Experimental hints about the existence of dark matter

A brief overview of hints of dark matter - signals (two of which were found in the sky, and four - under the ground), which may mean that these particles of dark matter are doing something interesting. A couple of signals may be true, but not all six, because some of them contradict each other. This should not worry you: such a situation is perfectly normal for advanced science; Research is difficult, and most of the hints of something amazing turn out to be mirages - statistical accidents, hitherto unknown strangenesses, measurement problems, or simply trivial errors. In the case of, for example, the Higgs particle, we had several false alarms until finally the alarm turned out to be true. So we need to be patient and cautious and not lose hope; discoveries are rare, but happen.

Dark matter above head

The information received from the Fermi satellite hints that a stream of photons of certain energies emanates from the center of the Galaxy (about 135 GeV, that is, with an energy of mass approximately 143 times more than that of a proton). This could potentially become a sign of the presence of dark matter particles (these particles moving slowly in a circle should be especially numerous in the center of the Galaxy), which collide with each other, annihilate and turn into photons.

Briefly, it goes something like this: the energy conservation law ensures that the energy of two annihilating dark matter particles (mostly represented as mass energy, since dark matter particles move very slowly through the Galaxy) is converted into the energy of motion of two photons - therefore the energy each photon is equal to the mass of the dark matter particle multiplied by c 2 .

Do I need to worry about the fact that this signal may not be what it seems? A small problem is that standard wimp (a massive particle interacting with matter through weak nuclear interaction) cannot produce such a signal without giving out other signals that we should also see (for example, a huge amount of lower-energy protons) . But the popularity of wimps is slightly exaggerated, and other types of dark matter particles, which theorists have imagined for many years, are quite capable of doing everything necessary.

More serious concerns are that the signal does not just come from the center of the Galaxy, it also comes from the edge of the Earth’s limb , and possibly the Sun. Such behavior from the annihilation of dark matter can not be expected. And the fact that this signal appears in such strange places where it was not expected could mean that all this is just an unobvious problem with Fermi's photon detector. No one knows for sure yet.

Another example. In an experiment with a magnetic alpha spectrometer (born Alpha Magnetic Spectrometer, AMS) working on the ISS, a large “discovery” was recently announced (although most press releases forgot to mention that they just confirmed that the PAMELA experiment in 2008). PAMELA discovered, and AMS confirmed, and studied in more detail that there is a huge excess of high-energy positrons in outer space compared to what one would expect (positrons are antiparticles of electrons). The "extra" positrons have different energies from 10 GeV to at least 350 GeV - and then the AMS data no longer goes.

It is possible that these positrons appeared due to the annihilation of dark matter particles. But if so, it cannot be TM particles of the same type as the Fermi experiment sees in the center of the Galaxy. Any TM particles responsible for the signal with AMS would have a mass of more than 350 GeV / s 2 to produce 350 GeV positrons of energy, despite the fact that if the photons that Fermi sees are produced by TM particles, then such particles never would produce a positron with energy above 135 GeV. This follows only from the conservation of energy; if the mass of each of the two TM annihilating particles is equal to 135 GeV / s 2 , and they move rather slowly, due to which the energy of their motion is sufficiently small, the electrons and positrons produced in annihilation cannot have an energy greater than 135 GeV. So Fermi and AMS cannot both see the effects of the presence of TM - at least one of them sees something else.

As they said back in 2008 (and experimenters with AMS are careful to recognize), those positrons that PAMELA saw then, and what AMS sees now, can be generated by astrophysical effects, for example, by a nearby pulsar (a rapidly rotating star with powerful magnetic field, which can serve as a natural particle accelerator and become a source of additional electron-positron pairs). And as everyone knows since 2008 (and that experimenters with AMS had the negligence not to recognize), the simplest neutralino predicted by the supersymmetry theories (or any other wimps) cannot produce such powerful signals, unless there is a force unknown to this day increase the rate of annihilation. And even then, we would not have seen such positrons without other signals - if only we did not assume that this TM belongs to a very uncommon variety. Extraordinary theories are cool in their own way, but TM particles in such matters are not simple Wimps with supersymmetries, which were mentioned in AMS articles.

Dark matter underfoot

We continue. Does anyone remember the DAMA project (now DAMA / LIBRA )? They claim there is evidence of the existence of dark matter for more than ten years! And they do have some kind of signal! Maybe from dark matter, but maybe not.

You see, one of the smart ways to find TM is to let her find you. Simply place a piece or whole barrel of carefully selected and purified substance in a mine deep underground. (Descending into the earth greatly reduces the effects of cosmic rays - high-energy particles from deep space). Since TM must pass straight through ordinary matter, and rarely leave traces, the flow of TM particles will flow directly through the stone, into the mine and through the barrel of the material. And if you are very, very patient, one of these particles of TM can collide with the atomic nucleus inside your material, and this kick can become loud enough so that you can detect it if you have developed a clever enough experiment. This is exactly what DAMA, XENON, CoGeNT, CRESST, CDMS, and a bunch of other experiments are doing - and they have been doing for quite some time.

But doing it is harder than saying. Radioactivity — a process in which an atomic nucleus changes its type by spitting out one or two high-energy particles — can mimic the effects of a TM particle. (The process that imitates your “signal” - what you are trying to detect - is called the “background”). The background in detecting TM particles is often stronger than the signal itself, and experimenters need to understand all possible backgrounds very, very well if they want to find something so small.

But now, returning to DAMA, what can be done from a series of damn ingenious. During the year, the Earth moves around the Sun, and its speed relative to the average speed of the TM particles changes. This is similar to how if you ride a bike on a ring track on a windy day, sometimes the wind will blow in your face, and sometimes push in the back. Just as the wind power changes when you circle the track, so the wind speed from TM changes during the year. And if the probability that TM particles interact with the core depends on the relative speed of the two of them (which is done in many variants of what TM is), then the number of collisions with TM, measured in the experiment, should increase and decrease with a cycle per year. .

So instead of simply looking for signs of several collisions, which may simply be the result of radioactivity that you misunderstand, you may need to look for variations in the number of collisions during the year! If you convince yourself that radioactivity and other backgrounds alone cannot have an annual cycle, then any such type of oscillation is clear evidence of TM. Just as a breeder in a strong wind feels a very strong wind when traveling towards him, and a weaker one when traveling in a different direction, so the Earth in orbit around the Sun moves with greater or lesser speed relative to the TM particles that are nearby during the year. . This can lead to a fixation of the number of collisions with TM, cyclically varying throughout the year.

Unfortunately, even though it sounds beautiful, background phenomena can actually cycle through the year, possibly due to the fact that small temperature changes can cause more or less radioactive gases to circulate in the mine, or something like that. . So, although the data from DAMA / LIBRA uniquely demonstrate fluctuations in the number of collisions of candidate particles for TM, it is still not entirely clear whether this is really TM. So far, no one has been able to confirm their signals, but no one has been able to prove that this is a false alarm.

DAMA / LIBRA is not one. Recently, the CoGeNT experiment reported the discovery of an excess of possible collisions, the number of which, like that of DAMA / LIBRA, fluctuates throughout the year.

And that's not it. The CRESST experiment also reported a heap of candidates for TM particles that hit atomic nuclei in their detectors. There are several possible effects that can give candidates of this type - but, according to them, if you add up all these effects, you get about 42 candidates, and they have already seen 67, which is more than 4 standard deviations - this is pretty strong evidence that something is missing.

Finally, one more hint: the CDMS experiment reported on the fixation of three candidates for TM collisions in their pieces of silicon. They have silicon and germanium based detectors. A new result was obtained on the basis of data from silicon detectors. Since the silicon core is much lighter than the germanium core, silicon reacts better to it when it collides with lightweight TM particles. And it is very interesting!

But, as they themselves accurately state, it is hardly possible to call the result decisive. This is almost certainly not the result of background effects. At first glance, this is not obvious; the backgrounds they know should produce on average only half of the collisions, and the possibility of obtaining these three events is about 5% - not quite unbelievable when you consider how many unlikely things can happen in the experiment. But when they take into account the energies of these candidates for collisions, the probability drops to 0.2%. And here the matter becomes serious. But remember: all this means that either (a) they opened the TM, or (b) they discovered a still unknown background activity that gives a false signal.

If you put all these four experiments together, the news is both good and bad. The good news is that all four of these experiments — DAMA / LIBRA, CRESST, CoGeNT, and CDMS — correspond to TM particles somewhere in the range of 10 GeV / c 2 .

Moderately bad news is that the four dimensions do not match; of the probability of interaction of TM particles of a certain mass, the following experiments do not coincide, and differ up to ten times. This is shown in the figure below (taken from the CDMS paper ), where it is shown that the four different bands associated with the observations of the four experiments usually do not overlap. This means that at least two of these experiments should be false alarms.

The figure shows allowable and unacceptable areas (with 90% accuracy) as a function of the mass of the TM particle (horizontal axis) and the number of interactions with ordinary matter (vertical axis). DAMA / LIBRA, CRESST and CoGeNT are shown in yellow, brown and pink, respectively. New CDMS results are given in blue and blue; a black star is the best approximation. Note that there are no points where three or four sections would intersect at once. In this case, the results of the analysis in experiments XENON10 and XENON100 exclude all areas that lie above the light green and dark green lines, which includes all four other experiments.

Very bad news comes from the results of another experiment, which should (seemingly) be more sensitive to TM particles of this type than any other of these experiments. I mean the XENON100. For most of the signals in the XENON100, many candidate events must have occurred, dozens or even more. But for now, they only saw two. And it turns out that all these signals are excluded by the XENON100 experiment, as well as by a special analysis of its predecessor, XENON10. One can argue that the results of CoGeNT and CDMS are disproved barely, and therefore perhaps they should be taken seriously.

But the sobering fact is that in all these underground experiments, a small unrecorded background should manifest itself in the form of several additional low-energy collision candidates that will very strongly resemble what can be expected from low-mass TM particles.

As Professor Juan Collard, head of the CoGeNT experiment at the University of Chicago, said at a conference at the CUNY Science Center in New York a few years ago, the search for TM saga is likely to be a long history of discovering one unexpected background after another - and this story can continue for a long time, until TM is actually found, if at all, it is found in one of these experiments. And this is reflected in the many false alarms we have seen lately. Interestingly, Collard stopped making such statements after the CoGeNT began to receive a signal that can be interpreted as TM. But remember what you said, Juan. We remember.

In the meantime, it is for the sake of such enigmas that theoretical physicists live. Puzzle! Call! Invent a theory of TM so that the CDMS and CoGeNT experiments can easily detect its effect, but the XENON100 could not! Experiments work in different ways - CDMS and CoGeNT consist of silicon and germanium, respectively, and XENON100 uses a surprise! - A barrel of xenon. There are already many works on this topic. Most likely, it turns out that XENON100 is right, while CDMS and CoGeNT see some background. But maybe everything will be exactly the opposite.

To summarize: we have at least six hints about the existence of TM, for the most part not relevant to each other. A new hint of CDMS roughly matches CoGeNT; but if they both see TM, why doesn't the XENON100 see a strong signal? All these experiments are working to improve their methods and measurements, so if some of these hints really prove to be TM, we will soon see more examples of impressive evidence.

Source: https://habr.com/ru/post/409551/

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