What regulates the stableness of lighter particle?

[ndung200790]

Please teach me this:
We know that the massive particles can decay to lighter particles.The lighter particles are more stable than massive particles.But I do not understand why some light particles(e,p...)are absolutely stable(durable) but not relatively stable.
Thank you very much in advance.

 

[tom.stoer]

I don't understand the difference you are making between absolutely stable and relatively stable.

The allowed reactions are dictated by symmetry and energy-momentum-conservation.

For an electron to decay there must be lighter particles which can carry the electric charge. As there is no lighter partcile with electric charge -1 there is no electron decay (neglecting Bremsstrahlung).

 

[ndung200790]

''Absolutely stable'' means the life-time is equal infinite

 

[ndung200790]

Why there is not a series of particles with masses lighter and lighter(and so on) and still satisfying the symmetries?

 

[tom.stoer]

''Absolutely stable'' means the life-time is equal infinite

And what is relatively stable?

 

[mfb]

Why there is not a series of particles with masses lighter and lighter(and so on) and still satisfying the symmetries?

This is just an observation. The Standard Model can describe the observed particles, but there is no good reason why they should exist in that way.
Alternatively, you could apply the anthropic principle: If everything would decay everywhere, it is unlikely that life as we know it would be possible.

The proton might be unstable - this is the prediction of a lot of theories beyond the SM. But its lifetime is really long in that case.

The lighter particles are more stable than massive particles.

This is some sort of trend if you think about long-living mesons, top-quarks and W/Z, but not a fundamental rule. There are many light particles which decay quicker than some heavier particles. The interactions which are involved in the decay are the most important thing.

 

[AdrianTheRock]

Charge conservation is the reason why certain light particles are stable. For example the electron is stable because it is the lightest electrically charged particle.

 

[ndung200790]

But how about the stable chargeless particles eg. neutrinos?

 

[ndung200790]

''Relatively stable'' means finite life-time.

 

[mfb]

All unstable particle have a finite life-time.
Maybe you refer to particles which fly long enough to measure their flight distance (and therefore decay time) in particle detectors. These are usually particles which decay via the weak (or sometimes electromagnetic) interaction only, except the top-quark.

Neutrinos carry their lepton flavour number, assuming that they are not their own antiparticles - while the individual type of it (electron, muon, tau) is not conserved due to neutrino mixing, they still have a quantum number which has to be conserved. There is no lighter particle to decay into.