Geneva, Switzerland : In a Physics milestone, European Organization for Nuclear Research (CERN) scientists have finally observed the long-sought decay of Higgs boson. Six years after the Higgs boson was discovered, scientists at CERN have at last observed its decaying to fundamental particles known as bottom quarks, a milestone in the exploration of the “God particle,” according to a press release by CERN on Tuesday.
In a finding that caps years of exploration into the tiny particle known as the Higgs boson, researchers have traced the fifth and most prominent way that the particle decays into other particles
The discovery gives researchers a new pathway by which to study the physical laws that govern the universe.
The Standard Model of particle physics predicts that about 60 percent of the time a Higgs boson will decay to a pair of bottom quarks, the second-heaviest of the six flavors of quarks. Testing this prediction is crucial because the result would either lend support to the Standard Model, which is built upon the idea that the Higgs field endows quarks and other fundamental particles with mass, or rock its foundations and point to new physics.
This new discovery is a big step forward in the quest to understand how the Higgs enables fundamental particles to acquire mass. Many scientists suspect that the Higgs could interact with particles outside the Standard Model, such as dark matter – the unseen matter that does not emit or absorb light, but may make up more than 80 percent of the matter in the universe.
“Since the first single-experiment observation of the Higgs boson decay to tau-leptons one year ago, CMS, along with our colleagues in ATLAS, has observed the coupling of the Higgs boson to the heaviest fermions: the tau, the top quark, and now the bottom quark. The superb LHC performance and modern machine-learning techniques allowed us to achieve this result earlier than expected,” said Joel Butler, spokesperson of the CMS collaboration.
Physicists at Princeton University led one of the two main teams that today announced the detection of the Higgs particle via its decay into two particles called bottom quarks. This pathway is the last to be detected of the five main signature pathways that can identify the Higgs particle.
“We found it exactly where we expected to find it and now we can use this new pathway to study the Higgs’ properties,” said James Olsen, professor of physics and leader of the team at Princeton. “This has been a truly collaborative effort from the beginning and it is exciting to see the amplification of effort that comes from people working together.”
Long-sought because it confirms theories about the nature of matter, the Higgs particle exists only fleetingly before transforming into other, so-called “daughter” particles. Because the boson lasts only for about one septillionth of a second, researchers use the particle’s offspring as evidence of its existence.
These daughter particles are scattered among the shower of particles created from the collision of two protons at the Large Hadron Collider at the European Organization for Nuclear Research (CERN). The Higgs particle was observed for the first time in 2012 through three of the other modes of decay.
Of these lineages or decay patterns, the decay into two bottom quarks occurs most often, making up about 60 percent of the decay events from the Higgs, according to Olsen.
But at the LHC, the bottom-quark pattern is the hardest to trace back definitively to the Higgs because many other particles can also give off bottom quarks.
Quarks are tiny constituents of protons, which themselves are some of the building blocks of atoms. The bottom quark is one of the six types of quarks that make up the menagerie of particles in the “standard model” that explains matter and their interactions.
Discerning which bottom quarks came from the Higgs versus other particles has been the main challenge facing the LHC’s two Higgs detectors, the Compact Muon Solenoid (CMS) with which Olsen works, and its companion, ATLAS. The detectors operate independently and are run by separate teams of scientists.
Once produced, these bottom quarks split into jets of particles, making them hard to trace back to the original parent particles. Because of this background noise, the researchers required more data than was needed for the other pathways to be sure of their finding.
Both detectors are adept at spotting particles such as electrons, photons and muons, but are more challenged by quarks. The quarks due to their nature are not observed as free particles. Instead they are bound and appear as other particles such as mesons and baryons or decay quickly.
“It is a messy business because you have to collect all of those jets and measure their properties to calculate the mass of the object that decayed into the jets,” Olsen said.
The two detectors are massive, complex structures that sit at the end of the LHC tunnel where protons are accelerated and smashed together at high energy levels. The structures contain layers of smaller detectors arranged like layers of an onion.
The devices detect particles at each layer of the onion to reconstruct their paths. This allows researchers to trace the path of a particle back to its source in a manner analogous to following the trail of a firework’s light back to the place where the first burst occurred. By following many of these paths, the researchers can identify where and when the Higgs first formed in the proton-proton collision.
“The Higgs-to-bottom-quark decay is important because it is the most frequent decay, so a precise measurement of its rate tells us a lot about the nature of this particle,” said Christopher Palmer, an associate research scholar at Princeton.
The main challenge in detecting the bottom-quark decay mode was the amount of background bottom quarks produced by non-Higgs events. In addition to gathering more data from collisions, the researchers searched for a distinct way that the Higgs gave rise to the bottom quarks.
Olsen began working on this challenge 10 years ago, before the LHC had switched on and when physicists were running simulations on computers. Around that time, Olsen learned about a theoretical study showing that it is possible to find bottom quarks created from the Higgs recoiling off another particle, such as a Z boson or a W boson.
“It was an idea that nobody had before, to search for it in that channel, and the only question was whether it was possible experimentally and whether it really would pay off,” Olsen said. Olsen said it is exciting to see the work come to fruition. “This is a very satisfying moment.”
“This observationis a milestone in the exploration of the Higgs boson. It shows that the ATLAS and CMS experiments have achieved deep understanding of their data and a control of backgrounds that surpasses expectations. ATLAS has now observed all couplings of the Higgs boson to the heavy quarks and leptons of the third generation as well as all major production modes,” said Karl Jakobs, spokesperson of the ATLAS collaboration.
With more data, the collaborations will improve the precision of these and other measurements and probe the decay of the Higgs boson into a pair of much-less-massive fermions called muons, always watching for deviations in the data that could point to physics beyond the Standard Model.
“The experiments continue to home in on the Higgs particle, which is often considered a portal to new physics. These beautiful and early achievements also underscore our plans for upgrading the LHC to substantially increase the statistics. The analysis methods have now been shown to reach the precision required for exploration of the full physics landscape, including hopefully new physics that so far hides so subtly,” said CERN Director for Research and Computing Eckhard Elsen.
Southern Methodist University played important roles in the analysis announced by CERN Aug. 28, including:
- Development of the underlying analysis software framework (Stephen Sekula, SMU associate professor of physics was co-leader of the small group that included SMU graduate student Peilong Wang and post-doctoral researcher Francesco Lo Sterzo, that does this for the larger analysis for 2017-2018)
- Studying background processes that mimic this Higgs boson decay, reducing measurement uncertainty in the final result.
“The Standard Model is the recipe for everything that surrounds us in the world today. Sekula explained. “It has been tested to ridiculous precision. People have been trying for 30-40 years to figure out where or if the Standard Model described matter incorrectly. Like any recipe you inherit from a family member, you trust but verify. This might be grandma’s favorite recipe, but do you really need two sticks of butter? This finding shows that the Standard Model is still the best recipe for the Universe as we know it.”
Scientists would have been intrigued if the Standard Model had not survived this test, Sekula said, because failure would have produced new knowledge.
“When we went to the moon, we didn’t know we’d get Mylar and Tang,” Sekula said. “What we’ve achieved getting to this point is we’ve pushed the boundaries of technology in both computing and electronics just to make this observation. Technology as we know it will continue to be revolutionized by fundamental curiosity about why the universe is the way it is.
“As for what we will get from all this experimentation, the honest answer is I don’t know,” Sekula said. “But based on the history of science, it’s going to be amazing.”
Research at the LHC is funded by the U.S. Department of Energy and the National Science Foundation.
Image : In experiments conducted at the Large Hadron Collider, a team including Princeton University researchers has identified a long-sought pathway by which the Higgs boson decays into two other particles, known as bottom quarks. The Higgs boson is produced with another particle called a Z boson. The Higgs boson decays to bottom quark jets (shown in blue), and the Z boson then decays into an electron and a positron (shown in red).
Image Credit : Image courtesy of the CMS Collaboration