How to search for supersymmetry at the Large Hadron Collider

Original author: Matt Strassler
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If you ask a random person on the street: “how to search for supersymmetry?” Then he will most likely quickly switch to the other side. But if you ask this question on the street at CERN, the laboratory that controls the Large Hadron Collider, you will most likely get something like “Look for an unexpected number of collisions with jets and lacking energy.”

And such an answer can make you quickly go to the other side of the street. But he is not so inexplicable, he just needs a translation. It means the following:
It is necessary to search for an unexpectedly large number of collisions of protons with protons, in which signs appear as (a) quarks, antiquarks or gluons (particles inside protons and other hadrons) flying out of the collision with very high energy, as if from a gun (and creating splashes of particles , called " jets "), and (b) indefinable particles, invisibly flying away, and carrying with them a large amount of momentum and energy.

The purpose of this article is to explain to you why people will give a similar answer, and what are its strengths and weaknesses.

Preliminary information


You need to read an article about what supersymmetry is, and what its predictions mean. Briefly: each type of particles known to us in nature for supersymmetry requires one or two additional, which physicists usually call superpartners with similar properties, but differing in one aspect:

If the particle we already know is a boson, then its superpartner will be a fermion, and vice versa (in this article you can read about what bosons and fermions are).

To avoid contradictions with the already obtained data, supersymmetry must be hidden in a cunning way, which is why the second difference between the particle and its superpartner appears:

The mass of the superpartner is greater than the mass of the particle we already know.

In the most popular version of supersymmetry, the superpartner of each particle known to us is heavy enough to barely go beyond the power of previous experiments, but still be within the limits of the LHC capabilities.


Fig. 1: known to us particles and Higgs particles, as well as their superpartners ( blindblinds , snutrinos, squars , gluino, charginos and neutralinos ), predicted by supersymmetry. Heavier particles at the top.

The reason why many physicists believe that superpartners are likely to be within the limits of the LHC capabilities is that they believe that supersymmetry can be a solution to a riddle known as the problem of gauge hierarchy. If the superpartners are much harder, then the solution to the problem of the hierarchy will have to look somewhere else.

Suppose that physicists are right - why then do we need to look for collisions leading to the appearance of multiple jets (signs of high-energy quarks / antiquarks / gluons) and a large amount of missing energy (signs of invisible particles)?

Where does the answer “jet and missing energy” come from?


To begin, let me tell you what the physicists have in mind, and then I will tell you where it all came from.

This is what they think:


Rice. 2: two protons (perspective view) rush towards each other, and the top quark inside the near proton will soon collide with the top quark inside the far proton at the collision point.

Because the proton consists of quarks, antiquarks and gluons, which are affected by strong nuclear interaction, in collisions The protons at the LHC of all superpartners are the easiest to obtain super partners for them: squars, anti-welds, and gluino. For example, (in Figs. 2 and 3) in a collision of protons two upper quarks may collide and form two upper squark.


Fig. 3: Colliding upper quarks with fig. 2 produce a pair of upper squawks, each of which almost immediately splits into the upper quark and neutralino (a mixture of photon superpartners, Z particles and Higgs).

What will happen next? Like most particles, the squaria will decay. For what? In many variants of supersymmetry, the squars will decay into a quark and another superpartner, neutralino (a mixture of photon superpartners, Z particles and Higgs). Quarks carry a lot of energy and turn into jets, and neutralino will fly through detectors without a trace. Accordingly, we should see two high-energy jets, one for each quark, and signs that they bounce off something invisible and undetected.


Fig. 4: each high-energy quark from fig. 3 will turn into a stream of hadrons, and neutralino will escape unchanged

The collision itself and the appearance with the subsequent disintegration of the squars is shown in Fig. 3. The jets and neutralino flying out of the collision point are shown in fig. 4. What the detector actually sees — the only information received by scientists — is shown in fig. 5.

The apparent imbalance seen in fig. 5, where most of the substance goes to the right and up, but nothing goes to the left and down, for unsuccessful historical reasons and for brevity is called “missing energy”. In fact, this is “the missing impulse in directions perpendicular to the colliding rays” - a long phrase that partially explains the desire for brevity.


Fig. 5: the detector at the LHC (ATLAS or CMS) will detect two jets from fig. 4 in the form of localized electronic signals appearing when the particles pass through the tracking equipment and stop at the energy detector. Two neutralinos do not leave marks, and their presence can be judged only by the absence of something reflected from the jets.

If a pair of gluinoes appears instead, the situation will differ slightly. Usually, each of the two gluinoes will decay into quark, antiquark, and neutralino, so the detectors will again see the jets (in this case, four), along with the “missing energy” from the two neutralines.

It is this picture that appears in the imagination of physicists when they answer your question about the search for supersymmetry. To understand where it comes from, it is necessary to study the underlying assumptions.

Assumptions underlying the answer “jet and missing energy”


We will now do this logical journey - it is illustrated in Fig. 6. At the end of our tour you will be able, to some extent, to judge the strengths and weaknesses of this answer to your initial question.

The logic includes three main assumptions.

Assumption 1 : we assume that in nature there is an additional principle that supersymmetry itself does not require, and according to which in any physical process the number of superpartners can be changed to an even number (its technical name is R-parity preservation ; I’m reporting not because his name is very important, but because you could meet him somewhere else).

Why do theorists impose such a criterion? Without assumption 1, supersymmetry would predict the existence of new interactions between particles of matter, and usually they lead to the rapid decay of protons. And this conflicts with the data. The proton is extremely stable (fortunately - even a slow proton decay rate would kill us, melt the Earth, etc.). You can take a cistern with a billion trillions of trillions of protons, wait ten years, and not find a single decaying proton (yes, people tried to do that! For this you need 180,000 tons of water). So without assumption 1, supersymmetry and we would be dead.

But if Assumption 1 is true - R-parity is preserved - then these new interactions are banned. Supersymmetry plus the preservation of R-parity predicts a very, very long-lived proton, which corresponds (in a favorable case) to the data.

Note that this requirement of preserving R-parity is imposed not because it is required by supersymmetry, or on the basis of some theoretical principles. It is added because it requires compliance with the data. It is also a perfectly reasonable requirement from a theoretical point of view.

Assumption 2 : Of all the superpartners in nature, the Higgs particle partner will be the easiest, and therefore, this is one of the superpartners in Fig. 1: Gluino, squark, charged blindton, snaytrino, chargino or neutralino.

This assumption is controversial. First, if supersymmetry is true, then the graviton (gravity transporter) should also have a superpartner, gravitino - and its in fig. 1 no How heavy is gravitino? We do not know. In some versions of supersymmetry, it is as heavy as the heaviest superpartners in Fig. 1, squash and gluino. In other embodiments, it is much lighter, and it may even be lighter than an electron! That would violate assumption 2.

Or in nature there can be particles with very small masses, about which we do not yet know, because they are very difficult to create or detect - particles that are not affected by any of the three forces in fig. 1, electromagnetic, weak or strong nuclear interactions. Such particles are usually called "hidden", due to the fact that they are difficult to obtain, despite the low weight. (If we are talking about several types of hidden particles, they are often called the "hidden sector"). If supersymmetry is true, these particles also have superpartners — as mentioned in the article on supersymmetry, supersymmetry is a symmetry of space and time, so any type of particle moving in space and time should have a superpartner. And if any of these superpartners is easier than the lightest superpartner in fig. 1, then Assumption 2 is incorrect.

Assumption 2 is not required by experimental data. The best theoretical arguments against hidden particles suggest that nature is likely to be simple and elegant, and since hidden particles are extra rubbish, the probability of their existence is low (whether you are convinced by this argument or not is a matter of taste). The best argument against light gravitino - stable gravitino could cause many different problems in the Big Bang process. Assumption 2 is in favor of another argument related to the fact that the lightest superpartner can play the role of the dark matter of the Universe, but in order to understand it, you must first understand several of its additional consequences, so for now we will not go into it.

Guess 3: superpartners exposed to strong nuclear interactions — squars, anti-welds, and gluino — are most likely heavy, much heavier than other super-partners, although not so heavy as not to appear on the LHC frequently enough.

This assumption is shakier than the other two - what do you mean "heavy" and "often"? But instead of delving into such arguments, I will simply say that in many, many variants of supersymmetry, this turns out to be true. Theoretical calculations show that in many different cases these super-partners, exposed to strong nuclear interactions, turn out to be heavier than most of the rest. But it's not always the case.


Fig. 6: a logical chain leading physicists to searches for supersymmetry through collisions searches, the results of which are similar to fig. 5. SP - superpartners, LSP - the lightest super partners.

What follows from these assumptions? Some very important consequences; use fig to keep track of the chain. 6.

Assumption 1 has three key consequences:

  1. If you start without superpartners (as happens in the case of a collision of two protons), and get them after a collision, then there should be at least two of them. You can not start from scratch superpartners and get one.
  2. If you have a superpartner and it falls apart, there must be at least one superpartner among the results of the collapse (maybe three or five, but almost always one is obtained). You cannot start with one superpartner and get zero.
  3. The lightest superpartner cannot decay — it is a stable particle — since particles can only decay into particles of lesser mass, therefore if the lightest superpartner had decayed, this would mean that one superpartner would turn into zero superpartners.

How amazing! The existence of an unknown, yet stable particle — the lightest superpartner (LSP) follows from supersymmetry and the preservation of R-parity. What properties can such a particle have?

Suppose that this particle is affected by an electromagnetic or strong nuclear interaction. Then (i) in the early Universe during the Big Bang many such particles should have appeared; (ii) they would affect the abundance of various elements, such as lithium, during the Big Bang, so that this abundance would not be consistent with today's observations; (iii) they would still fly through the Universe, some of them would collide with the Earth, create exotic atoms that would have been discovered long ago after a careful search for new unusual atoms. Although it is worth a longer discussion, the main conclusion is that any new stable particle should not be exposed to electromagnetic and strong nuclear interactions.

So, considering this, follows from assumption 2? The lightest superpartner can be one of the neutrinos or one of neutralinos. All other superpartners (squeezes, sleptons, charginos and gluino) of known particles are subject to electromagnetic or strong nuclear interactions. For technical reasons, most (but not all) particle physics specialists prefer models in which neutralinos are the easiest superpartner. It can be an excellent candidate for a dark matter particle - which is an argument in favor of assumption 2. But even if the snutrino turns out to be the lightest, the argument in favor of searching for jets and missing energy remains almost the same, with some minor changes.

And, finally, Assumption 3 says that squarks and gluino are easy to obtain, and that they are relatively heavy. This means that they explode with relatively high energy; the energy and momentum carried by the quarks and neutralinos, into which they decay, are large. The resulting jets will carry high energies, and the missing energy will be large.

Therefore, I hope you can understand the idea enclosed in rice, 3, 4 and 5. If supersymmetry is true, then, logically, we will get heavy squari and gluino; they will decay into high-energy quarks and neutralinos; quarks will manifest themselves in the form of high-energy jets, which are easy to detect, and the presence of neutralinos, which we will not detect, will result from the imbalance of the jets' momentum.

Well, we will look for it, and either find it or not. What's next?


Consequently, if we see a large number of collisions with high-energy jets and lacking energy, then this is cool; perhaps we discovered supersymmetry. However, WARNING: other types of phenomena may create similar events — it may take years, and it will take a lot of work before we begin to gain confidence that we have found supersymmetry, or that we have found something something else that just looks like supersymmetry at first glance. Just that we will see something like rice. 5, will not mean that we received what is shown in Fig. 3!

But if we do not see an excess of such events, will this mean that supersymmetry is not exactly a property of nature? Before making such far-reaching existential conclusions about the Universe based on the interpretation of the experimental result, we must ask ourselves what could have gone wrong with the three assumptions listed (or with a couple of not so important ones that I did not give here). I have already told you something about what could go wrong, and although I will not go into it, you yourself can see that if we don’t find such events, all that we can conclude from this:

  • either supersymmetry is not a property of nature,
  • or supersymmetry is a property of nature, but with some of the three assumptions, something is wrong.

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