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While my life still continues being very busy, things may finally even get stabler… <br>

In the meantime, I would like to discuss one of my recent publications, available  <b><a href="https://arxiv.org/abs/1903.02477">here</a></b>, in which I summarize a plenary talk I have given at a <b><a href="https://indico.tlabs.ac.za/event/75/overview">conference in South Africa</a></b> last December.<br>

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<center> <img style="padding:5px" width="97%" src="https://i.postimg.cc/YS5GBnGf/top-models.png">
<sub>[image credits: <b><a href="https://www.steemstem.io/#!/@pab.ink">@pab.ink</a></b>]</sub> </center> 
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In this work, I present some calculations achieved in the context of a class of dark matter models that could be coined <b>the top models of dark matter</b>.<br>

Those models are very simple. They postulate the existence of a dark matter particle that is connected with the Standard Model through a mediator solely interacting with the top quark, the heaviest of all known particles.<br>

In our calculations, we make use of perturbative quantum chromodynamics (the quantum theory of the strong interactions) to <b>calculate precisely several dark matter observables</b> in cosmology and at the LHC. And by ‘<i>precisely</i>’, I mean much more precisely than what is usually done. From there, we verify <b>how viable is the model relatively to data</b>.

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<h2> <center><b>More on dark top models</b></center></h2>

As a starter, let us come back to the model and the assumptions behind it. First of all, this is a model for dark matter, so that it assumes that <b>dark matter exists</b>. 

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<center> <img style="padding:5px" width="97%" src="https://s17.postimg.cc/b7jjrj1a7/dmdm.jpg">
<sub> [image credits: <a href="https://commons.wikimedia.org/wiki/File:1e0657_scale.jpg">NASA</a> (public domain)]</sub> </center> 
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This is a strong assumption, but dark matter is evidenced by many indirect observations of a different nature (while theories without dark matter are not ruled out).<br>

Dark matter is motivated by the <b><a href="https://arxiv.org/abs/astro-ph/9506004">velocity of stars</a></b> in the sky. This can only be explained (with Newton’s laws of motion) if we assume a large amount of dark matter.<br>

<b><a href="https://arxiv.org/abs/1502.01591">Gravitational lensing</a></b>, <i>i.e.</i> the bending of light when it travels through space, also testifies for the presence of some invisible matter. This yields duplication of images, as for instance with the <b><a href="https://en.wikipedia.org/wiki/Einstein_Cross#/media/File:Einstein_cross.jpg">Einstein cross</a></b> where the same quasar is observed at different locations.<br>

We can also mention the properties of the <b><a href="https://arxiv.org/abs/1502.01589">cosmic microwave background</a></b>, a left over from the universe epoch where atoms have been formed, or the <b><a href="https://arxiv.org/abs/1404.1938">galaxy structures observed in the universe</a></b> today.<br>

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<center> <img src="https://s9.postimg.cc/9n8le64i7/dm_art.jpg">
<sub> [image credits: <a href="https://www.flickr.com/photos/mualphachi/20908548189">Maxwell Hamilton</a> (CC BY 2.0)]</sub> </center> 
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In order to build a simple model for dark matter that could explain the observations, we start from the <b><a href="https://www.steemstem.io/#!/@lemouth/some-thoughts-about-stem-on-steem-bonus-a-complete-introduction-to-particle-physics">Standard Model of particle physics</a></b>, and minimally add a single <b>dark matter particle species</b>.<br>

In addition, we need to design the way this dark particle interacts. The easiest option is to add a second particle that will interact with dark matter and one of the Standard Model particle at the same time, the so-called <b>mediator</b>.<br>

A kind a threesome option (as I discuss top models, I am allowed for the joke).<br>

In the case of dark top models, we impose that the involved Standard Model particle is the <b>top quark</b>, the heaviest of all particles (and that is often considered as connected to new phenomena by virtue of its very large mass).<br>

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<h2> <center><b>The relic abundance of dark matter</b></center></h2>

Observations exactly indicate <b>the amount of dark matter present in the universe today</b>. Therefore, we must make sure this is correctly predicted by our model. This can be calculated (more or less) very simply. We compute how dark matter annihilates and how it could be produced, and then follow the evolution of its quantity from the Big Bang until today.<br>

This calculation (see <b><a href="https://www.steemstem.io/#!/@lemouth/dark-matter-from-its-origins-to-its-freeze-out">here</a></b> for more information) more or less tells us that dark matter production stops when the universe becomes too cold (the particles inside it are not energetic enough to create dark matter), and that dark matter annihilation stops when the universe has expanded too much (dark matter is too diluted to annihilate).<br>

The results are given by the following figure.

<center> <img style="padding:5px" width="95%" src="https://i.ibb.co/9YJgYsL/fig-relic.png">
<sub>[image credits: <a href="https://arxiv.org/abs/1903.02477">arXiv</a>]</sub> </center> 

In this figure, we represent the dark matter mass on the <i>x</i>-axis, the difference with the mediator mass on the <i>y-</i>axis, and the color code indicates the strength of the dark matter interaction with the top quark (yellow means very strong, and blue means weak).<br>

The white points are configurations excluded because the predicted amount of dark matter does not match the observations. The very important novelty of our calculations concerns <b>the right part of the figure, which could not be correctly predicted (by a factor of 1000) without relying on precision predictions</b>.

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<h2> <center><b>Dark matter direct and indirect detection</b></center></h2>

Dark matter is searched for on Earth through direct detection and indirect detection experiments, as well as at colliders like the LHC. 

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<center> <img style="padding:5px" width="97%" src="https://i.postimg.cc/xCb6s3vD/lzdm.jpg">
<sub>[image credits: <a href="https://www6.slac.stanford.edu/news/2016-06-01-prototype-lux-zeplin-dark-matter-detector-tested-slac.aspx">SLAC</a>]</sub> </center> <br>
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In <b>direct dark matter experiments</b> (like <b><a href="https://www.steemstem.io/#!/@lemouth/a-new-proto-sensei-t-1551906490">Sensei</a></b> which I talked about two weeks ago), we take a large detector volume and we wait for a dark matter going through it and hitting one subatomic component of the detector material.<br>

<b><a href="https://home.cern/science/accelerators/large-hadron-collider">At colliders like the LHC</a></b>, we collide very energetic particles (normal matter). Thanks to the amount of available energy, this could lead to the production of dark matter (through intermediate mediators) together with some extra (normal) stuff. Whilst dark matter is invisible, the normal stuff is not. Using <b><a href="https://en.wikipedia.org/wiki/Conservation_of_energy">the conservation of energy</a></b> principle, we can then unravel the invisible through the properties of the visible.<br>

Finally, dark matter can also be <b>indirectly detected</b>. Dark matter may annihilate somewhere in the universe into normal matter. The products of this annihilation can then be detected on Earth, for instance in <b><a href="https://fermi.gsfc.nasa.gov/science/instruments/lat.html">satellite experiments</a></b> analyzing cosmic ray or gamma ray data.

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<h2> <center><b>Constraints on dark top models and take-home message</b></center></h2>

Let’s apply all the discussed constraints to our dark matter model. This gives the following figure. <b>All our predictions are precise and include quantum effects driven by the strong interactions.</b>

<center> <img style="padding:5px" width="95%" src="https://i.postimg.cc/rphYw4Cp/results-tpdm.png">
<sub>[image credits: <a href="https://arxiv.org/abs/1903.02477">arXiv</a>]</sub> </center> 

In the grey area, there is no way to get the correct amount of dark matter. The different colored areas then indicate regions excluded by one or the other class of constraints, and <b>the white area is what is viable with respect to data</b>. The interesting point is that only <b><a href="https://steemit.com/steemstem/@lemouth/towards-a-future-100">future collider experiments</a></b> will be able to access it. 

<h4>We thus need definitely need to build the next generation of colliders to be able to fully probe the model!</h4>

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<h2> <center><b>SteemSTEM</b></center></h2>


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