Thousands of protons accelerated to nearly the speed of light will collide today with an energy never before achieved by a particle accelerator. It will be the return of the Large Hadron Collider, the LHC, in Geneva, Switzerland, trying to answer some of the big questions about the Universe.
Operated by the European Laboratory for Particle Physics, this facility is the largest experiment ever built on Earth. Within its 27-kilometer ring, conditions are mimicked a split second after the Big Bang, the explosion that created the universe 13.7 billion years ago. At that time there were no atoms, only their indivisible components: the elementary particles. To this day, there are many unanswered 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 being completely annihilated in the struggle between matter and antimatter.
The large physics machine has been standing still since 2019, and the start-up process began in 2021, which culminates today with the observation of the first proton collisions at maximum power. The LHC will begin collecting scientific data on these particles at a record energy of 13.6 teraelectronvolts. This third set of experiments will begin after 4:30 p.m. Peninsula Time and will run continuously for nearly four years. The number of collisions, decays, 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 goals of the LHC in this new phase will be to generate millions of Higgs bosons. Without this particle, the universe as we know it could not exist, since it gives up its mass to the other elementary particles when it interacts with them. The Standard Model theory, formulated in the 1970’s, provides the exact value of each of these interactions. Any discrepancy between theory and what is observed at the LHC may reveal previously unknown mechanisms, forces or particles of nature. “We need to take the most accurate X-ray image of the Higgs boson ever made to confirm that it behaves as we expect,” summarizes Mario Martínez, a physicist at Atlas, one of the LHC’s large detectors.
In search of a “new physics”
Last April, a US experiment announced one of the biggest anomalies on record: the mass of the W boson is not what theory predicts. Last year, the LHC itself and another experiment in the USA also observed discrepancies in the behavior of the muon, another elementary particle. The LHC will likely be able to more accurately measure the properties of these particles and provide a definitive verdict on the existence of “new physics”. If so, it would be a much more important discovery than that of the Higgs boson, as it could explain what 95% of the universe is made of, made up of dark matter and dark energy totally unknown to us humans.
The theory, which describes the behavior of conventional matter, involves 17 types of elementary particles that appeared shortly after the Big Bang in three consecutive generations, each with more mass than the previous one. The increase in energy at the LHC from 13 teraelectronvolts to currently 13.6 teraelectronvolts will make it possible for the first time to study the decay of the Higgs into second-generation particles such as muons. “The mass differences between the different generations of elementary particles are enormous and we don’t know why,” explains Alberto Casas, a researcher at the Institute for Theoretical Physics in Madrid. “There are very strong reasons to believe that there is new physics and a theory superior to the current one to explain it. For the first time, the LHC will be able to look for it in uncharted energy levels,” he adds.
Casas is an expert on the problem of dark matter, which makes up 27% of the cosmos. Although invisible, physicists are convinced of its existence through indirect observations, such as the gravitational pull it exerts on stars and galaxies. So far, no experiment has been able to prove it directly. “Physicists know very well what dark matter is not, but we have no idea what it is. Of all the experiments trying to study it, the LHC is the only one that could produce dark matter particles,” emphasizes Casas.
The great advantage of the LHC is its ability to accumulate a large number of collisions between protons and their decay into elementary particles and to obtain results about their masses with high statistical reliability. In this “intermediate phase”, which starts today, the main task will be to carry out high-precision measurements of the Higgs and the rest of the particles, explains Alberto Ruiz of the Physics Institute of Cantabria. “Once this phase is complete, the LHC will stop improving its detectors and continue to increase the number of collisions it generates,” he says. In 2029 the accelerator will be working again and the amount of data collected to date will increase tenfold. From now until then, the possibility of discovering “new physics” is open, concludes Ruiz.
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