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Science

Scientists may be on the verge of smashing the Standard Model of particle physics

TOKYO -- Hopes are rising that a previously unknown particle has been discovered by the Large Hadron Collider, the particle accelerator that lies under the France-Switzerland border near Geneva. The LHC was the site of the discovery of the Higgs boson, for which the scientists who predicted its existence won the 2013 Nobel Prize in physics.

Data obtained from experiments conducted through last year indicates the possible existence of a new particle with roughly six times the mass of the Higgs boson. In tests this year -- which began in May -- the facility will try to determine whether or not the particle actually exists. While a number of theories have already emerged to explain the nature of this particle, its mere existence, if proven, could rewrite the existing standard theory of elementary particles.

The data suggesting the existence of the new particle was unveiled at a conference held in December 2015 by the European Organization for Nuclear Research, or CERN, which operates the LHC. Atlas and CMS, two international research groups that discovered the Higgs boson in 2012, each exhibited data suggesting that a particle with an energy -- and thus a mass -- of 750 gigaelectronvolts (GeV) had been born in experiments involving head-on collisions of protons accelerated to nearly the speed of light. The new particle immediately decayed, and two photons were observed shooting out from it. By measuring the energies of these two photons, researchers were able to calculate the mass of the underlying particle. This was the same method used to discover the Higgs boson.

A mass of 750GeV is considerably larger than that of any other elementary particle discovered so far, including the top quark (173GeV) and the Higgs boson (125GeV).

Extremely high statistical precision is needed in experiments looking for new particles with accelerators. This precision is measured in standard deviations, or sigma, for short. To claim that a new particle has been discovered, the observed results must have a certainty level of greater than five sigma, which means the probability of the results being a statistical fluke is around 1 in 3 million.

The Atlas experiment had a certainty level of just under 4 sigma, while that of the CMS experiment was around 3 sigma. But while each experiment on its own fell short of achieving the status of "discovery," the fact that data showing the presence of a particle with virtually the same mass was discovered through two independently executed experiments has raised hopes that it does indeed exist.

On May 9, CERN announced it had restarted operation of the LHC for the first time in half a year. It will be in operation through early November and will compile an enormous volume of experimental data, increasing the number of proton collisions from 300 trillion last year to 2,500 trillion, roughly 8 times as many.

University of Tokyo professor Shoji Asai, a joint representative of the Atlas Japan group, is not placing any bets on the outcome. "To be honest, whether or not this will be confirmed as a new particle is a 50-50 proposition," he said. "There is still a high likelihood of statistical scattering. Because sufficient data will have been accumulated within one month of the start of experiments, the conclusion of whether it is a new particle or not should be out in July or August."

Fundamental rewrite

Immediately after information about the new particle began to circulate, papers discussing its possible nature started cropping up. These can be broadly classified into three schools of thought, according to Asai.

The first argues that the new particle is in the same category as the Higgs boson. The current standard theory is that there are 17 types of elementary particles, including quarks, neutrinos and the Higgs boson. There is a theory that expands on this to say that partner particles, known as supersymmetric particles, exist for each of the elementary particles. Hence, the existence of a heavier supersymmetric partner to the Higgs boson has already been predicted. If the new particle turns out to be a supersymmetric particle, it would qualify as a discovery.

A collision experiment in the CMS experiment group at the LHC. A variety of particles are created through proton beam collisions (yellow lines). When a 750GeV particle was created, photons shot off in two directions (long green lines). (Courtesy of CERN)

The second possibility is that the data reveals the existence of "extra dimensions" beyond the familiar three-dimensional space. The superstring theory, a prominent candidate as the ultimate explanation of all particles and forces, holds that all elementary particles including the graviton, which conveys gravitational force, are tiny, vibrating strings and that the universe is composed of 10 dimensions. The reason we are only aware of three dimensions, according to the theory, is that the remaining ones are small and curled up. Strings vibrating in this small space could be observed as particles with masses about the same as that seen in the recent round of accelerator experiments. If the existence of extra dimensions can be confirmed, that alone would be a major first.

The third possibility is that the new particle is a "complex Higgs boson" made up of multiple elementary particles bound together. According to this line of thinking, the Higgs boson itself is not an elementary particle but rather consists of two unknown elementary particles, known as technifermions, stuck together. This would rewrite the Standard Model, built on 17 types of elementary particles, from the ground up.

Meanwhile, Asai's group is moving forward with its data analysis with a fourth possibility in mind: that the particle with a mass of 750GeV decays in two stages. In such a case, one conceivable scenario is that a new particle -- possibly dark matter -- is produced midway through that process.

The already-lively debate over the nature of this particle will likely continue to heat up. As Asai puts it, "If the new particle's discovery is real, its impact will be so enormous that the discovery of the Higgs boson will pale in comparison."

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