Superconducting cavities for the LEP accelerator
The ALEPH detector at LEP
The L3 detector at LEP
The OPAL detector at LEP
The DELPHI detector at LEP
Particle tracks in an old-style Bubble Chamber
Zeptoscale Physics: Particle Physics in Plain English.|
High Energy Physics research is about matter, force, space, and
time at the most fundamental level. It
takes you on a trip down through the atom, inside the
atomic nucleus, to the quantum world of subatomic particles and
elementary forces. Or it can take you out beyond the Solar System,
to Supernova explosions, gamma ray bursts, and supermassive
black holes, even to
cosmological distance scales of order the size of the known Universe;
it can go off to higher-dimensional worlds of curled-up extra spatial
dimensions; or it can be a journey in time, back to the Big Bang or to
the far future;
regardless of the direction, it is always, to me, a voyage of challenge and of
wonder. It is also one of evolution, as new discoveries push back the edge of
knowledge, and as new speculations form on what migth lie behind it.
Wilson Hall at Fermilab.
Over the last century, in the wake of Einstein's theory of relativity and
Bohr's quantum mechanics, came sea changes to our perception of almost all
fundamental aspects of Nature. With the new fundamental discoveries came
also a truly enormous number of successively more advanced
applications that were to shape our future everyday lives, like
semiconductors, electronics, lasers, computers, optical fibers,
superconductivity, and so on.
On the practical side, all that is required to
explore Nature at its most extreme, is a perception tuned to
very small wavelengths (so we can see really tiny things). In
physics, this is equivalent to using probes of extremely high energies. The
proper equipment for the expedition thus normally consists of, firstly,
the vehicle, a good-sized particle accelerator,
secondly, the engine,
an amount of electricity that otherwise might keep a small city comfortable,
and, thirdly, the eye, a hi-tech wonder composed of gas,
steel, lead, cryogenics, computer farms, and electronics, the detector.
The CDF Detector at
equipped, the tiny world of elementary particles, beginning at
length scales below a millionth of a nanometre, may be opened to human
investigation. (Theorists, of course, will undertake the same expedition
without all the equipment and are therefore much cheaper, though somewhat
Today, particle accelerators are capable of
reaching energy scales several billion times larger than those of
ordinary chemical reactions. The highest energies in the world are
currently achieved at the Tevatron accelerator at
Fermilab, which collides protons with antiprotons at energies
thousands of billions times larger than the energy released by the
combustion of an octane molecule in a car engine.
At these energies, the ordinary phases of matter (solids, liquids, and
gasses) are no longer meaningful; we are in a domain where the effects of
relativity and quantum mechanics are dominating; matter and antimatter
can be created and annihilated from the vacuum, and particles are not
localised but are described by fuzzy quantum wave functions, or
The DØ Detector at Fermilab
In fact, this fundamental `fuzziness' of Nature is one of the most striking
aspects of quantum physics. Observing the light from a faraway star,
for instance, sensitive particles in our eyes are being gently swayed
from side to side by small waves in the electromagnetic field,
the crests of which are a few hundred nanometres apart.
Due to the fuzziness of the light quanta, visible light
could never tell us about structures smaller than that. Again, that is why
`particle physics' and `high energy physics' are synonymous; to resolve
things that are, at the largest, 100 million times smaller than the distance
between two crests of a wave of visible light, we need probes of extremely
short wavelengths. That is precisely what is obtained by accelerating matter
to high energies. The higher the energy, the better our 'vision' in
this 'fuzzy' quantum land.
Returning to antimatter, the discovery of the anti-electron, or
positron, dates as far back as 1932. Today, antipartners have been
found for all of the known matter particles. These antiparticles
have exactly the same masses as their particle
counterparts, but they carry diametrically opposite charges. The
billion dollar question, of course, is why?
In the quantum theory of fields, this phenomenon found
a very elegant and natural
explanation. Matter and antimatter appear simply as two different states of
the same field; so you can't have one without the other. A more
intuitive way of looking at antimatter is that
antimatter in fact acts just like matter travelling backwards
through time. Think about it.
The discovery of antimatter: positron track in a cloud chamber.
[C.D. Anderson, Phys. Rev. 43, 491 (1933).]
Matter and force are also intimately related - by symmetries. Among the
true success stories of 20th century theoretical physics was the
discovery of 'gauge forces'. These are forces that have no
existence 'by themselves'. Instead, they arise, naturally and
elegantly, almost as if by magic,
as simple consequences of the symmetry properties of matter.
These symmetries, called gauge symmetries, are not spatial
symmetries, rather they are symmetries of the quantum charges carried
by the matter fields, like the phase invariance of small electric
quark event recorded by the DØ detector at Fermilab
My own principal interest lies with a very colourful
but quite complicated force of nature, the so-called Strong Force. As
it is believed to arise as a consequence of symmetry properties of the
matter quanta, here called quarks (eg nuclear matter, like protons and
neutrons, is made up of quarks). However, the symmetry is not quite as
that governing the familiar electromagnetic force. Instead of
just having positive and negative charge, there exist 3 different kinds of
charge for the strong force,
with corresponding anticharges.
These 3 different kinds of "strong
charge" are suggestively labelled red, green, and blue
colours (and anticolours: cyan, magenta, and yellow,
respectively), since they mix with each other in a way
similar to that of ordinary coloured light. Moreover, the force
quanta, called gluons,
themselves carry strong charge (and even more of it than the
quarks!) --- a fact which
causes the strong force to give us considerably larger headaches than simple
The most immediate consequence may be described as
anti-screening. Drop a coloured particle in vacuum, and fix it in
place. Then watch what happens as you move away from it,
carrying an anticharge with you. Anti-screening means that
the colour charge will seem to increase steadily as you withdraw (more
precisely, as the resolution scale is decreased), causing a stronger
and stronger attraction. In fact, you won't reach many
before the colour field between the two charges narrows into
a string-like flux tube of high energy density that pulls at
you in a tight, elastic-like manner.
That can't go on forever. If you are not very strong, you'll give up
and let the charges stay together. If you are more determined and keep
pulling harder and harder, at some point your continued
exertions will have pumped enough energy into the field to set the vacuum
boiling! Drawing on the colour field energy, virtual quark-antiquark
pairs living as quantum fluctuations in the vacuum may make the
transition to become real
in a manner that screens and severs the connection between the
two original charges. This mechanism effectively confines colour
charges to the realm of sub-femtometre distances, since any attempt
to separate charges by more than that will immediately be neutralised
by the creation of compensating charges from the vacuum. Hence quarks
and gluons are hadronized at ordinary energies, ie they are
locked up inside colour neutral composite particles, like the proton
and neutron, collectively called hadrons.
This process of hadronization is extremely hard to calculate. In fact,
it is so hard it has not even been rigorously proven mathematically
to exist in the theory.
Some of my work has focussed on how hadronization occurs in certain
interesting topologies of the colour field, for instance the field you
would get if you could put a stick of dynamite inside a proton, blow
it up, and watch how the fragments subsequently hadronizes into
blobs of hadronic matter, mostly pions, kaons, and nucleons.
In fact, it is something along these lines
that occurs inside the collision halls at the Tevatron accelerator
right here at Fermilab as you read this, several times every microsecond.
The Tevatron and Main Injector at Fermilab
Such collisions also easily involve hundreds of (calculable)
interfering subprocesses, in addition to the goings-on just
mentioned. Highly complicated physics models and extensive
computer algorithms are often needed to make the calculations
tractable. This work, which also takes a lot of my time, has the added bonus
that it not only allows us to study theoretical predictions in detail,
it also provides high-accuracy computer simulations of particle
collisions which can be used by experimenters for detector design and
performance studies, and for calibration of the final experiment.
Finally, who knows what may be discovered tomorrow? From the
experimental point of view, the search is on for anything that
might not agree with the predictions of the so-called Standard Model
of Particle Physics, which incorporates the state of the art of our
present day knowledge. Scores of
measurements are continually being carried out, testing one or another facet of
the Standard Model. Meanwhile, within the limits permitted by existing data,
The Large Hadron Collider at CERN, Geneva, scheduled to turn on in
Despite its many successes the Standard Model is not likely to be `the end of
the line'; there are missing pieces and internal inconsistencies in the
model, which point towards the existence of hitherto undiscovered properties
of Nature, some of them may be just around the corner!
Naturally, it is exciting to speculate on what these properties
might be, and how best to search for them.
The game is basically to come up with an alternative theory that
- agrees with present data,
- `solves' one or more problems in the
Standard Model, preferably without introducing new ones,
- gives predictions
which are testable at one or more future experiments,
- is the one for
which evidence is found at those experiments, this last condition being the
one required to go to Stockholm.
The LEP tunnel, where the
LHC is currently being
At the most general level, the open-minded model-builder
has several starting options available.
It is possible, for instance, that there could exist new kinds of fundamental
matter. Some possibilities are the existence of something called a
4th generation of
fermions, and models with more Higgs bosons (or other -ons). It is also
possible that what we think of as being fundamental matter, is
not. Ideas along these lines are compositeness (substructure in quarks and
leptons), technicolour (compositeness of the Higgs boson), and even string
theory (where point-like particles are replaced by strings or even
There could also exist a `fifth force' in Nature, i.e. a new gauge
symmetry. Familiar examples are here models with Z' bosons, so-called
Left-Right symmetric models, and also technicolour models. The
known forces might also turn out all to be but different aspects of one
'original' force, as in Grand Unified Theories. And
finally, our concepts of space--time are not inviolate either. There could
exist a new kind of space--time symmetry (called supersymmetry).
Or relativistic (Lorentz) invariance could be violated to some small
when thinking of space and time it is natural to ask why the
universe appears to have exactly one time and three space dimensions.
The last decade has seen an explosion of activity on this
question, with the emergence of many new theories incorporating one or
several extra dimensions, which are then curled up in more or less
I have focussed mainly on the consequences
of a hypothetical new type of space-time
symmetry called supersymmetry, or SUSY for short. If this symmetry really
is a property of Nature, there should exist a large variety
of hitherto unseen exotic particles, which should be
detectable in experiments at forthcoming particle accelerators.
Conversely, if the experiments at some point do reveal that such particles
exist in Nature, we may have discovered a completely new type of symmetry
of space and time!
LHC experiments: ATLAS (left) and CMS (right).
Another topic which has caught my interest in the past
is what happens when you 'invert' space (sort of like turning it
inside out by performing a point reflection through the origin)
and simultanously exchange matter by antimatter.
This 'parity-plus-charge-conjugation' operation, called CP for short,
is almost a symmetry of the theory of the natural forces, but not
quite. For instance, violation of CP symmetry
shows up in decays of heavy b (beauty)
quarks living inside bound states, so-called B mesons.
One of the ways
of exploring physics beyond the Standard Model, is by observing the
magnitude of this violation,
and checking whether the Standard Model alone is capable
of explaining all the data, or whether 'something else' is
required. This 'something else' might well arise if, for instance, the
Universe is supersymmetric.
To end on a philosophical note, it is worth thinking about the fact
that the matter-antimatter
asymmetry is actually a very important aspect of Nature to us,
since we, and all the stars and galaxies that we can see,
are made of matter, not antimatter.
If the laws of Nature were exactly matter-antimatter symmetric,
for instance, it is difficult to see how we could have come to exist at all.