# feynman diagrams

#### JeffMv2

Nuclei of carbon-14 are radioactive. They disintegrate into nitrogen-14, giving off radioactive particles. The following diagram describes beta decay ( −) at a fundamental level

Identify the particles that are emitted from the nucleus when it decays: It's emitting a positron, neutrino and is the down quark being changed into an up quark?
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#### topsquark

Math Team
It is mostly correct. There are two more details you will need.
1) The W particle (the wavy line) means that the decay is done using a weak channel. (In fact, the d quark has a charge of -1/3 and the up quark has a charge of +2/3, so since Feynman diagram interactions conserve charge we know that the W has to carry a charge of -1 so it's a $$\displaystyle W^-$$.)

2) We have an electron, a neutrino, and a u coming out of this. We have to conserve lepton number as well. The d quark has a lepton number of 0, the electron has a lepton number of 1, and the u quark has a lepton number of 0, so the neutrino must be an electron anti-neutrino, which has a lepton number of -1. (We have to call it an electron anti-neutrino because there are muon and tau neutrinos as well.)

-Dan

JeffMv2

#### JeffMv2

This is somewhat related to this so I'll aks you how do we know that nuclei are not held together by just electromagnetic and gravitational forces?

#### Maschke

This is somewhat related to this so I'll aks you how do we know that nuclei are not held together by just electromagnetic and gravitational forces?
I'm a complete amateur but my understanding is that first, the nucleus contains positively charged protons and neutral neutrons. So the protons should fly apart from like-like repulsion. And they're far too small for gravity to be much of a factor. Gravity is negligible by many orders of magnitude for objects with such small mass. So there must be something else, the strong (or weak?) nuclear force. I'm a little fuzzy on those, the weak force has something to do with neutrinos.

I also recently ran across a physics video mentioning that we don't actually know much about gravity at tiny scales for tiny masses. For all we know Newtonian or Einsteinian gravity might be slightly different at tiny scale.

JeffMv2

#### topsquark

Math Team
This is somewhat related to this so I'll aks you how do we know that nuclei are not held together by just electromagnetic and gravitational forces?
I'm a complete amateur but my understanding is that first, the nucleus contains positively charged protons and neutral neutrons. So the protons should fly apart from like-like repulsion. And they're far too small for gravity to be much of a factor. Gravity is negligible by many orders of magnitude for objects with such small mass. So there must be something else, the strong (or weak?) nuclear force. I'm a little fuzzy on those, the weak force has something to do with neutrinos.

I also recently ran across a physics video mentioning that we don't actually know much about gravity at tiny scales for tiny masses. For all we know Newtonian or Einsteinian gravity might be slightly different at tiny scale.
Pretty much what maschke said. It's the strong nuclear force that holds the nucleus together. The weak nuclear force is what governs nuclear decay. (That's a bit of an oversimplification but nuclei aren't usually at a high enough energy for them to decay by the strong force. But it's theoretically possible.)

As to gravitation the numbers come out pretty good using just the strong and electromagnetic forces in the nucleus so any correction due to gravitational forces is apparently negligible on nuclear scales.

-Dan

JeffMv2