Why are protons and neutrons the only common baryons?

Question

OK, so I'm just starting to study particle physics on my own, so excuse me if my questions are a little 'element'-ary. ;->

As far as I've been able to comprehend, neutrons and protons are triquark baryons consisting of ddu and uud quarks respectively. As long as color charge is preserved, why can't we have other up-down combinations (uuu, ddd.) Is there something significant with the baryon number of 1 that prevents these other combos?

Answer 1

and it's a good thing too.

other baryons would be anti-protons and anti-neutrons, among other things. nobody has satisfactorily explained yet why our universe is mostly matter and not a mix. although a mix would tend to go out in a blaze of glory.

Answer 2

They aren't the only baryons

Particle Symbol Makeup Rest massMeV/c2 Spin B S

Lifetime(seconds> Decay Modes

Proton p uud 938.3 1/2 +1 0 Stable ...

Neutron n ddu 939.6 1/2 +1 0 920 pe-νe

Lambda Λ0 uds 1115.6 1/2 +1 -1 2.6

x10-10 pπ-, nπ0

Sigma Σ+ uus 1189.4 1/2 +1 -1 0.8

x10-10 pπ0, nπ+

Sigma Σ0 uds 1192.5 1/2 +1 -1 6x10-20 Λ0γ

Sigma Σ- dds 1197.3 1/2 +1 -1 1.5

x10-10 nπ-

Delta Δ++ uuu 1232 3/2 +1 0 0.6

x10-23 pπ+

Delta Δ+ uud 1232 3/2 +1 0 0.6

x10-23 pπ0

Delta Δ0 udd 1232 3/2 +1 0 0.6

x10-23 nπ0

Delta Δ- ddd 1232 3/2 +1 0 0.6

x10-23 nπ-

Xi Cascade Ξ0 uss 1315 1/2 +1 -2 2.9

x10-10 Λ0π0

XiCascade Ξ- dss 1321 1/2 +1 -2 1.64

x10-10 Λ0π-

Omega Ω- sss 1672 3/2 +1 -3 0.82

x10-10 Ξ0π-, Λ0K-

Lambda Λ+c udc 2281 1/2 +1 0 2x10-13 ...

Answer 3

Here's something to think about: in particle physics a modification of Murphy's Law holds: "If something can decay, it will". A heavy particle will have to decay to a lighter particle or a collection of particles with lower total mass.

* Photons, gluons, and gravitons are stable because they have no mass and hence cannot decay.

* Electrons are stable because there is no lighter particle with charge, and charge must be conserved.

* Neutrinos are stable because there is no lighter particle with lepton number, although they can change into each other.

* The up quark is stable because it is the lightest particle with baryon number.

Protons and neutrons are the lightest baryons. All other baryons are heavier than this. A proton is uud, and a neutron is udd.

Neutrons are slightly heavier than protons, and as such they will decay when left alone. However, a neutron inside a stable nucleus is also stable, because the attractive forces between protons and neutrons lower the neutron's potential energy. This works only because the mass difference is very small. If it were much bigger, neutrons would decay anyways, and atoms could not exist.

There are heavier quarks that we can make baryons out of, although they have not been found anywhere except inside particle accelerators. The strange, charm, and bottom quarks. But any particle containing one of these will be heavier than the corresponding particle containing only up and down quarks, and can decay.

This isn't the whole story, though. There are four delta particles made up of only up and down quarks: Δ++ (uuu), Δ+ (uud), Δ0 (udd), and Δ-(ddd). Note that Δ+ is similar to the proton and Δ0 is similar to the neutron. However, all four of these particles are over twice as heavy as protons and neutrons, due to the three quarks having parallel spin (don't ask me how that works, I don't know).

So, the reason why protons and neutrons are the only common baryons is that all other baryons are unstable.