Schlagwort: ‘dark matter’
Looking for Dark Matter: Dark(ness) at the end of a tunnel
In the last post, we introduced the “Shake It, Make It, Break It” approach for dark matter detection and talked about shaking dark matter to deduce its properties (what is usually called Direct Detection). Of course, the ideal way to study dark matter would be to create it in our laboratories, which brings us to the second approach: Make It.
Attempts to make dark matter are carried out in particle colliders. To make dark matter we destroy light (aka visible matter). But before we go down that road, let’s have quick look at the Standard Model of Particle Physics. This model describes the physics of everything visible around us. It tells us the structure of atoms, the workings of three of the four forces governing all interactions in nature, the mechanism by which particles gain mass, the mechanism by which they decay. Essentially, it is a summary of our knowledge of particle physics. And although the standard model has been an incredible success, it appears to be somewhat incomplete. For example, we don’t yet have a particle that mediates gravity, the fourth force. We don’t know conclusively why neutrinos have mass. Or why the Higgs mass appears fine-tuned. The incompleteness of the Standard Model provides a strong motivation for additional particles. If dark matter is composed of particles, it could be one of the missing puzzle pieces in the Standard Model. And much like in an actual puzzle, we can deduce the properties of the missing piece by the space its absence has created. We can figure out which Standard Model particles are likely to talk to dark matter and build robust models around these interactions. With this basis for dark-visible interactions, we can look for them at colliders.
A particle collider is used to accelerate particles to high energies, smash them together and study the resulting debris to understand the physics of nature at small (length) scales. One can look for dark matter at colliders by figuring out whether this debris matches our expectation from the Standard Model (which doesn’t include dark matter). A simple way to do this (in principle) is by using the law of conservation of energy– in any physical process, the total energy of the system remains conserved. The initial energy of the particle beams is something we know from an experiment’s design. The total energy after a collision can be reconstructed from the energy of the particles we detect. If these two numbers don’t match, we know that some of the energy has been carried away by “invisible” particles which could be potential dark matter candidates. (In practice, this is much harder to do which is one of the reasons we have an entire subgroup of physicists (theorists and experimentalists) devoted to working out the intricacies of collider physics.)
Another way to look for dark matter at colliders is by studying how the Standard Model particles produced in a collision decay. Consider the Z-boson. In the Standard Model, it can decay into quarks, leptons or neutrinos. We know the total decay width of the Z-boson which characterizes the probability that a Z-boson would decay. We can also get measurements on the individual probability of a Z-boson decaying into quarks, leptons, and neutrinos. A mismatch between these probabilities is a hint that a Z-boson decays into something else which is invisible to our detectors. Once again, we can deduce the presence of dark matter by its absence.
We’ve known for quite some time that there is more to the Universe than meets the eye. To understand it, we must exhaust all avenues available to us. Crashing particles being one of them.
Looking for Dark Matter: Trembles beneath the surface
The ever-elusive nature of dark matter is of interest not only from a theoretical point of view –we are, after all, missing 85% of the Universe– but also from an experimental one. How exactly does one study something invisible? We can point our telescopes at the sky and study how light bends around clusters of apparent emptiness, hinting that there must be something there. We can do this with ancient light (commonly known as the CMB) and figure out exactly how much of this invisible substance is present in our Universe. We can study the structure of galaxies and galaxy clusters, the motion of stars bound to them. Everywhere we look, it seems, there is evidence for something massive lurking in the shadows. But none of these things tell us anything about what a dark matter particle* actually looks like. We cannot “trap” a dark matter particle for it can, quite literally, slip through walls. We don’t have access to it in the way we have access to, say electrons. But as humans, we are tenacious. And as scientists, we are creative. The minor problem of dark matter’s invisibility won’t stop us in our quest for answers.
In this series of (hopefully) short posts, we’ll cover the three conventional avenues of dark matter detection. The elevator pitch for these techniques? Shake It, Make It or Break It. ‘It’ being dark matter.
Shaking dark matter is as fun as it sounds. In the simplest of terms, it involves waiting for a dark matter particle to strike a target atom placed inside a detector. As a result of the collision, the state of the target atom changes. It could either get ionized, resulting in the production of free electrons which we can ‘see’ in the detector. It could absorb the energy of the dark particle and then release it in the form of a photon. Or it could release this energy as heat. In all of these cases, the collision results in a visible/detectable signature.
There are two important questions to ask now:
1. Where does the initial dark matter particle come from?
2. How can we be sure that the electrons/photons/heat we detect is actually caused by this collision?
The answer to the first questions is fairly straight-forward. From various other measurements, we know that there is a constant (ish) dark matter flux through Earth. At any given instant, we are being bombarded with dark matter particles. To get an idea of how strong this bombardment is, consider these numbers. The dark matter flux (total dark matter mass passing through one cubic centimeter every second) is approximately 
. (GeV is the standard unit of mass in physics. 1 GeV is approximately 
kg.) So if we assume a dark matter mass of 1 GeV, this amounts to roughly 0.3 dark matter particles per centimeter cubed. The average volume of a human body is 66400 cubic centimeters. Which means that in one second, about 20,000 dark matter particles zip through every person on Earth. Given that dark matter is weakly interacting, it is no surprise that we don’t notice this constant flux. But given that the flux is substantial, we can hope to detect these particles by building detectors that are large enough. A larger detector volume means more dark particles passing through and hence a higher probability of collision.
Which automatically brings us to question two. How can we be sure that the cause of a signal in our detector is a dark matter-target atom collision. The short answer to this is we’re extremely meticulous. For every experiment, there is a ‘background’ that we need to be aware of. A background is basically “noise” which makes it hard to see the actual signal. In this case, any other particle colliding with the target would give rise to a background signal. A major part of these experiments is to account for all possible backgrounds and reduce this noise as much as possible. The first step in doing this is to build the detector underground. The Earth’s crust provides a natural shielding against most of the stray particles hanging around in our corner of the universe. Beneath that are layers of concrete to further stop any particles getting inside the detector. Note that since the dark matter particles are incredibly weakly interacting, they have no problem shuttling through the Earth’s crust or any of our other protective layers. Another important factor is the choice of the target atom itself. Since we want to avoid spurious signals, an atom that decays radioactively or is otherwise reactive would be a poor choice for the target. Some of the best target atoms are inert gases such as Xenon or Argon.
This is (very briefly) how a conventional direct detection experiment works**. We hide out in the depths of Earth, waiting. The obvious follow-up question is what happens if we don’t see a signal. Does it mean the dark matter paradigm is dead in the water (or liquid Xenon)? The answer is Not Quite. The lack of a signal is valuable information as well. For example, it can be used to infer that the dark matter-target interaction is weaker than we thought (meaning the probability of a collision is even smaller resulting in the absence of a signal). In this way, we can use the “null results” of an experiment to set limits on the strength of the dark matter — standard model interaction. So even though we don’t “detect” dark matter, we end up with more knowledge than we started with. And that, in the end, is the true spirit of science.
Up next: Can we make dark matter at colliders?
*The assumption that dark matter is composed of particles is well-motivated but it might very well be that dark matter is something more exotic such as primordial black holes.
**Detectors operate on the same principle (some kind of dark matter — particle collision) but can be experimentally realised in different ways.
TTK Outreach: A Beginner’s Guide to Dark Matter
In the post-truth society that we live in it is easy to fall down the rabbit hole of doubting every scientifically held belief. To wonder if NASA is hiding proof of intelligent extra-terrestrial life (they’re not), or if people at CERN are rubbing their hands plotting something nefarious (nope) or whether the Big Bang theory is a Big Bad Lie (it really…isn’t). But don’t worry, we at TTK have got you covered. Every Wednesday we answer your questions live on Twitter and every whenever-this-author-stops-procrastinating-day we give you a more elaborate explanation of some of the most frequently asked questions.
Today on the agenda: Dark Matter — what it is and why you should be reasonably sure of its existence.
Simply stated, dark matter is a kind of matter that doesn’t interact with light. This means we can’t “see” it in the conventional sense. As you would expect, this makes studying dark matter a bit difficult. But if there is one redeeming quality in humankind, it is that we don’t shy away from the seemingly impossible. Of course, the question remains, if we can’t see dark matter and if it doesn’t interact all that much with other things, how do we know that it exists in the first place? The answer comes to you in four parts.
1. Galaxy Rotation Curves
Some of the earliest indirect evidence of dark matter comes from galaxy rotation curves. A rotation curve is a plot of the orbital speed of stars or visible gas present in a galaxy as a function of their distance from the galactic center. If we assume that the total mass of a galaxy is only composed of normal or ‘visible’ matter, the farther we move away from the center (where most of this mass is concentrated), the lower the orbital speeds should get. This is what happens in the Solar system. Since the Sun accounts for most of the mass percentage, the planets farthest from it revolve slowly as compared to the ones close by.
However, measurements of galactic rotation curves don’t agree with this prediction at all. Instead of decreasing with distance, the orbital speeds of outlying stars appear to either stagnate to a constant value or increase. This points towards the possibility of an additional contribution to the mass of a galaxy from something we can’t see. Maybe something dark?
2. The Bullet Cluster
Another smoking gun for dark matter is the Bullet Cluster. It is composed of two colliding galaxy clusters, the smaller of which looks like a bullet. Galaxy clusters are a busy place and when they collide, chaos ensues. The stars, far apart as they are, mostly survive the collision without a story to tell (aka pass through). The particles present in the galactic plasma, however, smash and ricochet and radiate a lot of energy.
Galactic plasma makes up most of the baryonic (visible) mass of a cluster so we can derive a mass-profile for the cluster from this radiated energy. We can also model the mass-profile by studying the lensing effects of clusters. Because massive objects bend light, we can figure out their mass distribution by studying how they distort light from surrounding clusters. If the entire mass of a cluster is just the baryonic mass, these two mass-profiles should coincide. What we find instead, is that they are in exact opposition. In the image above, the pink regions are where the baryonic mass is present. The blue regions show where the total mass of each cluster is concentrated. The zero-overlap between the two implies the presence of a non-baryonic, invisible source of mass. Moreover, it purports that most of the mass of a cluster is non-baryonic or dark. (In fact, roughly 80% of the universe’s matter content is dark!)
Quick Side Note: Keep in mind that the colors are for purely representative purposes! The radiation emitted by the galactic plasma doesn’t fall within the visible spectrum. Similarly, the blue is where the experiments tell us dark matter is concentrated.
3. Large Scale Structure Formation
An interesting question to ask cosmologists is why does the universe have a structure? How do we go from a more or less homogeneous particle soup to well-defined clusters of galaxies and then even to clusters of clusters of galaxies? The simple answer to this question is fluctuations. Tiny fluctuations right after the Big Bang lead to overdensities and underdensities of matter. As the universe expands, these fluctuations also grow on account of gravity and we end up with clumps of matter which would eventually form stars, galaxies, galaxy clusters, etc. There is one small problem with this line of reasoning though. We know that the early universe was dominated by radiation (or light). And light exerts pressure. So even as the fluctuations would cause matter to clump, radiation would cause it to homogenize. In the end, the fluctuations would be nearly wiped out and we wouldn’t have the kind of structure that we see today.
Dark matter solves this problem. It is massive and it doesn’t interact with light. Formation of dark matter lumps would aid the ‘clumping’ of normal, baryonic matter and give rise to structure despite the homogenizing effect of radiation.
4. Cosmic Microwave Background
The CMB can be regarded as a picture of the baby universe. And though at first glance it might look like random splotches of paint, it provides deep insights into what the universe looked like billions of years ago. Any cosmological model that we create has to be in agreement with this map. By specifying initial conditions — for instance, percentage of matter, dark matter and radiation — we should end up with density fluctuations as observed here. The best model we currently have is the ΛCDM. As you might have guessed, the DM here stands for dark matter. It is only when we include dark matter in the model that our predictions line up with the data.
These are just a few of the reasons we believe that dark matter exists. And even though we haven’t detected anything like a dark matter particle (yet), everywhere we look the universe seems to suggest that it must be there. If you still don’t understand why you should believe in it, (and as a reward for reading these 1000+ words), here’s a (dark) analogy:
Exploring dark matter with IceCube and the LHC
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 http://arxiv.org/abs/1411.5917 and http://arxiv.org/abs/1509.07867. 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
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.
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.
We 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
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
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 »