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the quantum story
280
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
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,
7
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
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.
8
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
7
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.
8
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.
the quantum story
282
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
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-
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
0
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
0
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
0
, 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 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
0
events, with masses around 81 GeV
and 93 GeV, respectively.
the quantum story
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.
9
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.
9
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.
1
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
29
The Standard Model
Geneva, September 2003
1
Observations of Z
0
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
0
.
the quantum story
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
0
. 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.
2
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
eld theories in the gauge group SU(3) × SU(2) × U(1). It describes the
2
The 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
e
u
r
d
r
c
r
s
r
t
r
b
r
u
g
d
g
c
g
s
g
t
g
b
g
u
b
W
+
g
rg
-
g
r
-
g
g
rb
-
g
r
-
b
g
bg
-
g
b
-
g
g
d1
g
d2
W
Z
0
d
b
c
b
s
b
t
b
b
b
ν
e
γ
ν
μ
ν
τ
μ
τ
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’.
the quantum story
288
3
The neutrino mass in such a generation would have to be heavier than half the mass of
the Z
0
.
4
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
0
, 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
0
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
0
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,
3
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.
4
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
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.
5
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:
5
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.