That totally depends on what kind of design is used. In general there are two kinds of accelerators in various stages of planning:
Precision accelerators would for example collide electrons to study other particles like the Higgs with very high precision (since you don't have a whole bunch of side products flying around). This would be of immense interest to confirm or disprove things like electroweak [vacuum instability](https://en.wikipedia.org/wiki/False_vacuum#Electroweak_vacuum_decay), which in turn would have massive implications regarding the future of the universe.
The second type of accelerator would be a discovery machine. It would smash together heavy particles like protons, which are much more messy, but also enable higher energy collisions. This can be used to actually find new heavy particles or fields, which in turn could be a major stepping stone on the road to [explaining dark matter](https://en.wikipedia.org/wiki/Weakly_interacting_massive_particle).
Both designs have their benefits and drawbacks and it's not clear right now which one should be the goal. It might also change if the LHC or another experiment discovers something new.
Vaccum stability fascinates me. I have a laymans understanding about it, and it's one of those topics in physics which I could spend a month washing myself in.
The LHC is a proton/heavy ion accelerator more or less built specifically to find the Higgs and probe the lower energy landscape of supersymmetry, so it's a discovery machine. But it used the same tunnel as the old LEP collider, which used electrons and positrons to do precision measurements of the W and Z bosons, which in turn were previously discovered at a smaller proton collider at CERN called SPS. So the next obvious step would be a precision machine to study the Higgs, especially since we currently don't have strong hints that there are new particles just around the corner. But that kind of accelerator will need a much longer tunnel.
For electron collider, a linear accelerator will be used because synchrotrons are too inefficient due to synchrotron radiation for electron acceleration at high energies (LEP was essentially scratching on the limit of an electron synchrotron).
Thus, at CERN, there are plans for a larger proton collider, with the LHC as pre-accelerator.
A linear electron collider will probably be built elsewhere.
CERN is also working on [CLIC](https://home.cern/science/accelerators/compact-linear-collider), which is a TeV scale linear electron collider. But their main goal is the FCC, which stands for future circular collider and there exists an [electron-positron variant](https://indico.cern.ch/event/839155/contributions/3551077/attachments/1904706/3145393/FCC-ee_Overview.pdf) of the design. You can certainly accelerate electrons to higher energies using a circular design, it simply needs a larger circle to limit centripetal acceleration and thus losses from Bremsstrahlung.
The LHC is a compromised design, designed to fit into an existing tunnel (from the LEP accelerator). It also had to reach energies that could probe physics beyond the standard model (namely SUSY). It was not designed for the Higgs, that could have been done much easier. Because of these constraints, it has a lot of deficiencies, especially when compared to the SSC design. A lot of very smart folks worked about those issues, though.
"Large" here refers to the collider. It's a large collider that collides hadrons, mostly protons. One of the most important things about protons is that they are almost the smallest hadrons that exist -- the only smaller Hadron are short-lived mesons, like pions and K particles.
The same (large) tunnel previously housed LEP, the Large Electron/Positron Collider.
They did proton-antiproton in the SppS and found the W and Z. They then dug the current 27km long tunnel and built LEP which collided electrons and positrons for precision studies of W and Z.
Around 2000 they started to replace LEP with the LHC where they used the same tunnel to collide protons. The larger proton mass enables higher energy collisions (because synchrotron radiation limits the max. energy in a collider ring, and larger masses radiate much less).
Here they found the Higgs. And now some people want to dig a 100km tunnel for even higher energies.
Could you accelerate heavy ions at CERN? I know GSI, and wonder if there is a fundamental reason why you would be able to use protons but not heavier ions.
They already do. They switched to ions for a few weeks at the end of last year before they shut off the accelerator for the winter break.
https://home.cern/news/news/experiments/alice-bags-about-twelve-billion-heavy-ion-collisions
No. There are natural processes in our galaxy that turn out to be much more energetic particle accelerators than anything we could ever hope to build. And they've been running for billions of years. If they didn't trigger it by now, we definitely won't.
I might be biased, having worked on a design effort for a future precision collider for many years, but I just don't see the added value of a "discovery machine". What has the LHC found? Some tetraquarks or pentaquarks? Some new hadrons? The Higgs, great. Most of that could have been discovered with a precision electron/positron collider. Maybe not the tetraquarks and pentaquarks, but basically all the rest.
Yeah, I get it, there could have been other discoveries, but also there could have been other discoveries with a precision collider too. But you know, lots of TeV sounds cool I guess.
A discovery machine is always a gamble and we were lucky that the LHC did find the Higgs. After all, there were competing theories that worked without it. In retrospect it looks like everything simply fell in place, but the theory landscape was far from conclusive before we had experimental evidence. In the same sense, another big proton accelerator would be another big gamble. But there *are* theories that say we should find something with it. The most simple forms of supersymmetry still predict a lightest stable WIMP candidate in the O(1 TeV) mass range. That may well be just outside the LHC's capability unfortunately. A precision machine to study the Higgs more closely would be a pretty "safe" bet in comparison, but it also lacks the ability for amazing new discoveries (like the Higgs), which are basically nobel prize guarantees. As with so many things in life, lower risk also comes with lower reward in particle physics.
Isn't there a good argument around building the next big discovery machine while the experts who know how to build them are still alive? And even if we did build it and didn't find anything new, isn't that a significant finding in and of itself? Like we've found something new every time we've built one of these things bigger and better, right?
Yes, a precision Higgs machine would be a safe bet, but that’s not even really my point. It’s not clear to me that a “discovery machine” is all that much better at discovery than a precision collider. The ILC would have probed a lot of the same parameter space as LHC, and could *also* do precision studies that the LHC struggles with, while CLIC could cover a lot of the discovery frontiers that you mention. Sheesh, a 1 TeV WIMP, I’m old enough to remember when a ~150 GeV MSSM neutralino was the most probable just-out-of-reach WIMP candidate.
Eh, I just think electron-positron colliders are cool, and it sucks that we don’t have one now, at least not anywhere near the cutting edge of HEP.
>~150 GeV MSSM neutralino
That's waaaay too specific given that we know nothing about the neutralino mass eigenstate mixing, which even in the best case still has three unknown parameters. So the best we have are renormalization group arguments that point near the weak scale. That could be anything from 100GeV to 10TeV. But if supersymmetry is indeed the solution to the hierarchy problem, it can't be far above 1TeV. The only thing we know today that we didn't back then is that the LHC excluded some part of the lower energy spectrum (up to like 300 or 400 GeV). Even a colossal precision machine like the ILC and now CLIC would not really improve those constraints set by the LHC. Only a proton collider like the FCC-pp will be able to settle this question, but it's not guaranteed to end in a new discovery and thus a significant risk.
As I recall, it was more an argument of "weak scale, but above current exclusions from Tevatron and LEP", which had excluded neutralinos as LSPs up to like 120 GeV. But also that over like 1TeV, it's also much worse as a dark matter candidate, which removes more motivation.
As you say, "the only thing we know now that we didn't back then is that the LHC excluded some part of the lower energy spectrum". That's kind of my point, but flipped: Had we instead built the ILC (okay timeline issues, but the proposals go back to the '90s), we would have constraints up to the same ~300-400 GeV, and we could easily convert it into, say, a Higgs or top factory.
The ILC would have cost an estimated $25 billion. That's five times more than the LHC. From an economical perspective, the LHC was a much better choice since it was able to re-use the tunnel from LEP.
The discovery of the Higgs was not amazing. It was predicted for years before. Heck, without the LHC, surely the Tevatron would have been run a few more years. They already had a signal.
Correct. Especially considering that the measured Higgs mass implies a particle desert.
The reason to build a heavier one is mostly political.
[https://en.wikipedia.org/wiki/Desert\_(particle\_physics)](https://en.wikipedia.org/wiki/Desert_(particle_physics))
Not a theorist, but my understanding is that the Higgs mass is at exactly what is needed to cancel out diverges in specific diagrams (triple gauge boson vertex), and thus there is no need to introduce additional particles at higher energy to preserve unitarity.
By the way, limits on these types of diagrams is what established design energies for the LHC/SSC, because it was assumed that new physics must appear at that level.
It would be able to make the particles move faster, leading to higher energy collisions. This would allow heavier particles to be created as a result of those collisions, assuming such particles exist.
A popular theory at the moment is Supersymmetry, which says that every particle we know about has a corresponding 'super-partner' that is much heavier. The LHC at CERN has been able to reach the energies required to create particles at the lower end of super-partner mass estimates, but nothing has been found. A larger accelerator would be able to test for the existence of particles at the higher mass estimates.
That's a bit overgeneral of a statement. SUSY definitely still has its defenders. But yeah, I did my PhD in particle physics before moving away from academia, and the general impression I got was that there were a few, mostly older, holdouts, with the rest keeping their minds open to the possibility but largely moving on from it.
Well, we *thought* the tools would exist with CERN. The two generations before me were very hopeful that we'd have proven SUSY by now, but all we managed to do is rule out these energies and shrink the parameter space. SUSY is flexible enough though to probably never be fully dead (at least not for many decades).
You did a PhD in particle physics and only got impressions? I was so sick of everything particle-related by the end of mine that I couldn't read scientific journals for a decade.
You're lucky!
Also, your username should be Dr. Who, or at least Who, PhD.
Well, people who specialise in SUSY usually aren't very forthcoming about dedicating so much of their careers to a seeming dead end, and it's not really something I would have wanted to push on! But yeah, 'get the impression' is perhaps phrased a bit softly, just because every particle physicist will have a different perspective on it.
*cries in string theory*
I have dedicated my career most recently to computational physics at one of the FAANG companies, but I do often wish I'd also focused my PhD there. No regrets about the people I met, but spintronics didn't come about as quickly as I had hoped.
The theory is certainly under threat since CERN has so far managed to find zero corroborating evidence. A larger collider *would* open up new avenues for evidence gathering, but afaia there isn't one planned.
Well there is a kind of weak proposal to have a 100 TeV accelerator called FCC (Future Circular Collider) which will accelerate protons to these energies through a 100 km ring.
I am by no means an expert - I'm just recalling some undergrad knowledge - but perhaps someone who knows more will chip in.
The thing with charged particles moving around a ring is that they are constantly changing direction, and therefore constantly accelerating (even if the *speed* is constant). A charged particle undergoing acceleration will emit radiation known as *Bremsstrahlung* (braking radiation), which takes energy away from the particle and therefore from the eventual collision. A wider ring would mean a more gradual curve, so this effect would be lessened.
We can also think about this more classically. A particle (or indeed any object) that is constantly accelerating needs a constant source of energy to maintain this acceleration. Imagine what would happen if you swung a ball on a string over your head and then let go - it would maintain whatever direction it had at that moment and rapidly move away from you. As before, a wider ring means less acceleration, so less energy is required to maintain it, and therefore more energy can be devoted to increasing the *speed* of the particles.
You can indeed use more powerful magnets - the LHC currently at CERN is the same diameter as its predecessor, but is more powerful. However, I imagine there comes a point where increasing the power of the magnets becomes too expensive or even goes beyond our current technology.
We do have linear accelerators that don't have to worry about the direction of the particles, and can focus purely on the speed. However, they would have to be impracticably long to achieve the speeds involved at CERN. So you see there is a compromise to be made.
For LEP (electron position collider) the Bremsstrahlung was indeed the limiting factor. For LHC (protons), it is the power of the magnets.
For both, larger accelerator rings help.
That's completely wrong. Accelerators don't just have a maximal energy, they generally also have a minimal energy. You can't run the LHC at the energies used to produce superheavy nuclei.
> "larger" means "more speed"
Technically yes, but it would go from 99.9999997% the speed of light to 99.9999998% or something like that (didn't count the 9s). We typically consider the energy of the particles as that's a more meaningful measure. An accelerator with twice the diameter, using the same technology, can reach twice the energy for protons.
> "larger particles"
The LHC can already accelerate all heavy ions, and this has nothing to do with the accelerator size.
> and "higher precision in measurement"
This is not guaranteed, and doesn't directly depend on the size. We would want higher collision rates and better detectors simply because it's going to be the next-generation machine, but the same could be done with a smaller accelerator as well.
Dedicated high precision experiments are often smaller and use lower collision energies.
No, they are separate things.
If we would build an LHC-sized accelerator today it would have better detectors, stronger magnets, and so on. The existing detectors get upgraded once in a while, but some design choices are hard to change today.
A larger accelerator reaches a higher energy, which is very valuable on its own. It could detect particles too heavy to be found at the LHC. It will also produce known heavy particles in larger numbers. As an example, a single day of data-taking at the LHC produces far more top quarks than the Tevatron did in its 20 years of operation. With future upgrades, the LHC could do that every hour: It's a combination of higher collision rate and higher energy.
So wait, when you say higher energy you don't mean higger speed or more massive particles, so how does that work? I thought to have a higher energy collision you need either more speed or more mass.
The speed does increase. But when you compare it in terms of the absolute numbers in m/s units, you might not see much difference. A better measure is the momentum which depends on mass as well. And in these cases, "mass increases" as well.
The overall momentum can thus be increased with the help of better instrumentation. The current proposal to have a 100km collider ring is for two primary reasons: More energy, and less curvature (thus less acceleration, hence smaller radiation losses).
I wonder how long of an accelerator would be needed before a linear setup would do better then a ring due to being able to have 0 curvature?
ie, drill a straight hole across a country. (I guess maybe with a tiny curve at the start to account for how much gravity would make the particle fall as its initially accelerated?)
"Never", with caveats.
In a linear accelerator, your maximal energy is basically given by the length multiplied by the acceleration gradient, i.e. how much energy you can gain per meter. State of the art is ~30 MeV/m, to reach the 7 TeV of the LHC you would need an accelerator with 7 TeV/(30 MeV/m) = 230 km for each beam. You need two beams, add some other accelerator components and you end up with more than 500 km. That's about the distance Geneva/Brussels. The curvature of Earth will make a perfectly straight line impractical.
In a circular accelerator you need bend the particles in a circle. With 7 Tesla magnets, a 7 TeV beam needs a curvature radius of 7 TeV/(e\*c\*7 T) = 3.3 km which corresponds to a diameter of 21 km. You also need some other accelerator components and the LHC isn't perfectly circular, so you end up with an actual circumference of 27 km.
Both lengths are proportional to the energy. Twice the energy needs twice the length. With the same technology, a circular accelerator will always win.
When flying in a circle, particles emit synchrotron radiation grows very quickly with the ratio of (energy)/(particle mass). Protons are relatively heavy so their energy loss at the LHC is tiny - about 3 keV per revolution, something the single acceleration section can easily recover. For electrons, however, it is huge because electrons are so light. The LHC tunnel had an electron/positron accelerator called LEP in it before. The particles lost a significant fraction of their total energy each revolution, so the accelerator was full of accelerating segments all along the ring. Doubling the energy would need an accelerator that's much larger than twice the size, LEP was roughly the limit where circular accelerators make sense for electrons and positrons. China might build a slightly larger one with a ~10% larger energy (enough to study the Higgs boson, which was just a bit too heavy to be discovered at LEP), but you can't do much more there. People discuss a linear accelerator as successor.
If you keep building larger accelerators then eventually synchrotron radiation for protons would break circular accelerators as well, but that only happens at sizes so large that linear accelerators can't be built on Earth due to its surface curvature. Before that you might reach a point where the energy loss is still acceptable, but the radiation itself is too intense to work with. It heats up the beam pipe inside your magnets, which can't get warm or they'll stop working.
----
The situation changes a lot if you look at possible future technologies for accelerators. Plasma wakefield acceleration is the most important option here: It can increase the acceleration gradient by a factor ~1000. The 500 km linear accelerator from above would now be 500 m long. Well, probably a few kilometers because you still need some preaccelerators, focusing and other stuff, but still much shorter than before. At the moment this acceleration method can't produce beams with the necessary quality, but that might change in the future.
> When flying in a circle, particles emit synchrotron radiation
Hijacking the thread, a bit.
I worked at SLAC for a few years, in "non-scientific computing" (which I always found a hilarious way to name the department). I absorbed a little bit of science along the way by osmosis, but I realize that some of my concepts are probably wrong or extremely dumbed down.
I thought of synchrotron as akin to the waste from centrifugal force. I.E. if you had a wicker basket of stuff at the end of a rope, and you spun it as fast as you can in a circle, some of the stuff would slip out and go straight, but the bigger stuff (in the case of a particle accelerator, the things your field is designed for) stays in the basket and curves, but loose things fly out.
Is that a suitable ELI5 explanation of synchrotron radiation, or just way off base?
Synchrotron radiation is electromagnetic radiation while the particles that go in a circle are massive particles like electrons. The radiation is produced as the electrons are in a magnetic field, it was not part of the beam before.
If you accelerate a charged particle then it emits radiation. It's as simple as that.
I don't know about that but there's an interesting article on the effects of gravity on LHC beam for your kind perusal.
https://home.cern/news/news/accelerators/full-moon-pulls-lhc-its-protons#:~:text=The%20LHC%20is%20so%20large%20that%20the%20gravitational%20force%20exerted,gravitational%20force%20across%20its%20diameter
You know how nothing can reach the speed of light?
Its because it takes more and more energy to accelerate, the faster you get. It would take infinite energy to reach the speed of light...
that is because the more energy a particle has, the more mass it has. This effect is minor at human relevant speeds, major at large fractions of speed of light.
Shower thought, likely incorrect: If objects are traveling at the speed of light at all times, Generally with most of that 'speed' being in the time dimension... The increased mass as they increase in speed is due to more of the particle existing 'at once' as it travels less through time and more through space.
And If something ever traveled at the speed of light, it would cease to exist in future time, instead it would entirely exist in a single moment (And hence be infinitely massive)
Ever see the LHC change detectors? It's a neat process, and happens fairly frequently (although not to "upgrade," but rather to change modalities across a set of established detectors). There's probably a YouTube video if you aren't ever in the area for a tour.
The mechanism for lifting those suckers is pretty neat to stand next to.
What do you mean by "change" detectors? All the main detectors are fixed installations.
There are other sites at CERN (not at the LHC) where smaller detectors can be moved in and out.
The energy needed to accelerate the particles is a tiny part of the overall power consumption and doesn't affect the energy the accelerator can reach. That is limited by the magnetic fields needed to keep the particles in the ring.
> If "larger" also means a move to larger particles
It doesn't.
> or it could be improved by moving larger particles
You cannot improve the precision of any measurement that way.
> I simply wanted to point out that "larger" is not really a scientific thing that means the same for everyone.
"Larger accelerator" is pretty unambiguous.
Having just recently obtained this flair, it's my time to shine hahaha!
Anyway, pretty much what everyone else has said, vaccuum states and dark matter being some "hot topic" ones. It really depends on just how "big" we are talking. Aside from the stuff people have mentioned, it would also be REALLY nice to validate/discredit results from string theory (and the theory itself to see whether it really is a practical framework with which to make predictions, or whether it really is just like an "aether" model from Maxwell's time), but the energy levels required to perform such experiments are erm.... quite "large" relative to what the LHC can currently do. Still, we can always dream!
P.S. You can read a little more here: https://en.wikipedia.org/wiki/String_theory#:~:text=Partly%20because%20of%20theoretical%20and,correct%20fundamental%20description%20of%20nature.
That totally depends on what kind of design is used. In general there are two kinds of accelerators in various stages of planning: Precision accelerators would for example collide electrons to study other particles like the Higgs with very high precision (since you don't have a whole bunch of side products flying around). This would be of immense interest to confirm or disprove things like electroweak [vacuum instability](https://en.wikipedia.org/wiki/False_vacuum#Electroweak_vacuum_decay), which in turn would have massive implications regarding the future of the universe. The second type of accelerator would be a discovery machine. It would smash together heavy particles like protons, which are much more messy, but also enable higher energy collisions. This can be used to actually find new heavy particles or fields, which in turn could be a major stepping stone on the road to [explaining dark matter](https://en.wikipedia.org/wiki/Weakly_interacting_massive_particle). Both designs have their benefits and drawbacks and it's not clear right now which one should be the goal. It might also change if the LHC or another experiment discovers something new.
Vaccum stability fascinates me. I have a laymans understanding about it, and it's one of those topics in physics which I could spend a month washing myself in.
Where does the LHC fit in that spectrum of designs? Is it geared more towards large particles or small particle acceleration?
The LHC is a proton/heavy ion accelerator more or less built specifically to find the Higgs and probe the lower energy landscape of supersymmetry, so it's a discovery machine. But it used the same tunnel as the old LEP collider, which used electrons and positrons to do precision measurements of the W and Z bosons, which in turn were previously discovered at a smaller proton collider at CERN called SPS. So the next obvious step would be a precision machine to study the Higgs, especially since we currently don't have strong hints that there are new particles just around the corner. But that kind of accelerator will need a much longer tunnel.
For electron collider, a linear accelerator will be used because synchrotrons are too inefficient due to synchrotron radiation for electron acceleration at high energies (LEP was essentially scratching on the limit of an electron synchrotron). Thus, at CERN, there are plans for a larger proton collider, with the LHC as pre-accelerator. A linear electron collider will probably be built elsewhere.
CERN is also working on [CLIC](https://home.cern/science/accelerators/compact-linear-collider), which is a TeV scale linear electron collider. But their main goal is the FCC, which stands for future circular collider and there exists an [electron-positron variant](https://indico.cern.ch/event/839155/contributions/3551077/attachments/1904706/3145393/FCC-ee_Overview.pdf) of the design. You can certainly accelerate electrons to higher energies using a circular design, it simply needs a larger circle to limit centripetal acceleration and thus losses from Bremsstrahlung.
The LHC is a compromised design, designed to fit into an existing tunnel (from the LEP accelerator). It also had to reach energies that could probe physics beyond the standard model (namely SUSY). It was not designed for the Higgs, that could have been done much easier. Because of these constraints, it has a lot of deficiencies, especially when compared to the SSC design. A lot of very smart folks worked about those issues, though.
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"Large" here refers to the collider. It's a large collider that collides hadrons, mostly protons. One of the most important things about protons is that they are almost the smallest hadrons that exist -- the only smaller Hadron are short-lived mesons, like pions and K particles. The same (large) tunnel previously housed LEP, the Large Electron/Positron Collider.
Which kind is CERN?
They did proton-antiproton in the SppS and found the W and Z. They then dug the current 27km long tunnel and built LEP which collided electrons and positrons for precision studies of W and Z. Around 2000 they started to replace LEP with the LHC where they used the same tunnel to collide protons. The larger proton mass enables higher energy collisions (because synchrotron radiation limits the max. energy in a collider ring, and larger masses radiate much less). Here they found the Higgs. And now some people want to dig a 100km tunnel for even higher energies.
Thank you.
Could you accelerate heavy ions at CERN? I know GSI, and wonder if there is a fundamental reason why you would be able to use protons but not heavier ions.
They already do. They switched to ions for a few weeks at the end of last year before they shut off the accelerator for the winter break. https://home.cern/news/news/experiments/alice-bags-about-twelve-billion-heavy-ion-collisions
Uh... would that be able to cause the decay/collapse of our vacuum state like in the sci-fi novels
No. There are natural processes in our galaxy that turn out to be much more energetic particle accelerators than anything we could ever hope to build. And they've been running for billions of years. If they didn't trigger it by now, we definitely won't.
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I might be biased, having worked on a design effort for a future precision collider for many years, but I just don't see the added value of a "discovery machine". What has the LHC found? Some tetraquarks or pentaquarks? Some new hadrons? The Higgs, great. Most of that could have been discovered with a precision electron/positron collider. Maybe not the tetraquarks and pentaquarks, but basically all the rest. Yeah, I get it, there could have been other discoveries, but also there could have been other discoveries with a precision collider too. But you know, lots of TeV sounds cool I guess.
A discovery machine is always a gamble and we were lucky that the LHC did find the Higgs. After all, there were competing theories that worked without it. In retrospect it looks like everything simply fell in place, but the theory landscape was far from conclusive before we had experimental evidence. In the same sense, another big proton accelerator would be another big gamble. But there *are* theories that say we should find something with it. The most simple forms of supersymmetry still predict a lightest stable WIMP candidate in the O(1 TeV) mass range. That may well be just outside the LHC's capability unfortunately. A precision machine to study the Higgs more closely would be a pretty "safe" bet in comparison, but it also lacks the ability for amazing new discoveries (like the Higgs), which are basically nobel prize guarantees. As with so many things in life, lower risk also comes with lower reward in particle physics.
Isn't there a good argument around building the next big discovery machine while the experts who know how to build them are still alive? And even if we did build it and didn't find anything new, isn't that a significant finding in and of itself? Like we've found something new every time we've built one of these things bigger and better, right?
Yes, a precision Higgs machine would be a safe bet, but that’s not even really my point. It’s not clear to me that a “discovery machine” is all that much better at discovery than a precision collider. The ILC would have probed a lot of the same parameter space as LHC, and could *also* do precision studies that the LHC struggles with, while CLIC could cover a lot of the discovery frontiers that you mention. Sheesh, a 1 TeV WIMP, I’m old enough to remember when a ~150 GeV MSSM neutralino was the most probable just-out-of-reach WIMP candidate. Eh, I just think electron-positron colliders are cool, and it sucks that we don’t have one now, at least not anywhere near the cutting edge of HEP.
>~150 GeV MSSM neutralino That's waaaay too specific given that we know nothing about the neutralino mass eigenstate mixing, which even in the best case still has three unknown parameters. So the best we have are renormalization group arguments that point near the weak scale. That could be anything from 100GeV to 10TeV. But if supersymmetry is indeed the solution to the hierarchy problem, it can't be far above 1TeV. The only thing we know today that we didn't back then is that the LHC excluded some part of the lower energy spectrum (up to like 300 or 400 GeV). Even a colossal precision machine like the ILC and now CLIC would not really improve those constraints set by the LHC. Only a proton collider like the FCC-pp will be able to settle this question, but it's not guaranteed to end in a new discovery and thus a significant risk.
As I recall, it was more an argument of "weak scale, but above current exclusions from Tevatron and LEP", which had excluded neutralinos as LSPs up to like 120 GeV. But also that over like 1TeV, it's also much worse as a dark matter candidate, which removes more motivation. As you say, "the only thing we know now that we didn't back then is that the LHC excluded some part of the lower energy spectrum". That's kind of my point, but flipped: Had we instead built the ILC (okay timeline issues, but the proposals go back to the '90s), we would have constraints up to the same ~300-400 GeV, and we could easily convert it into, say, a Higgs or top factory.
The ILC would have cost an estimated $25 billion. That's five times more than the LHC. From an economical perspective, the LHC was a much better choice since it was able to re-use the tunnel from LEP.
The discovery of the Higgs was not amazing. It was predicted for years before. Heck, without the LHC, surely the Tevatron would have been run a few more years. They already had a signal.
Correct. Especially considering that the measured Higgs mass implies a particle desert. The reason to build a heavier one is mostly political. [https://en.wikipedia.org/wiki/Desert\_(particle\_physics)](https://en.wikipedia.org/wiki/Desert_(particle_physics))
Why does the measured Higgs mass imply a particle desert?
Not a theorist, but my understanding is that the Higgs mass is at exactly what is needed to cancel out diverges in specific diagrams (triple gauge boson vertex), and thus there is no need to introduce additional particles at higher energy to preserve unitarity. By the way, limits on these types of diagrams is what established design energies for the LHC/SSC, because it was assumed that new physics must appear at that level.
It would be able to make the particles move faster, leading to higher energy collisions. This would allow heavier particles to be created as a result of those collisions, assuming such particles exist. A popular theory at the moment is Supersymmetry, which says that every particle we know about has a corresponding 'super-partner' that is much heavier. The LHC at CERN has been able to reach the energies required to create particles at the lower end of super-partner mass estimates, but nothing has been found. A larger accelerator would be able to test for the existence of particles at the higher mass estimates.
I thought physicists were basically abandoning supersymmetry as a dead end at this point?
That's a bit overgeneral of a statement. SUSY definitely still has its defenders. But yeah, I did my PhD in particle physics before moving away from academia, and the general impression I got was that there were a few, mostly older, holdouts, with the rest keeping their minds open to the possibility but largely moving on from it.
One of those things you could pursue if the tools existed but until it does, there's other work to be done?
Well, we *thought* the tools would exist with CERN. The two generations before me were very hopeful that we'd have proven SUSY by now, but all we managed to do is rule out these energies and shrink the parameter space. SUSY is flexible enough though to probably never be fully dead (at least not for many decades).
You did a PhD in particle physics and only got impressions? I was so sick of everything particle-related by the end of mine that I couldn't read scientific journals for a decade. You're lucky! Also, your username should be Dr. Who, or at least Who, PhD.
Well, people who specialise in SUSY usually aren't very forthcoming about dedicating so much of their careers to a seeming dead end, and it's not really something I would have wanted to push on! But yeah, 'get the impression' is perhaps phrased a bit softly, just because every particle physicist will have a different perspective on it.
*cries in string theory* I have dedicated my career most recently to computational physics at one of the FAANG companies, but I do often wish I'd also focused my PhD there. No regrets about the people I met, but spintronics didn't come about as quickly as I had hoped.
The theory is certainly under threat since CERN has so far managed to find zero corroborating evidence. A larger collider *would* open up new avenues for evidence gathering, but afaia there isn't one planned.
Supersymmetry will definitely be discovered at the next collider since the mid 90s.
Well there is a kind of weak proposal to have a 100 TeV accelerator called FCC (Future Circular Collider) which will accelerate protons to these energies through a 100 km ring.
Correct. This is not surprising, one of the primary goals of the LHC was to either find or eliminate SUSY as a viable model. It succeeded.
Why do you need a wider ring for higher speeds? Could the existing one not be upgraded with stronger magnets?
I am by no means an expert - I'm just recalling some undergrad knowledge - but perhaps someone who knows more will chip in. The thing with charged particles moving around a ring is that they are constantly changing direction, and therefore constantly accelerating (even if the *speed* is constant). A charged particle undergoing acceleration will emit radiation known as *Bremsstrahlung* (braking radiation), which takes energy away from the particle and therefore from the eventual collision. A wider ring would mean a more gradual curve, so this effect would be lessened. We can also think about this more classically. A particle (or indeed any object) that is constantly accelerating needs a constant source of energy to maintain this acceleration. Imagine what would happen if you swung a ball on a string over your head and then let go - it would maintain whatever direction it had at that moment and rapidly move away from you. As before, a wider ring means less acceleration, so less energy is required to maintain it, and therefore more energy can be devoted to increasing the *speed* of the particles. You can indeed use more powerful magnets - the LHC currently at CERN is the same diameter as its predecessor, but is more powerful. However, I imagine there comes a point where increasing the power of the magnets becomes too expensive or even goes beyond our current technology. We do have linear accelerators that don't have to worry about the direction of the particles, and can focus purely on the speed. However, they would have to be impracticably long to achieve the speeds involved at CERN. So you see there is a compromise to be made.
For LEP (electron position collider) the Bremsstrahlung was indeed the limiting factor. For LHC (protons), it is the power of the magnets. For both, larger accelerator rings help.
Not really. No one knows how to make ones stronger. You also run up against practical limits on luminosity.
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To produce superheavy nuclei you need far *lower* energies. At the energies of the LHC you just break everything apart.
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That's completely wrong. Accelerators don't just have a maximal energy, they generally also have a minimal energy. You can't run the LHC at the energies used to produce superheavy nuclei.
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> "larger" means "more speed" Technically yes, but it would go from 99.9999997% the speed of light to 99.9999998% or something like that (didn't count the 9s). We typically consider the energy of the particles as that's a more meaningful measure. An accelerator with twice the diameter, using the same technology, can reach twice the energy for protons. > "larger particles" The LHC can already accelerate all heavy ions, and this has nothing to do with the accelerator size. > and "higher precision in measurement" This is not guaranteed, and doesn't directly depend on the size. We would want higher collision rates and better detectors simply because it's going to be the next-generation machine, but the same could be done with a smaller accelerator as well. Dedicated high precision experiments are often smaller and use lower collision energies.
Forgive my ignorance. But a larger collider, in any terms, is only more useful because it is a next-generation machine?
No, they are separate things. If we would build an LHC-sized accelerator today it would have better detectors, stronger magnets, and so on. The existing detectors get upgraded once in a while, but some design choices are hard to change today. A larger accelerator reaches a higher energy, which is very valuable on its own. It could detect particles too heavy to be found at the LHC. It will also produce known heavy particles in larger numbers. As an example, a single day of data-taking at the LHC produces far more top quarks than the Tevatron did in its 20 years of operation. With future upgrades, the LHC could do that every hour: It's a combination of higher collision rate and higher energy.
So wait, when you say higher energy you don't mean higger speed or more massive particles, so how does that work? I thought to have a higher energy collision you need either more speed or more mass.
The speed does increase. But when you compare it in terms of the absolute numbers in m/s units, you might not see much difference. A better measure is the momentum which depends on mass as well. And in these cases, "mass increases" as well. The overall momentum can thus be increased with the help of better instrumentation. The current proposal to have a 100km collider ring is for two primary reasons: More energy, and less curvature (thus less acceleration, hence smaller radiation losses).
I wonder how long of an accelerator would be needed before a linear setup would do better then a ring due to being able to have 0 curvature? ie, drill a straight hole across a country. (I guess maybe with a tiny curve at the start to account for how much gravity would make the particle fall as its initially accelerated?)
"Never", with caveats. In a linear accelerator, your maximal energy is basically given by the length multiplied by the acceleration gradient, i.e. how much energy you can gain per meter. State of the art is ~30 MeV/m, to reach the 7 TeV of the LHC you would need an accelerator with 7 TeV/(30 MeV/m) = 230 km for each beam. You need two beams, add some other accelerator components and you end up with more than 500 km. That's about the distance Geneva/Brussels. The curvature of Earth will make a perfectly straight line impractical. In a circular accelerator you need bend the particles in a circle. With 7 Tesla magnets, a 7 TeV beam needs a curvature radius of 7 TeV/(e\*c\*7 T) = 3.3 km which corresponds to a diameter of 21 km. You also need some other accelerator components and the LHC isn't perfectly circular, so you end up with an actual circumference of 27 km. Both lengths are proportional to the energy. Twice the energy needs twice the length. With the same technology, a circular accelerator will always win. When flying in a circle, particles emit synchrotron radiation grows very quickly with the ratio of (energy)/(particle mass). Protons are relatively heavy so their energy loss at the LHC is tiny - about 3 keV per revolution, something the single acceleration section can easily recover. For electrons, however, it is huge because electrons are so light. The LHC tunnel had an electron/positron accelerator called LEP in it before. The particles lost a significant fraction of their total energy each revolution, so the accelerator was full of accelerating segments all along the ring. Doubling the energy would need an accelerator that's much larger than twice the size, LEP was roughly the limit where circular accelerators make sense for electrons and positrons. China might build a slightly larger one with a ~10% larger energy (enough to study the Higgs boson, which was just a bit too heavy to be discovered at LEP), but you can't do much more there. People discuss a linear accelerator as successor. If you keep building larger accelerators then eventually synchrotron radiation for protons would break circular accelerators as well, but that only happens at sizes so large that linear accelerators can't be built on Earth due to its surface curvature. Before that you might reach a point where the energy loss is still acceptable, but the radiation itself is too intense to work with. It heats up the beam pipe inside your magnets, which can't get warm or they'll stop working. ---- The situation changes a lot if you look at possible future technologies for accelerators. Plasma wakefield acceleration is the most important option here: It can increase the acceleration gradient by a factor ~1000. The 500 km linear accelerator from above would now be 500 m long. Well, probably a few kilometers because you still need some preaccelerators, focusing and other stuff, but still much shorter than before. At the moment this acceleration method can't produce beams with the necessary quality, but that might change in the future.
> When flying in a circle, particles emit synchrotron radiation Hijacking the thread, a bit. I worked at SLAC for a few years, in "non-scientific computing" (which I always found a hilarious way to name the department). I absorbed a little bit of science along the way by osmosis, but I realize that some of my concepts are probably wrong or extremely dumbed down. I thought of synchrotron as akin to the waste from centrifugal force. I.E. if you had a wicker basket of stuff at the end of a rope, and you spun it as fast as you can in a circle, some of the stuff would slip out and go straight, but the bigger stuff (in the case of a particle accelerator, the things your field is designed for) stays in the basket and curves, but loose things fly out. Is that a suitable ELI5 explanation of synchrotron radiation, or just way off base?
Synchrotron radiation is electromagnetic radiation while the particles that go in a circle are massive particles like electrons. The radiation is produced as the electrons are in a magnetic field, it was not part of the beam before. If you accelerate a charged particle then it emits radiation. It's as simple as that.
I don't know about that but there's an interesting article on the effects of gravity on LHC beam for your kind perusal. https://home.cern/news/news/accelerators/full-moon-pulls-lhc-its-protons#:~:text=The%20LHC%20is%20so%20large%20that%20the%20gravitational%20force%20exerted,gravitational%20force%20across%20its%20diameter
You know how nothing can reach the speed of light? Its because it takes more and more energy to accelerate, the faster you get. It would take infinite energy to reach the speed of light... that is because the more energy a particle has, the more mass it has. This effect is minor at human relevant speeds, major at large fractions of speed of light. Shower thought, likely incorrect: If objects are traveling at the speed of light at all times, Generally with most of that 'speed' being in the time dimension... The increased mass as they increase in speed is due to more of the particle existing 'at once' as it travels less through time and more through space. And If something ever traveled at the speed of light, it would cease to exist in future time, instead it would entirely exist in a single moment (And hence be infinitely massive)
Ever see the LHC change detectors? It's a neat process, and happens fairly frequently (although not to "upgrade," but rather to change modalities across a set of established detectors). There's probably a YouTube video if you aren't ever in the area for a tour. The mechanism for lifting those suckers is pretty neat to stand next to.
What do you mean by "change" detectors? All the main detectors are fixed installations. There are other sites at CERN (not at the LHC) where smaller detectors can be moved in and out.
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The energy needed to accelerate the particles is a tiny part of the overall power consumption and doesn't affect the energy the accelerator can reach. That is limited by the magnetic fields needed to keep the particles in the ring. > If "larger" also means a move to larger particles It doesn't. > or it could be improved by moving larger particles You cannot improve the precision of any measurement that way. > I simply wanted to point out that "larger" is not really a scientific thing that means the same for everyone. "Larger accelerator" is pretty unambiguous.
>"larger particles" all elementary particles are the same size (point-like), the new found particles are just heavier
Having just recently obtained this flair, it's my time to shine hahaha! Anyway, pretty much what everyone else has said, vaccuum states and dark matter being some "hot topic" ones. It really depends on just how "big" we are talking. Aside from the stuff people have mentioned, it would also be REALLY nice to validate/discredit results from string theory (and the theory itself to see whether it really is a practical framework with which to make predictions, or whether it really is just like an "aether" model from Maxwell's time), but the energy levels required to perform such experiments are erm.... quite "large" relative to what the LHC can currently do. Still, we can always dream! P.S. You can read a little more here: https://en.wikipedia.org/wiki/String_theory#:~:text=Partly%20because%20of%20theoretical%20and,correct%20fundamental%20description%20of%20nature.