Why physics needs and deserves a post-LHC collider

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Accelerating particles into circles, bending them with magnets and colliding them with particles or anti-particles of high energy is one of the most powerful ways to probe the new physics in the Universe. To find what the LHC can not, we must go to higher energies and / or higher accuracies, which a bigger tunnel will provide absolutely.CERN / FCC study

The Large Hadron Collider is the most powerful particle accelerator ever built by mankind. By obtaining higher energies and more collisions with these energies than ever before, we have pushed the boundaries of particle physics beyond their former limits. The achievements of the thousands of scientists who built the LHC and its detectors, conducted the experiments and collected and analyzed the data can not be overstated.

He is best known for finding the Higgs boson, but nothing outside the standard model. Some even consider what the LHC found disappointing, as we have not yet discovered any new unexpected particles. But this masks the greatest truth of experimental science of all types: to truly know the fundamental nature of the Universe, you must ask him questions about himself. At the moment, the LHC is our best tool to do that, with its next high-brightness upgrade. If we want to continue learning, we must also prepare to go beyond the LHC.

Inside the LHC, where the protons intersect at 299,792,455 m / s, just 3 m / s of the speed of light. As powerful as the LHC is, we need to start planning the next generation of colliders if we want to discover the secrets of the Universe that exceed the capabilities of the LHC.CERN

The reason the LHC is such a powerful tool is not limited to the data it collects. Of course, it collects an incredible amount of data, colliding packets of protons with other proton packets at 99.999999% of the speed of light every few nanoseconds. The collisions produce debris that scatters through the huge detectors built around collision points, recording traces of outgoing particles and allowing us to reconstruct what has been created and how.

But there is another critical element in this story: understanding the standard model of elementary particles. Each particle in the universe obeys the laws of particle physics, which means that there are couplings and interactions between particles, real and virtual.

The Higgs boson, with its now known mass, is coupled to the quasks, leptons and W and Z bosons of the standard model, giving them mass. The fact that it is not directly coupled to the photon and gluons means that these particles remain massless. Photons, gluons, and W-and-Z bosons couple to all particles that are subject to strong and weak electromagnetic nuclear forces, respectively. If there are additional particles, they can also have these couplings.TriTertButoxy at English Wikipedia

Have mass? You are in a relationship with the Higgs. This includes the Higgs boson, which couples to itself.

Do you have electric charges, weak or strong? You couple to the appropriate bosons: photons, W-and-Z or gluons, respectively.

And this is not the end, like all these pairs of bosons are playing too. For example, the proton is composed of three quarks: two high quarks and one low quark, which are coupled to the strong force via the gluons. However, if we change the mass of the top quark from 170 GeV to around 1000 GeV, the mass of the proton would increase by about 20%.

As better theoretical experiments and calculations have emerged, our understanding of the proton has become more sophisticated, with the coming into play of gluons, marine quarks and interactions between them and the quarks of valence. Even the top quark, the heaviest of all, deeply affects the mass of the proton.Brookhaven National Laboratory

In other words, the properties of the particles of which we are aware depend on the full range of all other particles, even those that we have not yet detected. If we are looking for something beyond the standard model, the most obvious way is to create a new particle and simply find it.

But what we are much more likely to do in practice is:

create a lot of particles that we already know,
calculate what are, for example, decay rates, branching rates, scattering amplitudes, etc., only for the standard model,
measure what are actually these decay rates, branching ratios, scattering amplitudes, etc.,
and compare with the predictions of the standard model.

If what we observe and measure is identical to what the standard model predicts, then all that is new – and we know that new things must exist in the universe – does not change our observables any more than it does. ;measurement uncertainty. Up to now, this is what all LHC colliders have revealed: particles that behave perfectly in accordance with the standard model.

The standard model of particle physics represents three of the four forces (except gravity), the complete suite of discovered particles and all their interactions. The possibility of building on Earth additional particles and / or interactions that can be discovered with colliders is a questionable subject, but we will only know the solution if we explore beyond the known boundaries of energy and precision.Contemporary Physical Education Project / DOE / NSF / LBNL

But there must be new particles, and they could be detectable by pushing the boundaries of experimental particle physics. The options include new physics, new forces, new interactions, new couplings or a whole series of exotic scenarios. Some of them are scenarios that we have not even considered yet, but the dream of particle physics is that new data is leading the way. As we remove the veil from our cosmic ignorance; as we search the boundaries of energy and precision; as we produce more and more events, we start getting data like never before.

If we can look at meaningful data that take us from 3 to 5 decimals, we start to become sensitive to particle couplings that we can not create. Signatures of new particles may appear as a very small correction to Standard Model predictions, and the creation of large numbers of decaying particles such as Higgs bosons or higher quarks could reveal them.

The Future Circular Collider is a proposal to build, for the 2030s, an LHC successor with a circumference of up to 100 km, almost four times the size of today's underground tunnels. Once built, the FCC will mean Frontier Circular Collider.CERN / FCC study

That's why we need a future collider. A solution that exceeds the capabilities of the LHC. And surprisingly, the next logical step is not to go to higher energies, but to lower them with much greater accuracy. This is the first step in CERN's plans for the FCC: the circular collider of the future. In the end, a hadron-hadron collider, in the same tunnel, could break the 100 TeV threshold for collisions: an increase of seven times higher than the maximum energy of the LHC. (You can play with an interactive app here to see what the increase in energy and the number of collisions make to reveal the unexplored boundaries of physics.)

Most people do not remember, but before the LHC, this same 27 km tunnel was home to a different collider: LEP. LEP represented the large electron-positron collider, where instead of protons, electrons and their antimatter counterparts (positrons) were accelerated at incredibly fast speeds and fused together. This presented both a huge advantage and a huge disadvantage compared to proton-proton colliders.

The magnitude of the future proposed Future Circular Collider (FCC) compared to the LHC currently at CERN and Tevatron, which was previously operational at Fermilab.Pharito / Wikimedia Commons

Electrons and positrons are almost 2000 times lighter than protons, which means they can get closer to the speed of light much faster than protons can do with the same energy. LEP accelerated the electrons to energies up to 104.5 GeV, a speed of 299,792,457,9964 meters per second. At the LHC, protons reach much higher energies: 6.5 TeV each, about 60 times more than LEP energies. But their speed is only 299,792,455 m / s. They are much slower.

The lower maximum energies of electrons and positrons are explained by the fact that their masses are so light. The charged particles emit energy when they are in magnetic fields, according to a process called synchrotron radiation. The higher your load / mass ratio, the more you radiate, which limits your maximum speed. The electron-positron colliders are dedicated to low energies; it's their disadvantage.

Particles and antiparticles of the standard model have now all been detected directly. The last stronghold, the Higgs boson, dropped at the LHC early this decade. All of these particles can be created at the LHC energies, but they could be created in greater abundance and with better measurable properties in a next-generation electron-positron collider.E. Siegel / Beyond the galaxy

But their advantage is that the signal is perfectly clean. Electrons and positrons are fundamental fundamental particles. If you have an electron and a positron at energies of, say, 45.594 GeV each, then you can produce Z bosons with a rest mass of 91.188 GeV / c²) spontaneously and in great abundance. If you can adapt your center of mass energy to the resting mass of the particle (or particle pairs, or particle / antiparticle pairs) you want to create, via the Einstein software E = mc²you can basically build a plant to produce all the unstable particles you want.

In a future collider, this means that we produce W, Z, top quarks (and antitop) and Higgs bosons at will. When you build a particle accelerator, its radius and the strength of its magnetic fields determine the maximum energy of your particles. With the future future 100 km circular collider proposed, even in the collision between simple electrons and positrons, we can manufacture each particle of the standard model at will, in large quantities and as many times as we wish.

The observed Higgs decay channels compared to the standard model contract, with the latest ATLAS and CMS data included. The agreement is both staggering and frustrating. By 2030, the LHC will have about 50 times more data, but the details of many decay channels will still be known only a few percent. A future collider could increase this accuracy by several orders of magnitude, thus revealing the existence of potential new particles.André David, via Twitter

Even at lower energies than the LHC, a larger electron-positron collider has the potential to probe physics like never before. For example:

If new particles exist with energy below 10 TeV (and up to 70 TeV for some new physics classes), their indirect effects must appear in the production and disintegration of the particles of the standard model or in the mass relations that occur. unite them.
We can further investigate how Higgs couples couple to standard model particles, including themselves, as well as particles beyond the standard model.
We can determine if there are other "invisible" disintegrations, where the products are invisible, beyond the standard model neutrinos.
We can measure all the decays of short-lived particles (like the Higgs boson or the top quark, or even the b-quarks and τ leptons) with a greater and unprecedented accuracy.
We can search for, limit, and in some cases exclude exotic particles, not only from supersymmetry, but also from other scenarios, such as sterile neutrinos.
And, potentially, we can even learn how to break the electroweak symmetry and what kind of transition (with quantum tunnel or not) breaks it.

The vertices shown in the Feynman diagrams above contain all three Higgs bosons meeting at a single point, which would allow us to measure the self-coupling of Higgs, a key parameter in the understanding of fundamental physics .Alain Blondel and Patrick Janot / arXiv: 1809.10041

Before considering a collider at higher energies, the construction of a precisely tuned collider capable of creating all known particles in abundance is obvious. Considerable resources have already been invested in a linear collider for electrons and positrons, such as CLIC and ILC proposed, but similar technologies would also apply to a large circular tunnel in which electrons and positrons accelerate. and collide with the inside.

It's a way to push the boundaries of physics back to unexplored territory using existing technology. No new invention is needed, but the unique advantage of a future circular lepton collider is that it could be improved.

In the early 2000s, we replaced LEP with a proton-proton collider: the LHC. We could also do it for this future collider: move to the collision of protons once the electron-positron data collected. There is a hint of new physics, beyond the standard model, about the energies that a future collider is realizing – problems ranging from baryogenesis to the problem of hierarchy through to the enigma of dark matter – the proton-proton collider will actually produce these new particles.

When two protons collide, the marine quarks, the gluons and, beyond, the interactions in the field are not the only ones that can collide. All can provide information on the rotation of each component and allow us to create new potential particles if sufficiently high energies and brightness are achieved.CERN / CMS Collaboration

To better understand the self-coupling of Higgs, a hadron-hadron collider of about 100 TeV will be the ideal tool, producing more than 100 times the number of Higgs bosons that the LHC will never create. . A proton-proton version of a future circular collider can use the same tunnel as the lepton-lepton version and will use next-generation technology for its electromagnets, reaching a field strength of 16 T, double the the magnetic power of the LHC. (These magnets will be a formidable technological challenge for the next two decades.) This is an ambitious plan that allows us to predict at least two colliders in the same tunnel.

A future hadron-hadron collider of a future circular collider will also measure the rare Higgs boson decays, such as two-muon decays or a Z-boson and a photon, as well as the coupling of the Higgs-top quark to a accuracy of about 1%. If there are new bosons, fundamental forces or signs of electroweak scale baryogenesis, or even a factor of about 1000, the proton-proton incarnation of the future future circular collider will find the evidence . Neither an electron-positron collider nor the LHC can do it.

In total, the FCC hadron-hadron version will collect 10 times more data than the LHC will ever collect (and 500 times more than today), while reaching energies seven times higher than maximum of the LHC. This is an incredibly ambitious proposal, but it will be within our reach by 2030, if we plan to achieve it today.

When you collide with high-energy electrons with hadrons (such as protons) moving in the opposite direction at high energy, you can acquire the ability to probe the internal structure of hadrons like never before.Joachim Meyer; DESY / HERA

There is also a "phase III" that probes the boundaries of physics in a different way: by colliding high-energy electrons in one direction with high-energy protons in the other. Protons are composite particles, composed of quarks and gluons within, as well as a sea of virtual particles. Electrons, via processes such as deep inelastic diffusion, constitute the best conventional microscope for probing the internal structure of protons. If we want to understand the structure of matter, electron-proton collisions are the way forward and the FCC would push back the frontier where previous experiments, such as the DESY HERA Collider, had taken us.

Between the indirect effects that an electron-positron collider could see, the new direct particles that may come from proton-proton collisions and the better understanding of the mesons and baryons that an electron-proton collider will bring, we have all the reasons to hope that new physical signal could emerge.

What do we do next, if there is new physics? And if new particles were discovered at these higher energies? And then?

The V-shaped track in the center of the image is probably a muon decomposing into an electron and two neutrinos. The high energy trajectory with an elbow is evidence of a degradation of particles in the air. By colliding positrons and electrons with specific and adjustable energy, muon-antimuon pairs could be produced at will, providing the particles needed for a future muon collider.Scottish Science and Technology Tour

We do not necessarily need to build an even larger collider to better study them. If new physical data is generated at a very high energy scale, we could explore them in depth with a "phase IV" potential for a future circular collider: a muon-antimon collider in the same tunnel. The muon is like an electron: it's a point particle. It has the same price, except that it is about 207 times heavier. It means very good things:

it can reach much higher energies by reaching the same speeds,
it provides a clean signature, adjustable energy,
and unlike electrons, because of the much lower charge / mass ratio, its synchrotron radiation can be neglected.

It's a brilliant idea, but also a daunting challenge. The disadvantage is singular but important: muons disintegrate with an average life of only 2.2 microseconds.

An earlier (now extinct) design plan for a full-scale muon-antimuon collider at Fermilab, the source of the world's second most powerful particle accelerator.Fermilab

This is not a dealbreaker, however. Muons (and antimuons) can be made very efficiently by two methods: the first by collision of protons with a fixed target, producing charged pions decomposing into muons and antimuons, and the second by collision of positrons on the right. around 44 GeV with resting electrons producing muon / antimuon pairs directly.

We can then use magnetic fields to bend these muons and antimuons into a circle, accelerate them and hit them. If we make them go fast enough in a relatively short time, the temporal dilation effects of Einstein's relativity will keep them alive long enough to collide and produce new particles. We could, in principle, reach energies of about 100 TeV with a clear signal in a muon collider this way: about 300 times more energetic than a future electron / positron collider.

There is certainly a new physics beyond the standard model, but it might not appear before much higher energies than a terrestrial collider could ever achieve. It is also possible that new extraordinary physical models exist for small masses or energies, but with couplings too small to be detected by a terrestrial collider. Whatever the scenario, the only way to know is to watch.RevueUnivers.ca

Before the discovery of the Higgs, we used the term "nightmare scenario" to describe what it would be like if the LHC found the standard Higgs model and nothing else. In reality, it is not a nightmare to discover the Universe as it is. There may be no additional particles or abnormal behavior, beyond the standard model, to be discovered with a terrestrial collider that we could eventually build, that is true. But there could also be many new unexpected discoveries at scales and precisions that the LHC will be unable to access.

The only way to know the truth about our universe is to ask these questions. Determining the laws of nature and the behavior of particles is a step forward for human knowledge and the whole scientific enterprise. The only real nightmare would be that we stop exploring and give up even before having a look at it.

The author thanks Panos Charitos, Frank Zimmermann, Alain Blondel, Patrick Janot, Heather Gray, CERN Markus Klute and Matthew McCullough for their extremely useful and informative discussions and emails regarding the potential of a future LHC collider. .

Have mass? You are in a relationship with the Higgs. This includes the Higgs boson, which couples to itself.

Do you have electric charges, weak or strong? You couple to the appropriate bosons: photons, W-and-Z or gluons, respectively.

But what we are much more likely to do in practice is:

create a lot of particles that we already know,
calculate what are, for example, decay rates, branching rates, scattering amplitudes, etc., only for the standard model,
measure what are actually these decay rates, branching ratios, scattering amplitudes, etc.,
and compare with the predictions of the standard model.

But there must be new particles, and they could be detectable by pushing the boundaries of experimental particle physics. Les options comprennent une nouvelle physique, de nouvelles forces, de nouvelles interactions, de nouveaux couplages ou toute une série de scénarios exotiques. Certains d'entre eux sont des scénarios que nous n'avons même pas encore envisagés, mais le rêve de la physique des particules est que de nouvelles données ouvrent la voie. Alors que nous retirons le voile de notre ignorance cosmique; alors que nous sondons les frontières de l’énergie et de la précision; à mesure que nous produisons de plus en plus d'événements, nous commençons à obtenir des données comme jamais auparavant.

Si nous pouvons examiner des données significatives qui nous prennent de 3 à 5 décimales, nous commençons à devenir sensibles aux couplages aux particules que nous ne pouvons pas créer. Les signatures de nouvelles particules peuvent apparaître comme une très petite correction des prédictions du Modèle standard, et la création d'un grand nombre de particules en décomposition telles que les bosons de Higgs ou les quarks supérieurs pourrait les révéler.

The Future Circular Collider est une proposition visant à construire, pour les années 2030, un successeur du LHC dont la circonférence peut atteindre 100 km, soit près de quatre fois la taille des tunnels souterrains actuels. Une fois construit, le FCC signifiera «Frontier Circular Collider».Étude CERN / FCC

C'est pourquoi nous avons besoin d'un futur collisionneur. Une solution qui dépasse les capacités du LHC. Et étonnamment, la prochaine étape logique n'est pas d'aller vers les énergies plus élevées, mais de les baisser avec une précision bien supérieure. C’est la première étape des plans proposés au CERN pour la FCC: le collisionneur circulaire du futur. En fin de compte, un collisionneur hadron-hadron, dans le même tunnel, pourrait casser le seuil de 100 TeV pour les collisions: une augmentation de sept fois supérieure à l'énergie maximale du LHC. (Vous pouvez jouer avec une application interactive ici pour voir ce que l'augmentation d'énergie et le nombre de collisions font pour révéler les frontières inexplorées de la physique.)

La plupart des gens ne s'en souviennent pas, mais avant le LHC, ce même tunnel de 27 km abritait un collisionneur différent: le LEP. LEP représentait le grand collisionneur électron-positon, où, au lieu de protons, les électrons et leurs homologues antimatière (les positrons) étaient accélérés à des vitesses incroyablement rapides et fusionnés ensemble. Cela présentait à la fois un énorme avantage et un énorme inconvénient par rapport aux collisionneurs proton-proton.

L'ampleur du futur collisionneur circulaire futur (FCC) proposée par rapport au LHC actuellement au CERN et au Tevatron, qui était auparavant opérationnel à Fermilab.Pcharito / Wikimedia Commons

Les électrons et les positrons sont presque 2000 fois plus légers que les protons, ce qui signifie qu'ils peuvent se rapprocher beaucoup plus rapidement de la vitesse de la lumière que les protons ne peuvent le faire avec la même énergie. Le LEP a accéléré les électrons jusqu’à des énergies maximales de 104,5 GeV, soit une vitesse de 299 792 457,9964 mètres par seconde. Au LHC, les protons atteignent des énergies bien plus grandes: 6,5 TeV chacun, soit environ 60 fois plus que les énergies du LEP. Mais leur vitesse n’est que de 299 792 455 m / s. Ils sont beaucoup plus lents.

Les énergies maximales inférieures des électrons et des positrons s'expliquent par le fait que leurs masses sont si légères. Les particules chargées émettent de l'énergie lorsqu'elles se trouvent dans des champs magnétiques, selon un processus appelé rayonnement synchrotron. Plus votre rapport charge / masse est élevé, plus vous rayonnez, ce qui limite votre vitesse maximale. Les collisionneurs électron-positron sont voués aux énergies basses; c'est leur inconvénient.

Les particules et les antiparticules du modèle standard ont maintenant toutes été détectées directement. Le dernier bastion, le boson de Higgs, a chuté au LHC au début de cette décennie. Toutes ces particules peuvent être créées aux énergies du LHC, mais elles pourraient être créées en plus grande abondance et avec des propriétés mieux mesurables dans un collisionneur électron-positon de nouvelle génération.E. Siegel / Au-delà de la galaxie

Mais leur avantage est que le signal est parfaitement propre. Les électrons et les positrons sont des particules fondamentales fondamentales. Si vous avez un électron et un positron à des énergies de, disons, 45,594 GeV chacune, vous pouvez alors produire des bosons Z (de masse au repos de 91,188 GeV / c²) spontanément et en grande abondance. Si vous pouvez adapter votre énergie de centre de masse à la masse au repos de la particule (ou des paires de particules, ou des paires de particules / antiparticules) que vous souhaitez créer, via le logiciel Einstein E = mc², vous pouvez essentiellement construire une usine pour produire toutes les particules instables que vous voulez.

Dans un futur collisionneur, cela signifie que nous produisons des W, des Z, des quarks top (et antitop) et des bosons de Higgs à volonté. Lorsque vous construisez un accélérateur de particules, son rayon et la force de ses champs magnétiques déterminent l'énergie maximale de vos particules. Avec le futur collisionneur circulaire futur de 100 km proposé, même lors de la collision entre électrons simples et positrons, nous pouvons fabriquer chaque particule du modèle standard à volonté, en grande quantité et autant de fois que nous le souhaitons.

Les canaux de désintégration de Higgs observés par rapport au contrat de modèle standard, avec les dernières données d’ATLAS et de CMS incluses. L'accord est à la fois stupéfiant et frustrant. D'ici 2030, le LHC aura environ 50 fois plus de données, mais les précisions sur de nombreux canaux de désintégration ne seront encore connues que de quelques pour cent. Un futur collisionneur pourrait augmenter cette précision de plusieurs ordres de grandeur, révélant ainsi l'existence de nouvelles particules potentielles.André David, via Twitter

Même à des énergies inférieures à celles du LHC, un collisionneur électron-positon plus grand a le potentiel de sonder la physique comme jamais auparavant. Par exemple:

Si de nouvelles particules existent avec une énergie inférieure à 10 TeV (et jusqu’à 70 TeV pour certaines classes de physique nouvelle), leurs effets indirects doivent apparaître dans la production et la désintégration des particules du modèle standard ou dans les relations de masse qui les unissent.
We can further study how the Higgs couples with Standard Model particles, including itself, as well as beyond-the-Standard-Model particles.
We can determine if there are additional "invisible" decays, where the products are unseen, beyond the Standard Model neutrinos.
We can measure all the decays of short-lived particles (like the Higgs boson or the top quark, or even the b-quarks and τ leptons) to greater, unprecedented precision.
We can search for, constrain, and in some cases rule out exotic particles, not just from supersymmetry, but from other scenarios, such as sterile neutrinos.
And, potentially, we can even learn how the electroweak symmetry breaks, and what type of transition (involving quantum tunneling or not) breaks it.

The vertices shown in the above Feynman diagrams all contain three Higgs bosons meeting at a single point, which would enable us to measure the Higgs self-coupling, a key parameter in understanding fundamental physics.Alain Blondel and Patrick Janot / arXiv:1809.10041

Before we ever consider a collider at higher energies, building a precisely-tuned collider capable of creating all the known particles in abundance is a no-brainer. There have already been considerable resources invested in a linear collider for electrons and positrons, like the proposed CLIC and ILC, but similar technologies would also apply to a large circular tunnel with electrons and positrons accelerating and colliding inside.

It's a way to push the frontiers of physics to uncharted territory using technology that already exists. No new inventions are necessary, but the unique benefit of a future circular lepton collider is that it could be upgraded.

In the early 2000s, we replaced LEP with a proton-proton collider: the LHC. We could do that for this future collider as well: switching to colliding protons once the electron-positron data is collected. If there's any hint of new, beyond-the-Standard-Model physics at the energies a future collider achieves — addressing problems from baryogenesis to the hierarchy problem to the puzzle of dark matter — the proton-proton collider will actually make these new particles.

When two protons collide, it isn't just the quarks making them up that can collide, but the sea quarks, gluons, and beyond that, field interactions. All can provide insights into the spin of the individual components, and allow us to create potentially new particles if high enough energies and luminosities are reached.CERN / CMS Collaboration

To understand the Higgs self-coupling even better, a ~100 TeV hadron-hadron collider will be the ideal tool, producing over 100 times the number of Higgs bosons than the LHC will ever create. A proton-proton version of a Future Circular Collider can use the same tunnel as the lepton-lepton version and will employ next-generation technology for its electromagnets, reaching field strengths of 16 T, which is double the LHC's magnet strength. (These magnets will be a formidable technological challenge for the next two decades.) It's an ambitious plan that allows us to plan for at least two colliders in the same tunnel.

A future hadron-hadron collider at a Future Circular Collider will also measure rare decays of the Higgs boson, like decays to two muons or a Z-boson and a photon, as well as the Higgs-top quark coupling to ~1% precision. If there are new bosons, fundamental forces, or signs of baryogenesis at the electroweak scale or even a factor of ~1000 higher, the proposed proton-proton incarnation of the Future Circular Collider will find the evidence. Neither an electron-positron collider or the LHC can do this.

All told, the hadron-hadron version of the FCC will collect 10 times as much data as the LHC will ever collect (and 500 times as much as we have today), while reaching energies that are seven times higher than the LHC's maximum. It's an incredibly ambitious proposal, but one that's within our reach by the 2030s, if we plan for it today.

When you collide electrons at high energies with hadrons (such as protons) moving in the opposite direction at high energies, you can gain the ability to probe the internal structure of the hadrons as never before.Joachim Meyer; DESY / HERA

There's also a "phase III" that involves probing the frontiers of physics in an entirely different way: by colliding high-energy electrons, in one direction, with high-energy protons in the other. Protons are composite particles, made up of quarks and gluons on the inside, along with a sea of virtual particles. Electrons, via processes like deep-inelastic-scattering, are the best proverbial microscope for probing the internal structure of protons. If we want to understand the substructure of matter, electron-proton collisions are the way to go, and the FCC would push the frontier far past where previous experiments, like the HERA collider at DESY, have taken us.

Between indirect effects that an electron-positron collider might see, the direct new particles that might arise from proton-proton collisions, and the greater understanding of mesons and baryons that an electron-proton collider will bring, we have every reason to hope that some new physical signal might emerge.

What do we do next, then, if there is new physics there? What if there are new particles that are discovered at these higher energies? What next?

The V-shaped track in the center of the image is likely a muon decaying to an electron and two neutrinos. The high-energy track with a kink in it is evidence of a mid-air particle decay. By colliding positrons and electrons at a specific, tunable energy, muon-antimuon pairs could be produced at will, providing the necessary particles for a future muon collider.The Scottish Science & Technology Roadshow

We don't necessarily need to build an even bigger collider to study them better. If there is new physics up at a very high energy scale, we could probe it in depth with a potential "phase IV" for a Future Circular Collider: a muon-antimuon collider in the same tunnel. The muon is like an electron: it's a point particle. It has the same charge, except it's approximately 207 times heavier. This means some extremely good things:

it can reach much higher energies by achieving the same speeds,
it provides a clean, energy-tunable signature,
and unlike electrons, because of the much lower charge-to-mass ratio, its synchrotron radiation can be neglected.

It's a brilliant idea, but also a tremendous challenge. The drawback is singular but substantial: muons decay away with a mean lifetime of just 2.2 microseconds.

An earlier design plan (now defunct) for a full-scale muon-antimuon collider at Fermilab, the source of the world's second-most powerful particle accelerator.Fermilab

This isn't a dealbreaker, though. Muons (and antimuons) can be made very efficiently through two methods: one by colliding protons with a fixed target, producing charged pions which decay into muons and antimuons, and a second by colliding positrons at right around 44 GeV with electrons at rest, producing muon/antimuon pairs directly.

We can then use magnetic fields to bend these muons and antimuons into a circle, accelerate them, and collide them. If we get them going fast enough in a short enough timescale, the time dilation effects of Einstein's relativity will keep them alive long enough to collide and produce new particles. We could, in principle, reach energies of ~100 TeV with a clean signal in a muon collider this way: approximately 300 times as energetic as a future electron/positron collider.

There is certainly new physics beyond the Standard Model, but it might not show up until energies far, far greater than what a terrestrial collider could ever reach. It's also possible that new, beyond-the-Standard Model physics may exist at small masses or energies, but with couplings too small for a terrestrial collider to probe. Irrespective of which scenario is true, the only way we'll know is to look.Universe-review.ca

Prior to the discovery of the Higgs, we used the term "nightmare scenario" to describe what it would be like for the LHC to find the Standard Model Higgs and nothing else. Realistically, it is no nightmare to discover the Universe exactly as it is. There may not be any additional particles or anomalous, beyond-the-Standard-Model behavior to discover with any terrestrial collider we could possibly build, it's true. But there could also be plenty of new, unexpected discoveries at scales and precisions that the LHC will be incapable of accessing.

The only way to know the truth about our Universe is to ask it these questions. Figuring out what the laws of nature are and how particles behave is a step forward for human knowledge and the entire enterprise of science. The only true nightmare would be if we stopped exploring, and gave up before we ever looked at all.

The author thanks Panos Charitos, Frank Zimmermann, Alain Blondel, Patrick Janot, Heather Gray, Markus Klute, and Matthew McCullough of CERN for incredibly useful, informative discussions and emails concerning the potential for a future, post-LHC collider.

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Why physics needs and deserves a post-LHC collider

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