No, the Higgs boson was detected a few years ago, but was named and predicted in the 1960s. As far as I know, there's no more room in the Standard Model for additional particles.
If we find new particles and also come up with a theory that extends the SM to describe them, that new theory _becomes_ the standard model. It's a moving target that basically represents the best picture we have to account for everything we've seen so far. There are already a few things already known to be wrong with the standard model, for instance we don't have a clear mechanism for the neutrino masses and you could argue that it's missing a dark matter candidate (assuming DM can be described by particle physics to begin with). The exciting thing about this (potential) 750GeV resonance is that it would indicate new physics that isn't already known to be missing from our picture of the SM.
> If we find new particles and also come up with a theory that extends the SM to describe them, that new theory _becomes_ the standard model.
I don't know enough history to know if this was the case in the 1970s or 80s, but I strongly suspect it wouldn't play out this way now. The existing "standard model" has been so stable for so long (30 years or more) that it is now treated as a very specific thing. When theorists talk about possible expanded systems with additional particles, they tend to give them names: the "MSSM" is the "Minimal Supersymmetric Standard Model", for example, and if that proved to be an accurate description of our universe I think it would carry the MSSM label forever. I expect that even decades from now when we (hopefully) have a well-established broader model in place, the term "standard model" will still refer to the same low-energy sector of that theory that it does today.
The standard model is not a complete model of all elementary particles. It is a model of all known elementary particles.
There are reasonable outstanding predictions for:
- More Higgs bosons
- A Graviton
- Another 17 super symmetrical pairs to the existing 17 known particles.
- One or more dark matter particles
We don't know which (if any) of those will end up coming true, but they could all add new particles to the standard model. It is also entirely reasonable that new particles outside of the ones I listed could be added and incorporated into the model.
I didn't mean to imply the standard model was the thing predicting them. Only that scientists have predicted them and if they were to be discovered, the standard model would likely be extended to include them.
Suppose you have a series of 10 pairs of number coming from the universe, lets call them particles. After crunching the numbers you discover they fit perfectly in a linear function y=mx+b. Yep, two constants, one parameter and you can explain all those particles. Obviously, more particles could exist if they fit your curve, and you in fact discover, in time, 15 more of them!
Wow the model works, it predicted a bunch of awesome stuff. You and your peers name it "Standard Model" since it's so good. It's still just a name though.
Now suppose one more particle comes up and it doesn't fit the line at all. Sure you could "extend" the model by converting it to a Laplace polynomial with 24 more coefficient just to accomodate that new particle, but you can't honnestly say it's the same model anymore.
