# Exchange force carrier between two protons

1. Aug 9, 2012

### CAF123

I know that the virtual exchange particle between two electrons is the photon and this explains why they repel each other (ie virtual photon is spontaneously emitted from one electron A causing it to recoil. As the virtual photon hits electron B, it imparts momentum causing it to move away so that electron A and B move away in the opposite direction, as needed for conservation of momentum).

My question is can a Z particle also be the exchange carrier here because it too is neutral? I have the same question for the case of two protons.
Many thanks

2. Aug 9, 2012

### Bill_K

CAF123, You could regard the Z meson as a force carrier, like a photon, but there's a big difference -- the photon is massless, so the force field it produces (a Coulomb field) is long range, i.e. 1/r. The Z meson, on the other hand, is massive, very massive: 90 GeV. This means the force field it produces is short range (a Yukawa field), i.e. one that falls off exponentially, with a range equal to the Compton wavelength. The Compton wavelength of a 90 GeV particle is about .001 fermi, or 1/1000 the diameter of a proton.

Needless to say, at such a short range the interaction between two electrons (or two quarks) will be dominated by other effects, and so regarding the Z meson as having produced a force field is simply not a useful concept.
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Might as well take this opportunity to point out that your attempt to visualize this interaction ignores its quantum nature. You're imagining that the momentum they exchange is in the same direction as the spatial separation between them. Therefore it appears to cause a repulsion. (And so, often we are asked next, then how can opposite charges possibly attract??) In fact, the interaction includes an integration over ALL momenta: forwards, backwards and sideways. The intensity of the interaction generates an effective potential, and the resulting force between the charges arises from the gradient of this potential.

3. Aug 10, 2012

### CAF123

I believe another reason is that the electromagnetic interaction at this scale is a lot stronger (order of $10^8$ ) than that of the weak interaction

4. Aug 10, 2012

### Bill_K

Nope, this is false. If you're talking about the interaction between the W or Z meson and other particles, it is about the same strength as electromagnetism.

The weak interactions only appear weak, due to the large masses of the W and Z. In a beta decay, for example, the W is not directly observed. It's virtual, an intermediate product of the decay. And since the decay energy is so much less than the W mass, the W is far off the mass shell, and this reduces the decay rate.

They're not intrinsically weak, just short range.

5. Aug 14, 2012

### Durandarte

Protons are made up of quarks, which transmit forces via photons (electromagnetism) and gluons (strong force) themselves.

Z bosons cant mediate the interaction between two electrons or protons due to their mass. That is, they have mass so they cant travel as far as photons or gluons (both massless), which is required for the force particles of electromagnetism.

Additionally, the EM force has been observed repeatedly to travel at the speed of light, something impossible for the Z (which has mass), a direct consequence of special relativity.