In our world, we have many classes of forces that can act on the objects surrounding us. All those forces however result from only four fundamental interactions: gravity, electromagnetism, the strong force and the weak force.
The Standard Model of particle physics focuses on the last three of these forces, gravity being negligible for an elementary particle. While this allows for a successful description of nature, physicists have nevertheless been trying to unify these forces into a single one for a long time.
In such a Grand Unified Theory, all known interactions can be seen as different facets of a single force.
[Credits: Hal Gatewood]
In this blog, I discuss one of my recent research articles, in which we studied a particle physics model emerging from a Grand Unified Theory. Our motivation followed from the dark side of that model: It not only features a way to unify all known interactions, but also offers a solution to the dark matter problematics.
Fundamental interactions in a nutshell
In particle physics, everything can be related to symmetries.
This is in particular true for the fundamental interactions: they are related to the so-called gauge symmetries. Without digging too much into details, one imposes a theory without interaction to be invariant under some transformations, and the fundamental interactions come for free as a result.
The strength in this approach is that it explains most particle physics data available today.
In practice, an interaction between two elementary particles is seen as an exchange of a gauge boson (or a force carrier), a particle that mediates one of the fundamental interactions.
For instance, an electromagnetic interaction will be seen as the exchange of a photon between two other particles.
There hence are three sets of gauge bosons in the Standard Model. Photons mediate the electromagnetic force, the W and Z-bosons mediate the weak force and the gluons mediate the strong force.
Unification and its dark matter consequences
With grand unification, all fundamental interactions are combined into a single force. Similarly, the photon, the gluons, the W and the Z bosons are merged and seen as the different facets of the mediator of the unified force. The pattern featured by the fundamental interactions is hence greatly simplified.
In addition, Grand Unification also combines the elementary building blocks of matter. Several different known particles (quarks, electrons, neutrinos, etc.) become different excitations of a single species.
However, the grand unified symmetry imposes the way to do so, which automatically leads to the prediction of new particles.
In the model investigated in my article, many new particles are around. This includes new force carriers, new Higgs bosons and a new partner for the electron and the neutrino: the scotino (this name was introduced 10 years ago).
The scotino is a new weakly-interacting neutral particle. In other words, it possesses the properties necessary for being a dark matter candidate. However, is scotino dark matter a viable option for dark matter?
Scotino dark matter
In our article, we have verified under which conditions the scotino could viably be dark matter and have properties not violating any observation. The results can be shown (for instance) in the figure below in which we have scanned over the numerous model parameters.
The x-axis represents the mass of the scotino, and each (blue or red) point on the figure consists in one scenario not excluded by searches for a sign of a new interaction at the Large Hadron Collider at CERN, those searches being relevant for any grand unified framework. The red points are the interesting ones: the amount of dark matter is exactly what is needed by cosmology.
In addition, there are various experiments on Earth aiming at directly detecting dark matter. They have not observed anything so far. This leads to the excluded grey region. The y-axis of the figure is relevant for those experiments, as it represents the interaction rate of a scotino with normal matter.
We are left with two clusters of red points. In other words, the scotino mass is not free anymore. Moreover, for all those red points, it turned out that the mass of many other new particles was fixed too. The model has thus become very predictive.
In our work, we further investigated to which extent those particles could be observed during the future LHC runs. The results were quite amazing: the model should either be discovered at the LHC, or mostly excluded!
Summary - scotino dark matter may be behind the door
In one of my latest research articles, I studied a framework featuring the unification of the fundamental interactions. This model predicts the existence of many new particles, one of them having the properties required by dark matter.
We have first verified under which conditions those properties were compatible with collider and cosmological data. The findings were very surprising: cosmologically viable scenarios could be realised. All those scenarios predicted that the masses of many new particles were fixed, independently of the scenario, despite of the large amount of freedoms in the model. There is thus not so many fundamentally different setups!
Moreover, we have shown that those scenarios had great chances of being detectable in a close future at the LHC at CERN, so that the model (and scotino dark matter) has great chances of being either discovered or excluded in the next few years.
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