Farewell to Reality (13 page)

This was all fine in theory, but what of experiment?

The electro-weak theory makes three principal predictions. First, if the weak nuclear force really does require three force carriers, then the exchange of one of these — the Z
⁰
— should result in weak force interactions involving no change in charge. To all intents and purposes, these interactions look just like interactions involving the exchange of a photon. The physicists call such interactions ‘weak neutral currents' — they involve the weak force and result in no exchange of electrical charge (they are neutral).

Such currents were identified in particle accelerator experiments performed at CERN in Geneva in 1973, and subsequently at the US National Accelerator Laboratory (which was renamed Fermilab in 1974).

Second, Weinberg had predicted the masses for all the weak force carriers. At the time he made these predictions there was no particle accelerator large enough to observe them. But in the years that followed, a new generation of particle colliders was constructed in America and at CERN. The discovery of the W particles at CERN was announced in January 1983, with masses 85 times that of the proton, just as Weinberg had predicted. The discovery of the Z
⁰
was announced in June that year, with a mass about 101 times that of a proton.
¹²

The third prediction concerns the existence of the Higgs boson. Given that the Higgs mechanism allows the masses of the weak force carriers to be predicted with such confidence, the existence of a Higgs field — or something very like it — seems a ‘sure thing'. However, there are alternative theories of symmetry-breaking that do not require a Higgs field, and there remain problems with the electro-weak theory which erode our confidence somewhat and suggest that we might not yet have the full story.

The question of whether or not the Higgs boson exists in nature is therefore of fundamental importance.

On 4 July 2012, scientists at CERN's Large Hadron Collider declared that they had discovered a new particle ‘consistent' with the standard model Higgs boson. After hearing presentations from the two detector collaborations, ATLAS and CMS, CERN director-general Rolf Heuer declared: ‘As a layman I would say that I think we have it. Do you agree?'
¹³

The new boson was found to have a mass around 133 times that of a proton
¹⁴
and interacts with other standard model particles in precisely
the way expected of the Higgs. Apart from some slight anomalies, notably an observed enhancement in the decay into two photons (H → γγ), the new boson's decay modes to other particles have the ratios expected of a standard model Higgs. Whilst the ATLAS and CMS experiments were clear that this is a boson, neither could be clear on the precise value of its spin quantum number, which on the basis of the experimental results could be 0 or 2. However, the only particle anticipated to have spin-2 is the graviton, the purported carrier of the force of gravity. Spin-0 is therefore much more likely.

Although further research is required to characterize the new particle fully, the default assumption is that this is indeed a Higgs boson. But
which
Higgs boson? The standard model needs just one to break the electro-weak symmetry, though there are theories that extend beyond the standard model which demand rather more. The only way to find out precisely what kind of particle has been discovered is to explore its properties and behaviour in further experiments.

CERN commented:

Positive identification of the new particle's characteristics will take considerable time and data. But whatever form the Higgs particle takes, our knowledge of the fundamental structure of matter is about to take a major step forward.
¹⁵

Although the Higgs mechanism was invoked to explain how the carriers of the weak force acquire their mass, it is now understood that this is the mechanism by which
all
elementary particles gain mass. In a book published in 1993, American physicist Leon Lederman emphasized
^*
the fundamental role played by the Higgs boson and called it the
God particle.

Not many practising physicists like it, but it is a name that has stuck.

Three (actually, six) quarks for Muster Mark!

The flurry of experimental activity that established the ‘zoo' of particles in the late 1950s demanded some simplifying theoretical scheme.
Significant advances were made in the early 1960s. Further patterns identified by American theorist Murray Gell-Mann and Israeli Yuval Ne'eman called attention to the possibility that the hadrons might actually all be composite particles made of three even more elementary particles then unknown to experimental science.

The patterns suggested that hadrons such as the proton and neutron should no longer be considered to be elementary particles, but are instead composed of smaller constituents. But there was a big problem.

When Gell-Mann's colleague Robert Serber broached this idea over lunch at Columbia University in New York in 1963, Gell-Mann was initially dismissive.

It was a crazy idea. I grabbed the back of a napkin and did the necessary calculations to show that to do this would mean that the particles would have to have fractional electric charges — -
⅓
, +
⅔
, like so — in order to add up to a proton or neutron with a charge of plus or zero.
¹⁶

By this time, more than fifty years had elapsed since the idea of a fundamental unit of electrical charge had been established, and there had been no hints that there might be exceptions. But despite these worrying implications, there was no doubting that a system of smaller elementary particles did provide a potentially powerful explanation for the pattern of hadrons. Gell-Mann called these odd new particles ‘quarks'.
^*

At that time the pattern of particles demanded three quarks, which were called ‘up', with a charge of +
⅔
, ‘down', with a charge of -
⅓
, and ‘strange', a heavier version of the down quark, also with a charge of -
⅓
. The baryons known at that time could then be formed from various permutations of these three quarks and the mesons from combinations of quarks and anti-quarks.

In this scheme the proton consists of two up quarks and a down quark, with a total charge of +1. The neutron consists of an up quark
and two down quarks, with a total charge of zero. Beta radioactivity could now be understood to involve the conversion of a down quark in a neutron into an up quark, turning the neutron into a proton, with the emission of a W
^-
particle.

Hints that there might be a fourth quark emerged in 1970. This was a heavy version of the up quark with a charge of +
⅔
, and was called ‘charm'. It was now understood that the neutrino was paired with the electron (thus it is now called the electron neutrino). The muon neutrino was discovered in 1962. It seemed possible that the elementary building blocks of material substance consisted of two ‘generations' of matter particles. The up and down quarks, the electron and electron neutrino formed the first generation. The charm and strange quarks, muon and muon neutrino formed a heavier second generation.

Most physicists were generally sceptical that a fourth quark was needed. But when another new particle, called the J/ψ, was discovered in 1974 simultaneously at Brookhaven National Laboratory in New York and the Stanford Linear Accelerator Center (SLAC) in California, it was realized that this must be a meson formed from charm and anti-charm quarks. Here was physical evidence that the charm quark exists. The scepticism vanished.

Such was the physicists' commitment to the emerging standard model of particle physics that when the discovery of yet another, even heavier, version of the electron — called the tau — was announced in 1977, it was quickly accommodated in a third generation of matter particles. American physicist Leon Lederman found the upsilon at Fermilab in August 1977, a meson consisting of what had by then come to be known as a bottom quark and its anti-quark. The bottom quark is a heavier, third-generation version of the down and strange quarks with a charge of -
⅔
.

The discoveries of the top quark and the tau neutrino were announced at Fermilab in March 1995 and July 2000 respectively. Together they complete the heavier third generation of matter particles, which consists of the top and bottom quarks, the tau and tau neutrino. Although further generations of particles are not impossible, there is some reasonably compelling experimental evidence to suggest that three generations is all there is.

Asymptotic freedom and the colour force

The quark model was a great idea, but at the time these particles were proposed there was simply no experimental evidence for their existence. Gell-Mann was himself rather cagey about the status of his invention. He had argued that the quarks were somehow ‘confined' inside their larger hosts and, wishing to avoid getting bogged down in philosophical debates about the reality or otherwise of particles that could never be seen, he referred to them as ‘mathematical'.

But experiments carried out at SLAC in 1968 provided strong hints that the proton is indeed a composite particle, made up of even smaller, more elementary constituents. It was not clear that these constituents were necessarily quarks, and the experimental results suggested that, far from being held together tightly, they were actually rattling around inside the proton as though they were entirely free. How could this be squared with the notion of quark confinement?

This puzzle was cleared up in 1973 by Princeton theorists David Gross and Frank Wilczek, and independently by David Politzer at Harvard. When we imagine a force acting between two particles, we tend to think of examples such as gravity or electromagnetism, in which the force grows stronger as the particles move closer together. But the strong nuclear force doesn't behave this way. The force exhibits what is known as
asymptotic freedom.
In the asymptotic limit of zero separation between two quarks, the particles feel no force and are completely ‘free'. As the separation between them increases beyond the boundary of the proton or neutron, however, the strong force tightens its grip.

It is as if the quarks are fastened to each end of a piece of strong elastic. When they are close together, the elastic is loose. There is little or no force between them. But as we try to pull the quarks apart, we begin to stretch the elastic. The force increases the harder we pull.

Building on earlier work, Gell-Mann, German theorist Harald Fritzsch and Swiss theorist Heinrich Leutwyler now developed a quantum field theory of the strong nuclear force. In addition to the quark ‘flavours' — up, down, strange, etc. — they introduced a new variable which they called ‘colour'. Each quark can possess one of three different colour ‘charges' — red, green or blue.

Baryons are formed from three quarks each of a different colour, such that their total ‘colour charge' is zero and the resulting particle is
‘white'. For example, a proton may consist of a blue up quark, a red up quark and a green down quark. A neutron may consist of a blue up quark, a red down quark and a green down quark. The mesons, such as pions and kaons, consist of coloured quarks and their anti-coloured anti-quarks, such that the total colour charge is zero and the particles are also ‘white'.

In this model, quarks are bound together by a ‘colour force', carried by eight massless particles called gluons, which also carry colour charge and, like the quarks, are confined inside the hadrons. Gell-Mann called the theory
quantum chromodynamics,
or QCD.

In essence, this completes the standard model of particle physics. The model consists of three generations of matter particles, a collection of force particles and the Higgs boson (Figure 2). The interactions of these particles are described by a combination of electro-weak field theory and QCD. The electro-weak theory is itself a combination of weak force theory (sometimes referred to as quantum flavour dynamics, or QFD)
^*
and QED, their distinction forced as a result of the Higgs mechanism. We can therefore think of the standard model as the combination QCD × QFD × QED.

There is, at the time of writing, no observation or experimental result in particle physics that cannot be accommodated within this framework.
^**
It is not the end, however, as we will see.

The construction of mass

I have a heavy glass paperweight on the desk in front of me. Where, exactly, does the
mass
of this paperweight reside?

The paperweight is made of glass. It has a complex molecular structure consisting primarily of a network of silicon and oxygen atoms bonded together. Obviously, we can trace its mass to the protons and neutrons which account for 99 per cent of the mass of every silicon and oxygen atom in the structure. Not so very long ago, we might have stopped here, satisfied with our answer.