Experiment demonstrates decay of the Higgs boson in components of matter | AGÊNCIA FAPESP

Evidence corroborates the hypothesis that the boson generates the mass of the particles that constitute matter. The discovery was announced in Nature Physics by a group that included Brazilians (image: CMS)

Experiment demonstrates decay of the Higgs boson in components of matter

July 23, 2014

By José Tadeu Arantes

Agência FAPESP – The direct decay of the Higgs boson to fermions – corroborating the hypothesis that the Higgs boson generates the mass of the particles that constitute matter – has been proven at the Large Hadron Collider (LHC), the giant experimental complex maintained by the European Organization for Nuclear Research (CERN) on the border between Switzerland and France.

An announcement of the discovery has just been published in the journal Nature Physics by the group of researchers associated with the Compact Muon Solenoid (CMS).

On the CMS international team, composed of nearly 4,300 members (including physicists, engineers, technical personnel, students and administrative staff), there are two groups of Brazilian scientists: one headquartered at the Center for Scientific Computing (NCC) at São Paulo State University (Unesp) in São Paulo and another at the Brazilian Center for Physics Research of the Ministry of Science, Technology and Innovation (MCTI) and the State University of Rio de Janeiro (UERJ), in Rio de Janeiro.

“For the first time ever, the experiment measured the decay of the Higgs boson to bottom quarks and tau leptons. And it showed that they are consistent with the hypothesis that the masses of these particles are also generated through the Higgs mechanism,” said physicist Sérgio Novaes, professor at Unesp, to Agência FAPESP.

Novaes heads up the Unesp group in the CMS experiment and is the principal investigator in the thematic project “São Paulo Research and Analysis Center” (Sprace), which is associated with the CMS and supported by FAPESP.

The new finding strengthened the conviction that the object whose discovery was officially announced on July 4, 2012, is indeed the Higgs boson. The Higgs boson is the particle that confers mass to other particles, according to the Standard Model, which is the theoretical framework that describes the components and the interactions that are supposedly the basis of the material world.

“Since the official announcement of the discovery of the Higgs boson, a lot of evidence has been collected that shows that the particles generated correspond to the predictions in the Standard Model. The studies basically involved its [the putative Higgs boson’s] decay to other bosons (particles responsible for the interactions of matter), such as photons (bosons that interact electromagnetically) and the W and the Z (weak interaction) bosons,” Novaes said.

“However, even after accepting that the Higgs boson was responsible for generating the masses of W and Z, it was not clear if it would also generate the masses of fermions (particles that constitute matter, such as quarks, which make up protons and neutrons, and leptons such as electrons) because the mechanism is slightly different, involving what’s called the ‘Yukawa coupling [interaction]’ between these particles and the Higgs field,” he went on to say.

The researchers looked for direct evidence that the decay of the Higgs boson in these fields of matter obeyed the formula in the Standard Model. However, this was no easy task: because the Higgs boson confers mass, it has the tendency to decay to the more massive particles, such as the W and Z bosons, which have masses nearly 80 and 90 times greater than protons, respectively.

“Besides this, there are other complicating factors. In the case of the bottom quark, for example, a bottom-antibottom quark pair can be produced many other ways in addition to the decay of the Higgs. So these other possibilities needed to be filtered. And in the case of the tau lepton, the probability of decay of the Higgs to this particle is very small,” Novaes explained.

“Just to get an idea, for every trillion collisions conducted at the LHC, there is one Higgs boson event. Of these, fewer than 10% correspond to the decay of the Higgs to a pair of taus. Furthermore, a pair of taus could also be produced much more frequently in other ways, such as from a photon,” he said.

To convincingly prove the decay of the Higgs boson to a bottom quark and tau lepton, the CMS team needed to collect and process an immense amount of data. “That is why our article in Nature took so long to be published. It was literally harder than finding a needle in a haystack,” Novaes said.

But what was interesting, according to the researcher, was that even in cases such as these, when it was thought that the Higgs might contradict the Standard Model, this did not occur. The experiments were very much in agreement with the theoretical predictions.

“It’s always surprising to find agreement between experiment and theory. For years, the Higgs boson was considered to be nothing more than a mathematical artifice that would provide internal coherence to the Standard Model. Many physicists never even thought it would be discovered. They searched for this particle for nearly half a century and ended up accepting it only due to the absence of an alternative theory to account for all the predictions with the same margin of error. So these results we’re now obtaining at the LHC are really spectacular. We’re usually surprised when science goes wrong. But the real surprise is when it goes right,” Novaes said.

“In 2015, the LHC is expected to run with twice as much energy. The expectation is that it will reach 14 teraelectronvolts (TeV) (14 trillion electronvolts). At this level of energy, the proton bunches will accelerate at more than 99.99% of the speed of light. It’s exciting to think about what we may discover,” he said.

The article “Evidence for the direct decay of the 125 GeV Higgs boson to fermions”(doi:10.1038/nphys3005), with collaboration from the CMS, may be read at http://nature.com/nphys/journal/vaop/ncurrent/full/nphys3005.html.



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