In touch with dark matter at the Large Hadron Collider: possible or preposterous?

A new day dawns at the Swiss-French border

It is 4am in a small village close to the Swiss and French border. The sound of a cock crowing rings through the air. However, this sound does not signal the dawn of a new day but the injection of a new beam in the world’s most powerful particle accelerator. I am in the ATLAS control room. Instead of a rural scenery, the room is packed with computer monitors, projectors, and loudspeakers. The audio signal tells us, the shift crew, that the start of a new run is imminent and we have to get the detectors ready for another round of data-taking.

The ATLAS detector in the underground cavern at CERN. Picture taken by the author.

The ATLAS detector is a multi-purpose particle detector and perhaps the biggest 100 Megapixel camera ever built. It was designed by thousands of particle physicists and engineers to record the debris created in the interactions of the two colliding proton beams.

After my shift, on my way back to the CERN hostel, I look up to the clear night sky above the Jura mountains. These moments make me remember the questions which my five-year-old self used to ask: “What is the Universe made of?”

Over the last century, a fairly compelling answer to this question has evolved. The immodestly titled “Standard Model of Particle Physics” enumerates the elementary building blocks of matter. I like to compare the particles in the Standard Model to Lego blocks. Much like the creations of children playing with these building blocks, everything around us is made up of three different kinds of elementary particles and their mutual interactions.

Below my feet, the Large Hadron collider continues its operation, providing data which to date confirms this scientific narrative. A story too good to be true? Perhaps, as there is a catch.

Dark matter in our universe

The particles in the Standard Model make up only a fraction of the matter content in the Universe. According to astrophysical observations, there is more matter in the Universe than just the visible matter which we experience in everyday life. Even the matter provided by all the planets, stars, galaxies and even the enormous amounts of intergalactic dust is not enough: the major fraction of the Universe is made up from something which cannot be observed directly in astronomy and which we don’t understand well – so-called “dark matter”.

The only properties of dark matter we are certain about are its mass and its lack of observable interactions. These properties are established by the gravitational pull of dark matter on other (visible) astrophysical objects.

Although most of our current understanding of dark matter originates from indirect observations, remarkably, dark matter particles could be produced at the Large Hadron Collider. In my PhD at the Max-Planck-Institute for Physics, I investigated data collected at the Large Hadron Collider with ATLAS in the years 2015 – 2018 for traces of such dark matter particles.

At first glance, such an endeavour must seem preposterous: even if dark matter particles are produced at the Large Hadron Collider, they would still be invisible to the detectors. If we picture the ATLAS data as photographs of the collisions, then in searches for these elusive particles, we are blindfolded.

We need a clever way to detect the dark matter particles in a different way. Do you recall playing “blind man’s buff” at a child birthday party? When being blindfolded, you need to rely on your sense of touch for identifying your surroundings. In dark matter searches, a similar strategy is pursued: although dark matter particles are invisible, their potential effect on other objects is not. When other particles are produced in the same proton collision, they recoil against the dark matter particles. Therefore, we can use these other particles as a “cane” to search for dark matter.

The key to the invisible universe: missing transverse momentum

Of course, this is a simplified picture. The rigorous justification for this type of dark matter searches comes from one of the pillars of physics which can be traced back to Isaac Newton: momentum conservation.

Let us examine a typical collision of two proton beams: Prior to the collisions, the protons in the Large Hadron Collider happily fly along the first beam pipe, accelerated close to the speed of light and unaware of their eventual and terrible fate: in the centre of the ATLAS detector the proton beam intersects with a second beam, resulting in spectacular collisions of protons being smashed together. Unlike in car crashes, not only the debris of the protons shoots through the layers of detector material. The energy in the proton collisions is so large that even pairs of new particles can be created. 

What about the momentum balance prior and after the collision? Prior to the collision, the total momentum is fully aligned with the beam pipe. All protons circle in the beam, there is no movement in the plane perpendicular to the beam pipe. The net momentum in the so-called transverse plane is zero.

By the means of momentum conservation, this must also hold after the collision took place. If a pair of particles is created in the collision, the particles must fly in exactly opposite directions in the transverse plane to allow their momentum components to cancel out. If the net momentum of all particles created in the collision is not zero, then some invisible particle must have escaped the detectors.

Therefore, the associated observable is called “missing transverse momentum” – the missing momentum component in the plane transverse to the beam pipe.

Missing transverse momentum (illustration). In the transverse plane, perpendicular to the proton beam, two b-quarks are observed by the ATLAS detector. However, there is no visible particle to counteract the b-quark momentum! The missing momentum component for the net transverse momentum to balance to zero is called the “missing transverse momentum”.

Finally! This observable allows us to infer the presence of dark matter particles which might be produced in the collision. The missing transverse momentum is at the heart of dark matter searches performed with ATLAS data. The other defining property of these searches is the type of other particle which is recoiling. I investigated searches for dark matter produced with W/Z bosons, Higgs bosons and even pairs of W/Z bosons.

Is it that easy? Just searching for collision events with large missing transverse momentum and you can claim to have discovered dark matter?

Experimental challenge: invisible background

Of course, the answer is no. Not only do these searches need to account for every possible way, in which performance effects of the detectors or some particle reconstruction inefficiencies could lead towards fake missing transverse momentum: there are even particles described by the Standard Model which are invisible to the detector. These particles are called neutrinos and are sometimes referred to as “ghost particles” because of their extremely feeble interaction with other particles. By the time you read the article, a large numbers of solar neutrinos will have passed through your body and continue on their way without leaving any trace. Similarly, they produce the same signature as dark matter. Is all hope lost in detecting dark matter among background processes?

Histogram showing the number of selected collision events with a specific amount of missing transverse momentum. The observed data in this example are shown by black dots with error bars. Overlaid are stacked histograms for the theoretical prediction how many of these events are expected if there is only the neutrino background or if there is additional dark matter which was produced in the proton collision.

Fortunately, theoretical physicists have calculated how likely such background processes occur. In practice, the observed collision events are categorised by the missing transverse momentum and displayed as a histogram. A histograms shows how many events are associated to a certain range in missing transverse momentum. Similarly, the predictions from the theorists for the contribution of neutrino background processes and the potentially additional contribution from dark matter particles is categorised. In the end, the data are compared with the prediction to decide what is more likely: just the background hypothesis, or perhaps the dark matter hypothesis?

Search for dark matter in events with Higgs bosons

Now that we have established how to search for dark matter, let us look at the results of the real thing. Below, you can see another histogram, which shows the missing transverse momentum distribution measured in a search for dark matter produced in association with a Higgs boson. Of course, the distribution looks more complicated than the example above (e.g. there is a more detailed list of potential background processes and the prediction for dark matter particles is not stacked but overlaid), but the general structure is the same.

Missing transverse momentum distribution observed with the full Run-2 ATLAS data. Figure taken from ATLAS-CONF-2021-006.

The message from the experiment is clear: all data points agree perfectly with the background from the Standard Model. There is no sign for events occurring with missing transverse momentum which could be associated to dark matter particles created at the Large Hadron Collider.

Does that mean that all the time and effort in the ATLAS control room, in designing algorithms for data analysis, countless hours of statistical analysis, all has been in vain? The opposite is the case! These null-results from the Large Hadron Colliders are vital information for theoretical physicists pondering how dark matter could be realised in our Universe. Ultimately, experiment is the final arbiter of a theory in empiric sciences. Therefore, these results challenge models of dark matter particles and help to rule out certain hypotheses, paving the way to a better understanding of the dark matter puzzle.

Getting a job in academia

Job advice for early-career researches from an early-career researcher: recollections from a seminar meeting with junior group leaders talking about their careers in academia.

Getting your PhD is only the beginning. For many, the period of research as a doctoral candidate will be the most intensive, challenging, and joyful time of their life. In particular, research in experimental particle physics combines a stimulating environment with enthusiastic and very bright colleagues working on tough problems. All these elements require creativity, coding and technical skills, (time) management, and a large amount of collaboration and networking.

For many, the PhD research will be the first serious encounter with doing research on their own. Admittedly, as a master student you are doing your master project which culminates into a thesis. However, most students are supervised on a regular basis by more experienced researchers and unlike PhD students don’t have the extended period for thinking deep and experimenting with a variety of approaches.

Hopefully, the PhD project ends with a successful defence and the intellectual joy of having written a treatise on a topic on which the PhD is now an expert. There are at least as many reasons for not pursuing a career in science than there are in favour of it. Although the environment and the topic of your work is most engaging and satisfies ones natural curiosity, the job market for scientists in academia is tough.

In physics, a typical career path involves working as a postdoc after having done your PhD. In this fixed-term position, you can gain experience and broaden your view for becoming expert in more topics and more importantly, developing new research ideas.

My path has lead me to DESY in Hamburg, a vibrant centre for accelerator physics, photonics, and (for me) most importantly, particle physics. The DESY group is a welcoming and stimulating environment and collaboration with PhD students, other postdocs and scientists with permanent positions is great. Another perk of being at DESY Hamburg is the strong environment provided by the Quantum Universe excellence cluster, which brings together scientists from University of Hamburg and DESY.

Particularly noteworthy is the level of support for postdocs preparing them for getting a permanent position in academia. A plethora of trainings and counselling offers are provided by the PIER Education Platform for Postdocs.

On the 5th February 2021, a special format was offered for the first time by the  PIER Education Platform (PEP) for Postdocs in collaboration with the QU/CHAMPP Postdoc Council: the 1st Postdoc Experience Exchange Round table (PEER) Meeting.

Here, five successful early career researchers based in Hamburg talked about their personal experience and gave advice on how to get a junior position. All researchers were somehow linked to a physics background and (more relevant for me), most of them even worked in experimental particle physics.

I came into the meeting with a few questions in my mind, such as which grants are particularly appealing and what to keep in mind when applying to them, and how they balanced risky and very likely achievable projects in their applications.

The meeting started with five short introductions. Common themes of their stories were continuous building of leadership in large collaborations by taking over positions with responsibility, writing short-authorship papers as opposed to the large author lists of the collaborations, and doing a postdoc outside of Germany (when aiming for a permanent position in Germany).

The leadership skills are inevitable for leading a junior group, so it is very advisable (and I have to add: also most enjoyable) to supervise PhD students as a postdoc. So are management qualities which can be learned and demonstrated by taking over coordinating roles in collaborations, such as a convenership or the lead of an analysis team. The key word here was the “visibility” in large collaborations.

Writing papers with short author lists can make the difference in an application for tenure or a junior group. Not only do they set an applicant apart from others, they also provide room to demonstrate creativity and own ideas. Two early career researchers remarked how their own “hobby” or side-projects, which they followed and published in small authorship papers, ultimately helped them to get a permanent position. Again, small papers increase your “visibility”.

Finally, being abroad helps in several ways. Firstly, it broadens your horizon by exposing you to different styles of lecturing and supervision. Secondly, some programs of the DFG and the EU are designed to “bring the Germans back to Germany”.

It surprised me, how organically their academic path grew from one project to another. Apparently, it doesn’t hurt to practice your story and think of your “elevator pitch CV” in terms of storytelling. Almost every panelists used words such as “narrative” or “story”. One even admitted, that of course their stories sound very logically and organic: they have practised them over a hundred times in their career.

Speaking of practice: another advice was to sit down and write a lot of proposals. Like a novelist has to write in order to become a good writer, a scientist also has to practice his or her trade. Here, it can also help to read a lot of other successful proposals in your area and to analyse great papers for their writing. As a postdoc, you should think of the bigger picture and think of a research plan for the next years. Having to do the thinking in a grant application is a good way to organise your thoughts (e.g. in your third year as a postdoc).

There are several career options for a permanent position. Almost nobody mentioned the “classical” path: doing a habilitation and applying for a university professorship. Almost all focused on junior group leader positions and junior professorships. In the words of a panelist: “It is much more fun to do the job and demonstrate that you can do it well afterwards than to prove that you will be able to do the job well.” Also, concerning tenure-track-positions: “It is much easier if you are evaluated only against your own performance than against the performance of all your competitors.”

Most German schemes for junior group positions have their deadlines within 6 years after having obtained your PhD. Of the various grants and options, a few were mentioned several times:

For an Emmy-Noether junior group position, there is a tight time window of about four years after having obtained your PhD. In particular the Emmy-Noether program is specifically for fostering talent and considers whether precisely you are the right person to do the proposed research (“individual academic excellence”). If successful, it doesn’t hurt to negotiate with your university if they will grant you the title of a junior professor. If you don’t ask, most universities will not consider this option. For Emmy-Noether positions, teaching is also relevant. Typically, the Emmy-Noether junior group career path is embedded in a university context.

The Helmholtz Young Investigator groups on the other hand have a strong focus on research. After successful evaluation, “tenure” as a staff scientist at a Helmholtz institution can be granted. As the Helmholtz research centres don’t focus on lectures, the focus here is clearly research and leadership.

Finally, the ERC starter grant is a very prestigious way for negotiating a permanent position at a university. Here it should be noted, that you apparently can get blocked for the next round if your application is too bad. Therefore, one panelist suggested to apply already four or five years after your PhD to have a second chance before the deadline of seven years expires. Also for the ERC grants, going abroad can benefit your case in several ways (as discussed above).

The most important advice for your application is to ask successful applicants in your field. You should know them or at least have some connection, such as their institution being the same as your designated institution for your application.

The second most important advice is to stick exactly to the template, if there is such a thing. It doesn’t help if you write your project proposal first and then have to re-write it to adapt it to the template. Also, the template helps you to think over crucial elements in your proposal.

What I found very interesting was the answer of the panelists to my question how they decided the amount of risk in their grant proposals. Here, the advice was to layer the research plan and include some almost fail-safe projects with solid results, based on which more risky and innovative projects can follow. It also is beneficial to include “fallback solutions” or alternative directions to demonstrate that you are aware of potential failure of your projects.

But what if “you don’t make it in academia”? First of all, a career in industry can be equally fulfilling as doing research in a research centre or university context. Is there a thing such as being “too old for industry”? It depends, according to the panelists. If you did relevant research or gained experience with coding, machine learning / data science, fabrication during your postdoc, these years are by no means “wasted”. Otherwise, you need to accept that your salary will be not as good as that of somebody who went into industry right after the PhD.