The top quark, the heaviest and most short-lived elementary particle known, has long been thought to decay too quickly to form bound states. However, a new result from the CMS Collaboration, presented this week at the Rencontres de Moriond conference, strengthens last year's observation that top quarks may, in fact, briefly pair up with their antimatter counterparts. This fleeting bound state – known as toponium – would be the most massive composite particle ever observed, completing the family of quark–antiquark states bound by the strong nuclear force.

Most matter around us is made of atoms, in which electrons cling to protons through the electromagnetic force. But protons themselves are not elementary. They belong to a broad family of composite particles called hadrons, in which quarks are held together by the strong nuclear force. Among them, the simplest are pairings of a quark with its own antiquark, which provide an especially clean window on the workings of the strong force. For decades, such states have been known for every type of quark but the most elusive: the top.

First discovered more than 30 years ago at the Tevatron accelerator near Chicago, the top quark has been extensively studied ever since, with experiments at the LHC going so far as to measure quantum entanglement between top quarks and antiquarks. Even when produced alongside its antiquark, the top typically decays before any bound state can form. Yet the hundreds of millions of top quark–antiquark pairs produced at the LHC, effectively making it a top-quark factory, provide such an enormous dataset that the rarest phenomena can leave a detectable trace.

The first hints of toponium appeared in searches for heavy Higgs-boson-like particles that could decay into a top quark–antiquark pair. An unexpected excess of collision events was observed at a mass close to twice the mass of the top quark, which is more characteristic of a bound state rather than a new fundamental particle. Detailed studies by the CMS and ATLAS experiments confirmed this excess using events in which both top quarks decay into leptons (electrons or muons).

The new CMS study approaches the problem from a different angle, examining events in which one top quark decays into a bottom quark, a charged lepton and a neutrino while the other decays into quarks that produce sprays, or “jets”, of particles. “Isolating the signal in this decay channel was challenging,” says Otto Hindrichs, a researcher at the University of Rochester who developed a new AI-assisted technique to reconstruct these collision events.

“Instead of reconstructing the mass of the top quark–antiquark pair directly, we focused on the relative velocity of the top quark and antiquark,” explains Yu-Heng Yu, a graduate student involved in the analysis. “If they form a bound state, their relative velocity should be much smaller than when they are produced independently,”

These new techniques proved highly effective. They resulted in the observation of an excess with a statistical significance of more than five standard deviations – the gold standard for a discovery in high-energy physics. The result provides a new, statistically independent confirmation of toponium production.

“Toponium is heavier than the heaviest known atomic nucleus, oganesson, making it the most massive bound state ever observed,” says Regina Demina, leader of the CMS group at the University of Rochester. “Its discovery deepens our understanding of the strong nuclear force and its ability to bind the fundamental constituents of matter.”

Find out more on the CMS website.

According to the theory of supersymmetry, there is a mirror world of hypothetical particles that could help resolve several physics puzzles, such as the surprisingly small mass of the Higgs boson and the nature of dark matter. The ATLAS Collaboration at the Large Hadron Collider (LHC) has conducted new searches for these so-called supersymmetric (SUSY) particles using machine-learning techniques. The results of these searches, presented this week at the Moriond conference, have placed some of the strongest bounds yet on the properties of SUSY particles.

Supersymmetry proposes that each particle in the Standard Model has a “superpartner”. The higgsino is the SUSY counterpart of the Higgs boson and is the subject of many SUSY searches. But detecting the higgsino, if it exists, is far from simple. The higgsino would not appear on its own but as a mixture of other SUSY particles, creating states known as neutralinos and charginos. Theorists predict that the lightest neutralino could be stable and, therefore, a strong candidate for dark matter. The other, heavier neutralinos and charginos would decay into this stable SUSY particle. However, these decays are expected to produce very little energy and the resulting low-energy particles would be extremely difficult to detect.

By deploying machine-learning techniques, the ATLAS Collaboration has been able to significantly improve the experiment’s sensitivity to low-energy particles. ATLAS now reports the results of two new searches for signs of SUSY particles in analyses of data from the LHC’s second run, which was collected between 2015 and 2018.

One of these searches involved hunting for signs of a disappearing track left by a chargino decaying into a stable neutralino, which is invisible to the detectors, and a low-energy pion. The pion follows a highly curved trajectory that is extremely difficult to identify in a busy proton–proton collision, causing the chargino’s track to “disappear”. The ATLAS Collaboration additionally searched for signs of heavier neutralinos decaying into the lightest and only stable neutralino and two low-momentum leptons, such as electrons. The researchers deployed neural networks to search deep into the low-momentum region of pions and leptons to find signs of them being produced through the decay of SUSY particles.

No signs of these SUSY particles were observed in either of these searches. However, these results have now set some of the most stringent limits yet on the masses and lifetimes of charginos and neutralinos, superseding the longstanding limits set by the Large Electron–Positron Collider, the LHC’s predecessor.

These limits help guide future searches for SUSY particles at the LHC and the High-Luminosity LHC. The search continues for the mirror world of SUSY.

The LHCb experiment at CERN’s Large Hadron Collider (LHC) has discovered a new particle consisting of two charm quarks and one down quark, a similar structure to the familiar proton, but with two heavy charm quarks replacing the two up quarks of the proton, thus quadrupling its mass. The discovery, presented at the ongoing Moriond conference, will help physicists better understand how the strong force binds protons, neutrons and other composite particles together.

Quarks are fundamental building blocks of matter and come in six flavours: up, down, charm, strange, top and bottom. They usually combine in groups of twos and threes to form mesons and baryons, respectively. Unlike the stable proton, however, most of these mesons and baryons, which are collectively known as hadrons, are unstable and short-lived, making them a challenge to observe. Producing them requires smashing together high-energy particles in a machine such as the Large Hadron Collider (LHC). These unstable hadrons will quickly decay, but the more stable particles that are produced as a result of this decay can be detected and the properties of the original particle can therefore be deduced.

Researchers have used this approach many times to find new hadrons, and the new particle just announced by the LHCb Collaboration brings the total number of hadrons discovered by LHC experiments up to 80.

“This is the first new particle identified after the upgrades to the LHCb detector that were completed in 2023, and only the second time a baryon with two heavy quarks has been observed, the first having being observed by LHCb almost 10 years ago,” says LHCb Spokesperson Vincenzo Vagnoni. “The result will help theorists test models of quantum chromodynamics, the theory of the strong force that binds quarks into not only conventional baryons and mesons but also more exotic hadrons such as tetraquarks and pentaquarks.”

In 2017, LHCb reported the discovery of a very similar particle, which consists of two charm quarks and one up quark. This up quark is the only difference between this particle and the new one, which has a down quark in its place. Despite the similarity, the new particle has a predicted lifetime that is up to six times shorter than its counterpart, due to complex quantum effects. This makes it even more challenging to observe.

By analysing data from proton–proton collisions recorded by the LHCb detector during the third run of the LHC, the LHCb Collaboration observed the new baryon with a statistical significance of 7 sigma, well above the threshold of 5 sigma required to claim a discovery.

“This major result is a fantastic example of how LHCb’s unique capabilities play a vital role in the success of the LHC,” says Mark Thomson, CERN Director-General. “It highlights how experimental upgrades at CERN directly lead to new discoveries, setting the stage for the transformative science we expect from the High-Luminosity LHC. These achievements are only possible thanks to the exceptional performance of CERN’s accelerator complex and the teams who make it all work and to the commitment of the scientists on the LHCb experiment.”

Further information:
LHCb presentation at Moriond is available here.
LHCb news article.