Today's article should be considered as sort of a sequel to the last entry on the Higgs boson seach, and once again is inspired by the local media taking an interest in the possible discovery of a Higgs boson in the new year. (on a side note, I would like to remind the local media that a boson is a subatomic particle with specific spin properties, while a bosun is a member of the crew on a boat - there is a difference :) )

In the last few years there has been a lot of interest in searching for the Higgs boson, and a lot of money spent on experiments to aid in the hunt. But the question then remains - once it is found what is next for the particle physics community? What do we do with the expensive equipment when the primary goal is completed? As it turns out there are a lot of interesting theories to test and most of them can be tested in the same experiments.

Complicated Higgs: The Higgs boson that is being actively searched for is the simplest possible model - just a single particle that gives mass to all other particles. But there are theories and motivations for having multiple Higgs particles with each giving mass only to certain particles. And it is possible, perhaps even probable, that a second Higgs would be more massive than the first and both would have unique properties.

Dark Universe: On of the great mysteries of the last few decades has been the existence of dark energy and dark matter in the Universe. Based on the properties of background radiation in space, we know that 95% of the energy of the Universe is in forms that have not yet been discovered - and of that about two thirds is not even in the form of matter. Most candidates for explaining both the dark matter and the dark energy components involve massive particles that could be produced at particle accelerators like the LHC, and discovery of either would be a far bigger event in physics than the Higgs discovery.

Extra Dimensions: This one is a little more exotic, but very interesting from both a theoretical and experimental perspective. We have always known of three dimensions in our Universe - because we can observe them in everyday life. Then a century ago it was found that time behaved as a fourth dimension, albeit with different properties than the rest. Immediately there was speculation that the Universe could contain a fifth dimension, but it would have to be so small that even modern experiments wouldn't detect it. Then 13 years ago several groups discovered a loophole in the laws of physics that allowed for larger dimensions, on the scale of 1mm in size, and those are very easy to observe in particle accelerators if they exist. Soon after many theories of higher dimensions were developed which could potentially explain many of the observed properties of subatomic particles. At present, the limits on their size put them right at the limit of what the LHC can measure, but it is still possible that hints of their existence will be found in the next few years.

Supersymmetry: Another exotic theory, but with some motivations from the Standard Model and an interesting connection to the Higgs boson as well. The mass of the Higgs in theoretical models depends on the mass and coupling strength of every particle that it is connected to - which is every particle in the Universe! This results in a huge predicted mass that doesn't match up with the other properties of the Higgs. One very attractive solution is to require nature to have a symmetry property in which each particle has an identical twin with a slightly different spin to it, such that the effects on the Higgs mass cancel out. This is known as supersymmetry, and although it is a beautiful theory the simple fact is that none of these twins has ever been observed. (Supersymmetry can still exist, but it has to be broken enough to make the partners much heavier). And so another active search at the LHC and elsewhere is to find these supersymmetric partners to the Standard Model, and that could happen at any time since we don't know where they will be discovered.
Fourth Generation: The Standard Model of particle physics contains three generations, with each generation having the exact same group of particles and properties, but each generation being significantly heavier than the previous one. As yet no one can explain having three generations rather than two or four or a dozen. So another potential discovery at the LHC would be some or all of a fourth generation. There aren't really strong reasons to expect it, or to guess at where it would be found, but there are no theories or symmetries that can definitively exclude these particles.

And these are just a few of the more common examples of possible discoveries in particle physics in the coming years. There are many other lesser known theories that could also have an effect on the measurements and experiments. And so although the Higgs has been the primary goal for many years, it is far from the final piece of the puzzle. I predict that before the LHC is decommissioned it will have produced many revolutions in the world of particle physics.