would dwarf that already giant machine.
Each collider would be so big that the
country of Liechtenstein could easily fit
inside — t wice.
At a location yet to be determined in
China, the Circular Electron Positron
Collider would collide electrons and
positrons at 240 billion electron volts,
according to a plan released in November
and championed by Wang and the
Institute of High Energy Physics. The
accelerator could later be upgraded to
collide protons at higher energies. Scientists say they could begin building the
$5 billion to $6 billion machine by 2022
and have it ready to go by 2030.
And at CERN, the proposed Future
Circular Collider would likewise operate in stages, colliding electrons and
positrons before moving on to protons.
The ultimate goal would be to reach
proton collisions with 100 trillion electron volts, more than seven times the
LHC’s energy, according to a January
report from an international group of
Meanwhile, scientists have shut down
the LHC for two years while they upgrade
the machine to function at a slightly
higher energy. Further down the line, a
souped-up version known as the High-Luminosity LHC could come online in
2026 and would increase the proton
collision rate by at least a factor of five
while maintaining the same energy
(SN Online: 6/15/18).
Portrait of the Higgs
When the LHC was built, scientists were
fairly confident they’d find the Higgs
boson. But with the new facilities, there’s
no promise of new particles. Instead,
the machines will aim to catalog how
strongly the Higgs interacts with other
known particles; in physicist lingo, these
are known as its “couplings.”
Measurements of the Higgs’ couplings
may simply confirm expectations of
the standard model. But if the obser-
vations differ from expectations, the
discrepancy could indirectly hint at the
presence of something new, such as the
particles that make up dark matter.
Some scientists are hopeful that
something unexpected might arise.
That’s because the Higgs is an enigma.
The particles condense into a molasses-like fluid. As for why that happens, “we
have no clue,” says theoretical particle
physicist Michael Peskin of Stanford
University. But that fluid pervades the
universe, slowing particles down and
giving them heft.
Another puzzle is that the Higgs’ mass
is a million billion times smaller than
expected (SN Online: 10/22/13). Certain
numbers in the standard model must be
fine-tuned to extreme precision to make
the Higgs this small, a situation physicists find unnatural.
The weirdness of the Higgs suggests
that other particles might be out there.
Scientists previously thought they had
an answer to the Higgs quandaries, via
a theory called supersymmetry, which
posits that each known particle has
a heavier partner (SN: 10/1/16, p. 12).
“Before the LHC started, there were
huge expectations,” Abramowicz says.
Some scientists claimed the LHC would
quickly find supersymmetric particles.
“Well, it didn’t happen,” she says.
The upcoming colliders may yet find
evidence of supersymmetry or otherwise hint at new particles, but this
time around, scientists aren’t making
promises. “In the past, some people
have clearly oversold what the LHC
was expected to deliver,” says theoreti-
cal particle physicist Juan Rojo of Vrije
University Amsterdam. When it comes
to any new colliders, “we should avoid
making the same mistake if we want to
keep our field alive for decades to come.”
Researchers around the world are now
hashing out priorities, making a case for
new colliders and other particle phys-
ics experiments. European physicists,
for example, will meet in May to work
on a document, the European Particle
Physics Strategy Update, to guide
research there in 2020 and beyond.
One thing is certain: The proposed
accelerators would explore unknown
territory, with unpredictable results.
The unanswered questions surrounding
the Higgs boson make it the most obvi-
ous place to look for hints of new physics,
Peskin says. “It’s the place that we haven’t
looked yet, so it’s really compelling.” s
Accelerator Style Location Particles
Energy (electron volts)
circular Europe Protons 13 trillion
Linear Collider 20 km linear Japan Electrons and positrons 250 billion
11 to 50 km
linear Europe Electrons and positrons 380 billion to 3 trillion
positrons 240 billion
positrons 90 billion to 365 billion
Protons 100 trillion
Leveling up Potential new accelerators that would study the Higgs boson are compared
with the Large Hadron Collider, which discovered the particle. SOURCES: CERN, L. EVANS, IHEP, Y. WANG
Scientists at CERN are planning a particle
accelerator called the Future Circular Collider
that would have a circumference nearly four
times as large as the Large Hadron Collider’s.
LHC 27 km