Lawrence Krauss - The Greatest Story Ever Told--So Far

by Why Are We Here (pdf)

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.

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  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

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  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

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  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

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  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

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  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

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  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

  ͥ͟͞
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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

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  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