What Will the LHC Find?

With the Large Hadron Collider almost ready to turn on, it’s time to prepare ourselves for what it might find. (The real experts, of course, have been preparing themselves for this for many years!) Chad Orzel was asked what we should expect from the LHC, and I thought it would be fun to give my own take. So here are my judgments for the likelihoods that we will discover various different things at the LHC — to be more precise, let’s say “the chance that, five years after the first physics data are taken, most particle physicists will agree that the LHC has discovered this particular thing.” (Percentages do not add up to 100%, as they are in no way exclusive; there’s nothing wrong with discovering both supersymmetry and the Higgs boson.) I’m pretty sure that I’ve never proposed a new theory that could be directly tested at the LHC, so I can be completely unbiased, as there’s no way that this experiment is winning any Nobels for me. On the other hand, honest particle phenomenologists might be aware of pro- or con- arguments for various of these scenarios that I’m not familiar with, so feel free to chime in in the comments. (Other predictions are easy enough to come by, but none with our trademark penchant for unrealistically precise quantification.)

  • The Higgs Boson: 95%. The Higgs is the only particle in the Standard Model of Particle Physics which hasn’t yet been detected, so it’s certainly a prime target for the LHC (if the Tevatron doesn’t sneak in and find it first). And it’s a boson, which improves CERN’s chances. There is almost a guarantee that the Higgs exists, or at least some sort of Higgs-like particle that plays that role; there is an electroweak symmetry, and it is broken by something, and that something should be associated with particle-like excitations. But there’s not really a guarantee that the LHC will find it. It should find it, at least in the simplest models; but the simplest models aren’t always right. If the LHC doesn’t find the Higgs in five years, it will place very strong constraints on model building, but I doubt that it will be too hard to come up with models that are still consistent. (The Superconducting Super Collider, on the other hand, almost certainly would have found the Higgs by now.)
  • Supersymmetry: 60%. Of all the proposals for physics beyond the Standard Model, supersymmetry is the most popular, and the most likely to show up at the LHC. But that doesn’t make it really likely. We’ve been theorizing about SUSY for so long that a lot of people tend to act like it’s already been discovered — but it hasn’t. On the contrary, the allowed parameter space has been considerably whittled down by a variety of experiments. String theory predicts SUSY, but from that point of view there’s no reason why it shouldn’t be hidden up at the Planck scale, which is 1015 times higher in energy than what the LHC will reach. On the other hand, SUSY can help explain why the Higgs scale is so much lower than the Planck scale — the hierarchy problem — if and only if it is broken at a low enough scale to be detectable at the LHC. But there are no guarantees, so I’m remaining cautious.
  • Large Extra Dimensions: 1%. The idea of extra dimensions of space was re-invigorated in the 1990’s by the discovery by Arkani-Hamed, Dimopolous and Dvali that hidden dimensions could be as large as a millimeter across, if the ordinary particles we know and love were confined to a three-dimensional brane. It’s a fantastic idea, with definite experimental consequences: for one thing, you could be making gravitons at the LHC, which would escape into the extra dimensions. But it’s a long shot; the models are already quite constrained, and seem to require a good amount of fine-tuning to hold together.
  • Warped Extra Dimensions: 10%. Soon after branes became popular, Randall and Sundrum put a crucial new spin on the idea: by letting the extra dimensions have a substantial spatial curvature, you could actually explain fine-tunings rather than simply converting them into different fine-tunings. This model has intriguing connections with string theory, and its own set of experimental predictions (one of the world’s experts is a co-blogger). I would not be terribly surprised if some version of the Randall-Sundrum proposal turned out to be relevant at the LHC.
  • Black Holes: 0.1%. One of the intriguing aspect of brane-world models is that gravity can become strong well below the Planck scale — even at LHC energies. Which means that if you collide particles together in just the right way, you could make a black hole! Sadly, “just the right way” seems to be asking for a lot — it seems unlikely that black holes will be produced, even if gravity does become strong. (And if you do produce them, they will quickly evaporate away.) Fortunately, the relevant models make plenty of other predictions; the black-hole business was always an amusing sidelight, never the best way to test any particular theory.
  • Stable Black Holes That Eat Up the Earth, Destroying All Living Organisms in the Process: 10-25%. So you’re saying there’s a chance?
  • Evidence for or against String Theory: 0.5%. Our current understanding of string theory doesn’t tell us which LHC-accessible models are or are not compatible with the theory; it may very well be true that they all are. But sometimes a surprising experimental result will put theorists on the right track, so who knows?
  • Dark Matter: 15%. A remarkable feature of dark matter is that you can relate the strength of its interactions to the abundance it has today — and to get the right abundance, the interaction strength should be right there at the electroweak scale, where the LHC will be looking. (At least, if the dark matter is thermally produced, and a dozen other caveats.) But even if it’s there, it might not be easy to find — by construction, the dark matter is electrically neutral and doesn’t interact very much. So we have a chance, but it will be difficult to say for sure that we’ve discovered dark matter at the LHC even if the accelerator produces it.
  • Dark Energy: 0.1%. In contrast to dark matter, none of the energy scales characteristic of dark energy have anything to do with the LHC. There’s no reason to expect that we will learn anything about it. But again, maybe that’s because we haven’t hit upon the right model. It’s certainly possible that we will learn something about fundamental physics (e.g. supersymmetry or extra dimensions) that eventually leads to a breakthrough in our understanding of dark energy.
  • Strong Dynamics: 5%. Quantum Chromodynamics (QCD), the theory that explains the strong nuclear force as arising from strongly-interacting gluons coupled to quarks, is a crucial part of the Standard Model. An underappreciated feature of QCD is that the dynamics of quarks breaks the electroweak symmetry even without the Higgs boson — unfortunately, the numbers don’t work out for it to be the primary mechanism. However, an interesting alternative to the standard idea of a Higgs boson is to imagine a new “QCD-like” force that operates at even higher energies; one venerable idea along these lines is known as technicolor. For a long time now technicolor theories have been struggling to remain compatible with various experimental bounds; but theorists are clever, and they keep coming up with new ideas. I wouldn’t be completely surprised if a new strongly-interacting force was discovered at the LHC, although it’s a bit of a long shot.
  • New Massive Gauge Bosons: 2%. Another Standard-Model-like thing that could show up is a massive gauge boson from a spontaneously broken symmetry (or more than one), similar to the W and Z bosons of the weak interactions — you will hear about searches for Z-prime bosons or W-prime bosons. As far as I know they don’t solve any pressing problems, but lots of things in the universe don’t solve any problems, and nevertheless exist.
  • New Quarks or Leptons: 2%. The final Standard-Model-like thing we could find is a new “generation” of fermions (matter particles) — strongly-interacting quarks and non-strongly-interacting leptons. We don’t expect to, for the following indirect reason: each generation includes a neutrino, and neutrinos tend to be fairly light, and the existence of new light fermions is strongly constrained both by particle physics experiments and by Big Bang Nucleosynthesis. (If there are more light particles, the energy density of the universe is just a bit larger at any fixed temperature, and the universe therefore expands faster, and you therefore make a bit less more Helium. [Shouldn’t post late at night — see below.])
  • Preons: 1%. Historically, when we smash particles together at high energies, we find out that they were made of even smaller particles. The possibility that quarks and leptons are made of smaller constituents — preons — has certainly been taken very seriously, although none of the models has really caught on.
  • Mysterious Missing Energy: 15%. Particles that are long-lived, neutral, and weakly interacting — including dark matter particles and gravitons — can only be found indirectly at a collider like the LHC. You are smashing things together, and if the total energy of the resulting particles you detect is less than that of the initial particles you smashed, you know that some invisible particles must have escaped as “missing energy.” But what? If you have a specific theory, you can match carefully to the expected dependence on the initial energy, the angle of scattering, and so forth. But if you don’t … it will be hard to figure out what is going on.
  • Baryon-Number Violation: 0.2%. As Mark is explaining, there are more baryons than anti-baryons in the universe, and most of us think that the asymmetry must have been dynamically generated somehow. Therefore, some process must be able to change the number of baryons — but we’ve never observed such a process. And we probably won’t; in most models, violation of baryon number is far too rare to be visible at the LHC. But there is certainly no consensus about how baryogenesis happened, so we should keep an eye out.
  • Magnetic Monopoles, Strangelets, Q-Balls, Solitons: 1%. These aren’t really new particles, but composite objects of one form or another. Even if they exist in nature, the violent inner chambers of a particle collider might not be the best environment in which to make them.
  • Unparticles: 0.5%. One of the most recent hot topics in particle theory, unparticles are a suggestion from Howard Georgi that you could detect what looks like a fractional number of new particles, if there were a set of fields with perfect scale invariance (no masses or other parameters to judge their “size”). It’s undeniably clever, although the connection to reality still seems a bit tenuous. (Although.)
  • Antimatter: 100%. We detected antimatter long ago! In 1932, to be precise. It is no longer a mystery.
  • God: 10-20%. More likely than stable black holes, but still a long shot.
  • Something that Has Never Been Predicted: 50%. Here is my favorite thing to root for. Particle theorists have been coming up with new models for so long without being surprised by new experimental results, some of them have forgotten what it’s like. Nature has a way of throwing us curve balls — which is not only something to be anticipated, it’s something to be very grateful for. Surprises are how we learn things.
  • Something that Has Been Predicted, but Not Listed Above: 2%. I certainly haven’t included every idea ever proposed; if some model that not many people took seriously turns out to be right, someone will have some excellent gloating opportunities.
  • Absolutely Nothing: 3%. It’s always possible that we won’t find anything really new, not even the Higgs. If that turns out to be the case — well, suffice it to say that there will be great wailing and gnashing of teeth. It’s not a prospect I am especially worried about, but reality is what it is, and I’m sure we will find a way to move forward if that’s the case.

Now let’s turn the damn machine on, already!

Update: pretty pictures! Via Swans on Tea.

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202 Responses to What Will the LHC Find?

  1. Garbage says:

    three important points:

    A) “there

  2. BlackGriffen says:

    I think there will be great wailing and gnashing of teeth if the result is, “Exactly what you would expect from the Standard Model, and nothing more.” Regardless of whether or not that includes the Higgs. I mean, what could be worse news for a theorist than, “The current model checks out, no tweaks needed.”

  3. Building304 says:

    It seems to me that exclusion of the Higgs boson is far more interesting than it’s discovery at this point. With the constraints we have on likely Higgs masses, an exclusion will force us to rethink far more than a discovery will. Granted that it will mean countless bottles of champagne go unused in CERN, but I think we can take the hit if it means overthrowing a key element of the standard model.

  4. Ijon Tichy says:

    I’m putting my money on the “Something that Has Never Been Predicted” option. It will be very interesting indeed if the Higgs boson is not found by the LHC.

  5. mr.Mglobe says:

    I really wonder how you got number 10^(-25) .

  6. Eric Habegger says:

    It seems odd to me that most individuals accept the idea of entanglement but hardly any yet connect it to the idea of mass. It seems far more likely to me mass is just a form of very stable entanglement of all submicroscopic particles. And related to that was a previous comment that implied the only thing missing from the standard model. Far from it – think about that the fact that SM predicts if enough protons are observed one will be detected to decay – but that’s never occurred and they’ve had enough time to use that result to start drawing some conclusions about the standard model.

  7. Pieter Kok says:

    Eric Habegger, quantum opticians have made maximally (and less) entangled photons for decades, and there is no sign whatsoever that it has anything to do with mass. They don’t slow down, their frequency does not change, nothing.

  8. Van says:

    Eric H,
    The SM does NOT predict that the proton is unstable; this is a generic prediction of grand unified theories (GUTs).

  9. Mark says:

    Actually – the standard model does predict that the proton is unstable, through the baryon number anomaly. (But you certainly should never see this effect at colliders).

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  11. Seth Zenz says:

    If you convert your percentages into betting odds, I’d bet against you on most of them. Supersymmetry at 60% seems like a particularly good one to bet against, for two reasons:

    1. If TeV-scale SUSY really exists, it’s pretty surprising that we haven’t seen hints of it already.
    2. Even if there are lots of SUSY events, convincing the physics community that it’s definitely SUSY–and not something else with new, heavy particles–won’t be easy. Measuring spin is hard at the LHC experiments, and I seem to recall that it will take more than the five years you’ve allowed.

    On the bright side, I like your odds on “Something that Has Never Been Predicted.”

  12. JTankers says:

    Where do you get your odds for Stable Black Holes? Many physicists believe that all black holes are stable.

    Read Wikipedia article on Hawking Radiation. Hawking Radiation may not exist according to several peer reviewed papers.

    The operators of the Large Hadron collider have a vested interest in propaganda that implies assured LHC safety.

    Dr. Rossler theorizes that Hawking Radiation is not possible and if micro black holes are created they will destroy the planet in 50 months to 50 years.

    LHCFacts.org

  13. Sean, you seem to imply that finding nothing at all would be a very bad thing. I would have thought that ruling out a vast swathe of theories would be extremely interesting. Others have mentioned not finding the Higgs as an example of this, but there are certainly other theories which would also become unrealistically tightly constrained or squeezed out all together. Surely for theoretical physicists this is right up there with seeing something which has never been predicted.

  14. Lawrence B. Crowell says:

    My bet is for finding the Higgs, signatures of SUSY and the 50% “something not predicted.” As for black holes? — errmm, it would be interesting if there are some scattering amplitudes found which correspond to some dualality with black hole interiors via brane physics. I definately hope frankly that preons or related rishons are not found. That will really muck up the waters IMO. If missing energy is found it would appear that we are all in for some repeat of the history with neutrinos — maybe this will be dark matter.

    Lawrence B. Crowell

  15. Alex R says:

    Mark writes: Actually – the standard model does predict that the proton is unstable, through the baryon number anomaly.

    Is this correct?

    My recollection is that the standard model baryon number anomaly conserves baryon number modulo the number of fermion generations — presumably 3. A proton, with baryon number 1, could only “decay” to states with baryon number -2, 4, etc., all of which have higher energy than a single proton. So an isolated proton should be absolutely stable. (I suppose that a helium-3 or helium-4 nucleus could decay to a state with baryon number 0 or 1 respectively, and that we could call this “proton decay”, but I still wouldn’t say that this renders the proton unstable.)

  16. Count Iblis says:

    The deuteron can decay in a positron and an anti-muon neutrino. The deuteron lifetime is about 10^(218) years.

  17. Mike says:

    Go warped extra dimensions! 10% chance, you say!?!

    I’ve spent the total of about a couple days learning/thinking about this possibility, but that small amount of time in no way reflects my excitement for this possibility. The reason is, it seems to me, if warped extra dimensions are the explanation for hierarchy, this means that experimental verification of the fundamental theory is within our reach!

    “Within our reach” is a vague term — surely it would take an enormous effort to sort things out, and probably it would take more powerful accelerators than we have in production. But I’m young, and I can hope that over the next 40 years I have to work on these things, the main gist of it might be sorted out. That hope is far more tenuous if quantum gravity sits at 10^18 GeV.

  18. optimistic says:

    What probability would you give to the possibilty that whatever new physics comes out of the LHC will lead, however indirectly, to something uber-cool such as a solution to the world’s energy crisis, or warp drive?

  19. onymous says:

    As “Garbage” notes above, Randall-Sundrum is really a subset of strong dynamics. And while it’s great fun to play with, from a more theoretical point of view it’s not even clear that such theories exist; that is, they might just be effective theories with no good UV completion. The “landscape” of CFTs at strong ‘t Hooft coupling, with no supersymmetry, is pretty much completely mysterious. So I would put “strong dynamics” as significantly more likely than “warped extra dimensions.”

  20. andy.s says:

    Incidentally, why is there a high energy threshold for producing gravitons? If they’re massless, like photons, they should exist at all energies.

  21. Ellipsis says:

    Mark — re: your comment above: here’s a fun discussion — someday within the next 20-30 years, we’ll hopefully see sphaleron processes at colliders. As you know, one would “only” have to boost the energy of the LHC by a factor of 3-5 or so (although maybe “only” should be in two sets of quotes…), whereas to see the SM baryon number anomaly at a water cherenkov tank you would need to make a larger version of Super-K by 2 orders of magnitude or so. IMO, colliders (i.e. an energy-upgraded LHC) may get there first — and my opinion is the LHC probably will. Do you think a 100x SuperK (or alternative) will arrive before a 4x LHC?

  22. Jason Dick says:

    JTankers,

    Where do you get your odds for Stable Black Holes? Many physicists believe that all black holes are stable.

    Doubtful. But regardless, we’re not talking about stable black holes in general, just stable black holes produced in the LHC. That eventually is obscenely unlikely due to constraints from high-energy cosmic rays.

    The basic argument is thus: if high-energy collisions can produce black holes, most of them will be charged (since the colliding particles won’t often have the same charge). As charged particles, they will experience copious amounts of friction in traveling through matter, and will come to rest within the Earth. And this will have been going on since the formation of the Earth some 4.5 billion years ago, many with masses in excess of a million times what the LHC is capable of producing. Therefore, the mere fact that the Earth still exists is strong evidence that such events simply do not occur. And if you want to throw anthropics into the mix, just bear in mind that other planets and stars still exist, too. Therefore the possibility that the LHC will produce anything dangerous (such as a stable black hole) is vanishingly small.

  23. Haelfix says:

    Something that could be filed under ‘nothing’, would be hidden sector proposals. Where you can hide all sorts of funky interactions in the strongly coupled sectors and the effects, if any, would be very indirect. They could look like dramatic violations of theory, even though secretely they are not.

    I’d put the chances for ‘predicted but not listed above’ slightly higher as well. There are so many models!

  24. How is there a 100% chance of antimatter, but a 3% chance of nothing? I though antimatter was not nothing. 103% ftw?
    Unless you mean the nothing is actually on the “turn it on and have it blow up” scale instead of the “oh snap, stuff is coming out” scale.

  25. graviton383 says:

    Having worked on essentially every possibility on your list over the years, I also vote for `something we haven’t thought of yet’.