# Understanding the Weak Nuclear Force: Effects on Quarks & Leptons

• Honorable_Death
The weak nuclear force is a force that mediates beta decay and other effects on atoms. It is mediated by the W+, W-, and Z bosons, and it is explained in terms of the quantum mechanics of particles.

#### Honorable_Death

Whenever i research the weak nuclear force i always get how it is the cause of beta decay and other ways it effects an atom, but i can never get a clear answer to what the force actually is, what does it do? i know that it effects quarks and leptons but i have no idea how it effects them, can anyone explain it to me?

Honorable_Death said:
Whenever i research the weak nuclear force i always get how it is the cause of beta decay and other ways it effects an atom, but i can never get a clear answer to what the force actually is, what does it do? i know that it effects quarks and leptons but i have no idea how it effects them, can anyone explain it to me?

im not an expert at this, but ill try to explain.
the force is mediated by the W+, W-, and Z bosons according to the standard model. These bosons have mass, unlike the photon which has zero mass, so they can only operate over short distances, hence the range of the weak force as compared to the electromagnetic force. The W bosons mediate flavor and charge changes between quarks and leptons, while the Z bosons mediate effects which do not change the flavor or charge. W bosons will decay to produce a quark-antiquark pair, or a lepton and a neutrino/antineutrino. The Z boson decays into a quark-antiquark pair or a lepton-antilepton pair.
Beta decay of a neutron is mediated by a virtual W- boson, which converts one of the down-quarks (charge -1/3) into an up-quark (charge +2/3). Charge is conserved because the virtual W- boson carries away -1 charge (the change from -1/3 to 2/3 is +1). The virtual W- boson decays into an electron and an electron-antineutrino.
I don't know the maths behind this, but I'm guessing that the quantum explanation for this is that the probability amplitudes of the up-down-down quark configuration of the neutron are non-zero across a potential barrier, so the down quarks have a non-zero chance of crossing this potential barrier to decay into an up quark and a W- boson.

HD
Most explanations amount to an analogy compared to something you can understand.
As you likely know the weak force (at least to a great extent) is understood in the math to be “unified” with EM.
If you have a good understanding of the QM and/or the math that goes into all that; you should have no problem relating to an explanation in those terms.

Most of us don’t – so let me share with you how I like to think of the weak force.

Protons being of like charge of course repel each other.
And at the close proximity in the nucleus there is nothing on Earth that could hold them together like that. Except of course the STRONG force. Hence its name.

But how strong is the strong force? We know it’s not unlimited, atoms do divide.

As we go up in atomic number the atoms get larger and larger. The Electrostatic part of the EM force gets larger on an individual proton from the repulsive force of more and more protons. Define this as an increasing weak force. Also dependent on the configuration of those + charges and how they move inside the nucleus. Even the random movement of the electrons outside may have some influence on the net instantaneous weak force working on a individual proton. If all these elements combine such that at some random perfect combination just one proton reaches a weak force push that the strong force cannot hold it back, - well all hell breaks lose.
Once it happens, the end results depend on the EM, Weak, and Strong forces interactions on all the pieces. We see it as radiation, nuclear power, or an explosion.

How do we figure the exact odds, of a split happening. The movement of all the up and down quarks, and shape of the electron cloud if it might be important, is all a bit much to calculate. Much easier to put something in a lab, just make a measurement and call it radioactive decay.

Somehow down at the quark level a proton has both strong and weak forces working and will last a very long time by itself before decaying. A neutron on it’s own not so long.
But together in the form of a nucleus, the strong force remains organized in a way much harder for the weak force to find a configuration to break it up, until we go way up in the atomic numbers.

Works for me anyway as to “how” it works, let me know if that makes sense to you.

RB

I'll steer clear of the question of what the weak force actually is, because the answer depends on what "actually is" means to you. As for what the weak force does, there are two answers, depending on which of the weak-force-carrying particles is involved. If it's the $W^+$ or $W^-$, the basic result is to cause fundamental particles (quarks and leptons) to change their "flavor". For example, when a neutron decays into a proton, a down-quark changes into an up-quark by emitting a virtual $W^-$. Or a muon-neutrino can change into a negatively-charged muon by emitting a virtual $W^+$, which in turn can induce a down-quark to change into an up-quark, giving the overall reaction $\nu_{\mu} + n \rightarrow \mu^- + p$.

If it's the $Z^0$ that's involved, I suppose you can think of it as a much-weaker version of the electromagnetic interaction, that violates conservation of parity (whereas the EM interaction does conserve parity). Like the photon, it doesn't change the "flavors" of the particles that it interacts with.

RandallB said:
HD
divide.

As we go up in atomic number the atoms get larger and larger. The Electrostatic part of the EM force gets larger on an individual proton from the repulsive force of more and more protons. Define this as an increasing weak force. Also dependent on the configuration of those + charges and how they move inside the nucleus. Even the random movement of the electrons outside may have some influence on the net instantaneous weak force working on a individual proton. If all these elements combine such that at some random perfect combination just one proton reaches a weak force push that the strong force cannot hold it back, - well all hell breaks lose.
Once it happens, the end results depend on the EM, Weak, and Strong forces interactions on all the pieces. We see it as radiation, nuclear power, or an explosion.

RB

Your describing the weak nuclear force as EM, in fact u said in the top of this paragraph that the increasing EM is defined as an increasing weak force, what's the difference between EM and the weak nuclear force?

Honorable_Death said:
Your describing the weak nuclear force as EM, in fact u said in the top of this paragraph that the increasing EM is defined as an increasing weak force, what's the difference between EM and the weak nuclear force?
NO, I did not define W as being EM.
I am looking at the “Electrostatic part” part of EM and how how it relates to an “increasing Weak force”.
All this does is look at the question; how large can a nucleus get before the repulsive charges it it are large enough to override the strong force? (the larger the atom the more likely it’s radioactive) The complex structure that must sum these charge forces relate to a Weak Force finding a way to become strong enough to overcome the Strong force. Including how it must be problematical, i.e. decay.

The analogy doesn’t directly relate to W+ W- etc. But, it does to the idea the Weak cannot be explained, except as a part of the unified “Electro-Weak Force”, all three working together.

RB

Honorable_Death said:
whats the difference between EM and the weak nuclear force?

For one thing, EM interactions conserve parity, but weak interactions don't. Lee and Yang suggested in the 1950s that weak interactions might not conserve parity, and Wu did the first experiment that demonstrated it. Lee and Yang won the Nobel Prize for this.

http://www.bnl.gov/bnlweb/history/nobel/nobel_57.asp

http://www.physics.ucla.edu/~cwp/Phase2/Wu,_Chien_Shiung@841234567.html

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But what does the weak nuclear force do that isn't already explained by EM?