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Photo: CERN
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Experiments at the Large Hadron Collider
made a major discovery, but the world’s
highest-energy particle accelerator is just
getting started.
By Ashley WennersHerron
and Kathryn Jepsen
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The Large Hadron Collider, the largest particle accelerator in the
world, started colliding particles more than three years ago. Since
then, scientists have published more than 700 papers detailing the
knowledge they have gained at the cutting edge of particle physics.
Undisputedly, the most famous insight so far has been the discovery
of what could be the long-sought Higgs boson. This particle is thought
to arise from the fluctuation of the invisible “Higgs field” that pervades the
universe, imparting mass to particles that interact with it. Without the
Higgs field, our world would be a much different place.
Even during the excitement of that discovery, thousands of scientists—
more than 1800 of whom are based in the United States—continued the
important work of analyzing the continuing flood of new data pouring out
of their detectors.
There is still much to learn about the new, Higgs-like particle. And there
is still much more territory to cover in the search for new physics. The
LHC will expand its reach dramatically when scientists crank its energy
from 4 trillion to 6.5 trillion electronvolts in 2015.
Beyond discovery
In the LHC, superconducting magnets steer two beams of protons in opposite
directions along a 17-mile ring more than 300 feet beneath the border of
Switzerland and France. The beams cross paths in four locations along
the ring. When a proton from one beam collides with a proton from the
other, the energy of the collision can convert into mass, creating for
a moment new particles.
Massive particles created in collisions are unstable and quickly decay
into less massive particles, leaving a whole zoo of particles for scientists
to study.
Since the LHC turned on, the ATLAS, CMS, LHCb and ALICE experiments—
along with the smaller experiments TOTEM and LHCf—have discovered
a total of three particles.
“At the LHC, the streetlamps are just beginning to turn on, and we can
see under some of the lampposts now,” says John Ellis, a theorist and
professor at King’s College London. “Eventually, the pools of light will join
up and we’ll be able to see everything.”
In December 2011, one of the lamps revealed something new. The ATLAS
collaboration announced the first particle discovery at the LHC—a quark
and antiquark bound together named X b (3P) (pronounced kye-bee-threepee).
Although it had been predicted for years, it took the high rate of
collisions in the LHC to finally expose the particle. Scientists are still studying
it to understand how the quark and antiquark tie together through the
strong nuclear force, which makes the nucleus of an atom stick together, too.
The CMS collaboration found the limelight just a few months later, in May
2012, when they announced the discovery of the excited baryon Ξ b (pronounced
sai-bee), a particle composed of three quarks, including a bottom
quark. Scientists are now analyzing the particle; their work may reveal
insight into how quarks bind together.
And then, in July 2012, both the CMS and ATLAS collaborations
announced the discovery of a new particle that could be the Higgs boson.
Searching for new particles is just one continuing function of the LHC
experiments. Now that scientists have uncovered new particles, they have
another focus—finding out more about them.
The new Higgs-like particle, for example, seems to fulfill at least the
minimum role of the Higgs boson, as it interacts with particles in more or
less the expected way. But observations of the new particle’s properties—
its spin, parity and detailed interactions—could show it to be a different kind
of Higgs than the one predicted by the Standard Model, the theory used to
explain the makeup and interaction of particles and forces in our universe.
If it turns out that the particle is not the Standard Model Higgs boson,
scientists will learn that there are new phenomena whose descriptions may
require new underlying principles. One popular alternative model under
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The Large Hadron Collider drives
two beams of particles on a
collision course around a 17-mile
ring located more than 300 feet
underground at the border of
France and Switzerland.
Photo: CERN
symmetry | spring 2013
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Physicist Despina
Hatzifotiadou navigates a
maze of color-coded wires
at the ALICE detector, one
of six detectors at the Large
Hadron Collider.
Photo: antoniosaba/CERN
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investigation is called supersymmetry. It posits that each particle of the
Standard Model has a related, more massive partner particle. In this model
and others, there would be more than one Higgs boson. Alternatively, it
could be that the Higgs boson is made of other, even smaller particles.
Or it could be that the Higgs exists in more than our three dimensions
of space.
“We could be looking at a new framework,” says Joao Varela, a physicist
with the Portuguese institute LIP and CMS deputy spokesperson. “It may
not be the Standard Model or even supersymmetry. It might be something
else entirely.”
Conversely, if the Higgs turns out to be the particle scientists expected
to find, physicists will have finally discovered every piece predicted in the
Standard Model.
More than new particles
Yet even with a Standard Model Higgs, questions will remain in particle
physics theory.
Particle physics research encompasses three intertwining frontiers:
the energy frontier, the intensity frontier and the cosmic frontier. Energy
frontier experiments involve converting energy into mass at particle colliders
such as the LHC; intensity frontier experiments use intense beams of
particles to study rare processes and make high-precision measurements;
cosmic frontier experiments use the cosmos as a laboratory and also
study particles that reach Earth from distant sources.
Work at all three frontiers aims in part to resolve a major contradiction
in particle physics theory. The masses of force-carrying particles such
as the Higgs boson, the W boson and the Z boson are all relatively similar,
between 80 and 125 times the mass of the proton. Within the Standard
Model, there is no explanation for why the masses of these particles—each
associated with a force that governs interactions between particles—
should have these values, nor why the Higgs mass should be so similar to
the other two. In fact, theorists have argued that these values are “unnatural”
in the Standard Model, and that the findings beg for an explanation.
Theorists have proposed many new models that can account for these
strangely low masses. All of these new models require the addition of new
fundamental particles to the Standard Model. Conveniently, some of the
extra particles predicted are good candidates to fill the role of dark matter,
the matter that scientists have found indirect evidence for in cosmic
frontier experiments but have never observed directly.
So far, LHC experiments have not found these extra fundamental particles.
(The two new particles found thus far, other than the Higgs-like boson,
are composite, not fundamental.) But even if they did, there would be
another hitch: Adding particles to fix the problems of the inexplicably light
Higgs and invisible dark matter causes a different kind of trouble. The
contradiction appears in something called flavor physics.
Some particles come in multiple copies with different masses. These
iterations are called flavors. Neutrinos, rarely interacting particles that are
a favorite subject of intensity frontier experiments, come in three flavors.
Likewise, there are three types of electrically charged leptons: the electron,
muon and tau. Quarks, the particles that make up atomic nuclei, come in
different types as well: up, down, charm, strange, bottom and top.
Sometimes a particle will transform from one flavor to the next. Based
on the Standard Model, scientists can predict how often this should happen,
if at all.
Scientists’ current predictions are calculated based on the Standard
Model. But if there’s something beyond that model, one or more undiscovered
particles, scientists expect to find that their Standard Model predictions for
flavor mixing do not precisely match their experimental results.
So far, that has not been the case. Measurements of flavor mixing at
intensity frontier experiments and energy frontier experiments—including
LHCb, CMS and ATLAS—have conformed nicely with standard predictions.
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“It’s the tension between the frontiers that’s really exciting,” says Andrew
Cohen, a theorist at Boston University. “We’re investigating this fundamental
mystery of the energy scale of the W, Z and Higgs bosons on all three fronts.”
Results from the LHC experiments will continue to provide essential
contributions toward resolving this conflict. Studying the properties of the
new, Higgs-like particle and conducting direct searches for new particles
will be the tasks of the ATLAS and CMS experiments. Flavor physics and
indirect searches for new particles are more the specialty of LHCb. In their
own ways, CMS, ATLAS and LHCb are all working to make more and more
precise measurements to more rigorously test the Standard Model.
The ALICE experiment has a slightly different specialty: delving into
understanding the behavior of the early universe. ALICE was designed to
study collisions of heavy ions, which produce a very hot state of matter
called the quark-gluon plasma. Scientists think the universe began in this
state, a primordial soup from which everything around us grew. ALICE
results, like those from the other LHC experiments, may have an important
impact on all three frontiers of particle physics.
More to come
Starting in March 2013, the LHC’s long shutdown will give scientists, engineers
and technicians the opportunity to upgrade the machine to run close to its
design energy. Each beam will operate at 6.5 trillion electronvolts.
Scientists expect to collect data from more than 200 quadrillion particle
collisions after the machine switches back on in 2015. At higher energies,
they will be able to see even more interesting events.
“The same amount of data at a higher energy is worth more,” says Ian
Hinchliffe, a physicist with Lawrence Berkeley National Laboratory
and member of the ATLAS collaboration. “With the planned upgrades, we’ll
increase the LHC’s sensitivity by a factor of 10.”
Albert Einstein’s famous theory of relativity states that the energy of
a particle is related to its mass; the two are different sides of the same
coin. The LHC puts the theory to work, pumping up particles to high energies
and smashing them into one another in order to transform that energy
into mass in the form of new particles.
Collisions at higher energies can create particles with more mass. At
13 trillion electronvolts, the LHC will be able to access a new realm of
masses and states of matter never before seen in manmade accelerators.
“The increase of energy gives a much greater reach, particularly for
heavy objects with a higher mass,” says Andy Lankford, a physicist with
the University of California, Irvine, and deputy spokesperson for ATLAS.
“It gives us the ability to explore the unknown.”
A high-energy future
Exploration at the LHC has only just begun.
“There are many reasons to be excited for the next five to 10 years and
beyond,” says Joe Incandela, a physicist with the University of California,
Santa Barbara, and CMS spokesperson.
Through careful studies, scientists will determine the properties of the
new, Higgs-like particle. They will find out whether the Standard Model
is a done deal or whether it has steered them astray. And they’ll have the
opportunity to find the unexpected.
Nature certainly has more mysteries for scientists to explore, and once
the accelerator begins running near full capacity in 2015, researchers will
have even better tools to search for new physics.
“We are at the beginning,” says Aleandro Nisati, who leads the ATLAS
collaboration’s studies of how the LHC upgrades will expand the potential
of physics analyses. “This is a new, big chapter in high-energy physics.”
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University of California,
Santa Barbara, physicist
and CMS Spokesperson
Joe Incandela watches a
continually updating display
of recent collision
events on a screen in the
CMS control room.
Photo: CERN
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