Double trouble Higgs | symmetry magazine

In 2011, physicist Javier Duarte and friends walked the surface roads above the 17-mile radius of the Large Hadron Collider, an underground particle accelerator on the French-Swiss border. Eventually they completed the tour, but it took them four increasingly agonizing hours.

“The first half was very comfortable, but the second half was much tougher,” he says. “I hadn’t trained properly and wasn’t super prepared.”

A new pair of running shoes or some high-carb snacks might have improved Duarte’s performance—but only marginally. Finding his stride and making good time took Duarte more than just refinement. He had to fundamentally change his attitude towards running.

Three years and an extensive training program later, Duarte tackled a much longer run, finishing the Chicago Marathon in three hours and 15 minutes.

Duarte once again finds himself in a marathon, one with a hard time limit and an elusive finish line. He’s looking for evidence of Higgs self-coupling, a process so rare scientists fear they’ll never be able to detect it at the LHC.

Scientists initially predicted that even after combining the data from the two general-purpose detectors at the LHC, and even with an upgrade from the LHC to the high-luminosity LHC, “we would still be a little short of fully observing this phenomenon concretely,” says Duarte, who is now a professor at the University of California, San Diego, and whose research is supported by the US Department of Energy.

To find this rare process, which can only be observed when Higgs particles are produced in pairs, scientists at the ATLAS and CMS experiments needed a new approach. Today, they’re rethinking how they filter, process, and analyze their data — a change they hope will get them across the finish line.

Higgs boson, Higgs field

The Higgs boson is an elementary particle discovered in 2012 by the ATLAS and CMS experiments. Ever since the Higgs discovery, scientists have tried to learn everything they can: its mass, its properties, its lifetime, and how it interacts with other elementary particles. But one thing scientists have yet to see is how the Higgs interacts with itself. “This is the missing piece,” says Liza Brost, an ATLAS physicist at the DOE’s Brookhaven National Laboratory.

Seeing how the Higgs interacts with itself will give scientists a window into the Higgs field, an invisible medium that permeates the entire universe and is responsible for giving fundamental particles their mass. While scientists have made many measurements of the Higgs boson, the properties of the Higgs field are still mysterious.

“We want to learn more about the Higgs field and specifically the shape of its potential and which Higgs field configurations cost the least energy,” says CMS physicist Cristina Mantilla Suarez, a postdoctoral fellow with Lederman at the Fermi National Accelerator Laboratory. “It sounds theoretical, but it has enormous implications.”

According to Duarte, the shape of the Higgs potential is a kind of cosmic blueprint by which the energy released in the Big Bang solidified into the stable matter we see today.

“The Higgs is intimately connected to the evolution of the universe, how the universe became what it is, and its ultimate destiny,” says Duarte. “The Higgs connects big cosmic questions with microscopic scales.”

A rare event

Although the Higgs field exists everywhere, Higgs bosons have only a one in a billion chance of materializing in collisions in the LHC. The collisions convert the energy of the colliding particles into mass and instantaneously create new particles.

After more than a decade of operation, scientists estimated that the LHC produced about 7.5 million Higgs bosons. But they estimate that in quadrillions of collisions, the LHC has only produced about 4,500 Higgs boson pairs. This is the only way scientists can see how the bosons interact with each other.

“The production of Di-Higgs is so incredibly rare, but we know it exists,” says Suarez. “The challenge is that it can be very easily confused with other processes that look the same.”

Higgs bosons are so short-lived that scientists cannot see them directly. Instead, scientists use gigantic detectors to capture and measure the particles that the Higgs boson produces as it decays.

In the search for Higgs boson twins, both CMS and ATLAS have turned their attention to the Higgs boson’s most common decay product: bottom quarks, which are produced about 58% of the time.

Unfortunately for Higgs researchers, bottom quarks are a popular decay path for more than just Higgs bosons. “There are many ways to make two bottom quarks,” says Duarte.

Tearing out the tiny fraction of bottom quarks that originally came from a Higgs boson – unlike anything else – is remarkably difficult, as a Where’s Waldo? Book in which all the characters are dressed in red and white stripes and are far too small to see.

Two ways of looking at a Higgs

For this reason, researchers from CMS and ATLAS study specific subsets of bottom quark events.

Although Higgs bosons are microscopic, they are not exempt from certain laws of motion. “If I throw a soccer ball to my friend while walking past him, the ball flies almost perpendicular to my path,” says Duarte. “But if I’m doing that from a car going at 60mph then that football will have a lot of forward momentum.”

Applied to subatomic scales, this means that Higgs bosons with a lot of forward momentum will eject bottom quarks with a lot of forward momentum. Duarte and his CMS colleagues realized this could help them.

Because of their unique interactions, high-momentum Higgs bosons are more easily produced than other lighter particles that also produce bottom quarks. So it is more likely that “boosted” bottom quarks come from Higgs bosons.

“We get fewer signal events, but also fewer background events,” says Duarte. “The ratio is going in our favor. These amplified bottom quarks from the Higgs also look very different from the other sources, which we can use to our advantage.”

In contrast, Brost and her colleagues at ATLAS looked for the lower end of the energy spectrum. Brost studies double Higgs events, in which one Higgs boson turns into a pair of bottom quarks and the other Higgs boson turns into two photons, the particle form of light. Although this process is very rare – only 0.26% of all double Higgs decays – it is also phenomenally clear.

“This is how we designed the calorimeters in our detector; to see photons,” says Brost. “This was one of our strongest channels.”

rise of the machines

Scientists are using new sophisticated computing tools to narrow the search even further.

Traditionally, physicists isolate interesting particle collision events using what Brost calls a “cut and count” technique. She compares it to cutting off mold (undesirable background events) from cheese (valuable signal events).

“It’s an issue of how often we’re willing to slice versus how much cheese we’re losing,” says Brost.

More restrictive requirements help scientists remove unwanted background events. But it also means they lose more of their signal. To minimize signal loss, physicists have turned to machine learning. A machine learning algorithm can learn the differences between signal and background and remove the uninteresting events with fine precision.

“For the first time we applied machine learning techniques [on ATLAS data], it blew everything out of the water,” says Brost. “It was so much more delicate than anyone expected. Then we had to spend the next year or two trying to prove to everyone that it wasn’t wrong.”

At CMS, Suarez was developing machine learning algorithms for other projects when she realized they could be applied to finding Higgs pairs.

When Suarez first implemented a new algorithm, she was cautiously optimistic. “New experimental tools need to be validated and tested,” says Suarez. “This tool was so new that it wasn’t widely available. It took a lot of work to incorporate it into our analysis and apply it to the data.”

She worried that if she couldn’t get the new software to work, she would have wasted several months without showing anything. But according to Suarez, “sometimes you have to take the risk”.

Hot on the trail

The risk was worth it. Using the new software and machine learning techniques, Suarez and her colleagues discovered that their analysis is just as sensitive as the techniques that CMS scientists have been honing for years.

“Any new method can become a significant improvement if we combine them all together, especially in the long term,” says Suarez. “We are constantly developing new ways to study Higgs self-coupling earlier and with less data. The sooner we know how strongly the Higgs interacts with itself, the sooner we can answer many more questions.”

These new methods also help the ATLAS researchers. Brost and her colleagues have been performing back-of-the-envelope calculations to see how their improved particle reconstruction and data filtering will affect their search for duplicate Higgs bosons during the LHC’s recently launched Run 3.

“We scribbled on the back of the envelope and we’re temptingly close,” says Brost. “It’s an exciting time.”

To move from evidence to discovery in this realm, scientists will predict that they will still need the power of the High-Luminosity LHC, which is scheduled to become operational in 2029. It will increase the collision rate by at least a factor of 5 over a decade of planned operations. With so much more data, scientists will be able to study in detail how Higgs particles interact with each other.

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