麻豆传媒

LOOK around you. There鈥檚 matter everywhere. Your body alone contains perhaps
fifty billion billion billion protons. Out in space, matter is much rarer, but
it鈥檚 still there. And here鈥檚 the rub: by rights, all this matter shouldn鈥檛 exist
at all. According to prevailing theories, the big bang forged equal amounts of
matter and its nemesis, antimatter, which should then have annihilated each
other in bursts of pure light. So the Universe should be filled with light, not
stars and planets and gas.

鈥淣aively, one would expect antimatter to be exactly the opposite of matter in
all respects, including in the early Universe,鈥� says Ken Peach of the Rutherford
Appleton Laboratory in Oxfordshire. But to create the Universe we see today, he
says, a preference for matter must have arisen a fraction of a second after the
big bang. It only needed to be a tiny imbalance, with as little as one extra
particle of matter surviving out of every billion created in the primordial
inferno.

No one knows exactly what caused that imbalance鈥攂ut physicists think
they know how to find out. The secret may lurk in the behaviour of peculiar
particles called mesons, unstable mixtures of both matter and antimatter.
Researchers first discovered that matter and antimatter can act differently by
studying the K meson, or kaon, in 1964. Kaons, however, are relatively simple
particles and reveal only a tiny part of the picture. To better understand the
matter-antimatter imbalance, physicists needed to study the kaon鈥檚 big brother,
the B meson.

But without machines capable of generating large numbers of B mesons,
researchers were stuck. And that鈥檚 how things have stood for three decades.
Within months, however, not just one but a host of particle accelerators will be
capable of creating B mesons. A race to understand one of our Universe鈥檚 deepest
secrets has begun. Leading the charge are new accelerators in California and
Japan called 鈥淏 factories鈥�, which will spawn B mesons by the tens of millions,
starting this spring.

At stake is far more even than a clearer picture of the birth of matter in
our Universe. The B meson experiments may also endanger the standard model, the
theoretical edifice that describes nature鈥檚 fundamental forces and particles.
The standard model has barely changed for more than a quarter of a
century鈥攂ut to explain matter鈥檚 supremacy, physicists may need to invoke
forces beyond it.

In 1967, Soviet physicist Andrei Sakharov looked at what was necessary for a
matter preference to arise and persist in the cauldron of the early Universe.
One possibility was that a minuscule difference in the rate of decay of matter
and antimatter at high energies resulted in there being a fraction more matter
than antimatter. Then, after all the antimatter was annihilated, some matter
would have been left over to form today鈥檚 Universe. The crucial initial
difference, Sakharov speculated, could have been a phenomenon called
charge-parity violation鈥擟P violation for short鈥攚hich had first been
seen three years earlier.

Physicists had assumed that if one could magically change all particles in
two fashions, by switching their charges (that is, swapping matter for
antimatter) and reversing their 鈥減arities鈥� (reflecting them in a mirror and
flipping them upside down), the particles would still behave in exactly the same
way. In other words, if the idea of CP conservation were right, there would be
no way to distinguish between matter and antimatter flipped in this way.

That looking-glass notion is indeed true for the dominant forces in the
Universe: electromagnetism and the strong force. However, research on kaons
shattered the notion that it was also true for the weak force, which governs
radioactive decay.

Kaons and other mesons exist as the fleeting union of an ordinary quark (the
main building block of matter) and an antiquark. In 1964, a sensational
experiment at Brookhaven National Laboratory in New York by James Cronin and Val
Fitch鈥攚ho shared the Nobel prize in 1980鈥攕howed that one type of
kaon occasionally turns into its antikaon. Crucially, this happens less often
than the reverse process, by about 1 part in 500. That is, there is a small
preference for matter over antimatter.

However, physicists now know that kaons cannot tell the whole story. The
Cronin and Fitch experiment showed that this bias exists, but revealed little
about its intricate details. So physicists at the Fermi National Accelerator
Laboratory (Fermilab) in Illinois and the European laboratory for particle
physics (CERN) in Switzerland are trying to fill in the blanks. They are engaged
in what Cronin, now at the University of Chicago, calls 鈥渉eroic efforts鈥� to
measure a second oddity in the decay rates of kaons and antikaons that pops up
only about once in every 10 million events. But even if they do detect another
CP violation in kaons, most theorists believe that the decays of kaons are too
simple to flesh out the rest of the picture.

Bulky B mesons, on the other hand, include 鈥渂ottom鈥� quarks, which are much
heavier than the 鈥渟trange鈥� quarks in kaons. And just as a large stone can
shatter into a great many fragments, when Bs decay they produce a far richer
menagerie of particles than the lighter kaons. Physicists suspect that B meson
decays involve many different types of CP violation, and will thus reveal
aspects of the relationship between matter and antimatter that kaons cannot.
鈥淚t鈥檚 as if kaons represent a 3-note piano, whereas B mesons give you the whole
keyboard,鈥� says Peach. 鈥淓ach tune you can play gives you different information
about the source of CP 惫颈辞濒补迟颈辞苍.鈥�

This is why physicists in California and Japan have built massive machines
dedicated to producing B mesons and anti-B mesons (see 鈥淪warms of Bs鈥�). These 鈥淏
factories鈥� will have to pump out many millions of these unusual particles. 鈥淚f
you don鈥檛, forget it,鈥� says Jonathan Dorfan, director of the B factory at the
Stanford Linear Accelerator Center (SLAC). 鈥淵ou have to stock the shelves every
day with B mesons.鈥�

This is because most Bs and anti-Bs behave just as one would expect if CP
conservation were valid. Only once every 10 000 decays or so will a particle
decay in a way that might reveal differences in how matter and antimatter
behave. So detecting this difference means looking for a rare imbalance in an
already rare type of reaction.

The various chains of particles produced by the decay of one particle are
called channels. 鈥淎 B meson can decay in hundreds of ways, but for CP violation
there are a few flagship channels,鈥� says Helen Quinn, a SLAC theorist. The first
is the so-called 鈥済olden channel鈥�, in which a B meson yields two other mesons, a
鈥淛/psi鈥� and a 鈥淜 short鈥�, a type of kaon (see Diagram).
The rate at which that occurs from a B versus an anti-B may hint at a similar inequity that
favoured matter over antimatter after the big bang.

鈥淭his will be the first measurement of CP violation outside of the kaon
system, so everyone wants to do it first,鈥� says Quinn. 鈥淚t鈥檚 not too rare, not
too hard to find and very cleanly predicted. We should see the rate difference
in the decays relatively soon.鈥�

If that was all each B factory did, it would hardly justify the $300
million cost. However, if they perform as intended, the factories will belch out
enough Bs to illuminate matter-antimatter asymmetries in rarer channels seen
less than once in every million events. This will let physicists go well beyond
merely detecting CP violation. Measuring several rarer channels could vindicate
the standard model鈥檚 description of CP violation鈥攐r unveil types of
matter-antimatter asymmetry that can be explained only by new theories.

The standard model already falls woefully short of explaining the Universe鈥檚
preponderance of matter. The part of it that describes weak forces operating on
light and heavy quarks does contain a smidgen of CP violation, based on the
observed asymmetry between kaons and antikaons. But this isn鈥檛 enough to explain
the Universe: the model predicts about a million billion times fewer protons
than we see today. The hope is that the B factories will bridge that gap by
revealing what amounts to a stronger source of CP violation in the early
Universe.

For instance, the B factories may show that a simple addition to the standard
model would suffice to beef up the levels of CP violation it predicts in the
early Universe. This could involve a force-carrying particle called the Higgs
boson, which is believed to endow quarks, electrons and other particles with
mass.

鈥淭he way CP violation comes about is intimately connected with the Higgs
mechanism,鈥� Quinn says. But she adds: 鈥淲hen we try to look beyond the standard
model today, we are speculating and nothing more. It鈥檚 a frustratingly good
迟丑别辞谤测.鈥�

Adding a new kind of Higgs boson, or even more than one, to the standard
model might fix any shortcomings exposed by the B factories. Alternatively, CP
violation might stem from a new fundamental force, the 鈥渟uperweak鈥� force
proposed in the 1960s by Lincoln Wolfenstein of Carnegie Mellon University. This
force would operate only among certain quarks and would produce no other
observable effects. 鈥淭hat鈥檚 a haunting possibility,鈥� says Cronin. 鈥淚f that plays
a role, it would be extremely hard to learn anything more about CP
惫颈辞濒补迟颈辞苍.鈥�

Marathon task

The B factories at SLAC and the High Energy Accelerator Research Organization
(KEK) in Tsukuba, Japan, should provide new insights into these possibilities,
but some physicists caution that they are unlikely to answer all the questions.
According to Bruce Winstein of the University of Chicago, who studies kaons at
Fermilab, painting the full picture of CP violation will probably require
machines that produce Bs even more copiously than the B factories.

B-factory physicists disagree, but admit that there鈥檚 a long way to go.
鈥淲e鈥檙e engaged in both a 100-yard dash and a marathon,鈥� says physicist David
Hitlin of SLAC and the California Institute of Technology. 鈥淭he real solutions
to the science problems will come out after many years.鈥�

In the long run, a planned B meson experiment at CERN鈥檚 Large Hadron Collider
(LHC), which will become the world鈥檚 most powerful physics machine in 2005, will
blow everyone else out of the water. 鈥淚t will produce a trillion Bs per year,鈥�
says Neville Harnew of the University of Oxford. 鈥淭he disadvantages of the huge
number of background events, as compared with those at an electron-positron
collider such as SLAC and KEK, are completely outweighed by the huge advantage
in statistics.鈥� LHC will also spit out heavier B mesons composed of bottom and
strange quarks and antiquarks, possibly yielding exotic new types of
matter-antimatter asymmetry.

But for now, all eyes are on the race to the golden channel, the first sign
of CP violation in B mesons. For this dash, at least, both the B factories
already have competition. Some observers say there鈥檚 a 50-50 chance that
physicists at proton colliders will beat them to it. Fermilab鈥檚 potent Tevatron
machine will churn out 10 billion B mesons per year after an upgrade in 2000,
although they will be buried in an avalanche of other particles. Researchers are
learning how to unravel those horribly complex collisions and could get results
within two to three years. Even before its upgrade, this machine is generating
enough particles to suggest that there is indeed a skew between B mesons and
their antiparticle kin, as Fermilab physicists announced in December. It鈥檚 an
intriguing prelude, but not yet enough to claim detection of the golden
channel.

A novel experiment at the Hadron Electron Ring Accelerator (HERA) in Hamburg,
Germany, also will produce a flood of Bs. 鈥淚f we are delayed and have to cope
with competition from those groups, we鈥檒l really be in trouble,鈥� says Stephen
Olsen of the University of Hawaii, who is part of the KEK team.

As the two B factories prepare to turn on their machines, physicists on both
sides of the Pacific are expressing delight with the friendly nature of the race
so far. 鈥淚t鈥檚 a very open competition,鈥� says Dorfan of SLAC. 鈥淲e share
everything with each other. Right now it鈥檚 an even race, and there鈥檚 no reason
to believe they won鈥檛 be as successful as we are.鈥�

However, the pressure to be first is still there. 鈥淧hysicists in the
high-energy community here think that if we don鈥檛 do a really good job on the B
factory, our next project will suffer tremendously in terms of funding,鈥� says
Kazuo Abe of KEK. Theorist Anthony Sanda of Nagoya University agrees: 鈥淲e can鈥檛
keep coming in second and asking for more money from the government.鈥�

Beyond the first golden channel, for which the theory is unambiguous, no one
can say what secrets the Bs will reveal. Cronin, whose 1964 experiment triggered
this extended pursuit, doubts there鈥檒l be any easy answers. 鈥淲e are just as
likely to be caught by a surprise that nobody thought of, rather than finding
the one definite thing beyond the standard model,鈥� he says. 鈥淚 wouldn鈥檛 even
dare imagine what that might be.鈥�

The B factories at the Stanford Linear Accelerator Center (SLAC) and the High
Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, will create B
mesons by smashing breathtaking numbers of electrons into their antimatter
counterparts, positrons. The 鈥渇actories鈥� are aptly named, for they are the first
machines designed to make specific particles. This will let physicists study
their decays without having to sift through a blizzard of high-energy
debris.

The two factories are broadly similar. Electrons and positrons whirl at
nearly the speed of light in opposite directions within two adjacent rings (2.2
kilometres around at SLAC, 3 kilometres at KEK). The particles travel in
needle-shaped bunches about a centimetre long, separated by metre-long gaps of
empty space. Strong magnets line the rings and tightly confine each bunch, which
contains tens of billions of particles.

Both machines will use a nifty trick. They accelerate electrons to higher
energies than the positrons, so when the bunches collide, the resulting
particles鈥攁bout a third of which are pairs of B and anti-B
mesons鈥攕hoot off in one direction. This means that during their brief
lifetime of about 1.5 billionths of a second, the B mesons will travel far
enough for physicists to separate the points in space where they decay. Without
this 鈥渁symmetric鈥� boost, the two mesons would stay near the collision zone and
be indistinguishable to particle detectors. This untried innovation is essential
to expose any disparities in decay rates between B mesons and their
antiparticles鈥攕o both teams are anxious to see if it works.

The KEK factory incorporates another daring approach. Its beams will collide
at a slight angle, rather than head-on as at SLAC. This lets KEK physicists jam
particles together more tightly, because the beams won鈥檛 interfere with each
other except at the spot where the bunches clash. SLAC has to use extra magnets
to squeeze the beams together and bend them apart; this produces X-rays that
could confuse the detector. SLAC鈥檚 design absorbs the X-rays, but only by
circulating fewer bunches in the beams. As a result, KEK may generate 100
million pairs of B and anti-B mesons a year, compared with SLAC鈥檚 goal of 30
million.

Hundreds of physicists worldwide have collaborated to build hulking
1000-tonne detectors needed to record the paths and energies of the particles.
KEK鈥檚 detector is called Belle, while SLAC鈥檚 is dubbed BaBar after the elephant
in Laurent DeBrunhoff鈥檚 books. Considerably larger and squarer than its
namesake, BaBar nevertheless is painted an appropriate shade of grey.