Read The Higgs Boson: Searching for the God Particle Online
Authors: Scientific American Editors
The Higgs Boson: Searching for the God Particle (21 page)
The Higgs
Physicists are tackling all these post–Standard Model
mysteries, but one essential aspect of the Standard Model also
remains to be completed. To give mass to leptons, quarks, and
Wand Z bosons, the theory relies on the Higgs field, which has
not yet been directly detected.
The Higgs is fundamentally unlike any other field. To understand
how it is different, consider the electromagnetic field.
Electric charges give rise to electromagnetic fields such as those
all around us (just turn on a radio to sense them). Electromagnetic
fields carry energy. A region of space has its lowest possible
energy when the electromagnetic field vanishes throughout
it. Zero field is the natural state in the absence of charged particles.
Surprisingly, the Standard Model requires that the lowest
energy occur when the Higgs field has a specific nonzero value.
Consequently, a nonzero Higgs field permeates the universe, and
particles always interact with this field, traveling through it like
people wading through water. The interaction gives them their
mass, their inertia.
Associated with the Higgs field is the Higgs boson. In the
Standard Model, we cannot predict any particle masses from
first principles, including the mass of the Higgs boson itself.
One can, however, use other measured quantities to calculate
some masses, such as those of the Wand Z bosons and the top
quark. Those predictions are confirmed, giving assurance to the
underlying Higgs physics.
Physicists do already know something about the Higgs mass.
Experimenters at the LEP collider measured about 20 quantities
that are related to one another by the Standard Model. All the
parameters needed to calculate predictions for those quantities
are already measured—except for the Higgs boson mass. One can
therefore work backward from the data and ask which Higgs
mass gives the best fit to the 20 quantities. The answer is that the
Higgs mass is less than about 200 giga-electron-volts (GeV). (The
proton mass is about 0.9 GeV; the top quark 174 GeV.) That
there is an answer at all is strong evidence that the Higgs exists.
If the Higgs did not exist and the Standard Model were wrong,
it would take a remarkable coincidence for the 20 quantities to
be related in the right way to be consistent with a specific Higgs
mass. Our confidence in this procedure is bolstered because a
similar approach accurately predicted the top quark mass before
any top quarks had been detected directly.
LEP also conducted a direct search for Higgs particles, but
it could search only up to a mass of about 115 GeV. At that very
upper limit of LEP’s reach, a small number of events involved
particles that behaved as Higgs bosons should. But there were
not enough data to be sure a Higgs boson was actually discovered. Together the results suggest the Higgs mass lies between
115 and 200 GeV.
LEP is now dismantled to make way for the construction of
the LHC, which is scheduled to begin taking data in four years.
In the meantime the search for the Higgs continues at the Tevatron
at Fermilab. If the Tevatron operates
at its design intensity and energy and does not lose running
time because of technical or funding difficulties, it could confirm
the 115-GeV Higgs boson in about two to three years. If the
Higgs is heavier, it will take longer for a clear signal to emerge
from the background. The Tevatron will produce more than
10,000 Higgs bosons altogether if it runs as planned, and it
could test whether the Higgs boson behaves as predicted. The
LHC will be a “factory” for Higgs bosons, producing millions
of them and allowing extensive studies.
There are also good arguments that some of the lighter superpartner
particles predicted by the MSSM have masses small
enough so that they could be produced at the Tevatron as well.
Direct confirmation of supersymmetry could come in the next
few years. The lightest superpartner is a prime candidate to
make up the cold dark matter of the universe—it could be directly
observed for the first time by the Tevatron. The LHC will
produce large numbers of superpartners if they exist, definitively
testing whether supersymmetry is part of nature.
Effective Theories
To fully grasp the relation of the Standard Model to the
rest of physics, and its strengths and limitations, it is useful to
think in terms of effective theories. An effective theory is a description
of an aspect of nature that has inputs that are, in principle
at least, calculable using a deeper theory. For example, in
nuclear physics one takes the mass, charge and spin of the proton
as inputs. In the Standard Model, one can calculate those
quantities, using properties of quarks and gluons as inputs. Nuclear
physics is an effective theory of nuclei, whereas the Standard
Model is the effective theory of quarks and gluons.
From this point of view, every effective theory is open-ended
and equally fundamental—that is, not truly fundamental at
all. Will the ladder of effective theories continue? The MSSM
solves a number of problems the Standard Model does not solve,
but it is also an effective theory because it has inputs as well. Its
inputs might be calculable in string theory.
Even from the perspective of effective theories, particle physics
may have special status. Particle physics might increase our
understanding of nature to the point where the theory can be
formulated with no inputs. String theory or one of its cousins
might allow the calculation of all inputs—not only the electron
mass and such quantities but also the existence of spacetime and
the rules of quantum theory. But we are still an effective theory
or two away from achieving that goal.
Sidebar: The Standard Model
THE PARTICLES
Although the standard model needs to be extended, its particles suffice to describe the everyday world (except for gravity) and almost all data collected by particle physicists.
Matter Particles (Fermions)
In the Standard Model, the fundamental particles of ordinary matter are the electron, the up quark (u) and the down quark (d). Triplets of quarks bind together to form protons (uud) and neutrons (udd), which in turn make up atomic nuclei (above). The electron and the up and the down quarks, together with the electron-neutrino, form the first of three groups of particles called generations. Each generation is identical in every respect except for the masses of the particles (see grid, scroll below). The values of the neutrino masses in the chart are speculative but chosen to be consistent with observations.
Force Carriers: Bosons
The Standard Model describes three of the four known forces: electromagnetism, the weak force (which is involved in the formation of the chemical elements) and the strong force (which holds protons, neutrons and nuclei together). The forces are mediated by force particles: photons for electromagnetism, the W and Z bosons for the weak force, and gluons for the strong force. For gravity, gravitons are postulated, but the Standard Model does not include gravity. The Standard Model partially unifies the electromagnetic and weak forces—they are facets of one “electroweak” force at high energies or, equivalently, at distances smaller than the diameter of protons.
One of the greatest successes of the Standard Model is that the forms of the forces—the detailed structure of the equations describing them—are largely determined by general principles embodied in the theory rather than being chosen in an ad hoc fashion to match a collection of empirical data. For electromagnetism, for example, the validity of relativistic quantum field theory (on which the Standard Model is based) and the existence of the electron imply that the photon must also exist and interact in the way that it does—we finally understand light. Similar arguments predicted the existence and properties, later confirmed, of gluons and the W and Z particles.
The Source of Mass
In addition to the particles described above, the Standard Model predicts the existence of the Higgs boson, which has not yet been directly detected by experiment. The Higgs interacts with the other particles in a special manner that gives them mass.
Deeper Levels
Might the Standard Model be superseded by a theory in which quarks and electrons are made up of more fundamental particles? Almost certainly not. Experiments have probed much more deeply than ever before without finding a hint of additional structure. More important, the Standard Model is a consistent theory that makes sense if electrons and quarks are fundamental. There are no loose ends hinting at a deeper underlying structure. Further, all the forces become similar at high energies, particularly if supersymmetry is true. If electrons and quarks are composite, this unification fails: the forces do not become equal. Relativistic quantum field theory views electrons and quarks as being pointlike—they are structureless. In the future, they might be thought of as tiny strings or membranes (as in string theory), but they will still be electrons and quarks, with all the known Standard Model properties of these objects at low energies.
THE RULES OF THE GAME
The standard model describes the fundamental particles and how they interact. For a full understanding of nature, we also need to know what rules to use to calculate the results of the interactions. An example that helps to elucidate this point is Newton’s law,
F = ma
.
F
is any force,
m
is the mass of any particle, and a is the acceleration of the particle induced by the force. Even if you know the particles and the forces acting on them, you cannot calculate how the particles behave unless you also know the rule
F = ma
. The modern version of the rules is relativistic quantum field theory, which was invented in the first half of the 20th century. In the second half of the 20th century the development of the Standard Model taught researchers about the nature of the particles and forces that were playing by the rules of quantum field theory. The classical concept of a force is also extended by the Standard Model: in addition to pushing and pulling on one another, when particles interact they can change their identity and be created or destroyed.
Feynman diagrams (a–g, below), first devised by physicist Richard P. Feynman, serve as useful shorthand to describe interactions in quantum field theory. The straight lines represent the trajectories of matter particles; the wavy lines represent those of force particles. Electromagnetism is produced by the emission or absorption of photons by any charged particle, such as an electron or a quark. In a, the incoming electron emits a photon and travels off in a new direction. The strong force involves gluons emitted (b) or absorbed by quarks. The weak force involves W and Z particles (c, d), which are emitted or absorbed by both quarks and leptons (electrons, muons, taus and neutrinos). Notice how the W causes the electron to change identity. Gluons (e) and Ws and Zs (f) also self-interact, but photons do not.
Diagrams a through f are called interaction vertices. Forces are produced by combining two or more vertices. For example, the electromagnetic force between an electron and a quark is largely generated by the transfer of a photon (g). Everything that happens in our world, except for gravity, is the result of combinations of these vertices. —G.K.
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