Thousands of protons accelerated to almost the speed of light will collide today with an energy never before achieved by a particle accelerator. It will be the return to action of the Large Hadron Collider, the LHC, in Geneva (Switzerland), which tries to answer some of the big questions about the universe.
This facility operated by the European Laboratory for Particle Physics is the largest experiment ever built on Earth. Within its 27-kilometer-diameter ring, conditions that existed fractions of a second after the Big Bang, the explosion that created the universe 13.7 billion years ago, are emulated. At that time there were no atoms yet, only their indivisible components: the elementary particles. There are still many questions about what happened then so that the elementary particles began to unite to form a luminous universe with stars, galaxies and habitable planets instead of completely annihilating themselves in a clash between matter and antimatter.
The great physics machine had been stopped since 2019, and in 2021 the start-up process began, culminating today, with the observation of the first proton collisions at maximum power. The LHC will begin taking scientific data on these particles at a record energy of 13.6 teraelectronvolts. This third batch of experiments will begin after 4:30 p.m. peninsular time and will last almost four years of uninterrupted operation. The number of collisions, disintegrations and other subatomic interactions will be 20 times greater than during the first, which culminated in the discovery of the Higgs boson just 10 years ago.
One of the main objectives of the LHC in this new stage will be to generate millions of Higgs bosons. Without this particle, the universe as we know it could not exist, since it gives its mass to the rest of the elementary particles when interacting with them. The Standard Model theory formulated in the 1970s provides the exact value of each of these interactions. Any deviation between theory and what is observed at the LHC can reveal hitherto unknown mechanisms, forces or particles of nature. “We have to get the most precise X-ray of the Higgs boson that has ever been done to confirm that it behaves as we expect,” summarizes Mario Martínez, a physicist at Atlas, one of the LHC’s large detectors.
Last April, a US experiment announced one of the largest anomalies recorded to date: the mass of the W boson is not what theory predicts. The previous year, the LHC itself and another experiment in the US also observed discrepancies in the behavior of the muon, another elementary particle. The LHC is likely to be able to measure the characteristics of these particles more precisely and deliver a final verdict on the existence of “new physics.” If there is, it would be a much more important discovery than that of the Higgs boson, since it could begin to explain what 95% of the universe is made of, composed of dark matter and dark energy that we humans are completely unaware of.
The theory that describes the behavior of conventional matter includes 17 types of elementary particles that appeared moments after the Big Bang in three successive generations, each one with more mass than the previous one. The increase in energy at the LHC —which goes from 13 teraelectronvolts to the current 13.6— will make it possible to study for the first time the decomposition of the Higgs into second-generation particles, such as muons. “The differences in mass between the different generations of elementary particles are enormous and we don’t know why,” explains Alberto Casas, a researcher at the Institute of Theoretical Physics in Madrid. “There are very strong reasons to think that there is a new physics and a theory superior to the current one to explain it. For the first time the LHC is going to be able to search for it at unexplored energy levels”, he adds.
Casas is an expert on the problem of dark matter, which makes up 27% of the cosmos. Although it is invisible, physicists are convinced of its existence by indirect observations, such as the pull of gravity it exerts on stars and galaxies. So far no experiment has been able to detect it directly. “Physicists know very well what dark matter is not, but we have no idea what it is. Among all the experiments that try to study it, the LHC is the only one that could generate dark matter particles”, highlights Casas.
The great asset of the LHC is its ability to accumulate a large number of collisions between protons and their decomposition into elementary particles and obtain results on their masses with high statistical reliability. In this “intermediate” phase that begins today, the main task will be to make high-precision measurements of the Higgs and the rest of the particles, explains Alberto Ruiz, from the Physics Institute of Cantabria. “Once this phase is finished, the LHC will be shut down to improve its detectors and further increase the number of collisions it produces,” he details. In 2029 the accelerator will work again and will multiply by 10 the amount of data accumulated so far. Starting today and until then, the possibility of discovering “new physics” is open, concludes Ruiz.
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