Une dualité nouvelle et surprenante a été découverte en physique théorique des particules. La dualité existe entre deux types de processus de diffusion qui peuvent se produire dans les collisions de protons réalisées dans le Grand collisionneur de hadrons (LHC) au CERN in Switzerland and France. Surprisingly, the fact that this link can be made indicates that there is something in the fine intricacies of the standard model of particle physics that is not entirely understood. The standard model is a subatomic-scale picture of the world that describes all particles and their interactions, so when surprises occur, there is cause for attention. The scientific article has now been published in the journal *Physical Review Letters*.

#### Duality in physics

The notion of duality occurs in different areas of physics. The most well-known duality is arguably the particle-wave duality in quantum mechanics. The classic double-slit experiment demonstrates how light acts as a wave, but Albert Einstein received his Nobel Prize for demonstrating how light behaves as a particle.

The strange thing is that light is actually *both and neither of the two* at the same time. There are simply two ways we can look at this entity, light, and each comes with a mathematical description. Both with a completely different intuitive idea, but still describe the same thing.

“What we have now found is a similar duality,” Matthias Wilhelm, Assistant Professor at the Niels Bohr International Academy, explains. “We calculated the prediction for one scattering process and for another scattering process.

Our current calculations are less experimentally tangible than the famous double-slit experiment, but there is a clear mathematical map between the two, and it shows that they both contain the same information. They are linked, somehow.”

#### Theory and experiments go hand in hand

At the Large Hadron Collider, we collide a lot of protons – in these protons there are a lot of smaller particles, the subatomic particles gluons and quarks.

In the collision, two gluons from different protons can interact and new particles are created, such as the Higgs particle, resulting in intricate patterns in the detectors.

We map how these patterns look, and the theoretical work done in relation to the experiments aims to describe precisely what goes on in mathematical terms, in order to create an overall formulation, as well as to make predictions that can be compared to the results of the experiments.

About CERNCERN is an acronym for Conseil Européen pour la Recherche Nucléaire, and the aim was to benefit from sharing the expenses this type of research entails – something that would be too costly for a single country to bear.

There are presently 23 member states. But equally important was the international openness and the peaceful sharing of scientific progress in our knowledge about our world.

“We calculated the scattering process for two gluons interacting to produce four gluons, as well as the scattering process for two gluons interacting to produce a gluon and a Higgs particle, both in a slightly simplified version of the standard model.

To our surprise, we found that the results of these two calculations are related. A classical case of duality. Somehow, the answer for how likely it is for one scattering process to happen carries within it the answer for how likely it is for the other scattering process to happen.

The strange thing about this duality is that we don’t know why this relation between the two different scattering processes exists. We are mixing two very different physical properties of the two predictions, and we see the relation, but it is still a bit of a mystery wherein the connection lies,” Matthias Wilhelm says.

#### The duality principle and the application of it

According to current understanding, the two should not be connected – but with the discovery of this surprising duality, the only proper way to react to it is to investigate further.

Surprises always signify that there is something we now know that we don’t understand. After the discovery of the Higgs particle in 2012, no new, sensational particles have been discovered. The way we hope to detect new physics now is by making very precise predictions on what we expect to happen, then comparing them with very precise measurements of what nature shows us, and seeing if we can find deviations there.

Niels Bohr and CERNNiels Bohr was among the visionary researchers who, by the end of the 1940s, initiated the creation of an international research institution, which would allow researchers to collaborate on uncovering “what the universe is made of and how it works,” as the CERN mission states.

The idea was and continues to be to push the boundaries for our knowledge on the world we live in.

We need a lot of accuracy, both experimentally and theoretically. But with more precision comes harder calculations. “So where this could be leading is working in order to see if this duality can be used to get a sort of “mileage” out of it, because one calculation is simpler than the other – but still it gives the answer to the more complicated calculation,” Matthias Wilhelm explains.

“So if we can settle for using the simple calculation we may use the duality to answer the question that would otherwise require more complicated calculations – But then we really need to understand the duality.

It is important to note, though, that we are not there yet. But usually, the questions that arise from unexpected behavior of things are a lot more interesting than an orderly and expected outcome.”

Reference: “Folding Amplitudes into Form Factors: An Antipodal Duality” by Lance J. Dixon, Ömer Gürdogan, Andrew J. McLeod and Matthias Wilhelm, 15 March 2022, *Physical Review Letters*.

DOI: 10.1103/PhysRevLett.128.111602