“In the good old days, a man would spend a year at his desk doing calculations and then send a telegram to an observatory: ‘Place the telescope at such and such position and you will see a new planet.’ The planets were very polite and positioned themselves accordingly, as in a well-organised ballet. Nowadays, atomic particles appear suddenly, out of the blue, doing somersaults. The physics of yesteryear was a bit like ballroom dancing to Mozart, while now it’s more like a fairground with halls of mirrors, labyrinths, target-shooting booths and men hawking phenomena.
And astronomy, which was a demure home-loving girl, hard working and modest, now has a younger brother who messes up the house, turns the attic into a powder keg, asks excruciating questions and makes up outlandish stories.”
Ernesto Sabato “Física escandaloso”, in Uno y el Universo (1945)
Rather than giving you a standard introduction to the modern formulation of particle physics, I am going to take a historical approach. An approach that is more human, after all, with its many paths – mistaken, corrected, retraced and reconsidered. Attempts to clear a path through the jungle in the hope that scientific process will, sooner or later, lead us to the truth. And in any case, it is impossible to understand what motivates a theory without knowing the circumstances that favoured it and the scientific frustration that it was trying to escape from.
The ultimate goal of particle physics is to discover the foundations that form the basis of nature. Clearly, the first step in this quest is to understand what shapes our day-to-day reality by means of accessible energies. As such, the classical period (1897-1932) of particle physics covers the discovery of the electron, the proton and the neutron.
The “discovery” of the photon should also be included in this initial stage, although it is difficult to attribute it to any single individual. For our purposes, the interesting thing here is the revolutionary idea that Max Planck put forward in 1900: that electromagnetic radiation is quantized. In other words, that it comes in small packets, a notion that resolved the phenomenon that was later known as ultraviolet catastrophe. In 1905, Einstein put forward an even more daring hypothesis: that quantization is actually a property of the electromagnetic field. Even though experimental results appeared to confirm Einstein’s theory, the scientific community found it difficult to accept his claim because it seemed to take them back to Newton’s particle theory of light, which had by then been replaced by the wave theory of light. These two approaches, wave and particle, where later reconciled in the wave-particle duality theory: All matter has both wave and particle characteristics, and can behave as one or the other depending on the specific experiment.
So an idea that initially appeared to be a step backwards in scientific progress ended up bringing innovation. Einstein’s proposal offered a new insight: unlike classical electrodynamics, in which each electron contributes and responds to the electromagnetic field, in quantum field theory the electromagnetic field is quantized as photons. We perceive the interaction as an ongoing exchange of photons between the two charges, with each electron constantly emitting and absorbing them. The force-mediating particles – in this case, photons – play the role of “messengers” in the interaction.
Even though the knowledge generated in this first period was a simple (but satisfactory for the time) answer to the question “what is matter made of?” it sowed the seeds for the three big ideas of the middle period (1930-1960) of particle physics: Yukawa’s meson, Dirac’s positron and Pauli’s neutrino.
The first question that challenged the classical period was: “what keeps the nucleus together?” In 1934, Hideki Yukawa proposed that this binding energy was strong interaction, a new force that would also theoretically be quantized, in keeping with the logic of electromagnetic interaction. Yukawa called the mediators of this force pions . Nature provided researchers with the means to observe these pions for the first time in 1947, in the form of the very-high energy particles from outer space called cosmic rays that constantly bombard the earth. When these particles collide with atoms in the upper atmosphere, innumerable secondary particles – including pions – are created and steadily rain down on us.
In 1927, another tornado shook the foundations of our understanding of particle physics: in the equation that carries his name, Paul Dirac described free electrons with relativistic energy, and found that every equation had a negative energy solution for each positive energy solution. These negative energy solutions were later reformulated as the positive energy states of a different particle: the positron, which is the antiparticle of the electron. The dualism of the Dirac equation goes deeper still: for every particle, there is an antiparticle with the same mass and the opposite charge. A few years later, in 1932, Carl D. Anderson found the first experimental proof of the positron. The antiproton was detected at the Bevatron, Berkeley, in 1955, followed by the antineutron a few years later.
So the bestiary of known particles gradually expanded to include not just particles but also their corresponding antiparticles. The particles had peered into Alice’s looking-glass, and an age-old companion had turned up for each of them.
The third upheaval of the middle period came in 1930 with the study of nuclear beta decay. The experimental range of kinetic energy for electrons varied within a certain range instead of being fixed, as researchers would have expected in the decay of two bodies. Wolfgang Pauli suggested an explanation that seemed more reasonable than abandoning the law of conservation of energy: that another particle that we don’t see is emitted in the process. This particle, which is apparently “invisible” due to its extremely weak interaction with matter, was called a “neutrino”.
For a short time, it seemed possible that the biggest problems of elementary particle physics had been solved. But this calm was disturbed once again in late 1947 when George Rochester and Clifford Butler published photographs taken in a cloud chamber showing a previously unknown neutral particle that had a mass at least twice as great as a pion, and moved in a spiral pattern: it was named “kaon”. This was quickly followed by the appearance of many more mesons, and, some time later, the discovery of baryons. By this time, “meson” (meaning “medium weight”) and “baryon” (“heavy weight”) were simply labels that classified particles according to mass and mode of decay.
Up until this point, cosmic rays had offered an excellent free source of elementary particles with very high energy compared to the particles that are commonly found in everyday life. But cosmic rays had two drawbacks: the fact that they move through reasonably large detectors at low frequencies, and, more importantly, the fact that they cannot be controlled. So it became necessary to produce a laboratory version of the collisions caused by the impact of cosmic rays in the atmosphere, or in other words, to design our own controlled experiments in order to be able to study nature in its most primordial form. As Julio Cortázar puts it in a beautiful small piece in Cronopios and Famas:
A small cronopio was looking for the key to the street door on the night table, the night table in the bedroom, the bedroom in the house, the house in the street. Here the cronopio paused, for to go into the street he needed the key to the door.
The first of these “keys” – the first modern particle accelerator, the Cosmotron at Brookhaven – began operating in 1952. And it was soon possible to reproduce the “strange” particles (mesons and baryons) that had been observed using cosmic rays and to study them in the laboratory.
As David J. Griffiths writes in Introduction to Elementary Particles, the garden which seemed so tidy in 1947 had grown into a jungle by 1960. It was necessary to find the “periodic table” of elementary particles. This classification, put forward in 1961-1964, was called the “Eightfold Way” (a reference to the noble eightfold path of Buddhism), and the Mendeleev of elementary particle physics was Murray Gell-Mann, who came up with a scheme that ordered the baryons and mesons in complex geometric patterns according to their strangeness number and electric charge. An amazing discovery was made when the baryons were organised into decuplets: nine of the particles were experimentally known, but the last one, which completed the decuplet (the one at the bottom), had not yet been observed. As the prophet of particles, Gell-Mann predicted that this particle would be discovered, and deduced its properties from its position in the decuplet. In 1964, “omega-minus” (W–) was discovered, just as he had predicted. Gell-Mann had in a sense deciphered the message of “The Gold-Bug” that would lead experimental physicists to find the buried treasure.
The Eightfold Way was not just a classification of particles. It also provided some notion of the underlying structure, which led to the advent of the modern age of particle physics.
Why did the hadrons (mesons and baryons) form these peculiar patterns? The explanation came in 1964, when Gell-Mann and George Zweig independently suggested that hadrons were made up of even more elementary particles – quarks – which can in turn be grouped triangularly (up, down and strange were the only known quarks at the time).
According to the quark model, baryons are made up of three quarks, while mesons consist of a quark and an antiquark. Combinations of these quarks create the entire menagerie of baryons and mesons: the bestiary of “strange” particles had been placed under the microscope and its underlying structure laid bare. It was like examining a snowflake and discovering the fractal substructure in a seemingly indivisible whole.
Although the quark model included some singularities that were difficult to understand at the time, such as the fact that we cannot see single isolated quarks (“confinement”), it managed to explain the nature of a new particle that had been discovered in 1974, the J/psi meson, which had an extraordinarily long lifespan (1000 times longer than any comparable particle!). This property clearly pointed to a new physics. Indeed, this new particle was made up of a new type of quark which was called a “charm” – a quark that had already been predicted by James Bjorken and Sheldon Lee Glashow years earlier, given that we would theoretically expect to find an equal number of leptons and quarks (remember that, at this time, scientists only knew of electrons, muons and their corresponding neutrinos, forming the lepton family, and “up”, “down” and “strange” in the quark family). The tau lepton was discovered some time later, and it was soon joined by the “beauty/bottom” quark in 1977 and the “truth/top” quark in 1995.
Meanwhile, as some scientists hunted for quarks, others were embarking on a search for intermediate vector bosons. In keeping with the photon’s role in electromagnetic interaction, there had to be an intermediate boson that acted as a “messenger” in the interaction involved in beta decay. Its properties were soon predicted by the electroweak theory suggested by Glashow, Abdus Salam and Steven Weinberg. The bosons that mediate in weak interaction were called W+, W- and Z. A proton-antiproton collider (Super Proton Synchotron, SPS) was designed and constructed at the CERN in order to produce these particles that are extremely heavy compared to the ones that had been produced so far. In 1983, the CERN announced the discovery of the W and Z bosons.
Thus, following the discovery of the J/psi meson – a discovery that became known as the “November Revolution” –, a flood of new elementary particles completed the table that we can now find in any book of particle physics, which is perfectly balanced in terms of the number of leptons and quarks.
Not long ago, we witnessed another revolution: the discovery of the Higgs boson, the final piece of the Standard Model jigsaw puzzle, a fabulous creature that played hide-and-seek for fifty years. In order to hunt down particle physic’s own “snark”, researchers had to design the LHC collider and its particle detectors with enough ingenuity and innovation to capture it:
“You may seek it with thimbles – and seek it with care; you may hunt it with forks and hope; you may threaten its life with a railway-share; you may charm it with smiles and soap.”Lewis Carroll in The Hunting of the Snark
With the discovery of the Higgs boson on 4 July 2012, the map of the Standard Model appears to be complete. But we know that the same cannot be said for the map of reality, and that fundamental questions that lie outside of the scope of the Standard Model remain unsolved. It’s a bit like Alice holding a mirror to her book so that she could read the poem Jabberwocky. Now we are missing the mirror. Its reflection is the Holy Grail that particle physicists seek.
The history of physics, in particular particle physics, proves that it has always been necessary to take a fresh look, to climb a tree and scan the horizon, to live in the treetops like Italo Calvino’s Baron in the Trees, in order to discover the fundamental structure of a reality that is much more universal that the one that surrounds us here and now.
Or, as Julio Cortázar puts it in his Narraciones y Poemas:
“With cronopios or without them, but never leaving their world of play: the serious job of playing when you are looking for other doors. Other means of accessing the extraordinary, even if only to make the ordinary more beautiful by suddenly seeing it in a different light, taking it out of its usual compartments. To define it anew, and better.”
Griffiths, David. Introduction to Elementary Particles. 1987.
Sabato, Ernesto. Uno y el Universo. 1945.
Cortázar, Julio. Cronopios and Famas. 1962.
Carroll, Lewis. The Hunting of the Snark. 1876.
 We now know that the pion field that keeps the atomic nucleus together is just the residual effect of the strong force that acts on the internal components of hadrons, quarks, and that the gluon is the particle that mediates in the interaction.