Top partners at the Large Hadron Collider

Top partners are a thing in particle physics, mostly in the same way top models are a thing in everyday life. With this blog post, I am back with communicating about my own research, and a scientific publication that came out two weeks ago.

The topic concerns the production of top partners at the Large Hadron Collider (the LHC) at CERN in some specific mode that is very relevant for the future.

In our article, we achieved some new precision theoretical calculations to estimate more correctly the properties of these top partners.

But before going straight into details, I will define what are top partners, and why they get a very high ranking in the LHC program of searches for new phenomena. Such a journey in the world of the elementary particles will bring us straight… to the Higgs boson, that is somehow always in the way.

After this brief recap, I will discuss the current experimental status and why we investigated the single production of a top partner at the LHC. Partners can indeed be top and single sometimes… This was the stupid physicist’s joke of the day…

---

A FEW WORDS ABOUT THE PROBLEMS OF THE STANDARD MODEL

---

The Standard Model of particle physics is one of the most tested theories of all time. After fixing a handful of parameters, predictions for thousands and thousands of quantities all (mostly) agree with experiments run in very different conditions.

However, there are very good conceptual reasons to consider that the Standard Model is not the end of the story in our understanding of the fundamental laws of nature.

It can instead be seen as the tip of an iceberg, such that the entire iceberg is actually a more fundamental theory to be unraveled (hopefully at the LHC).

In other words, the Standard Model must be completed by new particles and/or interactions that are expected to fix its conceptual issues.

One of the main existing issues with the Standard Model consists in the so-called hierarchy problem.

This hierarchy problem is connected to the existence of two fundamental energy scales in the theory, the electroweak scale associated with the weak interactions and the Planck scale at which gravity cannot be neglected anymore so that the Standard Model ceases to be valid. These two scales are very different, the Planck scale being 100.000.000.000.000.000 larger than the electroweak scale. We thus have an important hierarchy between them, and this is where the wording ‘hierarchy problem’ comes from.

The reason why this is a problem originates from the Higgs boson. This hierarchy impacts the boson and its properties by a lot, so that the parameters have to be tuned up to their 30^th decimal to get predictions matching the observations. This is not very satisfactory…

This is finally where the top quark enters our story. The top quark is the heaviest of all known fundamental particles and is thus the particle that is the most strongly coupled to the Higgs boson. Correspondingly, it consists in the main driving force behind the hierarchy problem.

---

THE TOP QUARK AND THE NEED FOR NEW TOP MODELS

---

In the previous paragraph, I introduced the so-called hierarchy problem and mentioned that the existence of two very different energy scales in the theory were putting the Higgs boson into troubles.

This occurs through quantum effects, as quantum mechanics rules the microscopic world. Those quantum effects receive contributions from every single particle coupled to the Higgs boson.

However, each particle couples to the Higgs with a strength proportional to its mass. The quantum effects will thus be more important for heavier particles.

The net impact is hence mainly driven by the heaviest of all known particle, the top quark.

In many extensions of the Standard Model, partners to the top quark are postulated and their role is simple: compensating the top quark influence on the Higgs properties. This cures the hierarchy problem automatically and makes the theory stable with respect to the quantum effects above-mentioned.

---

PRODUCING TOP PARTNERS AT THE LHC

---

As we are now all convinced that we must have some top partners extending the Standard Model in one way or the other, it is time to explain how those guys could be produced at the LHC at CERN so that they could potentially be discovered. In my work, we focused on top partners as predicted in certain classes of theories in which the new particles are composite.

All the quarks of the Standard Model are sensitive to the strong interactions, as the top quark and its potential partners.

The LHC is in addition a proton-proton collider, so that its dynamics is driven by the strong interaction too.

We therefore expect a very copious production of pairs of top partners, as for any new particle that would be strongly interacting. The structure of the strong interactions indeed favors pair production over single production.

However, the rate is suppressed by the particle mass: the more massive a particle is, the smaller will be the rate. This stems from Einstein who explained that mass is energy and from the fact that the energy available at the LHC is constant.

Producing two particles hence costs a lot, especially when we compare with the production of a single particle. In the case of the production of a pair of top partners at the LHC, the strong interactions are saving us as they compensate the high energy price. And this holds as long as the particle is not too heavy.

In contrast, the production of a single top partner is suppressed because of the weak interactions that drive the corresponding mechanism. Even if the energy price is cheaper, the production rate turns out to be smaller. Weak is weaker than strong, somehow.

---

SUMMARY AND DISCUSSION ABOUT MY OWN RESEARCH

---

In the above part of this post, I have motivated the reasons why we expect that there exist new particles connected to the heaviest of all known particles, the top quark. Indeed, such partners are a necessity in order to avoid quantum instabilities related to the properties of the Higgs boson.

Those partners are traditionally expected to be pair-produced by virtue of the strong interactions. However, the strong interaction must compensate the important energy price associated with the production of two particles compared with one (since mass equals energy).

But today, there is no sign of any top partner at all, so that the lower bound on their mass is now large. Single production, associated with a smaller energy price, hence starts to be relevant. In other words, the minimum acceptable mass is too large for the strong interactions to compensate the price in energy, so that the weak production of a single top partner is now the winning process.

In my latest article, we have investigated the production of a single top partner. The problem was that there are two ways, both valid, to make predictions for the production rates and the properties of those singly-produced top partner.

We have performed precision calculations in both these ways including corrections originating from quantum effects driven by the strong interaction. It turned out that the results obtained with both methods are now compatible for what concern the properties of the top partners.

The ambiguity that we lifted was only an artefact of the small precision of the calculations performed up to now. The LHC experimental collaborations are now using these results to test the model in real life: in data!

---

STEEMSTEM

SteemSTEM aims to make Steem a better place for Science, Technology, Engineering and Mathematics (STEM). For more information about the SteemSTEM project, feel free to follow @steemstem, have a look to the last news from the project, check out our new app or read our latest distilled.

Please consider to support the project by voting for the SteemSTEM Witness (@stem.witness) or delegating to @steemstem for a ROI of 65% of our curation rewards (quick delegation links: 50SP | 100SP | 500SP | 1000SP | 5000SP | 10000SP).

We are on our way to make Steem a crucial medium for science communication, and it is still time to contribute to this effort!