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 10^15 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 [strike]less[/strike] 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.
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.