The Higgs Boson: Searching for the God Particle (33 page)

New Particle Resembling Long-Sought Higgs Boson
Uncovered at Large Hadron Collider

By John Matson

NEW YORK—The city that never sleeps was mostly asleep. The bars were closed.
But at 4:45 A.M., inside a library on Columbia University's Manhattan campus,
Michael Tuts was getting ready to pop the champagne.

The physicist had good reason to celebrate. The massive team of scientists of which
he is a part—3,000 researchers working on the ATLAS experiment at Europe's
Large Hadron Collider—had just announced the discovery of a new particle. The
particle looks an awful lot like the long-sought, and long-hypothetical, Higgs boson,
most famous for explaining why elementary particles, such as quarks, have mass. A
competing, comparably sized experiment, known as CMS, had arrived at a very
similar finding at the collider facility.

Both research teams announced their results during a morning seminar at CERN,
the European laboratory for particle physics that operates the Large Hadron
Collider, or LHC. But the morning start in Geneva meant that U.S. physicists and
other curious observers were tuning in to the announcement during the predawn
hours. Tuts and his Columbia colleagues decided to host a viewing party at the
campus library, with a live video feed from CERN as well as coffee, cookies, soft
drinks and chips. About 50 people, many of them students, turned up for the event,
which began around 2:30 A.M.

Unlike some past announcements centered on the Higgs in the past few years,
which have produced as much ambiguity and confusion as anything else, this one
did not disappoint. ATLAS physicists said that their most recent data reveal the
presence of an unknown particle with a mass of about 126.5 GeV, or 126.5 billion
electron-volts. An electron-volt is a physicist’s unit of mass or energy; for
comparison, the proton has a mass of about 1 GeV. The CMS collaboration found
evidence for a new particle with a mass of 125.3 GeV.

Crucially, both teams' findings appear exceptionally robust. In physics terms, evidence for a new particle requires a “3-sigma”
measurement, corresponding to a 1-in-740 chance that a random fluke could explain the observations, and a claim of discovery requires
a 5-sigma effect, or a 1-in–3.5 million shot that the observations are due to chance. In December representatives of the two experiments
had announced what one called “intriguing, tantalizing hints” of something brewing in the collider data. But those hints fell short of the
3-sigma level. The new ATLAS finding met not just that level of significance but cleared the gold standard 5-sigma threshold, and CMS
very nearly did as well, with a 4.9-sigma finding.

"This is the payoff," Tuts said after the two teams had announced their latest analyses in the Higgs hunt. "This is what you do it for."
Peter Higgs himself, who was in Geneva for the seminar along with other eminent physicists who developed the theory, sounded a
similar note after the ATLAS and CMS teams had unveiled their conclusions. "For me, it's really an incredible thing that it's happened in
my lifetime," Higgs said to the audience at CERN. He was among a half-dozen physicists who in the 1960s proposed what is now known
as the Higgs mechanism, hypothesizing the existence of a field permeating all of space, along with an associated particle. The field
imparts particles with mass by exerting a sort of drag on them, slowing them down much like a human being slows down when she tries
to walk through water instead of air.

The newfound particle fits the bill for the Higgs boson, but the researchers cautioned that more work is needed to compare the
properties of the particle to those predicted for the Higgs. After all, the LHC’s detectors cannot identify the Higgs directly. The LHC
accelerates protons to unprecedented energies of four trillion electron-volts (4 TeV) before colliding a clockwise-traveling proton beam
with a counterclockwise beam. From the smash-up new particles emerge, some of them existing for just an instant before decaying to
other particles.

In the case of the Higgs, physicists can only infer its existence and its properties from the more mundane particles it decays to
produce—say, gamma-ray photons or pairs of electrons. The new particle has the right mass to be the Higgs and broadly decays as
predicted, although a few ambiguities remain. Fortunately, more data are right around the corner. "We have only recorded one third of
the data expected in 2012," ATLAS spokesperson Fabiola Gianotti of CERN said during her presentation. "This is just the beginning.
There is more to come."

Both Gianotti and CMS spokesperson Joe Incandela of the University of California, Santa Barbara, were greeted by large outbursts of
applause when they displayed the slides outlining the results of their Higgs search.

"There aren’t many discoveries like this," Columbia physicist Brian Cole told the group assembled on campus for the early-morning
viewing party. "This trumps, I would say, everything in my physics career.…So I hope you all remember this for the rest of your lives."

Just before sunrise, four Columbia undergraduates made their way out of the library and back across campus. Only two of them were
studying physics, they said—one was focusing on chemistry and another on math. But they all agreed that it had been worth staying up
late to see history in the making.

-Originally published: Scientific American online July 4, 2012

Tantalizing Hints of Elusive Higgs Particle Announced

By Davide Castelvecchi

GENEVA—The two largest collaborations of physicists in history Tuesday presented
intriguing but tentative clues to the existence of the Higgs boson, the elementary
particle thought to endow ordinary matter with mass.

Representing the 6,000 physicists who work on two separate detectors at the Large
Hadron Collider (LHC), called CMS and ATLAS, two spokespersons said that both
experiments seemed to agree, as both their data sets suggested that the Higgs has a
mass close to that of about 125 hydrogen atoms. The LHC is an international facility
hosted by CERN, the European particle physics laboratory outside Geneva.

"We are talking of intriguing, tantalizing hints," said CMS spokesperson Guido
Tonelli, speaking to a room filled with dozens of journalists and TV crews. "It's not
evidence."

The experiments, in which protons traveling at nearly the speed of light collide
head-on, cannot directly detect the Higgs, because the boson would decay within a
fraction of a nanosecond into other particles. Instead, physicists must search
through the debris of many different types of particle decay to find precise
combinations of by-products that the Higgs would produce—and different chains of
particle decays may well have the same signatures. A particular combination that
appears more often than expected from other, "background" processes may signal
the presence of the Higgs. But if it does not appear often enough compared with the
expected background, it could just be a statistical fluctuation. Today, neither CMS
nor ATLAS could claim to have the "3-sigma" statistical significance needed to claim
evidence for a new particle—let alone 5 sigma for the accepted standard to claim a
discovery. (A 3-sigma result implies a fraction of a 1 percent chance of a statistical
fluke.) Instead, so far each experiment could only claim a statistical significance of
around 2 sigma.

Both the detectors and the LHC accelerator itself, however, have been performing better than expected; so all the ducks are now in a
row for settling the question soon, according to the researchers. "The nice thing to know is that by the end of 2012—sooner if we are
lucky—we should be able to say the final word," Fabiola Gianotti, the ATLAS spokesperson, said at the press conference.

"I find it fantastic that we have the first results on the search for the Higgs, but keep in mind that these are preliminary results. And
keep in mind that we have small numbers," said CERN Director General Rolf-Dieter Heuer in summarizing presentations that both
Tonelli and Gianotti gave during a CERN seminar earlier that day.

"I think the evidence is very encouraging, though it's still too early to be sure," comments Steven Weinberg, a leading theoretical
physicist at the University of Texas at Austin and a winner of the Nobel Prize in Physics.

A generation of high-energy physicists came of age studying and testing the Standard Model of particle physics, a theory devised in the
1970s that has withstood all experimental challenges. One final piece is missing, though, and it is one without which the whole model
could fall. Without the Higgs boson, physicists cannot explain how other particles have mass. The Higgs itself has mass, and going by
exclusion, researchers from the LHC and from its predecessor particle colliders were able narrow down the range of its value to between
115 and 140 giga–electron volts, or GeV. (One GeV is roughly the mass of a hydrogen atom.)

Together, the LHC detectors have now reduced the allowed range further: Tonelli said that according to CMS data its mass cannot be
greater than 127 GeV. That was not for lack of data—in fact, quite the opposite. "We were not able to exclude the range below 127 GeV
because of excesses," or more of certain particle by-products than would be expected in the absence of the Higgs, he remarked during
his seminar talk—which was an understated way of saying that the CMS experiment had actually seen hints of a Higgs existing and
having a mass of 124 GeV or so. ATLAS saw excesses in a similar range of energies, although the graphs did not quite line up—the
ATLAS data favor a Higgs around 126 GeV.

Not everyone is impressed with the new findings. The data are
"unconvincing," says Matt Strassler, a theoretical physicist at Rutgers University who was visiting CERN for the occasion. "I was a little
disappointed," he adds, that the results did not live up to the expectations and the rumors—some called it a "Higgsteria"—that had
circulated in the run-up to the announcement. On the other hand, he grants, no one expected to have a discovery at this stage—the
experiments have not yet amassed enough data.

Vivek Sharma, Higgs search coordinator at the CMS collaboration, agrees that the two experiments have a small discrepancy on what
the supposed Higgs mass would be, and that tantalizing hints of new physics from other experiments have often turned out to be
statistical anomalies. "People should curb their enthusiasm," he cautions.

Joe Lykken, a theoretical physicist at Fermi National Accelerator Laboratory in Batavia, Ill., who is a member of the CMS collaboration,
is more optimistic about the discrepancy. "Even though we are only seeing hints of the Higgs boson, it is encouraging that the ATLAS
and CMS hints seem to be consistent with each other," he says.

A Higgs with a mass of 125 GeV would fit with a hypothesized extension of the Standard Model called supersymmetry, which posits that
every known particle has a heavier, as-yet-undiscovered partner. "The low-mass Higgs is not so bad for supersymmetry, to say it
diplomatically," CERN's Heuer said.

The LHC first fired up in September 2008, but within a week it was crippled by a serious accident that put it out of order for more than
a year. "It was a big setback," says Lyn Evans, a CERN accelerator physicist who oversaw the construction and commissioning of the
LHC from 1994 until his retirement a year ago. After repairs, however, the machine restarted in 2009 and has delivered more collisions
than predicted, enabling the ATLAS and CMS collaborations to amass data five times faster than expected.

As recently as a year ago, one would not have thought that the LHC would make so much progress in its Higgs search by the end of 2011,
observes Dmitri Denisov, spokesperson for the DZero experiment, one of the detectors at Fermilab's recently retired Tevatron collider.
"It performed better than anyone expected," Denisov says.

If the Higgs really exists, it will answer the long-standing question of how matter gets its mass. It will also reveal the nature of the
connection between two fundamental forces, the weak nuclear force and the electromagnetic force—a relationship termed the
electroweak interaction. The two forces were unified for the first instants of our universe, but now they behave differently. Weinberg
says the new results suggest that "it should be possible to reach a definite decision about whether this is the particle associated with the
breakdown of the symmetries of the electroweak theory. I'll bet that it is."

-Originally published: Scientific American online, December 13, 2011