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Supersymmetry is a theory about childhood.
In early youth this child was, like most children,
bright, uncomplicated and impetuous.
In its world nothing was hidden,
convoluted or masquerading as something else.
But then it lost its innocence.
The harmony snapped,
and the scars of this trauma
settled permanently in the child¹s features
as it became cool and slugish,
Thus the universe grew up.
Theoretical physicists are fascinated
by this coming-of-age
because they believe it explains
the universe¹s current state.
They recon that the laws of physics,
now relatively complicated,
are the debris of much simpler rules
that held sway in the earliest times.
Over the past few years
they have sketched a rough picture of these primordial rules
and how they might have given rise to the present ones.
Experiments might soon confirm their theories,
by observing new fundamental particles called Osparticles¹.
Indeed, on the basis of results leaking out
of the workd¹s particle-physics laboratories,
some physicists suspect that sparticles,
or their effects, have already been seen.
Others think their colleagues are being over enthusiastic.
The mood is fractious.
The current set of laws,
known as the standard model,
already seem quite simple.
It says that everything is made
of tiny particles called fermions.
These exert forces on one another
by exchanging other particles called bosons,
like cricketers throwing balls back and forth.
That is a pretty economical theory.
But fermions come in many varieties.
And so do bosons and the forces they represent.
Two, the strong and weak nuclear forces,
act only over short distances; the others,
electromagnetism and gravity, are long range.
And if gravity were as powerful
as the strong nuclear force
(which holds nuclei together),
a mote of dust would have
the gravitational pulling power
currently enjoyed by the sun.
The standard model, though it predicts well how particles behave,
and tell you what each force does,
does not tell you how they are connected
(except in the case of the electromagnetic
and weak nuclear forces).
And connected, say physicists, they must be:
however variegated, these particles and forces
are part of the structure of the universe.
A key to finding the link lies in the idea of symmetry.
The standard-model laws of physics
are already symmetrical in lots of ways.
For instance, all four forces laws
have directional symmetry - they do not depend
on the direction that your laboratory is pointing in.
There are also more abstract sorts of symmetry.
For example, the electromagnetic force is charge symmetric:
a pair of positive charges exactly like a pair of negative ones
(the partner repel each other).
Swapping positive for negative
makes no difference to the outcome.
And electromagnetism itself
contains two symmetrical elements,
electricity and magnetism,
that are interwined and obey similar rules.
In summary, the more symmetry,
of whatever sort, a law has,
the more widely it applies.
So you might expect
some sort of grand symmetry
between the four forces,
if they have a common origin.
But none has been observed.
Different fermions respond to different forces
like pegs fitting with holes.
If swapped, they no longer fit.
However, in the early 1970¹s,
it was noticed that the four forces
do have something in common:
not a symmetry, but an asymmetry.
It is the one between fermions and bosons.
They can have similar properties,
such as mass and charge,
but they play very different roles.
So a new symmetry was proposed.
Each fermion, it was suggested,
has a hitherto unnoticed partner particle,
similar in most respects, but a boson.
Likewise, each boson has a corresponding fermion.
These siblings were named superpartner particles,
or sparticles, and their relationship, supersymmetry or SUSY.
This changed the picture entirely.
The standard model speaks of a disparate bunch of particles
with few apparent connections between them.
Adding the superpartners completed the set.
For physicists, their emergence was
like watching the monster rise out of Loch Ness
and realising that the separate humps
protruding from the surface
all belong to one creature.
Supersymmetry theory also irons out several wrinkles
in the cobbled-together standard model.
For one thing, standard model calculations
of how particle should behave give,
in some cases, infinitely large answers.
These have to be smoothed down
by mathematical sleight of hand.
Including superpartners in the sum
turns out to give infinities in the opposite direction
that neatly cancel out the first lot.
secondly, the standard model
requires yet another bit,
known as the Higgs mechanism,
to be tacked on to explain
why particles have the masses they do.
In supersymmetry theory,
most to the surprise
of its original proponents,
the Higgs mechanism is implicit.
The main attraction of the theory, however,
is that it links all the forces
with a common thread.
Which begs the question:
why do they seem different?
They were, if the theory is correct, once the same.
Just after the universe was born,
all four forces had equal strength.
Like electricity and magnetism
in the present universe,
they were symmetrical parts of a single force.
This was supersymmetry¹s fleeting moment of glory.
Then things cooled and the symmetry Obroke¹.
Imagine a marble in an open box with two compartments.
Shake the box energetically enough
and the marble can jump between them.
Reduce the energy and at some point
it will Ochoose¹ one side and stay there.
Its previous symmetry is broken.
The universe, likewise, is no longer
energetic enough to maintain supersymmetry.
Hence the billions spent on particle accelerators.
Inside these machines, physicists
help nature to recover its memory.
They collide fermions at high speed
(and therefore high energy and temperature),
recreating the conditions of the early universe.
In 1982, at the European particle-physics center,
CERN, one of the early symmetries was restored.
Enough energy was developed to reunify
the electromagnetic and weak-nuclear forces.
To do the same thing with supersymmetry
would be a trifle expensive.
It would require anaccelerator somewhere
between the size of the earth and that of the galaxy.
But symmetry-breaking should have left clues
at more attainable energies. These are the sparticles.
Today¹s accelerators might just be powerful enough
to provide a taste of supersymmetry
by creating the occasional sparticle.
The latest candidate has come from
the Fermilab accelerator in Batavia, Illinois.
A two-year run of experiments has produced
a single strange event that looks like
the creation of a sparticle - the ¹selectron¹,
partner to the electron - and its antiparticle.
The researchers led by Henry Frisch
of the University of Chicago,
were reluctant to publish.
But some theorists have heard, and like it.
Gordon Kane, at the University of Michigan,
argues that it is, if real,
the first direct evidence of supersymmetry.
It would also, he says, fit
with some better confirmed indirect evidence.
For six years CERN¹s Large Electron-Positron Collider (LEP)
and the Stanford Linear Collider in California
have been producing millions of bosons
known as the Z particle.
zs are unstable, and break down
into various combinations of smaller particles.
One combination, the bottom quark and antiquark,
has been turning up a little more often
than the standard model says it should.
Dr. Kane reckons that is due to zs
interacting with Ovirtual¹ sparticles
-ephemeral versions of the real thing
that quantum theory predicts
should appear briefly from the vacuum
and then vanish.
It is a tempting idea.
Supersymetry does predict that Fermilab
would produce, on average,
one pair of sparticles every two years.
On the other hand,
one pair does not a confirmation make.
OOne event¹ says Jorge López
of Rice University in Houston, Texas,
Ois statistically the same as zero¹.
(Una golondrina no hace verano)
He thinks it could as easily be
a glitch in the detector, or perhaps a rare blip
that is in fact allowed by the standard model.
As for the extra bottom quarks, he says,
supersymmetry alone cannot account for them all.
More power is on the way.
Next month LEP will start
a high-energy run that might turn up
another couple of sparticles.
In 1998 it is being upgraded,
as is Fermilab the following year.
Their results may support Dr. Kane.
Or they may hang physicists
back on their tenterhooks.
Physicists like Dr. Kane want to believe
sparticles have been found,
because they want to believe in supersymmetry,
a far neater theory than the standard model.
Besides, confirming supersymmetry
might have wider cosmological implications.
There is one sparticle that, if stable
(and the Fermilab event, if genuine implies that it is)
could make up the Ocold dark (and therefore invisible) matter¹
that cosmologists need to account for the universe¹sshape.
And supersymetry also leads towards
an even more general Otheory of everything¹
- superstring theory - that is now in revival
after several years of stagnation.
Theories of everything have little or no practical application.
There is no need to know what the fundamental particles
in an aeroplane are doing to work out how it will fly.
But physicists are convinced that a universe
born from a single point must be ruled by a single law,
albeit one with many consequences.
This law would explain its own breakdown,
and the universe¹s transition from a small, unified,
but boring fireball to its present interesting state.
Dr. Kane feels this would be
Oa truly primary theory,
the only possible theory that is self-consistent.¹
It would be the most successful piece
of child psychology ever done.
