PASADENA, Calif.—Physicists have announced that the Large
Hadron Collider (LHC) has produced yet more tantalizing hints for
the existence of the Higgs boson. The European Center for Nuclear
Research (CERN) in Geneva, the international team of thousands of
scientists—including many from the California Institute of
Technology (Caltech)—unveiled for the first time all the data
taken over the last year from the two main detectors at the LHC:
the Compact Muon Solenoid (CMS) and ATLAS (A Toroidal LHC
ApparatuS). The results represent the largest amount of data ever
presented for the Higgs search.
The Higgs boson is a hypothesized particle that endows every
other particle with mass, and is the presumed last piece of the
so-called Standard Model, the theory that describes how every
particle interacts. According to physicists, the discovery of the
Higgs boson, in whatever form it may take, is crucial for
understanding the fundamental laws of physics.
The team says they have seen what they call "excess
events"—a slight surplus of particle-collision events over
what would be expected if the Higgs didn't exist. This suggests
that the particle might have a mass between 115 and 127
gigaelectron volts (GeV, a unit of mass; in comparison, the mass of
a proton is about 1 GeV). While the physicists can't yet claim
discovery of the elusive particle, they are closer than ever,
having ruled out a very large range of the Higgs's possible masses
with great certainty.
But if physicists do not find the Higgs in the remaining mass
range between 115 and 127 GeV, then it means the particle—if
it exists at all—is of a more exotic form, requiring new
theories of physics. Although remarkably successful, the Standard
Model is incomplete, and an exotic form of the Higgs could help
point the way toward a more complete theory—which physicists
say is an exciting challenge.
"This is the beginning of a game-changing time for particle
physics," says Harvey Newman, professor of physics. Along with
professor of physics Maria Spiropulu, Newman leads Caltech's group
that works on the CMS detector.
The LHC searches for the Higgs boson by slamming together
protons at near-light speeds, producing new particles like the
Higgs in the process. The problem is that the particle is
exceptionally short-lived, decaying into other smaller particles
within a tiny fraction of a second of its birth. To find the Higgs,
physicists have to pick through the remains of each proton
collision and reconstruct what happened.
If the collisions successfully produce a Higgs, then the
particle can decay in several ways, depending on its mass. The
Caltech team helped analyze three of these decays, called channels,
in which the Higgs either decays into two photons, a pair of
particles called W bosons, or another pair called Z bosons.
Graduate students Yong Yang, Yousi Ma, Jan Veverka, and Vladlen
Timciuc are all studying the photon-photon channel, searching for
the Higgs as well as possible signs of new physics phenomena.
Caltech Tolman Postdoctoral Scholar Emanuele Di Marco, helped lead
the analysis of the W-boson channel for the CMS team. Spiropulu,
graduate student Chris Rogan, and other colleagues have also done
preparatory studies for the Z-boson channel, which they published
in the Physical
Review in 2010.
"The impressive data produced by the CMS is the result of the
experiment's ability to cleanly identify and precisely measure the
energies of photons, electrons, and positrons," says Adi Bornheim,
a Caltech staff scientist who heads the electromagnetic calorimeter
(ECAL) detector group at CMS. Composed of 76,000 crystal detectors
and weighing in at more than 90 tons, the calorimeter measures the
energy of the electrons and protons produced by LHC collisions with
"For the last 17 years, our group has led the way in
constructing and calibrating the calorimeter, which has to be
extremely precise for these demanding studies," adds Marat
Gataullin, an assistant scientist at Caltech.
The collision experiments in 2010 and 2011 at the LHC operated
only at half their designed energy levels, the researchers say.
Still, the experiments exceeded design specifications for the
proton beam's focus and intensity, producing about one quadrillion
(a million billion) proton collisions and resulting in millions of
gigabytes of data. Even though the latest experiments were more
complicated than ever, the scientists in the ATLAS and CMS teams
say they were able to analyze the data in record time, using new
technology pioneered and developed by Newman's group at
In order to confirm once and for all whether the Higgs exists as
physicists understand it—or if they'll need to come up with
new theories—the LHC will need be cranked up to collide
protons with more energy. "We now need more data-three or four
times the data that we expect the LHC to deliver in 2012," Newman
says. The LHC is currently operating at around seven teraelectron
volts (a TeV is a thousand times larger than a GeV), and scientists
are considering boosting up the energy in the next year, which will
help in the search. The LHC is designed to smash protons using
energies as high as 14 TeV. "We foresee reaching 14 TeV by 2015,
boosting the intensity by ten times starting in around 2022, and
cranking up the energy to 33 TeV starting about 20 years from now,"
he adds. "This will open a vast new realm for exploration, and will
surely revolutionize our understanding of the nature of matter and
forces at the most basic level."
The Caltech CMS group, which includes eight graduate students
and several postdocs, engineers, and technical staff, is working on
many other projects in addition to the Higgs search, such as
exploring supersymmetry (a theory that says every particle has a
"supersymmetric" partner), searching for other exotic, theoretical
particles, and developing new kinds of particle detectors.
"We're grateful for the achievements of the LHC team and our
colleagues with CMS," Spiropulu says. "We are working hard on the
final stage of improving the experiments and on publishing the
results-both about the Higgs and possible new, exciting theories of
physics-in the coming weeks and months."
For more information, go to the Caltech CMS website (http://hep.caltech.edu/cms), the
CMS public site (http://cms.web.cern.ch/), and the
American CMS site (http://uscms.org).