to a type of undertaking called a “light shining through a wall” experiment, which looks for light making a seemingly impossible journey through an opaque barrier.
The experiment uses two cavities, one above the other, that are bathed in liquid helium to keep them cold. One is held perfectly still, tuned to a specific frequency and kept as empty of photons as possible. The other cavity, which is intended to generate dark photons, has a special setup to adjust it to match the frequency of the other cavity in real time.
The experiment aims to catch photons produced in the adjustable cavity as they make their ghostly journey, through metal walls, into the empty cavity.
The journey begins when some of the 10 septillion photons that are bouncing around the tunable cavity convert into dark photons, which then pass through the wall of that cavity. Dark photons’ lack of interaction with mundane matter makes them invisible to us. It also renders the walls of the cavities, and everything else, intangible to them. Some of these dark photons will travel into the other cavity, and some fraction of those will revert into regular photons.
The appearance of these seemingly teleported photons signals the existence of their dark cousins. Sighting them would be the eureka moment.
The Dark SRF team at Fermilab is advancing the search for dark photons. Photo: Reidar Hahn, Fermilab
The Dark SRF difference
The success of Dark SRF’s simple design hinges on the extraordinarily fine calibration of the two chambers. The second chamber acts as a trap, capturing the reverted photons. But it will build up a noticeable number only if the two chambers’ frequencies precisely match. Otherwise, the photons’ journeys end with them being quickly absorbed by the second chamber’s walls, never to be seen, and the dark photon will continue to fly under the radar.
How fine must the calibration be? The required alignment is unforgiving: The roughly quarter-meter-long cavities must be perfectly positioned to within a billionth of a meter. That’s like correctly plotting the length of a regulation soccer field to within the length of a chromosome.
And once in harmony, these chambers become magnificently sensitive antennas. This is thanks to their high quality factor, a measure of how efficiently they retain energy. The higher the quality factor, the more photons the generating cavity produces, and the more sensitive to dark photons the receiving cavity becomes. The Dark SRF cavities have a quality factor of 1011 when chilled to 1.4 kelvins — the highest-efficiency engineered resonators in the world. Their quality factor leads to both a flood of potential progenitor photons and heightened sensitivity — both of which give scientists a fighting chance of plucking a dark photon from the vacuum.
Fermilab’s expertise in cryogenic cooling also contributes to Dark SRF’s ability to explore new ground in the search for dark photons. Any photons converted from dark photons must be picked out from a background crowd of other, normal photons generated by the cavity’s heat. In Dark SRF, the cavities’ cold 1.4-kelvin temperature helps reduce the background to a mere 1,000 in the receiving cavity. Researchers plan to modify the system in the future to operate at about 6 millikelvins, winnowing the number to less than one on average and providing the opportunity to search for a more elusive version of the dark photon.
“If you want to hunt for one photon, we are the only place in the world where you can do that with a cavity,” Grassellino said.
Blue sky science for the dark world
The Dark SRF experiment is an example of how technology and expertise developed for a particular purpose — designing efficient particle accelerators — finds use in another pursuit — searching for hidden particles.
Since it began operation in 2019, scientists on the Dark SRF Experiment have already made significant progress in their search. They’ve experimentally ruled out values for a particular quantity, called a kinetic mixing, that would point to the existence of a dark photon of a certain mass range (between 10 billionths and hundreds of millionths of an electronvolt). This narrows the possible values of the kinetic mixing by a factor of 1,000 compared to previous searches. Now they are pushing at the boundary of the parameter measurement by repeating the experiment using advanced quantum techniques.
If the experiment does find evidence of dark photons, it will introduce a whole world of new questions to be explored: How common are dark photons in the universe? Are dark photons the dark matter scientists have been eagerly searching for? Do dark photons interact with other dark matter similar to how photons do with regular matter? If so, do they reveal another part of our universe as complex as our own but previously invisible?
Whether the Dark SRF experiment discovers the dark photon or not, it will contribute to our understanding of the dark matter that we know is there but have yet to see.
“The path forward follows what we all learnt at school: In science we make a hypothesis, we test it.” Harnik said. “If we find it, ‘Hooray!’ If we don’t find it — science progressed.”
This work is supported by the DOE Office of Science.
Fermilab is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.