Researchers detect Higgs boson coupling with top quark

The detection of Higgs boson-top quark interaction at the LHC confirms theoretical predictions of the Standard Model of particle physics. It was only possible thanks to the very high level of energy reached by the LHC.

By José Tadeu Arantes  |  Agência FAPESP – The much anticipated Higgs boson-top quark coupling has finally been observed in the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, at CERN on the Franco-Swiss border. 

The event was detected independently by CMS and Atlas, the main teams working with the LHC, and announced at a joint press conference held in Bologna, Italy. An article has also been published in Physical Review Letters.

The result is a robust confirmation of the accuracy of the Standard Model of particle physics, which has been constructed collectively since the early 1960s.

“The Higgs boson participates in the process that produces the mass of all particles, so it was expected to interact with particles in proportion to their mass. In other words, the heavier the particle, the more it would interact with the boson. This highly specific property is unique to the Higgs boson, according to the Standard Model. Investigating experimentally whether this actually does happen is therefore a very strong way to corroborate the Standard Model,” said Sérgio Novaes, Full Professor at São Paulo State University (UNESP) in Brazil and leader of the group of researchers from São Paulo State who are members of the CMS international collaboration. 

“With light particles, coupling is small and hard to measure,” Novaes told Agência FAPESP. “So there were great expectations about the coupling of the Higgs boson with the top quark, which is a very heavy particle, even heavier than the Higgs boson itself, with a mass exceeding 172 GeV/c². At long last, we succeeded in detecting and measuring this interaction. We drew the conclusion that, as predicted by the Standard Model, the Higgs boson does in fact couple with the top quark in proportion to its mass. It was a major confirmation of the Standard Model.”

Observation of the Higgs boson-top quark interaction was only possible due to an increase in the LHC’s energy. In this interaction, a collision of two protons creates a top quark-top antiquark pair, each with more than 172 GeV/c², and a Higgs boson with approximately 125 GeV/c², which corresponds to almost the mass of 500 protons. Thus, at the collider’s current energy level of 13 TeV (13 trillion electron volts), the proton-proton collision produces a mass equivalent to 500 protons, and the rest of the initial energy is manifested as the energy of the particles produced. Here, it is worth recalling that energy converts into mass according to Einstein’s famous equation E = mc², where E is energy, m is mass, and c is the speed of light in a vacuum.

In addition, the greater the collider’s energy, the better the definition between two observed points. At the LHC’s current energy level, it is possible to distinguish between two points located only 10^-18 m apart. For the sake of comparison, this distance is a billion times smaller than the scale on which nanotechnology operates (10^-9 m).

Theoretical adventure

The Higgs boson – named for the British physicist who proposed its theoretical existence, Peter Higgs, born in 1929 and winner of the 2013 Nobel Prize for Physics – was incorporated into the Standard Model in the 1960s to solve an abstract theoretical problem: the model needed to contain an item that gave mass to particles that had to have mass and at the same time to remain “renormalizable”, i.e., capable of making predictions. 

This requirement remained a dilemma until US physicist Steven Weinberg – winner of the 1979 Nobel Prize for Physics with Pakistan’s Abdus Salam and Sheldon Glashow, another US scientist – had the idea of adding the so-called “Higgs mechanism” to the model.

“There was no experimental evidence of the Higgs boson’s existence,” Novaes said. “Proposing it was more a theoretical adventure than an experimentally verifiable hypothesis, so much so that it took 45 years for the particle to be finally detected and announced, on July 4, 2012.”

The difficulty of experimental confirmation is easy to understand. With a mass of approximately 125 GeV/c², more than 133 times the mass of a proton, the Higgs boson is the second most massive particle in the Standard Model after the top quark. It can be produced only for a tiny fraction of a second in extremely high-energy contexts — such as those supposed to have existed just after the Big Bang or those now achieved by the LHC.

“During that period of 45 years, no other hypothesis arose to endow particles with mass and at the same time explain the interaction between them. I’ve worked on this since my master’s. It’s a huge pleasure for me to have participated in the detection of the Higgs boson in 2012 and to see another confirmation of this theoretical proposition now,” said Novaes, currently at CERN in Geneva, Switzerland.

Origin of mass

The statement that the Higgs boson gives particles mass is sometimes misinterpreted because it may conjure up a picture of a particle delivering mass to another in a concrete event, which is not at all the case.

The best tool available to describe this level of nature is field theory. In quantum mechanics, particles are not microscopic bodies, as they are in classical physics, but quantum field excitations. Each particle is actually the quantum of a certain field. The photon is the quantum of the electromagnetic field. The electron is the quantum of the electron field; the Higgs boson is the quantum of the Higgs field; and so on.

“The Higgs field permeates all space, and its quantum is the Higgs boson. It manifests itself through Higgs bosons, just as the electron field manifests itself through electrons and the electromagnetic field manifests itself through photons. According to the Standard Model, particles are given mass by the Higgs field. When particles manifest themselves in space, they interact with the Higgs field. The greater the interaction, the greater the mass,” Novaes explained.

For example, while the up and top quarks are identical in charge (2/3) and spin (1/2), they differ enormously in mass. The top quark’s mass is almost 80,000 greater, and its interaction with the Higgs field is proportional.

“The fact that the Higgs coupling constant is proportional to the mass of the particles with which it interacts is a universal prediction of the Standard Model,” Novaes said. “This prediction had already been corroborated in the case of lighter particles. Now, the coupling with the top quark further reinforces the model’s effectiveness in describing the elementary particles and their interactions.”

The detection of the Higgs boson-top quark coupling required the surmounting of great experimental difficulties. One is that the three particles that result from the collision (the top quark, top antiquark, and Higgs boson) decay very rapidly into other objects. The top quark decays into the W boson and the bottom quark. The W decays further into other particles. 

The bottom quark is copiously produced in proton collisions, so it is a major challenge to distinguish the bottom quark originated by the top quark from a background highly abundant in bottom quarks. Moreover, the Higgs boson also decays into various objects. All this decay happens in a context in which some 40 interactions are proceeding at the same time.

“The final state detected is very complex and requires fantastic big data engineering so that the signal of interest can be extracted from this superabundant background. It’s the old story of looking for a few needles in a haystack,” Novaes said.

The “haystack” truly is colossal. Every 25 billionths of a second during the LHC’s activity, two beams, each with 100 billion protons, collide. These collisions in the LHC generate the largest amount of data ever produced on the face of the Earth.

The article “Observation of tt H production” (doi: https://doi.org/10.1103/PhysRevLett.120.231801) by A. M. Sirunyan et al. (CMS Collaboration) is published at journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.231801.