Observation of dark matter annihilation signs
Dark matter, constituting most of the matter in the universe, is not easy to see. It is dark. And yet there is one way by which dark matter (TM) can, in a sense, shine.
And what is it? If TM consists of particles attributed to itself by antiparticles (as is the case with photons, Z particles and Higgs particles, and probably neutrinos), then it is possible that two TM particles will meet each other and annihilate (just as they can annihilate an electron with a positron, or two photons), turning into something else that we can probably detect - for example, two photons, or any other particle and its antiparticle. Whether we are able to detect this effect depends on many things unknown to us. But there is nothing wrong with looking for this phenomenon, and there is a very good reason to try.
How do we hope to find it?
Fig. one
First we need to look at the center of our galaxy, the Milky Way. Just as an accident is likely to be seen in dense traffic at rush hour, collisions of dark matter particles are most likely to be seen where its density is greatest. And the greatest is in the centers of galaxies. The reason is that (see Fig. 1) large chunks of dark matter form around galaxies and stars - in fact, most of the Milky Way's mass is dark matter, distributed over a coarse sphere, although its exact structure is unknown and probably quite complicated. The stars and the large atomic clouds from which they are formed constitute a rotating disk with spiral arms, located inside this large sphere and having a ball of stars ( bulge ) in the center. Stars in the disk and bulge probably accumulate in the places with the highest concentration of TM. So, collisions and subsequent annihilation, leading to the appearance of particles that we can detect, can occur near the center of the galaxy, so we need to develop scientific tools that can look in this direction and look for hints that such annihilations are happening.
Unfortunately, it is not so easy to get hints, since there are not too many types of known particles that, being created in the annihilation of dark matter not far from the center of the Galaxy, can reach the Earth. The only sufficiently long living particles capable of reaching the Earth are electrons, antielectrons (positrons), protons, antiprotons, several other stable atomic nuclei (helium), neutrinos, antineutrinos, and photons. But neutrinos (and antineutrinos) are extremely difficult to detect, and almost all other particles are electrically charged, so their paths are bent and twisted in the magnetic field of the Galaxy, which is why they never reach the Earth. It also ensures that if they came to us, we could not say whether they came from the center of the galaxy or not. The photons remain as the only particles that, firstly, can move directly from the center of the Galaxy to the Earth, and secondly, are easily detected.
Fig. 2
A good hint of the annihilation of TM can be given by unusual high-energy photons coming from the center of the Galaxy, and more practically from nowhere (Fig. 2).
However, this strategy has many obstacles. In the center of the galaxy there are many unusual astronomical objects that also emit high-energy photons. How to distinguish photons emanating from the annihilation of TM, and photons coming from an unknown class of stellar processes, which may be more common in the center of the Galaxy than anywhere else?
Answer: not easy, except for one special case. If TM particles (with a certain mass, for example, M) can sometimes annihilate, turning into exactly two photons, then for both these photons the energy of motion will be equal (with very good accuracy) to the mass energy Mc 2 of dark matter particles. The reason is simple - it is described in the article on the annihilation of particles and antiparticles and is indicated in fig. 3
Fig. 3
If the particle and antiparticle are practically at rest, then the energy of each of them is almost completely contained in the mass and is almost exactly equal to Mc 2 . The pulses of both are almost zero. The energy and momentum are conserved; therefore, the total energy is approximately equal to 2 Mc 2 before and after annihilation. When a particle and an antiparticle turn into another particle and an antiparticle, the energies of both of them will be equal to Mc 2 . Usually it will be a mixture of mass energy and motion energy. In the case when the final particle and antiparticle turn out to be photons that have no mass and, accordingly, mass energy, all their energy will be the energy of motion.
We do not know the mass M of the particle TM, and we do not know the energy of the total photons. But since both all electrons have the same mass, and all protons have the same mass, so all TM particles have the same mass, each TM annihilation will lead to the appearance of two photons with an energy almost equal to Mc 2 . And this means that if we measure high-energy photons emanating from an area close to the center of the Galaxy using a special telescope and plot the number of photons from their energy, we should expect that many astrophysical processes will create many photons with different energies that will form smooth background, but the processes occurring with TM, will add a handful of photons of the same energy - a burst rising above the background (see. Fig. 4). It is almost impossible to imagine an astronomical object, any strange star that would be simple enough to create such a burst - so a signal in the form of a narrow burst would be a clear indication of the process of annihilation of pairs of TM particles.
Fig. four
And this is a very powerful way to search TM. It will not work if TM particles are not antiparticles for themselves and cannot annihilate. It will not work if TM particles do not very often produce photons during annihilation. But it can work. And there are already attempts, the most interesting of which is the use of the Fermi space gamma telescope , an experiment with a satellite operating in space and measuring photons coming from all corners of the sky, including those coming from the center of the Galaxy.
And what is it? If TM consists of particles attributed to itself by antiparticles (as is the case with photons, Z particles and Higgs particles, and probably neutrinos), then it is possible that two TM particles will meet each other and annihilate (just as they can annihilate an electron with a positron, or two photons), turning into something else that we can probably detect - for example, two photons, or any other particle and its antiparticle. Whether we are able to detect this effect depends on many things unknown to us. But there is nothing wrong with looking for this phenomenon, and there is a very good reason to try.
How do we hope to find it?
Fig. one
First we need to look at the center of our galaxy, the Milky Way. Just as an accident is likely to be seen in dense traffic at rush hour, collisions of dark matter particles are most likely to be seen where its density is greatest. And the greatest is in the centers of galaxies. The reason is that (see Fig. 1) large chunks of dark matter form around galaxies and stars - in fact, most of the Milky Way's mass is dark matter, distributed over a coarse sphere, although its exact structure is unknown and probably quite complicated. The stars and the large atomic clouds from which they are formed constitute a rotating disk with spiral arms, located inside this large sphere and having a ball of stars ( bulge ) in the center. Stars in the disk and bulge probably accumulate in the places with the highest concentration of TM. So, collisions and subsequent annihilation, leading to the appearance of particles that we can detect, can occur near the center of the galaxy, so we need to develop scientific tools that can look in this direction and look for hints that such annihilations are happening.
Unfortunately, it is not so easy to get hints, since there are not too many types of known particles that, being created in the annihilation of dark matter not far from the center of the Galaxy, can reach the Earth. The only sufficiently long living particles capable of reaching the Earth are electrons, antielectrons (positrons), protons, antiprotons, several other stable atomic nuclei (helium), neutrinos, antineutrinos, and photons. But neutrinos (and antineutrinos) are extremely difficult to detect, and almost all other particles are electrically charged, so their paths are bent and twisted in the magnetic field of the Galaxy, which is why they never reach the Earth. It also ensures that if they came to us, we could not say whether they came from the center of the galaxy or not. The photons remain as the only particles that, firstly, can move directly from the center of the Galaxy to the Earth, and secondly, are easily detected.
Fig. 2
A good hint of the annihilation of TM can be given by unusual high-energy photons coming from the center of the Galaxy, and more practically from nowhere (Fig. 2).
However, this strategy has many obstacles. In the center of the galaxy there are many unusual astronomical objects that also emit high-energy photons. How to distinguish photons emanating from the annihilation of TM, and photons coming from an unknown class of stellar processes, which may be more common in the center of the Galaxy than anywhere else?
Answer: not easy, except for one special case. If TM particles (with a certain mass, for example, M) can sometimes annihilate, turning into exactly two photons, then for both these photons the energy of motion will be equal (with very good accuracy) to the mass energy Mc 2 of dark matter particles. The reason is simple - it is described in the article on the annihilation of particles and antiparticles and is indicated in fig. 3
Fig. 3
If the particle and antiparticle are practically at rest, then the energy of each of them is almost completely contained in the mass and is almost exactly equal to Mc 2 . The pulses of both are almost zero. The energy and momentum are conserved; therefore, the total energy is approximately equal to 2 Mc 2 before and after annihilation. When a particle and an antiparticle turn into another particle and an antiparticle, the energies of both of them will be equal to Mc 2 . Usually it will be a mixture of mass energy and motion energy. In the case when the final particle and antiparticle turn out to be photons that have no mass and, accordingly, mass energy, all their energy will be the energy of motion.
We do not know the mass M of the particle TM, and we do not know the energy of the total photons. But since both all electrons have the same mass, and all protons have the same mass, so all TM particles have the same mass, each TM annihilation will lead to the appearance of two photons with an energy almost equal to Mc 2 . And this means that if we measure high-energy photons emanating from an area close to the center of the Galaxy using a special telescope and plot the number of photons from their energy, we should expect that many astrophysical processes will create many photons with different energies that will form smooth background, but the processes occurring with TM, will add a handful of photons of the same energy - a burst rising above the background (see. Fig. 4). It is almost impossible to imagine an astronomical object, any strange star that would be simple enough to create such a burst - so a signal in the form of a narrow burst would be a clear indication of the process of annihilation of pairs of TM particles.
Fig. four
And this is a very powerful way to search TM. It will not work if TM particles are not antiparticles for themselves and cannot annihilate. It will not work if TM particles do not very often produce photons during annihilation. But it can work. And there are already attempts, the most interesting of which is the use of the Fermi space gamma telescope , an experiment with a satellite operating in space and measuring photons coming from all corners of the sky, including those coming from the center of the Galaxy.
Source: https://habr.com/ru/post/409807/