The Higgs Boson: Searching for the God Particle (16 page)

It may well be a decade or two before the next level in the structure of matter comes clearly into view (if, again, there is another level). What is needed is a sound theoretical picture, one that is self-consistent, that agrees with all experiments and that is simple enough to explain all the features of the standard model in terms of a few principles and a few fundamental particles and forces.
The correct picture, whether it is a grand unified theory or a composite model of the quarks and leptons, may already exist in some embryonic form. On the other hand, it is also possible the correct theory will emerge only from some totally new idea. In the words of Niels Bohr, it may be that our present ideas
"are not sufficiently crazy to be correct."

-Originally published: Scientific American 248(4), 53-68. (April 1983)

The Asymmetry between Matter and Antimatter

by Helen R. Quinn and Michael S. Witherell

As far as humans can see into the
universe, an essential imbalance
strikes the eye. Stars,
planets, asteroids, rocks—everything is
made of matter. Essentially no antimatter
is evident.

Is this imbalance the result of an accident,
a chance occurrence during the
birth of the universe? Or is it an inevitable
outcome of some asymmetry in the
laws of nature? Theorists believe that
the excess of matter comes from fundamental
disparities in how matter and
antimatter behave. These differences
amount to violations of a symmetry
called charge-parity reversal, or CP.

After years of effort, experimental and
theoretical physicists have found a natural
way for CP symmetry to be broken
within the prevailing theory of particle
physics, called the Standard Model. Curiously,
the amount of CP violation the
model predicts is too small to explain
the matter excess in the universe.

This finding is a vital clue that not all
is well with the Standard Model: unknown
factors are very likely at play.
Two new accelerators, just now being
completed in California and in Japan,
will soon begin to probe violations of
CP, with the aim of understanding
whether the Standard Model needs to
be revamped or replaced. These accelerators,
which will produce enormous
numbers of particles called
B
mesons,
are known as asymmetric
B
factories.
They are the latest tool in the search for
physics beyond the Standard Model.

Everything known about the elementary
properties of matter is encapsulated
in the Standard Model. It describes
all the hundreds of observed particles
and their interactions in terms of a few
types of fundamental constituents: six
quarks and six leptons. (The leptons are
light particles, such as the electron, the
neutrino and their relatives.) In addition,
each quark or lepton comes
with an antiparticle, which has
the same mass but opposite sign
for some quantum numbers
such as electric charge. These ingredients
are arranged in three
generations of increasing mass,
the first of which provides the primary
constituents of matter.

The Standard Model describes three
kinds of interactions among particles:
the familiar electromagnetic force as
well as the strong and the weak forces.
(For objects of such low mass, gravity is
too weak to be of interest.) Strong interactions
confine quarks, which are
never seen alone, within composite objects
such as protons. Weak interactions
cause instability—in particular, the slow
decays of all the more massive quarks
and leptons into objects of lower mass.
All these forces are transmitted by
specialized particles that also appear in
the Standard Model: the photon, the
gluon, and the W and Z bosons. Last,
the theory requires an as yet unobserved
Higgs particle, whose interactions
are held responsible for the masses
of the quarks and leptons as well as
for much of their behavior.

Essential to the story of CP violation
is a family of composite objects called
mesons. A meson contains one quark
and one antiquark, in an equal mixture
of matter and antimatter. A set of mesons
of great significance is the kaons,
or K mesons, which contain a strange
quark or antiquark along with up or
down quarks and antiquarks. Similar in
many respects are the
B
mesons, which
contain a bottom quark or antiquark
paired with up or down partners.

Beyond the Standard

Despite its manifold successes in describing
the behavior of matter,
deep questions remain about the Standard
Model. Physicists do not understand
the mechanisms that determine
the model’s 18 parameters. For the theory
to describe the world as we know it,
some of those parameters must have
very finely tuned values, and no one
knows why those values would apply.
More fundamentally, we do not understand
why the model describes nature
at all—why, for instance, should there
be exactly three generations of leptons
and quarks, no more or less? Finally,
aspects of the theory that involve the
Higgs particle are all untested. The Large
Hadron Collider, now under construction
at CERN, the European laboratory
for particle physics near Geneva,
will, however, allow the Higgs to be observed
if its properties are as predicted
by the Standard Model. The Higgs is
believed to lie behind most of the mysteries
of the Standard Model, including
the violation of CP symmetry.

A theory of physics is said to have a
symmetry if its laws apply equally well
even after some operation, such as reflection,
transforms parts of the physical
system. An important example is
the operation called parity reversal, denoted
by P. This operation turns an object
into its mirror reflection and rotates
it 180 degrees about the axis perpendicular
to the mirror.
In mathematical terms, parity reverses
the vectors associated with the object.

A theory has P symmetry if the laws
of physics are the same in the parity-reversed
world as in the real world. Particles
such as leptons and quarks can be
classified as right- or left-handed depending
on the sense of their internal
rotation, or spin, around their direction
of motion. If P symmetry holds, righthanded
particles behave exactly the
same as left-handed ones.

The laws of electrodynamics and the
strong interactions are the same in a
parity-reflected universe. But in a famous
experiment in 1957 Chien-Shiung
Wu of Columbia University and her
collaborators found that the weak interactions
are very different for particles
of different handedness. Peculiarly, only
left-handed particles can decay by means
of the weak interaction, not right-handed
ones. Moreover, so far as we know
there are no left-handed neutrinos: these
particles are always right-handed. Because
neutrinos have only weak interactions
with the rest of the universe, this
asymmetry is attributed to the weak
force. So the weak force violates P.

Another basic symmetry of nature is
charge conjugation, or C. This operation
changes the quantum numbers of
every particle into those of its antiparticle.
Charge symmetry is also violated in
weak interactions: antineutrinos are not
left-handed, only right-handed.

Theorists combine C and P to get the
operation CP, which turns all particles
into their antiparticles and also reverses
the direction of all vectors. When subjected
to CP, the left-handed neutrino becomes
a right-handed antineutrino. Not
only does the right-handed antineutrino
exist, but its interactions with other particles
are the same as they are for lefthanded
neutrinos. So although charge
and parity symmetry are individually
broken by neutrinos, in combination
their dictates would seem to be obeyed.

Much to the surprise of physicists, the
story of CP turned out to be far from
simple. A mathematical theorem proved
in 1917 by German mathematician
Emmy Noether states that every symmetry
implies the existence of a related
quantity that is conserved, or immutable.
For instance, the fact that spacetime
is the same in all directions—that
is, has rotational symmetry—leads to
the conservation of angular momentum.
Noether’s theorem implies that if
charge parity were an exact symmetry
of nature, then a quantity called CP
number would be conserved.

CP Violated

A particle and its antiparticle moving
in opposite directions with equal
energies form a pair with charge-parity
symmetry: the CP operation does not
change the system (taken as a whole),
except that its mathematical representation
acquires an overall factor. This
factor is the CP number.

Either C or P, if acting twice on a system,
returns it to the original state. This
property is expressed as C
²
= P
²
= 1
(where 1, the identity operation, imparts
no change at all). As a result, the CP
number can be only +1 or –1. If nature
has perfect charge-parity symmetry,
Noether’s theorem rules that no physical
state with CP number –1 can transform
into a state with CP number +1.

Consider the electrically neutral kaons.
The
K
⁰
consists of a down quark and
an antistrange quark, whereas the anti-
K
⁰
consists of an antidown quark and a
strange quark. Because CP transposes
quarks and antiquarks, it would turn
each kaon into the other instead of leaving
it unchanged. Hence, neither of these
kaons has a definite CP number. Theorists
can, however, construct a pair of
kaons with definite CP numbers by superposing
the wave functions for
K
⁰
and anti-
K
⁰
. By the rules of quantum
mechanics, these mixtures correspond
to real particles and have definite mass
and lifetime.