The Standard Model of Particle Physics -- where we are at

[Trakk]

The Standard Model has been very successful so far, but there are lots of things that don't fit.

• Empirical
  • Neutrino masses
  • Matter-antimatter asymmetry
  • Gravity
  • Dark matter
  • Dark energy
  • Inflation
• Theoretical
  • Sources of neutrino masses -- seesaw mechanism?
  • Strong CP violation -- why isn't it observed?
  • Higgs-particle instability at around GUT energies
  • Elementary-fermion unification at GUT energies
  • Gauge unification at GUT energies

GUT = Grand Unified Theory

There is a lot of work being done to resolve these problems. Here is some:

• Neutrino-oscillation experiments, searches for neutrinoless beta decay
• Nucleon electric-dipole moments
• Proton decay and bound-neutron decay
• Dark-matter direct detection
• Detection of dark-matter annihilation radiation
• Continued search for supersymmetric and other BSM particles at the Large Hadron Collider
• Cosmic Microwave Background: search for evidence of inflation-generated gravitational waves
• Post-Newtonian gravitational effects, the best-known tests of general relativity and similar theories
• ...

Some of these experiments could well give us some interesting results in coming years.

 

[RC1970]

GUT = GOD

May the force be with you. However, be sure you choose the correct force.

 

[Michael]

The Standard Model has been very successful so far, but there are lots of things that don't fit.

• Empirical
  • Neutrino masses
  • Matter-antimatter asymmetry
  • Gravity
  • Dark matter
  • Dark energy
  • Inflation

I would argue that all but the first thing on your list (neutrino masses) are utterly and completely irrelevant in terms of the standard particle physics model. Nothing requires that gravity be a quantum effect, so there's really no requirement that gravity is related to a carrier particle to begin with, and the rest of your list relates to one specific cosmology hypothesis, not particle physics. FYI, there are cosmology models that do not require such things in the first place and that are perfectly congruent with the stand particle physics model.

While it's "possible" that a non standard particle physics model has merit, there's certainly no requirement that the standard particle physics model is incorrect. Perhaps Lambda-CDM has never had physical merit in the first place, and the standard particle physics model is simply correct and complete.

 

[Trakk]

I included them as problems because they show that there is a lot that's observed that's outside the Standard Model. One can argue about whether the SM was *intended* to include them, but that's another story.

But I'll go into detail about the SM, using Wikipedia as a source. First, its low-energy spectrum of particles. I'll list them by these quantum numbers (spin, electric charge, QCD multiplet number). Spin is quantum-mechanical angular momentum in units of hbar (Planck's constant / 2*pi, often set to 1 in theoretical work). Electric charge is in units of the elementary charge. The QCD multiplet number is more complicated. Here are its values:

• 1 -- colorless (no QCD interaction)
• 3 -- 3 color states, "red", "green", "blue"
• 3* -- 3 anticolor states, "antired = cyan", "antigreen = magenta", "antiblue = yellow"
• 8 -- 8 color-anticolor states: the 3*3 = 9 possible with the colorless mixture of them subtracted out

(Quantum numbers), (antiparticle QN's if different), name(s), mass(es) in GeV/c^2 (c often set to 1 in theoretical work also)
(0, 0, 1) - Higgs particle - 126
(1/2, 2/3, 3), (1/2, -2/3, 3*) - Up, charm, top quarks - ~0.0023, ~1.275, 173.07
(1/2, -1/3, 3), (1/2, 1/3, 3*) - Down, strange, bottom quarks - ~0.0048, ~0.095, ~4.18
(1/2, 0, 1), (1/2, 0, 1) - Electron, muon, tau neutrinos - around 10^(-11) - 10^(-10) (observed; SM: 0)
(1/2, -1, 1), (1/2, 1, 1) - Electron, muon, tau charged leptons - 0.000511, 0.1057, 1.777
(1, 0, 1) - Photon - 0 - carries electromagnetic force
(1, 0, 8) - Gluon - 0 - carries QCD force
(1, 1, 1), (1, -1, 1) - W - 80.4
(1, 0, 1) - Z - 91.2

A complicated mess? Certainly. But if you see some patterns in the quantum numbers, then you've got a good start in particle physics. The masses are another story, and the only nonzero ones that have successfully been predicted from some theory are the W and Z ones. Those are the result of "electroweak symmetry breaking" (EWSB), and that is from the Higgs particle's self-interaction giving it a nonzero ground-state field value. Almost all the other SM particles interact with the Higgs particle, and that particle sort of always being present makes an always-present interaction, and that interaction produces all their masses.

Finally, some non-SM-particle quantum numbers:
(2, 0, 1) - Graviton - 0 - carries gravitational force
(0, 0, 1) - Inflaton - ~10^(16)? - caused cosmic inflation (mass is rough estimate from its energy scale)
(?, 0, 1) - Dark-matter particle - ? - (spin, mass unknown)

 

[Trakk]

Without electroweak symmetry breaking, the Standard Model looks rather different. First the "gauge symmetry", described by "symmetry groups". With EWSB, it has two parts, SU(3) for QCD and U(1) for electromagnetism, combined as SU(3)*U(1). Without EWSB, it has three parts, SU(3) for QCD again, SU(2) (QCD-like but shrunken and resembling quantum-mechanical angular momentum) for "weak isospin", and U(1) for "weak hypercharge": SU(3)*SU(2)*U(1). Furthermore, the elementary fermions break into "left-handed" and "right-handed" parts, each of which is a separate multiplet. The Higgs particle bridges them, giving the combinations their EWSB low-energy masses.

Quantum numbers: (spin, handedness if present: L/R, weak hypercharge, weak-isospin multiplet number, QCD multiplet number). No masses here because everything but the Higgs particle is massless without EWSB.

(0, 1/2, 2, 1), (0, -1/2, 2, 1) - Higgs particle (4 modes, 3 of them get "eaten" by the W and the Z in EWSB to make them massive)
(1/2, L, 1/6, 2, 3), (1/2, R, -1/6, 2, 3*) - Left-handed quark (both up-like and down-like)
(1/2, L, -2/3, 1, 3*), (1/2, R, 2/3, 1, 3) - Right-handed up-like quark
(1/2, L, 2/3, 1, 3*), (1/2, R, -1/3, 1, 3) - Right-handed down-like quark
(1/2, L, -1/2, 2, 1), (1/2, R, 1/2, 2, 1) - Left-handed lepton (both neutrino and electron-like)
(1/2, L, 0, 1, 1), (1/2, R, 0, 1, 1) - Right-handed neutrino (for neutrino masses)
(1/2, L, 1, 1, 1), (1/2, R, -1, 1, 1) - Right-handed electron-like lepton
(1, 0, 1, 8) - Gluon: SU(3)
(1, 0, 3, 1) - W particle: SU(2) - charged and neutral W's
(1, 0, 1, 1) - B particle: U(1) - not the B meson
In EWSB, the neutral W and the B mix to make the photon and the Z.

A big mess, you might be thinking. Indeed it is. But one can find patterns in it, like
(weak hypercharge) = (integer) + (1/2 if weak-isospin multiplicity is even, 0 otherwise) + (-1/3 if QCD multiplicity is 3, +1/3 if it is 3*, 0 otherwise). It's patterns like that which suggest Grand Unified Theories.

 

[Trakk]

Now for supersymmetry or SUSY, a much-studied extension of space-time symmetry. It makes every elementary particle have a related one with spin differing by 1/2. All their other quantum numbers are the same. Unbroken SUSY makes partners have the same mass, but we don't observe unbroken SUSY. Therefore, SUSY must be a broken symmetry, with particles' superpartners having different masses.

In the Minimal Supersymmetric Standard Model (MSSM), we get lots of new particles. Here is its EWSB low-energy limit:
(0, 0, 1) - additional neutral Higgs particle (2 of them)
(0, 1, 1), (0, -1, 1) - charged Higgs particle
(1/2, 0, 1) - neutralinos (4 of them) - mixture of photino, zino, and neutral higgsinos
(1/2, 1, 1), (1/2, -1, 1) - charginos (2 of them) - mixture of wino and charged higgsino
(1/2, 0, 8) - gluino
The elementary fermions have SUSY partners called sfermions ("scalar fermions"), including squarks and sleptons, in turn including the likes of stops and selectrons and sneutrinos. They have the same quantum numbers, but with spin 0 instead of 1/2.

Without EWSB, it has
(0 and 1/2, L, 1/2, 2, 1), (0 and 1/2, R, -1/2, 2, 1) - "up Higgs"
(0 and 1/2, L, -1/2, 2, 1), (0 and 1/2, R, 1/2, 2, 1) - "down Higgs"
The elementary fermions have spin 1/2 replaced by spin "0 and 1/2".
The gauge particles (gluon, W, B) have spin 1 replaced by spin "1 and 1/2".

Among non-SM particles, the graviton gets a SUSY partner, the gravitino:
(3/2, 0, 1)
(with EWSB) combined with the graviton as
(2 and 3/2, 0, 1, 1)
(without EWSB)
Supersymmetric gravity is often called supergravity (SUGRA), though it is not superstrong gravity. I'm not going into detail about the more elaborate SUSY and SUGRA scenarios, other than to note that they typically involve particles with several different spins related to each other.

Seems like more complexity rather than simplicity. I'm not giving mass estimates because they jump all over the place. But SUSY has the nice feature that it may one day explain why there are spin-1/2 particles, because otherwise, there is no physical motivation for them the way there is for the graviton and the gauge particles.

 

[Trakk]

Now for Grand Unified Theories.

The simplest that includes the Standard model is the Georgi-Glashow SU(5) theory. All the gauge particles go into one multiplet, and the elementary fermions three multiplets per generation:
Right-handed neutrinos
Left-handed leptons and right-handed down-like quarks
Left-handed quarks, right-handed up-like quarks, and right-handed electron-like leptons

The SU(5) theory involves extra gauge and Higgs particles, and these particles can cause protons and neutrons to decay. Observing proton decay can thus test GUT's. It's extremely slow, with a half-life over about 10^(32) years at last report, and that number is getting to what some GUT's predict.

It also predicts that the bottom quark and the tau lepton should have the same mass at GUT energies.

The next step up is the SO(10) theory, and it goes even further. All the Higgs particles go into one multiplet, and all the elementary fermions go into one multiplet per generation. That is a bit too successful in unification, because it predicts that each generation of elementary fermion ought to have the same mass, and that cross-generation decays should not happen.

Then we get theories like E6 and E8. The latter one can unite *all* the Standard-Model particles into one multiplet. With E8, we get into string-theory territory, with the prospect of unifying with gravity.

So we get (string theory) -> E8 -> E6 -> SO(10) -> SU(5) -> (MS)SM -> low-energy SM
by a long series of symmetry breakings.

 

[Michael]

SUSY theory seems to be in big trouble after the first round of LHC results. I"d be shocked if SUSY theory is redeemed by future LHC results at this point. Thus far not a single sparticle has been observed, and there's a plethora of them in SUSY theory. Furthermore, some of the "oddball" decay options of standard theory seem to be right on the money in terms of what it predicts, and the electron roundness tests poked holes in many of the SUSY models.

 

[Trakk]

The "roundness" tests are electric-dipole tests -- how much a particle's charge distribution departs from sphericity.

As to the LHC and SUSY, the LHC's operations divide SUSY particles into two main categories: with and without QCD interactions. That is because QCD interactions are relatively strong even at LHC energies. The LHC is a proton-proton collider, and at its energies, it is effectively a quark and gluon collider. Quarks are electronlike particles, interacting much like electrons but with QCD interactions also. Gluons are photonlike particles, but with QCD interactions only. So the LHC can most easily produce anything that one can produce with QCD interactions.

So among SUSY particles, the QCD-interacting ones are squarks and gluinos, SUSY partners of the quarks and gluinos. So they should be the easiest for the LHC to make, their masses aside. The others will be much more difficult. There is the further problem of background -- other processes that can make events that impersonate these particles' decays. Squarks and gluinos should be produced at great enough rates to make them easy to see above their backgrounds, but the others will be much worse, and will be much like the Higgs particle. Consider what was necessary to observe the Higgs particle's decays -- several months of running before its decays started being prominent enough for the particle's existence to be unambiguously evident.

 

[Michael]

By the way, I'm *highly* impressed with your knowledge of particle physics. That particular field is exceptionally difficult to understand IMO. I certainly still struggle with many of the most *basic* of ideas.

 

[nebulaJP]

The Standard Model has been very successful so far, but there are lots of things that don't fit.

Maybe that is because it's incorrect? What about this model?

 

[sfs]

Maybe that is because it's incorrect? What about this model?

That doesn't seem to be a physics model. It looks like somebody waving their hands about a subject they've read about. And then right near the beginning we have this statement: "[The Standard Model] sees all reality as particles that divide into light-like bosons that don’t collide and matter-like fermions that do." That's pretty bad -- bosons do collide in the Standard Model.

 

[Trakk]

(Me: ) The Standard Model has been very successful so far, but there are lots of things that don't fit.

Maybe that is because it's incorrect?

It would be a limiting case of another theory, just as Newtonian mechanics is.

Of the known non-SM phenomena, neutrino masses and mixing angles are successfully handled as extensions of the Standard Model.

The dark-matter particle has yet to be detected nongravitationally, but if it ever is, then its interactions may give several clues about its nature. For instance, there are several experiments that have been done for detecting DM particles colliding with detector-material nuclei, and more such experiments in the works. If several such experiments succeed, then we may get numbers for spin-independent interactions with protons and neutrons, and possibly also spin-dependent ones. That could provide some good tests of the hypothesis that DM particles are the Lightest Supersymmetric Particle, and even give some clues for SUSY parameter values.

The others are more difficult, since they are not likely to give very much detail.

 

[Uncle Mikey]

I think Dark Matter fits in right about there...

 

[Uncle Mikey]

Inflation?

That's easy...

Matthew 13:33
"Another parable spake he unto them; The kingdom of heaven is like unto leaven, which a woman took, and hid in three measures of meal, till the whole was leavened"

The Expanding Universe...

 

[Uncle Mikey]

Dark Energy...

 

[Uncle Mikey]

Matter vs. Antimatter...

 

[Uncle Mikey]

Dark Matter...

 

[Uncle Mikey]

Four Forces of Nature...

 

[Uncle Mikey]

Planck Scale and Branes...