Engineering models to test dark matter signals at colliders

in StemSocial4 months ago

France being a very secular country (well, at least it is supposed to be one), those living here end up with a bunch of catholic holidays. Life without contradictions is actually not funny, is it? As a consequence, I spent some offline time with the family last week, enjoying a four-day week-end in which I practiced some of my favourite hobbies beyond physics: having fun with my kids, gardening, outdoor time, cooking (barbecue, meat-smoking), etc.

This week is in addition quite intense. I participate to the Planck 2022 conference which is organised 10 minutes away from my office. An in-person conference is really nice as it offers the possibility to meet again friends and colleagues I haven’t seen for a long time… Let’s hope an in-person Hivefest will eventually happen later this year. As promised to @gentleshaid, I’ll try to grab some muons from the conference and share them with everybody on Hive.

As a consequence to all of this, my activities on chain slowed down a bit and turned out to be delayed (I apologise to anyone still waiting for a reply). In addition, my weekly blog on particle physics is over-due, as well as an update relative to our citizen science project on Hive. Moreover, I still owe a few French versions of my blogs to the Hive French-speaking community. I promise: later this week (or next week), everything should (will?) get back to normal.

In the meantime, Hive brought me to Twitter, and some of my physicist contacts there brought to my attention an article addressing dark matter models suitable for investigations at particle colliders (like CERN’s Large Hadron Collider). After thinking a bit, I decided to dedicate a blog (I mean: this blog) about how simplified models are actually built. The reason is that this actually brings complementary pieces of information to the blog of last week, which addresses one of the darkest secrets of the universe (i.e. dark matter) and how to get insights on it at particle colliders.

[Credits: Original image from geralt (Pixabay)]

In the next part of this blog, I first provide a small recap about dark matter. For longer (non-recap) versions of this recap, please see this good old blog, as well as the post of last week. After this, I discuss the main details of the topic considered this week, and introduce an example in which a simplified model suitable for dark matter searches at particle colliders is built.

As usual, those in a hurry could move on directly with the summary section of this blog, and the others could just buckle up and chill.

Dark matter, dark matter, dark matter… Why?

Usually, when it is time to introduce dark matter, I jump straight to key motivations. However, by doing so I forget mentioning very important fine-prints (as correctly pointed out by @stayoutoftherz in the discussion of the previous blog). Therefore, let’s start with this today, for a change!

Since 100 years ago, many cosmological observations challenge our understanding of the manner our universe works. Finding an explanation for those gave rise to the so-called standard model of cosmology. Such a model has the particularity to be extremely simple (simplicity is always great when it works), and to feature very few free parameters, dark matter and dark energy.

We can fit the model to data and see that it explains very well a lot, although some anomalies are (reasonably) in the way. Whereas the standard model of cosmology works well, it is important to stress that this is a model and that it lacks a physical foundation. Other options exist and are not excluded. However, they do not provide a fit to data that is as good (which is why the standard model of cosmology is so appealing and acknowledged by most).

[Credits: Vis-sns (CC BY-SA 4.0)]

To summarise, the standard model of cosmology consists of one model, few parameters and many explanations to observed phenomena.

Those observations include for instance galaxy rotation curves. The latter show that stars lying very far from galactic centres rotate much faster than what is expected from gravity and the amount of visible matter in the universe. Something may be there, invisible and waiting…

Whereas this was one of the initial facts that has led to our understanding of cosmology as it is today, we cannot ignore the cosmic microwave background. It consists of a fossil radiation left over from the time atoms formed, and it provides a very precise footprint of the universe at this time (i.e. 380,000 years after the Big Bang). The standard model of cosmology fits the observation very neatly!

Moreover, standard cosmology describes quite well the structure of the universe in terms of how galaxies are organised in the universe. This is illustrated on the image above, although this only shows the universe when it was a fresh little boy of 1,000,000,000 years old.

Colliders and dark matter - a supersymmetry-inspired model

Whereas the standard model of cosmology and its successes motivate the assumption of dark matter, we have no clue about the nature of the latter. For that reason, anything (okay, mostly anything) is allowed. This does not render the situation practical from a particle collider standpoint, as we end up with gazillions of model that should be tested. In other words, particle physicists are… outnumbered!

For that reason, we designed a few simplified models that are representative of many complete and well motivated setups. This is precisely one of the ideas which I have tried to convey in my last blog. Moreover, among all existing simplified models (we have a small number of them on the market), some of them have strong underlying motivations.

For instance, supersymmetry consists of one of the most popular class of theories beyond the Standard Model of particle physics. I won’t spend time in detailing what is supersymmetry, why it is great and why it is a nice way to solve some of the conceptual limitations of the Standard Model. If this is of any interest to you, please follow this link. This is not needed for the following.

I instead simply consider one of the most common setups arising in supersymmetry and that is capable to provide a particle responsible for dark matter in the universe. This last sentence contains a strong assumption. Whereas we so far accepted the idea of dark matter, we now must admit that there is an elementary particle related to it. In addition, in the supersymmetric context (as well as in many dark matter models), the dark matter particle is connected to the Standard Model.

[Credits: Jason Jacobs (CC BY 2.0)]

This connection is not a direct one (this originates from the fact that a dark matter particle has to be stable), but goes through an intermediate particle playing the role of a heavier mediator. Supersymmetry provides us various candidates for such a mediator, each option differing by how the mediator connects to the quarks or the leptons of the Standard Model.

In order to design our simplified model for dark matter, we thus begin by following a supersymmetric inspiration. This defines a dark matter setup in which we have a dark matter particle that connects through a mediator to either the quarks of the Standard Model, or to the leptons.

In terms of free parameters, we therefore have two new physics masses (that of the dark matter and that of the mediator), and the theory predicts the value of all new couplings that dictate the connection between the mediator, the dark matter and the Standard Model. The next step is to relax this and make the couplings free parameters.

So we have a great simple model, at least in the sense that it has dark matter and finds its roots in a concrete well-motivated framework. However, is this viable? This question is addressed in the publication discussed today.

The incredible bulk

Let’s first emphasise again a very important point.

Whereas inspired by supersymmetry, the above simplified model for dark matter phenomenology is more general than the initial supersymmetric model. In supersymmetry, the new couplings are predicted to be equal to some value. On the contrary, in the simplified model we can fix them to any value. This is the power of simplified models, somehow. We start from a motivated framework and then generalise it.

A typical and concrete example of a motivated realisation of the simplified model consists of the so-called bulk region of the minimal supersymmetric parameter space (cf. the subtitle of this section). Here, the dark matter is connected to the Standard Model through hypercharge interactions.

I promise, ‘hypercharge’ is an existing word. However, I don’t enter into details as this is not the subject of this blog. Instead, I only mention that hypercharge interactions can be seen as admixtures of electromagnetic and weak interactions (two out of the four known forces). That’s all.

[Credits: Piotr Siedlecki (public domain)]

There is however an important consequence when we calculate the amount of dark matter present in our universe. In this bulk configuration of the simplified model, it turns out that enforcing predictions and data to agree leads to mediators that are light particles.

This is a big problem. The Large Hadron Collider is looking very widely for new phenomena and new particles, and it has not found any signal (yet?). There is now way guys and girls like our mediators could have escaped detection.

Supersymmetry-inspired dark matter at all costs!

This leads to a weird contradiction. We have a simplified setup whose most typical realisations are excluded by data. There are however three ways out (pffffeew), and it is important to consider them all and not sending anything to the graveyard.

The first one is to say that we don’t care. The simplified model is more general than the initial model and allows for exploration on general grounds (as all new masses and couplings can be tuned freely). It is thus not a big deal if we probe signals living far from any motivated model configuration. If by any chance there is a signal in data, then it will be the time to connect the simplified model to something more concrete (and we can trust physicists to design hundreds of ways…).

There are however to more physically sound options to make the simplified model an interesting avenue. These are more ‘top-down’: we start from the complete theory and try to see under which conditions the simplified model can survive. From there, we can somehow define favourite regions of the model’s parameter space, and we can study the associated phenomenology.

One of these options fits in the so-called non-minimal path that I discussed in the blog of last week. An extension of the mediator sector of the theory is sufficient to get the right amount of dark matter. Of course, we don’t do this blindly (physics must be solid!). We use instead the fact that in any supersymmetric model many new particles lie around. It is then sufficient to decide that a few of them are relevant.

[Credits: geralt (Pixabay)]

For the last option, the idea is to keep the content of the initial simplified model minimal, and instead make the mediator only slightly heavier than the dark matter. This particle spectrum is said to be compressed. A direct consequence is that the mediator plays a much greater role in the early universe, and it thus naturally impacts the amount of dark matter left today.

Let’s put ourselves at the very beginning of the history of our universe. The amount of dark matter that is observed today stems from processes that were occurring at that moment. In particular, the key process is that in which a pair of dark matter particles annihilates into a pair of Standard Model particles, together with the reverse reaction (the production of a pair of dark matter particles from the annihilation of two Standard Model particles).

If the mediator is close in mass to the dark matter, then extra reactions must be considered. These include the production of a pair of mediators, as well as the associated production of one mediator and one dark matter. Of course, the reverse processes are important too.

Therefore, if we have a configuration in which the dark matter and the mediator have almost the same mass, we must account for all above-mentioned processes. Then, we naturally obtain an amount of dark matter in agreement with data, making the model viable.

From there, we need to investigate the consequences of those viable scenarios in the context of all present and future experiments. This allows us to see where we go, and also which probes of new phenomena are missing. Equivalently, we need to answer the question about the configurations of the model uncovered, and how to fill those gaps. This is however not the topic of this blog.

Summary: building simplified models for dark matter at colliders

In this blog, I discussed once again dark matter, in the context of the models we use to probe it at particle colliders.

The starting point of our story is the so-called standard model of cosmology that is simple and that allows for an explanation to many phenomena observed in the universe. This model has a very cool property: there is an invisible substance called dark matter. The next question to answer (in this context) is thus about the possible nature of dark matter. What is it exactly?

If we assume that there is an elementary dark particle that plays the role of dark matter and that interacts in some way with known particles, then we end up with a game to which particle colliders can contribute. This however requires to simulate signals, which is an impossible task due to too many potential models and theories featuring dark matter. For that reason, particle physicists came out with simplified models.

In this blog, I explain how such a simplified model could be constructed. The starting point is a well motivated framework for physics beyond the Standard Model, like supersymmetry. In such a model, we have a particle that can play the role of dark matter, and it comes together with a connection to the Standard Model. This connection occurs through mediator particles. The next step is to remove everything from the model, excepted the dark matter and the mediator.

Now, we can do some calculations and verify under which circumstances the amount of dark matter is the correct one relative to data. And this is where it becomes tricky: the mediator particle must generally be light, which is in contradiction with its non-observation at CERN’s Large Hadron Collider.

We are thus offered three ways to move on.

  • We generalise the model by making the predicted couplings free, and use the new ad hoc setup for exploration.
  • We make the model non-minimal by complicating the mediator sector on the basis of what supersymmetry allows us to do, so that we could fit the right amount of dark matter and be compliant with experimental bounds on new particles.
  • We keep minimality and focus on an option in which the mediator is only slightly heavier than the dark matter. In this case, more processes are relevant in the early universe, allowing to get the right amount of dark matter together with a heavy enough mediator.

It is then up to us, physicists, to study the phenomenological implications of the three setups and make sure all options are covered in present and future searches. The major issue is not to waste any possibility to discover something new!

I stop here for today, and I hope you all enjoyed this blog. I would be happy to read your comments, feedback and suggestions. In addition, please be ready for a change of topic soon (neutrinos). Note that I may skip next week as another long week-end is in order (France, France, France).


I led some students through a book study of Dr. Rupert Sheldrake's Science Set Free last year. I chose the book because it presents a quintessentially skeptical view of everything we consider 'known' and 'proven by science', and my goal was to get students to think broadly and critically (a skill that is sorely missing in academia today).

In the book, Sheldrake begins with an emphasis on the paranormal. He argues that, whereas the vast majority of people report having experienced paranormal activity at some point in their lives, 'paranormal activity' is terribly misnamed.

He also talks a bit about dark matter and dark energy, and the fact that the two combined account for (based on presumed models) over 90% of the 'known' matter and energy in the universe. Taken from that perspective, it seems that scientists (especially those probing the extreme cases of reality, and seeking deep fundamental truths, like yourself) should focus much more on how to explain or understand rare but widely-observed phenomena.

Also, take 'miracles' for example. Many of the things we assume to be 'impossible' within the context of modern physics, such as turning water into wine, are certainly theoretically possible, given the presumed existence of dark matter and how little we know about it. I am curious about how your simplified models might change if you added into the framework whatever dark-matter-to-observable-matter conversions would be required to explain the various 'miracles' that have been recorded throughout history. No doubt some (or many) of the various supernatural phenomena that have been observed across the ages are, in fact, illusory or otherwise 'false'. However, if even only a fraction are 'true' or have some level of truth associated with them, it might lead to some new discoveries in fields like yours.

Sheldrake also talks about the fact that 'perpetual motion machines' within the context of dark matter and dark energy are entirely possible, meaning that a machine could conceivably be constructed that converts dark energy or dark matter to observable energy. Such a machine would appear 'impossible' because it presumably violates the first law of thermodynamics -- even though it only does so if you narrowly define matter and energy as observable matter and observable energy.


I am so happy you were spending time with your family. That's the best reason to take a little time off from Hive.

The incredible bulk


As always, you defy the stereotype of the humorless scientist. You inform and entertain.

A new idea, I think (for me, anyway). A mediator particle. The pieces of the puzzle fall into place, or so it seems. Great that Twitter has offered an opportunity to meet others in the physics community.

Thanks for another informative, entertaining blog. Enjoy your time with family and your time off. This, after all, is what most of us really work for.

 4 months ago  

You seem to be the only one who noticed the "incredible bulk" fancy subtitle! at least one person noticed. Therefore the joke is a success.

To be honest, they used it in the study I mentioned in this blog, and I really loved it. Which is why I reused it! ^^

As always, thanks for passing by and reading this blog!

Enjoy your time with family and your time off. This, after all, is what most of us really work for.

PS: I am now isolated at home, in a not so great shape :/

in a not so great shape

I am so very, very sorry. It is in my nature to worry and so I will worry about you. Please let us know when you are feeling better. This is going to sound silly, but there have actually been some studies that show a correlation between levels of vitamin D (you probably know this) and COVID infection/severity. Not all studies show this but make sure your vitamin D levels are good. Doesn't hurt.

Feel better @lemouth. Rest.

With very warm regards, AG

 4 months ago (edited) 

Today was much better. I still slept a lot (relative to my usual amount), but I manage to spend a couple of hours outside, gardening (that's good for D-vitamins ;) ). Tonight I am feeling very well, and I am fully confident that I will be able to attend my panel meeting tomorrow (I have 14 interviews planned).

What I don't say (there is always a catch) is that drugs help me (I need them for the remaining muscle pains and sore throat).

Thanks for asking about my state! I hope you are fine.


drugs help me (I need them for the remaining muscle pains and sore throat).

It is wonderful that they help. And yes, the outdoors, that's perfect. Good for the body and good for the spirit.

You must have a robust immune system. You met the beast, and you beat it 😇

 4 months ago  

You met the beast, and you beat it 😇

We will see this at my next antigen test (tonight if I have time, or tomorrow). ^^

Those observations include for instance galaxy rotation curves. The latter show that stars lying very far from galactic centres rotate much faster than what is expected from gravity and the amount of visible matter in the universe. Something may be there, invisible and waiting…

Or perhaps not...

Did you know that computer models of Birekland currents were able to create spiral galaxy formation? Completely without any of the "fancy dark stuff". I find that quite interesting to say the least.


Also, the simulation almost perfectly explained the flat rotation curve "mystery". In the plasma/electric universe it is not a mystery at all as it is the predicted and anticipated feature in galaxy rotation!


If you are intrigued, here is a link to a paper describing this in more detail:

 4 months ago  

Or perhaps not...

Here I must disagree. The starting assumption in my post is the standard model of cosmology. In this case, what I wrote is fine. There is no disagreement between theory and data. Of course, we can start from a different assumption, and conclusions will then be different.

As said in the previous discussion, for now the standard model of cosmology provides the best fit to data (when we globally account for as many observations as possible). This does not mean this is the true theory of nature (and in fact, the standard model of cosmology can only be seen as an effective description as it lacks physical foundation). It only means that we have a simple setup that works greatly.

To my knowledge, any other proposed option (that is not excluded by data) does not work that well. Therefore, any other paradigm should do as good to be considered seriously.

For what concerns the electric universe, I think that it is an idea that is probably worth to further investigate (but not by me; we need to make choices on what we work). The real scientific foundations, namely the mathematical formulation of the theory that leads to predictions, still seems to be missing. Once this will be there, together with predictions for the many observables that are correctly explained by the lambda-CDM model, maybe then it will get more interests from the community.

PS: I am not convinced by the last plot. There seems to be large difference between the upper and lower panels, and the absence of error bars makes it impossible to conclude anything. I would hence not say we have a 'perfect' explanation.

I think it's great that you are open minded about an alternative theory in cosmology. I argued with many mainstream scientists that dismissed all theories not in line with orthodoxy right out of hand...

PS: I am not convinced by the last plot.

I agree, it lacks quantification (although that might be because the author links herself to that study/paper and didn't provide much more details about it). But I think the point of that plot/my argument is that a model completely devoid of dark matter/energy could create something so closely resembling observed data. If I were a cosmologist, I would be intrigued by that to say the least and ask broader questions such as: are the fundamental assumptions that I am working with correct?

No religion but we'll keep the holidays. It's like eating Oreos, throw away the biscuit but keep the cream!

 4 months ago  

For which reason would you eat an Oreo? This could hurt you! However, I tend to agree, let's keep the holidays ;)

I really enjoyed your description about dark matter

 4 months ago  

Thanks a lot for passing by!

 4 months ago  

Can't wait to see what muons looks, feels, or taste like

 4 months ago  

I failed in catching muons. I only caught COVID... :(


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