Finally, physics’s zoo of subatomic
particles is full. Scientists have almost certainly snared the Higgs
boson, the last particle waiting to be roped into the fold.
Decades after it was proposed, the
Higgs emerged in the shards of particle collisions at the world’s
most powerful accelerator, the Large Hadron Collider at the CERN
laboratory near Geneva. Physicists announced the discovery on July 4
during a seminar at the lab.
“We have now found the last missing
cornerstone of the standard model,” said Rolf-Dieter Heuer, CERN’s
director-general. “It’s the beginning of a long journey to
investigate all the properties of this interesting particle.”
The particle’s mass is around 125
billion electron volts, or about 133 times the mass of a proton. CERN
captured the Higgs in two huge experiments, each of which
independently reached the gold-standard statistical level for
confirming the particle’s discovery.
One of the theorists who first proposed the particle nearly five decades ago joined in the
all-around congratulations. “It really is an incredible thing that
it’s happened in my lifetime,” said Peter Higgs of the University
of Edinburgh.
In one respect, finding the Higgs
simply confirms the standard model, physicists’ framework for
understanding the particles that make up the universe and the forces
that govern them. But the discovery also opens new areas to explore,
including alternate versions of the standard model that could explain
some of the biggest unanswered questions about the cosmos.
The Higgs traces back to 1964, when
several physicists independently dreamed up the idea of an energy
field that would have permeated the early universe (and persisted to
the present). “In all honesty we were trying to solve a more modest
problem,” said theorist Carl Hagen of the University of Rochester
in New York. In certain theoretical calculations, particles with zero
mass kept inconveniently popping up: In trying to get rid of those
particles, Higgs, Hagen and others realized that once the universe
cooled enough from its initial Big Bang, this energy field would have
had to emerge.
Like a puddle of
molasses, the field resists the motion of particles moving through
it. Such resistance to motion, or inertia, is the defining quality of
mass. Subatomic particles therefore acquire differing amounts of mass
depending on how strongly they interact with the energy field.
Known as the Higgs field, its existence
also required a new particle — the Higgs boson. (Bosons are a class
of fundamental particles defined by their quantum properties.)
Finding the Higgs was a major goal of the Superconducting Super
Collider, an atom-smasher that was being built beneath Waxahachie,
Texas, when the U.S. Congress canceled it under budget pressures in
1993. The Fermi National Accelerator Laboratory, in Batavia, Ill.,
also chased the Higgs until shutting down its biggest machine last
year.
Today, CERN scientists hunt the Higgs
by smashing two beams of protons together at the $10 billion LHC. Out
of a trillion proton-proton collisions, perhaps one will create a
Higgs particle, which then decays almost instantaneously into other
particles. Sensitive detectors placed at the sites of these smashups
look for signatures of several ways the Higgs might have decayed.
“It’s not a needle in a haystack — it’s much worse than
a needle in a haystack,” said Joe Lykken, a theoretical physicist
at Fermilab.
If each of the LHC's 500 trillion collisions were
represented by a grain of sand, they would fill an Olympic-sized swimming pool, said Joe Incandela, a
physicist at the University of California, Santa Barbara and a
spokesman for one LHC experiment. Yet the grains from the signals of
interest — the possible Higgses — would cover only the tip of
your finger.
Both experiments looked at multiple
ways the Higgs could decay, such as into two photons or into two Z
particles.
One of the LHC’s two main detectors,
the CMS experiment, found signs of a particle with a mass of 125.3
billion electron volts, plus or minus 0.6 billion electron volts,
Incandela said. The statistical strength of a signal is measured by a
quantity called sigma: A five-sigma result, considered the standard
to claim a discovery, means there is a 1-in-3.5 million chance that a
statistical fluke could have created a signal of that magnitude or
greater.
In three of five decay paths studied,
CMS found the Higgs with a statistical significance of 5.1 sigma.
Adding in the other two channels, which have relatively little data,
lowered that to 4.9 sigma — but the results are still consistent
with a Higgs being there, said physicist Elizabeth Simmons of
Michigan State University.
The competing ATLAS experiment spotted
a new particle with a mass of 126.5 billion electron volts, with a
statistical uncertainty at a 5.0 sigma level when combining the decay
paths it examined. Independent physicist Philip Gibbs combined data
from both ATLAS and CMS, using only the decay in which the Higgs
produces two photons, to come up with an unofficial six-sigma signal.
The Higgs masses found by both
experiments are consistent with one another given the uncertainty
ranges in each measurement, said ATLAS spokeswoman Fabiola Gianotti
(though she did not give a numerical error range for her experiment).
Both teams will also present their work this week at the
International Conference on High-Energy Physics in Melbourne,
Australia.
“It’s a great day for particle
physics and it’s really a profound discovery about how nature
works,” said Pier Oddone, director of Fermilab.
CERN won the transatlantic race to find
the Higgs after Fermilab’s proton-antiproton accelerator shut down
last September. On July 2, in their final analysis, Fermilab
physicists reported that their data could narrow the Higgs mass range
only to between 115 billion and 135 billion electron volts, with a
statistical significance of 2.9 sigma (SN
Online: 7/2/12).
Since April the LHC has been colliding
beams at energies of 8 trillion electron volts — 4 trillion
electron volts in each beam — at four times the energy of
Fermilab’s machine. Lab officials have decided to extend the LHC’s
current run by up to three months to gather as much data as possible
before it shuts down for two years for a major upgrade to 14 trillion
electron volts.
Now that the Higgs has almost certainly
been found, scientists are looking forward to learning more about it.
So far, the particle seen in the experiments looks like the Higgs as
predicted by the standard model, Heuer said, but slight differences
could still exist. He compared the task to trying to determine from
afar if a person approaching is your best friend or your best
friend’s twin. Only when the person gets close enough can you
determine which one it is. LHC measurements should soon reveal
whether the particle’s properties match those predicted by the
standard model, or whether new physics might be at work.
“Confirmation of theory is
satisfying, but it would be more eventful if there were significant
disagreements and controversies to resolve,” said Frank Taylor, an
MIT physicist who works on the ATLAS collaboration.
One well-loved
extension of the
standard model is a theory known as supersymmetry, which holds that
all known particles have a heavy supersymmetric partner as yet
unseen. The concept opens up all sorts of areas to explore. Some
versions of supersymmetry, for instance, predict that at least five
kinds of Higgs boson should exist, although only the lightest would
be detectable at the LHC. Other supersymmetric particles may account
for dark matter, the mysterious stuff that makes up most of the
matter in the universe but which scientists have yet to identify.
If supersymmetry is right, the LHC has
a shot at detecting many of these new particles as it continues its
Higgs-refining quest. “It’s the path to answer these other
questions,” said Gordon Kane, a theoretical physicist at the
University of Michigan whose work has predicted a Higgs in the mass
range found.
Fermilab’s Rob Roser said far more is
yet to come from the LHC. “They’re in a new regime of energy and
statistical precision, and this may not be the only surprise we have
this year from them.”
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