Observation of an excess at the top quark pair production threshold in the single-lepton channel

The universe is built from fundamental particles, the smallest known building blocks of matter. Among these, the top quark holds a unique and fleeting place. It is the heaviest of all known elementary particles, a fact that makes it incredibly unstable. Because it is so massive, it decays, or falls apart, almost instantly after it is created. In fact, it decays so quickly—trillions of times faster than the time it takes for other particles to bind together into larger structures—that it usually vanishes before it can form any kind of composite object. This rapid decay means that, unlike other heavy particles, the top quark typically does not have time to become part of a larger family of particles before it disappears.

However, theoretical physics suggests that under very specific conditions, two top quarks might briefly hold hands, forming a short-lived, quasi-bound pair before they decay. Scientists call this hypothetical state "toponium." It is not a stable particle you could catch in a jar, but rather a fleeting moment where the two heavy quarks orbit each other just long enough to leave a distinct fingerprint in the data. Finding this state would be a major triumph for our understanding of the strong force, the fundamental interaction that holds atomic nuclei together. It would confirm that even the most unstable particles can exhibit bound-state behavior, offering a rare glimpse into the quantum mechanics of the heaviest matter in the universe.

A researcher working with the CMS experiment at the Large Hadron Collider in Switzerland has now found strong evidence for this elusive state. Using data from billions of proton collisions, they have observed an unexpected excess of events right at the energy threshold where top quark pairs are created. This finding, published in a new report, provides an independent confirmation of a similar discovery made previously by the same researcher in a different type of collision event. The result suggests that these fleeting top quark pairs are indeed forming a bound state, consistent with predictions from quantum theory.

The search was conducted using data collected between 2016 and 2018, representing a massive dataset of 138 inverse femtobarns of collisions at an energy of 13 tera-electronvolts. The researcher focused on a specific type of collision outcome known as the single-lepton channel. In these events, a pair of top quarks is produced, and one of them decays into a charged particle (an electron or a muon) and a neutrino, while the other decays into a pair of quarks that turn into jets of particles. This channel is particularly valuable because it occurs about six times more often than the previously studied "dilepton" channel, where both top quarks decay into charged particles. This higher frequency provides a much larger sample of events to analyze, increasing the chances of spotting a subtle signal.

Detecting the toponium signal in this channel is not straightforward. The main challenge is that the signal is easily blurred by the complex environment of the collision. When top quarks are created, they are often moving very fast, and their decay products can be difficult to distinguish from the background noise of other particle interactions. Furthermore, the precise mass of the top quark is a critical factor; even a tiny uncertainty in this value can shift the predicted location of the signal, making it hard to find. To overcome these hurdles, the researcher developed a new way of looking at the data. Instead of relying solely on the total mass of the top quark pair, which is sensitive to measurement errors, they used the relative velocity between the two top quarks. This variable is more robust against experimental uncertainties and provides a clearer view of the region where the bound state is expected to appear.

In addition to velocity, the researcher examined the angles at which the decay products fly apart. The way these particles are distributed in space carries information about the spin and parity of the top quark pair. If the quarks are forming a bound state, their spins should be correlated in a specific way that differs from the random correlations seen in ordinary top quark production. By combining the velocity data with these angular measurements, the researcher created a powerful tool to separate the potential signal from the background. They used advanced computer simulations and machine learning techniques to reconstruct the events, matching the detected particles back to their parent top quarks with high precision.

The analysis revealed a significant excess of events in the data compared to the standard model prediction. The standard model, which describes the known laws of physics, predicts a smooth distribution of events as the energy of the collision increases. However, the researcher observed a distinct bump in the data right near the threshold where the top quark pair is created. This excess corresponds to a total production cross section of 5.1 ± 0.9 picobarns. The statistical significance of this observation is 6.1 standard deviations, a level of certainty that in particle physics is considered a definitive observation. This means there is less than a one-in-a-billion chance that the observed excess is a random fluctuation of the background noise.

The observed signal is consistent with a simplified model of a color singlet pseudoscalar toponium state. This model, motivated by nonrelativistic quantum chromodynamics, describes the top quark pair as a bound state with specific quantum properties. The researcher also tested an alternative model based on a different theoretical approach involving a Green's function reweighting. Both models yielded consistent results, with the alternative model showing a similar enhancement in the same mass range. The fact that two different theoretical descriptions both align with the data strengthens the conclusion that the excess is real and not an artifact of a specific calculation method.

This discovery is particularly important because it confirms a previous finding made by the same researcher in the dilepton channel. The fact that the signal appears in two different decay channels, with different backgrounds and reconstruction challenges, provides a robust and independent verification. It rules out the possibility that the earlier result was a statistical fluke or a specific error in the analysis of one channel. The new analysis also demonstrates that the signal is not an artifact of the specific way the data was processed, as the results hold up even when using different variables and reconstruction techniques.

The researcher was careful to account for all possible sources of error. They considered uncertainties in the energy scale of the detectors, the efficiency of identifying particles, and the theoretical calculations used to predict the background. Even with these uncertainties, the signal remained clear and distinct. The leading sources of uncertainty were related to the modeling of the top quark mass and the theoretical predictions for the production of top quark pairs. However, these uncertainties were not large enough to explain away the observed excess. The fit to the data showed that the background-only hypothesis, which assumes no toponium exists, is excluded with high confidence.

The implications of this finding extend beyond just confirming the existence of toponium. It provides a new laboratory for studying the strong force in a regime where it is not fully understood. The top quark is unique because it is the only quark that decays before it can hadronize, meaning it does not form stable composite particles like protons or neutrons. Observing a bound state of top quarks challenges our understanding of how the strong force operates at such short timescales and high energies. It suggests that the force is strong enough to bind these heavy particles together, even for a fleeting moment, before they decay.

The study also highlights the power of modern particle physics experiments. By combining vast amounts of data with sophisticated analysis techniques, researchers can probe the universe at scales that were previously inaccessible. The ability to reconstruct complex events with high precision and to distinguish subtle signals from overwhelming backgrounds is a testament to the advancements in detector technology and computational methods. This work opens the door for further studies of the top quark and its interactions, potentially leading to new insights into the fundamental structure of matter.

In summary, the CMS collaboration has observed a significant excess of events near the top quark pair production threshold in the single-lepton channel. This excess is consistent with the formation of a toponium state, a quasi-bound pair of top quarks. The observation, with a significance of 6.1 standard deviations, confirms a previous finding in the dilepton channel and provides strong evidence for the existence of this elusive state. The result is a major step forward in our understanding of the strong force and the behavior of the heaviest known elementary particle. It demonstrates that even the most unstable particles can exhibit bound-state behavior, offering a new window into the quantum world.