The Higgs Boson: Searching for the God Particle (20 page)

A difficulty in searching for the Higgs boson is that its mass is virtually unconstrained. As determined by experiment,
the mass must be greater than about 5 GeV. Theory presents no clue as to how heavy the Higgs boson could be, except the particle would generate some of the same difficulties it has been designed to solve if its mass were 1 TeV, which is approximately 1,000 times the mass of the proton. At that point theory suggests the weak vector bosons could no longer be viewed as elementary particles; they could be composite structures made of smaller particles.

The notion of a composite structure is, of course, nothing new in the history of physics. At the beginning of the article I mentioned five known layers of structure: molecules, atoms, nuclei,
nucleons (protons and neutrons) and quarks and leptons.

In considering the Higgs boson as a composite structure it is only a small step to suppose such "fundamental"
particles as quarks and leptons are really composite structures made from still smaller particles. In a sense the notion of a sixth layer of structure, one beyond quarks and leptons, brings me full circle. Traditionally the way to account for free parameters has been to go to a deeper layer of structure. The success of composite models in predicting energy levels of atoms and nuclei suggests that mass could also be predicted by going to a deeper layer of structure. The fact that in the standard model the Higgs boson is responsible for all observed masses implies that, even if in the end there is no such thing as a Higgs boson,
there is at least a common source for all masses. Searching for the Higgs boson could ultimately be the same as searching for a deeper structure of elementary particles.

-Originally published: Scientific American 255(5), 76-84 (November 1986)

The Dawn of Physics Beyond the Standard Model

By Gordon Kane

Today, centuries after the search began for the fundamental
constituents that make up all the complexity and beauty
of the everyday world, we have an astonishingly simple answer—it takes just six particles: the electron, the up and the
down quarks, the gluon, the photon and the Higgs boson.
Eleven additional particles suffice to describe all the esoteric phenomena
studied by particle physicists. This is
not speculation akin to the ancient Greeks’ four elements of
earth, air, water and fire. Rather it is a conclusion embodied in
the most sophisticated mathematical theory of nature in history,
the Standard Model of particle physics. Despite the word
“model” in its name, the Standard Model is a comprehensive
theory that identifies the basic particles and specifies how they
interact. Everything that happens in our world (except for the
effects of gravity) results from Standard Model particles interacting
according to its rules and equations.

The Standard Model was formulated in the 1970s and tentatively
established by experiments in the early 1980s. Nearly
three decades of exacting experiments have tested and verified
the theory in meticulous detail, confirming all of its predictions.
In one respect, this success is rewarding because it confirms that
we really understand, at a deeper level than ever before, how nature
works. Paradoxically, the success has also been frustrating.
Before the advent of the Standard Model, physicists had become
used to experiments producing unexpected new particles or other
signposts to a new theory almost before the chalk dust had
settled on the old one. They have been waiting 30 years for that
to happen with the Standard Model.

Their wait should soon be over. Experiments that achieve collisions
that are higher in energy than ever before or that study certain key phenomena with greater precision are on the verge of
going beyond the Standard Model. These results will not overturn
the Standard Model. Instead they will extend it by uncovering
particles and forces not described by it. The most important
experiment is occurring at the upgraded Tevatron collider
at Fermi National Accelerator Laboratory in Batavia, Ill., which
began taking data in 2001. It might produce directly the still elusive
particles that complete the Standard Model (Higgs bosons)
and those predicted by the most compelling extensions of the theory
(the so-called superpartners of the known particles).

Significant information is also beginning to come from “B
factories,” particle colliders running in California and Japan
configured to create billions of b quarks (one of the 11 additional
particles) and their antimatter equivalents to study a phenomenon
called CP violation. CP (charge-parity) is the symmetry
relating matter to antimatter, and CP violation means that antimatter
does not exactly mirror matter in its behavior. The amount
of CP violation observed so far in particle decays can be accommodated
by the Standard Model, but we have reasons to expect
much more CP violation than it can produce. Physics that goes
beyond the Standard Model can generate additional CP violation.

Physicists are also studying the precise electric and magnetic
properties of particles. The Standard Model predicts that electrons
and quarks behave as microscopic magnets with a specific
strength and that their behavior in an electric field is determined
purely by their electric charge. Most extensions of the
Standard Model predict a slightly different magnetic strength
and electrical behavior. Experiments are beginning to collect
data with enough sensitivity to see the tiny effects predicted.

Looking beyond the earth, scientists studying solar neutrinos
and cosmic-ray neutrinos, ghostly particles that barely interact
at all, have recently established that neutrinos have masses,
a result long expected by theorists studying extensions of the Standard
Model. The next round of experiments will clarify the
form of theory needed to explain the observed neutrino masses.

In addition, experiments are under way to detect mysterious
particles that form the cold dark matter of the universe and to
examine protons at higher levels of sensitivity to learn whether
they decay. Success in either project would be a landmark of
post–Standard Model physics.

As all this research proceeds, it is ushering in a new, data-rich
era in particle physics. Joining the fray by about 2007 will be the
Large Hadron Collider (LHC), a machine 27 kilometers in circumference
now under construction at CERN, the European laboratory
for particle physics near Geneva. A 30-kilometer-long linear electron-positron collider
that will complement the LHC’s results is in the design stages.

As the first hints of post–Standard Model physics are
glimpsed, news reports often make it sound as if the Standard
Model has been found to be wrong, as if it were broken and
ready to be discarded, but that is not the right way to think
about it. Take the example of Maxwell’s equations, written
down in the late 19th century to describe the electromagnetic
force. In the early 20th century we learned that at atomic sizes
a quantum version of Maxwell’s equations is needed. Later the
Standard Model included these quantum Maxwell’s equations
as a subset of its equations. In neither case do we say Maxwell’s
equations are wrong. They are extended. (And they are still used
to design innumerable electronic technologies.)

A Permanent Edifice

Similarly, the Standard Model is here to stay. It is
a full mathematical theory—a multiply connected and highly
stable edifice. It will turn out to be one piece of a larger such edifice,
but it cannot be “wrong.” No part of the theory can fail
without a collapse of the entire structure. If the theory were
wrong, many successful tests would be accidents. It will continue
to describe strong, weak and electromagnetic interactions at
low energies.

The Standard Model is very well tested. It predicted the existence
of the W and Z bosons, the gluon and two of the heavier
quarks (the charm and the top quark). All these particles were
subsequently found, with precisely the predicted properties.

A second major test involves the electroweak mixing angle,
a parameter that plays a role in describing the weak and electromagnetic
interactions. That mixing angle must have the same
value for every electroweak process. If the Standard Model were
wrong, the mixing angle could have one value for one process,
a different value for another and so on. It is observed to have the
same value everywhere, to an accuracy of about 1 percent.

Third, the Large Electron-Positron (LEP) collider at CERN,
which ran from 1989 to 2000, looked at about 20 million Z
bosons. Essentially every one of them decayed in the manner expected
by the Standard Model, which predicted the number of
instances of each kind of decay as well as details of the energies
and directions of the outgoing particles. These tests are but a few
of the many that have solidly confirmed the Standard Model.

In its full glory, the Standard Model has 17 particles and
about as many free parameters—quantities such as particle masses
and strengths of interactions.
These quantities can in principle take any value, and we learn the
correct values only by making measurements. Armchair critics
sometimes compare the Standard Model’s many parameters
with the epicycles on epicycles that medieval theorists used to
describe planetary orbits. They imagine that the Standard Model
has limited predictive power, or that its content is arbitrary,
or that it can explain anything by adjusting of some parameter.

The opposite is actually true: once the masses and interaction
strengths are measured in any process, they are fixed for
the whole theory and for any other experiment, leaving no freedom
at all. Moreover, the detailed forms of all the Standard
Model’s equations are determined by the theory. Every
parameter but the Higgs boson mass has been measured. Until
we go beyond the Standard Model, the only thing that can
change with new results is the precision of our knowledge of the
parameters, and as that improves it becomes harder, not easier,
for all the experimental data to remain consistent, because
measured quantities must agree to higher levels of precision.

Adding further particles and interactions to extend the Standard
Model might seem to introduce a lot more freedom, but
this is not necessarily the case. The most widely favored extension
is the Minimal Supersymmetric Standard Model (MSSM).
Supersymmetry assigns a superpartner particle to every particle
species. We know little about the masses of those superpartners,
but their interactions are constrained by the supersymmetry.
Once the masses are measured, the predictions of the MSSM
will be even more tightly constrained than the Standard Model
because of the mathematical relations of supersymmetry.

Ten Mysteries

If the Standard Model works so well, why must it be
extended? A big hint arises when we pursue the long-standing
goal of unifying the forces of nature. In the Standard Model, we
can extrapolate the forces and ask how they would behave at
much higher energies. For example, what were the forces like in
the extremely high temperatures extant soon after the big bang?
At low energies the strong force is about 30 times as powerful
as the weak force and more than 100 times as powerful as electromagnetism.
When we extrapolate, we find that the strengths
of these three forces become very similar but are never all exactly
the same. If we extend the Standard Model to the MSSM, the
forces become essentially identical at a specific high energy. Even better, the gravitational force approaches
the same strength at a slightly higher energy, suggesting
a connection between the Standard Model forces and gravity.
These results seem like strong clues in favor of the MSSM.

Other reasons for extending the Standard Model arise from
phenomena it cannot explain or cannot even accommodate:
1. All our theories today seem to imply that the universe
should contain a tremendous concentration of energy, even
in the emptiest regions of space. The gravitational effects of
this so-called vacuum energy would have either quickly
curled up the universe long ago or expanded it to much
greater size. The Standard Model cannot help us understand
this puzzle, called the cosmological constant problem.
2. The expansion of the universe was long believed to be
slowing down because of the mutual gravitational attraction
of all the matter in the universe. We now know that the expansion
is accelerating and that whatever causes the acceleration
(dubbed “dark energy”) cannot be Standard Model
physics.
3. There is very good evidence that in the first fraction of a
second of the big bang the universe went through a stage of
extremely rapid expansion called inflation. The fields responsible
for inflation cannot be Standard Model ones.
4. If the universe began in the big bang as a huge burst of energy,
it should have evolved into equal parts matter and antimatter
(CP symmetry). But instead the stars and nebulae
are made of protons, neutrons and electrons and not their antiparticles
(their antimatter equivalents). This matter asymmetry
cannot be explained by the Standard Model.
5. About a quarter of the universe is invisible cold dark matter
that cannot be particles of the Standard Model.
6. In the Standard Model, interactions with the Higgs field
(which is associated with the Higgs boson) cause particles
to have mass. The Standard Model cannot explain the very
special forms that the Higgs interactions must take.
7. Quantum corrections apparently make the calculated
Higgs boson mass huge, which in turn would make all particle
masses huge. That result cannot be avoided in the Standard
Model and thus causes a serious conceptual problem.
8. The Standard Model cannot include gravity, because it
does not have the same structure as the other three forces.
9. The values of the masses of the quarks and leptons (such
as the electron and neutrinos) cannot be explained by the
Standard Model.
10. The Standard Model has three “generations” of particles.
The everyday world is made up entirely of first-generation
particles, and that generation appears to form a consistent
theory on its own. The Standard Model describes all three generations,
but it cannot explain why more than one exists.

In expressing these mysteries, when I say the Standard Model
cannot explain a given phenomenon, I do not mean that the
theory has not yet explained it but might do so one day. The
Standard Model is a highly constrained theory, and it cannot
ever explain the phenomena listed above. Possible explanations
do exist. One reason the supersymmetric extension is attractive
to many physicists is that it can address all but the second and
the last three of these mysteries. String theory (in which particles
are represented by tiny, one-dimensional entities instead of
point objects) addresses the last three. The phenomena that the Standard Model cannot
explain are clues to how it will be extended.

It is not surprising that there are questions that the Standard
Model cannot answer—every successful theory in science has increased
the number of answered questions but has left some unanswered.
And even though improved understanding has led to
new questions that could not be formulated earlier, the number
of unanswered fundamental questions has continued to decrease.

Some of these 10 mysteries demonstrate another reason why
particle physics today is entering a new era. It has become clear
that many of the deepest problems in cosmology have their solutions
in particle physics, so the fields have merged into “particle
cosmology.” Only from cosmological studies could we
learn that the universe is matter (and not antimatter) or that the
universe is about a quarter cold dark matter. Any theoretical understanding
of these phenomena must explain how they arise as
part of the evolution of the universe after the big bang. But cosmology
alone cannot tell us what particles make up cold dark
matter, or how the matter asymmetry is actually generated, or
how inflation originates. Understanding of the largest and the
smallest phenomena must come together.