Felix Kahlhoefer

Research interests
My research combines particle physics and astrophysics related to the search for dark matter and focuses on the complementarity of different search strategies. The aim of my work is to find new techniques to compare different kinds of experiments in order to maximise the chance of a discovery and the amount of information that can be gained from a successful detection of a dark matter signal.

Darkness Visible: The Hunt For Dark Matter
For a general introduction into the topic of dark matter I refer to this talk, which I gave as part of the lecture series A Morning of Theoretical Physics aimed for alumni of the University of Oxford.

The research performed by the GAMBIT Collaboration is described in this article.

For more details have a look at the article below, which I wrote for the physics faculty newspaper De Vakidioot of Utrecht University.

More advanced readers may be interested in my Review of LHC Dark Matter Searches.

Exploring the Dark Side of the Universe
Almost everything we know about the universe and its past has been deduced from observing the skies and measuring the light and other kinds of radiation emitted from astrophysical and cosmic sources. But what if parts of the universe are dark? What if there are unknown particles – or even large undiscovered objects – that do not emit, absorb, or reflect light? To learn about this so-called dark matter, we need radically different detection strategies, which have been developed and improved over the past decades. The more our sensitivity increases, the more we begin to understand how limited our knowledge about the universe is, and what amazing puzzles it still contains.

Even though dark matter cannot be seen directly, its gravitation can influence nearby objects. Therefore, we can indirectly search for dark matter by looking for traces of its gravitational interactions. One way is to measure the velocities of stars that orbit the centre of a galaxy. We know that the gravitational force is responsible for keeping stars on their orbits. The larger the velocity of stars with an orbit of a given radius, the larger the amount of mass required within that radius to keep the stars on track. Measuring the velocities of stars at different distances from the galactic centre – so-called rotation curves – therefore enables us to determine the total mass distribution of a galaxy.

At the same time, we can directly measure the distribution of stars and gas and their contribution to the total mass of a galaxy. Comparing the observed distribution of luminous mass and the inferred distribution of gravitating mass, we can now start looking for a mismatch. The results are spectacular: At large distances from the galactic centre stars are observed to rotate much faster than expected from the amount of luminous matter that we see. The obvious conclusion is that there must be another – invisible – contribution to the total mass of galaxies.

This reasoning is in close analogy to the discovery of the planet Neptune. Before the outermost planet of the solar system was first identified, it was predicted by astronomers because the orbit of Uranus deviated slightly from the expectations – pointing towards a missing mass in the solar system. Nevertheless, these arguments do not always work. When astronomers observed a similar discrepancy in the motion of Mercury, they postulated the existence of a new planet, Vulcan, even closer to the sun. This planet was never discovered, and a few years later it became clear that the anomalous motion of Mercury was not due to the existence of a new planet, but because Newton's theory of gravity becomes inaccurate close to the sun. With a more complete description of gravity, using Einstein's theory of general relativity, it was possible to explain the motion of Mercury without the need for an additional planet.

In analogy, one could be worried that Newton's theory of gravity is inaccurate at very large distances and that galactic rotation curves can be explained without need for dark matter. Indeed, a theory called Modified Newtonian Dynamics (MOND) provides a perfect alternative explanation for these observations. The crucial question is therefore, whether one can find additional pieces of evidence to support the bold claim that there is dark matter in the universe. As before, we can only trace its gravitational interactions. One of the central predictions of general relativity is that matter bends space-time in such a way that light no longer travels in straight lines, leading to a distortion of the images that we observe – similar to the effect of a lens. In other words, if we observe objects that appear distorted, this is an indication for gravitating matter along the path of light. This technique, called gravitational lensing, enables us to determine the total mass and the mass distribution of large astrophysical objects such as galaxy clusters. Comparing the results of gravitational lensing measurements with the mass of the galaxies that we directly observe, we once again find a large discrepancy between gravitating and visible matter (see figure below). The case for dark matter becomes more and more convincing.

But how much dark matter is there in total in the universe? To answer this question, we have to look back in time to the early moments after the Big Bang, when the universe was still very hot and dense. We can learn about this epoch by studying the Cosmic Microwave Background (CMB), radiation that was emitted when the universe was only 380,000 years old and that has taken over 10 billion years to reach the earth. The structure of this radiation, known as CMB anisotropies, depends sensitively on the total amount of matter in the universe and can be measured with great accuracy. Comparing measurements to cosmological predictions, one finds that there must be five times more dark matter than visible matter in the universe.

So, after all these observations and measurements, we have come to the conclusion that we understand less than 20% of the matter of the universe (in fact, including the mysterious dark energy, which seems to be responsible for the accelerating expansion of the universe, we actually understand less than 5% of the universe). The only thing we know about dark matter is that it is fundamentally different from all the matter we know. It interacts so little (neither with light, nor with ordinary matter) that it must be composed of a completely new, yet undiscovered, particle.

What can we do to improve on this awkward situation? First of all, we can look at those regions where the largest dark matter densities are expected, such as the centre of the Milky Way. If the density is high enough, dark matter particles are expected to collide with each other, and in the process they may annihilate into particles which are detectable with conventional telescopes or satellites – just like a positron is most easily observed when it annihilates with its anti-particle, the electron, producing two gamma-rays of well-defined energy. But gamma rays are not the only thing to look for – anti-protons, positrons or neutrinos are all promising candidates, which could provide details about the nature of dark matter.

An alternative approach is based on the fact that – as we know from the measurements of rotation curves – dark matter particles are not just in the Galactic centre, but everywhere in the Milky Way. Consequently, we expect there to be dark matter in the solar system – even at the position of the Earth – like a gas of particles moving at several hundred kilometers per second. These particles constantly arrive at the surface of the Earth: every second hundred thousands of them pass through an area the size of your thumbnail. Almost all of the dark matter particles will come out on the other side of the Earth without having interacted with anything (indeed, most of them haven't interacted with anything for billions of years). But if they do interact, for example with nuclei, they can transfer large parts of their energy in the scattering process. Even if the probability for such an interaction is tiny – since there are so many dark matter particles, we have a good chance of observing recoiling nuclei, provided we construct a suitable detector. Such a detector has to be built deep underground, to be shielded from cosmic rays – dark matter particles have of course no problem to travel through huge layers of rock. At present several such detectors are in operation worldwide. Their performance is absolutely mind-blowing: Even if over the course of one year only one atom per kilogram target material experiences a dark matter collision, one would get a clear signal in the detector.

An even more adventurous path is to attempt to create dark matter in the laboratory by recreating the conditions shortly after the Big Bang. This goal is exactly what the Large Hadron Collider at CERN has been constructed to achieve by colliding protons at very high energies, similar to the energies in the very early universe. Unfortunately, we already know that dark matter particles have very weak interactions, so if we manage to produce them at the LHC, they will just escape from the detector without leaving a trace. The good news is that the detectors at LHC are able to "see" even such invisible particles, because they can tell if something disappears in a collision (for example, because momentum conservation seems to be violated when only taking visible particles into account). Thus, there are excellent chances that the LHC will soon have first measurements of dark matter production.

So far, the interactions of dark matter remain invisible – but the search has only just begun. Over the coming years, the LHC will increase both its energy and the luminosity of the beam, new underground detectors will be built, some of them having several tons of target material and telescopes will reach unprecedented sensitivity. No matter whether we first observe dark matter annihilation, scattering or production, we will immediately begin to learn about its particle properties. What is its mass? Does it have spin? Does it interact via any of the known forces of nature or via completely new, yet undiscovered, interactions? Within the next few years, we will hopefully begin to answer these questions and thereby solve one of the greatest puzzles of particle physics.