The Large Hadron Collider (LHC), the world's largest atom smasher, opens for business today. Basically, it crashes subatomic particles into each other at very high speeds, in an efford to pack enough energy into a very small space to allow subatomic phenomena that are only possible with large amounts of energy to unfold.
All known large subatomic particles are inherently unstable and hence not detected with any frequency in nature. It takes massive amounts of energy to bring them into being, and then we observe them indirectly, by gathering information about the debris left over which is observed when they decay into smaller particles. The rules of quantum mechanics, further, place fundamental limitations on how much information can be gathered from any one atom smashing, so we have to repeat them many times to get a good composite picture of what is going on.
Also, many stable subatomic particles don't normally appear in isolation. They naturally bind themselves to other subatomic particles. So, the only way we can observe them is through their effect on particles in decay chains from big atom smash ups that disturb these stable particles that are generally found only bound to other particles.
It will be months before any interesting results are expected. Analysis of the results requires statistical probing of months of collisions to sort the chaff of already well understood collision events from the wheat of new physics. Quantum mechanics makes the outcome of atom smashing a random event, and it is the events that only happen infrequently in the highest energy collisions that are the most interesting.
For many years, the potential of almost all previous such experimental tools has been exhausted, leaving theorists to ponder what will happen in larger collisions largely unbounded by experimental evidence in the meantime.
Almost no professional physicists think that the particles described in the "standard model of particle physics" include all particles that could or do exist. Many physicists think that at least two particles remain to be discovered: the Higgs Boson and the graviton, and theory suggests that if the Higgs Boson exists, that the LHC should be capable of detecting its existance.
In the standard model, the Higgs boson basically is responsible for inertia, while the graviton would be a "spin 2" particle that mediates the gravitational force between particles in much the same way that the photon mediates the electromagnetic force between particles.
A large number of physicists think that there are, at least, a few more, which are necessary to explain the absence of a particle that is a good fit for "cold dark matter." Some physicists think that there are a whole host of undescribed particles, particularly proponents of a theory known as supersymmetry. There are also many physicists who expect to see evidence of dimensions beyond the usual three in space and one in time. Given speculative nature of the theory that predicts these kinds of new particles (or a number of other possible particles under other theories, such as "fourth generation" particles similar to the existing three generations of neutrinos, electrons and quarks, but with higher masses), no one knows for sure whether the LHC will detect them.
The LHC will almost certainly rule out reams of theoretical physics proposals, but disproven theories have a tendency to mutate to accomodate the limitations of new data, rather than disappearing entirely.
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I personally expect that LHC will produce fewer new particles than many people expect. I would not be at all surprised to see the LHC rule out the existence of a Higgs boson. I also would not be surprised to learn that the LHC has revealed no new fundamental particles beyond the standard model, a result which would disfavor supersymmetry, and no indications of additional dimensions, a result which would disfavor string theory.