YOU might think that nothing would be allowed to spoil the birthday party of
a centenarian. But alas for the poor electron鈥攚hich was discovered this
month in 1897鈥攖he party poopers are already at the door. The question
they鈥檙e asking is whether the electron is truly fundamental.
After 99 years of probing, the electron still seemed to be a fundamental
building block of matter鈥攖hat is, it behaved like a point with no internal
structure. However, particle physicists have long had an uneasy feeling that the
electron is more than it seems; that it may be made up of other particles. Just
in the past few weeks, this suspicion has been heightened by a mysterious
series of high-energy collisions in a particle accelerator in Hamburg,
Germany.
It鈥檚 possible that these collisions could just be chance events. But if they
are not, the electron could be toppled from its pedestal, and physicists forced
to totally rethink their view of the subatomic world. 鈥淭his could certainly be
the most exciting development in particle physics in the last twenty years,鈥�
says Roger Cashmore, professor of experimental physics at the University of
Oxford.
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So why do scientists doubt the electron鈥檚 fundamental nature? The story goes
back to earlier this century. The electron was joined in 1919 by the proton and
13 years after that by the neutron to make up an elite band of subatomic
particles.
But then in 1937, researchers studying cosmic rays discovered the muon. This
is a particle with a mass 210 times that of the electron, but apparently just
like it in every other respect. Its discovery upset theorists鈥� tidy view of the
atomic world. 鈥淲ho ordered that?鈥� asked the Nobel prize-winning physicist Isidor
Rabi of the muon. That question was given an extra resonance in the 1970s, with
the discovery of a third, still heavier electron-like particle, the tau, which
weighs in at roughly 3500 times the electron鈥檚 mass.
Why nature鈥檚 basic inventory should include three varieties of electron is
still one of the leading questions challenging today鈥檚 particle physicists. In
fact the question is broader than this. Today, the list of all fundamental
particles can be split into three 鈥済enerations鈥�.
The first generation contains the electron, the electron neutrino and the two
most common quarks, known as down and up. On the face of it, this generation is
enough to build all the everyday stuff of the Universe. Down and up, for
example, are the particles that make up the proton, the neutron and a host of
short-lived particles. Yet, there are still two more generations. There are
another two neutrinos to go with the muon and the tau, and two more pairs of
quarks鈥攕trange and charm, and bottom and top (see
Diagram).
These particles can also be divided another way according to their
properties. The chargeless neutrinos and the electron, muon and tau, which each
carry the same charge, denoted by e, are known collectively as leptons.
The remaining particles, the quarks, have charges of either +2e/3 or
鈭抏/3. The main difference between these two groups is that the quarks
interact via the strong nuclear force, which keeps them bound within protons,
neutrons and other particles. The leptons, by contrast, do not feel this
force.
Now 12 particles (or 24 if you count their antiparticles as well) seems a
large number of fundamental objects, and for many years physicists have asked
whether this rich pattern of particles reflects a more fundamental, underlying
structure. Elsewhere, when nature exhibits such ordered patterns, the variety
has proved to stem from some deeper structure with the ability to arrange itself
in many ways. The 92 natural elements, for example, are built from just
electrons, protons and neutrons, while the many strongly-interacting objects
found in particle accelerators can all be understood in terms of the six
quarks.
So are the quarks and leptons manifestations of a simpler, common set of more
basic building blocks? One reason for suspecting that this may be so is the
relationship between their electric charges. Matter is electrically neutral
because of the balance between the negative charge of the electrons in atoms and
the positive charge of the protons in the atomic nuclei. Yet the protons are
built from quarks, two with a charge +2e/3, and one with charge 鈭�
e/3.
Appealing particles
These fractions must be exact: if they were only slightly different, matter
in bulk would be electrically charged and the forces upon it would far exceed
the pull of gravity. Yet it is difficult to imagine how a fraction of a charge
could come about. Much more appealing is the idea that electric charge is in
fact carried in units of e/3 by particles that are constituents of both
leptons and quarks.
The first serious attempt to build a composite model for quarks and leptons
dates back to 1974, and the work of the recently deceased Nobel prizewinner
Abdus Salam and physicist Jogesh Pati of the University of Maryland. Their idea
was to 鈥渂uild鈥� the quarks and leptons from combinations of a smaller number of
hypothetical 鈥減reons鈥� with different properties. Some had electric charge, some
had the colour charge, which underlies the strong force (鈥淲orld of Quarks鈥�,
快猫短视频, Inside Science, 10 July 1993), and some had a property that
determined the generation to which the composite quark or lepton would belong.
But there were problems from the start in building the quarks and leptons with
the correct electric charges. Moreover, the model predicted the existence of
many particles that did not correspond to any known particle.
Since then, there have been many attempts at composite models, but they all
run into difficulties. 鈥淚 have never come across a composite model that does not
become more complicated than the system it sets out to explain,鈥� says Graham
Ross, head of theoretical physics at the University of Oxford. But, he adds,
鈥渢here is a no no-go theorem鈥�. In other words, there is no reason to think that
all composite models are automatically doomed to fail.
So is there any physical evidence that quarks and leptons are made of
something smaller? To see finer and finer detail with light, you simply turn up
the magnification on a microscope. In particle physics, it鈥檚 a matter of turning
up the energy. The higher the energy with which two particles collide, the
closer they get to one another and the more likely it is that their
constituents鈥攊f they have any鈥攚ill interact and produce new
particles.
Close encounters
Ernest Rutherford turned up the energy in 1909 and discovered the atomic
nucleus, with a diameter of around 10-14 metres. Some 70 years later even
higher energy collisions revealed quarks inside the proton at a scale of about
10-16 metres. Today鈥檚 particle colliders are reaching distances down to 10-18
metres, but the quarks and leptons have remained resolutely fundamental.
Once you can bring particles very close together, it also becomes possible to
test the idea that quarks and leptons are composite particles by searching for
new interactions between those constituents. In the theory that describes the
interaction of quarks and leptons, the Standard Model, particles interact by
exchanging force-carrying particles called gauge bosons. These include the
gluons, which transmit the strong force, W and Z bosons for the weak nuclear
force, and photons for the electromagnetic force. The range of each force is
determined by the mass of its boson鈥攖he larger the mass, the shorter the
range over which it acts and so the closer together two particles must be
brought before the effects of that boson can be seen.
The discovery of an interaction between the constituents of quarks and/or
leptons would have a profound impact on physics because it would have to involve
an entirely new force that acted over a very short range. If it acted over
larger distances, it would already have been seen in experiments. The fact that
this force has not been seen also means that the boson associated with it must
be heavier than the W and Z bosons, which are themselves a hundred times heavier
than the proton.
In fact, by comparing the behaviour of particles in experiments with models
that predict a short-range force, physicists can set a maximum limit on the
range of that force. This, in turn, gives an idea of the distance between any
constituents of the electron. The best limits come from the powerful Large
Electron Positron collider at CERN, the European centre for particle physics in
Geneva. Results from the LEP show that the mass of the boson for any new
interaction within the electron must be greater than around 1.6
teraelectronvolts, or nearly 2000 proton masses. In other words, the boson would
act over a distance of just 1.2 脳 10-19 metres. So if the electron is not a
point, but is made up of other particles, it must be smaller than this.
Reshuffle
Another way that the electron鈥檚 underlying structure could manifest itself is
through the formation of new particles. These could arise from a reshuffling of
the constituents in high-energy collisions. The new particles could, for
example, be excited states of leptons or quarks. These would contain the same
constituents as the known leptons and quarks, but the contents would be moving
around at higher energies, giving the effect of a larger mass.
If this were to happen, the new particles would exist for only a very short
time. The excited states would almost immediately revert to a more normal
configuration with lower mass. The excess energy would be carried away, perhaps,
by emitting a photon. Experiments at the Hadron Electron Ring Accelerator (HERA)
at the DESY laboratory in Hamburg, indicate that if excited electrons exist then
they must have masses greater than about 85 gigaelectronvolts (about 90 proton
masses), otherwise we would have seen them already.
The constituents of leptons and quarks could also combine in unexpected ways
to form new particles. This is one way to produce hypothetical, hybrid particles
known as leptoquarks. In the Standard Model, quarks and leptons interact only
via the weak and electromagnetic forces so the discovery of a leptoquark would
once again demand the existence of a new nuclear force.
Leptoquarks could be composite particles, consisting of, for example, a quark
bound with an electron or other lepton, or they might be structureless
particles. In the latter case, the leptoquark would be created at the expense of
an interacting quark and electron in much the same way that a photon is created
when a particle and an antiparticle annihilate each other.
Whatever form the leptoquark takes, it would live only briefly before
decaying rapidly back to the electron and quark from which it was created. But
though brief, this existence would be enough to hint at theories that link
quarks and leptons more intimately鈥攊n particular the grand unified
theories that attempt to unite the strong, weak and electromagnetic forces as a
single force at high energy.
Interest in all these exotic possibilities has increased enormously in recent
weeks following the intriguing events at HERA. Researchers there have been
colliding positrons (antielectrons) with protons and studying the debris. Among
the events they have recorded are some very high-energy positron-quark
collisions, which are turning up at a rate that is several times that predicted
by the Standard Model. So speculation abounds that this could be the first clue
to the existence of some new interaction or particle along the lines of
leptoquarks.
For Cashmore, who leads the British contingent working on ZEUS, one of the
two experiments at HERA, these collisions are potentially revolutionary. But his
view is tempered with caution. 鈥淲e do have to remember that we only have a
handful of events,鈥� he says. Proving the existence of a new transitory particle
is all down to statistics, and 鈥渟tatistics can be unkind鈥�, says Cashmore. Until
the number of mysterious collisions increases, physicists at HERA will not know
for sure what they are dealing with.
One type of elementary particle which could be produced by the annihilation
of a positron and a quark is a new kind of quark called a squark. This beast has
been predicted by particular kinds of supersymmetric theories which propose that
every known particle has a so-far unseen 鈥渟uperpartner鈥�. This new symmetry, or
supersymmetry, neatly ties up some of the loose ends that exist in the Standard
Model鈥攁nd also represents a step closer to the grand unified theories.
What is really happening at HERA has yet to be seen. Last month the machine
was started up again and it will run until October. 鈥淒oubling the statistics
this year will tell us if we have indeed found something new,鈥� says Ralph
Eichler of the Swiss Federal Institute of Technology in Zurich and spokesman for
the other experiment at HERA, known as H1. 鈥淲e will be tuning our detector for
maximum efficiency in order to observe every bit of these rare events.鈥� These
investigations certainly hold the promise of being the most exciting glimpse for
years into the fundamental nature of the electron and the forces that govern its
behaviour.