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.
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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 interaction, 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 acc
ompanying 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 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 required both an intense beam and one whose initial polarization was well determined.
The best place to perform these experiments was at the Stanford Linear Accelerator, a two-mile-long electron linear accelerator built in 1962 that was the longest and straightest structure that had ever been built. In 1970 polarized beams were introduced, but not until 1978 was an experiment designed and run with the sensitivity required to look for weak-electromagnetic interference in electron scattering.
While the successful observation of neutral currents in 1974 meant that the Glashow-Weinberg-Salam theory began to have wide acceptance among theorists, what made the 1978 SLAC experiment so important was that in 1977 two atomic physics experiments had reported results that, if correct, convincingly ruled out the theory.
In our story thus far, light has played a crucial role, illuminating (if you will forgive the pun) our understanding not only of electricity and magnetism, but space, time, and ultimately the nature of the quantum world. So too it was realized that light could help probe for a possible electroweak unification.
The first great success of quantum electrodynamics was the correct prediction of the spectrum of hydrogen, and eventually other atoms. But if electrons also feel the weak force, then this will provide a small additional force between electrons and nuclei that should alter—if slightly—the characteristics of their atomic orbits. For the most part these are unobservable because electromagnetic effects swamp weak effects. But weak interactions violate parity, so the same weak-electromagnetic neutral current interference that was being explored using polarized electron beams can produce novel effects in atoms that would vanish if electromagnetism was the only force involved.
In particular, for heavy atoms, the Glashow-Weinberg-Salam theory predicted that if polarized light was transmitted through a gas of atoms, then the direction of the polarization of the light would be rotated by about a millionth of a degree, due to parity-violating neutral current effects in the atoms through which the light passed.
In 1977 the results of two independent atomic physics experiments, in Seattle and Oxford, were published in back-to-back articles in Physical Review Letters. The results were dismaying. No such optical rotation was seen at a level ten times smaller than that predicted by the electroweak theory. Had only one experiment reported the result, it would have been more equivocal. But the same result from two independent experiments using independent techniques made it appear definitive. The theory appeared to be ruled out.
Nevertheless, the SLAC experiment, which had begun three years earlier, was well under way, and since all of the experimental preparation had begun, the experiment was approved to begin to take data in early 1978. Because of the earlier null results from the atomic physics experiments, the Stanford collaboration added several bells and whistles to the experiment so that if they saw no effect, they could guarantee that they could have seen such an effect were it there.
Within two months the experiment began to show clear signs of parity violation, and by June 1978 the scientists announced a nonzero result, in agreement with the predictions of the Glashow-Weinberg-Salam model, based on measured neutrino neutral current scattering, which measured the strength of the Z interaction.
Still, questions remained, especially given the apparent disagreement with the Seattle/Oxford results. At a talk at Caltech on the subject, Richard Feynman, characteristically, homed in on a key outstanding experimental question and asked whether the SLAC experimentalists had checked that the detector responded equally well to both left-handed and right-handed electrons. They hadn’t, but for theoretical reasons they had had no reason to expect the dete
ctors to behave differently for the different polarizations. (Feynman would famously get to the heart of another complex problem eight years later after the tragic Challenger explosion, when he simply demonstrated the failure of an O-ring seal to the investigating commission and to the public watching the televised proceedings.)
Over the fall the SLAC experiment refined their efforts to rule out both this concern and others that had been raised, and by the fall they reported a definitive result in agreement with the Glashow-Weinberg-Salam prediction, with an uncertainty of less than 10 percent. Electroweak unification was vindicated!
To date, I don’t know if anyone has a good explanation of why the original atomic physics results were wrong (later experiments agreed with the Glashow-Weinberg-Salam theory) except that the experiments, and the theoretical interpretation of the experiments, are hard.
But a mere year later, in October 1979, Sheldon Glashow, Abdus Salam, and Steven Weinberg were awarded the Nobel Prize for their electroweak theory, now validated by experiment, that unified two of the four forces of nature based on a single fundamental symmetry, gauge invariance. If the gauge symmetry hadn’t been broken, hidden from view, the weak and electromagnetic interactions would look identical. But then all of the particles that make us up wouldn’t have mass, and we wouldn’t be here to notice. . . .
This is not the end of our story, however. Two out of four is still only two out of four. The strong interaction, which had motivated much of the work that led to electroweak unification, had continued to stubbornly resist all attempts at explanation even as the electroweak theory took shape. No explanation of the strong nuclear force via spontaneously broken gauge symmetries met the test of experiment.
The Greatest Story Ever Told—So Far Page 21