The model that describes fundamental particles and interaction forces in physics, known as the Standard Model, provides a comprehensive description of natural phenomena. Nevertheless, it leaves several questions unanswered and it remains incomplete: it excludes gravity and is not able to provide a description of how the world is functioning at extremely high temperatures. All the things we see around us are made of electrons, protons and neutrons. Protons and neutrons, in turn, are made of even smaller elementary particles called up and down quarks. The family of elementary particles however contains much more than quarks, it includes many other types of particles that can only be seen through a number of peculiar interaction processes. The study of elementary particles and their interactions thus represents a great tool in the construction of an ultimate model that would provide us with a complete description of nature.

What we learnt from smashing protons together

One of the most straightforward and fun ways to see what is inside of an object is to smash it with another one. If you smash them really hard, they would shatter into smaller elementary particles. Banging particles against one another can lead to the creation of a top quark, the heaviest particle of all. It was discovered in high energy proton-antiproton collisions at the Tevatron collider at the Fermilab laboratory in 1995. Since then, the top quark has been extensively studied in order to precisely determine its main parameters, such as mass and charge. Another intriguing particle, called the Higgs boson and for a long time the last missing piece of the Standard Model, was finally observed in proton-proton collisions at the Large Hadron Collider (LHC) at CERN in 2012. This resulted in Francois Englert and Peter Higgs receiving the Nobel Prize, as they predicted its existence almost fifty years earlier. The Brout-Englert-Higgs boson is the key ingredient in ‘granting mass’ to all known elementary particles (more about this below). Besides the unraveling of its basic properties, one of the fundamental questions is how it interacts with the other particles: does it behave in a consistent way as determined by the Standard Model or is there some room for anomalous effects, that cannot be explained within this model?

The Standard Model provides a comprehensive description of natural phenomena. Nevertheless, it leaves several questions unanswered: it excludes gravity and it fails to describe how the world is functioning at extremely high temperatures. (Picture: Wikimedia Commons)

Top quark’s walk in the Higgs field. Image: Kirill Skovpen

TOP QUARK is The king of the forest

While chasing possible answers to the unresolved questions of our universe, one can easily spot the very distinctive tracks left by top quarks in the forest of elementary particles. Quarks, as well as many other elementary objects, acquire mass through their interaction with the Higgs field, and the strength of this interaction is expected to be proportional to the mass of the particle. The mass of the top quark is nearly five orders of magnitude larger than the mass of the lightest quark in the Standard Model and it exceeds the mass of the runner-up, the bottom quark, by almost 40 times. This makes our top quark the uncontested king of the forest. The extremely large mass of the top quark relative to other particles’ masses suggests that there is a big chance that it leaves heavy footprints in the omnipresent Higgs field. This makes the top quark an ideal candidate for studying Higgs interactions.

Connecting the dots

We can study our universe by using telescopes to observe its structures at immense scales, as well as by revealing the subatomic composition of matter via particle collisions. The scales of the very big and the very small are nevertheless strongly connected. The top quark has a special bond with the Higgs particle and we can learn a lot if we listen carefully to the conversations that these two particles are having. The study of the top-Higgs interaction can indicate whether there is a chance for us to travel to a parallel universe to start filming a sequel to ‘Interstellar‘. All joking aside, these tiny particles can change everything in our understanding of the universe.

An artist’s impression of a particle interacting with the Brout-Englert-Higgs field. Image: Daniel Dominguez/CERN

Higgs boson and top quarks

There are several possible ways of creating the Higgs boson or the top quark in high energy collisions. However, the probability to create these two particles at the same time is very small and we can only find these results in a tiny fraction of the millions of collisions recorded at the Large Hadron Collider. The physics of proton-proton collisions suggested that this top-Higgs production would most likely be in the form of two top quarks and one Higgs boson in the detector, the so-called ttH production. This ttH production currently provides the only direct method to measure the strength of the interaction between the two particles. In the ttH production it is feasible to directly detect the created top quarks and Higgs bosons, so that that the strength of the top-Higgs interaction can be inferred straight from the measurement of the observed number of such events in a particle detector. The study of the ttH process thus provides a very clear way to measure the strength of this fundamental interaction. This is why physicists all over the world got so excited when CERN announced the first actual observation of Higgs – top quark events in early June.

This is what ttH production in the particle detector looks like in the final stage. The Higgs particles and top quark created during the proton-proton collision decay quickly, producing an even larger number of other particles. This ‘debris’ is what’s actually being picked up by the detector. Physicists have to analyse and recombine these puzzle pieces to find out what actually happened during the earlier stages of the collision. Image: CMS/CERN


On June 4, 2018, scientists at CERN declared the first direct observation of top-Higgs interactions. The top quark and the Higgs boson are both unstable particles that disappear shortly after their creation to produce various types of other particles. The main challenge of the experimental analysis of the ttH events is the precise recombination of all these final particles from the traces they left in the detector back into initial top quarks and Higgs bosons. A reliable detection of all these final objects and its eventual recombination via an experimental analysis has been made possible by some very sophisticated and highly performant algorithms. The success of these analysis techniques is due to an enormous effort coming from the expertise of a large group of people involved in this fascinating discovery.

Interviews with physicists at CERN on the new ttH results. Source: CERN

What’s next?

The observation of the ttH process marks an important milestone in our understanding of the properties of the Higgs boson by giving us the first direct measurement of the strength of the Higgs boson interaction with quarks. At first glance, the results indicate that the top quark behaves as predicted by the Standard Model when it interacts with the Higgs boson. To fully establish this fact and to get a better grasp on the physics behind it, we will need to study a lot more collisions at the LHC. It is important to mention that last year, we observed the first evidence of the decay of the Higgs boson to a pair of bottom quarks at CERN. This paves the way to the study of the Higgs boson interaction also with the second heaviest quark of the Standard Model, which nicely complements our ongoing studies with the top quark.

We are witnessing the first top quark landing on the Higgs surface and we are only starting to make our first steps on it. Future exploration of this unknown territory will tell us whether we can find any traces of new and unknown particle species that could cast doubts on the heavy-weight leadership of the top quark and the Higgs boson. Either way, at the end of our journey these two particles will join the list of the most studied fundamental specimens found in our universe.