to catch a Higgs
are setting a trap for an as-yet-unseen elementary particle, a
piece of subatomic matter that may lead to a whole new realm of
It is as if a heavy gust of wind whipped
through the world of elementary particle physics this past year,
swirling around the Swiss-French border, blowing newspaper pages
from Glasgow to Chicago, and lifting eyebrows to precarious heights
before releasing them to drift earthward again. The source of
the gale was none other than the Higgs boson, a piece of subatomic
matter that a group of Chicago physicists and several thousand
other scientists around the globe have been chasing for their
researchers believe the Higgs may have revealed itself at least
once in the Large Electron-Positron (LEP) collider at CERN in
Geneva last summer, and perhaps as many as four times. The glimpses-really
no more than shadows scuttling along the wall of LEP's cavernous
detector, if they were indeed even that-came just before the accelerator
was scheduled to shut down, clearing construction space for another,
much more powerful accelerator opening in five years. After heated
international debate, copious data checking, and a two-month extension
of LEP's run time, the $6 billion construction plan for the next
particle collider could be put off no longer. As the lights went
out at LEP in December, CERN's 3,000 physicists were left with
a single, clean, yet perplexing image and the sense that something
wonderful had whirled by.
Oreglia, associate professor in physics, the Fermi Institute,
and the College and a veteran Higgs hunter, was part of an eleventh-hour
working group assembled at CERN to examine the odd data signals
and attempt to determine whether they were hints of a Higgs. The
working group resolved the question as a parent might curb the
expectations of an overly excited child: yes, it is very exciting,
but we'll have to wait and see.
fundamental ideas of science, Albert Einstein insisted, are essentially
simple and, therefore, comprehensible to everyone.
simply, the Higgs boson is the subatomic particle that gives other
particles their mass. Named for Peter Higgs, the retired Scottish
theorist who proposed its existence in the early 1960s, the Higgs
is a hot object of pursuit for experimental physicists because,
after so much waiting, finally seeing it in the lab would
be the first step toward understanding its nature and properties.
does it mean for a particle to "give" another particle
its mass? When the Higgs hubbub began late last year, many well-meaning
journalists explained mass as the weight of something,
and the Higgs as the mechanism that gives things weight. But mass
is not the reason why a hunk of steel weighs a ton and a gooseberry
weighs a few grams. That's a function of gravity. Rather, mass
is why a thing has inertia, causing it naturally to resist a change
of motion, which is why a gooseberry and a hunk of steel both
stay right where they are until some intervening force (late-season
rain, a forklift) moves them.
Higgs boson is the particle that carries the inertia. The more
tightly it gloms on to other particles, the less likely they are
to be swayed by intervening forces. Melvyn J. Shochet, the Elaine
M. & Samuel D. Kersten Jr. professor in physics, the Fermi
Institute, and the College, describes the phenomenon created by
the Higgs as, in essence, sticky muck: "It's like soldiers
walking through mud. Interaction with the mud slows them down
and makes it much harder to walk." In fact, particle physicists
believe all of Nature is covered in a blanket of mud, and they
would like very much to scoop up a glob from this Higgs field
to study how, exactly, it gloms onto other particles.
they won't have to wait another five years to do it. Just as LEP
was shutting down in December, another particle accelerator 40
miles outside Chicago was preparing to fire up in March to resume
the hunt for the Higgs. The Tevatron accelerator, located at the
Fermi National Accelerator Laboratory in Batavia, Illinois, has
undergone extensive equipment upgrades, increasing the intensity
of its colliding beams and the machine's ability to tag data like
that seen at LEP. Shochet and Henry J. Frisch, professor in physics,
the Fermi Institute, and the College, are among the 2,000 scientists
conducting experiments at Fermilab.
Higgs, if it exists, will be the latest in a series of subatomic
particles discovered during the last century. The fermions, named
for the Chicago Nobel physicist, are the building blocks of atoms.
They include three pairs of quarks (top and bottom, up and down,
and the fancifully named charmed and strange) and three pairs
of leptons (the electron, muon, and tau and their corresponding
bosons, on the other hand, carry the forces that interact with
the fermions, causing them to stick together, have inertia, and
disintegrate under radioactive conditions. Four of the five predicted
bosons have already been seen in the lab. The photon, or the particle
of light, carries the electromagnetic force, which binds atoms
together to create molecules and, eventually, berries and steel.
The gluon carries the strong force, which holds atomic nuclei
together by binding together quarks to form protons and neutrons.
The Z and W bosons carry the weak force, which governs radioactive
decay, the demise of all unstable particles. Last but not least,
the Higgs boson carries the sticky muck that gives the fermions
inertia, or mass.
the roster of fermions and bosons and their interactions create
the Standard Model, the summary of scientists' present understanding
of Nature. Every physicist will tell you it's an imperfect model-or,
more precisely, an inelegant model, because it requires
parameters such as mass to be factored in, rather than accounting
for them on its own-but it's what they have for now, and the Higgs
is an important part of it.
most natural way for it all to work," Shochet explains, "would
be for all the elementary particles to have no rest mass whatsoever-to
have mass that only came from kinetic energy, the energy from
motion, and potential energy, the energy associated with things
interacting. But we know that's not the case."
the Standard Model has Nature doing a most unnatural thing:
imparting masses to each of the particles with random abandon:
allowing the photon to be massless and the neutrinos to be near-massless,
while the most massive particle, the top quark, measures in at
175 billion electron volts and the second-most massive, the bottom
quark, is a mere 5 billion electron volts. (Shochet explains an
electron volt as "how much energy an electron gets when it
goes through a one-volt battery"-a measurement system that
draws on Einstein's theory of relativity and the notion that mass
and energy are interchangeable. So the particles' masses
are, in essence, how much energy they deposit when they go splat
against the walls of the accelerator detectors.)
the mass conundrum begins with finding the Higgs.
the poet William Blake wrote, is eternal delight. The search for
the Higgs boson is a quest for energy: high enough levels of it,
more accurate measurements of it, a better understanding of it.
The small step between delight and light would not
be lost on the visionary poet. Says Henry Frisch, a visiting scientist
at Fermilab, "It's really intensity - it's luminosity
- that we need. Which is to say we need more collisions."
collisions is, in theory, simple work. Researchers begin with
hydrogen, the simplest and lightest element, with one proton and
one electron. LEP isolates the electron, while the Tevatron uses
the proton. Huge numbers of the particles are cooled and stored,
as are the particles' antimatter, which carries a reverse electric
charge. The collider at LEP used antielectrons (or positrons),
while Fermilab uses antiprotons.
accelerators are built in circular tunnels-four miles in circumference
at Fermilab, 17 at CERN-with magnets and high-voltage accelerating
devices placed at regular intervals around the loop. The particles
are released into the tunnel, and as they race around the loop,
they pick up energy from the high-voltage field, fattening them
for the slaughter ahead.
the antiparticles are also fed into the tunnel but, because they
have an opposite charge, they travel in the opposite direction.
Like "threading a needle," Frisch says, magnets direct
the flow of the looping particles and antiparticles into a vacuum
pipe a few inches across. Heavily concentrated, their plump little
bodies collide. At Tevatron, the collision occurs at an energy
of 1.8 trillion electron volts, ten times the energy achieved
at LEP. The particles and antiparticles annihilate each other.
Splatters of energy go hurtling off in every direction.
point at which the particle beams collide is in the epicenter
of a huge computerized detector. The machine, an onion-like device
with layers of specialized forensic materials, keeps track of
the energy's direction, how much is splattered, and how much the
splatters bend as they pass through a magnetic field.
particles that hurtle out of the collision are not necessarily
pieces of the protons and antiprotons. Rather, slamming together
two beefed-up particles releases enough energy to create a flurry
of new, highly unstable particles. These particles make it a few
millionths of an inch out of the collision point before they can't
resist their inherent weakness any longer, decaying into more
stable particles such as photons and electrons.
inevitability of malfunction aside, the process is continual:
24 hours a day, every day of the year, for years and decades at
a time. The result at Tevatron will be something like 5 million
collisions per second. The odds of finding a Higgs in those millions
and billions and trillions of collisions is slim: maybe three
in a year if the physicists are lucky. That's why increasing the
number of collisions - that delightful concept called luminosity
- is so critical.
it also presents an incredible challenge: how, for Pete Higgs's
sake, do you keep track of and sort through all those collisions?
Luckily, physicists have been doing this long enough to know what
to expect from the constant spray of colliding particles. They
know the properties of all the particles that can possibly come
hurtling out (except the Higgs), which means they know each particle's
preferred direction of splatter, how much energy (or mass) it
deposits in the detector, and how much each particle bends as
it passes through a magnetic field.
physicists also generally know what kinds of particles to expect,
and how often those particles will show themselves. Since Tevatron
collides protons (which are made up of quarks), they know to expect
huge numbers of insignificant quarks to come spraying out. They
call this quark-spray "background," the chaff from which
they must separate the wheat. The wheat, that single golden grain,
they call a "signal." The image left behind by LEP when
it closed down was a clean, beautiful signal with all the properties
of a Higgs.
their fruits you shall know them," the apostle Matthew wrote
almost 2,000 years ago-an apt description of how physicists hope
to know a Higgs when they see one.
can't actually see into the beam pipe, and even if they could,
too many collisions are occurring and the golden grains decay
too quickly to be glimpsed by a human or mechanized eye. So researchers
use clues from a detector to retrace the particles' steps back
to the collision point. Each step backward in time tells a little
more until, eventually, they can identify and measure a particle
with enormous precision.
are hand-waving descriptions of particle physics, and these carry
you a certain distance," explains James E. Pilcher, professor
in physics, the Fermi Institute, and the College. "But we
feel much happier and more confident in our progress when we can
really make precise predictions. If given enough time and energy
after we do the calculations and make a prediction, then we can
do some magnificent measurements to compare observation with our
begins in the detector's outer layer, at the 10-meter-wide "calorimeter."
As its Latin root implies, the calorimeter measures the "heat"-that
is, the energy-deposited by splattering particles. Chicago is
one of 150 institutions contributing pieces to the calorimeter
to be constructed at CERN. Anyone who takes a wrong turn in the
Fermi Institute on campus may come upon such a piece-shaped like
an oversized slice of fruit tart, four feet in length, eight inches
deep, slightly more than a foot at its widest end and tapering
to half that at the vertex. Threaded with fiber optic cable and
assembled, the pieces eventually will form a hulking 192-piece
crust to wrap around the beam pipe at CERN's Large Hadron Collider
signals begin in the calorimeter as light," Pilcher explains.
Scintillations are converted by the fiber-optic lines into electrical
pulses, and the detector records the signals. The next step backward
is to measure the bend of the particles as they pass through a
magnetic field before entering the calorimeter. A particle's willingness
to bend tells physicists its charge. One step deeper into the
detector, and physicists arrive at layers of silicon wrapped around
the vacuum beam pipe. As particles hurtle out of the vacuum, they
pass through the silicon, leaving behind a series of dot-like
signals. Physicists have simply to connect the dots and see the
"track" of a single particle.
the vast majority of cases, the dots don't connect or they lead
directly back to the collision point, both of which indicate background-those
superfluous quarks that fly out of a parent proton or antiproton
during annihilation. But very, very rarely the dots lead to a
point other than the collision. And that is one of those interesting
signals indicating enough energy was achieved to kick out some
other highly unstable particle which decayed too quickly to be
seen. Taken together, the deposits in the calorimeter, the trajectory
in the magnetic field, and the tracks left in the silicon tell
exactly what was kicked out.
that it isn't quite that simple. Given the luminosity of the machines-the
million collisions occurring each second-there's just too much
data to be stored for study. Explains Frisch, "These events
are on the order of a quarter of a megabyte each. We make a million
a second. So in principle, the detector is capturing 250 gigabytes
per second"-compared to, say, an Apple G4 with its much-lauded
four gigabytes of memory-"but we have no way to store
that much data. It sits on the detector for a little while, and
then we have to make the decision. Imagine you're taking pictures
with a digital camera at the rate of 250 gigabytes per second.
And you have to look at every one of those pictures and say, yes,
I want that one but not that one. That's what we've done here
at Chicago: picture recognition."
five years Frisch, Shochet, and a team of physicists have created
the new computerized trigger at Fermilab that will recognize the
golden grains of wheat among superfluous piles of chaff.
what, after all this, will a Higgs look like when one is finally
caught? It will look a lot like the signal left by LEP: four jets
of spray going in very specific directions, with very specific
trajectories, leaving behind very specific quantities of energy,
all of which will indicate the final particles into which two
unstable particles decayed before they could reach the silicon
and leave tracks of their own.
than 40 years of crunching the numbers of the Standard Model with
Mr. Higgs's theoretical particle as a function of the ever-increasing
levels of energy reached at these machines tells physicists exactly
how the four jets will add up. Two jets will be the sprays of
fermions that result when a W or a Z boson decays. Physicists
have seen enough to know a W or a Z by the fruits of their decay.
Two jets will spray in the opposite direction and come from something
that's never been seen before, something whose decay products
are two bottom quarks. It will be a W or a Z plus whatever decays
into two bottom quarks because the mathematics say so: add up
the end products, and the total equals the energy achieved during
physicists do know, Mark Oreglia explains, is that the Higgs "likes
mass." Of course it does: its nature is to glom onto things,
and the heavier the particle available to glom onto, the happier
(and more easily detected) the Higgs is. After 15 years LEP was
only able to give a possible glimpse of a Higgs at the very end
of its life. The physicists pushed the machine as hard as it could
be pushed and then finally agreed to close it down and move on.
The working group recognized how incredibly interesting that single,
perplexing image was, but the odds of producing another golden-grain
signal with a Z and two bottom quarks were too slim and the cost
of putting off construction of the LHC was too high.
the delightfully luminous Fermilab accelerator, with ten times
the energy of LEP and a smart new trigger able to recognize golden
grains in ten millionths of a second, see a Higgs? The answer
is still only maybe, though the odds are better. Chicago
theoretical physicist Carlos Wagner devotes his work to predicting
how long it will be before the Tevatron might trap a Higgs. He
estimates six years at least, by which point the LHC in Geneva
will be starting up with nearly eight times more energy than Tevatron
and 100 times more luminosity (effectively moving the "energy
frontier" from U.S. to European ground-a fact lamented repeatedly
by the Chicago physicists). Still, Oreglia says, another signal
like LEP's is not enough. Physicists need about ten clean signals
to say with certainty they've seen a Higgs. Then they'll spend
many more years creating Higgs events and studying them with greater
and greater precision to understand its properties.
even then, says Oreglia, the Higgs will not be the final, glorious
answer many well-meaning journalists have proclaimed, often calling
it "the God particle," after the title of a 1993 book
on the Higgs by physics Nobel laureate and Chicago professor emeritus
Leon M. Lederman-a metaphor that causes Oreglia to visibly cringe.
Seeing a Higgs is merely one step toward understanding that sticky
notion of mass. "Right now without the Higgs boson,"
Oreglia says, "a mass is nothing more than a parameter in
a theory"-that is, a set of numbers with no rhyme or reason
that must be plugged into the Standard Model rather than being
apparent on their own. How much mass is generated by the Higgs
field-how exactly the Higgs gloms onto other particles to cause
their all-over-the-map masses-is what physicists call its "coupling"
physicists begin to quantify many years from now how a Higgs couples
to each particle, they hope finally to see a pattern in the numbers.
And then, says Oreglia, a new theory may take the place of the
Standard Model-still an incomplete summary, but more complete
than we have now.
ultimate goal in all of this theoretical brainstorming, number
crunching, calorimeter assembling, silicon triggering, and continual
running of experiments is a single - Einstein would say simple
- unified understanding of Nature, where all the rest masses
of particles are zero, where all the forces end up as simply manifestations
of the same force, and where astrophysicists' theories of outer
space are explained by what happens in subatomic space and vice
not just about the Higgs. The stakes are much higher," says
Frisch. His meaning is clear: the possibilities are endless, we
have only to fire up the machines and see what we will see.