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We are always told that there are the four fundamental forces or interactions of nature: gravitation, electromagnetism, and the weak and strong forces. We know that gravitation is attractive, that electromagnetism can be attractive or repulsive depending on the electric charge of the interacting particles, and that the strong force is attractive between quarks.

But when the weak force is mentioned, the description is always something such as 'responsible for radioactive decay', but there is no mention of whether this force is attractive or repulsive. So my question is: is the weak force/interaction attractive or repulsive?

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Since the electroweak interaction is mediated by spin 1 bosons, it is the case that "like (charge) repels like and opposites attract".

In the electroweak case, the charges in question are weak isospin and weak hypercharge.

For weak isospin, there are two isospin charges (or flavors), up and down, and their associated anti-charges, anti-up and anti-down.

So:

  • up repels up (anti-up repels anti-up)
  • down repels down (anti-down repels anti-down)
  • up attracts down (anti-up attracts anti-down)
  • up attracts anti-up (down attracts anti-down)
  • up repels anti-down (down repels anti-up)

For weak hypercharge, there is just one type of charge and its associated anti-charge.

So:

  • hypercharge repels hypercharge (anti-hypercharge repels anti-hypercharge)
  • hypercharge attracts anti-hypercharge

Note that electric charge is a certain mixture of weak isospin and weak hypercharge.

Since (left-handed) particles carry both weak isospin and weak hypercharge, both must be taken into account to determine which particles attract or repel under the electroweak interaction.

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    $\begingroup$ Why "up attracts down" ? $\endgroup$
    – Trimok
    Sep 13, 2013 at 16:12
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    $\begingroup$ @Trimok because opposite weak charges attract. If you're asking why the weak force is that type of force instead of a different one, you're going beyond what is currently known. It's possible that a unified theory (ex string theory) might be able to answer that question at some point in the future. It's also possible that such a theory will end up trading one set of magic-numbers-to-make-it-work for another set (for string theory the shape of the extra dimensions of space). $\endgroup$
    – Dan Is Fiddling By Firelight
    Sep 13, 2013 at 17:08
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    $\begingroup$ @DanNeely : "because opposite weak charges attract" . It explains "up attracts anti-up", but not "up attracts down"(It is a different color), or I missed something important. $\endgroup$
    – Trimok
    Sep 13, 2013 at 17:44
  • $\begingroup$ @Trimok "up" and "down" are also opposites $\endgroup$
    – OrangeDog
    Jun 29, 2020 at 9:36
  • $\begingroup$ Another take: weak charge as an effective $Z$-boson coupling constant, approximately orthogonal to electric charge. $\endgroup$
    – rob ♦
    Aug 11, 2021 at 4:52
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The weak force is mediated by the W and Z bosons. The W and Z bosons are spin - 1 particles, like photons. The Higgs is spin - 0 (scalar field) and the graviton is spin - 2; attractive.

If a particle is spin - 1, it could be either attractive or repulsive, depending on the situation. So, like the Photon (Electromagnetism), it could be either.

You may thus want to know about the Weak Isospin.

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