The W boson: part 2

The Electroweak Theory

Lisi Goodlett

Electroweak Theory:

Sheldon Lee	Abdus Salam	Steven

Glashow	Weienberg

In 1979, Glashow, Salam, and Weienberg were awarded a nobel prize in physics

"for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current"

What is Electroweak Theory?

This is the theory that unifies the weak and electromagnetic forces.

What does it mean to unify forces?

In the early part of last century studies of electricity and magnetism seemed to be investigations of two different phenomena.

Later studies with moving charges showed that an electric current was generated in a wire when a magnet was moved in its vicintity.

Today, these effects as described by one theory-electromagnetism- which was introduced by Maxwell (1865).

Weak Nuclear Force:

The weak force is responsible for Beta-decay of a nucleus.

Has a short range of interaction, about 10^-17m.

A way to explain this short range is to assume the carrier of the force is very heavy.

To explain many known decays, the W boson must exist in both positively and negatively charged states (charged current weak interactions).

Weak force can violate parity

Neutrinos are uncharged and participate in weak interactions (neutral current weak interactions).

Z boson is uncharged and mediates neutral current weak interactions.

What is Gauge Invariance?

A particle can be described by its wave function.

A uniform wave is completely defined by its amplitude, wavelength, and phase.

Changing the phase is equivalent to moving it left or right.

Gauge invariance basically requires that any measurement made on the wave function should be independent of any number of phase changes made on it.

Application to Electroweak Theory:

Applying gauge invariance to the electroweak theory is slightly more complicated because the force carriers have mass.

In short, it requires the introduction of the Higgs boson.

The Higgs would be uncharged, and have a spin zero, it would give mass to particles- but it's mass is not predicted.

Types of Weak Charges:

Have described the weak force as interacting with weak charge, but there are actually two different types of weak charges.

Weak isospin and weak hypercharge

W bosons carry the force between the weak isospin of particles.

B boson carries the force between weak hypercharges.

The W bosons carry electric charge and are expected to occur as W+, W- and W° states, whereas the B° boson carries no electric charge.

Why don't we see W° and B° in nature?

These two states each carry a part of the electroweak interaction, and mix together to produce the observable photon and Z° boson.

Gauge invariance predicts detailed properties of the W-, W+, W°, and B° bosons.

The exact form of mixing between the two, which produces the photon and Z°, is not predicted.

This undefined parameter is usually written as sin^2(theta) (known as the Wienberg angle).

Predictions:

Masses of W-, W+, and Z° bosons can be predicted in terms of measured quantity sin^2(theta).

For W+ and W- bosons, approx. (37.3/sin(theta)).

For Z°, approx. (M(w)/cos(theta)).

Rachel Goodlett

2003-11-04T17:13:07