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Why are things heavy? The Higgs Boson might have a
Title-> Armies of physicists struggle to discover proof of a
Scot's brainchild.. (Peter Higgs; Higgs boson) (study of
mass and the cause of heaviness)
Authors-> Mann, Charles C.
Twenty men in boots and hard hats pick their way through a construction
site a few hundred yards from the border between Switzerland and France.
The morning is hot and dry for May, but the ground is nonetheless muddy
from the continued movement of bulldozers and trucks. The center of
activity is a vast hangarlike structure or, more precisely, the pit at
its center, more than 30 yards deep and almost as wide, cut into the
soft French soul.
A tall, stoop-shouldered man named Samuel C. C. Ting leads the group.
He is a physicist from the Massachusetts Institute of Technology, a
Nobel Prize winner. He wears dark suits, immaculately clean, of no
particular style, and dark ties, perfectly knotted, of no particular
hue. Authority rests lightly on his shoulders. He and the people
walking behind him are working on a colossal physics experiment; they
come from China, the United States and a dozen other countries.
Known unromantically as L3, the experiment is designed to answer what
would appear to be a fundamental question: Why are things heavy? Why,
in a gravitational field, do they have weight? Or, as physicists would
phrase the question: What is mass? We think mass is created by the
actions of a type of subatomic particle known as the Higgs boson, named
for Peter Higgs, the Scottish physicist who was one of the first to
predict its existence. Yet scientists have never seen it, and are not
certain where to look. Ting thinks his expertment can find the Higgs,
if it exists. He may discover it, which could ultimately prove as
significant an achievement as splitting the atom. Then, too, the
discovery might just as well have no practical consequences. Or, the
Higgs may not exist, after all.
The wind, keening in the open struts of the elevator, follows the
scientists into the hole. Because nearby residents objected to the
ugliness of the construction, an earthen berm stands between the hangar
and the surrounding farms. As the car descends, the berm sweeps out of
view; then the Jura Mountains, too, are hidden. By the end of the
elevator ride, the scientists have journeyed into a world of their own
making. It is a world some of them have never seen, despite having
worked on the experiment for years. Then, when the elevator finally
stops, jaws drop and a silence falls over the group.
Standing in the center of the pit is an octagon the size of an office
building (p. 108), its huge side doors yawning open. It is an
electromagnet, made of more metal than the Eiffel Tower, wound with
hundreds of miles of cable. Some of the scientists are startled, even
dismayed, by what is not there: the thousands of tons of equipment that
remain to be stuffed into the pit. The magnet and all the apparatus
that goes with it together constitute the heart of L3-the detector that
may, just may, come up with the Higgs boson.
"When we first put the plans for this experiment down on paper, I knew
the dimensions were big," Ting says at last. Usually his voice is
toneless, careful, without edge even when administering the sharpest
rebuke; you keep thinking you have missed bits of sentences. Now,
however, he hesitates"But it isn't till you see it that you get a
feeling for how . . . big . . . it really is. I went down here a
couple of days ago and I thought, 'This is crazy.' I thought, 'It can't
be this size."'
Seeking an answer better than Aristotle's Why are things heavy?
Artistotle thought objects are pulled to earth because they wish to
return to it, and for much of Western history nobody came up with a
better explanation. It was left for Kepler, Newton and their fellows to
realize that heaviness came from the interaction of gravity with some
other quality, which we today call "mass." It is the quality of an
object that causes it to have inertia: that means you have to use force
to get it moving if it is standing still, and you have to use force to
stop it if it is already moving. You must use force whether you are on
the surface of a planet or floating weightless between the stars. Mass
is a quality an object always has; it is different from weight, which
changes, or even disappears, with changing circumstances. Take an
astronaut who, when he is on Earth, weighs 180 pounds. The moon's
gravitational field is only one-sixth as strong as Earth's, so when that
same astronaut is standing on the moon, he will feel he only weighs
one-sixth as much. When he is in free-fall in a spacecraft, we say he
is "weightless." His mass, however, is always the same 180 pounds.
Well into this century, physicists knew a lot about how mass behaved but
did not know whence it derived or, more simply, what it was.
"Mass is the great question of physics," says Abdus Salam, a Nobel
Prize-winning physicist who is director of the International Centre for
Theoretical Physics at Trieste. "We have fruitlessly banged our heads
against it for centuries."
Today the search for the origins of mass has become, to the dismay of
some researchers, one of the largest and most costly undertakings in the
history of science. On both sides of the Atlantic, thousands of
scientists and support staff are working to build the enormous machines
that will be used in the quest. The L3 group, led by 450 physicists
from 39 institutions, is but one of six groups of experimenters hoping
to solve the mystery of mass.
The quest began, at least indirectly, during the . 1950s, when
physicists first began to work out their grand unified theoires, schemes
in which the four fundamental forces in nature are only different
manifestations of one and the same thing (SMITHSONIAN, May 1983). Part
of the idea is that each force has an agent, a subatomic particle, that
produces the effects we see. They are known collectively as gauge
bosons. Photons, for example, "carry" the electromagnetic force,
whether in the form of light waves, radio waves, x rays or gamma rays,
among others. Mesons carry the strong force that holds thc nucleus of
an atom together. Scientists still have not seen gravitons, the
particles postulated to carry gravity; and until recently, physicists
had not even seen the particlcs, known as intermediate vector bosons or
the W and Z particles, that carry the weak force, an obscure force
central to the process of radioactive decay.
It was in the atmosphere of looking for unification theories that
physicists such as Salam and Sheldon Glashow, now of Harvard, working
independently, were toying with the idea that electromagnetism and the
weak force could be brought together and seen as different
manifestations of the same thing-the electroweak force. In the late
1950s, Glashow and Salam developed similar theories, but hit the same
stumbling block: while much of the mathematics in each theory worked,
there were major discrepancies between the particles that carried the
forces. Photons have no mass at all; but even before they were actually
seen, W and Z particles were predicted to be heavy-very heavy. (Many
physicists had calculated their mass and had eventually come up with a
similar ballpark figure. Each W or Z weighs more than 80 times what an
entire hydrogen atom does.) Yet the equations of Glashow and Salam said
that on some deeper level, the Ws and Zs should have no mass, like their
sibling the photon. "It was a problem," Glashow has admitted. "It was
a bit like saying a Ping-Pong ball and a bowling ball are somehow the
same thing."
By the early 1960s, Glashow had begun to look into other areas of
physics, but his childhood friend Steven Weinberg, now at the University
of Texas, and Salam each continued looking for a way to describe the two
forces as one. At the same time, a Scottish theorist, Peter Higgs, was
at the University of Edinburgh working on a somewhat related problem.
For several years the shy, soft-spoken, excessively modest Higgs had
been investigating various aspects of field theory, which is the notion
that all of space is saturated by a field, much as the field of a magnet
saturates the region near it. Like the field of a magnet, the
hypothetical field Higgs created had ephemeral particles that acted as
its agents: these eventually came to be known as "Higgs bosons."
Higgs had been engaged in intense doodling, fiddling with equations,
adding in one variable and taking out another to learn more about the
nature of his hypothetical field. It was at this point, around 1964,
that he noticed something extraordinary. If he added in his own field
equations (and thus the Higgs bosons) to equations somehat analogous to
those of Salam or Glashow, he found that certain particles in the
original equation behaved in an astonishing fashion. They began with
zero mass and then mathematically "ate up" other,unwanted particles in
the field (ones that had made a mess of the mathematics in the theory he
was investigating) emerging with mass.' (The unknown
particles turned out to be the W and Z particles, but Higgs did not know
that at the time.) Higgs published his fingings-described by Glashow as
a "terrifically loony idea: an idea than nobody would ever have paid
attention to"- with little fanfare. Eventually, Salam and Weinberg
independently heard about his work and realized it might solve their
problem. Each added Higgs' equations into the ones they had oecn
working on and-to their great delight-found that the particles that
acquired mass in Higgs equations corresponded to the W and Z particles
in their own. Higgs' hypothetical boson provided the mechanism by which
the electromagnetic and weak forces could finally be seen as one.
Higgs bosons might have been entirely forgotten if a groundswell of
theoretical and experimental work in the 1970s had not led to the
acceptance of the electroweak theory-and to Nobel Prizes for Glashow,
Salam and Weinberg in 1979. Four years later the W and Z were
discovered by a team led by Carlo Rubbia and Simon van der Meer, who
quickly received Nobels for their efforts. Today the electroweak theory
is inscribed in the textbooks, except for one not-so-little worry:
nobody has seen even the slightest trace of the Higgs. "And without the
Higgs," jokes Paul Lecoq, a French member of the L3 collaboration,
"Messrs. Glashow, Weinberg and Salam may eventually have to give back
those little prizes they got in Sweden."
A globe-trotting Higgs scare-her
In a long, high laboratory, Ulrich Becker wedges his way through a
forest of electronic equipment and the cables and welding equipment
necessary to hook it together. A big man with the square face and wiry,
stand-up hair of a friendly monster, Becker has been one of Ting's chief
collaborators for two decades. Born and raised in the Federal Republic
of Germany, he is now a member of the international fraternity of
science: citizen of West Germany, resident of France, worker in
Switzerland, taxpayer of the United States. Like Ting, he is officially
on the faculty of MIT but, like Ting, he has spent the past decade
traveling the globe in quest of the Higgs.
"Put naively," Becker says"most of the progress in particle physics for
the past 20 years, this electroweak theory and everything else, depends
on the supposition -the guess-that there is this field, the Higgs field,
that saturates the whole Universe, and that particles get their mass
from interacting with it. You might say it's a little like a magnet,
with some kinds of metal being more attracted than others. Particles
that are more 'attracted' to the Higgs have more mass. The picture is
very nice, theoretically speaking-yet there 'is not one shred of
evidence for it."
Ting's L3 group is based at the European Organization for Nuclear
Research, still known to physicists around the world by its old acronym,
CERN. The main campus is in a suburb of Geneva, but the laboratory has
grown with physics itself: the L3 assembly area is in France, a few
miles from the Swiss border. (Customs is a constant problem; recently,
Becker recalls, he had to cross the frontier 12 times in a single
afternoon; the border guard threatened to seize his car if he came
through again that day.) A great circular tunnel connects the campus and
the L3 pit; it houses an almost completed particle accelerator called
the LEP, the Large Elect ron-Positron collider.
Accelerators are devices that smash subatomic particles into a target or
into one another with enormous energy; in the resultant collisions,
matter changes into energy and energy into matter, and the Hying
fragments provide clues to the laws of nature. They are usually built
in the shape of a wheel; as particles move around the circumference,
they are boosted to higher and higher speeds by powerful electric
fields. Electrons, which at rest are very light, can be pushed to
within a whisker of the speed of light. At such relativistic speeds,
however, the electron becomes heavy and difficult to turn in the
necessary continuous circle. It takes huge amounts of electrical power
to keep bending the beam of electrons, which keep losing energy in the
form of x rays as they are forced to turn. To smooth out the turns,
accelerators have become bigger and bigger around, not to mention more
and more costly. The old proton accelerator at Brookhaven National
Laboratory on New York's Long Island is just half a mile around, The
main accelerator ring at the Fermi National Accelerator Laboratory in
Batavia, Illinois, is nearly 4 miles around. The LEP in Geneva, when it
is commissioned in July, will be 17 miles aroundthe largest in the
world. Someday it may be eclipsed in its turn by the planned
Superconducting Super Collider (SSC) in West Texas, which will have a
circumference of about 53 miles-and which some investigators think may
ultimately have the best chance of finding the Higgs.
The LEP will achieve extraordinary energy levels in two ways. First,
instead of having the beam smash into stationary targets, the LEP will
have two beams moving in opposite directions. It will be the difference
between a car running into a tree and one car colliding head-on with
another. Second, the electrons will collide with their antimatter
counterpart, positrons, producing total annihilation. Some collisions
will produce Zs, the neutral agents of the weak force, A few of these,
theory predicts, will fall apart into Higgs bosons, which in turn will
decay into a spray of other particles. The Zs and Higgses will live so
briefly-trillionths of a trillionth of a second -that they will be
impossible to see directly. But the end product, the spray of other
particles, can be seen by complex detectors, and from it the researchers
hope to infer the existence and properties of the Higgs (pp. 106-07).
When the accelerator cranks up this summer, four experiments, all
multimillion-dollar collaborations, will begin work. L3 is the biggest,
the most expensive and the most directly aimed at the Higgs. The project
began in 1979, when Ting, Becker and a small number of other physicists
started designing an ambitious machine, a detector, that they hoped
would trap the Higgs if, indeed, it exists. "We asked CERN for the
biggest hole they would give us," coleader Hans Hofer of Zurich says.
"It turned out that we could fill it up completely with a detector the
size of a four-story building." This is the octagon in the pit, a
massive tube stuffed with electronics. Electrons and positrons will
collide at its center; different detector parts will measure different
particles flying outward from the violence.
"I hope," Becker says drily, "that Mr. Higgs is pleased by how much
trouble we have gone to in trying to find his little particle." His
train of thought is interrupted by the screech of a million fingernails
being dragged across a million blackboards as CERN workers drag a
20-foot-high metal rack across the concrete floor "You know," Becker
shouts over the noise "nobody sees Higgs! He just sits up there in
Edinburgh! My graduate students come to me all the time, asking me 'Who
is this man Higgs?' Does he really exist?"'
On the wall behind Becker is a painting of an object vaguely resembling
a Ferris wheel, It is the section of the detector to which Becker has
devoted nearly ten years of his life; the completed structure is more
than 30 feet in diameter, yet measures the trajectories of particles
called muons to within a thousandth of a millimeter. Within the wheel
is a lacework of laser lights, each centered on a photosensitive cell.
Like an onion, Becker explains, the detector consists of layers of
instruments, each intended to measure a specific type of particle. Each
layer takes years of work, costs millions of dollars and requires
delicate negotiations among the dozens of institutions and nations
involved. Each brings unexpected benefits to industry: by giving one
order to a company that made plastic whiskers for teddy bears, for
example, L3 launched the firm into the fiber-optics business.
As Becker speaks, fatigue informs his voice; he has accumulated a
quarter of a million miles in air travel. There are shadows under his
eyes and he often finds he needs to vent his frustrations by working on
his car. "Nobody would choose to work this way, on projects so big," he
says "But we have to, if we're going to chase after something like the
Higgs. And, obviously, we have to hope the people at Stanford don't get
there ahead of us."
Frustration and fury in a California lab
A group of men and women gather, shirtsleeved and disheveled, early one
June morning at a laboratory just south of San Francisco. Outside, the
California sky is as blue as heaven; inside, the light is dim and
migraine green. In a chair against the wall slumps a round-faced man
with bags beneath his tired eyes: the Nobel Prizewinning physicist and
Stanford Linear Accelerator Center (SLAC) director, Burton Richter, who
has designed the project on which they are working. He has staked the
rest of his career, and those of everyone else in the room, on its
completion. Richter, as it happens, shared his Nobel with Sam Ting in
1976 for their separate but simultaneous discovery of yet another
subatomic particle, the J or Psi (SMITHSONIAN, July' 1975). Silent as
stone, Richter watches as, one after another, his colleagues recount the
small, infuriating problems of the day before. Blown electronics.
Inappropriate settings. Cooling difficulties. Human error. The list
goes on and on. The project is more than a year behind schedule, and
Richter's fury is as palpable in the room as a heavy fog.
"We're getting close," says Andrew Hutton, the tall, bearded Englishman
acting as program deputy. "Close enough to proceed as if we were going
to start for real."
"Not as if," Richter says. His voice is quiet, but his vehemence is
unmistakable. "We are starting. It is real. We can't wait anymore on
this. From now on, if something goes wrong, work around it. You can't
do science waiting for everything to work right. I want us to start
taking data at 4 Em."
Hutton smiles. "You heard the man," he says. "In eight hours, we'll
try to make history."
Like their rivals at CERN, the scientists at Stanford are working on a
particle accelerator. Known as the SLAC Linear Collider (SLC), the
device is as intricate and baffling as an Escher drawing. Richter, its
designer, had a hand in the original design of the LEP When he
discovered that the United States would not pay for such a gigantic
machine, he tried to come up with a smaller device that would do the
same work, and at a fraction of the cost of a circular collider. The
linear collider is the result.
Shaped somewhat like a two-mile-long tennis racket, it sends two pulses
of particles -electrons and positrons -racing down the handle and around
opposite sides of the head until they meet with soundless violence at
the top. For most of the Journey, the particles move in a
straight line and do not lose energy by giving off x rays as they are
forced to turn. That means the machine can be much smaller, and hence
more quickly and inexpensively built, than a conventional accelerator.
Many physicists believe that the SLAC Linear Collider is a prototype for
future machines.
On the other hand, the two bunches of particles must collide on the
first pass, instead of having many chances as they spin millions of
times around a ring. The challenge is enormous: make two particle
aggregations, each the thickness of a human hair, each traveling at a
fraction less than the speed of light, smash together at the precise
center of a detector.
Although construction began after the LEP, the hope was to finish the
Stanford accelerator more than a year sooner, giving physicists there
the first chance to find a Higgs. If all goes well, the machine's
design will have been one of the most daring and successful gambles in
recent science, and physicists in the United States will have
leapfrogged their friends and competitors in Europe. If, that is, all
goes well.
"There is a bare chance we can find the Higgs first," says Patricia
Burchat, a young experimentalist at Stanford. "Like them, we will make
Zs, some of which will decay into Higgses. The question is whether our
machine will make enough Zs, and that, I'm afraid, is not at all clear."
"To get this project approved by the government," Richter says, "we cut
the budget to the point where there was some doubt we would be able to
build the machine. It was a risk, but with the U.S. budget deficit, it
was the only way we could get the money."
The result is a bricolage, new equipment attached to another accelerator
that was built in the 1960s. The jumble of old and new systems is an
engineer's nightmare: like an exotic sports car, it works fine on blocks
in the garage, but develops new problems on the road. The linear
collider is now months behind, and every moment of additional delay
further cuts the Americans' chances of getting there first,
At 4 that afternoon, Richter appears at the main control center for the
first test run. Ninety minutes later, he is forced to leave in
frustration: the machine is not working. With Richter gone, the staff
continues to work before the computer screens lining one end of the
long, dimly lighted room. The only sounds are the peeping of computer
signals, the hush of air-conditioning and the occasional murmur of a
dismayed voice. At a chair in the center, Hutton marks the log. A
Briton, he is one of the relatively few scientists to work both in
California and in Switzerland.
"We've done enough to show that the principle works," he says. "You can
make the bunches collide, and the next generation of accelerators will
be based on this one. But the curious thing is that a future triumph
will not be enough to make the lab look good today. For that, we need
to get this machine functioning ahead of our friends in Europe, and to
get some physics done." Hutton laughs tiredly; he has been working for a
long time without much sleep. "I must say that right now it would be
really nice-truly nice-if a Z turned up on the screen."
They broke their hearts in San Francisco
A punishing heat wave blankets San Francisco Bay at the end of July,
forcing the region to ration water. Faced with record 100-degree
temperatures, the Stanford accelerator's components begin to
malfunction. Water pumps break down. Flow switches give up the ghost.
Microprocessors burn out. The 8 A.M. meetings become sweaty litanies
of disaster. Five weeks after Richter ordered the machine to be turned
on for real, the laboratory has managed to accumulate a bare 21 hours of
data, Not a single Z has been seen. To make it worse, the Europeans are
ahead of schedule; in an initial test, their accelerator has taken seven
minutes to reach the goals scheduled to be achieved in seven days.
Richter has taken to haunting the control room, His face looks, says one
friend, as if he were barely restraining himself from biting his
fingernails. More and more, it is apparent that the jury-rigged machine
was built too quickly and too cheaply. Richter is spending much time
huddling with scientists like Martin Breidenbach, the wiry, intense
leader of the principal experiment. Weeks before, Breidenbach had been
happily splayed on his back, wrench in hand, beneath the team's
detector; now he is being drawn to the accelerator itself, for without
the machine there will be no particles to detect, no discoveries to
make.
Back in his office at CERN, Sam Ting tells the delegation from the
Soviet Union "I don't know what your rules are. I don't even care. What
I am saying is this: when the announcement of a discovery is made, the
people on the podium are the ones who get the credit. If you want your
scientists to get the credit they deserve, you will have to change your
policies about not letting them work outside the Soviet Union.
Obviously, the choice is up to you, not me." He smiles. "I am just a
scientist, a lone professor at MIT"
For the first time in the meeting, the Soviets laugh. Whatever they
think of their rules, experience has taught them that they are not
dealing with lone professors at MIT. The face of physics has changed
and research is now performed by huge collaborations, run by men with
fierce and powerful wills. Richter, at Stanford, is one; Ting, at CERN,
is another.
A moment later Ting says, "Nobody has spent all this time and money on
these projects to do ordinary physics. I will consider this experiment
to be an utter failure if someone finds the Higgs before we do or if we
make a technical mistake that prevents us from making a major
discovery." Tapping the symbol for the Higgs boson that he has written
on the blackboard, Ting repeats the words slowly. "A . . . major
discovery."
As the new year begins, tensions are running high at both CERN and
Stanford. The California team is still testing, but hoped to have the
machine on-line as early as March. In Geneva, physicists have finished
installing the components of their massive detector, are hoping to begin
testing in late winter or early spring, and plan to have the machine up
and running by fall. In Scotland, serenity reigns. Sitting in his
small office at the University of Edinburgh, Peter Higgs listens to the
details of the gargantuan effort to find the particle whose existence he
hypothesized so many years ago. "Of course I'm flattered by it all," he
says. Afternoon light comes in the window, the same light that is
filling the rolling hills outside with the colors of sunset"Of course I
like it when they give seminars on the search for the Higgs and so on.
And of course I think it's important to look for. But-you want to know
the truth-, When I consider the huge sums going for this, the lifetimes
spent on the search, I can't help but think: 'Good heavens, what have I
done?"'
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