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Comment by gibybo | Hacker News Reader

The standard model currently has 17 particles[1]. This number has grown over time, most recently when the Higgs boson was added a few years ago. The standard model makes no claim as to how many particles there can be, what their masses should be, what spin they should have, and so on. Those are unsolved questions outside of the scope of the standard model. Instead, it uses the particles we know about to make predictions about how they behave.

A new particle doesn't necessarily violate the standard model in the same way that discovering a new element wouldn't necessarily violate the periodic table. It could just expand the types of predictions that it is capable of making. It is also possible that a new particle could have properties that do violate constraints predicted by the standard model, but I don't know it well enough to know whether this particular particle does so.

[1] https://upload.wikimedia.org/wikipedia/commons/0/00/Standard...

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> the Higgs boson was added a few years ago

No, it was there from the beginning.

> The standard model makes no claim as to how many particles there can be

Nope, it does. That's kind of the whole point of it.

> what their masses should be

True, these are free parameters (except the massless bosons)

> what spin they should have

It does. Fundamental fermions have to be spin-1/2, gauge bosons spin-1 and the Higgs spin-0.

> A new particle doesn't necessarily violate the standard model in the same way that discovering a new element wouldn't necessarily violate the periodic table.

Depends if you mean a new fundamental particle or a new hadron. Discovering a new hadron is akin to adding a new element to the periodic table. Discovering a new fundamental particle means the Standard Model has to be extended or otherwise rewritten.

Okay, you're right.

I was thinking of the Standard Model as more of a moving target that incorporates everything we know about the fundamental particles and their forces. I thought the model started with a smaller number of particles [photon, electron, neutrino, muon neutrino, up quark, down quark, strange quark, gluon] and was expanded over time [higgs in 64, w/z bosons in 68, charm in 70, top/bottom in 73]. After further investigation it looks like no one actually called it the "Standard Model" until all of the 17 particles known today were included, so technically there have been no new particles added to the "Standard Model"

I think considering the Standard Model a moving target is still reasonable since if a new fundamental particle were to be discovered tomorrow, we would very likely just add it to the Standard Model and still call the new version the "Standard Model". However, I also realize now that treating it in this way is probably not appropriate given the context of mojoe's original question. I would edit my original comment to retract what I've said, but it seems I took too long to recognize my mistake and it will be forever immortalized in hacker news history :(

> I think considering the Standard Model a moving target is still reasonable

IMO no. The standard model isn't just a table full of particles, it's much more. One core part is is the description of how these particles behave, and the lagrangian. These could change much more drastically if a new particle turned up. This is because the particles in the standard model are intimately tied with underlying representation groups, which can't accomodate another particle. It's possible folks would still call it the "Standard Model", but I suspect such a drastic change would get a new name.

> If it turns out to be true, the chart linked below showing the 17 known elementary particles would be expanded to show an 18th.

no, actually the standard model would fundamentally break down. The math does not allow for such a heavy particle. No charts could save it.

That's not really the case. The standard model would need to be extended, sure (as the 750 GeV particle isn't predicted by the current SM). But there's no fundamental reason this can't be done; in fact in the weeks following this announcement by ATLAS/CMS, there were literally hundreds of theory models posted to the arXiv, claiming to extend the SM to account for the (potential) new resonance.

But the fact that there were hundreds of theory models posted means that there is no "standard model" for this new particle at the moment (if it is a real particle).

How would it fundamentally break down? Admittedly I don't know enough to know how a particle with these particular properties would affect it, I was just trying to make the point that a new particle doesn't necessarily break the standard model. I'll try to edit my post for more accuracy.

(It's been a while since I took this class, the following might well be wrong or grossly inaccurate)

The particles of the standard model are assumed to correspond to the irreducible representations (irrep) of the groups associated to the symmetries of the strong, weak and electromagnetic interactions. These groups are SU(3) (strong) and SU(2)×U(1) electroweak. The currently known particles correspond to the first few irreps of these groups, with the dimensionality of the irrep nicely matching the number of “similar” particles, see e.g. [1]. This means that, if the above assumptions hold, finding a single new particle requires there to be yet another irrep[2] which would imply that there are even more new particles corresponding to the other dimensions of this irrep.

[1] https://en.wikipedia.org/wiki/Standard_Model_%28mathematical...

[2] If this is even possible, I’m not quite sure, but SUSY does something similar I guess.

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.

Everything you list there is called physics beyond the Standard Model. It does not predict any of those.

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.

I'm greatly oversimplifying:

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.

So does the procedure usually "invent particles" where the theory does not include everything to fill the gap, and we look if there in fact is this kind of particle afterwards?

And now we found a particle but we didn't know there was a gap to fill?

I believe that is normally the case, but this wouldn't be the first time a particle was discovered before being predicted.

The muon was discovered before an accurate prediction for it was made. There were some known gaps that people tried to fill with particles related to muons, but no one quite predicted that there was a particle identical to an electron but with a larger mass.

The tau neutrino was discovered without any prior reason to believe it existed. We knew of neutrinos and they seemed to fill the only related gaps we had. It wasn't until the tau neutrino was experimentally detected that we even realized there was a gap for it.

> The tau neutrino was discovered without any prior reason to believe it existed. We knew of neutrinos and they seemed to fill the only related gaps we had. It wasn't until the tau neutrino was experimentally detected that we even realized there was a gap for it.

The tau neutrino was discovered in 2000. The discovery of the tau lepton 25 years prior heavily implied the existence of an associated neutrino.

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