Later in the week, Patricia and I joined one of the last tours of the new accelerator before it was turned on. Once the machine was operating and the protons were chasing around the 27-km circumference of the LHC, then the radiation level would be prohibitive and access to the machine would be severely restricted. We stood on the viewing deck of the enormous ATLAS detector, feeling overwhelmed by its sheer size. The amount of iron used to build just this one detector was equivalent to all the iron in the Eiffel Tower. Colorful cables wound through the complicated electronic parts of the detector, which appeared to us to be about the size of a European cathedral. Together with three other main detectors, ATLAS was the place where the protons would collide and produce a huge amount of particle debris, which would then be analyzed for years by a worldwide grid of computers.
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At this writing, five years later, the LHC has been shut down for two years of maintenance and upgrading to an energy of 13 to 14 TeV. During the past two years, there has been tremendous excitement about what the LHC may have discovered, with many physicists already popping the champagne corks to celebrate the discovery of the Higgs boson. The data accumulated before the LHC shutdown have been analyzed and found to be consistent with the standard-model Higgs boson. However, as we will learn in this book, there remain critical experimental issues to determine exactly what the new Higgs-like boson is, which need to be resolved after the machine starts up again in 2015.
Cracking the Particle Code of the Universe
1
What Is Everything Made Of?
The fifth-century BC Greek philosopher Leucippus, and his pupil Democritus (460 to 370 BC), presaged in broad strokes by more than 2,000 years the standard model of particle physics. They claimed that matter could be broken down to a basic unit. This claim was in contrast to Aristotle (384 to 322 BC), whose ideas, based on Plato’s, soon became the establishment view of the day, and who taught that the basic elements of the universe were the earthly elements fire, water, earth, and air, plus the heavenly element, ether. Leucippus and Democritus called their basic unit atom, which means “indivisible” or “uncutable” in Greek. By 1964, with the publication of the quark model by Murray Gell-Mann and the independent proposal by George Zweig of aces, the ancient Greek idea of a basic unit of matter had finally arrived—and the quarks, along with the leptons, would eventually be heralded as the basic units of all matter.1
FINDING THE BUILDING BLOCKS
Particle physics has a long history, reaching back to the 19th century and the discovery of atoms. The reductionist view that matter is constituted of tiny, indivisible units received its first serious validation in 1897, when J. J. Thomson of Cambridge University discovered the electron. Ernest Rutherford’s discovery around 1919 of the proton—the positively charged particle at the center of the hydrogen atom, with Thomson’s negatively charged electron buzzing around it—introduced another important unit of matter. It was not until 1932, when James Chadwick discovered the neutron with the first particle accelerator, that the dominance of the proton and electron as the only basic units of matter was broken. This was the birth of nuclear physics; the nuclei of more complicated atoms and molecules were discovered to be composed of different numbers of bound protons and neutrons. The radioactive beta decay—or transmutation of matter first discovered by Henri Becquerel in 1896—of some of these nuclei released a further unit of matter, called the neutrino, which was a massless, electrically neutral particle. At the time of its discovery, it was believed to have zero mass, and, obeying Einstein’s special relativity theory, it moved at the speed of light. The neutrino was first proposed by Wolfgang Pauli in 1931 to explain the observed missing energy in radioactive decays. He surmised that an unseen neutral particle was carrying it off. The neutrino was not confirmed experimentally until 1959, by Fred Reines and Clyde Cowan.
The famous Dirac equation, invented by Paul Dirac in 1928, provided a particle-wave description of the electron in accordance with Einstein’s special relativity. However, it had an interesting by-product; it also predicted the existence of antimatter. Carl Anderson’s detection in 1932 of the “positron,” a positively charged electron, was a triumph for Dirac. Eventually, this discovery led to the understanding that all the basic constituents of matter had antimatter partners. For example, the proton’s antiparticle is the antiproton.
As accelerators were developed that collided particles at higher energies, more particles and their antimatter partners were discovered. The muon, a kind of heavy electron, was found by Carl Anderson and Seth Neddermeyer at Caltech in 1936. This was followed by the discovery of the mesons, the particles that were, at that time, considered to be the carriers of the strong force holding atomic nuclei together, much as the photon carries the electromagnetic force. In 1935, Japanese physicist Hideki Yukawa had predicted that the protons and neutrons were bound in the atomic nucleus by the exchange of mesons, and after World War II, in 1947, Cecil Powell, César Lattes, and Giuseppe Occhialini of the University of Bristol detected these particles in cosmic rays.
GROUPING THE BUILDING BLOCKS
The electron, proton, neutron, neutrino, antimatter, meson, muon,…! What next? By the mid 1950s, the number of newly discovered particles at accelerators was so large that physicists began referring to them as the particle zoo. To make sense of the abundance of particles, physicists began categorizing them, using the ideas from group theory, which had been promoted by Hermann Weyl, John von Neumann, and other mathematicians. Murray Gell-Mann and Yuval Ne’eman played a significant role in this effort. Gell-Mann was a dominant figure in particle physics during the 1960s and 1970s. He was a professor of physics at Caltech, and won the Nobel Prize in 1969 for his contributions to particle physics and the discovery of quarks.
Figure 1.1 Gell-Mann’s Eightfold Way of baryons. Here, s stands for the quantum number strangeness, and q stands for electric charge.
SOURCE: tikalon.com.
During the early 1960s, before the advent of the quark model, Gell-Mann developed what he called the Eightfold Way, alluding to the Buddhist Noble Eightfold Path of right thought and actions leading to enlightenment. He proposed that what were now called the baryons (the proton, neutron, and their cousins with spin ½ and ) fitted into octets (Figure 1.1) and decuplets (Figure 1.2)—namely, patterns of eights and 10s—of the symmetry group from group theory called SU(3).
Figure 1.2 Gell-Mann’s Eightfold Way decuplet of baryons.
SOURCE: Wikipedia Commons.
One of the baryons in the decuplet was missing. There was nothing in the place where a particle should be at the base of the triangle. Gell-Mann, confident of his patterned scheme, predicted that the missing particle must exist. He called it the Ω-—the Omega minus, a negatively charged resonance.2 Gell-Mann’s classification scheme was validated in 1964 when a team of physicists from Brookhaven, the University of Rochester, and Syracuse University discovered the Omega minus particle.
Before wading any further into particle physics terminology, some brief explanations are in order. Spin is one of several parameters that characterize the different particles. It is an intrinsic quantum number that was discovered during the early development of quantum mechanics by Samuel Goudsmit and George Uhlenbeck in 1925. Spin is a degree of freedom that has meaning only in quantum mechanics. Its classical counterpart is the angular momentum of a body, such as the spinning of a top. However, the quantum mechanical spin does not have a classical interpretation. The spin of a particle is described by being either up or down in direction. Its magnitude can be integer values such as 0, 1, or 2 for bosons, and half-integer values such as ½ and for fermions. In physical units, the spins of the particles are given in multiples of Planck’s constant h; so, for example, the spin of the electron is ½ h.
Some of the other properties that characterize particles are electric charge, mass, and parity (left- or right-handedness). Group theory will be described in more detail in Chapter 3. Suffice it to say here tha
t, of the several mathematical groups that have been applied successfully to particle physics, the group SU(3) is one of the most important groups used by physicists to categorize particles.
In addition to the baryon octet and decuplet categories, there are octets of mesons having spin 0 (Figure 1.3) and spin 1 (Figure 1.4). Most of the particles that fitted into these meson octets, such as the eta meson and the electrically neutral K mesons, were already known in the 1960s or, if not, were soon discovered. Two of these particles were the rho and omega mesons with spin 1, which were thought, at the time, to be important force-carrying particles that bound protons and neutrons together.
A basic feature of the group SU(3) is the triplet. It implies that there could be three fundamental constituents of all the known hadrons, a category that includes baryons and mesons; hadros means “heavy” in Greek. Back in 1956, Japanese physicist Shoichi Sakata proposed that three particles are the building blocks of all others: the proton, neutron, and “Lambda (Λ) particle.” Initially, Murray Gell-Mann went along with this idea. The proton and neutron, collectively called nucleons, were interpreted in terms of the quantum number called isospin, which is a property of the proton- and neutron-like spin. Isospin means that hadrons with similar masses and the same spin but different electric charges were identical as far as the strong interactions within the nucleus were concerned.
Figure 1.3 Gell-Mann’s Eightfold Way spin-0 pseudoscalar (negative-parity) meson nonet (octet plus a singlet).
SOURCE: Wikipedia Commons.
Figure 1.4 Gell-Mann’s Eightfold Way spin-1 pseudoscalar meson nonet (octet plus a singlet).
SOURCE: Wikipedia Commons.
The Lambda particle is an electrically neutral, short-lived particle discovered in 1947. Sakata included the Lambda in the triplet to account for the quantum number “strangeness.” Abraham Pais first proposed a quantum number in 1952 to explain that certain particles, such as the Lambda, even though they were produced copiously in particle interactions, decayed slowly, having lifetimes typical of weak radioactive decays. This quantum number was dubbed strangeness by Gell-Mann. The idea of the triplet of proton, neutron, and Lambda was that all the other hadrons in the particle zoo were made up of these three particles bound together. This idea did not fit with the experiments at the time and was abandoned. However, the idea of a stable triplet of basic constituent particles for matter persisted.
“THREE QUARKS FOR MUSTER MARK!”
In 1963/1964, Murray Gell-Mann speculated on the basis of a suggestion by Robert Serber, a professor at Columbia University, that there were three basic constituents of all matter, which had not yet been observed. Gell-Mann understood that because these three constituent particles would make up protons and neutrons, which had a unit of positive electric charge and zero charge respectively, the charge of the constituent particles had to be fractional, ⅔ and ⅓, to create a plus-one or zero charge. Because no one had ever observed such fractional electric charges, this seemed an absurd idea. However, Gell-Mann continued to pursue the idea and published a letter in Physics Letters in 1964 titled, “A Schematic of Baryons and Mesons,” proposing that these fractionally charged particles played a mathematically important role in the basic constituents of matter.3 In his letter, he called the particles quarks, taken from James Joyce’s Finnegan’s Wake: “Three quarks for Muster Mark!”
Meanwhile, George Zweig, who was a Caltech graduate visiting CERN as a postdoctoral fellow, proposed independently at about the same time an idea that was very similar to Gell-Mann’s quark model. He called his basic constituents of the triplet aces. However, he did not publish his paper because senior physicists at CERN considered it to be too speculative. Zweig later recalled:
The reaction of the theoretical physics community to the ace [quark] model was generally not benign. Getting the CERN report published in the form that I wanted was so difficult that I finally gave up trying. When the physics department of a leading university was considering an appointment for me, their senior theorist, one of the most respected spokesmen for all of theoretical physics, blocked the appointment at a faculty meeting by passionately arguing that the ace model was the work of a “charlatan.”4
Gell-Mann and Zweig both agreed on the basic attributes of the quark model. There were three species of quarks. The “up” quark (u) and the “down” quark (d) have spin ½ and isospin ½ (called an isospin doublet). A third quark, called the strange quark (s), has isospin 0 (called an isospin singlet) and “strangeness” one. This third quark is needed to explain the existence of baryons carrying a nonzero strangeness number and also mesons like the positively charged K meson, which is composed of an up quark and an antistrange quark; and the neutral K meson, which is composed of a down quark and an antistrange quark. The three quarks—up, down, and strange—each have what is called baryon number equal to ⅓, so that the baryon number of the three quarks constituting a proton or neutron adds up to one.
The particle physics community was skeptical about the idea of protons and neutrons being made up of three quarks, and the need for fractional electric charge did not go down well either. Gell-Mann himself was not convinced that his quarks were real. He treated them as a productive way of visualizing how matter is constituted, and as a mathematical trick to make sense of his Eightfold Way model of particles.
During a visit Gell-Mann made to the University of Toronto in 1974, he told me that he had initially submitted his quark paper to Physical Review Letters but it was rejected. He told me that he then phoned the theory director at CERN at the time, Leon van Hove, and said that he was planning to submit his quark paper to Physics Letters B, where Van Hove was an editor. Van Hove asked him what a quark was, and he said it was a particle making up the triplet SU(3), which had fractional electric charge and baryon number ⅓. Van Hove was dismissive and didn’t think it was a good idea for Gell-Mann to submit the paper for publication. However, Gell-Mann did, and it was accepted and published by Physics Letters B.
Yet soon, in 1968, the Stanford Linear Accelerator (SLAC) was used to begin experiments to try to detect quarks by bombarding atomic nuclei with electrons. The idea was to use the latest accelerator technology to repeat the Rutherford experiments that had discovered protons inside atomic nuclei. Rutherford had scattered a beam of alpha particles (helium nuclei composed of two protons and two neutrons) off gold leaf, and had found a sufficient number of alpha particles scattering at large angles to conclude that they had hit hard objects—namely, the nuclei of the gold atoms. Similarly, in the SLAC experiment, if the angles of the scattering of electrons hitting the insides of nuclear targets were sufficiently large, this would signal a significant deflection of the electrons by hard, massive objects, and would provide at least indirect proof that quarks existed inside protons and neutrons. The experiments did indeed produce a distribution of scattering angles that strongly suggested that there were small objects inside protons.
Two kinds of experiments had already been performed in the linear collider to contribute to this conclusion. One was called elastic proton–electron scattering. These experiments showed that the proton was not a pointlike object like the electron; it had a diffused structure. This led physicist Robert Hofstadter to experimental investigations of the structure of the proton during the early 1950s, for which he won the Nobel Prize for Physics in 1961. Subsequently, a series of experiments of so-called deep inelastic scattering, in which scattering electrons and protons produced other particles, did not behave as expected with increasing energy. The data suggested that there were some hard objects inside the proton. These experiments were performed on the 22-GeV, two-mile long linear electron accelerator at SLAC beginning in 1967. This research heralded the new “hard-scattering” era of particle physics, compared with the soft-scattering era that had previously dominated experimental particle physics. Hard scattering refers to the hard objects inside protons that were hit by the electron beams whereas soft scattering refers to proton–proton collisions in which a plot o
f the scattering cross-sections displayed a rapid fall-off with increasing energy, indicating that these experiments were not investigating the interior of the protons.
Initially, the Stanford experimentalists were not able to explain these radical deep inelastic scattering results. They knew that something important had been observed, but they were unable to understand the implications fully. At that time, particle physicists had not accepted the reality of fractionally charged quarks inside protons and neutrons. James Bjorken, fondly known as “BJ” by his colleagues, who was a member of the theory group at SLAC, analyzed the experimental data and discovered a scaling relationship that could constitute proof of the existence of quarks inside the proton.5 He analyzed the SLAC data in a somewhat esoteric way, using what was called “current algebra sum rules,” and found that the electromagnetic structure of the proton scaled with a scaling parameter that consisted of the ratio of the energy loss by electrons radiating off photons and the energy of the new particles produced in the deep inelastic collision.
Cracking the Particle Code of the Universe Page 2
