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.
Yes, there are. Next time.
PS. For more on the Higgs mechanism, please click here.
Hey,
ReplyDeleteMe and my brother were checking up on this ( You are my friend in Facebook ). Can you give me some clear ideas on :
1: How / Why does different particles interact differently with the Higgs field? The Top Quark is super heavy while a photon is mass less. And there is no difference in the size of these particles. Most explanations went with particles with mass interact with higgs field, which is a very circular explanation.
2: Higgs Particle is super heavy with a detected 126GeV so how does this happen?
These are tough-looking questions, Christin. It is not just a question of I not knowing all the answers. I think science does not have many of the answers yet. Having said that, I do want to make some points:
ReplyDelete1. The Higgs particle mediates the Higgs field. The larger its mass, the shorter is the range of the interaction it mediates. The Standard Model, like any other good model, must be formulated in a self-consistent way. The model expects a certain mass (or interaction range) for it, and we seem to have found that mass experimentally.
2. The different particles interact with the Higgs field differently. That is how it has to be. How can particles be 'different', and yet have the same interaction strength with the Higgs field? The strength of the interaction depends on a whole lot of parameters; e.g. spin. What is surprising about that? The so-called 'size' of the particles is only one among many parameters one has to factor in.