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

by Why Are We Here (pdf)

  beam would be sufficiently compressed so that when the two beams

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  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.

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  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

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  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.

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  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

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  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,

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  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

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  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.

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  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

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  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