Whats known to be wrong with Standard Model

[azaharak]

I know the Higgs has been causing some issues. Could we have a list of the problems/issues with the standard model, aside from the recent unverified neutrino issue.


For instance, does anyone know where the spin of the proton comes from?


Thanks

 

[phinds]

There is some contention that it is in some way wrong that there are a huge number of physical constants that have to be discovered through measurement and then plugged into the Standard Model. Elegance would seem to say that it is really awful to have 23 numbers that you have to basically "make up" as far as the theories go because the theories do not predict any of them. I'm not knowledgeable enough in physics to have a strong feel for just how awful this is or is not but it all sounds pretty baggy to me.

Then, of course, there's gravity, which is pretty non-trivial.

 

[kurros]

Fine tuning - there is no 'natural' reason for the Higgs mass to be at the electroweak scale (which is where is has to be to match electroweak precision measurements). It receives large radiative corrections from any new massive particles which might exist, all the way up to the Planck scale, and for it to remain at a low mass there have to be miraculous cancellations between these various corrections. This was why supersymmetry was invented; the required miraculous cancellations happen automatically once you add it in. This is just a theoretical problem though, Nature could just turn out to have fine tuning, it doesn't have to care that we don't like it. I expect it really is a problem though.

Muon anomalous magnetic moment - measurements currently indicate that this is about 3-sigma different from what the Standard Model predicts, although there have been arguments that we are just calculating it wrong. Who knows.

Dark matter - pretty much the big one
Dark energy - although you could shift the blame for this elsewhere

There are a few little inconsistancies here and there in some precision collider measurements but not significiant enough to say for sure that they are a problem with the Standard Model I think.

As for your comment about proton spin, I'm not sure what you are talking about. The proton spin comes from the sum of the spins of the quarks inside. Is there something weird about it that I don't know?

 

[tom.stoer]

The proton spin comes from the sum of the spins of the quarks inside. Is there something weird about it that I don't know?

There was the so-called "proton spin crisis" or "nucleon spin crisis" b/c we were not able to explain the total proton spin based on some simple QCD mechanism ("valence quark spin"). Meanwhile the measurement of so-called polarized nucleon structure functions is much more accurate; in addition we have better models (again based on QCD) which allow us to take other contributions (gluon spin, quark and gluon angular momentum) into account. Afaik based on these considerations the nucleon spin can be explained pretty well.

You may have a look at http://arxiv.org/abs/0812.3535/

 

[azaharak]

No I don't think that's correct. The way I understand it is that those contributions from gluons only make up a small difference of the discrepancy. This is still an ongoing research area, I know they have been running experiments on this at BNL for years.

http://www.isgtw.org/feature/case-missing-proton-spin

This is more recent than that paper.http://www.bnl.gov/bnlweb/pubaf/pr/PR_display.asp?prID=1232

and here too.

This isn't necessarily implying an incorrect theory yet, just incomplete or not understood now. Possibly a major problem.

 

[kurros]

Hmm, awesome, cheers.

 

[humanino]

I would definitely not count the spin proton "puzzle" as "something wrong" with the standard model. You can only count it as a fundamental issue if you consider that we have something to understand about non-perturbative gauge theory (say AdS/QCD for instance, or "gravity as the square of a gauge theory"). It is plausible that research in the field of non-perturbative gauge theory will eventually lead to a reformulation of quantum field theory altogether. But as of today, it is a long shot. Lattice QCD allows for calculations of different contributions to the proton spin (including as Tom mentioned angular momentum). It is a very active field, but people who work on this do not call it "fundamental issue with the standard model" as they consider say the fine tuning.

Moments of GPDs and transverse-momentum dependent PDFs from the lattice
Gravity as the Square of Gauge Theory

 

[tom.stoer]

I would definitely not count the spin proton "puzzle" as "something wrong" with the standard model. You can only count it as a fundamental issue if you consider that we have something to understand about non-perturbative gauge theory (say AdS/QCD for instance, or "gravity as the square of a gauge theory"). [/url]

Agreed.

Proton spin (and many other observables) are practical issues in non-perturbative QCD; you "only" have to calculate them, but this can be very hard in practice. Other issues are fundamental.

But even if (in the far future) we can resolve most of these issues e.g. using SUSY models we are still confronted with the situation of a unreasonably complex but phenomenologically very successful theory. So most of the fundamental issues are located at a deeper level than "the standard model". They are relevant for a huge class of models.

 

[Oldfart]

From the bottom basement of my physics knowledge, I await an enormous simplification of the standard model. All of its complications seem to be too complicated to be real, as though they are derived to suit experimental results that no one fully understands. I'm not knocking the science behind the standard model, just looking for something much simpler.

 

[tom.stoer]

From the bottom basement of my physics knowledge, I await an enormous simplification of the standard model. All of its complications seem to be too complicated to be real, as though they are derived to suit experimental results that no one fully understands. I'm not knocking the science behind the standard model, just looking for something much simpler.

The problem is the following. Assume you were able to identify a new and elegant structure. How would you "break" this elegance and create the awfull standard model? I mean, in the end the standard model (the its physics) must emerge b/c we see it in experiments.

Look at Supersymmetry. The theories look rather nice and highly symmetric. Unfortunately we do not observe SUSY in nature, therefore the conclusion is that SUSY has to be broken at low energies. In order to do that you have to introduce Higgs bosons (plus SUSY-partners) and all elegance is ruined. The same applies to highly symmetric GUTs like SU(5) or SO(10). In the beginning the structure is nice but as soon as you want them to agree with nature you have to introduce some symmetry breaking which spoils its elegance.

Regardless which structure you may find, there must be some obscure mechanism which creates all the low-energy mess.

 

[kaksmet]

The problem is the following. Assume you were able to identify a new and elegant structure. How would you "break" this elegance and create the awfull standard model? I mean, in the end the standard model (the its physics) must emerge b/c we see it in experiments.

Look at Supersymmetry. The theories look rather nice and highly symmetric. Unfortunately we do not observe SUSY in nature, therefore the conclusion is that SUSY has to be broken at low energies. In order to do that you have to introduce Higgs bosons (plus SUSY-partners) and all elegance is ruined. The same applies to highly symmetric GUTs like SU(5) or SO(10). In the beginning the structure is nice but as soon as you want them to agree with nature you have to introduce some symmetry breaking which spoils its elegance.

Regardless which structure you may find, there must be some obscure mechanism which creates all the low-energy mess.

Very nice explanation Tom!