“Higgs Bozon is open. What's next? "

On June 7, 2018, the Academician of the Russian Academy of Sciences Valery Rubakov about the Khiggs boson and the research now at the tank was held at the Arkha Cultural and Educational Center . With the kind consent of Arkha, we publish the presentation of this lecture authorized by V. A. Rubakov, prepared by Boris Stern .

The opening of the Higgs boson was reported on July 4, 2012 at a seminar in Cern. It was said quite carefully: a new particle is opened and its properties are consistent with the predicted properties of Bozon Higgs. And over the next years, we gradually became more and more convinced that the properties were exactly the same as predicted by theorists, and in the most naive model. The most important thing is that this, as theorists say, is not just a new particle, but a representative of the new sector of elementary particles - the Higgs sector.

Let me remind you the main provisions of the standard model. The entire "zoo" of its particles fits one slide. Protons, neutrons, π-mesons-all these are composite particles. There are not so many elementary particles. This is a Lepton family, a family of quarks that make up the Fermonian sector. The second sector is particles responsible for their interactions: photons, W- and Z-bosons, glows and gravitons. Bozons interact not only with Fermines, but also among themselves. The most famous of the listed particles is photon.

The most interesting in their manifestations are gluons, it is they who connect the quarks in the proton so that it is impossible to melt. W- and Z-bosons are similar to the photon in their role, but they are massive and are responsible for the weak interactions that are related to electromagnetic, although they look different. There should also be a particle of graviton. After all, gravitational waves are already open, and where there are waves, there should be particles. Another thing is that we will never, apparently, can receive and register gravitons one by one.

And finally, the Higgs Bozon, which is a separate sector on our slide. This is another particle that stands apart in the entire “zoo”, consisting of a small number of different types.

What is Higgs Boson?

To begin with: what is boson? Each particle, like a top, has, as it were, the inner moment of rotation, or spin (this is a quantum mechanical phenomenon). There is a whole and semi -fingered back in units of a permanent bar. Particles with a back 1/2 or 3/2 (any semi -fingered spin) are called farmions. The bosons of the spin have a whole that leads to fundamental differences in the properties of these particles (bosons like to accumulate in one quantum-mechanical state, like photons in radio waves; Fermions, on the contrary, avoid this, due to which the electrons populate different atomic membranes.- Ed. ). So, in Bozon Higgs Spin is 0 (and this is also an integer).

Higgs Bozon is a heavy particle. Its mass is 125 GEV (for comparison: the mass of the proton is about 1 GEV, the mass of the most heavy particle, T-quarrels,-172 GWs). Higgs Bozon is electrically neutral.

New particles are opened on accelerators, they are born in the clashes of particles, in this case - in the clashes of protons. After that, the decay products of the desired particle are recorded. Higgs boson breaks up on average in 10–22 s. For a heavy particle, this is not such a short time-top quark, for example, lives 500 times less.

And Boson Higgs has many different ways to break up. One of the “golden channels” of decay - the decay into two photons is quite rare: so the Higgs boson breaks up in two cases out of a thousand. But this path is remarkable in that both photons are highly energy. In the rest system of the Higgs boson, each photon has an energy of 62.5 GEV, it is a great energy. These photons are clearly visible, you can measure the directions of their movement, energy. An even cleaner decay channel is the decay into four leptons: by two pairs E+and E–, on E+, E - and µ+, µ– or four muons. It turns out four high -energy charged particles, which are also clearly visible, they can measure energy and direction of departure.

How to find out that we see exactly the collapse of Bozon Higgs? Suppose we registered two photons. At the same time, there are many other processes leading to the birth of two photons. But if photons came from the decay of a particle, then on them you can determine its mass. To do this, you need to calculate the energy of two photons in the reference system, where they fly in opposite directions with the same energy - in the system of the center of mass. In our reference system, this is a well -defined combination of photon energies and the angle of expansion between them. It is called the invariant mass of the particle system. If photons - the decay products of the Higgs bosone, their invariant mass should be equal to the mass of the boson with an accuracy to measurement errors. The same thing if Bozon broke up into four particles.

In Fig. 2 shows the distribution of events over the invariant mass of two photons. The latter is postponed along the horizontal axis, and the number of events is postponed according to the vertical. There is a continuous background, and there is a “standing” in the area of ​​the invariant mass of 125 GEV. Perhaps you will laugh, but this "bloom" is the boson of Higgs. A similar peak is also outlined in the invariant mass of four leptons (E +, E -, µ +, µ -), into which it also breaks up. Only this occurs in one of ten thousand decays. That is, it is necessary to give rise to a million bosons of Higgs in order to accumulate one hundred decay into two lepton pairs. And it was done.

It is possible to measure the energy and direction of departure (therefore, the impulse) of a charged electron or muon with a much higher accuracy than in the case of photon. It is for this that the detector has a strong magnetic field: the curvature of the trajectory of the charged particle in the magnetic field allows you to determine its impulse (as well as the charge sign). In addition, there are little isolated leptons of high energies, and even more small in the number of four isolated lepts (isolated, i.e., outside the adron stream). Therefore, the background for decay into four lepton is small.

Finally, the researchers on the tank were selected by events in which the invariant mass of one pair of leptons of the opposite sign is equal to the mass of the Z-Bozone (Higgs decays into the real Z and virtual Z), which even more presses the background. But the decay into four leptons is actually not better than the decay into two photons, since the probability of decay into two photons is much higher, the errors in its measurement are compensated by greater statistics.

Why did the boson Higgs open only recently

There are two circumstances. Firstly, the desired particle is heavy. So, you need an accelerator for great energy. Secondly, you need to have a greater intensity of beams so that the number of clashes is sufficient. Physicists use the word "luminosity", reflecting the number of clashes per unit of time. You should have a lot of clashes.

Everything seemed to be fine with energy, because Tevatron, a collider in the United States, worked before the Great Hadron Collider. He had complete energy of 2 TEV. It seems good, because Bozon Higgs is 125 GEV. In principle, by energy, Tevatron could give birth to Higgs bosons. But he had insufficient luminosity. He did not have the born Higgs bosons.

A few words about tank

Big Adrone Collider is a structure notable in all respects. This is a superconducting accelerator-drive located underground. The length of its ring is 27 km, and all this ring consists of magnets that hold protons in this ring, superconducting magnets. At that time, when the tank was built, this was the last technological achievement. Now there are quite successful attempts to receive a more powerful magnetic field in magnets. But at that time it was the most. In general, all that is done there is the peak of modern technology, on the very edge of human capabilities.

First, the tank accelerated protons to the total energy of 7 fee, then 8 TEV. Each proton, encountering, had an energy of 4 TEV. Starting steadily working in 2010 on energy in 7 TEV, in 2011 the tank switched to energy of 8 TEW, and its project energy - 14 TEV. Now, for cunning technical reasons, up to 14 TEV still have not reached; Since 2015, the accelerator has been working on total energy 13 TEE. His luminosity is very high by all the standards, specialists in Cern, of course, are large masters. And actually the clashes of particles occur in four places, we are interested in two of them where the Atlas and CMS detectors are standing. This is what CMS looks like - a compact muon solenoid (Fig. 4).

The most extreme is a muon chamber that allows you to register and measure the parameters of muons that fly through the entire detector, sewing it through. All this is enclosed in the magnetic field in order to measure its impulse by curvature of the particle movement.

Atlas - even more. This is such a multi -storey building, completely clogged with equipment.

These detectors measure energy, impulses, directions of particle movement, determine that it was an electron, photon, muon or a strongly interacting particle such as a proton or neutron - all of them have their own signatures.

A separate interesting story is related to how the groups of physicists are arranged - collaborations that are engaged in this business. It is clear that such a gigantic machine to develop, create and maintain, remove and process data, make sure that nothing spoils, look for various events and interesting phenomena, and large commands are needed. They gather around the world. A characteristic figure is 3.5 thousand physicists in each collaboration, in Atlas and CMS. These groups are international: in addition to European specialists from America, Japan, China, Russia, etc. The total number of institutions is about 200; 150–200 in each of the collaborations. It is remarkable that this is a self -organizing system. This is a system that was organized “from below”, there were their “founding fathers”, who slowly surrounded interested physicists in the 1990s. A large number of people gathered, but there are no bosses except elected there, all are divided into groups, subgroups that answer each for their own, so it is all arranged. Despite the fact that these are people of various cultures, all this works. They did not quarrel, did not rush among themselves.

I must say that Russia can be proud and proud of the fact that we participate in all activities. In Cern and around him, everyone understands and emphasizes: Russia's contribution is quite significant and serious. A noticeable part of the accelerator was made in Novosibirsk. A significant part of the elements of detectors was also done with us. And our participants are many, from different cities, of different institutions. Approximately for money, resources and people, Russia accounts for 5–7% of the Tsern detectors (depending on a particular detector). What is quite normal for our country.

Why do you need Bozon Higgs

We move on to the theoretical part, maybe a little boring and dreary, but it seems to me that it is useful to understand and explain, at least with high quality, why it was Engler, Braut and Higgs suddenly decided that there should be a new particle. More precisely, Higgs decided that there should be a new particle, and Engler and Braut came up with the Bozona field.

First of all, we must recall that any particle is connected with the field. A particle is always a quantum of a certain field. There are an electromagnetic field, electromagnetic waves, and a photon - a quantum of an electromagnetic field is associated with them. Also here: Bozon Higgs is a quantum of a certain field. You might ask: why do you need a new field? Engler and Braut realized this the first.

Here you have to go a little to the side. All kinds of symmetry rule the world. For example, spatio-temporal, associated with shifts in time and space: a physicist tomorrow is the same as yesterday, physics here is the same as in China. The laws of conservation of energy and impulse are associated with these symmetry. There are less obvious, from the point of view of our everyday experience, symmetry - internal. For example, in electrodynamics there is a symmetry that leads to the law of conservation of an electric charge. She is not visible, except on the formulas, but she is. Together with the law of conservation of energy, this symmetry prohibits the electron to disintegrate. It is remarkable that the same symmetry forbids the photon to have a mass, and it really does not have it. Gluons are also massacre for the same reason - it is forbidden to have a mass of symmetry associated with the "color". “The color” is charged with quarks, and gluions are tied to “color”, like photons to the charge.

But the particles that are responsible for weak interactions are W- and Z-Bozons are massive. The trouble is that they are very similar to photons: the electron can be scattered on the electron, exchanging a photon, or maybe a Z-Bozone. The processes are very similar, I want to attribute to weak interactions the symmetry of the same type as they have electromagnetic (it is called calibration symmetry), but the mass W and Z - carriers of weak interaction - does not allow this, it violates calibration symmetry.

Why did this beautiful symmetry be broken? It turns out that this is a rather universal phenomenon in nature: many symmetries exist in the primary laws of nature, but are disturbed in the real universe. This phenomenon is called a "spontaneous violation of symmetry."

Imagine that you and I are little men who live in a constant magnet, in a magnetized piece of iron. We conduct an experiment with electrons: we get electron-positron pairs (we have a small accelerator there, we radiate electrons). So, these electrons fly to the magnet not in a straight line. Due to the fact that there is a magnetic field, they are “stuck” on it and fly in a spiral. We measure them and say: guys, we have a dedicated direction, our world is not isotropen, we have a dedicated axis on which electrons are wound.

But if we are smart theorists, we will guess that the point is not that the space has a highlighted direction, but that there is a magnetic field in this space. We will understand: if we managed to remove this magnetic field, then in space all directions would be equal. We will decide that there is symmetry in relation to rotation, but it is violated that there is a magnetic field in space. And if we were even smarter theorists, then, realizing that there is such a new field that provides a violation of symmetry, they said that there should be its quantum. And they would predict a photon. And it would be correctly predicted! Symmetry can be disturbed if there is a field spilled in space that violates this symmetry.

And in the physics of the microworld, this happens exactly. With some differences. The differences are that symmetry is not spatial, not relative to spatial rotations, as in magnet, but internal. And we have no iron here, this symmetry is broken right in a vacuum. Finally, unlike the magnetic field, a new field is needed here. This is the field of Engler, Brows and Higgs, which provides this violation. And the subtlety is that the magnetic field is a vector, it has a direction, but this field should be a scalary so as not to violate the symmetry regarding spatial rotations. It should not be directed anywhere. A particle of this field should have a spin equal to zero.

Such a picture was proposed and clothed in the formulas to Engler and Brows, then Higgs. But Engler and Braut somehow did not pay attention to the fact that their theory predicts a new particle. And Higgs, who published his work a little later, paid attention to this, and from the presentation of the reviewer, who asked if Higgs had some new things in the article that Engler did not say about Braut. Higgs thought, thought and said that there should be a new particle. Therefore, she was called the "boson of Higgs."

What next?

So far, everything was "in health." But there are questions. On the one hand, the picture with the Higgs boson is consistent. Formally, everything can be calculated, everything can be calculated by having the known parameters of this theory - communication constants, mass. But this picture does not bring final satisfaction. And one of the most important places that prevent physicists who do not calmly sleep is that in nature there are very different energy scale of interactions.

Strong interactions between quarks and glows have their own characteristic scale. This, roughly speaking, is the mass of the proton - 1 GEV. There is a scale of weak interactions, 100 GEV (masses W, Z, Higgs Bozon). And this scale is precisely the scale of the Higgs field - about 100 GEV. And that would be nothing, but there is still a lot of bar - a gravitational scale. Which is already 1019 GEV. And, of course, it is already strange: what kind of story is this, why are these scales are all different?

There is no such problem with the scale of strong interactions: there is a mechanism that allows you to understand the difference between this scale and gravitational (well, at least, replacement of our perplexity for the carpet). But with the scale of the Higgs Boson, the matter is bad. Why? Because, in fact, in nature there is a vacuum-a condition without particles. And this is not an absolute emptiness at all-in the sense that in a vacuum virtual processes occur all the time: the birth of a pair of particles and the fluctuation of the fields. Life is going on all the time. However, since this is a vacuum and there are no particles in it, we are not directly visible to us. And indirectly - very visible. For example, the processes of the birth of virtual pairs affect the properties of atoms, change their energy levels. Это давно известный лэмбовский сдвиг, вычисленный в 1930-х и измеренный в 1940-х. Влияют, как правило, не очень сильно. Вот этот лэмбовский сдвиг атомных уровней — всего лишь доли процента.

Но есть одно место, где вакуум «стреляет» на 100%. Это как раз масса бозона Хиггса. Выясняется, что если вы начнете учитывать рождение и уничтожение виртуальных частиц и наивно попытаетесь провести вычисление — сколько же эти процессы вкладывают в массу бозона Хиггса, — то убедитесь, что эти явления стремятся подтянуть массу бозона Хиггса к планковской массе. Они не дают бозону Хиггса быть легким.

И это, действительно, страшное дело. Очень хочется понять, почему реально в природе электрослабый масштаб такой маленький по сравнению с гравитационным масштабом 1019 ГэВ. Это объясняется, может быть, тем, что мы плохо знаем физику при не очень высоких энергиях, при энергиях масштаба 1 ТэВ. Дело в том, что если физика меняется на масштабе тераэлектронвольт, то, может быть, там и происходят чудеса: влияние вакуума почему-то оказывается маленьким, несущественным. Такая идея. Возможно, БАК еще не всё открыл, и должны быть новые явления, которые ему доступны. Его энергия, напоминаю, — 14 ТэВ. Правда, это столкновения протона с протоном. Кварк с кварком имеют энергию столкновения примерно в шесть раз меньше. Поэтому реальный масштаб энергии, который изучается БАКом, — это 2–3 ТэВ. Но все-таки это тот самый масштаб, на котором (как нам хотелось бы) может появиться новая физика, совершенно новые физические явления.

И я вам должен сказать, что на самом деле сейчас ситуация очень стремная. Потому что БАК уже поработал почти на своей проектной энергии — 13 ТэВ, он отлично на ней отработал 2017 год, и сейчас эта работа продолжается. И нет пока никаких — никаких! — указаний на эту новую физику, на которую мы всё надеемся. Все эти соображения, про которые я вам говорю, не подтверждаются. То ли еще светимости не хватило, столкновений маловато, статистики маловато. То ли что-то тут совсем не то, и все эти достаточно убедительные, но не стопроцентно железные аргументы, может быть, неправильны.

Какая может быть новая физика? Очень большие надежды были на суперсимметрию. Она замечательна тем, что это теория, в которой есть дополнительная симметрия по сравнению со всеми известными. Которая связывает частицы с целым и полуцелым спином — бозоны и фермионы. Кстати сказать, эта симметрия была предложена теоретиками здесь в Москве, в ФИАНе, в 1970-х годах.

В контексте физики элементарных частиц это значит следующее: если у вас есть кварк со спином 1/2, то у него должен быть партнер, которого недолго думая назвали скалярный кварк, — «скварк» со спином 0. У электрона должен быть партнер — скалярный электрон, у фотона партнером должно быть фотино со спином 1/2, у глюона — глюино, у гравитона — гравитино.

Кроме гравитино, все эти частицы, если они легкие, должны рождаться на Большом адронном коллайдере. Вообще, горячие головы говорили так: включится БАК — и первым делом найдут отнюдь не бозон Хиггса, а суперсимметрию. И это мнение разделялось не только многими теоретиками, но и бедными экспериментаторами, которым теоретики запудрили мозги. Однако суперсимметрия до сих пор не открыта, только есть ограничения на массы вышеперечисленных частиц. Вообще, уже не похоже, что суперсимметрия есть в природе при не очень высоких энергиях.

Почему суперсимметрия хороша? Оказывается, вклады виртуальных частиц в массу бозона Хиггса имеют разные знаки для разных спинов. При суперсимметрии бозонный и фермионный вклады сокращаются до нуля, и если у вас есть фотоны и фотино или W-бозоны и ви́но, то их вклады тоже сокращаются до нуля. Если массы частиц и их суперпартнеров разные — а это так и есть, нет скалярного электрона с такой же массой, как у электрона, это мы точно знаем, — то это сокращение происходит не до нуля. Но если массы суперпартнеров — в районе тераэлектронвольт, то как раз получается, что эти вклады имеют масштаб сотни гигаэлектронвольт, и тогда всё хорошо. Но это уже не работает. Уже сейчас ограничения на эти массы такие сильные, что данный механизм сокращения полностью не работает, 100 ГэВ не получить. Если наивно вычислять, то должно получиться что-то вроде 500–700 ГэВ для массы бозона Хиггса. Так что сейчас ситуация с поиском суперсимметрии очень напряженная.

Есть еще сценарии: например, бозон Хиггса может быть составным, не обязательно элементарным. И вообще, в физике конденсированных сред известны аналоги механизма Хиггса, и там аналог бозона Хиггса или хиггсовского поля не элементарный, а составной. Самый известный пример — это сверхпроводимость. В сверхпроводнике фотон как бы имеет массу, это так называемый эффект Мейснера. Теория Энглера — Браута — Хиггса — это почти один в один теория Гинзбурга — Ландау, которая была предложена лет за десять до Энглера — Браута — Хиггса.

Если бозон Хиггса — составной, тогда всё меняется, и огромные вклады от взаимодействия с вакуумом исчезают, появляется размер составной системы, как у протона. Если этот размер составляет 10–18 см, то соответствующая энергия системы оказывается разумной, при этом внутренняя структура пока остается неразличимой. У таких моделей есть свои предсказания, но опять-таки до сих пор ничего похожего на ускорителе не видно.

Может быть, мы чего-то сильно не понимаем, теоретики чего-то сильно не додумали, не открыли в своих головах. Конечно, на Большом адронном коллайдере есть программа поиска новых явлений, не опирающаяся на теоретические предсказания. Будем искать, где можем, «где есть фонари» — под ними и будем искать. И будем пытаться найти отличия от Стандартной модели везде, где только это можно сделать. До сих пор ничего этого нет, и Стандартная модель прекрасно работает.

В заключение скажу: сейчас мы находимся на очень интересном этапе развития физики элементарных частиц. С одной стороны, есть уверенность, что Стандартная модель — это еще не вся история. Есть еще пришедшие из космологии жесткие, однозначные свидетельства того, что Стандартная модель неполна, — в первую очередь это темная материя: во Вселенной есть массивные частицы, которые составляют темную материю, их по массе примерно в пять раз больше, чем обычного вещества.

Сейчас такая ситуация, что физика элементарных частиц снова стала экспериментальной наукой. В 1950–1960-е годы эта область физики была экспериментальной наукой, когда шли эксперименты, их результаты осмысливались и создавались теории. Однако на протяжении всей моей сознательной жизни всё было наоборот: теоретики делали предсказания, а экспериментаторы их подтверждали. Сейчас мы опять пришли к ситуации, когда мы полностью завязаны на эксперимент, не зная, что он нам покажет. Ждем, держим пальцы крестиком, но пока ничего интересного БАК нам не говорит. Кроме того, что есть бозон Хиггса…

Какая новая физика появится в конце концов, мы тоже не знаем. Так что ситуация интересная, важное открытие сделано, но каким будет следующее открытие, никто сегодня сказать не может. Может быть, это и хорошо, это заставляет нас напрягаться и думать, а экспериментаторов — искать новые явления. Надеюсь, что эти поиски завершатся успехом.

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