The Higgs Boson: Searching for the God Particle (30 page)

Illustration by Slim Films

Rocky Road

With all the novel technologies being prepared
to come online, it is not surprising that the LHC
has experienced some hiccups—and some more
serious setbacks—along the way. Last March a
magnet of the kind used to focus the proton
beams just ahead of a collision point (called a
quadrupole magnet) suffered a “serious failure”
during a test of its ability to stand up against the
kind of significant forces that could occur if, for
instance, the magnet’s coils lost their superconductivity
during operation of the beam (a mishap
called quenching). Part of the supports of
the magnet had collapsed under the pressure of
the test, producing a loud bang like an explosion
and releasing helium gas. (Incidentally, when
workers or visiting journalists go into the tunnel,
they carry small emergency breathing apparatuses
as a safety precaution.)

These magnets come in groups of three, to
squeeze the beam first from side to side, then in
the vertical direction, and finally again side to
side, a sequence that brings the beam to a sharp
focus. The LHC uses 24 of them, one triplet on
each side of the four interaction points. At first
the LHC scientists did not know if all 24 would
need to be removed from the machine and
brought aboveground for modification, a timeconsuming
procedure that could have added
weeks to the schedule. The problem was a design
fl aw: the magnet designers (researchers at Fermilab)
had failed to take account of all the kinds
of forces the magnets had to withstand. CERN
and Fermilab researchers worked feverishly,
identifying the problem and coming up with a
strategy to fix the undamaged magnets in the accelerator
tunnel. (The triplet damaged in the test
was moved aboveground for its repairs.)

In June, CERN director general Robert Aymar
announced that because of the magnet failure,
along with an accumulation of minor problems,
he had to postpone the scheduled start-up
of the accelerator from November 2007 to spring
of this year. The beam energy is to be ramped up
faster to try to stay on schedule for “doing physics”
by July.

Although some workers on the detectors
hinted to me that they were happy to have more
time, the seemingly ever receding start-up date
is a concern because the longer the LHC takes to
begin producing sizable quantities of data, the
more opportunity the Tevatron has—it is still
running—to scoop it. The Tevatron could find
evidence of the Higgs boson or something equally
exciting if nature has played a cruel trick and
given it just enough mass for it to show up only
now in Fermilab’s growing mountain of data.

Holdups also can cause personal woes
through the price individual students and scientists
pay as they delay stages of their careers
waiting for data.

Another potentially serious problem came to
light in September, when engineers discovered
that sliding copper fingers inside the beam pipes
known as plug-in modules had crumpled after a
sector of the accelerator had been cooled to the
cryogenic temperatures required for operation
and then warmed back to room temperature.

At first the extent of the problem was unknown.
The full sector where the cooling test
had been conducted has 366 plug-in modules,
and opening up every one for inspection and
possibly repair would have been terrible. Instead
the team addressing the issue devised a
scheme to insert a ball slightly smaller than a
Ping-Pong ball into the beam pipe—just small
enough to fit and be blown along the pipe with
compressed air and large enough to be stopped
at a deformed module. The sphere contained a
radio transmitting at 40 megahertz—the same
frequency at which bunches of protons will
travel along the pipe when the accelerator is
running at full capacity—enabling the tracking
of its progress by beam sensors that are installed
every 50 meters. To everyone’s relief, this procedure
revealed that only six of the sector’s
modules had malfunctioned, a manageable
number to open up and repair.

When the last of the connections between accelerating
magnets was made in November,
completing the circle and clearing the way to
start cooling down all the sectors, project leader
Lyn Evans commented, “For a machine of this
complexity, things are going remarkably smoothly,
and we’re all looking forward to doing physics
with the LHC next summer.”

-Originally published: Scientific American 298(2), 39-45 (February 2008)

The Coming Revolutions in Particle Physics

by Chris Quigg

When physicists are forced to give a single-word answer to the question of
why we are building the Large Hadron
Collider (LHC), we usually reply “Higgs.”
The Higgs particle—the last remaining undiscovered
piece of our current theory of matter—is the marquee attraction. But the full story is
much more interesting. The new collider provides
the greatest leap in capability of any
instrument in the history of particle physics. We
do not know what it will find, but the discoveries
we make and the new puzzles we encounter
are certain to change the face of particle physics
and to echo through neighboring sciences.

In this new world, we expect to learn what
distinguishes two of the forces of nature—electromagnetism
and the weak interactions—with
broad implications for our conception of the everyday
world. We will gain a new understanding
of simple and profound questions: Why are
there atoms? Why chemistry? What makes stable
structures possible?

The search for the Higgs particle is a pivotal
step, but only the first step. Beyond it lie phenomena
that may clarify why gravity is so much
weaker than the other forces of nature and that
could reveal what the unknown dark matter
that fills the universe is. Even deeper lies the
prospect of insights into the different forms of
matter, the unity of outwardly distinct particle
categories and the nature of spacetime. The
questions in play all seem linked to one another
and to the knot of problems that motivated the
prediction of the Higgs particle to begin with.
The LHC will help us refine these questions and
will set us on the road to answering them.

The Matter at Hand

What physicists call the “Standard Model” of
particle physics, to indicate that it is still a work
in progress, can explain much about the known
world. The main elements of the Standard Model
fell into place during the heady days of the
1970s and 1980s, when waves of landmark
experimental discoveries engaged emerging theoretical
ideas in productive conversation. Many
particle physicists look on the past 15 years as
an era of consolidation in contrast to the ferment
of earlier decades. Yet even as the Standard
Model has gained ever more experimental
support, a growing list of phenomena lies outside
its purview, and new theoretical ideas have
expanded our conception of what a richer and
more comprehensive worldview might look like.
Taken together, the continuing progress in
experiment and theory point to a very lively
decade ahead. Perhaps we will look back and
see that revolution had been brewing all along.

Our current conception of matter comprises
two main particle categories, quarks and leptons,
together with three of the four known fundamental
forces, electromagnetism and the
strong and weak interactions. Gravity is, for the moment, left to the side.
Quarks, which make up protons and neutrons,
generate and feel all three forces. Leptons, the
best known of which is the electron, are immune
to the strong force. What distinguishes these
two categories is a property akin to electric
charge, called color. (This name is metaphorical;
it has nothing to do with ordinary colors.)
Quarks have color, and leptons do not.

The guiding principle of the Standard Model
is that its equations are symmetrical. Just as a
sphere looks the same whatever your viewing
angle is, the equations remain unchanged even
when you change the perspective from which they
are defi ned. Moreover, they remain unchanged
even when the perspective shifts by different
amounts at different points in space and time.

Ensuring the symmetry of a geometric object
places very tight constraints on its shape. A
sphere with a bump no longer looks the same
from every angle. Likewise, the symmetry of the
equations places very tight constraints on them.
These symmetries beget forces that are carried
by special particles called bosons.

In this way, the Standard Model inverts Louis
Sullivan’s architectural dictum: instead of “form
follows function,” function follows form. That
is, the form of the theory, expressed in the symmetry
of the equations that define it, dictates the
function—the interactions among particles—
that the theory describes. For instance, the strong
nuclear force follows from the requirement that
the equations describing quarks must be the
same no matter how one chooses to defi ne quark
colors (and even if this convention is set independently
at each point in space and time). The
strong force is carried by eight particles known
as gluons. The other two forces, electromagnetism
and the weak nuclear force, fall under the
rubric of the “electroweak” forces and are based
on a different symmetry. The electroweak forces
are carried by a quartet of particles: the photon,
Z boson, W
⁺
boson and W
^–
boson.

Breaking the Mirror

The theory of the electroweak forces was formulated
by Sheldon Glashow, Steven Weinberg and
Abdus Salam, who won the 1979 Nobel Prize in
Physics for their efforts. The weak force, which
is involved in radioactive beta decay, does not act
on all the quarks and leptons. Each of these particles
comes in mirror-image varieties, termed
left-handed and right-handed, and the beta-decay
force acts only on the left-handed ones—a striking
fact still unexplained 50 years after its discovery.
The family symmetry among the left-handed
particles helps to defi ne the electroweak theory.

In the initial stages of its construction, the theory
had two essential shortcomings. First, it foresaw
four long-range force particles—referred to
as gauge bosons—whereas nature has but one:
the photon. The other three have a short range,
less than about 10
^–17
meter, less than 1 percent
of the proton’s radius. According to Heisenberg’s
uncertainty principle, this limited range implies
that the force particles must have a mass approaching
100 billion electron volts (GeV). The
second shortcoming is that the family symmetry
does not permit masses for the quarks and leptons,
yet these particles do have mass.

The way out of this unsatisfactory situation is
to recognize that a symmetry of the laws of nature
need not be reflected in the outcome of those
laws. Physicists say that the symmetry is “broken.”
The needed theoretical apparatus was
worked out in the mid-1960s by physicists Peter
Higgs, Robert Brout, François Englert and others.
The inspiration came from a seemingly unrelated
phenomenon: superconductivity, in which certain
materials carry electric current with zero resistance
at low temperatures. Although the laws
of electromagnetism themselves are symmetrical,
the behavior of electromagnetism within the superconducting
material is not. A photon gains
mass within a superconductor, thereby limiting
the intrusion of magnetic fields into the material.

As it turns out, this phenomenon is a perfect
prototype for the electroweak theory. If space is
filled with a type of “superconductor” that affects
the weak interaction rather than electromagnetism,
it gives mass to the W and Z bosons
and limits the range of the weak interactions.
This super conductor consists of particles called
Higgs bosons. The quarks and leptons also acquire
their mass through their interactions with
the Higgs boson. By obtaining mass in this way, instead
of possessing it intrinsically, these particles
remain consistent with the symmetry requirements
of the weak force.

The modern electroweak theory (with the
Higgs) accounts very precisely for a broad range
of experimental results. Indeed, the paradigm
of quark and lepton constituents interacting by
means of gauge bosons completely revised our
conception of matter and pointed to the possibility
that the strong, weak and electromagnetic interactions meld into one when the particles
are given very high energies. The electroweak
theory is a stunning conceptual achievement,
but it is still incomplete. It shows how the quarks
and leptons might acquire masses but does not
predict what those masses should be. The electroweak
theory is similarly indefinite in regard
to the mass of the Higgs boson itself: the existence
of the particle is essential, but the theory
does not predict its mass. Many of the outstanding
problems of particle physics and cosmology
are linked to the question of exactly how the
electroweak symmetry is broken.

Credit: Slim Films

Where the Standard Model Tells Its Tale

Encouraged by a string of promising observations
in the 1970s, theorists began to take the
Standard Model seriously enough to begin to
probe its limits. Toward the end of 1976 Benjamin
W. Lee of Fermi National Accelerator Laboratory
in Batavia, Ill., Harry B. Thacker, now
at the University of Virginia, and I devised a
thought experiment to investigate how the electroweak
forces would behave at very high energies.
We imagined collisions among pairs of W,
Z and Higgs bosons. The exercise might seem
slightly fanciful because, at the time of our
work, not one of these particles had been
observed. But physicists have an obligation to
test any theory by considering its implications
as if all its elements were real.

What we noticed was a subtle interplay
among the forces generated by these particles.
Extended to very high energies, our calculations
made sense only if the mass of the Higgs boson
were not too large—the equivalent of less than
one trillion electron volts, or 1 TeV. If the Higgs
is lighter than 1 TeV, weak interactions remain
feeble and the theory works reliably at all energies.
If the Higgs is heavier than 1 TeV, the weak
interactions strengthen near that energy scale
and all manner of exotic particle processes ensue.
Finding a condition of this kind is interesting
because the electroweak theory does not directly
predict the Higgs mass. This mass threshold
means, among other things, that something
new—either a Higgs boson or other novel phenomena—
is to be found when the LHC turns
the thought experiment into a real one.

Experiments may already have observed the
behind-the-scenes influence of the Higgs. This
effect is another consequence of the uncertainty
principle, which implies that particles such as
the Higgs can exist for moments too fleeting to
be observed directly but long enough to leave a
subtle mark on particle processes. The Large
Electron Positron collider at CERN, the previous
inhabitant of the tunnel now used by the
LHC, detected the work of such an unseen hand.
Comparison of precise measurements with theory
strongly hints that the Higgs exists and has
a mass less than about 192 GeV.

For the Higgs to weigh less than 1 TeV, as required,
poses an interesting riddle. In quantum
theory, quantities such as mass are not set once
and for all but are modified by quantum effects.
Just as the Higgs can exert a behind-the-scenes
influence on other particles, other particles can
do the same to the Higgs. Those particles come
in a range of energies, and their net effect depends
on where precisely the Standard Model
gives way to a deeper theory. If the model holds
all the way to 10
¹⁵
GeV, where the strong and
electroweak interactions appear to unify, particles
with truly titanic energies act on the Higgs
and give it a comparably high mass. Why, then,
does the Higgs appear to have a mass of no
more than 1 TeV?

This tension is known as the hierarchy problem.
One resolution would be a precarious balance
of additions and subtractions of large numbers,
standing for the contending contributions
of different particles. Physicists have learned to
be suspicious of immensely precise cancellations
that are not mandated by deeper principles.
Accordingly, in common with many of my
colleagues, I think it highly likely that both the
Higgs boson and other new phenomena will be
found with the LHC.

Supertechnifragilisticexpialidocious

Theorists have explored many ways in which
new phenomena could resolve the hierarchy
problem. A leading contender known as supersymmetry
supposes that every particle has an as
yet unseen superpartner that differs in spin. If nature were exactly supersymmetric,
the masses of particles and superpartners would
be identical, and their influences on the Higgs
would cancel each other out exactly. In that
case, though, physicists would have seen the
superpartners by now. We have not, so if supersymmetry
exists, it must be a broken symmetry.
The net influence on the Higgs could still be
acceptably small if superpartner masses were
less than about 1 TeV, which would put them
within the LHC’s reach.

Another option, called technicolor, supposes
that the Higgs boson is not truly a fundamental
particle but is built out of as yet unobserved
constituents. (The term “technicolor” alludes
to a generalization of the color charge that defines the strong force.) If so, the Higgs is not
fundamental. Collisions at energies around 1
TeV (the energy associated with the force that
binds together the Higgs) would allow us to
look within it and thus reveal its composite nature.
Like supersymmetry, technicolor implies
that the LHC will set free a veritable menagerie
of exotic particles.