The Higgs Boson: Searching for the God Particle (32 page)

Waiting for the Higgs

by Tim Folger

Underneath a relict patch of illinois prairie, complete with a small herd of grazing
buffalo, protons and antiprotons whiz along in opposite paths around a
four-mile-long tunnel. And every second, hundreds of thousands of them slam
together in a burst of obscure particles. It’s another day at the Tevatron, a particle
accelerator embedded in the verdant grounds of the 6,800-acre Fermi National
Accelerator Laboratory complex in Batavia, about 50 miles due west of Chicago.
There have been many days like this one, some routine, some spectacular; of the 17
fundamental particles that physicists believe constitute all the ordinary matter and
energy in the universe, three were discovered here. But there won’t be many more
such days. By October 1 the power supplies for more than 1,000 liquid-heliumcooled
superconducting magnets will have been turned off forever, the last feeble
stream of particles absorbed by a metal target, ending the 28-year run of what was
until recently the most powerful particle accelerator in the world.

For several hundred physicists here who have spent nearly two decades searching
for a hypothetical particle called the Higgs boson, the closure means ceding the
hunt—and possible Nobel glory—to their archrival, the Large Hadron Collider, a
newer, more powerful accelerator at CERN on the Swiss-French border. With its
17-mile circumference and higher energies, the LHC has displaced the Tevatron as
the world’s premier particle physics research instrument, a position it will retain
well into the next decade.

-Originally published: Scientific American 305(4), 74-79 (October 2011)

Waiting for the Higgs, With the Man Who Built the LHC

by Davide Castelvecchi

CERN

They call it “the machine.”

Thousands of physicists working at the LHC are looking for the Higgs boson and other new particles, and many of them have
contributed to building the gigantic detectors that are taking most of the media limelight these days.

But humming 100 meters under the Franco-Swiss border is the apparatus that makes it all possible. The “machine” is the collider itself:
the particle accelerator that delivers swarms of protons to the detectors—funneling them through intense magnetic fields, pumping
them with energy, and eventually smashing them into each other at an interaction point that is the width of a hair. Building particle
accelerators is an entirely different job than building particle detectors or looking for new particles. The specialists who do it are called
accelerator physicists.

Particle physicists live in a quantum world—that of the processes that destroy particles and create new ones and that underlie the
fundamental forces—and dream of discovering the new laws of nature for the 21st century.

Accelerator physicists toil in relative obscurity, with tools such as radio-frequency waves and giant tesla coils, and mostly rely on physics
that is more than a century old—classical electromagnetism, with a good dose of special theory of relativity.

While we all wait here in Geneva for tomorrow’s update on the Higgs boson, I met with Lyn Evans, who recently retired after four
decades as an accelerator physicist at CERN. During those years he took part in the inception of the LHC and, starting in 1994, he
oversaw its design and construction.

Evans picked me up at CERN’s visitors center this morning. We walked through a maze of connected hallways until we got out to his
car. A quick drive took us to another building toward the outer edge of this citadel of science.

There, we sat and chatted in his office. Like every other expert I talked to, Evans says that tomorrow’s announcement will only be a step
toward the Higgs, not the final answer. “It’s obvious to everybody that we don’t have enough data yet,” he says.

But to get more data faster, the particle physicists rely on the machine—and so far, the machine has delivered. This year CERN’s
accelerator physicists have been able to ramp up the intensity of the beams faster than expected, and to produce five times as many
collisions, than the particle physicists were hoping to get. “I think everybody is astonished—even I, a little bit” at how the machine has
performed so far.

It was not always this way. Only three years ago, the machine lay crippled after a severe accident. It happened at Sector 34 of the LHC
ring. On September 19, 2008, just over a week after the LHC first got started up, a cable connecting two of the 15-meter-long, 35-ton
magnets that form the LHC melted down, producing an electrical arc. Suddenly, the liquid helium that keeps magnets at their
superconducting temperature of 1.9 kelvin vaporized. Valves designed to release the resulting gas were not able to do so fast enough,
and a shock wave ensued–so violent that it gravely damaged 53 magnets.

“It was really hard to pick ourselves up from that one,” Evans says. At the time, he recalls, he was in the personnel department, and he
received a call from the accelerator’s control room. He quickly went down to inspect the damage, wearing a respirator as the tunnel had
filled with helium gas. Evans says it was not surprising that an electrical joint could fail. “It was the collateral damage that was
unexpected.”

The LHC cools helium to low temperatures to make the magnets superconducting, so that they can carry more current and create more
powerful fields. But at 1.9 kelvin, Evans explains–the helium is colder than that at the Tevatron, the LHC’s precursor at Fermilab, near
Chicago. In particular, it is below a critical temperature at which it becomes a superfluid.

Supefluidity is an exotic state of matter that drastically lowers viscosity, and thus it enables the liquid to soak the porous material the
magnet is made of, carrying any stray heat away more efficiently. (Superfluid helium also conducts heat 10,000 times better than any
other materials, Evans says.)

(As it happens, both the magnets’ superconductivity and the helium’s superfluidity are quantum effects, so it’s no longer quite true that
particle accelerators are based entirely on classical physics.)

While particle physicists gear up for big discoveries, the machine experts at CERN are already looking ahead to the upcoming
upgrade. In part as a result of the Sector 34 accident, CERN has decided to do a first run at half the energy. But in 2013, the lab will
completely shut down the accelerator for an entire year.

First, the CERN team will pump the liquid helium out. Part of it will be liquefied and stored, but CERN does not have enough storage
space for all of its 150 tons of it, so it will sell about half of it on the market. Then, they will circulate helium gas inside the machine to
slowly bring all of its 50,000 tons up to room temperature, a process that will take weeks. “There are constraints on the rate you can do
it,” Evans says: less-than-gentle temperature gradients could easily break things up.

During the shutdown, CERN will bring the LHC up to its design specs, and then the laborious cool-down process will begin, so the
accelerator can restart. Once again, it will be the machine people’s job to make all of that happen.

-Originally published: Scientific American online December 12, 2011

Is Supersymmetry Dead?

by Davide Castelvecchi

For decades now physicists have contemplated the idea of an entire shadow
world of elementary particles, called supersymmetry. It would elegantly
solve mysteries that the current Standard Model of particle physics leaves
unexplained, such as what cosmic dark matter is. Now some are starting to
wonder. The most powerful collider in history, the Large Hadron Collider
(LHC), has yet to see any new phenomena that would betray an unseen level of
reality. Although the search has only just begun, it has made some theorists
ask what physics might be like if supersymmetry is not true after all.

“Wherever we look, we see nothing—that is, we see no deviations from
the Standard Model,” says Giacomo Polesello of Italy’s National Institute of
Nuclear Physics in Pavia. Polesello is a leading member of the 3,000-strong
international collaboration that built and operates ATLAS, one of two cathedral-
size general-purpose detectors on the LHC ring. The other such detector,
CMS, has seen nothing, either, according to an update presented at a conference
in the Italian Alps in March.

Theorists introduced supersymmetry in the 1960s to connect the two basic
types of particles seen in nature, called fermions and bosons. Roughly speaking,
fermions are the constituents of matter (the electron being the quintessential
example), whereas bosons are the carriers of the
funda mental forces (the photon in the case of
electro magnetism). Supersymmetry would give
every known boson a heavy “superpartner” that
is a fermion and every known fermion a heavy
partner that is a boson. “It is the next step up
toward the ultimate view of the world, where we
make everything symmetric and beautiful,” says
Michael Peskin, a theorist at SLAC National
Accelerator Laboratory.

The monumental collider at CERN near Geneva
should have the oomph to produce those superparticles.
Currently the LHC is smashing protons
with an energy of four trillion electron volts (TeV)
apiece, up from 3.5 TeV last year. This energy is
divided among the quarks and gluons that make up
the protons, so the collision can generate new
particles with the equivalent of about 1 TeV of mass.
But despite the high expectations (and energies), so
far nature has not cooperated. LHC physicists have
been searching for signs of
particles new to science and
have seen none. If superparticles
exist, they must be
even heavier than many
physicists had hoped. “To put
it bluntly,” Polesello says,
“the situation is that we have
ruled out a number of ‘easy’
models that should have
showed up right away.” His
colleague Ian Hinchliff e of
Lawrence Berkeley National
Laboratory echoes him: “If
you look at the range of
masses and particles that
have been excluded, it’s quite impressive.”

Many are still hopeful. “There are still very viable
ways of building supersymmetry models,” Peskin
says. Expecting to see new physics after just a year
of data taking was unrealistic, says Joseph Lykken,
a theorist on the CMS team.

What has theorists on edge, however, is that for
super symmetry to solve the problems for which it
was invented in the first place, at least a few of the
superparticles should not be too heavy. To constitute
dark matter, for example, they need to weigh no
more than a few tenths of 1 TeV.

Another reason most physicists want some
superparticles to be light lies in the Higgs boson,
another major target of the LHC. All elementary
particles that have mass are supposed to get it
through their interaction with this boson and,
secondarily, with a halo of fleeting “virtual particles.”
In most cases, the symmetries of the Standard Model
ensure that these virtual particles cancel one
another out, so they contribute only modestly to
mass. The exception, ironically, is the Higgs itself.
Calculations based on the Standard Model yield the
paradoxical result that the boson’s mass should be
infinite. Superpartners would solve this mystery by
providing greater scope for cancellations. A Higgs
mass of around 0.125 TeV, as suggested by preliminary
results announced in December 2011, would
be right in the range where supersymmetry predicts
it should be. But in that case, the superparticles
would need to have a fairly low mass.

If that proves not to be the case, one explanation
is that heretofore underappreciated symmetries of
the Standard Model keep the Higgs mass finite, as
Bryan Lynn of University College London suggested
last year. Others say Lynn’s idea would provide at
best a partial explanation, leaving a vital role for
physics beyond the Standard Model—if not supersymmetry,
then one of the other strategies that
theorists have devised. A popular plan B is that
the Higgs boson is not an
elementary particle but
a composite of other particles,
just as protons are
com posites of quarks. Unfortunately,
the LHC simply
does not have enough data
to say much about that idea
yet, says CERN’s Christophe
Grojean. More exotic options,
such as extra dimensions
of space beyond
the usual three, may forever
lie beyond the LHC’s reach.
“Right now,” points out
Gian Francesco Giudice,
another theorist at CERN, “every single theory has
its own problems.”

As ATLAS and CMS continue to accumulate
data, they will either discover superparticles or
exclude wider ranges of possible masses. Although
they may never be able to strictly disprove supersymmetry,
if the collider fails to find it, the theory’s
usefulness may fade away, and even its most hardcore
supporters may lose interest. That would be
a blow not just to supersymmetry but also to even
more ambitious unified theories of physics that
presume it, which include string theory and other
approaches. LHC physicists take this
uncertainty in stride and expect the collider to find
some new and exciting physics—not just the physics
theorists had expected. Hinchliffe says, “The most
interesting thing we will see is something that
nobody thought of.”

-Originally published: Scientific American 306(5) 16-18 (May 2012)