June 27, 2005
The Mysteries of Mass
Physicists are hunting for an elusive particle that would reveal the
presence of a new kind of field that permeates all of reality. Finding that
Higgs field will give us a more complete understanding about how the universe
works
By Gordon Kane
Most people think they know what mass is, but they understand only part of the story. For instance, an elephant is clearly bulkier and weighs more than an ant. Even in the absence of gravity, the elephant would have greater mass--it would be harder to push and set in motion. Obviously the elephant is more massive because it is made of many more atoms than the ant is, but what determines the masses of the individual atoms? What about the elementary particles that make up the atoms--what determines their masses? Indeed, why do they even have mass?
We see that the problem of mass has two independent aspects. First, we need to learn how mass arises at all. It turns out mass results from at least three different mechanisms, which I will describe below. A key player in physicists' tentative theories about mass is a new kind of field that permeates all of reality, called the Higgs field. Elementary particle masses are thought to come about from the interaction with the Higgs field. If the Higgs field exists, theory demands that it have an associated particle, the Higgs boson. Using particle accelerators, scientists are now hunting for the Higgs.
The second aspect is that scientists want to know why different species of elementary particles have their specific quantities of mass. Their intrinsic masses span at least 11 orders of magnitude, but we do not yet know why that should be so. For comparison, an elephant and the smallest of ants differ by about 11 orders of magnitude of mass.
What Is Mass?
Isaac Newton presented the earliest scientific definition of mass in 1687 in his landmark Principia: "The quantity of matter is the measure of the same, arising from its density and bulk conjointly." That very basic definition was good enough for Newton and other scientists for more than 200 years. They understood that science should proceed first by describing how things work and later by understanding why. In recent years, however, the why of mass has become a research topic in physics. Understanding the meaning and origins of mass will complete and extend the Standard Model of particle physics, the well-established theory that describes the known elementary particles and their interactions. It will also resolve mysteries such as dark matter, which makes up about 25 percent of the universe.
Why is the Higgs field present throughout the universe? What is the Higgs field?
The foundation of our modern understanding of mass is far more intricate than Newton's definition and is based on the Standard Model. At the heart of the Standard Model is a mathematical function called a Lagrangian, which represents how the various particles interact. From that function, by following rules known as relativistic quantum theory, physicists can calculate the behavior of the elementary particles, including how they come together to form compound particles, such as protons. For both the elementary particles and the compound ones, we can then calculate how they will respond to forces, and for a force F, we can write Newton's equation F = ma, which relates the force, the mass and the resulting acceleration. The Lagrangian tells us what to use for m here, and that is what is meant by the mass of the particle.
But mass, as we ordinarily understand it, shows up in more than just F = ma. For example, Einstein's special relativity theory predicts that massless particles in a vacuum travel at the speed of light and that particles with mass travel more slowly, in a way that can be calculated if we know their mass. The laws of gravity predict that gravity acts on mass and energy as well, in a precise manner. The quantity m deduced from the Lagrangian for each particle behaves correctly in all those ways, just as we expect for a given mass.
Fundamental particles have an intrinsic mass known as their rest mass (those with zero rest mass are called massless). For a compound particle, the constituents' rest mass and also their kinetic energy of motion and potential energy of interactions contribute to the particle's total mass. Energy and mass are related, as described by Einstein's famous equation, E = mc2 (energy equals mass times the speed of light squared).
An example of energy contributing to mass occurs in the most familiar kind of matter in the universe--the protons and neutrons that make up atomic nuclei in stars, planets, people and all that we see. These particles amount to 4 to 5 percent of the mass-energy of the universe. The Standard Model tells us that protons and neutrons are composed of elementary particles called quarks that are bound together by massless particles called gluons. Although the constituents are whirling around inside each proton, from outside we see a proton as a coherent object with an intrinsic mass, which is given by adding up the masses and energies of its constituents.
The Standard Model lets us calculate that nearly all the mass of protons and neutrons is from the kinetic energy of their constituent quarks and gluons (the remainder is from the quarks' rest mass). Thus, about 4 to 5 percent of the entire universe--almost all the familiar matter around us--comes from the energy of motion of quarks and gluons in protons and neutrons.
The Higgs Mechanism
Unlike protons and neutrons, truly elementary particles--such as quarks and electrons--are not made up of smaller pieces. The explanation of how they acquire their rest masses gets to the very heart of the problem of the origin of mass. As I noted above, the account proposed by contemporary theoretical physics is that fundamental particle masses arise from interactions with the Higgs field. But why is the Higgs field present throughout the universe? Why isn't its strength essentially zero on cosmic scales, like the electromagnetic field? What is the Higgs field? T
he Higgs field is a quantum field. That may sound mysterious, but the fact is that all elementary particles arise as quanta of a corresponding quantum field. The electromagnetic field is also a quantum field (its corresponding elementary particle is the photon). So in this respect, the Higgs field is no more enigmatic than electrons and light. The Higgs field does, however, differ from all other quantum fields in three crucial ways.
The first difference is somewhat technical. All fields have a property called spin, an intrinsic quantity of angular momentum that is carried by each of their particles. Particles such as electrons have spin 1/2 and most particles associated with a force, such as the photon, have spin 1. The Higgs boson (the particle of the Higgs field) has spin 0. Having 0 spin enables the Higgs field to appear in the Lagrangian in different ways than the other particles do, which in turn allows--and leads to--its other two distinguishing features.
The second unique property of the Higgs field explains how and why it has nonzero strength throughout the universe. Any system, including a universe, will tumble into its lowest energy state, like a ball bouncing down to the bottom of a valley. For the familiar fields, such as the electromagnetic fields that give us radio broadcasts, the lowest energy state is the one in which the fields have zero value (that is, the fields vanish)--if any nonzero field is introduced, the energy stored in the fields increases the net energy of the system. But for the Higgs field, the energy of the universe is lower if the field is not zero but instead has a constant nonzero value. In terms of the valley metaphor, for ordinary fields the valley floor is at the location of zero field; for the Higgs, the valley has a hillock at its center (at zero field) and the lowest point of the valley forms a circle around the hillock. The universe, like a ball, comes to rest somewhere on this circular trench, which corresponds to a nonzero value of the field. That is, in its natural, lowest energy state, the universe is permeated throughout by a nonzero Higgs field.
The final distinguishing characteristic of the Higgs field is the form of its interactions with the other particles. Particles that interact with the Higgs field behave as if they have mass, proportional to the strength of the field times the strength of the interaction. The masses arise from the terms in the Lagrangian that have the particles interacting with the Higgs field.
Our understanding of all this is not yet complete, however, and we are not sure how many kinds of Higgs fields there are. Although the Standard Model requires only one Higgs field to generate all the elementary particle masses, physicists know that the Standard Model must be superseded by a more complete theory. Leading contenders are extensions of the Standard Model known as Supersymmetric Standard Models (SSMs). In these models, each Standard Model particle has a so-called superpartner (as yet undetected) with closely related properties [see "The Dawn of Physics beyond the Standard Model," by Gordon Kane; Scientific American, June 2003]. With the Supersymmetric Standard Model, at least two different kinds of Higgs fields are needed. Interactions with those two fields give mass to the Standard Model particles. They also give some (but not all) mass to the superpartners. The two Higgs fields give rise to five species of Higgs boson: three that are electrically neutral and two that are charged. The masses of particles called neutrinos, which are tiny compared with other particle masses, could arise rather indirectly from these interactions or from yet a third kind of Higgs field.
Theorists have several reasons for expecting the SSM picture of the Higgs interaction to be correct. First, without the Higgs mechanism, the W and Z bosons that mediate the weak force would be massless, just like the photon (which they are related to), and the weak interaction would be as strong as the electromagnetic one. Theory holds that the Higgs mechanism confers mass to the W and Z in a very special manner. Predictions of that approach (such as the ratio of the W and Z masses) have been confirmed experimentally.
Second, essentially all other aspects of the Standard Model have been well tested, and with such a detailed, interlocking theory it is difficult to change one part (such as the Higgs) without affecting the rest. For example, the analysis of precision measurements of W and Z boson properties led to the accurate prediction of the top quark mass before the top quark had been directly produced. Changing the Higgs mechanism would spoil that and other successful predictions.
Third, the Standard Model Higgs mechanism works very well for giving mass to all the Standard Model particles, W and Z bosons, as well as quarks and leptons; the alternative proposals usually do not. Next, unlike the other theories, the SSM provides a framework to unify our understanding of the forces of nature. Finally, the SSM can explain why the energy "valley" for the universe has the shape needed by the Higgs mechanism. In the basic Standard Model the shape of the valley has to be put in as a postulate, but in the SSM that shape can be derived mathematically.
Testing the Theory
Naturally, physicists want to carry out direct tests of the idea that mass arises from the interactions with the different Higgs fields. We can test three key features. First, we can look for the signature particles called Higgs bosons. These quanta must exist, or else the explanation is not right. Physicists are currently looking for Higgs bosons at the Tevatron Collider at Fermi National Accelerator Laboratory in Batavia, Ill.
Second, once they are detected we can observe how Higgs bosons interact with other particles. The very same terms in the Lagrangian that determine the masses of the particles also fix the properties of such interactions. So we can conduct experiments to test quantitatively the presence of interaction terms of that type. The strength of the interaction and the amount of particle mass are uniquely connected.
Third, different sets of Higgs fields, as occur in the Standard Model or in the various SSMs, imply different sets of Higgs bosons with various properties, so tests can distinguish these alternatives, too. All that we need to carry out the tests are appropriate particle colliders--ones that have sufficient energy to produce the different Higgs bosons, sufficient intensity to make enough of them and very good detectors to analyze what is produced.
A practical problem with performing such tests is that we do not yet understand the theories well enough to calculate what masses the Higgs bosons themselves should have, which makes searching for them more difficult because one must examine a range of masses. A combination of theoretical reasoning and data from experiments guides us about roughly what masses to expect.
The Large Electron-Positron Collider (LEP) at CERN, the European laboratory for particle physics near Geneva, operated over a mass range that had a significant chance of including a Higgs boson. It did not find one--although there was tantalizing evidence for one just at the limits of the collider's energy and intensity--before it was shut down in 2000 to make room for constructing a newer facility, CERN's Large Hadron Collider (LHC). The Higgs must therefore be heavier than about 120 proton masses. Nevertheless, LEP did produce indirect evidence that a Higgs boson exists: experimenters at LEP made a number of precise measurements, which can be combined with similar measurements from the Tevatron and the collider at the Stanford Linear Accelerator Center. The entire set of data agrees well with theory only if certain interactions of particles with the lightest Higgs boson are included and only if the lightest Higgs boson is not heavier than about 200 proton masses. That provides researchers with an upper limit for the mass of the Higgs boson, which helps focus the search.
The LEP collider saw tantalizing evidence for the Higgs particle.
For the next few years, the only collider that could produce direct evidence for Higgs bosons will be the Tevatron. Its energy is sufficient to discover a Higgs boson in the range of masses implied by the indirect LEP evidence, if it can consistently achieve the beam intensity it was expected to have, which so far has not been possible. In 2007 the LHC, which is seven times more energetic and is designed to have far more intensity than the Tevatron, is scheduled to begin taking data. It will be a factory for Higgs bosons (meaning it will produce many of the particles a day). Assuming the LHC functions as planned, gathering the relevant data and learning how to interpret it should take one to two years. Carrying out the complete tests that show in detail that the interactions with Higgs fields are providing the mass will require a new electron-positron collider in addition to the LHC (which collides protons) and the Tevatron (which collides protons and antiprotons).
Dark Matter
What is discovered about Higgs bosons will not only test whether the Higgs mechanism is indeed providing mass, it will also point the way to how the Standard Model can be extended to solve problems such as the origin of dark matter.
With regard to dark matter, a key particle of the SSM is the lightest superpartner (LSP). Among the superpartners of the known Standard Model particles predicted by the SSM, the LSP is the one with the lowest mass. Most superpartners decay promptly to lower-mass superpartners, a chain of decays that ends with the LSP, which is stable because it has no lighter particle that it can decay into. (When a superpartner decays, at least one of the decay products should be another superpartner; it should not decay entirely into Standard Model particles.) Superpartner particles would have been created early in the big bang but then promptly decayed into LSPs. The LSP is the leading candidate particle for dark matter.
The Higgs bosons may also directly affect the amount of dark matter in the universe. We know that the amount of LSPs today should be less than the amount shortly after the big bang, because some would have collided and annihilated into quarks and leptons and photons, and the annihilation rate may be dominated by LSPs interacting with Higgs bosons.
As mentioned earlier, the two basic SSM Higgs fields give mass to the Standard Model particles and some mass to the superpartners, such as the LSP. The superpartners acquire more mass via additional interactions, which may be with still further Higgs fields or with fields similar to the Higgs. We have theoretical models of how these processes can happen, but until we have data on the superpartners themselves we will not know how they work in detail. Such data are expected from the LHC or perhaps even from the Tevatron.
Neutrino masses may also arise from interactions with additional Higgs or Higgs-like fields, in a very interesting way. Neutrinos were originally assumed to be massless, but since 1979 theorists have predicted that they have small masses, and over the past decade several impressive experiments have confirmed the predictions [see "Solving the Solar Neutrino Problem," by Arthur B. McDonald, Joshua R. Klein and David L. Wark; Scientific American, April 2003]. The neutrino masses are less than a millionth the size of the next smallest mass, the electron mass. Because neutrinos are electrically neutral, the theoretical description of their masses is more subtle than for charged particles. Several processes contribute to the mass of each neutrino species, and for technical reasons the actual mass value emerges from solving an equation rather than just adding the terms.
Thus, we have understood the three ways that mass arises: The main form of mass we are familiar with--that of protons and neutrons and therefore of atoms--comes from the motion of quarks bound into protons and neutrons. The proton mass would be about what it is even without the Higgs field. The masses of the quarks themselves, however, and also the mass of the electron, are entirely caused by the Higgs field. Those masses would vanish without the Higgs. Last, but certainly not least, most of the amount of superpartner masses, and therefore the mass of the dark matter particle (if it is indeed the lightest superpartner), comes from additional interactions beyond the basic Higgs one.
Finally, we consider an issue known as the family problem. Over the past half a century physicists have shown that the world we see, from people to flowers to stars, is constructed from just six particles: three matter particles (up quarks, down quarks and electrons), two force quanta (photons and gluons), and Higgs bosons--a remarkable and surprisingly simple description. Yet there are four more quarks, two more particles similar to the electron, and three neutrinos. All are very short-lived or barely interact with the other six particles. They can be classified into three families: up, down, electron neutrino, electron; charm, strange, muon neutrino, muon; and top, bottom, tau neutrino, tau. The particles in each family have interactions identical to those of the particles in other families. They differ only in that those in the second family are heavier than those in the first, and those in the third family are heavier still. Because these masses arise from interactions with the Higgs field, the particles must have different interactions with the Higgs field.
Hence, the family problem has two parts: Why are there three families when it seems only one is needed to describe the world we see? Why do the families differ in mass and have the masses they do? Perhaps it is not obvious why physicists are astonished that nature contains three almost identical families even if one would do. It is because we want to fully understand the laws of nature and the basic particles and forces. We expect that every aspect of the basic laws is a necessary one. The goal is to have a theory in which all the particles and their mass ratios emerge inevitably, without making ad hoc assumptions about the values of the masses and without adjusting parameters. If having three families is essential, then it is a clue whose significance is currently not understood.
Tying It All Together
The standard model and the SSM can accommodate the observed family structure, but they cannot explain it. This is a strong statement. It is not that the SSM has not yet explained the family structure but that it cannot. For me, the most exciting aspect of string theory is not only that it may provide us with a quantum theory of all the forces but also that it may tell us what the elementary particles are and why there are three families. String theory seems able to address the question of why the interactions with the Higgs field differ among the families. In string theory, repeated families can occur, and they are not identical. Their differences are described by properties that do not affect the strong, weak, electromagnetic or gravitational forces but that do affect the interactions with Higgs fields, which fits with our having three families with different masses. Although string theorists have not yet fully solved the problem of having three families, the theory seems to have the right structure to provide a solution. String theory allows many different family structures, and so far no one knows why nature picks the one we observe rather than some other [see "The String Theory Landscape," by Raphael Bousso and Joseph Polchinski; Scientific American, September 2004]. Data on the quark and lepton masses and on their superpartner masses may provide major clues to teach us about string theory.
One can now understand why it took so long historically to begin to understand mass. Without the Standard Model of particle physics and the development of quantum field theory to describe particles and their interactions, physicists could not even formulate the right questions. Whereas the origins and values of mass are not yet fully understood, it is likely that the framework needed to understand them is in place. Mass could not have been comprehended before theories such as the Standard Model and its supersymmetric extension and string theory existed. Whether they indeed provide the complete answer is not yet clear, but mass is now a routine research topic in particle physics.
© 1996-2005 Scientific American, Inc.
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| Is space filled with a cosmic treacle whose stickiness gives particles their mass? The idea is not as crazy as it sounds, as leading physicist Frank Wilczek explains |
SOME TIME in the next few years a great
discovery will be unveiled, with appropriate fanfare. The headlines will
read "ORIGIN OF MASS DISCOVERED". Many readers will be blown away; many will
be cynical. Some will scratch their heads and wonder, what do these words
actually mean? One doesn't normally think of mass as something with an
origin. But a wise and happy few will be prepared. They will leaf through
their treasured back issues of New Scientist, fish out the right one,
and refresh their memories. Welcome back!
What will have been 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
(see "Messy mass"). 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 prefer
red 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 discrete 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
(see "The Higgs particle cookbook" below).
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, scien
tists 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. Remember the future headline, trumpeting the
discovery of the origin of mass? Honesty compels me to call the headline
writer to task: the statement is not entirely misleading, but it's
literally false. 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? Write to the editors, demanding a follow-up article.
Frank Wilczek is a theoretical physicist at the Institute for Advanced Study in Princeton
The Higgs particle cookbook
BECAUSE Higgs particles
interact most strongly with other high-mass particles, it is hard to make
them directly in the collisions of lightweights like electrons. Instead,
we reach the Higgs particle indirectly, through virtual Z or W bosons or
pairs of top quarks, which then decay into Higgs.
Messy mass
ONCE, we thought that the fundamental
laws of physics made no distinction between left and right - for any
behaviour you can observe in the real world, its mirror image can also
happen. So if you filmed the real world and its reflected image, someone
watching the movies later wouldn't be able to tell which was which. This
is called parity symmetry.
Then in 1956 Tsao-Dai Lee at Columbia University, New York, and Chen Ning
Yang at the Institute for Advanced Study, Princeton, suggested that the
weak interaction breaks parity symmetry. They turned out to be right. For
example, neutrons decay through the weak interaction into protons, electrons
and electron antineutrinos. The electrons emitted in this decay are moving
at nearly the speed of light, and they are also spinning. About 98 per
cent of them are left-handed, meaning that if you pointed the thumb of
your left hand in the direction of its motion your fingers would curl in
the direction the electron spins. This bias violates parity symmetry,
because it distinguishes left from right.
So the weak interaction likes left-handed particles (electrons, muons,
quarks and so on) and right-handed antiparticles. But, irritatingly, this
seems to be no more than a rule of thumb. Weak processes involving
right-handed particles or left-handed antiparticles are rare, but not absent.
Here's why. According to the theory of relativity, the laws of physics
should look the same to a moving observer. But consider an observer moving
in the same direction as a left-handed electron emitted in neutron decay,
but faster. They will see the electron going backwards, but spinning the
usual way--it will seem right-handed.
But what if the electron had zero mass? Then it would, like a photon,
always move at the speed of light. No observer could overtake the electron,
and the problem would no longer arise. In a world without mass, the
``rule of thumb'' that only left-handed particles and right-handed
antiparticles participate in the weak interaction could be an exact principle.
Political heavyweights
In 1993, researchers were
challenged by
William Waldegrave, then Britain's science minister, to
come up with a concise description of the Higgs particle.
For a complete list of the winners and their explanations see
The
Waldegrave Higgs Challenge.
David Miller from University College London won a bottle of champagne
for this "quasi-political explanation".
©
Copyright
New Scientist, RBI Limited 2001
In diagrams like those shown below,
only the particles with free ends extending
backwards exist for a noticeable time in the past, and only the particles
with free ends extending forwards exist for a noticeable time in the future.
The lines with no free ends have only a very fleeting existence and cannot be
observed - they are said to be virtual particles.
In part (A) of the diagram, we see how an electron and a positron create
a virtual Z boson, which then emits a Higgs boson and becomes real. This is
the process LEP experimenters hope to see. At the Tevatron, instead of
electrons, experimenters will use quarks and antiquarks (B) found within
their colliding protons and antiprotons, and produce W bosons in place of Zs.
(C) Alternatively at the Tevatron, and especially at the LHC, gluons - again
found within colliding protons - should make pairs of virtual top quarks
that will annihilate to form Higgs particles. This process is my own
contribution to the Higgs particle cookbook.
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 harder 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.
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 out 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.
Because the information is carried by clusters of people, and since it was
clustering which gave extra mass to the ex-prime minister, then the
rumour-carrying clusters also have mass.
The Higgs particle 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 mass 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.
There could be a Higgs mechanism, and a Higgs field throughout our Universe,
without there being a Higgs particle. The next generation of colliders will
sort this out.
These ideas are illustrated in
a series of cartoons.
For an even more whimisical approach see
What is the Higgs?, cartoons by Claus Grupen of CERN.
From New Scientist,
10 April 1999
Jiggling
the Cosmic Ooze