LESS than a year after toasting the discovery of the top quark, physicists at
the world鈥檚 most powerful particle accelerator seemed poised to uncork the
champagne yet again. In February, tantalising reports emerged from Fermilab,
near Chicago, that quarks just might be made up of something even smaller. Yet
the clues were frustratingly ambiguous, and the debate still rumbles on. Did the
researchers glimpse the heart of a quark, or was it simply a subatomic
mirage?
The Collider Detector at Fermilab (CDF), had recorded head-on collisions
between protons and antiprotons moving close to the speed of light in Fermilab鈥檚
Tevatron particle accelerator. The protons and antiprotons contain quarks, along
with particles called gluons that bind the quarks together. Occasionally, a
quark or gluon from a proton collides head-on with a quark or gluon from an
antiproton. When this happens, the energy from the collision generates jets of
exotic particles that shoot out in all directions.
Physicists have a pretty good idea how the quarks and gluons behave, and so
they know what the collision debris should look like鈥攊n particular how
many jets they should see at different energies. At low energies, they see
exactly the number of jets they expect. But at higher energies, the CDF data
threw up something unexpected鈥攁bout 50 per cent more jets than they
bargained for.
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This is exactly what you might expect if quarks have some sort of internal
structure鈥攑erhaps a hard core, or point-like constituents. This echoes the
discovery of the atomic nucleus in 1911. Students of Ernest Rutherford fired
alpha particles at gold foil and found that instead of passing easily through,
they occasionally bounced backwards. Rutherford realised that they had to be
striking a hard, central nucleus.
But most physicists would be astounded by the discovery of particles within
the quark. 鈥淚t鈥檚 far removed from the present theory,鈥� says William Carithers of
the CDF team. The current picture of the subatomic world is the so-called
standard model. This says that everything in nature is made up of different
combinations of six types of quark and six types of another family of particles
called leptons, which include electrons and neutrinos. Take these basic building
blocks, and the four forces that act between them, and you can work out what new
particles should exist.
So far this recipe has been astoundingly successful, and new particles have
turned up with exactly the right properties. But nonetheless, there are several
things the standard model doesn鈥檛 explain. Why do quarks have such a curious
spectrum of masses? For instance, the top quark is about 35 000 times heavier
than the 鈥渦p鈥� quark, and the standard model doesn鈥檛 say why.
Questions like these have prompted some physicists to look for something
beyond the standard model. Particles within quarks鈥攐ften dubbed
preons鈥攃ould be one answer. Perhaps you could bind preons together in a
way that neatly explains the masses we see. And wouldn鈥檛 that seem natural? As
accelerators have become more powerful the nucleus has fragmented鈥攆irst
into protons and neutrons, then into quarks. There is no law of physics that
says the progression must stop there.
But according to Chris Hill and Estia Eichten, theorists at Fermilab,
experimental evidence throws cold water on preons. First, if quarks are made up
of something smaller, we鈥檇 expect to have seen excited quark states a long time
ago. Shine photons of light (typically with a few electronvolts of energy) onto
atoms, and they can bump the electrons up to higher energy levels. Excited atoms
then lapse back to their original state by emitting the energy again. Protons
within the nucleus can be excited too, but at higher energies of around a few
hundred million electronvolts.
Excited quarks
If quarks contained some kind of structure that could be rearranged, they
should be excited in the same way. As a rough guess, Hill would expect this to
take about several thousand million electronvolts. But at energies up to
hundreds of times greater than this, Fermilab doesn鈥檛 see any signs of excited
quarks鈥攖hey still look just like points.
If the CDF experiment had suddenly uncovered quark substructure at
thousands of times the expected energy, this would be inexplicable in Hill鈥檚
view. It would suggest that whatever lies inside the quarks is incredibly
tightly bound, in a way that theory can鈥檛 yet accommodate. For about a decade,
people have been trying to come up with a theoretical way of binding preons
tightly together to generate the odd spectrum of masses for the quarks, but
without success. 鈥淭he models that people have cooked up are very ugly,鈥� says
Hill.
Today the task seems even more daunting in Eichten鈥檚 view, now that the top
quark has weighed in at a monstrous 170 times the mass of the proton. The models
are doomed, he says, unless experiments give them more to go on: 鈥淚t鈥檚 like
trying to solve a clueless puzzle. The models are going nowhere until we get
some clues from nature.鈥�
Eichten and Hill are not alone in believing that the Tevatron hasn鈥檛 turned
up the right clues. As the CDF researchers pointed out, there are many other
possible explanations for their extra jets than the footprints of preons.
A slight retuning of the prevailing picture of the structure of the proton
might be enough, for example. Along with the permanent population of one 鈥渄own鈥�
and two 鈥渦p鈥� quarks, protons (and antiprotons) contain a sea of gluons (See
鈥淐omplex heart of a simple proton鈥�, 1 July 1995, p 30). To complicate matters,
gluons can spontaneously mutate into a quark-antiquark pair. Calculation of the
number of jets at a given energy hinges on exactly how the total momentum of the
proton is distributed among all the quarks, gluons and the quark-antiquark
pairs.
If the gluons carry just slightly more momentum, Carithers says, quark-gluon
collisions could make a bigger contribution to the jets. This would mean just a
harmless adjustment to the theoretical picture. The CDF team plans to spend the
next few months analysing its data to see if adjusting the momentum could
explain the findings. But any revision would have to be minor because the new
distribution would also need to fit every other experimental result to date.
Forceful problems
There鈥檚 another, more radical alternative. Perhaps physicists have
miscalculated the power of the strong force, which governs the behaviour of
quarks and gluons. Theory suggests that the strong force weakens at higher
energies, and eventually disappears completely. But if the force died off a
little more slowly than predicted, there would be more interactions between
quarks and gluons, and an excess of high-energy jets.
Could the standard calculation be wrong? As it turns out, it鈥檚 not possible
to solve the equations exactly for the strength of the strong force. The usual
way round this is to come up with an approximate answer using a branch of
mathematics called perturbation theory. It鈥檚 a similar process to calculating
the circumference of a circle by approximating it as a polygon. A hexagon gives
a very rough approximation, and the more sides you add, the better the
approximation becomes. Perhaps, says Hans Jensen of the CDF team, the solutions
for the strong force to date have simply not incorporated enough terms.
But the mathematical problems may run deeper than that, according to Adrian
Patrascioiu of the University of Arizona at Tucson, and Erhard Seiler of the Max
Planck Institute in Munich. In some situations, perturbation theory works like a
treat. For instance, it predicts the properties of the electron to an accuracy
of ten parts in a thousand million. But in trying to deal with the strong force,
perturbation theory might be giving the wrong answers鈥攅ven at very high
energies, where it is deemed to be most trustworthy.
According to Patrascioiu, perturbation theory gives confusing, multiple
answers for the strength of the strong force. He says that this idea is
supported by alternative, number-crunching calculations, which suggest that the
force drops off more slowly than perturbation theory suggests, and never falls
to zero.
Unification trouble
If he鈥檚 right, physicists will also have to think again about many of the
ideas that have come to the forefront in recent years鈥攆or instance, 鈥済rand
unified theories鈥� which neatly merge the strong, electromagnetic and weak forces
at very high energies. These theories rely on the strong force fading away in
exactly the ways that the suspicious perturbation theory suggests.
Having taken the strong force back to the drawing board, Patrascioiu predicts
that the number of high-energy jets seen by CDF should increase by more than 40
per cent, in line with the actual results. He adds that two experimental results
at CERN have also hinted that the strong force falls away more gradually than we
think, although one of these has not yet been confirmed. 鈥淭he evidence is piling
up that something beyond the standard model is going on,鈥� says Patrascioiu.
鈥淲hile it could be new physics, we believe that our more economical explanation
stands the best chance.鈥�
Yet another possible explanation for the extra jets is that a new particle is
born in the collisions, and the extra jets are the debris of its decay. Guido
Altarelli of CERN speculates that the culprit could be the Z鈥欌�攁 possible
heavy relative of the so-called Z0 particle. In the standard model, the Z0 is
one of the carriers of the weak force, best known as the cause of radioactive
beta decay.
The beauty of the Z鈥� is that it could also lay another mystery to rest. The
standard model predicts that the Z0 should decay to give both
bottom-antibottom quark pairs, and charm-anticharm pairs. But at CERN鈥檚 Large
Electron-Positron Collider, the Z0 produces bottom-antibottom quarks about 1
per cent more often than the standard model predicts. Charm-anticharm pairs, on
the other hand, seem to appear slightly less often than they should.
Altarelli can explain these discrepancies, as well as the CDF results, if a
Z鈥� with a mass about 1000 times that of the proton turns up in the interactions.
At CERN, the Z鈥� could 鈥渕ix鈥� with the Z0. Sometimes the mixed state would decay
like a Z0, sometimes like a Z鈥�. The Z鈥� contribution would give an extra decay
route to bottom-antibottom at CERN. If it appeared at the Tevatron in the
highest energy collisions, it would decay to produce two jets鈥攑erhaps
giving the extra jets that CDF sees.
But even Altarelli would be surprised if this turned out to be correct. 鈥淚鈥檓
sceptical of the CDF results,鈥� he says. Like many other physicists, he suspects
that more thorough analysis could turn the CDF results on their head. 鈥淚t
happens all the time,鈥� says Altarelli. 鈥淚t鈥檚 no surprise because these
experiments are very difficult.鈥� Calibrating the detectors to measure the
energies of rare, high-energy jets is notoriously difficult. And at the highest
energy, the CDF results rest on only about six events鈥攏ot nearly enough in
Altarelli鈥檚 view to support any meaningful error estimates.
Shadows of doubt
Further doubts have come from CDF鈥檚 neighbouring research team, Fermilab鈥檚
DZero collaboration, which runs similar experiments on the same accelerator
ring. At a conference in Les Arcs in March, they announced preliminary results
based on about five times the amount of data in the original CDF analysis.
Although their counts of high-energy jets are consistent with the CDF results
within their error limits, they are also comfortably in line with the standard
model predictions.
But in spite of this, Carithers believes the CDF results are sound. 鈥淲e鈥檙e
convinced the measurement itself is correct,鈥� he says. And at the moment, no one
else can shed any light on the matter鈥攖he Tevatron is the only accelerator
powerful enough to reach these high energies. Both Fermilab teams hope to get to
the bottom of the problem by sifting through all of their data more thoroughly.
The CDF team hopes to have ruled out some of the options within a few
months.
If it eventually turns out that there鈥檚 no exciting new physics at work,
there will no doubt be further criticism of the CDF team for fuelling
鈥渙verblown鈥� headlines in the papers. Despite the fact that the CDF researchers
have been on the trail of this discrepancy in the high-energy jets since the
late 1980s, they have been accused of jumping the gun before the physics
community had the chance to reach a consensus. 鈥淭hey created a lot of publicity
about having found hints for composite quarks,鈥� says Altarelli. 鈥淚 think they
should have waited for confirmation.鈥�
But confirmation is a long time coming in the world of particle physics.
鈥淚t鈥檚 not like the kind of discovery that might have been made 100 or 150 years
ago in geography, where you cross a mountain range and see an ocean that nobody
knew about before鈥攁nd there鈥檚 no question that it鈥檚 there,鈥� says
Carithers. 鈥淩ather, it鈥檚 a picture that comes into focus a little bit at a time.
Even if we don鈥檛 know the full picture at any time, this is just a way of
sharing the excitement of our field. I hope that we will never be so anxious and
paranoid about the perception of our work that we never say anything.鈥�
Splitting the atom: protons in the nucleus are made up of quarks. Could these
contain yet smaller particles?
