beam would be sufficiently compressed so that when the two beams
ͣ͢͞
were brought together, steered by powerful magnets, any collisions
at all would be observed.
Convincing the CERN directorate to transform one of the world’s
most powerful accelerators, built in a circular tunnel almost eight
kilometers around at the French-Swiss border, into a new kind of
collider would have been difficult for many people, but Carlo
Rubbia, a bombastic force of nature, was up to the task. Few people
who got in Rubbia’s way were likely to be happy about it afterward.
For eighteen years he jetted every week between CERN and Harvard,
where he was a professor. His office was two floors down from mine,
but I knew when he was in town because I could hear him.
Moreover, Rubbia’s idea was good, and in promoting it he was really
suggesting to CERN that the SPS move up from an “also-ran”
machine to the most exciting accelerator in the world. As Sheldon
Glashow said to the CERN directorate when encouraging them to
move forward, “Do you want to walk, or do you want to fly?”
Still, to fly one needs wings, and the creation of a new method to
produce, store, accelerate, and focus a beam of antiprotons fell to a
brilliant accelerator physicist at CERN, Simon van der Meer. His
method was so clever that many physicists who first heard about it
thought it violated some fundamental principles of thermodynamics.
The properties of the particles in the beam would be measured at
one place in the circular tunnel, then a signal would be sent for
magnets farther down the tunnel to give many small kicks over time
to the particles in the beam as they passed by, thus slightly altering
the energies and momenta of any wayward particles so that they
would eventually all get focused into a narrow beam. The method,
called stochastic cooling, helped make sure particles that were
wandering away from the center of the beam would be sent back
into the middle.
ͤ͢͞
Together van der Meer and Rubbia pushed forward, and by 1981
the collider was working as planned, and Rubbia assembled the
largest physics collaboration ever created and built a large detector
capable of sorting through billions of collisions of protons and
antiprotons to search for a handful of possible W and Z particles.
Rubbia’s team was not the only one hunting for a W and a Z,
however. Another detector collaboration had been assembled and
was also built at CERN. Redundancy for such an important
observation seemed appropriate.
Unearthing a signal from the immense background in these
experiments was not easy. Remember that protons are made of more
than one quark, and in a single proton-antiproton collision a lot of
things can happen. Moreover, the W’s and Z’s would not be
observed directly, but via their decays—in the case of the W, into
electrons and neutrinos. Neutrinos would not be directly observed,
either. Rather the experimentalists would tally up the total energy
and momentum of each outgoing particle in a candidate event and
look for large amounts of “missing energy,” which would signal that
a neutrino had been produced.
By December 1982, a W candidate event had been observed by
Rubbia and his colleagues. Rubbia was eager to publish a paper based
on this single event, but his colleagues were more cautious, for good
reason. Rubbia seemed to have a history of making discoveries that
weren’t always there. In the meantime he leaked details of the event
to a number of colleagues around the world.
Over the next few weeks his “UA1” collaboration obtained
evidence for five more W candidate events, and the UA1 physicists
designed several far more stringent tests to ascertain with high
confidence that the candidates were real. On January 20, 1983,
Rubbia presented a memorable and masterful seminar at CERN
announcing the result. The standing ovation he received made it
ͥ͢͞
clear that the physics community was convinced. A few days later
Rubbia submitted a paper to the journal Physics Letters announcing
the discovery of six W events. The W had been discovered with
precisely the predicted mass.
The search was not over, however. The Z remained to be seen. Its
predicted mass was slightly higher than that of the W, and its signal
was therefore slightly harder to obtain. Nevertheless, within a month
or so of the W announcement, evidence for Z events began to come
in from both experiments, and on the basis of a single clear event, on
May 27 that year Rubbia announced its discovery.
The gauge bosons of the electroweak model had been found. The
significance of these discoveries for solidifying the empirical basis of
the Standard Model was underscored when, just slightly over a year
after making the announcement, Rubbia and his accelerator
colleague van der Meer were awarded the Nobel Prize in Physics.
While the teams that had built and operated both the accelerator
and the detectors were huge, few could deny that without Rubbia’s
drive and persistence and van der Meer’s ingenious invention the
discovery would not have been possible.
One big Holy Grail now remained: the purported Higgs particle.
Unlike the W and Z bosons, the mass of the Higgs is not fixed by the
theory. Its couplings to matter and to the gauge bosons were
predicted, as these couplings allow the background Higgs field that
presumably exists in nature to break the gauge symmetry and give
mass not to just the W and the Z, but also to electrons, muons, and
quarks—indeed to all the fundamental particles in the Standard
Model save the neutrino and the photon. However, neither the
Higgs particle mass nor the strength of its self-interactions was
separately determined in advance by then existing measurements.
Only their ratio was fixed by the theory in terms of the measured
strength of the weak interaction between known particles.
ͣ͜͞
Given conservative estimates of the possible magnitude of the
Higgs self-interaction strength, the Higgs particle mass was
conservatively estimated to lie within a range of 2 to 2,000 GeV.
What set the upper limit was that, if the Higgs self-coupling is too
big, then the theory becomes strongly interacting and many of the
calculations performed using the simplest picture of the Higgs break
down.
Aside from their necessary role in breaking the electroweak
symmetry and giving other elementary particles masses, these
quantitative details were therefore largely undetermined by
experiments up to that time—which is probably why Sheldon
Glashow in the 1980s referred to the Higgs as the “toilet” of modern
physics. Everyone was aware of its necessary existence, but no one
wanted to talk about the details in public.
That the Standard Model didn’t fix in advance many of the details
of the Higgs field didn’t dissuade many theorists from proposing
models that “predicted�
�� the Higgs mass based on some new
theoretical ideas. In the early 1980s, each time accelerators increased
their energies, new physics papers would come out predicting a
Higgs would be discovered when the machine was turned on. Then a
new threshold would be reached, and nothing would be observed.
To explore all the available parameter space to see if the Higgs
existed, a radically new accelerator would clearly have to be built.
I was convinced during all this time that the Higgs didn’t exist.
The spontaneous symmetry breaking of the electroweak gauge
symmetry did certainly occur—the W and the Z exist and have mass
—but adding a fundamental new scalar field designed by recipe
specifically to perform this task seemed contrived to me. First, no
other fundamental scalar field had ever been observed to exist in
nature’s particle menagerie. Second, I felt that with all of the
unknown physics yet to be discovered at small scales, nature would
ͣ͞͝
have developed a much more ingenious and unexpected way of
breaking the gauge symmetry. Once one posits the Higgs particle,
then the next obvious question is “Why that?” or more specifically
“Why just the right dynamics to cause it to condense at that scale,
and with that mass?” I thought that nature would find a way to break
the theory in a less ad hoc fashion, and I expressed this conviction
fairly strongly when I was interviewed for my eventual position at
the Society of Fellows at Harvard after getting my PhD.
Let’s recall now what the existence of the Higgs implies. It
requires not just a new particle in nature but an invisible background
field that must exist throughout all of space. It also implies that all
particles—not just the W and the Z particles but also electrons and
quarks—are massless in the fundamental theory. These particles that
interact with the Higgs background field then experience a kind of
resistance to their motion that slows their travel to less than the
speed of light—just as a swimmer in molasses will move more slowly
than a swimmer in water. Once they are moving at sub-light-speed,
the particles behave as if they are massive. Those particles that
interact more strongly with this background field will experience a
greater resistance and will act as if they are more massive, just as a
car that goes off the road into mud will be harder to push than if it
were on the pavement, and to those pushing it, it will seem heavier.
This is a remarkable claim about the nature of reality.
Remembering that in superconductors the condensate that forms is
a complicated state of bound pairs of electrons, I was skeptical that
things would work out so much more simply and cleanly on
fundamental scales in empty space.
So how to explore such a remarkable claim? We use the central
property of quantum field theory that was exploited by Higgs himself
when he proposed his idea. For every new field in nature, at least one
new type of elementary particle must exist with that field. How, then,
ͣ͞͞
to produce the particles if such a background field exists throughout
space?
Simple. We spank the vacuum.
By this I mean that if we can focus enough energy at a single point
in space, we can excite real Higgs particles to emerge and be
measured. One can picture this as follows. In the language of
elementary particle physics, using Feynman diagrams, we can think
of a virtual Higgs particle emerging from the background Higgs field,
giving mass to other particles. The left diagram corresponds to
particles such as quarks and electrons scattering off a virtual Higgs
particle and being deflected, thus experiencing resistance to their
forward motion. The right diagram represents the same effect for
particles such as the W and the Z.
We can then simply turn this picture around:
In this case energetic particles such as W’s and Z’s or quarks
and/or antiquarks or electrons and/or positrons appear to emit
virtual Higgs particles and recoil. If the energies of the incoming
particles are large enough, then the emitted Higgs could be a real
particle. If they aren’t, the Higgs would be a virtual particle.
Now remember that if the Higgs gives mass to particles, then the
particles it interacts with most strongly will be the particles that get
the largest masses. In turn this means that the particles most likely to
spit out a Higgs are the incident particles with the heaviest masses.
That means that light particles such as electrons are probably not a
ͣ͟͞
good bet to directly create Higgs particles in an accelerator. Instead
we can imagine creating an accelerator with enough energy so that
we can create heavy virtual particles that will spit out Higgs particles,
either virtual or real.
The natural candidates are then protons. Build an accelerator or a
collider starting with protons and accelerate them to high enough
energies to produce enough virtual heavy constituents so as to
produce Higgs particles. The Higgs particles, virtual or real, being
heavy, will quickly decay into the lighter particles that the Higgs
interacts with most strongly—once again either the top or bottom
quarks or W’s and Z’s. These will in turn decay into other particles.
The trick would be to consider events with the smallest number
of outgoing particles that could be cleanly detected, then determine
their energies and momenta precisely and see if one could
reconstruct a series of events traceable to a single massive
intermediate particle with the predicted interactions of a Higgs
particle. No small task!
These ideas were already clear as early as 1977, even before the
discovery of the top quark itself (since the bottom quark had already
been discovered, and all the other quarks came in weak pairs—up
and down, charm and strange—clearly another quark had to exist,
although it took until 1995 to discover it, a whopping 175 times
heavier than the proton). But knowing what was required and
actually building a machine capable of doing the job were two
different things.
ͣ͞͠
C h a p t e r 2 1
G OT H I C
C AT H E D R A L S
O F
T H E
T W E N T Y- F I R S T
C E N T U RY
The price of wisdom is above rubies.
—JOB 28:18
Accelerating protons to high enough energies to explore the
full range of possible Higgs masses was well beyond the capabilities
of any machine in 1978—when all the other predictions of the
electroweak theory were confirmed—or in 1983 when the W and
the Z had been discovered. An accelerator at least an order of
magnitude more powerful than the most powerful machine then in
existence was required. In short, not a collider, but a supercollider.
The United States, which for the entire period since the end of
the Second World War had dominated science and technology, had
good reason to want to build such a mac
hine. After all, CERN in
Geneva had emerged by 1984 as the dominant particle physics
laboratory in the world. American pride was so hurt when both the
W and the Z particles were discovered at CERN that six days after
the press conference announcing the Z discovery, the New York
Times published an editorial titled “Europe 3, U.S. Not Even Z-Zero”!
Within a week after the Z discovery, American physicists decided
to cancel construction of an intermediate-scale accelerator in Long
Island and go for broke. They would build a massive accelerator with
a center-of-mass energy almost one hundred times greater than the
CERN SPS machine. To do so they would need new
ͣ͞͡
superconducting magnets, and so their brainchild was named the
Superconducting Super Collider (SSC).
After the project was proposed by the US particle physics
community in 1983, the traditional scramble proceeded among
many different states to get a piece of the enormous fiscal pie for its
construction and management. After much political and scientific
wrangling a site just south of Dallas, Texas, in Waxahachie, was
chosen. Whatever the motivation, Texas seemed particularly
appropriate, as everything about the project, which was approved in
1987 by President Reagan, was supersize.
The huge underground tunnel would have been eighty-seven
kilometers around, the largest tunnel ever constructed. The project
would be twenty times bigger than any other physics project ever
attempted. The proposed energy of collisions, with two beams each
having an energy twenty thousand times the mass of the proton,
would be about one hundred times larger than the collision energy
of the machine at CERN that had discovered the W and the Z. Ten
thousand superconducting magnets, each of unprecedented strength,
would have been required.
Cost overruns, lack of international cooperation, a poor US
economy, and political machinations eventually led to SSC’s demise
in October 1993. I remember the time well. I had recently moved
from Yale to become chair of the Physics Department at Case
Western Reserve University, with a mandate to rebuild the
department and hire twelve new faculty members over five years.
The first year we advertised, in 1993–94, we received more than two
hundred applications from senior scientists who had been employed
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 29
