SOME time in the next few years a great discovery will be unveiled, with
appropriate fanfare. The headlines will read 鈥淥RIGIN OF MASS DISCOVERED鈥�. 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鈥檛 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 快猫短视频,
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鈥檛
mass just a fact of life? Not necessarily. In fact, ours would be a much simpler
world if particles didn鈥檛 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鈥檚 the problem? Well, the
existence of mass means that particles feeling the weak force don鈥檛 all spin in
the same way (see 鈥淢essy mass鈥�). It would be neater if they did.
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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鈥檛 we see
it?
To understand, imagine you鈥檙e 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鈥檛 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鈥攊t doesn鈥檛 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 掳C, the spins get enough energy to break
free from their neighbours and point in random directions again鈥攖he
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 鈥渧acuum鈥� 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 鈥渧irtual鈥� 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
Fran莽ois 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鈥檛 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鈥檚 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鈥攁 ripple in the cosmic molasses鈥攊s 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鈥攁n extension of the Standard Model that proposes a host
of extra fundamental particles, partners of the familiar bunch鈥攑redicts
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 鈥淭he
Higgs particle cookbook鈥�). 快猫短视频s 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鈥檒l do.
The Standard Model requires just one Higgs particle. But theories with more
symmetry imply several new particles鈥擧iggs 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
鈥渄ark鈥� 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鈥檛 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鈥檝e 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鈥檚 literally false. Actually the
lion鈥檚 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.
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.
In diagrams like these, 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鈥攖hey
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鈥攁gain found within colliding protons鈥攕hould make pairs of
virtual top quarks that will annihilate to form Higgs particles. This process is
my own contribution to the Higgs particle cookbook.
Once, we thought that the fundamental laws of physics made no distinction
between left and right鈥攆or 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鈥檛 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鈥檚 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鈥攊t 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 鈥渞ule of thumb鈥�
that only left-handed particles and right-handed antiparticles participate in
the weak interaction could be an exact principle.
In 1993, researchers were challenged by William Waldegrave, then Britain鈥檚
science minister, to come up with a concise description of the Higgs particle.
David Miller from University College London won a bottle of champagne for this
鈥渜uasi-political explanation鈥�
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鈥攖he clustering of the field around the particle鈥攇enerates
the particle鈥檚 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鈥檚 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.