The Higgs Boson: Searching for the God Particle (17 page)

The conservation of CP number would
explain an odd detail: the two “combination”
kaons, though apparently similar,
differ in their life spans by a factor
of about 500. The kaon with CP
number +1 can change to two pions, a
state that has the same CP number.
This decay proceeds rapidly, because
the kaon is massive enough to yield two
pions readily. But the kaon with CP
number –1 can decay only to another
state with CP number –1: three pions.
This latter breakdown takes time, because
the kaon has barely enough mass
to generate three pions. So when physicists
found a long-lived kaon in addition
to a short-lived one, they acquired
strong evidence that the combination
kaons obeyed CP symmetry.

This tidy picture was shattered in
1964, when in a groundbreaking experiment
at Brookhaven National Laboratory
on Long Island, James Christenson,
James Cronin, Val Fitch and René
Turlay observed that about one out of
every 500 of the long-lived kaons (those
with CP number –1) decays into two
pions. If CP were an exact symmetry of
nature, it would forbid such a decay.
Few experiments in particle physics
have produced a result as surprising as
this one. Theorists found it hard to see
why CP symmetry should be broken at
all and even harder to understand why
any imperfection should be so small.

In 1972 Makoto Kobayashi and Toshihide
Maskawa of Nagoya University
showed that charge parity could be violated
within the Standard Model if three
or more generations of quarks exist. As
it happened, only two generations of
quarks—the first, containing the up and
down, and the second, with the strange
and charm—were known at the time.
So this explanation began to gain currency
only when Martin L. Perl and
others at the Stanford Linear Accelerator
Center (SLAC) spied τ (tau) leptons,
the first particles of the third generation,
in 1975. Two years later experimenters
at Fermi National Accelerator
Laboratory in Batavia, Ill., found the
bottom quark. But only recently, with
the top quark being nailed down, also
at Fermilab, has the third generation
been completed.

Skewing the Universe

It is imaginable that the universe was
born skewed—that is, having unequal
numbers of particles and antiparticles
to begin with. Such an initial imbalance,
however, would be quickly eliminated
if the early universe contained any processes
that could change baryon number—
the number of matter particles minus
the number of antimatter particles.
(In extensions of the Standard Model
called Grand Unified Theories, such
processes would have been very common
soon after the big bang.) Theorists
prefer the alternative scenario, in which
particles and antiparticles were equally
numerous in the early universe, but the
former came to dominate as the universe
expanded and cooled.

Soviet physicist (and dissident) Andrei
Sakharov pointed out three conditions
necessary for this asymmetry to develop.
First, fundamental processes that
do not conserve baryon number must
exist. Second, during the expansion the
universe must not attain thermal equilibrium.
(When in thermal equilibrium,
all states of equal energy contain equal
populations of particles, and because
particles and antiparticles have equal
mass or energy, they would be generated
at the same rate.) Third, CP symmetry—essentially, the symmetry between
matter and antimatter—must be violated.
Otherwise any process that changes
the amount of matter would be balanced
by a similar effect for antimatter.

The prevailing theory holds that when
the universe was born, the quantum field
associated with the Higgs particle was
everywhere zero. Then, somewhere in
the universe, a bubble developed, inside
which the Higgs field assumed its present
nonzero value. Outside the bubble,
particles and antiparticles had no mass;
once inside, however, they interacted
with the Higgs field to acquire mass. But
as the bubble grew, particles and antiparticles
were swept through its surface
at unequal rates because of CP violation.
Any imbalances between matter
and antimatter thus created outside the
bubble were quickly corrected by processes
that change baryon number.

Such processes were extremely rare
inside the bubble, however, so the imbalance
was frozen in. By the time the
bubble had expanded to occupy the entire
universe, it contained more particles
than antiparticles. Eventually the
universe cooled to a point where particles
and antiparticles could no longer
be generated in collisions but would annihilate
when they found one another.
Unfortunately, when theorists calculate
how much of an imbalance between
matter and antimatter this mechanism
can create, it comes out too small—by
many orders of magnitude. This failure
suggests that there must be other ways in
which CP symmetry breaks down and
hence that the Standard Model may be
incomplete.

A fruitful place to search for more violations
is most likely among the B
mesons. The Standard Model predicts
the various decays of the
B
⁰
and the anti-
B
⁰
to be highly asymmetric. A
B
⁰
contains
a down quark bound to an antibottom
quark, whereas the anti-
B
⁰
consists
of an antidown quark and a bottom
quark. The
B
mesons behave much like
the kaons discussed earlier: the observed
B mesons consist of mixtures of the
B
⁰
and anti-
B
⁰
.

Consider the evolution of a
B
⁰
meson
produced at a certain instant. Some time
later an observer has a certain probability
of finding the same particle and also
some probability of finding its antiparticle,
the anti-
B
⁰
. This peculiar meson
state, oscillating between a given quarkantiquark
combination and its antiparticle,
is a remarkable illustration of
quantum mechanics at work.

The Bottom Line

To study CP violation, experimenters
need to study decays of
B
⁰
into
those final states that have a definite CP
number. Such decays should proceed at
a different rate for a particle that is initially
B
⁰
compared with one that is initially
anti-
B
⁰
. This difference will indicate
the extent of CP violation in the
system. But rather than resulting in the
one-in-1,000 effect seen in
K
⁰
decays, the
predicted asymmetry for
B
⁰
decays grows
so large that one decay rate can become
several times larger than the other.

Models other than the Standard often
have additional sources of CP violation—
sometimes involving extra Higgs
particles—in general offering any value
for imbalances in
B
⁰
decays. Thus, measuring
the pattern of asymmetries will
provide a clear test of the predictions.

When the bottom quark was discovered,
its mass was measured at around
five giga-electron volts (GeV), or about
five times the mass of a proton. Consequently,
theorists calculated that it
would take a little more than 10 GeV
of energy to produce two
B
mesons (because
the added down or antidown
quarks are very light). In the early 1980s
at Cornell University, operators of an
electron-positron collider—a machine
that accelerates electrons and positrons
into head-on crashes—tuned it so that
an electron-positron pair would release
an energy of 10.58 GeV on annihilating.
As predicted, this burst of energy
preferentially converts to
B
mesons,
providing a very rich source of the particles.
About one in four annihilations
results in a
B
meson and its antiparticle,
leaving behind no other particles at all.

At SLAC in 1983 experimenters found
an unexpectedly long lifetime of about
1.5 picoseconds for the
B
meson. The
extended life improved the chances that
a
B
⁰
would turn into an anti-
B
⁰
before
decaying, making CP-violating asymmetries
easier to observe. Furthermore, in
1987 experimenters at the Electron Synchrotron
Laboratory (DESY) in Hamburg,
Germany, measured this “mixing”
probability at 16 percent, making
it likely that the asymmetries would be
far larger than those for the
K
⁰
. Still,
these large asymmetries occur in relatively
rare decays of the
B
mesons. For
a true study of CP violation, a great
number of
B
mesons would be needed.

In 1988 at a workshop in Snowmass,
Colo., the major topic of interest was the
Higgs particle. A group of participants
also discussed CP violation, especially
in
B
mesons. It determined that a favorable
way to study the
B
mesons would
be with an electron-positron collider
tuned to 10.58 GeV in which the electron
and positron beams had different
energies. This rather unusual feature
would facilitate the measurement of a B
meson’s life span. Experimenters identify
the point of birth and the point of
death (that is, decay) of a
B
meson from
traces of particles in the detector. Dividing
the distance between these two
points by the calculated velocity of the
meson yields its life span. But an ordinary
electron-positron collider at 10.58
GeV produces two
B
mesons that are
almost at rest; the small distances they
move are hard to measure.

Pier Oddone of Lawrence Berkeley
National Laboratory had pointed out
that if the electrons and positrons have
different energies, the
B
⁰
mesons that
are produced move faster. For instance,
if the electron beam has an energy
of 9.0 GeV and the positron
beam an energy of 3.1
GeV, the
B
⁰
mesons move at
half the speed of light, traveling
about 250 microns (about one
hundredth of an inch) before
they decay. Such a distance can
yield a reasonably accurate measure
of the lifetime.

An accelerator facility with
two separate rings delivering
different energies to the electrons
and positrons would fit
the task. Each ring would have
to deliver very intense beams of
particles, obtaining a high rate
of collisions. Such a machine
came to be called an asymmetric
B factory: asymmetric because
of the different beam energies,
and
B
factory because of
the large numbers of
B
mesons
it would produce.

Teams at several laboratories
developed designs that could
generate about 30 million pairs
of
B
mesons a year. In 1993 the
U.S. Department of Energy and
the Japanese agency Monbusho
approved two proposals for
construction: one at SLAC in
California and the other at KEK, the
High Energy Accelerator Research Organization
in Tsukuba, Japan. The
SLAC project is utilizing the existing
linear tunnel to accelerate the positrons
and electrons. These will then be circulated
in separate rings newly constructed
in a 20-year-old tunnel and set to
collide at a point of crossing. The accelerator
construction cost $177 million.
The Japanese project is also employing
extant tunnels—those that previously
housed the Tristan collider.

Physicists and engineers are busy setting
up a large experiment that can identify
the rare decays of a
B
meson and
measure their positions to within the
requisite 80 microns. This accuracy is
obtained by using the silicon microstrip
technology that helped to unearth the
top quark [see “The Discovery of the
Top Quark,” by Tony M. Liss and Paul
L. Tipton; Scientific American, September
1997]. Experimenters aim to
identify almost every particle that emerges
from the decays of the
B
mesons in
order to isolate the rare events that
shed light on charge-parity questions.

In the BABAR detector that is being
built for SLAC, the silicon microstrip
will be the innermost layer, forming a
cylinder roughly 30 centimeters in diameter
and 60 centimeters long.
Outer layers will measure energy,
velocity and penetration power
for each particle created, allowing
physicists to reconstruct the
original events. More than 500
participants—including both of
us—from 70 institutions in nine
nations are building the detector
and also sharing its cost of $85
million. (It was, in fact, to facilitate
international collaborations
of this kind that the World Wide
Web was invented at CERN.)
The BELLE collaboration that is
building the Japanese experiment
is also international in scope,
with members from 10 countries.
Both
B
factories are scheduled
for completion later this
year, with the first data arriving
in early 1999.