Tiny particles can have huge amounts of energy

Dark matter: searching for clues in space

The unknown dark matter is particularly dense in areas of strong gravitational forces, for example in the vicinity of huge black holes in the centers of galaxies or in large star clusters. It could also have accumulated in the center of the earth or in the center of our sun. The dark matter particles were formed shortly after the Big Bang. Shouldn't we then be able to observe the annihilation of these particles in areas of great concentration of dark matter?

Dark matter probably consists of heavy particles that fly through space guided by gravitational forces and otherwise only have very weak interactions with other particles. Candidates for such WIMPs (Weakly Interacting Massive Particles) are the lightest supersymmetrical particle or the lightest Kaluza-Klein particle (see the articles “The Particle Doubler: Supersymmetry” and “Dark Matter: Mysterious Particles Hidden”). Since the matter in space still today, 14 billion years after the Big Bang, consists of around 80 percent dark matter, these WIMPs have to be stable. This is explained by the fact that the new particles also have a new property, a so-called parity, which cannot disappear without a trace. If so, the WIMPs must always have been created in pairs during the Big Bang.

Destructive WIMPs

Destruction of two WIMPs

Even though the WIMPs are stable, they can still go away. If concentrated energy can produce WIMP pairs, then conversely WIMP pairs can also radiate into energy, from which lighter particles then emerge, which we all know from the Standard Model (see article “Ingredients for a Universe”). To do this, however, our WIMPs have to come extremely close, and even then nothing happens very often because the forces between the WIMPs are very small.

But let's imagine a large center of gravity, for example a huge black hole, as it often occurs in the center of galaxies. Then the WIMPs should actually accumulate in the course of time in the vicinity of the hole, i.e. come much closer on average than in other spatial areas. Although the forces between the WIMPs are weak, with the large number of WIMPs buzzing around, every now and then two will annihilate. Of course, this cannot have made too much difference to the total amount of dark matter in the last 14 billion years, otherwise the universe would not contain so much of it today. But nevertheless it could be that such centers of gravity represent small hot spots in space, so to speak, in which a significant number of the mysterious WIMPs are radiated.

But which particles can in turn be formed in the WIMP annihilation? Amazingly, that is very easy to answer. In principle, all possible particles of the Standard Model can be generated, but almost all of them decay into lighter particles themselves, so that in the end only a few types of particles remain. On the one hand, there are photons and neutrinos, the only possible products that are electrically neutral. Electrons and protons as well as the corresponding antimatter particles, positrons and antiprotons, are created as electrically charged particles. And all these particles have a great energy, which is only limited upwards by the energy equivalent of the WIMP mass.

Witnesses from outer space

Fortunately, this is all very clear and even gives us several exciting opportunities to watch the radioactive WIMPs from Earth, so to speak. In contrast to the charged particles, the neutral particles are not deflected in the magnetic fields that exist everywhere in our galaxy and also in the entire universe, and are whirled around completely. Like rays of light, they therefore reach us on a straight path from the place of production, for example the cosmic centers of gravity.

In principle, we can “see” the centers of gravity with high-energy photons and neutrinos, if we succeed in detecting these messenger particles. The WIMPs reveal themselves not only through the location of the event, but also through the energy distributions of the photons and neutrinos, which are essentially determined by the mass of the WIMPs.

The charged particles, on the other hand, are deflected by magnetic fields in the universe and cannot reveal the directions of the sources of the WIMP annihilation. However, they provide another spectacular fingerprint of dark matter: positrons and antiprotons are antimatter. One would have to try to measure whether antimatter particles from space hit the earth and of course how often and with what energies. In addition, antimatter can collide with matter in interstellar space directly at the point where it occurs, and in the process itself annihilate. For example, if a positron collides with an electron, two photons, or more precisely X-rays, are created with an energy precisely determined by the mass of the electron and positron. So we should also try to detect such X-rays from space.

Astroparticle physics in search of dark matter in space

Light sensors of a neutrino telescope

Although the experimental requirements are high, the elementary particle physicists and astrophysicists have teamed up to search for all the described fingerprints of the annihilating dark matter in space. This has created a completely new sub-area, astroparticle physics, in which the detector technology of elementary particle physics is used to enable the observation of high-energy particles from space.

The detection of high-energy neutrinos is particularly difficult because neutrinos only exchange weak force effects with matter. Therefore, one needs huge detectors, preferably cubic kilometers in size, in which one of the many neutrinos passing through occasionally leaves a signal. But do not worry, the landscape will not be spoiled by gigantic detector cubes made by neutrino physicists. Rather, natural detection material is used, such as the water of a deep lake or sea or the ice of the Antarctic, into which a matrix of sensitive light sensors is sunk (see article on “Neutrinos”). When a neutrino interacts, it generates highly energetic charged particles that immediately lead to characteristic bluish flashes of light, the Cherenkov light, in the water or ice. This is measured and allows the direction and energy of the neutrino to be reconstructed. In addition, improved or cheaper detection methods are constantly being sought. For example, research is currently being carried out into whether the “impact” of a neutrino could perhaps be made audible with highly sensitive microphones.

Cherenkov telescopes from MAGIC

High-energy photons are much easier to detect than neutrinos, because they feel the electromagnetic force and therefore cannot penetrate deeply into matter. It has long been known how to detect and measure X-ray photons and the even more energetic gamma photons. The instruments required for this can be attached to satellites, for example, and launched into orbit with rockets. This also works quite well, but this method has practical limits.

If the energies are too high, the detectors would have to be much too big and heavy for meaningful measurements to be launched into orbit. In this case, as in the case of neutrinos, a natural detection medium, the earth's atmosphere, is used. When it hits the upper atmosphere, a high-energy gamma photon generates a huge rain of particles that penetrates deep into the atmosphere, and these particles again emit a flash of bluish Cherenkov light. This can be measured with reflecting telescopes, and again one obtains the flight direction and the energy of the original gamma photon.

INTEGRAL satellite

There are now many sources of X-ray and gamma radiation in space, up to and including photon energies that are around a hundred times greater than those that we can currently generate artificially at our particle accelerators. Whether this radiation is entirely of “ordinary” astrophysical origin, or whether dark matter is involved, is currently being intensively researched with increasingly better equipment. Exciting signs of dark matter fingerprints are already there, but the final word has not yet been spoken.

In contrast, no permanently radiating neutrino source has yet been discovered, but that should change when the new giant detectors go into operation. Neutrino astronomy has the advantage over astronomy with high-energy photons that sources inside the accumulations of matter become visible. The WIMPs could accumulate inside the earth or the sun and emit neutrinos from there, which reach our detector, since the earth and sun are practically "transparent" for neutrinos. Other particles produced by WIMP annihilation, such as photons, would be absorbed immediately and could never reach our detector.

The Alpha Magnetic Spectrometer at the ISS space station

The search for and measurement of antimatter radiation from the universe is also carried out with compact detectors for elementary particles, which are either brought to great heights with balloons or launched into earth orbit with rockets (see article “Antimatter in the universe”). These detectors detect particles and antiparticles directly as they pass through and determine their type and charge. Here, too, there are already a number of interesting observations that could be related to WIMP annihilation. However, one has to be careful, as very "common" sources of antimatter also exist in space. Our galaxy is interspersed with very high-energy radiation from protons and atomic nuclei, the so-called cosmic rays. When such a cosmic radiation particle hits an atomic nucleus in the interstellar medium, a great deal of energy is released, which materializes in the form of new particles and antiparticles.

So there is already a lot of evidence that suspiciously resembles the fingerprints of dark matter. The prospects of finally transferring dark matter with the improved instruments of the next generation are excellent.