equal number of religions, were required to build the accelerator and
ͤ͞͡
the detectors that monitor it—taking almost two decades to
complete the task. Its scale dwarfs that of all machines constructed
before it. And it was built for no more practical reason than to
celebrate and explore the beauty of nature.
Seen in this perspective, the cathedrals and the collider are both
monuments to what may be best about human civilization—the
ability and the will to imagine and construct objects of a scale and
complexity that requires the cooperation of countless individuals,
from around the globe if necessary, for the purpose of turning our
awe and wonder at the workings of the cosmos into something
concrete that may improve the human condition. Colliders and
cathedrals are both works of incomparable grandeur that celebrate
the human experience in different realms. Nevertheless, I think the
LHC wins, and its successful construction over two decades
demonstrates that the twenty-first century is not yet devoid of
culture and imagination.
Which brings me finally to the road to July 4, 2012.
By 2011 the LHC was cruising along, as one of the CERN officials
put it. The amount of data taken by October of that year was already
4 million times higher than during the first run in 2010, and thirty
times higher than had been obtained by the beginning of 2011.
At this point in the collection of data that physicists had been
waiting forty years for, rumors began to fly in the community. Many
of these came from the experimenters themselves. I have a part-time
position at Australian National University in Canberra, and the
International Conference on High Energy Physics was going to be in
held in Melbourne in July of 2012. Melbourne has a big LHC
contingent, and when visiting, I kept hearing how a greater and
greater possible mass range for the Higgs particle had been ruled out
by the experiments already.
ͤ͢͞
Many experimentalists relish being able to prove theorists wrong.
So it was in this case. One experimentalist had excitedly told me less
than six months before the meeting that the entire Higgs mass
region had been ruled out except for a narrow range between 120
and 130 times the mass of the proton. She expected that by July they
would be able to rule out that region too. As one who was skeptical
of the Higgs, I wasn’t unhappy to hear this. In fact, I was getting a
paper ready to explain why the Higgs might not exist.
On April 5, the situation got more interesting as the LHC center-
of-mass beam energy was increased slightly, to eight thousand times
the rest energy of the proton. This translated into an increased
potential for new particle discovery. By mid-June it was announced
that the leaders of the two main experiments, along with the director
general of CERN, would not be traveling to Melbourne for the
meeting, but would be presenting results remotely from a televised
conference on the morning of July 4 in the main colloquium room at
CERN—the same room where Rubbia had announced the discovery
of the W particles.
On July 4 I was at a physics meeting in Aspen, Colorado. Because
of the significance of the impending announcement, the physics
community there had set up a live remote presentation screen—so
that at 1:00 a.m. we could all sit and watch history unfold. About
fifteen of us showed up in the dark at the Aspen Center for Physics,
mostly physicists, but also a few journalists, including Dennis
Overbye from the New York Times, who knew he was going to have a
late night writing. As it turned out, so would I. The Times had asked
me for an essay for the following week’s Science Times section if
things worked out as expected.
Then the show began, and in the next forty-five minutes or so
spokespeople presented data from both of the two large detectors
that compellingly demonstrated the existence of a new elementary
ͤͣ͞
particle with mass of about 125 times the mass of the proton. After
the initial catastrophe in 2009, the LHC had functioned impeccably
—as had both the detectors. I and many of my colleagues were
amazed during the early months by the immaculately clean results
the detectors displayed regarding known background processes. So
we were not surprised that when something new appeared, these
detectors could find it, in spite of the unbelievably complicated
environment that the detectors were functioning in.
But more than this, the particle was discovered by looking
precisely at the decay channels that had been predicted for a
Standard Model Higgs particle. The relative decays into photons (via
intermediate top quarks or W’s) versus particles such as electrons
(via intermediate Z bosons) agreed more or less with what was
predicted, as did the production rate of the new particle in the
proton-proton collisions. Of the billions and billions of collisions
analyzed by the two detector collaborations up to that point, about
fifty potential Higgs candidates had been discovered. Many tests
needed to be performed to get a more definitive identification, but if
it walked like a Higgs and quacked like a Higgs, it probably was a
Higgs. The evidence was good enough that François Englert and
Peter Higgs were awarded the Nobel Prize in October of 2013, the
first year possible after the claimed discovery.
In February 2013, the LHC shut down and the machine was
upgraded so that it could finally run at its originally designed energy
and luminosity. By the final weeks before turnoff, the CERN mass-
storage systems had stored more than one hundred petabytes of
data, more info than in 100 million CDs. New results continued to
roll in from data that had not yet been analyzed before the first
announcement (including tantalizing hints of a possible new and
unexpected heavy particle, six times heavier than the Higgs, hints
that disappeared just as this book was being sent off to press).
ͤͤ͞
For a real discovery, the more data you have, the better it looks,
whereas anomalous results tend to disappear over time. This time
things looked good, almost embarrassingly so. If one compared five
different predicted decay channels into photons, Z particles, W
particles, tau particles (the heaviest known cousin of the electron),
and particles containing b quarks, to observation, the predictions of
the Standard Model Higgs, with no extra accessories, agreed
strikingly well.
From the angular distribution and energies of the decay products,
with a new larger sample of Higgs candidates, the LHC detectors
were able to explore whether the particle was indeed a scalar
particle, which would make it the first fundamental scalar ever
observed in nature. On March 26, 2015, the ATLAS detector at
CERN released results that showed with greater than 99 percent
confidence that the new particle was a spin 0 particle, with precisely
the proper parity assignment to be a Higgs sc
alar. Nature had shown
that it does not abhor scalar fields like the Higgs, as I for one had
thought. The existence of such a fundamental scalar changes a great
deal about what may be possible in nature, and people, including me,
began to consider scenarios we would never before have considered.
In September 2015, about a month before the first draft of this
book was written, the two large detectors ATLAS and CMS
combined their data from 2011 and 2012 and presented for the first
time a unified comparison of theory and experiment. The result—
involving a mammoth computational effort to take into account
separate systematic effects in each experiment, involving a total of
forty-two hundred parameters—showed with a residual uncertainty
of about 10 percent that the new particle had all the properties
predicted for the Standard Model Higgs.
This simple conclusion may seem almost anticlimactic, following
as it does a half century of directed effort by thousands of individuals
ͤͥ͞
—the theorists who developed the Standard Model and the others
who performed the incredibly complex calculations needed to
compare predictions with experiments, to determine background
rates, and so on, and the thousands of experimental physicists who
had built and tested and operated the most complex machine ever
constructed. Their story was marked by incredible heights of
intellectual bravery, years of confusion, bad luck and serendipity,
rivalries and passion, and above all the persistence of a community
focused on a single goal—to understand nature at her most
fundamental scales. Like any human drama, it also included its share
of envy, stubbornness, and vanity, but more important, it involved a
unique community built completely independent of ethnicity,
language, religion, or gender. It is a story that carries with it all the
drama of the best epic tales and reflects the best of what science can
offer to modern civilization.
That nature would be so kind as to actually use the ideas that a
small collection of individuals wrote down on paper, inspired by
abstract ideas of symmetry and using the complex mathematics of
quantum field theory, will always seem to me nothing short of
remarkable. It is hard to express the mixture of exhilaration and
terror that comes from the realization that nature might actually
work the way you are proposing it does when putting the final
touches on a paper, possibly late at night, alone in your study. I
suppose it may resemble the reaction Plato described that his poor
philosophers might have as they are dragged out into the sunlight
away from the cave for the first time.
To have discovered that nature really follows the simple and
elegant rules intuited by the twentieth- and twenty-first-century
versions of Plato’s philosophers is both shocking and reassuring. It
hints that the willingness of scientists to build an intellectual house
of cards that could come tumbling down at the slightest
ͥ͜͞
experimental tremor was not misplaced. It gives us courage to
continue to suppose, as Einstein had once expressed his amazement
about, that the universe on its grandest scale is fathomable after all.
After witnessing the announcement of the Higgs discovery on
July 4, 2012, I wrote the following:
The apparent discovery of the Higgs may not result in a better
toaster or a faster car. But it provides a remarkable celebration of
the human mind’s capacity to uncover nature’s secrets, and of the
technology we have built to control them. Hidden in what seems
like empty space—indeed, like nothing, which is getting more
interesting all the time—are the very elements that allow for our
existence.
By demonstrating this, last week’s discovery will change our
view of ourselves and our place in the universe. Surely that is the
hallmark of great music, great literature, great art . . . and great
science.
It is too early yet to judge or even fully anticipate what changes in
our picture of reality will result from the Higgs discovery at the LHC,
or the discoveries that may follow. Yet fortune does favor the
prepared mind, and it is at once the responsibility and the joy of
theorists such as me to ponder just that.
While nature may have appeared to be kind to us this time,
perhaps it was too kind. The epic saga I have described here may yet
provide a dramatic new challenge for physics and for physicists, and
an explicit reminder that nature doesn’t exist to make us
comfortable. Because while we may have found what we expected,
no one really expected to find just that and nothing else. . . .
ͥ͞͝
C h a p t e r 2 2
M O R E
Q U E S T I O N S
T H A N
A N S W E R S
A fool takes no pleasure in understanding, but only in expressing
his opinion.
—PROVERBS 18:2
In one sense, our story might end here, because we have come
to the limits of our direct empirical knowledge about the universe at
its fundamental scales. But no one says we have to stop dreaming,
even if the dreams are not always pleasant. Before July 2012 particle
physicists had two nightmares. The first was that the LHC would see
precisely nothing. For if it did, it would likely be the last large
accelerator ever built to probe the fundamental makeup of the
cosmos. The second was that the LHC would discover the Higgs . . .
period.
Each time we peel back one layer of reality, other layers beckon.
So each important new development in science generally leaves us
with more questions than answers. But it also usually leaves us with
at least the outline of a road map to help us begin to seek answers to
those questions. The discovery of the Higgs particle, and with it the
validation of the existence of an invisible background Higgs field
throughout space, was a profound validation of the bold scientific
developments of the twentieth century.
However, the words of Sheldon Glashow continue to ring true:
The Higgs is like a toilet. It hides all the messy details we would
rather not speak of. The Higgs field, as elegant as it might be, is
ͥ͞͞
within the Standard Model essentially an ad hoc addition. It is added
to the theory to do what is required to accurately model the world of
our experience. But it is not required by the theory. The universe
could have happily existed with a long-range weak force and
massless particles. We would just not be here to ask about them.
Moreover, the detailed physics of the Higgs is, as we have seen,
undetermined within the Standard Model alone. The Higgs could
have been twenty times heavier, or a hundred times lighter.
Why, then, does the Higgs exist at all? And why does it have the
mass it does? (Recognizing once again that whenever scientists ask
“Why?,” we really mean “How?”) If the Higgs did not exist, the world
we see would not exist, but surely that is n
ot an explanation. Or is it?
Ultimately to understand the underlying physics behind the Higgs is
to understand how we came to exist. When we ask, “Why are we
here?,” at a fundamental level we may as well be asking, “Why is the
Higgs here?” And the Standard Model gives no answer to this
question.
Some hints do exist, however, coming from a combination of
theory and experiment. Shortly after the fundamental structure of
the Standard Model became firmly established, in 1974, and well
before the details were experimentally verified over the next decade,
two different groups of physicists at Harvard, where both Glashow
and Weinberg were working, noticed something interesting.
Glashow, along with Howard Georgi, did what Glashow did best:
they looked for patterns among the existing particles and forces and
sought out new possibilities using the mathematics of group theory.
Remember that in the Standard Model the weak and
electromagnetic forces are unified at a high-energy scale, but when
the symmetry is spontaneously broken by the Higgs field condensate,
this leaves, at observable scales, two separate and distinct forces—
with the weak force being short-range and electromagnetism
ͥ͟͞
remaining long-range. Georgi and Glashow tried to extend this idea
to include the strong force and discovered that all of the known
particles and the three nongravitational forces could naturally fit
within a single fundamental larger-gauge symmetry structure. They
then
speculated
that
this
fundamental
symmetry
could
spontaneously break at some ultrahigh energy and short-distance
scale far beyond the range of current experiments, leaving two
separate and distinct unbroken gauge symmetries left over—
resulting in the separate strong and electroweak forces.
Subsequently, at a lower energy and larger distance scale, the
electroweak symmetry would break, separating that into the short-
range weak and the long-range electromagnetic force.
They called such a theory, modestly, a Grand Unified Theory
(GUT).
At around the same time, Weinberg and Georgi along with Helen
Quinn noticed something interesting—following the work of
Wilczek, Gross, and Politzer. While the strong interaction got
weaker as one probed it at smaller-distance scales, the
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 31
