RWTH Aachen Particle Physics Theory

Schlagwort: ‘dark matter’

Exploring dark matter with IceCube and the LHC

October 2nd, 2015 | by

Various astrophysical and cosmological observations point towards the existence of dark matter, possibly a novel kind of fundamental particle, which does not emit or reflect light, and which only interacts weakly with ordinary matter.


If such a dark matter particle exists, it can be searched for in different ways: direct detection looks for the elastic scattering of dark matter with nuclei in highly sensitive underground experiments, as Earth passes through our galaxy’s dark matter halo. Indirect detection experiments on Earth or in space look for cosmic rays (neutrinos, photons, or antiparticles) from the annihilation of dark matter particles in the centre of the Galaxy or in the Sun. And last but not least, if dark matter interacts with ordinary matter, it may be produced in high-energy proton collisions at the LHC.

To explore the nature of dark matter, and to be able to combine results from direct, indirect and collider searches, one can follow a more model-independent approach and describe dark matter and its experimental signatures with a minimal amount of new particles, interactions and model parameters. Such simplified or minimal models allow to explore the landscape of dark matter theories, and serve as a mediator between the experimental searches and more complete theories of dark matter, like supersymmetry.



About 1027 dark matter particles per second may pass through the Sun. They can loose some energy through scattering off protons and eventually be captured in the core of the Sun by the Sun’s gravitational pull. Dark matter particles in the Sun would annihilate with each other and produce ordinary particles, some of which decay into neutrinos. Neutrinos interact weakly with matter, can thus escape the Sun and could be observed by the neutrino telescope IceCube near the South Pole. Neutrinos therefore provide a way to search for dark matter in the core of the Sun.

At the LHC, dark matter may be produced in high-energy proton collisions. As dark matter particles interact at most weakly with ordinary matter, they would leave no trace in the LHC detectors. However, dark matter (and other novel weakly interacting particles) can be detected by looking at exotic signatures, where a single spray of ordinary particles is seen, without the momentum and energy balance characteristic for standard particle collisions (so-called mono-jet events, see right figure).


We have recently joined forces with members of the RWTH IceCube team to explore dark matter searches from neutrinos in the Sun and through dark matter production at the LHC, see and  We have considered a minimal dark matter model where we only add two new particles to the ordinary matter: a new dark matter fermion, and a new force particle, which mediates the interaction between the dark matter fermion and the ordinary matter. As no signal for dark matter has been observed, we can place limits on the masses of the dark matter particle and the new force particle, see figure to the left. We find a strinking complementarity of the different experimental approaches, which probe particular and often distinct regions of the model parameter space.

Thus only the combination of future collider, indirect and direct searches for dark matter will allow a comprehensive test of minimal dark matter models.

Axions, WIMPs or WISPs? Searching for dark matter

July 3rd, 2015 | by

PhD student Mathieu Pellen reports from a dark matter workshop in Zaragoza.

The quest for the understanding of dark matter is certainly one of the greatest challenges of the 21st century. It is thus an extremely hot topic in the particle physics community. 


The 11th Patras Workshop on Axions, WIMPs and WISPs has been held in the beautiful and hot city of Zaragoza (Spain) (21-26 June 2015). As the title indicates, the focus was on dark matter and more particularly on axions.

Axions have been originally proposed to solve the strong CP problem. They are light particles (of the order of an electron-Volt or even lighter). These can be detected in light-shinning-through-wall experiments or in low background underground laboratories like the one of Canfranc (which has been one the highlights of the conference). During the conference, several innovative experiments looking for axions, axion-like particles or dark photons have been presented. New mechanisms predicting the existence of light particles have been also proposed.

In addition to light particles, Weakly Interacting Massive Particles (WIMPs) are the best motivated solution to account for the dark matter observed in our Universe. WIMPs are studied in three different ways: the first is their production at collider experiments such as the Large Hadron Collider (LHC, Geneva). The second is the detection of nuclear recoils produced by dark matter particles scattering on heavy nuclei in underground facilities such as the Grand Sasso laboratory in Italy. Finally, when two dark matter particles annihilate in the galaxy, they produce cosmic rays of standard model particles. These can be detected in satellite-based experiments such as the Alpha Magnetic Spectrometer (AMS-02, partly built at the RWTH Aachen University) on the International Space Station (ISS).

My contribution to the conference focuses on the last possibility. I have reported exciting results on a project carried out with Leila Ali Cavasonza and Michael Krämer.CCnew2_09_14 Indeed, AMS-02 has reported an excess in the measurement of the positron flux (red date points, left figure) compared to standard expectations from astrophysical sources (green curve, left figure). This has triggered a lot of interest recently. The reason is that anti-particles are an extremely interesting observable when searching for dark matter. Indeed they are rarely produced from standard astrophysical sources. Thus the discovery of excesses in anti-particles fluxes could be already a smoking gun for the existence of dark matter. Nowadays, the dark matter contribution is believed to be sub-dominant in the AMS-02 observations. However, the absence of a “bump” – as expected from a from a dark matter signal – in the very smooth AMS-02 spectrum is a great opportunity to set limits on dark matter annihilation cross sections.

MathieuWe have derived new upper limits on the annihilation cross section using a new method that allows us to study dark matter with masses ranging from several TeV down to 1 GeV. In particular we have focused on the impact of massive electro-weak gauge bosons on these limits. Even if their contributions are limited, they are of prime importance as they produce all standard model particles when decaying. I have thus shown that there is a promising complementarity between different fluxes of anti-particles. This opens up new ways to exclude or find dark matter in the next few years using indirect detection.

Exploring dark matter through electroweak radiation

May 1st, 2015 | by

PhD student Leila Ali Cavasonza reports on a her recent work on indirect dark matter searches.

Investigating the nature of Dark Matter is certainly one of the most compelling and exciting goals of particle and astroparticle physics nowadays, both on the experimental and on the theoretical side.

It is now almost universally assumed that the Dark Matter consists of one or more new particles. According to the observations, this new particle is neutral, non relativistic, massive, weakly interacting and with a small self-interaction cross section. One of the most prominent candidate are the so-called Weakly Interactive Massive Particles (WIMPS).

These particles could interact with ordinary matter and be detected in the so-called direct detection experiments. Or they could be produced via annihilation of standard particles and discovered at colliders. Or they could annihilate into standard particles like photons, electrons and positrons or neutrinos and produce an excess in the fluxes of standard model particles observed in cosmic rays.


Indirect detection experiments, like the Alpha Magnetic Spectrometer (AMS, see left figure) measure the composition and the fluxes of the cosmic rays with very high precision in order to detect such possible excesses.

The AMS experiment has actually found a significant excess in the positron fluxes, that is at the moment not explained by standard astrophysics (figure below). On the other hand no excess has been observed for example in the antiproton fluxes.


The positron fraction measured by AMS (red circles) compared with the expectation from ordinary cosmic-ray collisions.

To explain this situation, the so called leptophilic models for Dark Matter have been introduced: According to these models the Dark Matter particles can annihilate only into leptons, like electrons and positrons, but not in hadrons, like the antiprotons.

However, the situation is not so simple. A very energetic electron produced via WIMPS annihilation can actually radiate a Z or W boson and produce at the end all stable standard particles, including antiprotons. To have a consistent picture and accurate predictions for the AMS experiment in the frame of leptophilic models it is then necessary to take into account the electroweak radiation off the standard model final state.

The so called fragmentation functions approximation has been developed in order to include these contributions in a simple and model independent way. In our paper arXiv: 1409.8226 we analyse the quality of this approximation. In particular we produce predictions for the AMS experiment within a simple leptophilic model including the electroweak radiation in the complete theory and with the approximation respectively. We then compare the predicted fluxes to understand how reliable the approximation is. It turns out that for some models the approximation is not valid. On the other hand, when valid, the approximation is actually very reliable and it is possible to obtain accurate predictions in a faster and simpler way.

Dark matter mysteries

July 20th, 2014 | by

The bullet cluster are two galaxy clusters roughly 3.8 billion light years away in the Carina constellation in the southern sky.  Galaxy clusters are gravitationally bound accumulations of galaxies. The bullet cluster is an object of particular interest: Since its discovery in 1995, it has been an object of study with different observation methods. In the optical light, there seem to be two separate galaxy clusters with a distance of roughly 0.7 Mpc. The X-ray observation reveals, that these two galaxy cluster collided in the past and are now separating again. The bullet cluster is a textbook example for such two objects interacting, leading to a bow shock which can be nicely studied in the X-ray image of the object. However, there is something else which is very interesting about this object: The collision separates two components of the galaxy clusters, namely the luminous mass of the cluster and the main mass components of the cluster, that can not be seen in the optical or X-ray region. This hints towards a large amount of dark matter taking part in the collision. And this makes it very interesting for particle physicists as well! Read the rest of this entry »