How dark matter is sought at the Large Hadron Collider

Dark matter is more elusive than lost car keys, and more mysterious than a badge burning on a car dashboard. It probably exists, and if it does, a large part of the matter of the Universe consists of it. It may consist of particles, and if so, and if scientists are lucky, then the Large Hadron Collider (BAC) will be able to create some of them. In any case, in the experiments conducted at the LHC, it is possible to look for such particles (although, perhaps, it will be easier to find the keys to the car).

In this article, I will try to answer obvious questions about how scientists with the LHC can observe the effects of a new, undetectable particle, and how they can get evidence that this particle really belongs to dark matter.

Detective: Do you want to draw my attention to anything else?
Sherlock Holmes: On a strange night with a dog.
Detective: But the dog did nothing at night.
Sherlock Holmes: That was strange.

- A.C. Doyle
')

How can BAC experiments detect undetectable?

Experiments on the BAC ATLAS and CMS can actually participate in the search for dark matter. This is not like finding keys, because in experiments no one hopes to detect dark matter directly. But after all, neither of them detects neutrinos directly!

Neutrinos, many times per second created in collisions of protons at the LHC, pass through ATLAS and CMS, without touching anything and leaving no traces. Despite this, ATLAS and CMS can conclude that neutrinos were produced — and they can use the same technology for dark matter. I will explain it now; it is pretty simple. And then I will explain a little more complicated thing - how can you distinguish dark matter from neutrinos.

Note: when I write "undetectable", I mean "undetectable in LHC experiments." Neutrinos cannot be detected at the LHC, but it is possible - with great difficulty and low probability - in completely different experiments. In such huge experiments, giant water tanks are involved, and in some cases they can only detect a few neutrinos a month! With dark matter, everything can be alike; many experiments have been designed for this.

The basic principle is the law of conservation of momentum. It is easy to illustrate, especially if you are clumsy enough. Take a glass of water, and sharply pour it straight down on the floor in the shower. As a result, splashes will appear. In fig. Figure 1 shows how water spreads in all directions and forms an approximately circular pattern on the floor. It is important that this happens in all directions. You will never see water splashing just to the left and not to the right. This happens as a result of the conservation of momentum.


Fig. 1: Impact of conservation of momentum. a) water sprinkles in all directions. b) the salute explodes in all directions. c) The plane flies forward as its turbines drive the air back. d) when firing a pistol, the bullet flies forward, and the pistol discards recoil. e) ejection down moves the rocket up.

You can come up with many examples in which the main role is played by the law of conservation of momentum. Details may vary, but the basic principle remains the same.




Fig. 2

In fig. 2 shows an experiment that you can repeat yourself. Inflate the ball, direct its neck towards you and release it. The ball will fly away from you. Why? Because the air from the ball rushes towards you - it can even be felt. But your friend, who is watching from the other side of the room, does not feel the outgoing air and does not see it. But if he knows the law of conservation of momentum, he can assume that the air must come out of the ball towards you - this is the only reason why the ball that was still starts to move away from you when you let it go. The ability to guess about the presence of something that you do not see, or to discover it in any way - this is the key idea of ​​the experiment.

The collision of two protons on the tank is like a spray of water in your soul, only the vertical axis is turned to the horizontal. The collision takes place in a frontal, on one axis - let's call it “the direction of the beam”, it goes from right to left in fig. 3. Let's call two other directions, from top to bottom and perpendicular to the image - transverse, or perpendicular to the direction of the beam.


Fig. 3

After the collision, dozens of particles (and other hadrons created due to the collision energy) appear and fly away, and they mostly fly in the direction of the beam. They are not very interesting to us - they are hard to measure, and they will not answer the questions of interest to physicists today. Also, there are particles with a very small momentum, which are also not important to us.

But sometimes some particles fly away in transverse directions and carry a large impulse - we are talking about their large “transverse impulse”. But the law of conservation of momentum suggests that since the initial protons did not have a transverse momentum, the resulting transverse momentum of all particles must be balanced. If one particle goes up, there must be one or more others going down. If a particle flies towards you, there must be those that fly from you.

A classic example of a collision is shown in Fig. 4. A collision of protons occurs at the center of the ATLAS detector, which has detected and measured traces of particles resulting from a collision. Then these traces were drawn on a computer so that scientists could see where they went. Most of the particles scattered left and right, and they are not shown here. Blue traces indicate the trajectories of particles with a very small momentum. But two yellow trails ending in yellow spots denote particles with high energies and pulses. One of them is an electron flying upwards. And even before we start another particle, we already know, from the conservation law, that at least one particle with a large transverse momentum must fly down. And here it is - a yellow mark at the bottom, which turned out to be an anti-electron, or a positron.


Fig. four

But in fig. 5 you can see another collision - from the CMS experiment. In it the electron flies upward, as in fig. 4. But not a single particle with a large transverse momentum flies down. What's happening?


Fig. five

Most likely, the particle flew down, but the experiment could not detect it. Since scientists know that:
• Neutrino and antineutrino cannot be detected on CMS,
• Electrons and antineutrinos often form together as a result of the decay of a W-particle,

It will be natural to assume that this is exactly what happens: the detected CMS electron flies up, the antineutrino flies down, which the CMS could not detect.

Of course, the question arises as to whether the impulse can be maintained. This is very unlikely - just look at a wide range of experiments conducted over several decades, including those that were conducted on ATLAS and CMS, and it will become clear that everything speaks in favor of maintaining momentum.

So far, everything has been sketchy and at a qualitative level, but it is important to understand that physicists can make accurate quantitative statements about the conservation of momentum. For example: if it is known that the momentum in the transverse directions is initially zero before the collision, then we can take all the moments from the transverse directions, add them as vectors, and expect their sum to be zero.

In the collision of protons, their momentum in transverse directions is zero. After a collision in ATLAS, the experiment measures all detectable particles. Some particles go in the direction of the beam, and they are not measured - but they do not have a transverse pulse. For some, the transverse momentum is negligible. But for some it may be great. If we add the transverse pulses, and their sum will be close to zero (no measurement would be perfect), we can conclude that ATLAS successfully detected all the particles. But if the sum is far from zero, it can be concluded that ATLAS could not detect one or several particles with a transverse momentum. These can be known particles — neutrinos — or unknowns, for example, dark matter.

Now you know that if dark matter particles appear in an ATLAS or CMS experiment, they cannot be detected. But experimenters will be able to assume, in the case where the sum of transverse pulses is non-zero, that one or more undetectable particles have been obtained.

Of course, the same thing happens when neutrinos are created in experiments - and this happens many times a second. So how can the LHC find out that something different from the neutrino was obtained on it? And how can scientists understand that this novelty is dark matter?

How can BAC experiments distinguish dark matter from neutrinos?

In the previous section, I explained how experimenters at ATLAS or CMS can learn that in one of the proton collisions one or more particles appeared that passed through an experiment without detection. But how can experimenters find out if they have found something new and surprising, for example, dark matter particles instead of the usual neutrinos that we have known for many decades? Why not just collect the usual suspects, instead of declaring that a new criminal has appeared in the city?

Simply put, it cannot be said what kind of undetectable particles appeared in this particular experiment. It is also usually unknown how many such particles have appeared. Instead, information is collected from a large number of collisions. Specifically, it follows from the comparison of the obtained data with the predictions of the equations used to describe the known particles and forces, and are called the "Standard Model". I will give you one example of how this works.

The easiest way to imagine that in the collision of protons was created two neutrinos, or two dark matter particles, or two certain undetectable entities. Suppose (fig. 6) that only these two particles have a significant transverse momentum (remember that many hadrons usually arise in collisions, but they usually fly away along the direction of the beam, and their transverse momentum is small). Then we will not see anything! For example, one of these particles can go up, the second - down, with the same in magnitude and opposite in the direction of the pulses - exactly as it was with the electron and the positron in Fig. 4. But if both particles are not detected, the transverse moment of the detected particles will look balanced, and we will not even know that undetectable particles were born there!


Fig. 6

But all is not lost. Usually, random high-energy gluons appear in collisions of protons at the birth of any particles with a large transverse momentum. Sometimes such a gluon (or several gluons) flies away in the transverse direction, also receiving a large transverse momentum. Then we will see something like fig. 7. Such an event is called a “mono-jet event], and it has a jet with a large transverse momentum (hadron splashes created by a gluon), ricocheting from“ nothing ”, probably from an undetected neutrino and anti-neutrino (from a decaying Z-particles).

Compare pic. 6 and fig. 7: Now we have a jet with a large transverse momentum, from which two undetected particles ricochet. Since we see the jet, we conclude that the transverse momentum of the observed particles is not balanced, and undetectable particles of some type were born.


Fig. 7

In fig. 8 shows the same collision as in fig. 7, only the direction of the beam on it is perpendicular to the image.


Fig. eight

Now a real example of a monojet observed in an ATLAS experiment. In the figure, the direction of the beam is perpendicular to the image.


Fig. 9

The ATLAS experiment has a bulbous structure and is equipped with sensors at several levels. The collision occurred exactly in the middle. In the “tracker” section, the trajectories of the particles that make up the jet are indicated. In the “calorimeter” sections (electromagnetic and hadron), the particle energy is marked with green and red spots. Note that there are no significant traces or spots anywhere, which means that the total transverse momentum is clearly not zero. Tracks leading up and to the left are too small transverse moment, and they go too close to the direction of the beam. Scientists believe that in this case, most likely, were obtained gluon, neutrinos and antineutrinos. But in fact, one cannot be sure which particles were produced in this collision.

The standard model allows with fairly good accuracy to predict in what percentage of proton collisions a certain shortage of transverse momentum will be observed. This is shown in fig. 10. The upper part of the blue area denotes the prediction of the Standard Model for the frequency with which neutrinos will appear with at least one jet (consisting of several components marked with different colors; blue is the greatest effect due to Z-particles generating neutrino pairs / antineutrino. The data are marked with black dots, and errors are shown with vertical bars.


Fig. 10. Data from CMS (black points) and predictions of the Standard Model (color sections). On the vertical axis - the number of events in which there is a certain shortage of transverse momentum; on the horizontal axis - the missing momentum E _T ^miss . Notice how well the data matches the predictions. The red line - the effect that gravitons would leave, disappearing in all additional dimensions - is obviously not confirmed. Note that the graph is logarithmic.

The dashed red line would be confirmed in the presence of gravitons, disappearing in additional dimensions . The data obviously coincide with the Standard Model and exclude the presence of gravitons. Also, the data do not agree (though not so obviously) with the possible appearance of dark matter particles (particles with a certain mass and interaction force), indicated by a solid blue line. If such particles appeared, then the last 2-3 points would be much higher.

In this example, you can see how well the Standard Model equations are used to predict known particles. They allow us to determine how often we should expect the appearance of a jet ricocheting from “nothing”, that is, from undetectable neutrinos. This prediction will coincide with the data, if no other types of undetectable particles appear in collisions at the LHC. And we expect that the predictions will not come true only if new types of undetectable particles appear on the LHC, and / or neutrinos will appear on it not in the way we know - for example, as a result of the decay of an unstable particle.

This is a general experiment strategy. We have a lot of predictions, a lot of measurements, by which we check the distribution of the missing transverse momentum in large groups of similar collisions. If we find that the predictions are not fulfilled, then something happens that is not explained by the standard model, that is, unknown undetectable particles appear, or known (neutrinos) but not in the way we expect.

Such a discovery would show that the Standard Model clearly does not describe all physics in the LHC, and would bring many awards to experimenters. But his interpretation would be extremely ambiguous! Even if we received particles of dark matter, it would be completely unobvious! We would only know that in some process unexpectedly often undetectable particles are born. The transition from them to particles of dark matter would be unjustified logically.

How can scientists distinguish between different possibilities and finally come to the conclusion about the discovery of dark matter? It will not be easy and can take many years, or even decades.

Two more examples

But first let me give you two more examples of how dark matter, or other undetectable particles, could manifest themselves. The newly discovered Higgs boson can sometimes decay into dark matter or something else undetectable. Such so-called. The “invisible” Higgs decays in the Standard Model occur extremely rarely, so if it turned out that they occur often, it would be an amazing discovery! And such decays are already looking for. The invisible Higgs decay cannot be observed directly, but Higgs are often composed of W-particles, Z-particles, or certain pairs of quarks (issuing specific jets are relatively close to the beam - see Fig. 11). And they can already be observed, as well as the shortage of the transverse impulse from the Higgs, disintegrating into undetectable particles. But, as usual, such a signal can be found in the Standard Model - when the Z-particle decays into neutrinos instead of the Higgs, which decays into dark matter. You can distinguish them only by counting the number of collisions of this type, and checking how much this number exceeds the predictions of the Standard Model.


Fig. 11. The Higgs particle (H) can arise together with two high-energy quarks, each of which gives rise to a high-energy jet (scattering hadrons). Such unusual jets ricocheted from the Higgs, whose decay into undetectable particles can lead to a large shortage of transverse momentum. But the same signal can occur when, as a result of a collision, a Z-particle is born, which decays into neutrinos and antineutrinos.

Another example: in many variants of particle physics considered by scientists, including, but not limited to supersymmetry, the equations predict the presence of a new electrically charged particle capable of decaying into dark matter. In this case, as a result of the collision of protons, the appearance of an electron (or muon) and an anti-electron (or anti-muon) and two dark matter particles, which remain undetected and give the missing transverse moment, cannot be called unusual.


Fig. 12

The only problem is that such particles can leave such a picture. When a positively charged W-particle and its antiparticle (a negatively charged W-particle) are born in collisions, these particles can break up into something that looks exactly like fig. 12, only instead of two dark matter particles, they will generate neutrinos and antineutrinos. The only way to find dark matter is to calculate. If, in addition to W, new particles are created, the number of collisions of this type will be more than expected. Interestingly, in the current data from the LHC, there are more collisions than expected - not so much to be very happy about this, but enough to carefully monitor how the LHC is collecting a large amount of data.

These are just three examples from the set. There are even more ideas about what dark matter can turn out to be than experts on dark matter, and in each case there are many variations on how dark matter can be created at the LHC. Therefore, experimenters are not sure how to search for it in experiments, and they are preparing a very wide and varied program of searches in order not to miss anything.

Even if new undetectable particles are found at the LHC, will it really be dark matter particles?

How can BAC experiments prove that they got dark matter? No At least by themselves. Even if they get a new type of undetectable particles, they will have to cooperate with at least one more experiment that can verify whether dark matter has actually turned out (a substance that the Universe is rich in). Mere information about the existence of some type of particles does not prove that it is these particles in the Universe that most of all. It can, like a neutrino, make up a small part of the matter of the Universe. Or none at all - if new particles are unstable (as is the case with most particles), and they will live long enough to fly unnoticed outside the BAK sensors before decaying, but small enough to disappear from the Universe shortly after the Big Bang .

In short, even if a new class of particles not detected by sensors are found at the LHC, experimenters will not be able to determine how many of these particles are in the Universe today. TANK is not intended for this.

What to do? TANK can be used to determine some properties of new particles, and to make certain assumptions. For example, in the previous section, I gave three examples of how undetectable particles can be discovered. In each case, the particles were obtained in a certain way. For example, if only these particles were generated, then after the collision a single jet was produced (Fig. 8). If particles were born from the Higgs decay, two high-energy jets from two defined quarks were obtained (Fig. 11).If they were generated during the decay of a new charged particle (Fig. 12), then this happened in the presence of a charged lepton and a charged antilepton (a charged lepton is an electron, muon, or tau). So, by observing what accompanies the new particles, and delving into the details of the missing transverse impulses, scientists can in principle create hypotheses about the nature of these new particles. They can be expressed in terms of equations that can be used to make predictions.

And now we are almost there. If you have a hypothesis about what a new particle is, you can ask yourself - how would dark matter behave if it consisted of particles of this type?

For example, one might ask how rarely would such particles react with ordinary matter? How much energy would remain after interactions? Knowing how much dark matter is in the universe, one can predict how often underground experiments, like LUX, XENON100, CDMS, etc. would receive signals of this type of dark matter. Perhaps this number is so large that the hypothesis has already been disproved? Or is it so small that such signals have not yet been received, but are large enough to receive them in the foreseeable future?

Another question: what happens if these dark matter particles meet somewhere in the center of our Galaxy or in the centers of nearby dwarf galaxies? Can they annihilate and produce visible particles, such as electrons, antielectrons, antiprotons, photons (possibly in the form of gamma radiation or X-rays)? And one may wonder whether these particles have already discovered such satellites and telescopes like PAMELA, FERMI-LAT, AMS, etc., or will they not detect them in the near future?

Only if and when we get enough information from the LHC (or colliders of the future) to formulate clear hypotheses about how new particles can behave and get accurate predictions of what can be expected from new experiments, and only when new experiments confirm though one of these predictions, it will be possible to seriously say that dark matter was discovered at the LHC.

Can it happen, can it happen soon? Of course.But, as you can see, for this we must be lucky several times in a row, so that although there is nothing impossible, we should not expect it very soon. Most likely, it will take quite a long time, perhaps a decade. And if dark matter consists of particles that can not be created at the LHC, or does not consist of particles at all, or it does not exist at all - well, the LHC will not tell us. He will just be silent about this. So we do not lose hope, scientists are looking for, but it is worth practicing other approaches to solving the great mysteries of the universe.