Their work covers both data analysis and the construction, operation and updating of the detector itself. All its researchers are actively involved in studies related to the top quark and the Higgs boson, as well as in the search for dark matter or supersymmetry signals, among other possible indications of new physics beyond the Standard Model.
The ATLAS experiment, one of the great LHC detectors, has taken an important step in understanding how the Higgs boson, the particle that gives mass to others, interacts with the quark top, the heaviest known elementary particle. For the first time, scientists from the ATLAS collaboration have conducted an optimized search specifically to study the joint production of a single top quark and a Higgs boson (called tH), an extremely rare process within the Standard Model and not yet observed experimentally. This result brings the scientific community closer to measuring more precisely the coupling between both particles, a key interaction of the Standard Model for particle physics.
The associated production of the Higgs boson with a pair made up of a top quark and an antiquark top has already been observed in ATLAS and CMS, but producing a Higgs boson together with a single top quark remains one of the least explored processes. This mechanism is particularly interesting because, due to how the coupling of the Higgs boson to the top quark intervenes in it, comparing its production rate with that predicted by the Standard Model could reveal signs of the so-called CP symmetry violation, a phenomenon related to the difference between matter and antimatter. Since the Standard Model does not fully explain why the Universe is composed almost exclusively of matter, any clues in this direction are particularly relevant.
For the first time, scientists from the ATLAS collaboration have conducted an optimized search specifically to study the joint production of a single top quark and a Higgs boson (called tH), an extremely rare process within the Standard Model and not yet observed experimentally. This result brings the scientific community closer to measuring more precisely the coupling between both particles, a key interaction of the Standard Model for particle physics
The doctors trained at IFIC, Pablo Martínez Agulló and Jesús Guerrero Rojas, under the direction of the research staff Susana Cabrera Urbán and Carlos Escobar Ibáñez (who also coordinates the ATLAS working group responsible for these analyses and others studying different Higgs boson disintegrations, as well as combining all experimental channels to increase sensitivity) have played an essential role in this work, which is part of their doctoral theses. Their analysis has focused on the disintegrations of the Higgs boson in different types of particles, which has made it possible to explore several experimental channels sensitive to tH production. Several leptons, a type of light particle, can be produced in these collisions: the most common are electrons, muons (heavier electrons) and tau leptons (heavier and shorter-lived).
Although the tH process is extremely rare (with an inclusive effective section of 74.3 fb) and has not yet been observed experimentally, the new ATLAS analysis has succeeded in establishing the strictest limits obtained so far for this process. Specifically, ATLAS states that the production rate of the tH process in LHC collisions does not exceed about 14 times the prediction of the Standard Model with 95% statistical reliability. This breakthrough not only demonstrates the enormous experimental challenge of studying such a weak signal among millions of collisions, but also confirms that the techniques developed (from the combined use of multiple channels to advanced artificial intelligence methods for lepton identification, Higgs boson decay reconstruction or signal-noise discrimination) are working at the level required for future searches.
In alternative scenarios where the interaction between the Higgs and the top quark would have a different behavior than predicted by the Standard Model, such as those in which this interaction would change by inverting matter for antimatter (what is known as a CP-impar component), the experiment obtains an even more restrictive upper bound with respect to the Standard Model scenario of 2.4. Although these figures indicate that their experimental observation cannot yet be confirmed, they represent an essential advance because they demonstrate that the sensitivity of the experiment is improving rapidly and that the methods developed are already reaching the level required for future data campaigns, especially in the high luminosity LHC (HL-LHC), allow to study with much greater sensitivity the coupling between the Higgs boson and the top quark, one of the most important interactions to understand why particles have mass and what role the Higgs boson plays in the evolution of the Universe.
Source: IFIC