ATOM & COSMOS
Future colliders will target the Higgs
Potential new accelerators plan to study the particle in detail
BY EMILY CONOVER
If particle physicists get their way, new
accelerators could one day scrutinize the
most tantalizing subatomic particle in
physics — the Higgs boson. More than
six years after discovering that particle
at the Large Hadron Collider, scientists
are planning enormous new machines
that would stretch for tens of kilometers
across Europe, Japan or China.
The 2012 discovery of the Higgs,
which reveals the origins of mass, put
the finishing touch on the standard
model, the overarching theory of particle
physics (SN: 7/28/12, p. 5). And it was a
landmark achievement for the LHC, the
world’s biggest accelerator, located at
the CERN laboratory near Geneva.
Physicists hope that delving further
into the mysteries of the Higgs will be
key to solving lingering puzzles of par-
ticle physics. “The Higgs is a very special
particle,” says physicist Yifang Wang,
director of the Institute of High Energy
Physics in Beijing. “We believe the Higgs
is the window to the future.”
The LHC — a ring 27 kilometers in
circumference, where protons are
accelerated to nearly the speed of light
and smashed together a billion times a
second — was great for discovering the
Higgs. But the accelerator is not ideal for
studying the particle in detail.
Physicists are clamoring for a collider
that can crank out oodles of Higgs bosons.
Blueprints for such machines have been
put forth, and researchers hope these
“Higgs factories” will reveal solutions to
glaring weak spots in the standard model.
“The standard model is not a complete
theory of the universe,” says experimen-
tal particle physicist Halina Abramowicz
of Tel Aviv University. For example,
the theory can’t explain dark matter,
an unidentified substance whose mass
is necessary to explain cosmic obser-
vations such as the motions of stars in
galaxies. Nor can the theory explain why
the universe is made up of matter, while
antimatter is exceedingly rare.
Carefully scrutinizing the Higgs might
point scientists toward the answers to
those questions, proponents of the new
colliders claim. But among scientists, the
desire for new, costly accelerators is not
universal, especially because it’s unclear
what exactly the machines will find.
Next in line
Closest to inception is the International
Linear Collider in northern Japan.
Unlike the LHC, in which particles zip
around a ring, the ILC would accelerate
two beams of particles along a straight
line, directly at one another over the
ILC’s 20-kilometer length. And instead
of crashing protons together, the accelerator would collide electrons and their
antimatter partners, positrons.
But last December, a multidisciplinary
committee of the Science Council of
Japan came down against the project
in a report, urging the government to
be cautious with its support and ques-
tioning whether the expected scientific
achievements justify the estimated cost
of about $5 billion.
Supporters argue that the ILC’s plan
to smash together electrons and posi-
trons, rather than protons, has some big
advantages. Electrons and positrons are
elementary particles — they have no
smaller constituents — while protons
are made up of smaller particles called
quarks. So proton collisions have more
useless particle debris to sift through.
Additionally, in proton smashups,
only a fraction of each proton’s energy
goes into the collision. In electron-positron colliders, particles bring the
full brunt of the accelerator’s energy
to bear, allowing scientists to tune the
energy of collisions to maximize the
number of Higgs bosons produced.
Likewise, because of the electrons’ and
positrons’ elementary nature, the ILC
would require only 250 billion electron
volts to produce Higgs bosons; the LHC
needs 13 trillion electron volts.
For the ILC, “the quality of the data
coming out will be much higher, and
there will be much more of it on the
Higgs,” says particle physicist Lyn Evans
of CERN. One in every 100 ILC collisions
would pump out a Higgs. That happens
once in 10 billion collisions at the LHC.
The Japanese government is expected
to decide about the collider in March.
If approved, the ILC should take about
12 years to build, Evans says. It could also
be upgraded later to increase its length
and the energy it can reach.
CERN has plans for a similar machine,
the Compact Linear Collider. It would
also collide electrons and positrons but at
higher energies than the ILC. The energy
would start at 380 billion electron volts
and increase to 3 trillion electron volts in
a series of upgrades. To reach those higher
energies, the Compact Linear Collider
will rely on a new type of accelerator.
Running in circles
Two other planned colliders, in China
and in Europe, would be circular like the
LHC. But at 100 kilometers around, both
A 20-kilometer-long accelerator planned for Japan, the International Linear Collider (design
illustrated), would slam together electrons and positrons to better understand the Higgs boson.