Saturday, February 4, 2012

13. The Standard Model and the Higgs Mechanism

I had started describing the 'standard model' of particle physics in Part 12. In this model, the electromagnetic interaction and the weak nuclear interaction have been unified into a single 'electroweak' interaction, and a quantum field theory of this unified interaction has been successfully formulated. Further, a quantum field theory of the strong interaction has also been established, but it has not been satisfactorily unified with other interactions (see below). A quantum theory of the gravitational interaction ('quantum gravity'), as also its unification with other interactions, is still 'work in progress'.

During the 1970s, physicists formulated a number of grand unification theories (GUTs) which aimed at unifying the electroweak interaction and the strong interaction. Most of them predicted that the proton should decay with a 'half life' of ~1032 years. Experimental verification of proton decay has not been effected unambiguously, so the GUTs have not been an unqualified success. But there has been an unexpected fallout. Ideas from the GUTs have found great applications in cosmology! A certain 'scalar field' in the GUTs has provided a strong basis for supporting the idea of cosmic inflation (cf. Part 8) postulated for the baby universe:

The gravitational interaction arose 10-43 seconds (the 'Planck time') after the Big Bang. The other three interactions were still in a 'grand-unified' state then. The spontaneous emergence (on further cooling of the universe) of a GUT scalar field 10-35 s after the Big Bang broke the 'GUT symmetry', and new interactions appeared one by one, the first to appear being the strong interaction. In parallel with the emergence of new interactions, the GUT scalar field also caused inflation, doubling the size of the baby universe every 10-34 s. This means that 100 doublings occurred within 10-32 s after the Big Bang. This inflation was so rapid that a quantum fluctuation just 10-20 the size of a proton could grow to a size of 10 cm in just 15 x 10-33 s. This is how the GUT scalar field kick-started the growth of the universe, preventing a gravitational-pull collapse of the quantum fluctuation that had initiated the universe at the Big Bang.

In the standard model there are 12 elementary particles of spin 1/2: six quarks and six 'leptons'. Further, each such fermion has a corresponding 'antiparticle'. For example, the antiparticle of an electron is a particle called the 'positron' (of positron-emission tomography (PET) fame). Thus there are 24 fermions in all.

The six quarks are: up, down, charm, strange, top, and bottom. And the six leptons are: electron, electron-neutrino, muon, muon-neutrino, tau, and tau-neutrino.

The quarks interact via the strong nuclear interaction. They form 'colour-neutral' composite particles called hadrons. [Now you know why the word 'hadron' appears in the particle-accelerator name 'Large Hadron Collider' (LHC).] The hadrons are either baryons, or mesons. The baryons (protons and neutrons) contain three quarks, and the mesons comprise of a quark and an antiquark.

There are three types of gauge bosons: (i) photons; (ii) W+, W‑, and Z particles; and (iii) gluons. Out of these, photons and gluons are massless. Photons mediate the electromagnetic interaction. There are eight types of gluons, and they mediate the interaction among quarks. The W+, W-, and Z gauge bosons have nonzero 'rest mass', and they mediate the (short-ranged) weak-nuclear interaction among quarks and leptons.

[A fourth type of gauge boson should also be mentioned here, although it has not been observed yet, namely the graviton. It is expected to be massless, and have spin parameter of value 2.]

In the standard model there is envisioned a 'Higgs field' that permeates all space. The 'Higgs particle' (or 'Higgs boson') is the quantum of this field, and it is expected to have nonzero mass. The Higgs field is a 'scalar field' (i.e., the Higgs particle has zero spin).

The Higgs field interacts with the various particles with different strengths. Particles that interact more strongly with it experience more resistance or drag to their motion, and thus appear more massive. Some particles, such as photons, do not interact with the Higgs field at all, and are therefore massless ('zero rest mass'). In this way, the mass of everything is determined by the existence of the Higgs field.

Very high temperatures or energies (comparable to those that prevailed soon after the Big Bang) have to be created in the laboratory for creating and detecting the Higgs particle. Recently there have been substantial indications from the experiments conducted at the LHC that the Higgs particle does indeed exist. If confirmed by further experiments, this would constitute a major accomplishment of the rational human mind, which could predict its existence by adopting the scientific method for understanding natural phenomena.

You might be wondering: Why so much fuss about a Higgs mechanism for imparting mass to elementary particles? What is so surprising or wrong about a particle having a mass? Why do we need a mechanism (the Higgs mechanism) for 'imparting' mass to the particles?

We have to remember that in the beginning there was only a radiation field, and the gravitational interaction. Particles with nonzero rest mass appeared via the Higgs mechanism. The Higgs field is a part of the theory of the electroweak interaction. It played a role in breaking the symmetry between the electromagnetic interaction and the weak interaction. This symmetry existed at very high temperatures, and was broken spontaneously as the universe cooled a little and the Higgs field appeared. Particles with nonzero rest mass arose when this symmetry was broken by the emergence of the Higgs field.

The Higgs field pervades all space, and the physical laws governing it have high symmetry. Yet the present actual state of the Higgs field has a spontaneously-broken or lower symmetry. The field takes a particular value at low energies, and pervades all space, rather like the energy associated with vacuum in quantum field theory. Particles acquire their masses by interacting with the pervading Higgs field.

One has to go beyond the standard model for creating a grand theory of how our universe came into existence. Are there particles even more elementary than 'the veritable zoo' of particles envisaged in the standard model?

Yes, there are. Next time.

PS.  For more on the Higgs mechanism, please click here.