"Physics with Dimuons":
it will get harder and more interesting

On 23-24 June, an international scientific meeting dedicated to the 20th anniversary of JINR's participation in the programme "Physics with Dimuons" of the CMS experiment at the LHC. The meeting organised by the Joint Institute for Nuclear Research focused on such topics as new physics, verification of the Standard Model, and future physics beyond the Large Hadron Collider (LHC).

Participants presented reports on the work of the CMS collaboration at the LHC, the search for dark matter at the collider, and the study of the Higgs boson discovered at CERN in 2012. This topic is so important that a separate session at the event was dedicated to the 10th anniversary of the discovery of the Higgs boson.

What has this particle given to basic science in terms of the Standard Model of elementary particles? Here's what the participants of the meeting said about it.

Dmitry Kazakov, Corresponding Member of the Russian Academy of Sciences, Director of BLTP JINR, stressed that the Higgs boson is not just a new particle but also a very important cornerstone of the Standard Model. "For the past 10 years the properties of this boson have been studied intensively, and it has been confirmed that this is indeed the particle that is needed to implement the mechanism of generation of elementary particle masses. It turned out that we discovered a particle performing exactly the functions required for the Standard Model. In this sense, its role is very important, and the alternatives discussed were immediately excluded. The variant that was implemented was the simplest but elegant, because a simple working scheme confirms the beauty of science," he said.

At first, 10 years ago, they said it was a particle similar to the Higgs boson, and it was important to study its properties, as Dmitry Kazakov said. It was necessary not only to know what the mass was equal to, but also to study the quantum numbers of the particle, to determine its parity and decay modes so that they corresponded exactly to what they predicted. It was necessary to find out how the mass of elementary particles and the interaction constant were related to the Higgs boson. "All this has been studied and obtained for various particles, first for heavier, then for lighter ones. This process has been going on for 10 years. Now we know a lot more about this particle than about the others, because it was given top priority," the speaker said.

Dmitry Kazakov noted that there may be phenomena that go beyond the minimum scheme that has been derived. So, in parallel with the study of the particle, there has been a search for similar other particles conducted, but they have not been detected. "The theory admits the existence of not one but many Higgs bosons, but no other such particles have been found yet, and this is one of the directions of the new search. I think that if there were more such particles, it would enrich the so-called Higgs sector. At any rate, it remains one of the most important areas of research in the physics of the Standard Model and beyond," he stressed.

Victor Kim, Deputy Head of the High Energy Physics Department of NRC KI - PINP, Doctor of Physics and Mathematics, said "We are surrounded by a vacuum, it has quantum properties. One of the quanta of this vacuum is the Higgs boson, which has become known for being a very unusual particle. It gives mass to all currently known particles. For the first time in fundamental physics, a new particle representing a vacuum has been discovered, which is unlike anything else. The properties of this particle still need to be clarified in terms of how it is related to other particles, both already discovered and still unknown."

Viсtor Kim explained that the Standard Model of elementary particles and their interactions in its simplest version uses only one type of Higgs boson, but there can be several. Moreover, in theory there are no restrictions on the number of types of Higgs bosons. By studying the properties of this particle we can understand a new physics and a new world unknown to humanity.

"Why is all this important? Because we still do not know what the Universe is made of. The visible matter that we observe and which is described by the Standard Model is only 5% of the mass and energy of the observable Universe. The evidence for this is indirect - we suggest it from the gravitational balance. The direct indication, however, is these new particles and interactions we are trying to find, and perhaps the Higgs boson will help us with that," he added.

Viсtor Kim said that colliders make it possible to obtain previously unknown particles, including those that were previously unattainable in terms of energy for formation under laboratory conditions, but may have existed in the first moments of the Universe. The physics of micro- and macrocosm is thus connected through the Big Bang theory, which says that in the earliest moments of the Universe there were elementary particles, even more elementary than the ones we know now.

"Therefore, the currently existing theory is called the Standard Model, not a theory, because we know that it is temporal and that there is an even deeper one. The Standard Model contains too many parameters and, furthermore, it does not include one of the types of interactions - gravitation, which seems to exist separately. What's more, we think that there should be new interactions that are not described by the Standard Model. And it may even be, as theorists suggest, that perhaps there are additional dimensions of space-time and other, interesting new physics. Therefore, the research does not stop. The LHC will soon enter a new phase, when its intensity will increase significantly, and at the same time, new accelerators are already being developed, because new technologies allow us to achieve new energies, new electronics allows us to achieve better precision, etc. The current state of physics suggests that the most pressing challenge now is a search for these more fundamental interactions and particles. The picture of the world that we have now is obviously incomplete," Viсtor Kim summed up.

Maria Savina, Leading Researcher at VBLHEP, Candidate of Physics and Mathematics devoted her report at the meeting to dark matter search at the LHC. She shared her opinion about the topic, "The occurrence of all the masses of observed particles known to us in the Universe is associated with the Higgs boson. So, if we are looking for some other masses, in particular, dark matter, we can consider the Higgs boson as a concept that will help us shed light on this question as well, and it is very important and necessary, because dark matter is five times more than ordinary matter and we know nothing about it. Accordingly, we start from the fact that the Higgs boson is connected by interaction with all ordinary particles, with the visible sector, and then assume that dark matter can also be connected with the Higgs boson. This is just a hypothesis, but it is sufficiently substantiated by the special role of the Higgs sector responsible for the emergence of mass."

She specified that the mass of dark matter is not necessarily acquired due to the Higgs boson that has already been observed and studied at the LHC. Specifically for dark matter, other, additional Higgs bosons may be responsible, which interact with our matter only indirectly, through a connection with our Higgs boson, and the latter is thus a kind of a bridge linking the dark and visible sectors. In this case, it is called the "Higgs portal to dark matter". If such a concept exists, then the Higgs boson can help researchers discover dark matter particles.

"Within the framework of the quantum field formalism used for the construction of the Standard Model and its extensions, one can construct a theory, or a model, which describes the interaction of dark matter particles and the Higgs boson," Maria Savina continued. She highlighted, "Any well-constructed model allows us to predict all possible processes with elementary particles proceeding according to the rules adopted in this model, and to obtain quantitative characteristics, in particular, the probabilities of these processes as a percentage. And after that, we can study the processes involving the Higgs boson at the LHC and see if there are any that differ from the allowed possibilities listed in the Standard Model and that may be consistent with the interaction with dark matter particles.

But how do we even understand what actually happened at the LHC, how are we looking for all this, how do we study the properties of a particle? We study experimental signatures, that is, signals in detector systems associated with the production of particles of a certain type. And we have well-developed techniques and reconstruction procedures, which allow us to firmly state that, say, this signal really corresponds to the production of this pair of particles, and another detected signal, respectively, to the production of some other finite configuration of particles. From this, according to what we detect at the end, that is, after the interaction, we can reconstruct what we had at the beginning. Of course, we can only do this if we have a theory or a model that allows us to quantify all possible processes. As said before, we need to set up such a model, that is to come up with, create, and then test it by comparing its predictions with those of the Standard Model. In our case, these are predictions for different modes of the Higgs boson decay into different final-state particles.

Thus, when studying different definite finite signatures, we pick up the situations when the Higgs boson has decayed into certain particles, that is, we record the facts of its decay through different channels. Further on, based on the number of these events and taking into account the parameters of the accelerator, you can quantify it, in decay proportions for each channel in terms given by the theory. You can then compare how closely the numbers obtained coincide with what the Standard Model predicts. The Higgs boson is unstable and necessarily disintegrates in a very short time. The inevitability of this event corresponds to 100% decay probability. But these 100%, or the total decay probability, are composed of definite percentage shares corresponding to different probabilities of the Higgs boson decay through different channels. Some decays are more probable, some are less probable, and others are highly improbable, or very rare. But their total sum should still be equal to 100%.

Where is the window into the new physics here? When the Higgs boson was just discovered, it was discovered only through three channels available in 2012. There were only a few dozens of Higgs bosons at the time. There are many of them being formed now though, the statistics for them is much better, and we can calculate these percentages much more accurately: in which case and into what it decays from the whole list allowed by the Standard Model. Modern-day precision is in principle enough to count all these channels and find out that they do not add to 100%. That's to say that there remains some window for unexplored and unregistered decay channels, which we assume still leaves us an opportunity to search for new physics. In particular, since the previous conference, news has broken that the available window for such channels has narrowed to less than 17%. That is, there is still about a 17% probability that the Higgs boson decays in a nonstandard way, but we cannot detect such a decay; relatively speaking, they remain "invisible" to us. These are not necessarily only dark matter particles; they could be other possible scenarios of new physics, which also change the decay picture.

I should also say that this figure is not necessarily a real indication that there are any new properties: it could well just be a lack of modern measurement precision. It is constantly improving; accordingly, the percentage share of possible new decay channels is systematically decreasing, from about 30% at the very beginning of the study of the Higgs boson's properties during Run 1 to the indicated current figure. Maybe it will continue to decrease until it completely disappears, and then we will be left with the Higgs boson with absolutely standard properties. We will see. In any case, a detailed study of the properties of the Higgs boson will allow us to conclude whether it is related to the particles of dark matter or not. At first, we were happy just to have discovered the Higgs boson. Further we have long studied its properties, and now we have more ambitious goals. We already believe that we can use the Higgs boson as a tool for searching for new physics."

Maria Savina added that scientists assume that new physics is more complicated than the Standard Model, and more complicated theories often give several different scalar Higgs bosons. So, the closer examination of the Higgs sector is of great importance not only in terms of search for dark matter particles, but also in terms of verification of many other theoretical concepts of new physics. Accordingly, another important area of research is the search for other Higgs bosons at the LHC, finding out what their masses and other quantum numbers (parity, charge) are. Theoretically, all Higgs bosons are arranged in such a way that they interact not only with other types of particles, like fermions, vector bosons, but also with each other, including self-action (interaction of a scalar boson with itself); such interactions are described by the Higgs potential. Therefore, we can try to test the properties of this potential, which has the simplest form in the Standard Model and is much more complicated in the extended Higgs sector. We can look for new Higgs bosons through their interaction with the already known one with a mass of 125 GeV and with studied quantum numbers that we have been observing at the LHC for 10 years. Specifically, if the proposed new Higgs bosons are lighter than our boson, it can decay into them. And if it is the other way round - those are heavier, the new heavy scalar can be expected to decay into one or two of our Higgs bosons. We can try to search for and study all this. Moreover, according to the processes corresponding to the self-action of the known Higgs boson with itself - the so-called triple and quadruple vertices of the Higgs interaction in the Standard Model - we can try to isolate non-standard additives, which will certainly be in the case of other Higgs bosons, through loop corrections to such vertices. It is very important to study exactly the properties of the observed boson. This task will be solved both during Run 3 and later in the high-luminosity LHC mode.

"Before the start of Run 3, all analyses of the previous Run 2 were almost completed, and we expect new data to be presented by a series of traditional "big summer" international conferences on particle physics. Unfortunately, we have not found anything yet, no signals from new physics, but nevertheless, the boundaries of possible values of masses and other parameters of the new particles will once again be moved soon. When we look for a particle and do not see it, we interpret the obtained experimental results in terms of restrictions on the allowed values of its parameters. Over time, we would thus close ever-increasing regions of these parameter spaces for different particles and different new theories, but a very large window of opportunities still remains. So for now, it's a systematic movement for us to narrow this window of opportunity," she said.

Sergei Shmatov, Co-Chairman of the Organising Committee of the meeting, Head of the CMS Physics Programme at JINR, Head of the CMS New Physics Sector at VBLHEP, Doctor of Physics and Mathematics: "Our meeting is dedicated to the 20th anniversary of the programme for CMS research with a pair of muons. During this time, we conducted a large amount of research, in which we were looking for the much-talked-about signals of new physics with the help of this pair of muons. We checked whether the Standard Model works as it should in the new energy range, and second, whether there is new physics. We searched for new particles, such as new gauge bosons and multidimensional gravitons, as well as for microscopic black holes using complementary channels, but did not find them. After two runs of the collider operation (Run 1 and Run 2), we reached a stage where the LHC peak energy had been achieved. Now the LHC is operating at the design peak energy, at its maximum capacity. No matter how complex experimental signatures we invent and how much progress we make in reconstruction techniques to study such signals, we still have a kinematic limit due to the fact that the LHC energy is 14 TeV."

Sergei Shmatov explained that there is further opportunity to get more data as part of the experiment, but it will be impossible to increase the energy. At the same time, for this research the energy is more critical than statistics. "Thus, in our classical studies with a pair of muons we come to a certain logical dead end," the speaker noted. "An increase in statistics by an order of magnitude leads only to small shifts in the space of the model parameters. We annually find it harder and harder to close the area of the unknown. That is why we have come to use additional channels: to watch a pair of muons plus something. Let us say, a pair of muons plus two b-quarks or some share of lost energy, and so on. Complications of these channels allow us to add new physics areas: for example, to watch signals from a potential dark matter candidate using this signal. So the research in which JINR is involved is gradually moving from simple, obvious signals to more complex ones, by adding increasingly sophisticated theoretical concepts that we can investigate experimentally."

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