NOTES on Standard Model - Quarks, Leptons etc (1 Viewer)

[alcalder]

Hi All. Since I wrote this out for one of your number, I thought I might as well post it here and it could be useful to some trying to tackle this last little tack on dot point in Q2Q.

This is a new addition to the new syllabus and I have a feeling it was kind of tacked on there without giving it much thought.

• discuss the key features and components of the standard model of matter, including quarks and leptons

​

Looking at GetSmart Physics - which cuts the course down a heap to the essentials (if you don't have it, try and look at it and see if it is the sort of thing that might help you) it lists these as the key points to know for this dot point:

Main components of standard model of matter:

• The "Big Four" forces - nuclear strong, electromagnetic, nuclear weak and gravity (what are they)
• "mediating particles" called BOSONS: i.e. gluons, photons, W and Z bosons
• quarks: their six flavours - up, down, strange, charm, top and bottom (held together by strong nuclear force holders called gluons - hence the name, I suppose)
• leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino
• particles & anti-particles - same mass but opposite charge

Now, if you are like me, all those words mean nothing. I'd be then drawing a tree to see how all these relate.

e.g.

Particles of matter

HADRON ------ classified as
BARYONS = three quarks and MESONS = one quark & 1 anti-quark

BARYONS e.g. NUCLEONS - protons & neutrons
MESONS e.g. pion (u + anti d) & koan (u + anti s)


HADRONS are composed of QUARKS held together by BOSONS

QUARKS
Spin 1/2 & 6 flavours -
up (u) charge = + 2/3 thus anti up charge = -2/3
down (d) charge = - 1/3
charm (c) charge = + 2/3
strange (s) charge = - 1/3
top (t) charge = + 2/3
bottom (b) charge = - 1/3
(NB: interesting to note that there are obvious pairs here, one is +2/3 and the other -1/3)

And 3 colours each - red blue green

[THUS you can put three quarks together and not disobey Pauli Exclusion principle. You can have an UP spin one way, UP spin the other way and UP spin the same as another BUT another colour. (see page 507 Jacaranda)]


[PROTONS & NEUTRONS contain only up and down quarks.
SO, if a proton has a charge of +1, it must be 3 quarks (2 x +2/3 + 1 x -1/3) = 2u + 1d
A neutron has no charge. So it must be 3 quarks (2 x -1/3 + 1x +2/3) = 1u + 2d]

(NB: Know this, I have seen HSC questions on this sort of thing. There is a whole list of other types of Baryons and Mesons on page 506 of Jacaranda - the only interesting column is SPIN and QUARK CONTENT but don't feel you need to know all of them, just know proton and neutron.)

BARYONS = 3 quarks
ANTIBARYONS = 3 antiquarks

------------------------------
BOSONS - force carriers
(ie the particles that are responsible for forces)

Gluon - carries strong nuclear force (glue together quarks)
Photon - carries electromagnetic force
W & Z bosons - carry the weak nuclear force
Graviton - carries gravity force (not yet discovered although they use it in Star Trek ;-) )


------------------------------
LEPTONS - fundamental particles (3 particles & 3 neutrinos)
electron
electron-neutrino
muon
muon-neutrino
tau
tau-neutrino

Quarks and Leptons obey Pauli exclusion principle with respect to spin.

​

It seems the texts are still very airy fairy on the details, probably because the details involve very complex mathematics WAY beyond the course. But if this still makes no sense, try drawing a concept map.

Relating it to something more concrete -

Quarks are LEGO pieces.
The Bosons are the little nobbles on the LEGO holding it all together - in this case GLUONS.
Hadrons are the models made from LEGO. Put three bits together and you get BARYONS (Neutrons and Protons have 3 bits of LEGO). Put two together you get MESONS (pions and kaons have 2 bits of LEGO).

BUT Quarks only like to go together if they are completely different with respect to spin. If the spin is the same they must have a different colour.

LEPTONS are not LEGO at all. They are children's wooden building blocks. They don't have nobbles because they do not use the strong nuclear force to stay together.

HOPE THIS HELPS EVERYONE.

 

[willC]

For a very complete explanation (i mean, you have to skip over heaps cause its too basic) theres always particleadventure.org. Theres a wonderful poster there too...

 

[jlnWind]

Hey i really like your analogy, it makes so much sense now!
Im unsure about the colour bit, do you mean that the lego pieces have to
have different colour-charge (if their next to eachother) because they obey the paul exclusion principle?

And i thought i should mention the Standard Model does not include GRAVITY.
Although gravity is considered a fundamental force its not considered part of the Standard Model.
I got this from Cern-http://pdg.web.cern.ch/pdg/particleadventure/frameless/gravity.html

Gravity is weird. It is clearly one of the fundamental interactions, but the Standard Model cannot satisfactorily explain it. This is one of those major unanswered problems in physics today.

In addition, the gravity force carrier particle has not been found. Such a particle, however, is predicted to exist and may someday be found: the graviton.

Fortunately, the effects of gravity are extremely tiny in most particle physics situations compared to the other three interactions, so theory and experiment can be compared without including gravity in the calculations. Thus, the Standard Model works without explaining gravity.

 

[alcalder]

Indeed, a model that included gravity is the holy grail of physics and I believe String Theory is an attempt to do so.

And with the LEGO and colour. If you have two lego pieces that go together that have everything the same (particularly in terms of spin) then yes, they must have different "colour". So you can have a blue top quark spinning one way and a blue bottom quark spinning the other but would need either a red or green quark top quark to complete the triplet spinning in either direction.