The Higgs Boson: Searching for the God Particle (27 page)

Many teams from the U.S. are now planning to work at the LHC. A sub-panel of the High Energy Physics Advisory Panel, chaired by Sidney D. Drell of the Stanford Linear Accelerator, recently emphasized to the U.S. Department of Energy the need to support such participation. Fortunately for those of us at U.C.L.A., our early involvement in the Compact Muon Solenoid guarantees our place in the LHC.

The discovery of the top quark gives physicists a more accurate tool in evaluating decays of the bottom quark. Now that the mass of the top is known, theorists can calculate the frequency of penguin processes involving top quarks. Knowing the top's contribution, they can more precisely gauge which FCNCs signal exotic particles.

The top quark could also decay in exotic ways that signal unusual physics. For instance, it might decay to a charm and two neutrinos, a decay mediated by technicolor or multiple Higgs particles. The high mass of the top—174 GeV—might be part of a general pattern, indicating that exotic particles are
even heavier than theorists had anticipated. They could range from hundreds of GeV to 1 TeV.

The observations of flavor-changing decays at Cornell and the limits on exotic particles from UA1 have put scientists in a new era of searches for phenomena beyond the Standard Model. With the profuse sources of
B
mesons experimenters will have in the near future, and information about top quarks, they can consolidate the early sightings of flavor-changing processes—and tease out the implications.

The story of flavor-changing neutral currents illustrates the role that “null” experiments—those that see nothing-have played in guiding the development of particle physics. We hope the 30 years of arduous searches will be rewarded in the not too distant future with more discoveries. Even before the Large Hadron Collider comes on line, physicists may be able to peel partially yet another layer from the elementary-particle onion.

-Originally published: Scientific American 271(3), 40-47 (September 1994)

Building the Next-Generation Collider

by Barry Barish, Nicholas Walker and Hitoshi Yamamoto

A new era in physics will open up when
the Large Hadron Collider (LHC)
extends the reach of subatomic particle
investigations to unprecedented energy scales.
But even before researchers initiate the first
high-energy collisions in the LHC’s giant storage
ring, located under the French-Swiss border,
they are already contemplating and working
toward the next great particle accelerator. And
the consensus choice of the particle physics
community is a proposed facility called the
International Linear Collider (ILC), a machine
more than 30 kilometers long that would smash
electrons and positrons together at velocities
very close to the speed of light. (The positron is
the antimatter counterpart of the electron, identical
in mass but opposite in charge.)

Far more powerful than previous electronpositron
colliders, the ILC would enable physicists
to follow up any groundbreaking discoveries
made by the LHC. The LHC is designed to
investigate the collisions of protons, each of
which is actually a bundle of three quarks
bound together by gluons (the particles carrying
the strong nuclear force). Because the quarks
and gluons within a proton are constantly interacting,
a proton-proton collision is an inherently
messy affair. Researchers cannot be certain
of the energy of each quark at the moment of the
collision, and this uncertainty makes it difficult
to determine the properties of novel particles
produced by the impact. But the electron and
positron are fundamental particles rather than
composites, so physicists working with an electron-
positron collider can know the energy of
each collision to great accuracy. This capability
would make the ILC an extremely useful tool
for precisely measuring the masses and other
characteristics of newly discovered particles.

More than 1,600 scientists and engineers
from nearly 300 laboratories and universities
around the world are now working on the design
of the ILC and the development of the detectors
that would analyze its particle collisions.
In February 2007 our design team released a
cost estimate for the machine: $6.7 billion (not
including the expense of the detectors). We have
done studies comparing the costs of locating the
ILC at three possible sites—CERN, the European
laboratory for particle physics near Geneva,
the Fermi National Accelerator Laboratory
in Batavia, Ill., and the mountains of Japan—and we are developing schemes for the governance
of a truly international laboratory. Although
the ILC’s price tag may seem steep, it is
roughly comparable to the costs of large science
programs such as the LHC and the ITER nuclear
fusion reactor. And if everything proceeds as
hoped, the ILC could start illuminating the
frontiers of particle physics sometime in the
2020s.

Birth of a Collider

In August 2005 about 600 physicists from
around the world gathered in Snowmass, Colo.,
to start planning the development of the ILC.
But the true beginnings of the project go back
to the commissioning of CERN’s Large Electron-
Positron (LEP) collider in 1989. The LEP
accelerated electrons and positrons in a storage
ring with a circumference of 27 kilometers, then
smashed the particles together, producing
impacts with energies as high as 180 billion
electron volts (GeV). It was clear, though, that
the LEP would be the largest collider of its kind,
because accelerating electrons and positrons to
energies in the trillion-electron-volt (TeV) scale—also known as the terascale—would require a
ring several hundred kilometers in circumference
and would be completely cost-prohibitive.

The major obstacle to a storage ring solution
is synchrotron radiation: relatively light particles
such as electrons and positrons happily radiate
their energy as they speed around the ring,
their paths continuously bent by the ring’s many
dipole magnets. Because these losses make it
progressively harder to accelerate the particles,
the cost of building such a collider is proportional
to the square of the collision energy: a
machine that doubled the LEP energies would
cost four times as much. (The energy losses are
not as severe for colliders that accelerate heavier
particles such as protons; hence, the tunnel
dug for the LEP ring is now being used by the
LHC.)

A more cost-effective solution is a linear collider,
which avoids synchrotron radiation by accelerating
particles in straight lines rather than
in a ring. In the ILC design, two 11.3-kilometer-long
linear accelerators, or linacs—one for electrons,
one for positrons—are aimed at each other,
with the collision point in the middle. The
downside is that the electrons and positrons
must be accelerated from rest up to the collision
energy on each pulse of the machine instead of
building up speed with each circuit of the storage
ring. To obtain higher collision energies,
one can simply build longer linear accelerators.
The cost of the facility is directly proportional
to the collision energy, giving linear colliders a
clear advantage over the storage ring concept at
the TeV scale.

At the same time that the LEP was being constructed
in Europe, the U.S. Department of Energy
was building a competing machine at the
Stanford Linear Accelerator Center (SLAC).
SLAC’s device, which was considered a proof of
principle of the linear collider concept, used a
three-kilometer-long linac to accelerate bunches
of electrons and positrons in tandem, boosting
them to energies of about 50 GeV. The
bunches were then magnetically separated and
bent around to bring them into a head-on collision.
Although SLAC’s machine—which operated
from 1989 to 1998—was not exactly a true
linear collider, because it employed only one linac,
the facility paved the way for the ILC.

Planning for a TeV-scale linear collider began
in earnest in the late 1980s and early 1990s
when several competing technologies were proposed.
As researchers developed these proposals
over the next decade, they focused on the
need to keep the linear collider affordable. Finally,
in August 2004, a panel of 12 independent
experts assessed the proposed technologies
and recommended a design conceived by the
TESLA group, a collaboration of scientists from
more than 40 institutions, coordinated by the
DESY research center in Hamburg, Germany.
Under this proposal, the electrons and positrons
would travel through a long series of vacuum
chambers called cavities. Constructed from the
metal niobium, these cavities can be superconducting—
when cooled to very low temperatures,
they can conduct electricity without resistance.
This phenomenon would enable the efficient
generation of a strong electric field inside the
cavities that would oscillate at radio frequencies,
about one billion times per second. This
oscillating field would accelerate the particles
toward the collision point.

The basic element of this superconducting radio-
frequency (SCRF) design is a one-meterlong
niobium cavity consisting of nine cells that
can be cooled to a temperature of two kelvins
(–456 degrees Fahrenheit). Eight or nine cavities
would be attached end to end in a string and
immersed in ultracold liquid helium in a tank
called a cryomodule. Each of the two main linacs in the ILC
would require about 900 cryomodules, giving
the collider about 16,000 cavities in all. Researchers
at DESY have so far constructed 10
prototype cryomodules, fi ve of which are currently
installed in FLASH, a laser at DESY that
employs high-energy electrons. The SCRF technology
will also be incorporated into DESY’s
upcoming European X-Ray Free-Electron Laser
(XFEL), which will string together 101 cryomodules
to form a superconducting linac that
can accelerate electrons to about 17.5 GeV.

Because the ILC’s linacs can be shorter (and
hence less expensive) if the cavities can generate
a stronger electric field, the design team has set
an aggressive goal of improving the performance
of the SCRF system until it can give the particles
an energy boost of 35 million electron volts
(MeV) for every meter they travel. Several prototype
cavities have already exceeded this goal,
but it remains a challenge to mass-produce such
devices. The key to high performance is ensuring
that the inner surface of the cavity is ultraclean
and defect-free. The preparation of the
cavities and their installation in the cryomodules
must be done in clean-room environments.

The ILC in a Nutshell

The ILC design team has already established the
basic parameters for the collider. The machine will be about 31 kilometers
long, with most of that length taken up by the
two superconducting linacs that will set up electron-
positron collisions with 500 GeV energies.
(A 250-GeV electron striking a 250-GeV positron
moving in the opposite direction will result
in a collision with a center-of-mass energy of
500 GeV.) At a rate of five times per second, the
ILC will generate, accelerate and collide nearly
3,000 electron and positron bunches in a onemillisecond-
long pulse, corresponding to an
average total power of about 10 megawatts for
each beam. The overall efficiency of the
machine—that is, the fraction of electrical power
converted to beam power—will be about 20
percent, so the two linacs will require a total of
about 100 megawatts of electricity to accelerate
the particles.

To produce the electron beam, a laser will
fire at a target made of gallium arsenide, knocking
off billions of electrons with each pulse.
These particles will be spin-polarized—all their
spin axes will point in the same direction—
which is important for many particle physics investigations.
The electrons will be rapidly accelerated
in a short SCRF linac to an energy of 5
GeV, then injected into a 6.7-kilometer storage
ring at the center of the complex. As the electrons
circulate and emit synchrotron radiation,
the bunches of particles will be damped—that
is, their volume will decrease, and their charge
density will increase, maximizing the intensity
of the beam.

When the electron bunches exit the damping
ring 200 milliseconds later, each will be about
nine millimeters long and thinner than a human
hair. The ILC will then compress each electron
bunch to a length of 0.3 millimeter to optimize
its acceleration and the dynamics of its subsequent
collisions with the corresponding positron
bunch inside the detector. During the compression,
the bunches will be boosted to an energy
of 15 GeV, after which they will be injected
into one of the main 11.3-kilometer-long SCRF
linacs and accelerated to 250 GeV.

Midway through the linac, when the particles
are at an energy of 150 GeV, the electron
bunches will take a small detour to produce the
positron bunches. The electrons will be deflected
into a special magnet known as an undulator,
where they will radiate some of their energy into
gamma rays. The gamma photons will be focused
onto a thin titanium alloy target that rotates
about 1,000 times per minute, and the impacts
will produce copious numbers of electronpositron
pairs. The positrons will be captured,
accelerated to an energy of 5 GeV, transferred
to another damping ring and finally sent to the
other main SCRF linac at the opposite end of
the ILC. Once the electrons and positrons
are fully accelerated to 250 GeV and
rapidly converging toward the collision
point, a series of magnetic
lenses will focus the high-energy
bunches to flat ribbon beams
about 640 nanometers (billionths
of a meter) wide and six
nanometers high. After the collisions,
the bunches will be extracted
from the interaction region
and removed to a so-called
beam dump, a target that can
safely absorb the particles and
dissipate their energy.

Every subsystem of the ILC will
push the technological envelope and
present major engineering challenges. The
collider’s damping rings must achieve beam
qualities several times better than those of existing
electron storage rings. What is more, the
high beam quality must be preserved throughout
the compression, acceleration and focusing
stages. The collider will require sophisticated
diagnostics, state-of-the-art beam-tuning procedures
and a very precise alignment of its components.
Building the positron production system
and aiming the nanometer-size beams at
the collision point will be demanding tasks.

Developing detectors that can analyze the
collisions in the ILC will also be challenging. To
determine the strengths of the interactions between
the Higgs boson and other particles, for
example, the detectors will need to measure the
momentum and creation points of charged particles
with resolutions that are an order of magnitude
better than those of previous devices.
Scientists are now working on new tracking and
calorimeter systems that will allow researchers
to harvest the rich physics of the ILC.

The Next Steps

Although the ILC team has chosen a design for
the collider, much more planning needs to be
done. Over the next few years, while the LHC
starts collecting and analyzing data from its
proton-proton collisions, we will strive to optimize
the ILC design to ensure that the electronpositron
collider achieves the best possible performance
at a reasonable cost. We do not yet
know where the ILC will be located; that decision
will most likely hinge on the amount of
financial support that governments are willing
to invest in the project. In the meantime, we will
continue to analyze the sample ILC sites in
Europe, the U.S. and Japan. Differences in geology,
topography, and local standards and regulations
may lead to different construction
approaches and cost estimates. Ultimately,
many details of the ILC design will depend on
exactly where the collider is built.