The Higgs Boson: Searching for the God Particle (28 page)

In any event, our planning will allow us to
move forward at full speed as soon as the scientific discoveries at the LHC reveal the best targets
for follow-up research. In parallel with the
technical design work, we are creating models
for dividing the governance of the ILC project so
that each constituency of physicists will have a
say. This ambitious undertaking has been truly
global in its conception, development and design,
and we expect it to be thoroughly international
in its construction and operation as well.

-Originally published: Scientific American 298(2), 54-59 (February 2008)

Higgs Won’t Fly

by Graham P. Collins

In a move that surprised and dismayed
many physicists, one of the
world’s leading laboratories has chosen
not to continue an experiment
that showed every sign of being on the
verge of discovering an elusive particle
that would have placed the capstone on
a century of particle physics. The experiment
was the last gasp of the venerable
Large Electron-Positron collider (LEP), located
near Geneva, Switzerland, and part
of the European laboratory for particle
physics (CERN). The particle was the
long-sought Higgs, which is profoundly
unlike any other particle discovered in
human history and is the final jigsaw
piece needed to complete the Standard
Model of particle physics. The decision
came down to the judgment of one man,
Luciano Maiani, CERN’s director general,
who chose to shut down LEP on schedule
to avoid delaying construction of
CERN’s next big experiment, the Large
Hadron Collider (LHC), which is slated to
be turned on in 2005.

Postulated independently by British
physicist Peter Higgs and others in 1964,
the Higgs plays a unique role in particle
physics. In one guise, the Higgs is a field
permeating the universe and giving the
other particles their mass. If the field were
turned off, the particles making up your
body would presumably fly apart at the
speed of light like so many photons. We
have no way of directly detecting the allpervasive
Higgs field, but its other guise—individual Higgs particles, like tiny concentrated
knots in the field—should be
producible in violent collisions at accelerators.
By studying the particle, physicists
can verify the theory and pin down the
Higgs’s many unknown properties.

In 2000 researchers optimized the 11-year-old LEP to conduct one last search
for the Higgs, pushing it to achieve collision
energies of 206.5 billion electron
volts (GeV)—about 14 GeV beyond its
original design parameters. Most likely the
Higgs would be too massive to fall within
LEP’s extended reach, but in the summer,
physicists saw signs of Higgs particles. Out
of millions of collisions, nine produced
Higgs candidates. A one-month extension
to LEP yielded additional results, sufficient
to conclude that the odds that the
results were noise were one in 250—a tantalizing
result but much too uncertain to
proclaim “discovery.” The data indicated
that the Higgs has a mass of about 115
GeV (the remaining collision energy goes
into creating a so-called Z particle at 91
GeV). By comparison, a proton is 1 GeV.
A 115-GeV Higgs would agree nicely with
predictions of supersymmetry models—
the idea that particles in the Standard
Model have “supersymmetric” partners.

Hoping to gain enough data to reduce
the odds of error below the one in a million
needed for a discovery, experimenters
pleaded for a year’s reprieve to LEP’s
scheduled dismantling, but after vigorous
debate they were turned down. It was
time to make way for the $4-billion LHC,
which is to occupy the same 27-kilometer-circumference tunnel as LEP. Running
LEP in 2001 would have cost CERN $65
million, including $40 million in civilengineering
contract penalties for delaying
the LHC.

Chris Tully, the Higgs coordinator for
one of the four LEP detectors and the person
responsible for combining the data
from all four, complains that what is most
frustrating is the perceived failure of
CERN’s scientific decision-making process.
Two different review boards discussed the
Higgs evidence and the extension request,
and both failed to recommend whether to
proceed or not. Each board had roughly
equal numbers of LEP and LHC scientists.
Tully feels that part of the problem was
the boards’ not keeping to their proper
terms of reference. For example, the LEP
Scientific Committee, instead of limiting
itself to the scientific issues, also considered
the potential effect on LHC finances.

Maiani’s decision could have been overturned
at a special November 17 meeting
of the CERN Council, representatives of
CERN’s 20 member countries—but again
the result was a deadlock, and so Maiani’s
decision stood. “CERN is following a scientific
program based on indecision,” Tully
says. Yet he doesn’t fault Maiani, who,
he considers, “made the wisest choice”
from the perspective of a director general,
who must give highest priority to the future
of the laboratory, meaning the LHC.

LHC advocates insist that the decision
was based on the science. Ana Henriques
Correia, who leads construction on part of
the LHC’s ATLAS detector, says, “The scientific
evidence [for Higgs] was not strong
enough to postpone LHC.” She points out
that a sizable chance remained of no discovery
by LEP even after a 2001 run.

Supporters argue that LEP was uniquely
positioned to discover or rule out a 115-GeV Higgs promptly: after 11 years LEP’s
experimenters had a very good understanding
of the performance of the accelerator
and its four detectors. By comparison,
the LHC’s extremely complicated
detectors are unknown quantities. Although
the LHC is scheduled to collide
its first protons in July 2005, collection of
scientific data will not begin until 2007—after the lengthy process of commissioning,
understanding and calibrating the
accelerator and its detectors. Furthermore,
CERN is discussing moving the start-up
date back to the end of 2005.

The opportunity to discover the Higgs
now passes to the Tevatron proton collider
at the Batavia, Ill., Fermi National Accelerator
Laboratory. The Tevatron discovered
the top quark in 1995 and starts
up again in March after a major upgrade.
But it will take until about 2006 to gather
sufficient data to claim discovery of the
Higgs, if it is near 115 GeV (the device
could see Higgs evidence up to 180 GeV).
Paul Grannis, a member of the D-Zero experiment
at the Tevatron, cautions that he
doesn’t know enough about the various
factors in play to second-guess the CERN
decision, but nonetheless he has “a hard
time imagining why they did not” choose
to continue. “We would be globally in so
much better shape if we knew whether the
Higgs were there or not, in trying to map
out the future [accelerator] program.”

These matters interest experimenters
planning what to build after the LHC.
The U.S., Japan and Germany are working
on plans for the next electron-positron
colliders, which will explore higher
energies than LEP had. These devices
would map out the detailed properties of
the Higgs and other new particles, such
as supersymmetric particles, expected to
be discovered at the LHC. A Higgs under
130 GeV favors supersymmetry, and physicists
understand very well what kind of
program is needed to find and study supersymmetry.
Above 130 GeV, “it is most
likely not supersymmetry,” Grannis says,
“and then we’re on a fishing expedition
to figure out what the hell is going on.”

-Originally published: Scientific American 284(2) 17-18 (February 2001)

The Large Hadron Collider

by Chris Llewellyn Smith

CERN

When two protons traveling at 99.999999 percent
of the speed of light collide head-on, the ensuing
subatomic explosion provides nature with 14 trillion
electron volts (TeV) of energy to play with. This energy,
equal to 14,000 times that stored in the mass of a proton at rest,
is shared among the smaller particles that make up each proton:
quarks and the gluons that bind them together. In most collisions
the energy is squandered when the individual quarks and
gluons strike only glancing blows, setting off a tangential spray
of familiar particles that physicists have long since catalogued
and analyzed. On occasion, however, two of the quarks will
themselves collide head-on with an energy as high as 2 TeV or
more. Physicists are sure that nature has new tricks up her sleeve
that must be revealed in those collisions—perhaps an exotic particle
known as the Higgs boson, perhaps evidence of a miraculous
effect called supersymmetry, or perhaps something unexpected
that will turn theoretical particle physics on its head.

The last time that such violent collisions of quarks occurred
in large numbers was billions of years ago, during the first picosecond
of the big bang. They will start occurring again in
2007, in a circular tunnel under the Franco-Swiss countryside
near Geneva. That’s when thousands of scientists and engineers
from dozens of countries expect to finish building the giant detectors
for the Large Hadron Collider (LHC) and start experiments.
This vast and technologically challenging project, coordinated
by CERN (the European laboratory for particle
physics), which is taking the major responsibility for constructing
the accelerator, is already well under way.

The LHC will have about seven times the energy of the Tevatron
collider based at Fermi National Accelerator Laboratory in
Batavia, Ill., which discovered the long-sought “top” quark in
experiments spanning from 1992 to 1995. The LHC will
achieve its unprecedented energies despite being built within the
confines of an existing 27-kilometer tunnel. That tunnel housed
CERN’s Large Electron-Positron Collider (LEP), which operated
from 1989 to 2000 and was used to carry out precision tests
of particle physics theory at about 1 percent of the LHC’s energy.
By using LEP’s tunnel, the LHC avoids the problems and vast
expense of siting and building a new, larger tunnel and constructing
four smaller “injector” accelerators and supporting facilities.
But bending the trajectories of the 7-TeV proton beams
around the old tunnel’s curves will require magnetic fields
stronger than those any accelerator has used before. Those fields
will be produced by 1,232 15-meter-long magnets installed
around 85 percent of the tunnel’s circumference. The magnets
will be powered by superconducting cables carrying currents
of 12,000 amps cooled by superfluid helium to –271 degrees
Celsius, two degrees above the absolute zero of temperature.

To carry out productive physics experiments,
one needs more than just
high-energy protons. What counts is the
energy of collisions between the protons’
constituent quarks and gluons, which
share a proton’s energy in a fluctuating
manner. The LHC will collide beams of
protons of unprecedented intensity to increase
the number of rare collisions between
quarks and gluons carrying unusually
large fractions of their parent
protons’ energy. The LHC’s intensity, or
luminosity, will be 100 times as great as
that of previous colliders and 10 times
that of the canceled Superconducting Super
Collider (SSC). The SSC would have
been a direct competitor to the LHC, colliding
20-TeV proton beams in an 87-
kilometer-circumference tunnel around
Waxahachie, Tex. The LHC’s higher intensity
will mostly compensate for the
lower beam energy, but it will make the
experiments much harder. Furthermore,
such large intensities can provoke problems,
such as chaos in the beam orbits,
that must be overcome to keep the beams
stable and well focused.

At four locations around the LHC’s
ring, a billion collisions will occur each
second, each one producing about 100
secondary particles. Enormous detectors—
the largest roughly the height of a
six-story building—packed with thousands
of sophisticated components will
track all this debris. Elaborate computer
algorithms will have to sift through this
avalanche of data in real time to decide
which cases (perhaps 10 to 100 per second)
appear worthy of being recorded for
full analysis later, off-line.

Unanswered Questions

As we study nature with higher-energy
probes, we are delving into the structure
of matter at ever smaller scales. Experiments
at existing accelerators have
explored down to one billionth of one billionth
of a meter (10
^-18
meter). The
LHC’s projectiles will penetrate even
deeper into the heart of matter, down to
10
^-19
meter. This alone would be enough
to whet scientific appetites, but pulses are
really set racing by compelling arguments
that the answers to major questions must
lie in this new domain.

In the past 35 years, particle physicists
have established a relatively compact picture—the Standard Model—that successfully
describes the structure of matter
down to 10
^-18
meter. The Standard Model succinctly characterizes
all the known constituents of matter
and three of the four forces that control
their behavior. The constituents of
matter are six particles called leptons and
six called quarks. One of the forces,
known as the strong force, acts on quarks,
binding them together to form hundreds
of particles known as hadrons. The proton
and the neutron are hadrons, and a
residual effect of the strong force binds
them together to form atomic nuclei. The
other two forces are electromagnetism
and the weak force, which operates only
at very short range but is responsible for
radioactive beta decay and is essential for
the sun’s fuel cycle. The Standard Model
elegantly accounts for these two forces as
a “unified” electroweak force, which relates
their properties despite their appearing
very different.

More than 20 physicists have won
Nobel Prizes for work that has contributed
to the Standard Model, from the
theory of quantum electrodynamics (the
1965 prize) to the discovery of the neutrino
and the tau particle (1995) and the
theoretical work of Gerardus ’t Hooft
and Martinus J. G. Veltman while at the
University of Utrecht (1999). Nevertheless,
although it is a great scientific
achievement, confirmed by a plethora of
detailed experiments, the Standard Model
has a number of serious flaws.

First, it does not consistently include
Albert Einstein’s theory of the properties
of spacetime and its interaction with matter.
This theory, general relativity, provides
a beautiful, experimentally very well
verified description of the fourth force,
gravity. The difficulty is that unlike general
relativity, the Standard Model is a fully
quantum-mechanical theory, and its
predictions must therefore break down at
very small scales (very far from the domain
in which it has been tested). The absence
of a quantum-mechanical description
of gravity renders the Standard Model
logically incomplete.

Second, although it successfully describes
a huge range of data with simple
underlying equations, the Standard Model
contains many apparently arbitrary features.
It is too byzantine to be the full story.
For example, it does not indicate why
there are six quarks and six leptons instead
of, say, four. Nor does it explain
why there are equal numbers of leptons
and quarks—is this just a coincidence? On
paper, we can construct theories that explain
why there are deep connections between
quarks and leptons, but we do not
know if any of these theories is correct.

Third, the Standard Model has an unfinished,
untested element. This is not
some minor detail but a central component:
a mechanism to generate the observed
masses of the particles. Particle
masses are profoundly important—altering
the mass of the electron, for example,
would change all of chemistry, and the
masses of neutrinos affect the expansion
of the universe. (A neutrino’s mass is at
most a few millionths of an electron’s
mass, but recent experiments show that it
is not zero. The scientists who led two pioneering
experiments that made this discovery
were awarded a share of the 2002
Nobel Prize for Physics.)

Higgs Mechanism

Physicists believe that particle masses
are generated by interactions with a
field that permeates the entire universe; the
stronger a particle interacts with the field,
the more massive it is. The nature of this field, however,
remains unknown. It could be a new
elementary field, called the Higgs field after
British physicist Peter Higgs. Alternatively,
it may be a composite object, made
of new particles (“techniquarks”) tightly
bound together by a new force (“technicolor”).
Even if it is an elementary field,
there are many variations on the Higgs
theme: How many Higgs fields are there,
and what are their detailed properties?

Nevertheless, we know with virtually
mathematical certainty that
whatever
mechanism is responsible, it must produce
new phenomena in the LHC’s energy
range, such as observable Higgs particles
(which would be a manifestation of
ripples in the underlying field) or techniparticles.
The principal design goal of the
LHC is therefore to discover these phenomena
and pin down the nature of the
mass-generating mechanism.

These new phenomena may be discovered
before the LHC comes into operation
by experiments at Fermilab’s
Tevatron, which started colliding beams
of protons and antiprotons again in 2001
after a major upgrade. These experiments
could find new phenomena beyond
the range already explored by LEP.
But even if they do “scoop” the LHC,
they will reveal only the tip of a new iceberg,
and the LHC will be where physicists
make comprehensive studies of the
new processes.

If the Tevatron does not observe these
new phenomena, then the LHC will pick
up the chase. The exploratory power of
the LHC overlaps that of LEP and the
Tevatron, leaving no gaps in which new
physics could hide. Moreover, high-precision
measurements made in the past
decade at LEP, the Stanford Linear
Accelerator Center and Fermilab have essentially eliminated worries that the
Higgs boson might be out of reach of the
LHC’s energy range. It is now clear that
either the Higgs boson or other new
physics associated with the generation of
mass will be found at the LHC.

Emulating the Big Bang

To address this kind of physics requires
re-creating conditions that existed
just a trillionth of a second after the big
bang, a task that will push modern technologies
to their limits and beyond. To
hold the 7-TeV proton beams on course,
magnets must sustain a magnetic field of
8.3 tesla, almost 100,000 times the
earth’s magnetic field and the highest ever
used in an accelerator. They will rely on
superconductivity: large currents flowing
without resistance through thin superconducting
wires, resulting in compact
magnets that can generate magnetic-field
strengths unobtainable with conventional
magnets made with copper wires. To maintain
the superconductivity under operating
conditions—with 12,000 amps of current—the magnets’ cores must be held at
–271 degrees C around 22.4 kilometers of
the tunnel. Cryogenics on this scale has
never before been attempted.