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Introduction
The Higgs Boson is currently thought
to be the elementary sub-atomic particle responsible for the property
of mass. The current situation is that the Higgs Boson has
not yet been formerly identified, but was predicted by and named
after the physicist Peter Higgs of Edinburgh University.
It is thought that it will explain
the reasons why some particles have little or no mass, while others
appear to have a lot of mass. The Higgs Boson may manifest
itself as a single particle or a group of particles. It may not
even exist.
If it does not exist, then a new
problem arises in either quantum mechanics or general relativity,
since an inconsistency arises in the large and small scale behaviour
of matter. This is in fact consistent with the so called
wave particle duality, still unexplained but widely observed in
experiments, and the self cancellation of single photons in split
beam experiments.
How Particles Acquire
Mass
By Mary and Ian Butterworth,
Imperial College London, and Doris and Vigdor Teplitz,
Southern Methodist University, Dallas, Texas, USA.
The Higgs boson is a hypothesised
particle which, if it exists, would give the mechanism by which
particles acquire mass.
Matter is made of molecules; molecules
of atoms; atoms of a cloud of electrons about one-hundred-millionth
of a centimetre and a nucleus about one-hundred-thousandth the
size of the electron cloud. The nucleus is made of protons and
neutrons. Each proton (or neutron) has about two thousand times
the mass of an electron. We know a good deal about why the nucleus
is so small. We do not know, however, how the particles get their
masses. Why are the masses what they are? Why are the ratios of
masses what they are? We can't be said to understand the constituents
of matter if we don't have a satisfactory answer to this question.
Peter Higgs has a model in which
particle masses arise in a beautiful, but complex, progression.
He starts with a particle that has only mass, and no other characteristics,
such as charge, that distinguish particles from empty space. We
can call his particle H. H interacts with other particles; for
example if H is near an electron, there is a force between the
two. H is of a class of particles called "bosons". We
first attempt a more precise, but non-mathematical statement of
the point of the model; then we give explanatory pictures.
In the mathematics of quantum mechanics
describing creation and annihilation of elementary particles,
as observed at accelerators, particles at particular points arise
from "fields" spread over space and time. Higgs found
that parameters in the equations for the field associated with
the particle H can be chosen in such a way that the lowest energy
state of that field (empty space) is one with the field not zero.
It is surprising that the field is not zero in empty space, but
the result, not an obvious one, is: all particles that can interact
with H gain mass from the interaction.
Thus mathematics links the existence
of H to a contribution to the mass of all particles with which
H interacts. A picture that corresponds to the mathematics is
of the lowest energy state, "empty" space, having a
crown of H particles with no energy of their own. Other particles
get their masses by interacting with this collection of zero-energy
H particles. The mass (or inertia or resistance to change in motion)
of a particle comes from its being "grabbed at" by Higgs
particles when we try and move it.
If particles do get their masses
from interacting with the empty space Higgs field, then the Higgs
particle must exist; but we can't be certain without finding the
Higgs. We have other hints about the Higgs; for example, if it
exists, it plays a role in "unifying" different forces.
However, we believe that nature could contrive to get the results
that would flow from the Higgs in other ways. In fact, proving
the Higgs particle does not exist would be scientifically every
bit as valuable as proving it does.
These questions, the mechanisms
by which particles get their masses, and the relationship amongs
different forces of nature, are major ones and so basic to having
an understanding of the constituents of matter and the forces
among them, that it is hard to see how we can make significant
progress in our understanding of the stuff of which the earth
is made without answering them.
The Need to Understand
Mass
By Roger Cashmore Department
of Physics, University of Oxford, UK.
What determines the size of objects
that we see around us or indeed even the size of ourselves? The
answer is the size of the molecules and in turn the atoms that
compose these molecules. But what determines the size of the atoms
themselves? Quantum theory and atomic physics provide an answer.
The size of the atom is determined by the paths of the electrons
orbiting the nucleus. The size of those orbits, however, is determined
by the mass of the electron. Were the electron's mass smaller,
the orbits (and hence all atoms) would be smaller, and consequently
everything we see would be smaller. So understanding the mass
of the electron is essential to understanding the size and dimensions
of everything around us.
It might be hard to understand
the origin of one quantity, that quantity being the mass of the
electron. Fortunately nature has given us more than one elementary
particle and they come with a wide variety of masses. The lightest
particle is the electron and the heaviest particle is believed
to be the particle called the top quark, which weighs at least
200,000 times as much as an electron. With this variety of particles
and masses we should have a clue to the individual masses of the
particles.
Unfortunately if you try and write
down a theory of particles and their interactions then the simplist
version requires all the masses of the particles to be zero. So
on one hand we have a whole variety of masses and on the other
a theory in which all masses should be zero. Such conundrums provide
the excitement and the challenges of science.
There is, however, one very clever
and very elegant solution to this problem, a solution first proposed
by Peter Higgs. He proposed that the whole of space is permeated
by a field, similar in some ways to the electromagnetic field.
As particles move through space they travel through this field,
and if they interact with it they acquire what appears to be mass.
This is similar to the action of viscous forces felt by particles
moving through any thick liquid. the larger the interaction of
the particles with the field, the more mass they appear to have.
Thus the existence of this field is essential in Higgs' hypothesis
for the production of the mass of particles.
We know from quantum theory that
fields have particles associated with them, the particle for the
electromagnetic field being the photon. So there must be a particle
associated with the Higgs field, and this is the Higgs boson.
Finding the Higgs boson is thus the key to discovering whether
the Higgs field does exist and whether our best hypothesis for
the origin of mass is indeed correct.
Politics,
Solid State and the Higgs
By David Miller Department
of Physics and Astronomy, University College, London, UK.
1. The Higgs Mechanism
Imagine a cocktail party of political
party workers who are uniformly distributed across the floor,
all talking to their nearest neighbours. The ex-Prime Minister
enters and crosses the room. All of the workers in her neighbourhood
are strongly attracted to her and cluster round her. As she moves
she attracts the people she comes close to, while the ones she
has left return to their even spacing. Because of the knot of
people always clustered around her she acquires a greater mass
than normal, that is she has more momentum for the same speed
of movement across the room. Once moving she is hard to stop,
and once stopped she is harder to get moving again because the
clustering process has to be restarted.
In three dimensions, and with the
complications of relativity, this is the Higgs mechanism. In order
to give particles mass, a background field is invented which becomes
locally distorted whenever a particle moves through it. The distortion
- the clustering of the field around the particle - generates
the particle's mass. The idea comes directly from the physics
of solids. instead of a field spread throughout all space a solid
contains a lattice of positively charged crystal atoms. When an
electron moves through the lattice the atoms are attracted to
it, causing the electron's effective mass to be as much as 40
times bigger than the mass of a free electron.
The postulated Higgs field in the
vacuum is a sort of hypothetical lattice which fills our Universe.
We need it because otherwise we cannot explain why the Z and W
particles which carry the weak interactions are so heavy while
the photon which carries electromagnetic forces is massless.
2. The Higgs Boson
Now consider a rumour passing through
our room full of uniformly spread political workers. Those near
the door hear of it first and cluster together to get the details,
then they turn and move closer to their next neighbours who want
to know about it too. A wave of clustering passes through the
room. It may spread to all the corners or it may form a compact
bunch which carries the news along a line of workers from the
door to some dignitary at the other side of the room. Since the
information is carried by clusters of people, and since it was
clustering that gave extra mass to the ex-Prime Minister, then
the rumour-carrying clusters also have mass.
The Higgs boson is predicted to
be just such a clustering in the Higgs field. We will find it
much easier to believe that the field exists, and that the mechanism
for giving other particles is true, if we actually see the Higgs
particle itself. Again, there are analogies in the physics of
solids. A crystal lattice can carry waves of clustering without
needing an electron to move and attract the atoms. These waves
can behave as if they are particles. They are called phonons and
they too are bosons.
There could be a Higgs mechanism,
and a Higgs field throughout our Universe, without there being
a Higgs boson. The next generation of colliders will sort this
out.
Of Particles, Pencils and Unification
By Tom KibbleDepartment
of Physics, Imperial College, London, UK.
Theoretical physicists always aim
for unification. Newton recognised that the fall of an apple,
the tides and the orbits of the planets as aspects of a single
phenomenon, gravity. Maxwell unified electricity, magnetism and
light. Each synthesis extends our understanding and leads eventually
to new applications.
In the 1960s the time was ripe
for a further step. We had a marvellously accurate theory of electromagnetic
forces, quantum electrodynamics, or QED, a quantum version of
Maxwell's theory. In it, electromagnetic forces are seen as due
to the exchange between electrically charged particles of photons,
packets (or quanta) of electromagnetic waves. (The distinction
between particle and wave has disappeared in quantum theory.)
The "weak" forces, involved in radioactivity and in
the Sun's power generation, are in many ways very similar, save
for being much weaker and restricted in range. A beautiful unified
theory of weak and electromagnetic forces was proposed in 1967
by Steven Weinberg and Abdus Salam (independently). The weak forces
are due to the exchange of W and Z particles. Their short range,
and apparent weakness at ordinary ranges, is because, unlike the
photon, the W and Z are, by our standards, very massive particles,
100 times heavier than a hydrogen atom.
The "electro-weak" theory
has been convincingly verified, in particular by the discovery
of the W and Z at CERN in 1983, and by many tests of the properties.
However, the origin of their masses remains mysterious. Our best
guess is the "Higgs mechanism" - but that aspect of
the theory remains untested.
The fundamental theory exhibits
a beautiful symmetry between W, Z and photon. But this is a spontaneously
broken symmetry. Spontaneous symmetry breaking is a ubiquitous
phenomenon. For example, a pencil balanced on its tip shows complete
rotational symmetry - it looks the same from every side. - but
when it falls it must do in some particular direction, breaking
the symmetry. We think the masses of the W and Z (and of the electron)
arise through a similar mechanism. It is thought there are "pencils"
throughout space, even in vacuum. (of course, these are not real
physical pencils - they represent the "Higgs field"
- nor is their direction a direction in real physical space, but
the analogy is fairly close.) The pencils are all coupled together,
so that they all tend to fall in the same direction. Their presence
in the vacuum influences waves travelling through it. The waves
have of course a direction in space, but they also have a "direction"
in this conceptual space. In some "directions", waves
have to move the pencils too, so they are more sluggish; those
waves are the W and Z quanta.
The theory can be tested, because
it suggests that there should be another kind of wave, a wave
in the pencils alone, where they are bouncing up and down. That
wave is the Higgs particle. Finding it would confirm that we really
do understand the origin of mass, and allow us to put the capstone
on the electro-weak theory, filling in the few remaining gaps.
Once the theory is complete, we
can hope to build further on it: a longer-term goal is a unified
theory involving also the "strong" interactions that
bind protons and neutrons together in atomic nuclei - and if we
are really optimistic, even gravity, seemingly the hardest force
to bring into the unified scheme.
There are strong hints that a "grand
unified" synthesis is possible, but the details are still
very vague. Finding the Higgs would give us very significant clues
to the nature of that greater synthesis.
Ripples at the Heart of Physics
By Simon Hands Theory Division,
CERN, Geneva, Switzerland.
The Higgs boson is an undiscovered
elementary particle, thought to be a vital piece of the closely
fitting jigsaw of particle physics. Like all particles, it has
wave properties akin to those ripples on the surface of a pond
which has been disturbed; indeed, only when the ripples travel
as a well defined group is it sensible to speak of a particle
at all. In quantum language the analogue of the water surface
which carries the waves is called a field. Each type of particle
has its own corresponding field.
The Higgs field is a particularly
simple one - it has the same properties viewed from every direction,
and in important respects is indistinguishable from empty space.
Thus physicists conceive of the Higgs field being "switched
on", pervading all of space and endowing it with "grain"
like that of a plank of wood. The direction of the grain in undetectable,
and only becomes important once the Higgs' interactions with other
particles are taken into account. for instance, particles called
vector bosons can travel with the grain, in which case they move
easily for large distances and may be observed as photons - that
is, particles of light that we can see or record using a camera;
or against, in which case their effective range is much shorter,
and we call them W or Z particles. These play a central role in
the physics of nuclear reactions, such as those occurring in the
core of the sun.
The Higgs field enables us to view
these apparently unrelated phenomenon as two sides of the same
coin; both may be described in terms of the properties of the
same vector bosons. When particles of matter such as electrons
or quarks (elementary constituents of protons and neutrons, which
in turn constitute the atomic nucleus) travel through the grain,
they are constantly flipped "head-over-heels". this
forces them to move more slowly than their natural speed, that
of light, by making them heavy. We believe the Higgs field responsible
for endowing virtually all the matter we know about with mass.
Like most analogies, the wood-grain
one is persuasive but flawed: we should think of the grain as
not defining a direction in everyday three-dimensional space,
but rather in some abstract internal space populated by various
kinds of vector boson, electron and quark.
The Higgs' ability to fill space
with its mysterious presence makes it a vital component in more
ambitious theories of how the Universe burst into existence out
of some initial quantum fluctuation, and why the Universe prefers
to be filled with matter rather than anti-matter; that is, why
there is something rather than nothing. To constrain these ideas
more rigorously, and indeed flesh out the whole picture, it is
important to find evidence for the Higgs field at first hand -
in other words, find the boson. There are unanswered questions:
the Higgs' very simplicity and versatility, beloved of theorists,
makes it hard to pin down. How many Higgs particles are there?
Might it/they be made from still more elementary components? Most
crucial, how heavy is it? Our current knowledge can only put its
mass roughly between that of an iron atom and three times that
of a uranium atom. This is a completely new form of matter about
whose nature we still have only vague hints and speculations and
its discovery is the most exciting prospect in contemporary particle
physics.
This definition was taken
from Science-Week
... ... *Higgs boson: Higgs fields
(named after Peter W. Higgs,University of Edinburgh, UK) constitute
a set of fundamental theoretical fields that induce spontaneous
symmetry breaking. In general, spontaneous symmetry breaking occurs
in systems whose underlying symmetry state is unstable. A Higgs
particle is associated with a Higgs field in the same way that
a photon is associated with the electromagnetic field. Higgs bosons
are massive mesons whose existence is predicted by certain theories.
Mesons are apparently composed of quark and anti-quark pairs;they
are produced by various high-energy interactions and decay into
stable particles.
This explanation was taken from New Scientist<
What may soon be discovered is a new kind of heavy, highly
unstable particle, the so-called Higgs particle. And we might
see it in just a few months, at one of two high-energy accelerators:
the Large Electron Positron collider (LEP) at CERN near Geneva
or the Tevatron at Batavia, Illinois.
The Higgs is more than just another expensive, highly unstable
particle: it embodies the mechanism that gives other fundamental
particles mass. But isn't mass just a fact of life? Not necessarily.
In fact, ours would be a much simpler world if particles didn't
have mass. For one thing, mass disfigures the theory of the weak
nuclear force. The weak force, as befits its name, is much weaker
than the strong force which holds atomic nuclei together and the
electromagnetic force that holds atoms together. But it does things
that no other interaction can: it causes the slow decay of various
otherwise stable particles, and it is the only interaction aware
of neutrinos. So what's the problem? Well, the existence of mass
means that particles feeling the weak force don't all spin in
the same way. It would be neater if they did.
That is merely untidiness; but there is another, more disturbing
problem with the particles that carry the weak force. All forces
in nature work by the action of such carrier particles; photons
carry the electromagnetic force, for example. And in 1954, Chen
Ning Yang and Robert Mills hypothesised the existence of particles
called vector mesons, generalised versions of the photon, which
looked like good candidates to carry the weak nuclear force. Then
in 1961 Sheldon Glashow used them in a theory that unified weak
and electromagnetic forces. According to this theory, vector mesons
are massless, like the photon. But unlike electromagnetism, the
weak force is short-ranged, a sign that its carrier particles
must have mass. To fix this, Glashow fudged the equations by just
sticking in a mass, without understanding where it came from.
Cosmic molasses
It would be easier, then, to understand an imaginary world with
only massless particles, forever whizzing around at the speed
of light. But we know that in our world particles do have mass.
So to get from that ideal world to ours, we need some kind of
cosmic molasses that fills all space and slows down these massless
speed demons. But if this molasses is everywhere, why can't we
see it?
To understand, imagine you're living in a bar magnet. An ordinary
magnet is really an extraordinary thing. For whereas the laws
of physics do not have a preferred direction, the magnet does:
its pole. Where does this direction come from? Each electron in
any material acts as a small magnet, pointing in the direction
of its spin axis. An isolated electron would be equally happy
with its spin in any direction, an indifference that we call rotational
symmetry. But in some materials, such as iron, neighbouring electrons
prefer to point in the same direction. Like insecure teenagers,
they don't care what they are doing, as long as they are all doing
the same thing. So to make all the electrons happy or, in more
dignified language, to obtain the configuration of minimum energy,
all the spins have to pick a common direction--it doesn't matter
which. That direction defines the magnetic pole.
The rotational symmetry of an isolated spin is gone, but not forgotten.
For if we heat an iron magnet above 870 °C, the spins get enough
energy to break free from their neighbours and point in random
directions again--the material loses its magnetism. If the iron
is later cooled, it will once again become magnetic. But the new
pole will usually point in a different direction from the old.
And rotational symmetry can reappear in another, subtler way.
Give the spins just a little energy, and you can make the preferred
spin direction (the local magnetic North) change slowly as a function
of location. Configurations in which the preferred direction varies
periodically are called spin waves. And just as quantum mechanics
parcels up light waves into photons, it parcels these spin waves
into particles known as magnons.
Particle swarm
Intelligent creatures living inside a magnet would be used to
seeing magnons, but they would have trouble figuring out why magnons
exist. Evolution would adapt their senses to ignore the unchanging
aspects of their environment. So what we think of as the material
of the magnet, they would commonly regard as empty space. And
it would seem obvious that there was a preferred direction to
space, because everything the creatures experienced would be coloured
by the pervasive magnetism of their world. Eventually, though,
some visionary might imagine the true situation: an underlying
set of laws with full rotation symmetry, a symmetry hidden by
the spontaneous alignment of spins in the pervading medium. Our
visionary would have deduced that the "vacuum" is really
a structured medium, explained the existence of magnons, and so
become a hero of physics.
This is just what happened on Earth. We have known since the 1930s
that our vacuum is really a swarm of short-lived "virtual"
particles, appearing and disappearing at random. But where is
the organised structure in this melee? The visionaries who first
saw it were Yochiro Nambu and Jeffrey Goldstone. In the early
1960s they noticed a symmetry by which the laws of physics stay
the same if certain particles are substituted for others. (It
would take an article several times the length of this one to
attach proper names and identifiable faces to these particles,
and unless you are a very unusual person you would not stay awake
to the end. Trust me.) But, just as in the magnet, at low temperature
the symmetry is broken: from the symmetrical swarm of virtual
particles, one kind condenses out in large numbers. So a preference
is formed among the otherwise interchangeable types of particles.
Instead of a preferred direction like the magnet, our space has
a preferred particle composition.
And this is where the cosmic molasses oozes into our story. In
1966 Peter Higgs of the University of Edinburgh, and his co-workers
Robert Brout and François Englert of the Free University in Brussels
added this idea to the theory of vector mesons. They discovered
that when the symmetry breaks, producing a condensate of virtual
Higgs particles, the vector mesons become massive.
Better still, interactions with the condensate could generate
the masses of all the other elementary particles, the quarks and
leptons. Nambu and Goldstone had constructed a form of cosmic
molasses using particles already known to exist. But this isn't
quite enough because it exerts too little drag on the vector mesons,
and none at all on the leptons. In 1967, however, Steven Weinberg
(and later Abdus Salam) postulated an additional stickier form,
and showed how it could give an improved, fudge-free version of
Glashow's weak interaction model. This stickier stuff is what
physicists usually mean when they talk about the Higgs condensate.
How can we test this extraordinary conception? We could try to
heat up the vacuum, by concentrating a lot of energy in a small
space, and watch to see if its symmetry is restored as the condensate
evaporates. All particles in this region would become massless.
Unfortunately, that will only happen at temperatures approaching
1016 kelvin. Although such temperatures were universal in the
early stages of the Big Bang, they are out of reach on Earth for
the foreseeable future. The Relativistic Heavy Ion Collider at
Brookhaven, New York, due to turn on this summer, will peak at
only 1013 kelvin.
Stir it up
A much more modest project is feasible, however. Rather than restore
symmetry completely, we can stir up the Higgs condensate a bit.
This being a quantum world, we can only stir it up in dis-crete
units. The minimal excitation--a ripple in the cosmic molasses--is
the Higgs particle.
How hard will it be to make this particle? Who gets to taste the
joy of discovery depends on the value of the Higgs mass, as does
the nature of particle physics. We can already narrow down the
range.
If the Higgs particle were lighter than 95 gigaelectronvolts (GeV),
about 100 times the mass of a proton, LEP would already have seen
it. If it were heavier than 600 GeV, virtual Higgs particles would
affect many particle reactions in a way that experiments have
already ruled out. And the promising theoretical idea of supersymmetry--an
extension of the Standard Model that proposes a host of extra
fundamental particles, partners of the familiar bunch--predicts
masses well below 200 GeV for the Higgs particle; probably between
100 and 130 GeV.
That is why so much excitement surrounds the upcoming explorations.
Scientists at LEP will drive their machine to the limits of its
energy and luminosity, pushing the mass window up to 105 GeV or
so within two years. Meanwhile, scientists at the Tevatron hope
to explore all the way up to 160 GeV. If they fail, then a final
effort will be made at the Large Hadron Collider (LHC) being constructed
in Geneva due to open around 2005. Its reach extends beyond 600
GeV. If that fails, we theoretical physicists will be exceedingly
embarrassed, and I hesitate to predict what we'll do.
The Standard Model requires just one Higgs particle. But theories
with more symmetry imply several new particles--Higgs galore.
The theory of supersymmetry predicts at least five Higgs-type
particles. In the most popular version, the lightest member of
the Higgs family has the properties we discussed above. There
is no consensus on the masses of the others, although they should
not be much heavier than 1000 GeV, and might be much lighter.
The masses of these particles will tell us how the supersymmetric
partners of ordinary particles hide themselves from us. At present
it is a big mystery, and wild concepts are in the air, including
their infection by otherwise inaccessible "dark" matter,
or exotic condensates living only in extra dimensions of space.
The LHC should shed light on this mystery.
More ambitious models that unify the strong and electroweak forces
predict a bizarre tribe of very much heavier Higgs particles.
We probably won't be able to make them directly anytime soon,
but we might sense the effect of their exchange as virtual particles.
Some of them can make protons decay, at rates close to current
experimental limits.
I hope I've conveyed why we physicists find cosmic molasses to
our taste, and look forward to sampling it soon, perhaps in several
varieties.
Afterword…. Actually the lion's share of ordinary mass, in
protons and neutrons, has nothing to do with Higgs particles.
It comes instead from the energy of the gluon field that holds
their constituent quarks together. Intrigued?
Frank Wilczek
is a theoretical physicist at the Institute for Advanced Study
in Princeton. |
