by Anil Ananthaswamy
11 November 2009
Magazine issue 2734
is a consulting
editor for New Scientist.
AS DAMP squibs go, it was quite a
spectacular one. Amid great pomp and ceremony - not to mention dark
offstage rumblings that the end of the world was nigh - the Large
Hadron Collider (LHC), the world's mightiest particle smasher,
fired up in September last year. Nine days later a short circuit and
a catastrophic leak of liquid helium ignominiously shut the machine
Now for take two. Any day now, if all goes to plan, proton beams
will start racing all the way round the ring deep beneath CERN, the
LHC's home on the outskirts of Geneva, Switzerland.
Nobel laureate Steven Weinberg is worried. It's not that he
thinks the LHC will create a black hole that will engulf the planet,
or even that the restart will end in a technical debacle like last
No: he's actually worried that the LHC
will find what some call the "God particle", the popular and
embarrassingly grandiose moniker for the hitherto undetected
"I'm terrified," he says.
"Discovering just the Higgs would really be a crisis."
Evidence for the Higgs would be the
capstone of an edifice that particle physicists have been building
for half a century - the phenomenally successful theory known simply
as the standard model. It describes all known particles, as
well as three of the four forces that act on them: electromagnetism
and the weak and strong nuclear forces.
depicts the decay of a Higgs particle following a collision of two
protons in the CMS experiment
It is also manifestly incomplete. We
know from what the theory doesn't explain that it must be just part
of something much bigger. So if the LHC finds the Higgs and nothing
but the Higgs, the standard model will be sewn up. But then particle
physics will be at a dead end, with no clues where to turn next.
Hence Weinberg's fears. However, if the theorists are right, before
it ever finds the Higgs, the LHC will see the first outline of
something far bigger: the grand, overarching theory known as
SUSY (supersymmetry), as
it is endearingly called, is a daring theory that doubles the number
of particles needed to explain the world.
And it could be just what particle
physicists need to set them on the path to fresh enlightenment.
So what's so wrong with the standard model? First off, there are
some obvious sins of omission. It has nothing whatsoever to say
about the fourth fundamental force of nature, gravity, and it
is also silent on the nature of dark matter. Dark matter is
no trivial matter: if our interpretation of certain astronomical
observations is correct, the stuff outweighs conventional matter in
the cosmos by more than 4 to 1.
Ironically enough, though, the real trouble begins with the Higgs.
The Higgs came about to solve a truly
massive problem: the fact that the basic building blocks of ordinary
matter (things such as electrons and quarks, collectively known as
fermions) and the particles that carry forces (collectively called
bosons) all have a property we call mass. Theories could see no
rhyme or reason in particles' masses and could not predict them;
they had to be measured in experiments and added into the theory by
These "free parameters" were embarrassing loose threads in the
theories that were being woven together to form what eventually
became the standard model. In 1964, Peter Higgs of the
University of Edinburgh, UK, and François Englert and
Robert Brout of the Free University of Brussels (ULB) in Belgium
independently hit upon a way to tie them up.
That mechanism was an unseen quantum field that suffuses the entire
cosmos. Later dubbed the Higgs field, it imparts mass to all
particles. The mass an elementary particle such as an electron or
quark acquires depends on the strength of its interactions with the
Higgs field, whose "quanta" are Higgs bosons.
Fields like this are key to the standard model as they describe how
the electromagnetic and the weak and strong nuclear forces act on
particles through the exchange of various bosons - the
W and Z particles, gluons and
photons. But the Higgs theory, though elegant, comes with a nasty
sting in its tail: what is the mass of the Higgs itself?
It should consist of a core mass plus
contributions from its interactions with all the other elementary
particles. When you tot up those contributions, the Higgs mass
balloons out of control.
The experimental clues we already have suggest that the Higgs's mass
should lie somewhere between 114 and 180 gigaelectronvolts - between
120 and 190 times the mass of a proton or neutron, and easily the
sort of energy the LHC can reach. Theory, however, comes up with
values 17 or 18 orders of magnitude greater - a catastrophic
discrepancy dubbed "the hierarchy problem".
The only way to get rid of it in
the standard model is to fine-tune
certain parameters with an accuracy of 1 part in 1034, something
that physicists find unnatural and abhorrent.
Three into one
The hierarchy problem is not the only defect in the standard model.
There is also the problem of how to
reunite all the forces. In today's universe, the three forces dealt
with by the standard model have very different strengths and ranges.
At a subatomic level, the strong force is the strongest, the weak
the weakest and the electromagnetic force somewhere in between.
Towards the end of the 1960s, though, Weinberg, then at Harvard
University, showed with Abdus Salam and Sheldon Glashow
that this hadn't always been the case. At the kind of high energies
prevalent in the early universe, the weak and electromagnetic forces
have one and the same strength; in fact they unify into one force.
The expectation was that if you extrapolated back far enough towards
the big bang, the strong force would also succumb, and be unified
with the electromagnetic and weak force in one single super-force
In 1974 Weinberg and his colleagues Helen Quinn and Howard
Georgi showed that the standard model could indeed make that
happen - but only approximately. Hailed initially as a great
success, this not-so-exact reunification soon began to bug
physicists working on "grand unified theories" of nature's
It was around this time that supersymmetry made its appearance,
debuting in the work of Soviet physicists Yuri Golfand and
Evgeny Likhtman that never quite made it to the west. It was
left to Julius Wess of Karlsruhe University in Germany and
Bruno Zumino of the University of California, Berkeley, to bring
its radical prescriptions to wider attention a few years later.
Wess and Zumino were trying to apply physicists' favorite
simplifying principle, symmetry, to the zoo of subatomic particles.
Their aim was to show that the division of the particle domain into
fermions and bosons is the result of a lost symmetry that existed in
the early universe.
According to supersymmetry, each
fermion is paired with a more
massive supersymmetric boson, and each boson with a fermionic
super-sibling. For example, the electron has the selectron (a boson)
as its supersymmetric partner, while the photon is partnered with
the photino (a fermion). In essence, the particles we know now are
merely the runts of a litter double the size (see diagram).
The key to the theory is that in the high-energy soup of the early
universe, particles and their super-partners were indistinguishable.
Each pair co-existed as single massless entities. As the universe
expanded and cooled, though, this supersymmetry broke down. Partners
and super-partners went their separate ways, becoming individual
particles with a distinctive mass all their own.
Supersymmetry was a bold idea, but one with seemingly little to
commend it other than its appeal to the symmetry fetishists. Until,
that is, you apply it to the hierarchy problem. It turned out that
supersymmetry could tame all the pesky contributions from the
Higgs's interactions with elementary particles, the ones that cause
its mass to run out of control.
They are simply cancelled out by
contributions from their supersymmetric partners.
"Supersymmetry makes the
cancellation very natural," says Nathan Seiberg of the Institute
of Advanced Studies, Princeton.
That wasn't all. In 1981 Georgi,
together with Savas Dimopoulos of Stanford University, redid
the force reunification calculations that he had done with Weinberg
and Quinn, but with supersymmetry added to the mix.
They found that the curves representing
the strengths of all three forces could be made to come together
with stunning accuracy in the early universe.
"If you have two curves, it's not
surprising that they intersect somewhere," says Weinberg. "But
if you have three curves that intersect at the same point, then
that's not trivial."
This second strike for supersymmetry was
enough to convert many physicists into true believers. But it was
when they began studying some of the questions raised by the new
theory that things became really interesting.
One pressing question concerned the present-day whereabouts of
supersymmetric particles. Electrons, photons and the like are all
around us, but of selectrons and photinos there is no sign, either
in nature or in any high-energy accelerator experiments so far. If
such particles exist, they must be extremely massive indeed,
requiring huge amounts of energy to fabricate.
Such huge particles would long since have decayed into a residue of
the lightest, stable supersymmetric particles, dubbed neutralinos.
Still massive, the neutralino has no electric charge and interacts
with normal matter extremely timorously by means of the weak nuclear
force. No surprise then that it is has eluded detection so far.
When physicists calculated exactly how much of the neutralino
residue there should be, they were taken aback. It was a huge amount
- far more than all the normal matter in the universe.
Beginning to sound familiar? Yes, indeed: it seemed that
neutralinos fulfilled all the requirements for the
dark matter that astronomical
observations persuade us must dominate the cosmos. A third strike
Each of the three questions that supersymmetry purports to solve -
the hierarchy problem, the reunification problem and the dark-matter
problem - might have its own unique answer.
But physicists are always inclined to
favour an all-purpose theory if they can find one.
"It's really reassuring that there
is one idea that solves these three logically independent
things," says Seiberg.
Supersymmetry solves problems with
the standard model, helps to unify nature's forces and explains the
origin of dark matter
Supersymmetry's scope does not end there. As Seiberg and his
Princeton colleague Edward Witten have shown, the theory can
also explain why quarks are never seen on their own, but are always
corralled together by the strong force into larger particles such as
protons and neutrons. In the standard model, there is no
mathematical indication why that should be; with supersymmetry, it
drops out of the equations naturally.
Similarly, mathematics derived from
supersymmetry can tell you how many ways can you fold a
four-dimensional surface, an otherwise intractable problem in
All this seems to point to some fundamental truth locked up within
"When something has applications
beyond those that you designed it for, then you say, 'well this
looks deep'," says Seiberg. "The beauty of supersymmetry is
Sadly, neither mathematical beauty nor
promise are enough on their own. You also need experimental
"It is embarrassing," says Michael
Dine of the University of California, Santa Cruz. "It is a lot
of paper expended on something that is holding on by these
Circumstantial evidence for
supersymmetry might be found in various experiments designed to find
and characterize dark matter in cosmic rays passing through Earth.
These include the
Cryogenic Dark Matter Search
experiment inside the Soudan Mine in northern Minnesota and the
Xenon experiment beneath the Gran Sasso mountain in central Italy.
Space probes like NASA's Fermi satellite are also scouring the Milky
Way for the telltale signs expected to be produced when two
neutralinos meet and annihilate.
The best proof would come, however, if we could produce neutralinos
directly through collisions in an accelerator. The trouble is that
we are not entirely sure how muscular that accelerator would need to
be. The mass of the super-partners depends on precisely when
supersymmetry broke apart as the universe cooled and the standard
particles and their super-partners parted company.
Various versions of the theory have not
come up with a consistent timing. Some variants even suggest that
certain super-partners are light enough to have already turned up in
accelerators such as the Large Electron-Positron collider -
the LHC's predecessor at CERN - or the Tevatron collider in
Batavia, Illinois. Yet neither accelerator found anything.
The reason physicists are so excited about the LHC, though, is that
the kind of supersymmetry that best solves the hierarchy problem
will become visible at the higher energies the LHC will explore.
Similarly, if neutralinos have the right
mass to make up dark matter, they should be produced in great
numbers at the LHC.
Since the accident during the accelerator's commissioning last year,
CERN has adopted a softly-softly approach to the LHC's restart. For
the first year it will smash together two beams of protons with a
total energy of 7 teraelectronvolts (TeV), half its design energy.
Even that is quite a step up from the
1.96 TeV that the Tevatron, the previous record holder, could
"If the heaviest supersymmetric
particles weigh less than a teraelectronvolt, then they could be
produced quite copiously in the early stages of LHC's running,"
says CERN theorist John Ellis.
If that is so, events after the
accelerator is fired up again could take a paradoxical turn. The
protons that the LHC smashes together are composite particles made
up of quarks and gluons, and produce extremely messy debris. It
could take rather a long time to dig the Higgs out of the rubble,
Any supersymmetric particles, on the other hand, will decay in as
little as 10-16seconds into a slew of secondary particles,
culminating in a cascade of neutralinos. Because neutralinos barely
interact with other particles, they will evade the LHC's detectors.
Paradoxically, this may make them
relatively easy to find as the energy and momentum they carry will
appear to be missing.
"This, in principle, is something
quite distinctive," says Ellis.
So if evidence for supersymmetry does
exist in the form most theorists expect, it could be discovered well
before the Higgs particle, whose problems SUSY purports to solve.
Any sighting of something that looks
like a neutralino would be very big news indeed. At the very least
it would be the best sighting yet of a dark-matter particle. Even
better, it would tell us that nature is fundamentally supersymmetric.
There is a palpable sense of excitement about what the LHC might
find in the coming years.
"I'll be delighted if it is
supersymmetry," says Seiberg. "But I'll also be delighted if it
is something else. We need more clues from nature. The LHC will
give us these clues."
String theory and
are two as-yet unproved theories about the make-up of the
universe. But they are not necessarily related.
It is true that most popular variants of string theory take a
supersymmetric universe as their starting point. String theorists,
who have taken considerable flak for advocating a theory that has
consistently struggled to make testable predictions, will breathe a
huge sigh of relief if supersymmetry is found.
That might be premature: the universe could still be supersymmetric
without string theory being correct.
Conversely, at the kind of energies
probed by the LHC, it is not clear that supersymmetry is a
precondition for string theory.
"It is easier to understand string
theory if there is supersymmetry at the LHC," says Edward
Witten, a theorist at the Institute of Advanced Studies,
Princeton, "but it is not clear that it is a logical
If supersymmetry does smooth the way for
string theory, however, that could be a decisive step towards a
theory that solves the greatest unsolved problem of physics:
gravity seems so different to
all the rest of the forces in nature.
If so, supersymmetry really could have
all the answers.