Baggott J. The Quantum Story: A History in 40 Moments

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the quantum story

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The violation is apparent, not real. In ‘sensing’ the speeds of the atoms

in the gas, the demon must have expended energy, leading to entropy

increases in some larger system. It can be argued that when the demon

and his measuring instruments are included, then the entropy of the total

system does indeed increase in accordance with the second law.

Van der Meer needed to do something rather similar. If it could be pos-

sible to ‘sense’ anti-proton energies distant from the desired beam energy

and somehow ‘nudge’ them back towards this energy, then the result would

be a concentration of anti-proton energies and an increase in the luminos-

ity of the beam. His 1968 experiments had hinted that this could be done.

He called it ‘stochastic cooling’. Pick-up electrodes positioned in one part

of the beam detect anti-protons whose energies deviate from the desired

beam energy. A signal is then sent to the other side of the beam storage

ring. The size of this signal is proportional to the extent of the deviation.

The signal is ampliﬁ ed and applied to a ‘kicker’ which applies an electric

ﬁ eld to the same section of the beam and deﬂ ects the offending particles

towards the centre of the beam orbit. By repeating this process many mil-

lions of times, the beam gradually converges on the desired beam energy.

The word stochastic simply means ‘random’, and the cooling refers

not to the temperature of the beam but to the random motions and the

energy spread of the particles contained within it.

In 1974 van der Meer carried out some tests of stochastic cooling using

the ISR. The results were not substantial, but they were sufﬁ cient to sug-

gest that the principle worked. Two years later a small storage ring was

converted and used for an Initial Cooling Experiment, which yielded

results in 1977 and 1978 that were greatly encouraging. It seemed that by

using the stochastic cooling technique it would be possible to create anti-

proton beams of sufﬁ cient luminosity to perform proton–anti-proton

collision experiments in the SPS. No new collider would be needed.

Proton and anti-proton beam energies of 270 GeV would combine

in the SPS to produce collisions with a total energy of 540 GeV, well in

excess of the energies required to reveal the W and Z particles.

Carlo Rubbia had begun to champion the proton–anti-proton collider

experiments at CERN in 1976. When the Initial Cooling Experiment was

shown to be successful in June 1978, Rubbia was given the go-ahead to

intermediate vector bosons

281

form a collaborative team of physicists to design the elaborate detector

facility that would be required to prove the existence of the W and Z

particles. As this was to be constructed in a large excavated area on the

SPS the collaboration was called Underground Area 1, or UA1. The team

would grow to include some 130 physicists.

This was still a long-shot, and the potential for disruption to the LEP

project remained. It was not an easy decision for the CERN Research

Board. But the Board decided to take a gamble. America had dominated

high-energy physics since the end of the Second World War, taking full

advantage of its lead during the slow recovery of post-war European

physics. American particle physicists had notched up a string of pres-

tigious discoveries. Although CERN physicists had been involved in

the discovery of weak neutral currents, new particles had so far eluded

them. Besides, Rubbia, notoriously difﬁ cult to work with,

would have

simply taken his proposal elsewhere if he hadn’t secured the go-ahead

from CERN. ‘Most likely, if CERN hadn’t bought Carlo [Rubbia]’s idea, he

would have sold it to Fermilab,’ Darriulat explained.

Six months after the decision a second, independent collaboration—

UA2—was formed under Darriulat’s leadership. This would be a smaller

collaboration, consisting of some 50 physicists, designed to provide

friendly competition with UA1. The UA2 detector facility would be less

elaborate (it would not be able to detect muons, for example), but would

nevertheless be able to provide independent corroboration of the UA1

ﬁ ndings.

Protons were ﬁ rst accelerated to 26 GeV in CERN’s proton synchrotron

and used to produce anti-protons from a stationary copper target.

The

anti-protons were then injected into a purpose-built anti-proton accu-

mulator (AA), one batch with energies around 3.4 GeV every few sec-

onds. The anti-protons were accumulated in the AA for a day or so,

and the stochastic cooling technique applied to converge the range of

Martinus Veltman wrote, of Rubbia: ‘When he was Director of CERN, he changed secretar-

ies at the rate of one every three weeks. This is less than the average survival time of a sailor on

a submarine or destroyer in World War II . . . ’. See Veltman, p. 74.

The CERN physicists had estimated that about a million protons hitting the target would be

required to produce two anti-protons, an estimate that proved to be a factor of two too high.

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anti-proton energies, before being re-injected into the proton synchro-

tron to be accelerated to 26 GeV.

The anti-protons were then passed to the SPS ring, where they joined

protons travelling in the opposite direction. The protons and anti-pro-

tons were then accelerated to 270 GeV, arranged in bunches each a few

nanoseconds (a few billionths of a second) in duration. The bunches of

protons and anti-protons would collide at six points around the ring. The

UA1 and UA2 detector facilities were positioned at two of these points.

Anti-protons were ﬁ rst injected into the AA in July 1980. In July 1981

the SPS accelerated the anti-protons to 270 GeV for the ﬁ rst time, and the

ﬁ rst collisions were registered. The UA1 and UA2 detector facilities were

designed to be rolled away from the ring to allow the SPS to be switched

back to operation as an accelerator with ﬁ xed targets, as the search for

the W and Z particles alternated with more conventional experiments.

The ﬁ rst opportunity to perform ‘full-blooded’ proton–anti-proton

collision experiments therefore came in the spring of 1982. But the UA1

detector facility was brought down by a contaminated compressed air

supply. There was no choice but to dismantle UA1 and clean its delicate

components, a task that would take many months.

As a consequence, two separate proton–anti-proton runs were merged

into a single run commencing in October 1982. UA1 and UA2 began log-

ging data. Not every collision was to be captured. It was anticipated that

collisions producing the W and Z particles would be very rare, so both

detector facilities were set so that they would respond only to selected

collisions meeting pre-programmed criteria. The collider would produce

several thousand collisions per second over a period of two months.

Only a handful of W- and Z-producing events were expected.

The detector facilities were programmed to identify events involving the

ejection of high-energy electrons or positrons at large angles to the beam

direction. Electrons carrying energies up to about half the mass of the W

would be the signature of the decay of W

−

particles. High-energy positrons

would likewise signal the decay of W

particles. Measured energy imbal-

ances (differences between the energies of the particles going into the colli-

sion versus those coming out) would signal the concomitant production of

anti-neutrinos and neutrinos, which could not be detected directly.

intermediate vector bosons

283

Preliminary results were presented at a workshop on the physics of

proton–anti-proton collisions in Rome in early January 1983. Rubbia,

uncharacteristically nervous, made the announcement. From the several

thousand million collisions that had been observed, UA1 had identiﬁ ed

six events that were candidates for W-particle decays. UA2 had identi-

ﬁ ed four candidates. Though somewhat tentative, Rubbia was convinced:

‘They look like Ws, they smell like Ws, they must be Ws.’ ‘His talk was

spectacular,’ wrote Lederman. ‘He had all the goods and the showman-

ship to display them with a passionate logic.’

On 20 January, CERN physicists packed into the auditorium to hear

two seminars delivered by Rubbia for UA1 and Luigi Di Lella for UA2. A

press conference was called on 25 January. The UA2 collaboration pre-

ferred to reserve judgement, but judgement was soon forthcoming. The

W particles had been found, with energies close to the predicted 80 GeV.

The UA1 collaboration published its results in the 24 February 1983 issue

of Physics Letters. The UA2 collaboration published its results in the same

journal less than a month later.

It had always been understood that the Z

would be a little more difﬁ cult

to hunt down. When the proton–anti-proton collider experiments were

resumed in April 1983, the CERN physicists pushed the SPS even harder.

At least the signature of the Z

decay would be easier to identify—elec-

tron–positron or muon–anti-muon pairs observed to carry never-before

seen energies.

The UA1 discovery of the Z

, with a mass around 95 GeV, was announced

on 1 June 1983 and published in Physics Letters on 7 July. This was based on

the observation of ﬁ ve events—four producing electron–positron pairs

and one producing a muon pair. The UA2 collaboration had accumu-

lated a few candidate events by this time but preferred to wait for results

from a further experimental run before going public. UA2 eventually

reported eight events producing electron–positron pairs. They published

their results in Physics Letters on 15 September 1983.

By the end of 1983, UA1 and UA2 between them had logged about a

hundred W

events and a dozen Z

events, with masses around 81 GeV

and 93 GeV, respectively.

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284

It had been a long journey, one that was, arguably, begun with Yang and

Mills’ seminal 1954 work on an SU(2) quantum ﬁ eld theory of the strong

force.

This was the theory which predicted massless bosons that had so

irked Pauli. In 1957 Schwinger had speculated that the weak nuclear force

is mediated by three ﬁ eld particles, and his student Glashow had subse-

quently reached for an SU(2) Yang–Mills ﬁ eld theory.

Much water had since been carried under the bridge. The massless

bosons had acquired mass through spontaneous symmetry-breaking

and the Higgs mechanism. The resulting theory had been shown to

be renormalizable. And now the intermediate vector bosons had been

found, precisely where they had been expected.

Rubbia and van der Meer shared the 1984 Nobel Prize for physics.

Actually, in 1938 Oskar Klein had suggested that the weak force might be mediated by ‘elec-

trically charged photons’.

285

The character of particle physics had changed. Many particle physicists would come to

regard the 1960s and 1970s as a golden age of their science. Back then, the commission-

ing of a new accelerator or collider was herald to the beginning of an exciting journey into

an unfamiliar and largely uncharted territory, populated by particles that were, at best,

only vaguely hinted at by the theorists. Particle discoveries were often surprising, if not

bewildering. They spoke of what we didn’t yet understand.

Now, it seemed, the task of the particle physicist was simply to conﬁ rm the existence

of particles that everybody knew must be there. The purpose of a new accelerator or col-

lider was to verify what physicists already believed to be true. This was now a journey into

increasingly familiar and charted territory. There were still surprises, still much that the

physicists didn’t yet understand, but the kinds of questions that were within the reach of

the experimentalists had to a large extent already been answered.

Flush with success from their search for the W and Z particles, CERN physicists

pushed to ﬁ nd the top quark. They failed, concluding that the mass of the top quark must

be greater than 41 GeV, almost ten times the mass of its third-generation partner, the bot-

tom quark.

By the early 1990s, competition between CERN and the Fermilab Tevatron, a

new proton–anti-proton collider capable of reaching collision energies of 1.8 TeV (almost

The Standard Model

Geneva, September 2003

Observations of Z

particle decays had revealed no evidence for creation of top–anti-top

pairs, suggesting that the mass of the top quark must be greater than a little less than half the

mass of the Z

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286

two thousand billion electron volts) had helped to push the lower mass limit of the top

quark to 77 GeV. And then on to 91 GeV, close to the mass of the Z

. CERN physicists

could not stretch the utility of their proton–anti-proton collider any further than a total

collision energy of 620 GeV. They dropped out of the race.

The discovery of the top quark was eventually announced at Fermilab on 2 March

1995, by two competing research teams each consisting of about 400 physicists.

Its mass

was an astonishing 175 GeV, equivalent to the mass of a rhenium nucleus and almost 40

times larger than the mass of the bottom quark. It was identiﬁ ed through its decay prod-

ucts. Energetic protons and anti-protons collide to produce a top–anti-top pair. Each of

these particles decays into a bottom quark and a W particle. One W particle decays into a

muon and a muon anti-neutrino. The other decays into an up and a down quark. The end

result is a collision which produces a muon, a muon anti-neutrino, and four quark jets.

Aside from the Higgs boson, the only particle that remained to be discovered was the

tau neutrino. Its discovery was announced at Fermilab ﬁ ve years later, on 20 July 2000.

With the discoveries of the top quark and the tau neutrino, the Standard Model was

virtually complete. Physicists faced the unprecedented situation that there were now no

experimental data that did not conform to the predictions of the quantum ﬁ eld theories

that formed the basis of the Standard Model.

From the very beginning of its history, the development of quantum physics had been

driven by disconcerting and inexplicable experimental results. Planck had experienced

the most strenuous work of his life forging a theoretical basis for his empirical radia-

tion formula. Stopped in a street in Copenhagen, Pauli had explained that the source

of his misery was the anomalous Zeeman effect. As the number and types of ‘elemen-

tary’ particles had exploded in the 1950s and 1960s, quantum ﬁ eld theorists occupied

an unfashionable backwater. At almost every stage, experiment had tended to outpace

theory, leaving the theorists scrambling for explanations.

Now that had all changed. There were now no disconcerting or inexplicable experi-

mental results. Theory had triumphed. And yet, it was also painfully apparent that we

were far from the end of physics.

The Standard Model comprises three types of Yang–Mills quantum

ﬁ eld theories in the gauge group SU(3) × SU(2) × U(1). It describes the

The ﬁ rst few pages of the papers reporting these results consist just of a long list of

names.

the standard model

287

interactions of three generations of matter particles through three kinds

of force, mediated by a collection of ﬁ eld particles or ‘force carriers’.

The everyday material substances with which we are most familiar

are composed of atoms. Atoms consist of a central nucleus of protons

and neutrons surrounded by ghost-like electron wave-particles. Protons

and neutrons are in their turn composed of up and down quarks. These

quarks, the electron, and electron neutrino are all spin-½ fermions and

together they form the ﬁ rst generation of matter particles in the Standard

Model. They are all that is needed to describe everything we can experi-

ence in the material world around us.

Each quark ﬂ avour (up, down) comes in three different colour vari-

eties—red, green, and blue. The quarks are bound inside protons and

Generation

Leptons

–

Quarks

Electromagnetic force

Weak nuclear force

Strong nuclear force

Matter ParticlesForce Particles

123

fig 19 The Standard Model of particle physics describes the interactions of three

generations of matter particles through three kinds of force, mediated by a collection

of ﬁ eld particles or ‘force carriers’.

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288

The neutrino mass in such a generation would have to be heavier than half the mass of

the Z

The number of neutrino species was determined to be 2.985±0.008. See Cashmore et al.,

p. 81.

neutrons by the strong colour force, which increases in strength as the

quarks are pulled apart. In consequence, the quarks are permanently

‘conﬁ ned’, forbidden from appearing without a chaperone. The colour

force is carried by coloured spin-1 gluons, which interact with the quarks

and with themselves.

The up and down quarks pair with anti-quarks to form the spin-0

pions. The positive pion, p

, is formed from the combination of an up

quark and an anti-down quark (ud

−

). The negative pion, p

−

, is formed

from a down quark and an anti-up quark (du

−

). The neutral pion, p

, is a

superposition of dd

−

and uu

−

combinations. These are relatively low-mass

particles (which is why they were discovered in the 1940s and 1950s)

and can be thought of as pseudo Nambu–Goldstone bosons. They are

responsible for mediating the interaction between protons and neutrons,

binding them together in the nucleus.

There are two further generations of matter particles, following the

pattern established by the ﬁ rst generation and differing only in the par-

ticle masses. The second generation consists of the strange and charm

quarks and the muon and muon-neutrino. The third generation consists

of the top and bottom quarks and the tau and tau-neutrino.

There are only three generations. Detailed studies of Z

decays using

the Large Electron–Positron (LEP) collider at CERN revealed that there can

be no more than three different kinds of neutrino. If there was a fourth, or

a ﬁ fth, further decay routes would be open to the Z

which would affect its

measured lifetime. These measurements do not rule out a radically differ-

ent generation of matter particles consisting of a very heavy neutrino,

but

there is no evidence for such a particle. It is therefore concluded that there

are just three neutrinos and, by inference, three particle generations.

The strong nuclear force acts on the quark colours. The weak nuclear

force, mediated by the spin-1 W and Z particles, acts on the quark ﬂ a-

vours. In beta radioactive decay, one of the more familiar manifestations

of the weak force, a down quark in a neutron is transformed into an up

the standard model

289

quark, turning the neutron into a proton, with the emission of a W

−

par-

ticle. The W

−

particle then decays into an electron and an anti-neutrino.

Mixing via weak-force interactions allows transitions between up and

strange quarks, and between down and charm quarks. This mixing is char-

acterized by another angle, known as the Cabibbo angle, named for Italian

physicist Nicola Cabibbo, with a measured value of about 13°. Further mixing

allows transitions between up and bottom, down and top, charm and bot-

tom, and strange and top quarks. This is a generalization of Cabibbo mixing,

called CKM mixing after Cabibbo and Japanese physicists Makoto Kobayashi

and Toshihide Maskawa. The CKM ‘matrix’ is characterized by three angles.

The measured mixing angle between ﬁ rst- and third-generation quarks is

about 0.2°. The measured angle for mixing second- and third-generation

quarks is about 2.4°. A fourth ‘angle’ reﬂ ects the relative phase of the coupling

between the quarks and is related to CP violation in weak-force decays.

Finally, the electromagnetic force, experienced between electrically

charged particles, is mediated by massless spin-1 gauge bosons. These

are the photons, ﬁ rst discovered by Planck in 1900 and championed by

Einstein in his ‘miracle year’, ﬁ ve years later.

Lurking somewhat mysteriously beneath this formalism is the Higgs

ﬁ eld, which pervades the vacuum and ﬁ lls the universe. Interactions

between massless particles and the Higgs ﬁ eld (or Higgs ‘condensate’)

endow the particles with mass. The amount of mass acquired reﬂ ects the

extent of the coupling between the particles and the ﬁ eld. The particle

of the Higgs ﬁ eld is the spin-0 Higgs boson, which has been elevated in

the Standard Model to the status of a ‘God particle’, responsible for the

masses of all the particles.

‘The Higgs particle itself has never been detected,’ wrote ‘t Hooft in

1995, ‘but its ﬁ e l d is being felt everywhere. If the Higgs were not there, our

model would have so much symmetry that all particles would look alike;

there would be too little differentiation.’

The Standard Model is a triumph of theoretical and experimental

physics. ‘t Hooft summarized the theory as follows:

It should be noted, however, that interaction with the Higgs ﬁ eld is not the only source of

what we recognize to be mass. In fact, 99 per cent of the mass of protons and neutrons is derived

from the energy of the gluon ﬁ elds that bind their constituent quarks together. In their turn,

protons and neutrons account for 99 per cent of the mass of every atom.