Everything we can perceive and touch, from animals to stars, is fundamentally constituted by two subatomic constituents: quarks, responsible for the atomic nucleus, and electrons, which form the clouds of negative electrical charge around the nucleus. In addition to these, there are other particles, such as photons, which are the basic unit of light, essential for technologies like lamps and Wi-Fi. Neutrinos, on the other hand, have a practically undetectable presence, constantly passing through the human body without causing any effect.
Currently, these particles are considered the smallest existing entities, the primary elements of reality, being impossible to subdivide according to current knowledge. However, the organization of these building blocks of the Universe presents singular characteristics.
The quarks that compose atoms are classified into two categories, called up and down. Although the terms 'up' and 'down' are informal nicknames, they have no meaning in the quantum context. There are six types of quarks in total: up and down, plus charm and top (heavier than up) and strange and bottom (heavier than down).
The top quark stands out for having a mass 85 thousand times greater than the up quark, making it the most massive particle within the Standard Model. Despite this extensive family of quarks, the vast majority existing is composed of the up and down types, which form the matter around us.
These quarks are observed under extreme conditions, such as in tests conducted at the Large Hadron Collider (LHC), the world's largest particle accelerator. This equipment consists of a 27 km underground ring located on the border between France and Switzerland. When a collision between particles results in an unusual entity, such as a bottom quark, it persists for a billionth of a second before undergoing a process called 'decay', generating other particles.
Quarks never occur in isolation; they appear in groups, either in triads (like in protons and neutrons that structure atoms) or in pairs. One of these pairs is the B0 meson, formed by a bottom quark and a down quark (where one of the components may be antimatter). Normally, when this meson decays, it decomposes into common particles with no notable relevance. However, occasionally, it transforms into a pair of muons, particles that are essentially heavier electrons, with approximately 200 times more mass, an event that occurs rarely, once every 10 billion occurrences.
Physicists have been trying to record this unusual phenomenon since 1984. The first confirmation was presented last Tuesday (3) at CERN, the European laboratory operating the LHC. If confirmed, this detection will represent the rarest particle decay ever recorded.
The result comes from LHCb, one of the four major detectors of the collider, whose name includes 'b' for *beauty*, another nickname given to the bottom quark. To locate this specific signal, a team of about 20 researchers, led by Harry Cliff from the University of Cambridge, analyzed data collected between 2011 and 2018, supplementing it with a new set of data from 2024 and 2025. This new batch of data was obtained after an upgrade that allowed the detector to capture information up to five times faster, aided by machine learning algorithms to filter noise from billions of collisions.
It is important to note that the finding has a statistical significance of 3.2 sigma. This means that the probability of the signal being merely a chance, a variation in the data that simulates the phenomenon, is approximately 1 in 1,500. Although this seems low, in particle physics, a discovery is only consolidated starting from 5 sigma, which corresponds to about 1 in 3.5 million. For this reason, researchers use the term 'evidence' instead of 'discovery', and the work still awaits review by other scientists.
Despite the rarity of the event, the Standard Model, the theory that describes all known particles, predicts the frequency of this decay with accuracy. Any deviation from what is predicted by the detectors would indicate a flaw in the Model, pointing to new scientific mysteries. This is compared to a little-used road: if an unexpectedly high volume of traffic is recorded, it suggests the existence of unmapped access points.
These unknown particles could reside in a scale called the 'zeptoverse', a domain of distances on the order of a zeptometer, which is one billionth of a trillionth of a meter. While the LHC can investigate distances of about 100 zeptometers, rare decays like this enable even deeper investigation, like listening into an inaccessible room. At these levels, answers to fundamental physics enigmas, such as the nature of dark matter and the imbalance between matter and antimatter in the Universe, might be hidden.
Currently, the Standard Model has no problems, as the decay follows the established equations. However, this confirmation is also valuable because it helps eliminate possibilities where new physics could be present. LHCb will continue collecting data until the early 2030s, and a future upgrade may increase the collection rate tenfold. Subsequently, expectations will fall on the successor to the LHC, which is not expected to become operational for several decades. Perhaps it is in this zeptoverse that the answer to why the Universe produced such limited-use quarks lies, something physicists still do not know, but which may be about to be revealed.
