Into the Unknown
At the world most sophisticated lab, thousands of physicists are working together to find a path beyond the standard model
What are we made up of? At the most fundamental level, one can ask the same question as what the universe is made up of. Currently, the standard model of particle physics is our best answer to the fascinating question. According to the standard model, all the matter in the universe, including galaxies, stars, planets, and even you, is made up of 25 elementary particles. The development of the standard model began in the 1960s and was completed mainly by the late 1970s. Besides the fermions and gauge bosons, there is only one more particle in the standard model: the Higgs boson, which gives masses to the other elementary particles.
Higgs boson was the last elementary particle to be discovered. However, it was proposed independently by several researchers in the early 1960s. After nearly half a century of particle-chasing, physicists hunted the elusive particle in 2012 at the Large Hadron Collider (LHC), the largest and by far the most powerful particle accelerator on earth. “By colliding protons at high energy and high luminosity, this powerful accelerator makes it possible to probe the matter on new scales and to thoroughly test the Standard Model,” says Prof. Yahya Tayalati, a physicist at University Mohammed V in Rabat, Morocco, who was involved in LHC project for two decades. The discovery slotted into place the final missing keystone of the standard model.
Through the victorious years of particle physics, researchers in other fields of physics have also tried their hands on the foundations of reality. In the 1930s, astrophysicists realized that galaxy clusters contain a lot more mass than all visible matter combined can possibly account for. Apparently, a new type of “dark matter” was needed to explain the observations. Since then, evidence of dark matter has piled up to the point that now no one doubts its existence. Still, no one knows what dark matter is made of. Astrophysicists say it is a type of particle that has no interaction with ordinary matter, a mysterious one that neither absorbs nor emits light. However, the horrifying fact is that dark matter is five times more abundant than visible matter.
In 1998, cosmologists surprisingly discovered that the expansion of the universe is accelerating. They can mathematically show that the mysterious accelerator, called “dark energy,” is nothing but the energy carried by empty space. Besides that, there is one more thing we know about dark energy for sure: 68 percent of the total matter-energy content of the universe consists of dark energy. In other words, we are living in a universe with a composition of 68% dark energy, 27% dark matter, and only 5% ordinary matter. All our knowledge about the building blocks of the matter is limited to that 5% ordinary matter.
Despite its enormous success, the standard model leaves several fundamental questions unanswered. “A major problem with the standard model is related to the origin of dark matter and dark energy, constituting nearly 95% of the energy density of the universe, remained totally unexplained, and the standard model fails at providing a viable candidate for the observed abundance of dark matter in the universe,” Tayalati says. However, this is not the only problem with the standard model. One of the most fundamental questions left open by this model is the gravitational interaction, which is totally ignored in the description of fundamental interactions. “All this and many other arguments suggest that this model is only an effective theory of a more fundamental model manifesting itself at higher energy,” he says.
Tayalati involvement with ATLAS, the largest general-purpose particle detector experiment at LHC, goes back to the project’s early days. He spent twenty years of his career in ATLAS, covering many topics ranging from hardware projects and detector operations to software development and physics analysis and measurements. His first involvement was the ATLAS Liquid Argon Presampler, in which he has contributed to all steps related to the construction, commissioning, and operation of this subsystem. The Presampler, which is used for photons and electrons detection, has proven to be very efficient and it is now widely used in many ATLAS physics measurements.
One of the recent achievements of Tayalati and his colleagues in ATLAS collaboration was the observation of Light-by-light (LbyL) scattering for the first time in 2019. This process is completely forbidden in classical electrodynamics but appears in quantum electrodynamics. The LbyL scattering is an extremely rare process which makes its measurement very difficult and inaccessible. Many attempts with other devices have been proposed without any success.
Regarding the ultra-peripheral high energy heavy-ion collisions at the LHC, the probability of this process gets enhanced, and researchers found an excellent opportunity to observe that. They hoped to detect the telltale signal with a simple topology of two scattered photons at the final state while the heavy ions escape the collisions. Eventually, using data collected by ATLAS, Tayalati and his colleagues reported 59 events while they expected only 12 from the background, and this was interpreted as the first observation of the LbyL scattering of photons.
They have also measured the probability of this process and what they obtained is very close to the theoretical predictions. It was a clear demonstration of how LHC can perfectly work as a photons collider. What makes this process very interesting is the fact that scattered photons could couple to any new particle, providing a promising way to probe physics beyond the standard model. “We in ATLAS explored the LbyL scattering to search for axion-like particles, which is a great candidate to dark matter. The study provides the most stringent limits to date on axion-like particle production,” Tayalati says.
Another fundamental question left unanswered by the standard model is about the mass of neutrinos. Besides their unique properties, what makes them the favored particle for many physicists, Tayalati is no exception, are their implications on cosmology and astrophysics. Neutrinos are the messengers transmitting information from the early stages of the universe. Detection of these long-time travelers could help in the understanding of the evolution of the universe. Furthermore, measurements from neutrino telescopes coupled to gravitation waves and photons detections have opened a new area of Multi-Messengers astrophysics in recent years.
Tayalati started his career as an experimental high-energy physicist with a Ph.D. degree from the University of Mohammed First, Oujda, Morocco. At that time, he proposed a solution to one of the problems in neutrinos physics, which was the observed deficit of neutrinos coming from the sun. Later he pursued the field by involvement in the ANTARES project, a neutrino detector residing 2.5 km under the Mediterranean Sea. “I have been involved in the early preparation and deployment of the ANTARES telescope,” he says. Due to his efforts, Morocco officially joined this international collaboration in 2011. Since then several students graduated with the ANTARES project. “I was the convener of the exotic physics group and with the Moroccan students we derived the strongest experimental limits on the existence of Magnetic Monopoles,”he says.
In recent years, Tayalati has started a new collaboration with KM3NeT, which is a large research infrastructure, in construction with the technology and the knowledge acquired from its predecessor ANTARES. “I convinced three universities in Morocco to join this international effort and to form an Astroparticle cluster in Morocco,” he says. This cluster allowed launching a pilot project, M1, to set up and operate a production line of optical modules for the KM3NeT neutrino telescope in Morocco.
Before the LHC first run, many physicists hoped that this fascinating machine would reveal some clues about what might lie beyond the standard model. However, everything seems standard so far. Tayalati believes events beyond the standard model are quite rare, so isolating and investigating those events needs a massive amount of data.
“Up to now, we have collected only 10% of data planned for the LHC program; this was sufficient to constraint or to reject many exciting theoretical models that introduce physics beyond the standard model. Certain versions of supersymmetry, for example, are less and less plausible,” he says. However, he thinks it is very early to judge the situation, and we have to wait for the subsequent runs.”
Tayalati believes the breakthrough will be “detecting a signal that can be interpreted as a candidate for dark matter or graviton. This will open a huge challenge for both experimenters and theorists to confirm such a discovery and interpret it within a universal model.”