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In the early universe the temperature was high enough for the electromagnetic force and weak force to act as one. My question is how exactly does that work, because the weak force uses the W and Z bosons while the electromagnetic force uses photons? I read that the high energy density allowed for the particles to be essentially identical (Article Link), so does that mean the W,Z and photon had the same mass and worked the same way?

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In the early universe the temperature was high enough for the electromagnetic force and weak force to act as one. My question is how exactly does that work, because the weak force uses the W and Z bosons while the electromagnetic force uses photons?

In 1967, Weinberg introduced a model incorporating a gauge theory based around local invariance with respect to the Lie group $SU (2) $ of rotation and phase changes in the (abstract) weak-isospin space. This is the space in which the weak force identical particles $e^-$ and $\nu_e $ ( electron neutrino) can be swapped with each other.

This theory also incorporates a $U (1) $ gauge symmetry, the "photon" analog single gauge quantum, which is not the true quantum of the electromagnetic interaction. The properties of the associated minimal interaction vertex of this $U (1) $ gauge quantum ( aka the $B^0$) and the electron did, almost but not quite, correspond to the minimal interaction vertex described by QED.

enter image description here

The $U (1)B^0$ and $SU (2)W^0$ field quanta are, besides both being electrical neutral, so similar to each other that, as the diagram above illustrates, it is impossible to know for certain which is responsible for the mutual electron repulsion demonstrated above.

So it's entirely possible that the exchanged quantum is a mixture of the two, if you are allowed take a well defined amount of $B^0$ and combine it with an equally well defined amount of $W^0$, to arrive at a combined field quantum with exactly the properties of the photon of QED.

In the very high energy regime of the early universe, this is what is conjectured to have occured and, as the temperature reduced, we see today the electroweak forces split into the electromagnetic force and weak force.

I read that the high energy density allowed for the particles to be essentially identical (Article Link), so does that mean the W,Z and photon had the same mass and worked the same way?

Please read anna's answer to this question, as she has far more knowledge than I do regarding particle physics and my answer would virtually be a duplicate of annas.

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Gauge bosons should have zero mass, and the W and Z at our times are very massive. The unification of the weak and electromagnetic interactions into one gauge theory at very high energies implied massless gauge bosons.

At those high energies all particles in the standard model table of particles would be massless. As the universe cools the gauge bosons acquire masses by symmetry breaking and only the photon stays massless.

The cost is the introduction a new basic field, the Higgs field which induces the breaking of the symmetry, and also has as its excitation the recently discovered Higgs boson. The Higgs field also induces the masses seen in the elementary particles table.

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    $\begingroup$ Does the mass of the W/Z/H boson actually change during this phase transition, or is it reasonable to think of the early/hot "unified" regime as being different from the present "broken symmetry" regime simply because if the temperature $kT$ is above the gauge boson mass then you can produce them for free? (If you'd prefer, I could expand to a follow-up question.) $\endgroup$ – rob Oct 8 '16 at 19:45
  • $\begingroup$ All the gauge bosons are massless until the temperature cools enough for the symmetry of the Higgs field to become broken. $\endgroup$ – Lewis Miller Oct 9 '16 at 0:29
  • $\begingroup$ @rob we are in quantum mechanics here, and the simplest model has a break into the specific vacuum expectation values for the Higgs field and the specific masses( once they were seen and measured). I suppose theorists being theorists one could devise a quantum mechanical .transition dependent on the energy scale ( after all coupling constants run) but I am not aware of such a model. Their differentiation into w z and photon happens at the transition, before that they were one . $\endgroup$ – anna v Oct 9 '16 at 2:36

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