A group of scientists from DESY and Universität Hamburg in collaboration with the University of Rochester in the US might be on the trail of something new in the data from the CMS detector, one of the two giant multi-purpose detectors at the Large Hadron Collider at CERN in Geneva. Focusing on LHC collisions that produce pairs of top quarks, they have found an interesting effect in the data that could point to something never seen before.

The top quark is the heaviest elementary particle in the Standard Model of particle physics. Because it is so heavy and decays so quickly, it is quite unique in the world of particle physics because it’s the only quark to be observed on its own – all other quarks can only ever be seen in bound states, i.e. forming new particles with other quarks. The top quark also shares a strong connection with the Higgs boson, which is responsible for giving mass to all elementary particles. Thus, the top quark plays a key role in exploring the Higgs boson itself and determining whether the Higgs field is more complex than we currently think—especially when it comes to finding the existence of possible heavier versions of the Higgs boson.

In their analysis, the team of Christian Schwanenberger, DESY scientist and professor at Universität Hamburg, and Alexander Grohsjean, scientist at Universität Hamburg, compared the CMS proton-proton collisions with the state-of-the-art theoretical predictions. “We observed an unexpected excess in the production of pairs of top quarks and their antiquarks near the mass threshold, where the available energy is just enough to create top quark and top antiquark,” explains Grohsjean. The effect was first seen in the data from 2016 and has become even more significant  having analysed also  the data recorded in 2017 and 2018. “Does the top quark form a bound state with its own antiparticle before decaying? We are not sure yet what we are seeing, but we are pretty sure it’s not just a statistical fluctuation,” Schwanenberger says.

Science at the LHC has an inbuilt proofing mechanism: there are two large multi-purpose detectors, CMS and ATLAS. In principle, discoveries can only be claimed when both detectors observe the same things. However, the team of DESY scientist Katharina Behr from the ATLAS collaboration does not observe a significant effect in their data. Their analysis is optimised for a slightly different range of top-quark energies, and so far cannot confirm the excess their CMS colleagues see. “The discussions among the two teams comparing our results, our hypotheses, and the theoretical models we use will be extremely exciting and have just started…,” says Laurids Jeppe, PhD student at DESY within the excellence cluster “Quantum Universe.”

The key to this analysis is a property of particles known as 'spin,' which can be thought of as an intrinsic form of angular momentum. Spin can have values of 0, ½, or 1. The Higgs boson, discovered in 2012, has a spin of 0, classifying it as a scalar particle. Heavier Higgs-like particles, predicted by some theories, could also decay into pairs of top quarks and may either be scalar (like the Higgs) or pseudoscalar, a distinct type of particle with different properties.

In this analysis, measuring spin-sensitive observables revealed that the observed excess in data aligns more with the pseudoscalar hypothesis than the scalar one. This raises crucial questions: Could this be evidence of a completely new fundamental particle, or might it represent a bound state of a top quark and an antiquark—both expected to behave as pseudoscalars? Could it be just a tricky effect due to an imperfect modeling of the background which involves very challenging calculations? Or, perhaps it's something even more unexpected, challenging our current understanding of particle physics

It has long been thought that the top quark is too short-lived to form bound states, unlike lighter quarks. However, this possibility isn’t entirely ruled out, especially in the threshold region, where top quarks and antiquarks move very slowly relative to each other.

“More theoretical work is needed to model the properties of this system reliably,” says Afiq Anuar, DESY postdoc. “Our next step will be to explore the origin of this fascinating new effect. Exciting times lie ahead!” 

If you'd like to know more read this paper by the CMS collaboration