W Boson

Source: The Hindu 

Why in News?

Recently, researchers from Collider Detector at Fermilab (CDF) Collaboration, in the US, announced that they have made a precise measurement of the mass of the W boson.

It has been stated that this precisely determined value did not match with the estimates from the standard model of particle physics.

What is W Boson?

The W boson was first seen in 1983 at CERN, located on the Franco-Swiss border.

In contrast to the photon, which is massless, the W bosons are quite massive, so the weak force they mediate is very short ranged.

European Organisation for Nuclear Research (CERN) is the world’s largest nuclear and particle physics laboratory and best known as operator of the Large Hadron Collider, which found the elusive Higgs boson in 2012.

Unlike the photon, which is electrically neutral, the W-plus and W-minus are both massive and charged.

By exchanging such W bosons, a neutron can change into a proton, for example:

This is what happens in beta decay, a radioactive interaction that takes place in the sun.

Thus, the W boson facilitates the interactions that make the sun burn and produce energy.

What is the Standard Model of Elementary Particle Physics?

The standard model of elementary particles is a theoretical construct in physics that describes particles of matter and their interaction.

It describes the elementary particles of the world as being connected by mathematical symmetry, just as two objects are connected by bilateral (left-right) symmetry.

These are mathematical groups generated by continuous transformations from, say, one particle to another.

According to this model there are a finite number of fundamental particles which are represented by the characteristic “eigen” states of these groups.

The particles predicted by the model, such as the Z boson, have been seen in experiments.

The last to be discovered, in 2012, was the Higgs boson which gives mass to the heavy particles.

Why is the Standard Model believed to be Incomplete?

Because it gives a unified picture of only three of the four fundamental forces of nature — electromagnetic, weak nuclear, strong nuclear and gravitational interactions — it totally omits gravity.

So, in the grand plan of unifying all forces so that a single equation would describe all the interactions of matter, the standard model was found to be lacking.

Also, it does not include a description of dark matter particles.

So far these have been detected only through their gravitational pull on surrounding matter.

How are the Symmetries related to Particles?

The symmetries of the standard model are known as gauge symmetries, as they are generated by “gauge transformations”.

Gauge transformations are a set of continuous transformations (like rotation is a continuous transformation). Each symmetry is associated with a gauge boson.

For example, the gauge boson associated with electromagnetic interactions is the photon. The gauge bosons associated with weak interactions are the W and Z bosons. There are two W bosons — W+ and W-.