W′ and Z′ bosons

In particle physics, W′ and Z′ bosons (or W-prime and Z-prime bosons) refer to hypothetical gauge bosons that arise from extensions of the electroweak symmetry of the Standard Model. They are named in analogy with the Standard Model W and Z bosons.

Types

Types of W′ bosons

W′ bosons often arise in models with an extra SU(2) gauge group. SU(2) × SU(2) is spontaneously broken to the diagonal subgroup SU(2)_W which corresponds to the electroweak SU(2). More generally, we might have n copies of SU(2), which are then broken down to a diagonal SU(2)_W. This gives rise to n²−1   W⁺′, W⁻′ and Z′ bosons. Such models might arise from quiver diagram, for example. In order for the W′ bosons to couple to weak isospin, the extra SU(2) and the Standard Model SU(2) must mix; one copy of SU(2) must break around the TeV scale (to get W′ bosons with a TeV mass) leaving a second SU(2) for the Standard Model. This happens in Little Higgs models that contain more than one copy of SU(2). Because the W′ comes from the breaking of an SU(2), it is generically accompanied by a Z′ boson of (almost) the same mass and with couplings related to the W′ couplings.

W and Z bosons

The W and Z bosons (together known as the weak bosons or, less specifically, the intermediate vector bosons) are the elementary particles that mediate the weak interaction; their symbols are W+, W−, and Z. The W bosons have a positive and negative electric charge of 1 elementary charge respectively and are each other's antiparticles. The Z boson is electrically neutral and is its own antiparticle. The three particles have a spin of 1, and the W bosons have a magnetic moment, while the Z has none. All three of these particles are very short-lived, with a half-life of about 6975300000000000000♠3×10⁻²⁵ s. Their discovery was a major success for what is now called the Standard Model of particle physics.

The W bosons are named after the weak force. The physicist Steven Weinberg named the additional particle the "Z particle", later giving the explanation that it was the last additional particle needed by the model – the W bosons had already been named – and that it has zero electric charge.