The Standard Model (of sub-atomic physics): Patrick's notes

 

The Standard Model (of sub-atomic physics)

 

The universe is comprised of twelve ‘fundamental particles’, governed by four ‘fundamental forces’. Our best understanding of how these particles and forces operate is called The Standard Model of particles and forces. It was developed in the 1970s and has explained many experimental results and predicted a wide variety of phenomena. The Standard Model covers only three of the fundamental forces – it doesn’t include gravity. Since most people are more familiar with gravity than with any of the other three forces (and certainly with the strong and weak forces), this might seem a major weakness of the Standard Model. However, gravity is a powerful force only on matter in bulk; at the sub-atomic scale of particle physics, gravity has but a negligible effect. So the Standard Model is still an effective model of the universe at a sub-atomic scale, despite its exclusion of gravity.

 

The twelve ‘fundamental particles’

The twelve fundamental particles are divided into two groups of six – quarks and leptons; and each group of six particles is divided into three pairs or ‘generations’. All stable matter consists of lighter, First Generation particles; heavier, Second and Third Generation particles quickly decay to their next most-stable level.

 

‘GENERATIONS’	QUARKS	LEPTONS
First (Lightest, most stable)	‘Up quark’	‘Electron’ (Electrical charge + mass)
	‘Down quark’	‘Electron-neutrino’ (Electrically neutral, no mass)
Second (Heavier, less stable)	‘Charm quark’	‘Muon’ (Electrical charge + mass)
	‘Strange quark’	‘Muon-neutrino’ (Electrically neutral, no mass)
Third (Also heavier, less stable)	‘Top quark’	‘Tau’ (Electrical charge + mass)
	‘Bottom quark’	‘Tau-neutrino’ (Electrically neutral, no mass)

 

The four ‘fundamental forces’

There are four fundamental forces in the universe: the strong force, the weak force, the electromagnetic force and the gravitational force. The four fundamental forces differ in their strength and range; and each force is carried by its own type of force carrier particle or ‘boson’. Thus, particles of matter exchange energy in discrete/fixed (as opposed to variable) amounts because they exchange bosons with each other.

 

	STRONG FORCE	WEAK FORCE	ELECTROMAGNETIC FORCE	GRAVITATIONAL FORCE
STRENGTH	Strongest	Weaker	Weaker still	Weakest
RANGE	Sub-atomic	Sub-atomic	Infinite	Infinite
ASSOCIATED BOSON	‘Gluon’	‘W and Z’ bosons’	‘Photon’	‘Graviton’ (Yet to be found)

 

A crude chronology of the universe

13.7 billion years ago	The Big Bang
A few billionths of a second later (Around 2,000 billion degrees – 100,000 times hotter than the Sun’s core)	Quarks and electrons emerged. At this stage, they existed freely in a new state of matter – ‘quark-gluon plasma’. (CERN experiments in the late 1990s supported this theory. The Large Hadron Collider’s ‘ALICE’ experiment will re-create these conditions, enabling scientists to study the quark-gluon plasma as it expands and cools, progressively creating the particles that constitute the matter of the universe.))
A few more billionths of a second later	Quarks aggregated into protons and neutrons. They are stuck together by gluons (the carrier particles of the strong force) so tightly that it has proved impossible so far to extract individual quarks or gluons.
Three minutes later	Protons and neutrons aggregated into nuclei
380,000 years later	Electrons were trapped in orbit around nuclei, forming the first atoms. These were mainly helium and hydrogen – still the most abundant elements in the universe.
1.6 million years later	Under gravity, clouds of gas began to form stars and galaxies.
‘Subsequently’	Heavier atoms, such a carbon, oxygen and iron, have been ‘cooked’ continuously in the hearts of stars and distributed throughout the universe each time a star ends as a supernova.

 

What remains?

The Standard Model can’t explain many things, including the existence of ‘dark matter’ and dark energy’, the origin of particle mass and the disappearance of anti-matter.

 

The ‘Dark’ side of the universe

the universe is defined not so much by the ‘things’ that make it up as by the void around them. Observations have shown that the description and chronology above accounts for only 4 per cent of the universe; the remaining 96 per cent consists of ‘dark matter’ (26 per cent) and dark energy’ (70 per cent). Neither emits electromagnetic radiation, so we detect them only through their gravitational effects. We know neither what each is nor what role it played in creating the universe. To answer those questions requires replacing the Standard Model.

 

 

Find the Higgs Boson!

In the 1970s, scientists found very close ties between the weak force and the electromagnetic force, enabling them to describe the two forces within the one theory – the basis of the Standard Model. This theory implies that electricity, magnetism, light and some types of radioactivity are all manifestations of the same force – the ‘electroweak force’. However, while the carriers of this ‘electroweak force’ – the ‘W and Z bosons’ - were discovered in 1983, for the theory of the ‘electroweak force’ to work mathematically, force-carrying particles should have no mass – which experiments have shown they do.

 

Higgs, Brout and Englert (1964?) suggested that immediately after the Big Bang, no particles of any sort had mass; but as the universe began to cool, an invisible force and associated boson were formed – ‘the Higgs field’ – and ‘Higgs boson’. This force prevails throughout the universe and any particles that interact with it are given mass by the Higgs boson. The more that particles interact with the Higgs field, the heavier they become; while particles that don’t interact with it have no mass. Finding the Higgs boson would explain why certain particles – but not all - have mass.

 

The Higgs boson is our current best guess at the origin of particle mass and is a key ingredient of the Standard Model. However, physicists have yet to find the Higgs boson. it is hard to identify, because we do not know its mass, so scientists look for it by searching a range of mass within which it is predicted theoretically to exist. This is what the Large Hadron Collider’s ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact Muon Solenoid) experiments will do. If the LHC fails to find the Higgs boson, physicist will have to pose a new explanation for the origin of particle mass.

 

 

Anti-matter

Matter and anti-matter have the same mass but opposite electrical charge; and for each particle of matter there is a corresponding anti-particle. For instance, the negatively charged electron has a positively charged anti-particle called the positron (found in 1932 occurring naturally in cosmic rays). When a particle and its anti-particle collide, they disappear in a flash, as their mass is transformed into energy.

 

For the past fifty years, scientists have created anti-particles as a matter of routine, but no-one has created anti-particles without creating the corresponding particles. In the same way, the Big Bang must have produced equal quantities of matter and anti-matter, but how, then, has the universe persisted, despite the co-creation of the mutually-annihilating particles and anti-particles? The LHC will investigate by studying the ‘beauty quark’ (‘b quark’).

 

 

String theory

We are used to the existence of three dimensions – height, width, depth – and even four when we add (after Einstein) time. String theory poses the existence of six more dimensions, each curled-up so small as to render it undetectable; and suggests that gravity is the weakest of the four ‘fundamental forces’ (and the graviton has yet to be found) because its effect is shared with these other dimensions. High-energy experiments cold prise-open these hidden dimensions just enough to allow particles to move between the 3-D world and other dimensions. This could be manifest as the sudden disappearance of a particle or the unexpected appearance of one!

 

Underlying String Theory is the idea that fundamental particles are not points or dots, but small loops of vibrating string. Thus, all the apparently diverse particles and forces in the universe are ‘merely’ different modes of oscillation of the same type of string.

 

The complexity of String Theory has so far prevented scientists from deriving testable hypotheses. Not only does String Theory involve the study of the geometry of unseen dimensions, but also the structure of each dimension seems arbitrary and can lead to different outcomes. For example, the extra dimensions can ‘curl-up’ in different ways, according to what shape and size is chosen; and some ‘curled-up’ dimensions are so small that it will be hard to obtain direct evidence that they exist.