(ORDO NEWS) — The Large Hadron Collider is returning to service after more than a three-year hiatus. In honor of this event.
The Large Hadron Collider returns to work . The largest and most powerful accelerator in history has been under maintenance and modernization for more than three years. Now engineers are gradually “revitalizing” it.
The giant will finally return to service this summer. Then a four-year cycle of experiments will begin, already the third in the history of the accelerator. The first cycle took place in 2010-2012, and the second, after modernization, in 2015-2018.
The Large Hadron Collider accelerates beams of protons and sends them towards each other. Almost all particles go “into the milk”, and no wonder: the task of hitting a proton with a proton is more difficult than hitting a bullet with a bullet.
But some of the protons still collide with each other. In these collisions, various new particles are born. Observing what exactly and how often is formed, scientists comprehend the laws of elementary particle physics.
The most important characteristic of an accelerator is the collision energy. The greater the energy of the colliding particles, the more massive the particles will be as a result of the impact (the energy goes into mass according to the famous formula E = mc 2 ). And the new upgrade brought the energy of proton collisions to a record 13.6 teraelectronvolts.
Another important quantity is the luminosity of the accelerator. This is the number of particle collisions per square centimeter of the accelerator section per second. The higher the luminosity, the more events physicists will register and the more information they will receive.
Modernization helped here too. It is expected that the ATLAS and CMS detectors will detect more collisions during the third “run” than during the previous two combined. The number of events at the LHCb detector will triple compared to previous launches.
And the number of collisions of heavy ions in the ALICE experiment will increase by 50 times. In addition, new detectors, FASER and [email protected] , will come into operation, specifically designed to search for phenomena that do not fit into the Standard Model.
By the way, what is this Standard Model, why are scientists so eager to break out of its limits? Let’s talk about this in more detail.
Everything in the world is made up of elementary particles. Any process – be it a handshake or a nuclear explosion – ultimately comes down to the interaction between particles.
What elementary particles exist in nature? How do they interact with each other? Why are they the way they are? Getting exhaustive and definitive answers to these questions is the blue dream of physicists.
From the school bench, we are familiar with the particles that make up the atom: electrons, protons and neutrons. But this is just the tip of the iceberg. In fact, there are many more particles in nature. Today there are more than 200 of them open.
The vast majority of particles are short-lived: once formed, they exist for only a tiny fraction of a second. But there is no reason to think about understanding the general laws of elementary particle physics by studying only long-lived particles like an electron or a proton.
It’s like trying to comprehend all the secrets of the Earth‘s biosphere, studying only thousand-year-old sequoias.
So, for experiments, physicists need a “factory” that produces short-lived particles. And there are such factories: they are called accelerators. In them, stable particles (usually protons or electrons) accelerate and collide with each other or with a stationary target. It is in these collisions that short-lived particles are born.
If you accelerate two cars well and send them towards each other, they will fly apart. In the world of elementary particles, there are other laws: a collision leads to the birth of new particles. We emphasize that this does not mean that the colliding protons consisted of these particles.
If cars were elementary particles, the result of a collision between two Muscovites would not be bumpers and wings flying in different directions, but, say, a brand new Bentley and a couple of motorcycles. Sounds strange, but it’s a fact.
As we have already mentioned, the mass of the produced particles depends on the collision energy. The energy of colliding particles depends primarily on their speed. And to accelerate the particle well, you need a large accelerator.
The first operational accelerators appeared in the early 1930s, and a race for their size began that lasted for decades. Each new stage in it meant a lot of freshly discovered particles.
Over time, the number of new particles that were found in accelerators and in cosmic rays began to even somewhat disturb physicists. It was necessary to find a system in this chaos, just as the periodic table expressed the elegant law behind the bewildering variety of chemicals.
And this system was found and tested in two legendary decades – the 1960s and 1970s. Below we will tell the amazing story of an era when discoveries were big and accelerators were small.
The Mystery of the Four Powers
Let’s deviate for now from the classification of particles and see what forces act between them. Already in the 1930s, physicists knew that there were four interactions between elementary particles: electromagnetic, gravitational, strong, and weak.
Any force acting on any object is reduced to one of these fundamental interactions. And without any of them, the existence of planets, stars and living beings would be impossible.
For example, without electromagnetic interaction, there would be no atoms, because electrons would not be attracted to the nucleus.
In addition, this interaction is responsible for the forces of friction and elasticity (thanks to the latter, we do not fall through the floor), all chemical reactions (including those occurring in a living cell), light emission and many other phenomena.
And without gravity, there would be no galaxies, stars and planets: it is the only force that holds such huge objects together.
The strong interaction keeps protons and neutrons in the nucleus of an atom, despite the electrical repulsion of protons.
Without it, there would be no chemical elements other than hydrogen, including those that make up our bodies. But they would not exist without the weak interaction, because only thanks to it, nuclear reactions became possible, which created the entire periodic table from primary hydrogen.
At the same time, the four fundamental forces are defiantly different from each other. For example, electromagnetic and gravitational forces act at any distances, while strong and weak forces act only at very small distances (smaller than an atom).
Interactions are still very selective in which particles are allowed to participate in them. Only particles that have a charge participate in the electromagnetic, in the gravitational – having a mass, in the strong – having the so-called color (which has nothing to do with color in the usual sense).
And in the weak – everything in general, except for particles – carriers of other interactions (photons, gluons and hypothetical gravitons).
The strength of these interactions is also very diverse. The strong force is, as the name suggests, the strongest. That is why it overpowers the electrical repulsion of protons and, fortunately for us, keeps them in the nucleus of the atom.
In second place is the electromagnetic interaction, which is much weaker. On the third – weak, which is weaker by several orders of magnitude.
And the gravitational one closes the list – so weak that we are unlikely to ever be able to measure the gravity between two separate particles. It becomes noticeable only when astronomical masses of matter come together.
In general, the four forces that shape our world are similar to each other, about like a banana, a penguin, an iceberg, and handcuffs.
It seems that they were simply snatched at random from a huge box in which everything in the world was piled up. This is not what you expect to find at the foundation of the world. Perhaps three elephants would look more decent: at least they are the same.
But perhaps this triumph of chaos is apparent? And if such symmetry and beauty are hidden behind external dissimilarity that it seems inevitable? Looking ahead, let’s say that it is, at least in part. Part of this beautiful harmonious picture is already known to us, but we can guess about the rest… and hope.
However, the discoverers had enough work even without these metaphysical questions. Until the 1960s, researchers simply did not have a working theory of either the strong or the weak force.
They asked clear quantitative questions (for example, how likely is particle A to become particle B) and did not know how to calculate the answer.
The particles involved in the strong interaction are called hadrons. The vast majority of particles known to us (including protons and neutrons) are just hadrons, so it is natural to start raking the Augean stables from them.
In the early 1960s, Murray Gell-Mann and Yuval Ne’eman saw a hidden order in the pile of known hadrons. It turned out something like a periodic table for particles. And it soon became clear that this analogy is deeper than it seems.
The place of an element in the periodic table is determined by how many protons (or, if you prefer, electrons, there are always equal numbers) in its atom.
Surprisingly, more than a hundred of these different chemical elements are obtained simply by sequentially adding another proton to the nucleus. Gell-Mann and, independently of him, George Zweig realized that the regularities in the properties of hadrons are also their internal structure.
They suggested that hadrons are composed of even smaller particles – quarks. There are only six kinds of quarks, and the properties of all hadrons are determined by what kind of quarks they are made of (and also by the state in which these quarks are).
Hadrons turned out to be “elementary, but not the most elementary particles.” The variety of dozens of hadrons was reduced to a shuffling of six quarks. The world has become noticeably simpler.
Looking at hadrons as Lego blocks with quark parts, the theorists assembled several new, as yet undiscovered, hadrons at the tip of a pen.
The first of these were soon successfully discovered, earning Gell-Mann the 1969 Nobel Prize in Physics. Experimenters are still discovering new hadrons (this is the main activity of the Large Hadron Collider). But they are all built from the same quarks according to the same principles.
We emphasize, however, that only hadrons, and not all particles, consist of quarks. Electrons, neutrinos and some other proud people (they have a common name – leptons) do not consist of quarks and do not participate in strong interaction.
Dealing with hadrons and quarks, theorists built, and experimenters tested the theory of strong interaction – quantum chromodynamics.
It turned out that the strong interaction is the exchange of special particles (gluons). Scientists have derived formulas that gluons and quarks obey. Knowledge of the strong interaction has evolved from a pile of disparate facts into a coherent system that physicists still use today.
In the same years, theorists struggled with the mysteries of the weak interaction. Steven Weinberg tried to build a theory of weak forces by analogy with the theory of electromagnetic forces (quantum electrodynamics). The latter was created in the 1930s.
As a result, Weinberg not only built the theory of weak interactions, but did much more. Diving deep into the analogy between the weak force and the electromagnetic force, the physicist suddenly discovered that the two forces were sides of the same coin.
At temperatures above 10 15 (thousand trillion) degrees, these forces cease to differ from each other. As well as cease to differ from each other such different particles as an electron (electrically charged and, therefore, participating in electromagnetic interaction) and a neutrino (according to the name, neutral).
At such temperatures, the neutrino and the electron can be considered one and the same particle. Other particles also form pairs that merge at a temperature above the threshold (for example, the u-quark ceases to differ from the d-quark).
This theory, which explained weak interactions and at the same time revealed their hidden unity with electromagnetic forces, is called the theory of electroweak forces. Weinberg completed it in 1967.
A year later, the Pakistani physicist Abdus Salam independently built almost the same theory, and some aspects of it were previously developed by Sheldon Glashow. The first experimental confirmation of the new theory appeared very quickly, so that in 1979 the trio of discoverers received the Nobel Prize.
Little giants of big physics
All these landmark discoveries were made on accelerators that are very small by today’s standards. For example, an important confirmation of the electroweak theory was received at the Proton Synchrotron (Proton Synchrotron) with a length of only 628 meters.
And one of the decisive confirmations of the Gell-Mann theory – the discovery of the c-quark – took place thanks to the Alternating Gradient Synchrotron with a length of 806 meters. History was made at installations ten times smaller than the LHC. Truly an age of miracles.
But one of the key predictions of the electroweak theory turned out to be too tough for these devices. Just as the electromagnetic force is carried by photons and the strong force by gluons, the weak force must also have carrier particles.
There are two of them: W-boson and Z-boson (in general, there are two W-bosons: positively and negatively charged, but they are each other’s antiparticles, so they have the same mass and other parameters).
The heavier the particle, the more energy it needs to be born (E = mc 2 , remember?). And hundred-meter installations could not reach the energies of the production of W- and Z-bosons. These particles were discovered in 1983 thanks to the seven-kilometer-long Super Proton Synchrotron.
Note that it is not so far from here to the LHC with its 27-kilometer ring. Checking all the new predictions of brilliant theories created in the 1960s and 1970s already required serious efforts. However, nothing surprising: when low-hanging fruits are plucked, only those hanging high remain.
The theory of Weinberg, Salam and Glashow was again brilliantly confirmed, but there was an unresolved issue in it.
We mentioned that above a certain temperature threshold, the weak interaction is indistinguishable from the electromagnetic one. From the point of view of the electroweak theory, this is not surprising: it describes both forces with the same equations.
On the contrary, the really difficult question is why do the interactions begin to differ when the temperature drops below the fatal point?
The answer was proposed by Peter Higgs in 1964. Without going into details, let’s say that his hypothesis required the existence of the Higgs boson – the third of the three cherished bosons of the electroweak theory.
The Standard Model predicted all the parameters of this particle, except for the mass. Mass, on the other hand, had to be measured experimentally and incorporated into the theory. The Higgs boson has been sought for decades at increasingly powerful accelerators.
They didn’t find it and shrugged it off: they say, it looks like it is even heavier than we thought. The long-awaited discovery turned out to be within the power of only the LHC.
The breakthrough was announced in 2012, 48 years after the publication of the Higgs. It turned out that the Higgs boson is more than 130 times heavier than the proton. Not surprisingly, the accelerators of the 1980s and 1990s failed to find it.
Simplifying the world
The electroweak theory, together with quantum chromodynamics, makes up the Standard Model of particle physics, the main theory of how almost everything works. There are only 24 fundamental particles in it (not counting antiparticles).
They do not consist of any even smaller parts, that is, truly elementary. This list contains 12 particles of matter: six quarks and six leptons (an electron, a muon, a tau lepton, and three types of neutrinos).
Another 12 particles – photon, W-boson, Z-boson, graviton and eight types of gluons – carry interactions: electromagnetic, weak, gravitational and strong, respectively. This is what the zoo of two hundred elementary particles came down to, almost all of which, in fact, turned out to be not so elementary.
With the creation of the Standard Model, the world has become amazingly elegant. An immense pile of fragmentary facts was replaced by an economical list of particles and working theories of three of the four interactions (alas, no one has succeeded in constructing a satisfactory quantum theory of the fourth – gravitational – yet).
Moreover, two of them – electromagnetic and weak – even united into one. Whatever discoveries may happen in the future, whatever new, even deeper order of things is discovered, the Standard Model will not lose its significance, just as its periodic table or the law of universal gravitation does not lose.
However, physicists are always striving to advance further in understanding the world. The unification of the two fields – electromagnetic and weak – gave rise to hopes for the unification of all four. This is what truly fundamental forces should be: not four fancy curiosities, but symmetrical facets of a beautiful crystal.
Of course, for this it is necessary at least to build a quantum theory of gravity. And it would be nice if the new theory also derived those parameters that are included in the Standard Model as input (for example, the mass and charge of an electron and other fundamental particles). Then it can rightfully be called the theory of everything .
Theories that combine the electroweak and strong forces are called grand unification theories. This is no longer the Standard Model, but its untested extensions. How to check them? It’s as easy as shelling pears: you need to get particles that are 10 15 ( thousand trillion ) times heavier than a proton.
To achieve this at the expense of the size of the booster, a ring as long as the orbit of Mars is required . And in order for the fourth, gravitational, to join the three interactions, it is possible that an accelerator the size of a galaxy will be needed.
In the meantime, the beauty and pride of modern science is the 27-kilometer LHC, which is somewhat short of the desired scale.
Successful and best
However, the neglect of the Large Hadron Collider is inappropriate: it is a super-successful project. With it, experimenters confirmed many of the Standard Model’s predictions that could not be tested in smaller setups.
Among these predictions that have come true is the discovery of dozens of new particles. Only 59 hadrons were discovered at the Large Hadron Collider.
But the creators of the gigantic installation, of course, hoped to go beyond the theories created in the era of hundred-meter accelerators. To receive data that does not fit into the Standard Model and calls for its extension – what could be more tempting?
Until this dream comes true. In general, there are some intriguing results , but quite fresh. It is not yet clear whether they will indeed require revision of theories. On the contrary, some of the theoretical extensions of the Standard Model have been debunked.
Well, humanity has reason to be proud of its theorists. The Standard Model explains data far beyond what it was once created from.
For a scientific theory, this is the most successful success in success. Only bad models have to be adjusted for every fresh fact. But – the experimenters sigh – how much we want a new era of discoveries!
Perhaps the third “session” of the LHC will finally allow us to break out into the vastness of new physics. And upon its completion, they plan to turn the LHC into the High Luminosity Large Hadron Collider, or HL-LHC. The number of colliding particles will increase sharply. This will help to catch rare exotic processes – if they exist, of course.
CERN plans to launch a 100-kilometer Future Circular Collider, which they intend to launch in 2040. The current LHC record holder will be only an auxiliary ring for this colossus. So for the LHC itself, the seven-kilometer Proton Super Synchrotron, the proud “discoverer” of W- and Z-bosons, became an auxiliary ring.
Smart uphill will not go?
However, there is hope to deduce consequences from extensions of the Standard Model that can be verified without accelerators. For example, some of the grand unification theories claim that protons decay over time, it’s just that their average lifetime is unusually long.
But if you take a large enough mass of matter, there are several protons in it that will decay right now. So far, such experiments have not been successful, but who knows …
And most importantly, it is possible that someday we will completely abandon accelerators in the current sense. The energy of collisions at the same LHC is high only by the standards of elementary particles. In fact, about the same amount of energy a fly would spend doing push-ups from the floor.
The trick is to concentrate all this energy in a single proton, which is as much thinner than a fly’s leg as it is smaller than the Sun itself. So far, we have not found a better way to do this than to drive a long-suffering proton in an electromagnetic field through a long tunnel.
This approach is almost a hundred years old, except that the tunnels are getting longer, the fields are more powerful, and the equipment is more perfect.
But is progress possible only on the path of “let’s do the same, but more”? You might want to hitch two hundred horses to a wagon, although it’s better to have a truck.
You can also want a ring the size of the orbit of Mars, but it’s better to come up with something more efficient. This is a challenge for physicists of the 21st century. If they can handle it, the dream theory might become the theory of reality in every sense of the word.
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