Beyond Higgs: The Search for New Particles That Could Solve Mysteries of the Universe

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Category: Research News

Published: Wednesday, September 07 2022 00:01

An elusive elementary particle called the Higgs boson is partly to thank for life as we know it. No other elementary particles in the early universe had mass until they interacted with a field associated with the Higgs boson, enabling the emergence of planets, stars and—billions of years later—us.

Despite its cosmic importance, scientists couldn’t prove the Higgs boson even existed until 2012, when they smashed protons together at the most powerful particle accelerator ever built: the Large Hadron Collider (LHC). A decade later, this massive machine, built in a tunnel beneath the France-Switzerland border, is up and running again after a series of upgrades. 

As the search for new particles starts anew, researchers like Assistant Professor of Physics Manuel Franco Sevilla find themselves wondering if they will discover anything beyond Higgs.UMD Assistant Professor of Physics Manuel Franco Sevilla is helping to upgrade the Large Hadron Collider (LHC) at CERN in Switzerland. In the above photo, he is shining a light on silicon sensors to measure whether their dark current increases—a sign that they are properly connected. Photo courtesy of Manuel Franco Sevilla.

“It’s a tough question because particle physics is at a juncture,” said Franco Sevilla, who is working at the LHC this semester. “It’s possible that we are currently in what physicists call a ‘nightmare scenario,’ where we discovered the Higgs, but after that, there’s a big desert. According to this theory, we will not be able to find anything new with our current technology.”

This is the worst-case scenario, but not the only possible outcome. Some scientists say the LHC could make another discovery on par with the Higgs boson, potentially identifying new particles that explain the origin of dark matter or the mysterious lack of antimatter throughout the universe. Others say new colliders—more powerful and precise than their predecessors—must be built to bring the field into a new era.

There are no easy answers, but Franco Sevilla and several other faculty members in UMD’s Department of Physics are rising to the challenge. Some are working directly with the LHC and future collider proposals, while others are developing theories that could solve life’s biggest mysteries. All of them, in their own way, are advancing the ever-changing field of particle physics.

Looking For Beauty

As Franco Sevilla will tell you, there’s beauty all around—if you know where to look. He is one of the researchers who uses the LHC’s high-powered collisions to study a particle called the beauty quark—b quark for short. It’s one of the components of “flavor physics,” which observes the interactions between six “flavors,” or varieties, of elementary particles called quarks and leptons. 

Because these particles sometimes behave in unexpected ways, Franco Sevilla believes that flavor physics might lead to breakthroughs that justify future studies in this field—and possibly even the need for a new particle collider. 

“Some of the most promising things that will break the ‘nightmare scenario’ and allow us to find something are coming from flavor physics,” he said. “We still haven't fully discovered something new, but we have a number of hints, including the famous ‘b anomalies.’”

These anomalies are instances where the b quark decayed differently than predicted by the Standard Model, the prevailing theory of particle physics. This suggests that something might exist beyond this model—which, ever since the 1970s, has helped explain the fundamental particles and forces that shape our world.

Some mysteries remain, though. Physicists still don’t know the origin of dark matter—an enigmatic substance that exerts a gravitational force on visible matter—or why there’s so much matter and so little antimatter in the universe.

Sarah Eno, a UMD physics professor who has conducted research at particle accelerators around the world, including the LHC, said these answers will only come from collider experiments.Sarah Eno, a physics professor at UMD, sits atop a model of a Large Hadron Collider (LHC) dipole magnet at CERN about 10 years ago. At the time, she was participating in LHC experiments and frequently spent her summers at the lab in Switzerland. Credit: Meenakshi Narain.
“What is the nature of dark matter? Nobody has any idea,” Eno said. “We know it interacts via gravity, but we don’t know whether it has any other kinds of interactions. And only an accelerator can tell us that.”

Better, Faster, Stronger

After the third (and current) run of the LHC ends, the collider’s accelerator will be upgraded in 2029 to “crank up the performance,” according to the European Organization for Nuclear Research (CERN), which houses the collider.

With the added benefit of stronger magnets and higher-intensity beams, this final round of experiments could be a game-changer. But once that ends, Eno said the LHC will have reached its limit in terms of energy output, lowering the odds of any new discoveries. The logical next step, she said, would be to construct a new collider capable of propelling the field of particle physics forward.

“The field is now trying to decide what the next machine is,” Eno said.

Around the globe, there are various proposals on the table. There is a significant push for another proton collider in the same vein as the LHC, except bigger and more powerful. Electron-positron colliders—both linear and circular in shape—have also been proposed in China, Japan and Switzerland.

Eno is one of the physicists leading the charge for the construction of an electron-positron collider at CERN. It would be the first stage of the proposed Future Circular Collider (FCC), which would be four times longer than the LHC. By the late 2050s, it would be upgraded to a proton collider, with an energy capacity roughly seven times that of the LHC. Eno, who was appointed one of the U.S. representatives for this project, said the electron-positron collider would allow physicists to study the Higgs boson with significantly higher precision.

“When an electron and positron annihilate, all their energy becomes new states of matter,” Eno said. “This means that when you’re trying to reconstruct the final state, you know the total energy of that final state. This allows you to do much more precise measurements.”

This proposal does come with some challenges. Circular colliders radiate off a lot of energy, making it difficult to accelerate electrons to high energy. Eno said this can be avoided by building a massive collider with a long tunnel (to the tune of 62 miles, in the case of the FCC), preventing electrons from losing steam as they whip around sharp corners.

The radiation problem has pushed some physicists towards another theoretical possibility: a muon collider. Muons are subatomic particles that are like electrons, but 207 times heavier, which keeps them from radiating as much. This would make them ideal candidates for collider research—if only they didn’t decay in 2.2 microseconds.

“If you talk to the muon collider proponents, their faces light up because it’s such a challenge,” Eno said. “And who doesn’t like a challenge?”

Clean Collisions 

One of Eno’s colleagues at UMD—Distinguished University Professor and theoretical physicist Raman Sundrum—endorses the muon collider idea. So much so that he and a team of physicists wrote a paper titled “The muon smasher’s guide,” which appeared in Reports on Progress in Physics in July 2022.

“We build colliders not to confirm what we already know, but to explore what we do not,” the research team wrote in their paper. “In the wake of the Higgs boson’s discovery, the question is not whether to build another collider, but which collider to build.”

They made the case for the world’s first muon smasher, arguing that these collisions would be “far cleaner” than proton collisions, which occur at the LHC. Unlike protons—a composite object made of quarks and gluons—muons are elementary particles with no smaller components. This would let physicists see only what they want to see, without any distractions.

“Muon collisions would make it easier to diagnose what’s going on,” Sundrum said. “When something extraordinary happens, it doesn’t get dwarfed by all of the mundane crashing of many parts.”

Considering that the Higgs boson only appears once in about a billion collisions at the LHC, this level of clarity and precision could make a world of difference. However, the more the field of particle physics advances, the more challenging it is to find something new.

“The Higgs was a needle in the haystack, but discovering newer particles could be even subtler and harder,” Sundrum said.Artistic rendering of the Higgs field. Credit: CERN

Despite these challenges, the LHC could still make a major discovery. Sundrum continues to develop theories that guide and inspire the field, including the idea that the LHC could find a “parent particle” that gave rise to all protons in the universe. If this comes to fruition, it would be worth building new colliders that could validate the LHC’s initial findings and provide a more complete picture of why matter dominates over antimatter in the universe, Sundrum said.

In the coming years and decades, physicists will continue to debate the pros and cons of various collider proposals. The outcome will depend partly on scientific advancements, and partly on political will and funding. Sundrum said it’s not cheap to build a collider—with some projects expected to cost $10 billion—but the discoveries that could come from these experiments are priceless.

“An enormous number of people find it very moving and interesting to know what it’s all about, in terms of where the universe came from, what it means and how the laws work,” Sundrum said. “Individually these experiments are expensive, but as a planet, I think we can easily afford to do it.”

Written by Emily Nunez

From Unexpected Opportunity to Game-changing Discovery

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Category: Department News

Published: Tuesday, September 06 2022 00:01

In the world of startups, opportunity can come knocking in strange ways. Six years ago, Didier Depireux (Ph.D. ’91, physics) was doing research at the University of Maryland when he was approached by Sam Owen, a young scientist who said he’d developed a device to treat motion sickness. Depireux was skeptical but decided to check it out. 

“Since I get very severe motion sickness, I made a deal with him,” Depireux recalled. “I said, ‘I’ll come over with my car and you can drive me around while I use the device. If I haven’t thrown up after 20 minutes while I’m in the back of the car reading, I’ll join the effort.’”

The two made plans to meet in Washington, D.C., on a muggy July afternoon.  Didier Depireux

“So, I go to Georgetown. The windows are down, it’s hot, it’s humid and I’m thinking I will not make it past the first turn,” Depireux explained. “Owen is driving and I’m in the back seat using his device and reading my cellphone. And for the first time in my life—and I’m over 50 years old—I was able to read in the back of a car and not get sick. I thought, ‘I need to join this, this is amazing.’”

Thanks to that strange summer ride-along, Depireux joined Owen in launching a startup called Otolith Labs to address inner ear-related conditions and their often debilitating symptoms. Otolith’s noninvasive vestibular system masking technology—designed for acute treatment of vestibular vertigo—received the FDA’s Breakthrough Device designation and clinical trials are ongoing, with support from investors including AOL founder Jack Davies and billionaire entrepreneur Mark Cuban.

All of this sets the stage for a major test that could lead to the startup’s ultimate goal—FDA approval as early as next year.

“In July we told the FDA we want to do a large-scale pivotal trial with hundreds of participants,” Depireux explained. “If all goes well, we’ll have a meeting next summer where the FDA will approve us and then the device will become available.”

For Depireux, it’s the latest step on a bigger mission that has guided his career.

Didier Depireux“It’s mostly relevance,” he explained. “I would like my life to make a difference, that’s the one thing that keeps me going.”

From philosophy to physics

Depireux was raised in Belgium. A bright, thoughtful boy, he grew up with a strong interest in science and theory, thanks to his father, a physics professor, and his mother, a chemistry teacher.

“I was always very science-y,” Depireux recalled. “Initially, I wanted to become a philosopher and I read this 800-page book—I think it was Kant—and at the end of it I was like, ‘I still don’t know the answer, and I’m not even sure I understand the question anymore.’ That’s when I thought that’s not a good fit for me.”  

Depireux eventually gravitated toward physics. After receiving his B.S. in physics from the University of Liège in Belgium in 1986, he began his graduate work in physics at the University of Maryland, where he focused on string theory and met Distinguished University Professor of Physics Sylvester James Gates Jr., who quickly became a mentor and friend.

“Jim had a huge impact on me. He was a fantastic person to work with and he had so much positive energy,” Depireux said. “I still remember late one night I was working on something, and I was stuck and I wrote to him, and he said, ‘I’ll come over, let’s work this out.’ So we had office hours at 10:30 p.m. just because I couldn’t solve a problem.”

Depireux earned his Ph.D. in 1991 and went on to do postdoctoral work in Quebec, Canada, before returning to College Park in 1994. Inspired by his wife Pamela, who was getting her Ph.D. in neuropharmacology, Depireux took on the challenge of modeling the brain and studying how it processes sound. By 2001, he was also teaching a gross anatomy class at the University of Maryland School of Medicine.

“I think, to this day, I am the only string theorist who has taught gross anatomy,” he reflected.

From his research on the brain and hearing, Depireux shifted his focus to tinnitus—disruptive ringing in the ears. He explored possible treatments and eventually teamed up with former UMD Bioengineering Professor Benjamin Shapiro who was already working on the drug delivery challenges Depireux was trying to solve.

“I wanted to get drug delivery to the ear but I didn’t know how to do it,” Depireux said. “He had this method with nanoparticles to deliver drugs and I had the target so we started working together.”

In 2013, the two launched Otomagnetics, a startup that has made major strides in developing noninvasive methods to treat inner ear diseases and more.

“We’ve gotten very nice results as far as drug delivery goes and Otomagnetics is still an ongoing concern,” Depireux explained, “But raising money for drug delivery is the real challenge, because to get drug delivery to the ear is going to take hundreds of millions of dollars, and that hasn’t happened yet.”

Going all-in on Otolith

Depireux balanced his time between Otomagnetics, his UMD research and teaching at the School of Medicine until 2016, when he experienced Owen’s experimental motion sickness device for the first time. Depireux saw so much potential with the device that he went all-in on Otolith. 

“You have to have pretty strong resilience to join a startup—I went for a year and a half without a salary or anything,” Depireux explained. “It’s not like we didn’t have money, we just needed all of the money to develop the device, get the patents in, all of the things we had to do.”

Though Otolith started with a motion sickness device, its co-founders hoped to make an even bigger impact by developing a device for vertigo, debilitating dizziness often caused by problems in the inner ear.

And they had a plan.

“For tinnitus or ringing in the ears, some patients get relief from a noise masker—they can still perceive their tinnitus, but the noise masker allows them to ignore the tinnitus,” Depireux explained. “So Sam, my cofounder said, ‘Why don’t we come up with a noise masker for the vestibular system?’”

That’s exactly what they did. Their novel device, worn like a headband, treats vertigo by applying localized mechanical stimulation to the vestibular system through calibrated vibrations. 

Depireux says he never would have made it this far without physics.

“My physics training really helped me,” he explained. “In physics, you have this huge problem and you have to break it down. If it’s intractable, you make it tractable, break it into small, simple things we can understand and then we can solve it.”

Promising results and personal stories

Clinical trials of Otolith’s investigational headband have yielded promising results. In the first of a series of ongoing clinical studies, 87.5% of the 40 participants reported a reduction in their vertigo within five minutes of turning on the device. But for Depireux, it’s the personal stories that are most rewarding.

“Somehow my phone number was listed as an emergency contact on clinicaltrials.gov, which I thought would be for emergencies only,” he said. “I’d have patients calling me in tears, telling me, ‘When my grandkids visit, I can finally bend down and pick them up, and it used to be that just bending down would send me into such vertigo that I would have to go to bed for days.’ Or ‘For the first time in years, I’ve been able to walk around the block.’ That’s what really motivates me.”

It's been Depireux’s goal all along—doing relevant research that changes people’s lives.

“We cannot help 100% of vertigo patients, no device does that,” he reflected. “But if we can help even half of those patients, that’s really my hope.”

Looking back on a career path that’s been anything but predictable, Depireux appreciates every challenge and setback that got him to where he is today.

“Something can feel like a failure when things go wrong, but then later you realize you really learned something from it,” he reflected. “I’m so grateful I was given the opportunity to come to the U.S. and study physics and do research in College Park, do this random walk in my career and finally end up doing something that I feel has given me great meaning in my life.”

Written by Leslie Miller

Faculty, Staff, Student and Alumni Awards & Notes

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Category: Department News

Published: Friday, September 02 2022 00:01

We proudly recognize members of our community who recently garnered major honors, began new positions and more.

Compact Electron Accelerator Reaches New Speeds with Nothing But Light

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Category: Research News

Published: Friday, September 02 2022 00:01

Scientists harnessing precise control of ultrafast lasers have accelerated electrons over a 20-centimeter stretch to speeds usually reserved for particle accelerators the size of 10 football fields.

A team at the University of Maryland (UMD) headed by Professor of Physics and Electrical and Computer Engineering Howard Milchberg, in collaboration with the team of Jorge J. Rocca at Colorado State University (CSU), achieved this feat using two laser pulses sent through a jet of hydrogen gas. The first pulse tore apart the hydrogen, punching a hole through it and creating a channel of plasma. That channel guided a second, higher power pulse that scooped up electrons out of the plasma and dragged them along in its wake, accelerating them to nearly the speed of light in the process. With this technique, the team accelerated electrons to almost 40% of the energy achieved at massive facilities like the kilometer-long Linac Coherent Light Source (LCLS), the accelerator at SLAC National Accelerator Laboratory. The paper was published in the journal Physical Review X on September 16, 2022

“This is the first multi-GeV electron accelerator powered entirely by lasers,” says Milchberg, who is also affiliated with the Institute of Research Electronics and Applied Physics at UMD. “And with lasers becoming cheaper and more efficient, we expect that our technique will become the way to go for researchers in this field.”  An image from a simulation in which a laser pulse (red) drives a plasma wave, accelerating electrons in its wake. The bright yellow spot is the area with the highest concentration of electrons. In an experiment, scientists used this technique to accelerate electrons to nearly the speed of light over a span of just 20 centimeters. (Credit Bo Miao/IREAP)

Motivating the new work are accelerators like LCLS, a kilometer-long runway that accelerates electrons to 13.6 billion electron volts (GeV)—the energy of an electron that’s moving at 99.99999993% the speed of light. LCLS’s predecessor is behind three Nobel-prize-winning discoveries about fundamental particles. Now, a third of the original accelerator has been converted to the LCLS, using its super-fast electrons to generate the most powerful X-ray laser beams in the world. Scientists use these X-rays to peer inside atoms and molecules in action, creating videos of chemical reactions. These videos are vital tools for drug discovery, optimized energy storage, innovation in electronics, and much more.  

Accelerating electrons to energies of tens of GeV is no easy feat. SLAC’s linear accelerator gives electrons the push they need using powerful electric fields propagating in a very long series of segmented metal tubes. If the electric fields were any more powerful, they would set off a lightning storm inside the tubes and seriously damage them. Being unable to push electrons harder, researchers have opted to simply push them for longer, providing more runway for the particles to accelerate. Hence the kilometer-long slice across northern California. To bring this technology to a more manageable scale, the UMD and CSU teams worked to boost electrons to nearly the speed of light using—fittingly enough—light itself.

“The goal ultimately is to shrink GeV-scale electron accelerators to a modest size room,” says Jaron Shrock, a graduate student in physics at UMD and co-first author on the work. “You’re taking kilometer-scale devices, and you have another factor of 1000 stronger accelerating field. So, you’re taking kilometer-scale to meter scale, that’s the goal of this technology.”

Creating those stronger accelerating fields in a lab employs a process called laser wakefield acceleration, in which a pulse of tightly focused and intense laser light is sent through a plasma, creating a disturbance and pulling electrons along in its wake. 

“You can imagine the laser pulse like a boat,” says Bo Miao, a postdoctoral fellow in physics at the University of Maryland and co-first author on the work. “As the laser pulse travels in the plasma, because it is so intense, it pushes the electrons out of its path, like water pushed aside by the prow of a boat. Those electrons loop around the boat and gather right behind it, traveling in the pulse’s wake.”

Laser wakefield acceleration was first proposed in 1979 and demonstrated in 1995. But the distance over which it could accelerate electrons remained stubbornly limited to a couple of centimeters. What enabled the UMD and CSU team to leverage wakefield acceleration more effectively than ever before was a technique the UMD team pioneered to tame the high-energy beam and keep it from spreading its energy too thin. Their technique punches a hole through the plasma, creating a waveguide that keeps the beam’s energy focused.

“A waveguide allows a pulse to propagate over a much longer distance,” Shrock explains. “We need to use plasma because these pulses are so high energy, they're so bright, they would destroy a traditional fiber optic cable. Plasma cannot be destroyed because in some sense it already is.”

Their technique creates something akin to fiber optic cables—the things that carry fiber optic internet service and other telecommunications signals—out of thin air. Or, more precisely, out of carefully sculpted jets of hydrogen gas.

A conventional fiber optic waveguide consists of two components: a central “core” guiding the light, and a surrounding “cladding” preventing the light from leaking out. To make their plasma waveguide, the team uses an additional laser beam and a jet of hydrogen gas. As this additional “guiding” laser travels through the jet, it rips the electrons off the hydrogen atoms and creates a channel of plasma. The plasma is hot and quickly starts expanding, creating a lower density plasma “core” and a higher density gas on its fringe, like a cylindrical shell. Then, the main laser beam (the one that will gather electrons in its wake) is sent through this channel. The very front edge of this pulse turns the higher density shell to plasma as well, creating the “cladding.” 

“It's kind of like the very first pulse clears an area out,” says Shrock, “and then the high-intensity pulse comes down like a train with somebody standing at the front throwing down the tracks as it's going.” 

Using UMD’s optically generated plasma waveguide technique, combined with the CSU team’s high-powered laser and expertise, the researchers were able to accelerate some of their electrons to a staggering 5 GeV. This is still a factor of 3 less than SLAC’s massive accelerator, and not quite the maximum achieved with laser wakefield acceleration (that honor belongs to a team at Lawrence Berkeley National Labs). However, the laser energy used per GeV of acceleration in the new work is a record, and the team says their technique is more versatile: It can potentially produce electron bursts thousands of times per second (as opposed to roughly once per second), making it a promising technique for many applications, from high energy physics to the generation of X-rays that can take videos of molecules and atoms in action like at LCLS. Now that the team has demonstrated the success of the method, they plan to refine the setup to improve performance and increase the acceleration to higher energies.

“Right now, the electrons are generated along the full length of the waveguide, 20 centimeters long, which makes their energy distribution less than ideal,” says Miao. “We can improve the design so that we can control where they are precisely injected, and then we can better control the quality of the accelerated electron beam.”

While the dream of LCLS on a tabletop is not a reality quite yet, the authors say this work shows a path forward. “There’s a lot of engineering and science to be done between now and then,” Shrock says. “Traditional accelerators produce highly repeatable beams with all the electrons having similar energies and traveling in the same direction. We are still learning how to improve these beam attributes in multi-GeV laser wakefield accelerators. It’s also likely that to achieve energies on the scale of tens of GeV, we will need to stage multiple wakefield accelerators, passing the accelerated electrons from one stage to the next while preserving the beam quality. So there’s a long way between now and having an LCLS type facility relying on laser wakefield acceleration.” 

This work was supported by the U.S. Department of Energy (DE-SC0015516, LaserNetUS DE-SC0019076/FWP#SCW1668, and DE-SC0011375), and the National Science Foundation (PHY1619582 and PHY2010511).

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Story by Dina Genkina

In addition to Milchberg, Rocca, Shrock and Miao, authors on the paper included Linus Feder, formerly a graduate student in physics at UMD and now a postdoctoral researcher at the University of Oxford, Reed Hollinger, John Morrison, Huanyu Song, and  Shoujun Wang, all research scientists at CSU, Ryan Netbailo, a graduate student in electrical and computer engineering at CSU, and Alexander Picksley, formerly a graduate student in physics at the University of Oxford and now a postdoctoral researcher at Lawrence Berkeley National Lab.