stumble upon a year later, was precisely the model proposed six
years earlier by his old high school friend Sheldon Glashow when he
responded to Schwinger’s challenge to find a symmetry that might
unify the weak and electromagnetic interactions. No other choice
could mathematically reproduce what we see in the world today.
Glashow’s model had been largely ignored in the interim because no
mechanism was then known to give the weak bosons masses. But
now such a mechanism existed, the Higgs mechanism.
Weinberg and Glashow, whose lives had crisscrossed since they
were children, would later share the Nobel Prize, along with Salam,
for completely independent discoveries of the greatest unification in
physical theory since Maxwell had unified electricity and magnetism
and Einstein had unified space and time.
͟͞͞
C h a p t e r 1 8
T H E F O G L I F T S
Their voice goes out through all the earth, and their words to the
end of the world.
—PSALM 19:4
You might expect that physicists around the world would
have thrown parties with fireworks when Weinberg’s paper came
out. But for the next three years following publication of Weinberg’s
theory, not a single physicist, not even Weinberg himself, would find
cause to reference the paper—now one of the most highly cited
papers in all of particle physics. If a great discovery about nature had
been made, no one had yet noticed.
After all, Maxwell’s unification made the beautiful prediction that
light was an electromagnetic wave whose speed could be calculated
from first principles, and lo and behold, the prediction was equal to
the measured speed of light. Einstein’s unification of space and time
predicted that clocks would slow for moving observers, and lo and
behold, they do, and in just the way he predicted. In 1967 the
Glashow-Weinberg-Salam
unification
of
the
weak
and
electromagnetic interactions predicted three new vector bosons that
were almost one hundred times heavier than any particle that had
been yet detected. It also predicted new interactions between
electrons and neutrinos and matter due to the newly predicted Z
particle that had not only not been seen, but a number of
experiments suggested did not exist. It also required the existence of
a new and as yet unobserved massive fundamental scalar boson, the
͟͟͞
Higgs particle, when no fundamental scalar particles were yet known
to exist in nature. And finally, as a quantum theory, no one knew if it
made sense.
Is it any wonder that the idea did not immediately catch fire?
Nevertheless, within a decade everything would change, resulting in
the most theoretically productive period for elementary particle
physics since the discovery of quantum mechanics. While a gauge
theory of the weak interaction started the ball rolling, what resulted
was far greater.
• • •
The first crack in the dike holding back the waters of progress came,
fittingly, with the work of Dutch graduate student Gerardus ’t Hooft,
in 1971. I always remember how to spell his name because a
particularly brilliant and witty former Harvard colleague, the late
Sidney Coleman, used to say that if Gerard had monogrammed cuff
links, they would need an apostrophe on them. Before 1971 many of
the greatest theorists in the world had tried to figure out whether the
infinities that plague most quantum field theories would disappear
for spontaneously broken gauge theories as they do for their
unbroken cousins. But the answer eluded them. Remarkably this
young graduate student, working under the supervision of a
seasoned pro—Martinus Veltman—found a proof that others had
missed. Often when presented with a new result, we physicists can
work through the details and imagine how we might have
discovered it ourselves. But many of ’t Hooft’s insights, and there
were many—almost all the new ideas in the 1970s derived in one
way or another from his theoretical inventions—seemed to come
from some hidden reservoir of intuition.
The other remarkable thing about Gerard is how gentle, shy, and
unassuming he is. For someone who became famous in the field
͟͞͠
when he was a student, one might have expected some sense of
privilege. But from the first time I met him—again when I was a
lowly graduate student—Gerard treated me as an interesting friend,
and I am pleased to say that relationship has continued. I always try
to remember this attitude when I meet young students who may
seem shy or intimidated, and I try to emulate Gerard’s open
generosity of spirit.
His supervisor Tini Veltman, as he is often called, couldn’t appear
more different. Not that Tini isn’t fun to talk to. He is. But he always
made explicitly clear to me the moment we started a discussion that
whatever I might say, I didn’t understand things well enough. I
always enjoyed the challenge.
It is important to note that ’t Hooft would never have approached
the problem if Veltman had not been obsessed with it, even as most
others gave up. The notion that one might ultimately extend the
techniques that Feynman and others had developed to tame
quantum electrodynamics to try to understand more complex
theories such as spontaneously broken Yang-Mills theory was simply
viewed as naïve by many in the field. But Veltman stayed with the
project, and he wisely found a graduate student who was also a
genius to help him.
It took a while for ’t Hooft’s and Veltman’s ideas to sink in and the
new techniques ’t Hooft had developed to become universally
adopted, but within a year or so physicists agreed that the theory that
Weinberg, and later Salam, had proposed, made sense. Citations of
Weinberg’s paper suddenly began to grow exponentially. But making
sense and being right are two different things. Did nature actually
use the specific theory that Glashow, Weinberg, and Salam had
suggested?
That remained the key open question, and for a while it looked as
if the answer was no.
͟͞͡
The existence of the new neutral particle, the Z, required by the
theory, was a significant addition, beyond the charged particles
suggested years earlier by Schwinger and others that were required
to change neutrons into protons and electrons into neutrinos. It
meant that there would be a new kind of weak interaction, not just
for electrons and neutrinos but also for protons and neutrons,
mediated by a new neutral-particle exchange. In this case, as for
electromagnetism, the identity of the particles interacting would not
change. Such interactions became known as neutral current
interactions, and the obvious way to test the theory was to look for
them. The best place to look for them was in the interactions of the
only particles in nature that just feel the weak interact
ion, namely
neutrinos.
You may recall that the prediction of such neutral currents was
one of the reasons that Glashow’s 1961 suggestion never caught on.
But Glashow’s model wasn’t a full theory. Particle masses were
simply put into the equations by hand, and as a result quantum
corrections couldn’t be controlled. However, when Weinberg and
Salam proposed their model for electroweak unification, all elements
that allowed for detailed predictions were there. The mass of the Z
particle was predicted, and as ’t Hooft had shown, one could
calculate all quantum corrections in a reliable way, just as one did for
quantum electrodynamics.
This was a good thing, and a bad thing because no wiggle room
was left to argue away any possible disagreements with observation.
And in 1967 there appeared to be such disagreements. No such
neutral currents had been observed in high-energy collisions of
neutrinos with protons, with an upper limit being set of about 10
percent of the rate observed for more familiar charge-changing weak
interactions of neutrinos and protons, such as neutron decay. Things
͟͢͞
looked bad, and most physicists assumed weak neutral currents
didn’t exist.
Weinberg had a vested interest in this quest, and in 1971 he
reasonably argued that there was still wiggle room. But this view was
not generally held by others in the community.
In the early 1970s, new experiments at the European
Organization for Nuclear Research (CERN) in Geneva were
performed using the proton accelerator there, which smashed high-
energy protons into a long target. Most particles produced in the
collision would be absorbed in the target, but neutrinos would
emerge from the other end—as their interactions are so weak that
they could traverse the target without being absorbed. The resulting
high-energy neutrino beam would then strike a detector placed in its
path that could record the few events in which neutrinos might
interact with the detector material.
A huge new detector was built, named Gargamelle after the
giantess mother of Gargantua, from the work of the French writer
Rabelais. This five-meter-by-two-meter “bubble chamber” vessel was
filled with a superheated liquid in which trails of bubbles would
form when an energetic charged particle traversed it, sort of like
seeing the vapor trail high in the sky of a plane that is itself not
visible.
Interestingly, when the experimentalists who built Gargamelle
met in 1968 to discuss their plans for neutrino experiments, the idea
of searching for neutral currents wasn’t even mentioned—an
indication of how many physicists thought the issue was then settled.
Of far more interest to them was the possibility of following up on
recent exciting experiments at the Stanford Linear Accelerator
(SLAC), where high-energy electrons had been used as probes to
explore the structure of protons. Using neutrinos as probes of
ͣ͟͞
protons might give cleaner measurements because the neutrinos are
not charged.
After the results of ’t Hooft and Veltman, however, in 1972,
experimentalists began to take the gauge theory description of the
weak interaction, and in particular the Glashow-Weinberg-Salam
proposal, seriously. That meant looking for neutral currents. The
Gargamelle collaboration had the capability to do this, in principle,
even though it hadn’t been designed for the task.
Most of the high-energy neutrinos in the beam would interact
with protons in the target by turning into muons, the heavier
partners of electrons. The muons would exit the target, producing a
long charged-particle track all the way to the edge of the detector.
The protons would be converted into neutrons, which would
themselves not produce a track but would collide with nuclei,
producing a short shower of charged particles that would leave
tracks. Thus, the experiment was designed to detect muon tracks, as
well as accompanying charged-particle showers, both arising as
separate signals of a single weak interaction.
However, sometimes a neutrino would interact with material
outside the detector, producing a neutron that might recoil back into
the detector and then interact there. Such events would consist of a
single strongly interacting shower of particles due to the colliding
neutron, with no accompanying muon track.
When Gargamelle began to search for neutral current events,
such isolated charged-particle showers without an accompanying
muon became just the signal the scientists needed to focus on. In
neutral current events a neutrino that interacts with a neutron or
proton in the detector doesn’t convert into a charged muon, but
simply bounces off and escapes the detector unobserved. All that
would be observable would be the recoiling nuclear shower—the
same signature produced by the more standard neutrino interactions
ͤ͟͞
outside the detector that produce neutrons that recoil back into the
detector and produce a nuclear shower.
The challenge, then, if the experiment was to definitively detect
neutral current events, was to distinguish neutrino-induced events
from such neutron-induced events. (This same problem has
provided the chief challenge to experimentalists looking for any
weakly interacting particles, including the presumed dark matter
particles that are being searched for in underground detectors
around the world today.)
The observation of a single recoil electron, with no other
charged-particle tracks in the detector, was observed in early 1973.
This could have arisen from the less frequent predicted neutral
current collisions of neutrinos with electrons instead of protons or
neutrons. But generally a single event is not enough to definitively
claim a new discovery in particle physics. However, it did give hope,
and by March of 1973 a careful analysis of neutron backgrounds and
observed isolated particle showers appeared to provide evidence that
weak neutral current interactions actually exist. Nevertheless, not
until July of 1973 did the researchers at CERN complete a sufficient
number of checks to be confident enough to claim a detection of
neutral currents, which they did at a conference in Bonn in August.
The story might have ended there, but unfortunately, shortly after
this, another collaboration searching for neutral currents rechecked
their apparatus and found that a previous signal for neutral currents
had disappeared. This produced significant confusion and skepticism
in the physics community, where once again neutral currents
seemed suspect. Ultimately the Gargamelle collaboration returned to
the drawing board, tested the detector using a proton beam directly,
and took a great deal more data. At a conference almost a year later,
in June 1974, the Gargamelle collaboration presented overwhelming
confirmation of the signal. Meanwhile the competing collaboration
ͥ͟͞
<
br /> had found the cause of its error and confirmed the Gargamelle
result. Glashow, Weinberg, and Salam were vindicated.
Neutral currents had arrived, and a remarkable unification of the
weak and the electromagnetic interactions appeared to be at hand.
But two loose ends still remained to be cleared up.
The existence of neutral currents in neutrino scattering validated
the notion that the Z particle existed, but this didn’t guarantee that
the weak interaction was identical to that proposed by Glashow,
Weinberg, and Salam, where the weak and the electromagnetic
interactions were unified. To explore this required an experiment
using a particle that participated in both the weak and the
electromagnetic interaction. The electron was ideal for this purpose
because these are the only two interactions it experiences.
When electrons interact with other charges by their
electromagnetic attraction, left-handed electrons and right-handed
electrons behave identically. However, the Glashow-Weinberg-
Salam theory required that weak interactions occur differently for
left-handed versus right-handed particles. This implied that careful
measurements of the scattering of polarized electrons—electrons
prepared initially in left- or right-handed states using magnetic fields
—off various targets should reveal a violation of left-right symmetry,
but not as extreme an asymmetry as that observed in neutrino
scattering—because the neutrino is purely left-handed. The degree
of violation in electron scattering, if it existed, would then reflect the
extent to which the weak interaction and electromagnetism were
mixed together in a unified theory.
The idea of testing for such interference using electron scattering
had actually been suggested as early as 1958 by the remarkable
Soviet physicist Yakov B. Zel’dovich. But it would take twenty years
for sufficiently sensitive experiments to actually take place. And as
͜͞͠
for the neutral current discovery, the road to success was full of
potholes and wrong turns along the way.
One of the reasons it took so long to test this idea is that the weak
interaction is weak. Because the dominant interaction of electrons
with matter is electromagnetic, the left-right asymmetry predicted
due to a possible exchange of a Z particle was small, smaller than
one part in ten thousand. To test for such a small asymmetry
Lawrence Krauss - The Greatest Story Ever Told--So Far Page 25
