In 2012, scientists on the CMS and ATLAS experiments at CERN’s Large Hadron Collider discovered the Higgs boson. Now, they’re looking for two Higgs bosons born from a single collision.

“The Higgs self-coupling has implications in understanding the origin and ultimate fate of the universe,” says Nan Lu, a postdoc at the California Institute of Technology supported by the US Department of Energy’s Office of Science. “This process is a lighthouse that guides the future of particle physics.”

Higgs bosons are the physical manifestations of the Higgs field, an invisible medium that is woven into the fabric of spacetime.

“Elementary particles obtain their masses through interaction with this Higgs field,” Lu says. “Without the Higgs field, all elementary particles would be massless and traveling at the speed of light. The universe would not look the same as it does today.”

The LHC can generate Higgs bosons by colliding protons and transmitting the stored energy into the Higgs field—much like a pebble striking the surface of a river and transferring its kinetic energy into a ripple. This process happens at a rate of about one in a billion collisions.

On even rarer occasions, this energy transfer can generate not one but two Higgs bosons at the same time. These Higgs boson twins could help scientists characterize a largely unmeasured facet of the Higgs mechanism: the shape of the Higgs potential.

The boson, the field and the Higgs potential

The Brout-Englert-Higgs mechanism—to use its full name—consists of three interlinking facets: the Higgs boson, the Higgs field and the Higgs potential.

“You can think about it like a river,” says Irene Dutta, a graduate student at Caltech. “The Higgs boson is a ripple, the Higgs field is the water, and the Higgs potential is the shape of the riverbed.”

The Higgs boson—the ripple—gives scientists a glimpse of the otherwise invisible Higgs field—the water. But underneath it all is the Higgs potential—the riverbed, or a mathematical function that determines the different possible energy states of the Higgs field.

“A river might seem calm and flat,” Dutta says, “but there could be a waterfall that leads to much lower ground that we cannot see from where we are.”

If the potential dropped and the Higgs field spontaneously fell into a lower energy state, the universe as we know it would evaporate.

“The current calculations indicate that we could be living in a false vacuum,” says Thong Nguyen, a graduate student at Caltech. “This means that at any moment, the Higgs field could tunnel through the potential barrier to a true negative-energy vacuum, creating an expanding singularity bubble that eventually swallows up the entire universe.”

(The universe has yet to disappear in its 14 billion years, at least, and physicists do not anticipate that it will happen anytime soon.)

The origin of matter

According to Nguyen, the Higgs field has already fallen from a high energy state into a lower one once before.

“Right after the Big Bang, when the universe was a hot and dense soup, the Higgs field was perfectly symmetrical and did not interact with other particles,” Nguyen says. “But as the universe cooled, the Higgs field underwent a phase transition. Symmetry broke, and then particles were able to interact with the Higgs field to acquire mass.”

This first transition could be the missing link in one of the biggest mysteries in physics: the dominance of matter over its equal-and-opposite counterpart, antimatter.

“Right after the Big Bang, we should have had equal amounts of matter and antimatter,” Lu says. “Today, there is a large amount of matter and almost no antimatter.”

The evolution of the Higgs field during the primordial universe could be responsible for this imbalance.

“If it’s a smooth transition, as our models predict, then the entire Higgs field would have cooled homogeneously, like water slowly freezing into ice,” Nguyen says. “But if it’s an abrupt transition, then bubbles could have formed and eventually expanded to fill the entire universe.”

These bubbles in the Higgs field could have serendipitously sheltered the small excess of matter that eventually formed everything.

Higgs self-coupling

The Higgs potential regulates the behavior of Higgs bosons, including their interactions with one another. If scientists can find and study Higgs-boson pairs, then they can work backwards and indirectly probe the Higgs potential’s shape.

“First we need to measure the rate of Higgs-boson pair production,” Lu says. “Then we want to measure the properties of these two Higgs bosons.”

The scarcity of this process makes it a classic needle-in-a-haystack problem.

“During Run II [which ended in December 2018], the LHC would have generated about 7.5 million Higgs bosons,” Dutta says. “But it would have only produced about 4500 Higgs-boson pairs.”

Higgs bosons are notoriously difficult to separate from look-alike subatomic processes. Even for a very clean signature—a Higgs boson decaying into two photons—there are 10 identical background events for every real Higgs boson.

“We’re completely swamped by the background,” Dutta says. “The di-Higgs production process is not an easy observation to make and our best chance of seeing it is with the High-Luminosity LHC upgrade,” now in full progress. 

Once this upgrade is completed later this decade, future runs will increase the total number of potential collisions scientists have to study by at least a factor of 10.

The overall US contributions to the LHC experimental research program and HL-LHC upgrade are funded by the US Department of Energy and the National Science Foundation. With the HL-LHC only a few years away, scientists are already honing their analysis methods.

They have begun to apply machine-learning techniques inspired by natural language processing. Nguyen is developing and training machine-learning algorithms to recognize the subtle differences between Higgs boson signatures and look-alike background processes (much like a natural-language-processing algorithm separates similar sounding words like “close” and “clothes.”)

“We can treat each particle like a word in a sentence,” Nguyen says.

Currently, scientists are working with only a few signatures for Higgs-boson pairs, but they hope to study more complex signatures over the next few years.

“This research is still in the early stages but moving very fast,” Lu says.

Latin American scientists have completed their roadmap for the next five years of physics research, marking the end of a two-year grassroots effort to plan for the future of experiments and partnerships in the region.

“One of the main goals was to find where collaboration could generate more impact for Latin American contributions to physics,” says Marcela Carena, head of the Theoretical Physics Department at the US Department of Energy’s Fermi National Accelerator Laboratory. “At this moment, there’s been no other Latin American body to guide such a strategic plan for the scientific community.”

Carena, originally from Argentina, was one of the 23 members of the Latin American Strategy Forum for Research Infrastructure (LASF4RI) preparatory group, the team responsible for collecting input from the Latin American and international theoretical and experimental physics community in three pilot areas of research: high-energy physics, astrophysics and cosmology. The scientists on the team represented 10 countries in the region—Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Mexico, Peru, Paraguay and Venezuela—as well as Japan, Europe (as represented by CERN) and the United States.

A plan for science and infrastructure

The final report contains 10 recommendations, beginning with a push for continued support of current and future projects in cosmology and astrophysics, ranging from those already in operation—such as Pierre Auger Observatory in Argentina—and those planned for the near-term—such as the BINGO telescope in Brazil and Vera Rubin Observatory in Chile—to those planned for start-up more than a decade from now—such as the South American Gravitational-Wave Observatory. 

Latin America is known for its clear skies and powerful telescopes; it provides the location for crucial observations that form many astrophysics and cosmology experiments. 

“There are a series of projects that are already being built in Latin America and will become operational in 2025 or 2026,” says Marta Losada, originally from Colombia and more recently a professor of physics and the dean of science at New York University Abu Dhabi, who chaired the preparatory group. “So that’s a high priority.”

Another high priority named in the report is ANDES, an underground laboratory proposed to be built in a tunnel connecting Argentina and Chile. The laboratory, protected from cosmic radiation that hits the planet’s surface, would allow scientists around the world to investigate dark matter and neutrinos, as well as conduct experiments in biology and geology. 

“You need to start building the capacity and the expertise for those experiments now, if you want to build them in the region,” Losada says.

The strategy identifies as areas of strength for the Latin American high-energy physics community both its involvement in collider physics, through international experiments at the Large Hadron Collider at CERN, and its involvement in neutrino physics, through the international Deep Underground Neutrino Experiment, managed by Fermilab. 

“This is important for both Europe and the United States, because Latin American scientists have been contributing for decades,” Carena says. “But this plan will allow a higher level of coordination and will allow Latin American scientists to be better contributors to international large-scale experiments.”

The report also describes the need to improve the region’s computing infrastructure and internet connectivity. The report says it is “fundamental to all experimental efforts” and will help scientists process the vast amounts of data generated at research sites, such as the under-construction Vera Rubin Observatory and the planned Cherenkov Telescope Array, both in Chile.

Support for stronger connections

By developing research infrastructure, encouraging global scientific participation and offering training opportunities, the report authors write that these efforts will help inspire future Latin American scientists and work to impede the “‘brain draining effect’ in the region.” 

The LASF4RI group also mentioned the need to help scientists align their research with funding options, which can be tough to navigate. “Latin America is a complex environment with many countries and many different funding agencies,” Carena says.

Prior to publishing the final report, the authors presented their recommendations to government officials and leaders of funding agencies in Latin America on October 27, 2020. Afterward, the officials at the fourth Iberoamerican Science and Technology Ministerial Meeting issued a declaration.

Carena says the declaration provides national and international validation for the work of local scientists, labs and universities. Losada adds that “it was important to reflect the size of the growing community of Latin American physicists, and for us to demonstrate how you should continue to protect that investment in knowledge and resources.” 

A regional roadmap

Latin American physicists began brainstorming what might go into a formal regional strategy—inspired by the longer-term plans developed for Europe and the United States—at professional conferences going back to 2016. And in early 2019, LASF4RI held its first workshops to create such a plan. 

“Before this, we really hadn’t even worked out our main areas of research in Latin America or our strengths very clearly. These are really important, deep questions,” Losada says.

Through extensive consultation and meetings, the team worked with physicists across disciplines to come up with the final recommendations. In 2020, the committee adjusted their meetings as the pandemic forced them to replace international in-person gatherings with Zoom calls.

The preparatory group reached out to scientific societies, gathered 40 scientific white papers, and then submitted a draft report to a high-level review by scientists outside the preparatory group. 

With an endorsement from the reviewers, the LASF4RI group finalized their report in November 2020.