# I What causes an elementary particle to "decay"?

#### mitrasoumya

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What causes an elementary particle to "decay" into other elementary particles? And where do these particles come from if they were not part of the original particle?

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#### .Scott

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I am not the best to answer this. But it's been four+ hours since you posted, so I'll take a shot at it.

The key thing you need for decay is something for the particle to decay into - while conserving mass, charge, spin, etc.
So, it's really a matter of what keeps a particle from decaying.

The electron is stable because there is no smaller particle with an electric charge.
In contrast, the Higgs boson is massive and quickly decays into an assortment of particles.

Here are some lecture notes on the topic:
https://www.physics.utoronto.ca/~krieger/PHY357_Lecture6.pdf

As for where those other particles come from, given enough energy and other conserved features (charge, etc) in the same place, it seems particles are always destined to form.

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#### FactChecker

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CORRECTION: As @.Scott states below, the OP was asking about elementary particles, not nuclei. So this answer does not apply. A more appropriate answer can be found at http://www.particleadventure.org/decay_intro.html with fundamental partical decay beginning at http://www.particleadventure.org/mediator.html .

I'll take an amateur stab at it:
There are opposing attractive and repelling forces, each with a probability distribution for the strength of the force. It starts out as one unit, so we know that the attractive forces are greater at that time. After a certain amount of time, the odds favor the random repelling force being greater than the attractive force at some point within that time. If that happens, it decays.
This all assumes that no other particle or energy is involved. Chain reactions are a different matter.

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#### .Scott

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I'll take an amateur stab at it:
There are opposing attractive and repelling forces, each with a probability distribution for the strength of the force. It starts out as one unit, so we know that the attractive forces are greater at that time. After a certain amount of time, the odds favor the random repelling force being greater than the attractive force at some point within that time. If that happens, it decays.
This all assumes that no other particle or energy is involved. Chain reactions are a different matter.

#### mfb

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And where do these particles come from if they were not part of the original particle?
They are newly produced, and the original particle stops existing at the same time.
Ultimately physics cannot answer "how" questions. It can give you tools to calculate how long the particle will live, what its decay products will be, their angular distribution and so on. But it cannot answer philosophical questions. Particle decays just happen.

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#### mitrasoumya

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Thanks to all.

I still want to know this: -
If a particle decays into other particles, does it not in a way suggest, that it could be a composite particle rather than an elementary particle?

#### king vitamin

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I still want to know this: -
If a particle decays into other particles, does it not in a way suggest, that it could be a composite particle rather than an elementary particle?
It does not suggest that by itself, since we have a very successful theory which contains elementary particles decaying. If you have a theory which claims that only composite particles decay, and that theory is internally consistent and makes even better predictions, this would certainly suggest your premise. (But such a theory should be written up, peer reviewed, and published before discussing it on these forums).

#### mitchell porter

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In string theory, it's because the string breaks.

#### mfb

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Thanks to all.

I still want to know this: -
If a particle decays into other particles, does it not in a way suggest, that it could be a composite particle rather than an elementary particle?
No. We even have direct evidence that they cannot all be composite particles. Consider for example the following two processes:
A free neutron decays to proton plus electron plus a neutrino: $n \to p + e^- + \bar \nu$. Can a neutron consist of these three particles?
Two protons can combine to (proton+neutron) plus positron plus neutrino: $p+p \to (np) + e^+ +\nu$. The first one is a deuterium nucleus. Does that mean a proton is made out of a neutron plus positron plus neutrino?
Clearly these two things can't be true together, you would run in a circle. You can even split the deuterium nucleus into proton and neutron and then watch the neutron decay via the first reaction.

#### mitrasoumya

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No. We even have direct evidence that they cannot all be composite particles. Consider for example the following two processes:
A free neutron decays to proton plus electron plus a neutrino: $n \to p + e^- + \bar \nu$. Can a neutron consist of these three particles?
Two protons can combine to (proton+neutron) plus positron plus neutrino: $p+p \to (np) + e^+ +\nu$. The first one is a deuterium nucleus. Does that mean a proton is made out of a neutron plus positron plus neutrino?
Clearly these two things can't be true together, you would run in a circle. You can even split the deuterium nucleus into proton and neutron and then watch the neutron decay via the first reaction.
I think both proton and neutron are 'composite' particles made of quarks. But what about the decay of quarks? They are supposed to be 'elementary'.

#### Orodruin

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You immediately have to dispell the notion of elementary particles being little balls or that they can be understood as such or that you can draw any conclusions from trying to treat them as such. They are quantum particles and quantum particles have certain properties. One such property is that, due to the interactions among quantum fields, some particle numbers are not conserved and the corresponding quantum particles may decay or be produced in decays as long as relevant conservation laws are satisfied.

#### mfb

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They are supposed to be 'elementary'.
And all measurements point to that. And yes, there are many of them.
But that's not what the argument I made was about. If a particle could only decay into particles it consists of, then either proton -> neutron+stuff or neutron -> proton+stuff would be impossible.

#### haushofer

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What causes an elementary particle to "decay" into other elementary particles? And where do these particles come from if they were not part of the original particle?
Conservation laws, i.e. symmetries. Symmetries dictate whether particles are stable or not.

Take e.g. the electron. The electron is the lightest lepton we know of. It is electrically charged, and (among others) energy/momentum conservation dictates that there is no particle to which it can decay. If there were a lighter version of the electron (a lighter lepton), the electron would decay to it.

#### mfb

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Neutrinos are leptons and lighter.
The electron is the lightest (electrically) charged lepton and charge is conserved.

#### mitrasoumya

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And all measurements point to that. And yes, there are many of them.
But that's not what the argument I made was about. If a particle could only decay into particles it consists of, then either proton -> neutron+stuff or neutron -> proton+stuff would be impossible.
Thanks for the explanation.

It could be that, I am being misled too much by my classical "common sense" notion.

But, I picture decay as some sort of a change. And I also think that whenever some "composite" stuff changes into another "composite" stuff, it is actually the "composition" that is changing.

For example when a neutron changes into a proton, it's internal structure changes from 2 down quarks and an up quark to 2 up quarks and a down quark.

But, quarks are elementary. Which perhaps means that they don't have an internal structure (may be I am wrong here). Then how does a down quark change into an up quark?

Is there something "inside" the quarks that changes?

I am sorry if my question sounds funny, as I am not very well versed about the quantum world. But, I honestly find it difficult to understand how something not having an "internal" structure can change into another thing also not having an internal structure.

#### mfb

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And I also think that whenever some "composite" stuff changes into another "composite" stuff, it is actually the "composition" that is changing.
Not necessarily. There are also hadrons with two up quarks and a down quark that can decay to a proton plus other particles.
But, quarks are elementary. Which perhaps means that they don't have an internal structure
Right.
Then how does a down quark change into an up quark?
In the same way e.g. a muon decays. One quark stops existing, the other one starts existing.
Is there something "inside" the quarks that changes?
No.
But, I honestly find it difficult to understand how something not having an "internal" structure can change into another thing also not having an internal structure.
Maybe, but that's what happens.

#### mitchell porter

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Another way to think about it is to focus on fields and their interactions. A particle is an excitation in a field, and particle decay means that excitation disappears from one field and appears in others, thanks to the interaction. You might find that more palatable.

#### dahoa

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Another way to think about it is to focus on fields and their interactions. A particle is an excitation in a field, and particle decay means that excitation disappears from one field and appears in others, thanks to the interaction. You might find that more palatable.
But there are interpretations where the particle is the primary thing. Then the field is just perturbations (or effect? how?) of the particle?

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