More on Particle Physics

Amazingly, all the objects we see around us – clouds, cars, people – forming a kaleidoscope of shapes, colours, and textures, are made up of a relatively small number of building blocks, just a hundred or so types of atoms: carbon, oxygen, uranium, and so forth. Moreover, any atom can be constructed out of just three still more fundamental building blocks: protons and neutrons, forming the atomic nucleus, and electrons, shrouding the nucleus in a strange quantum dance.

Physicists knew this picture of matter, as being composed of protons, neutrons, and electrons, by the early 1930s. However, the building blocks of matter – the elementary particles, as they are understood today – go deeper than this.  Moreover, the idea of elementary particles extends also to forces. For example, in trying to understand how light and other forms of the electromagnetic force interact with matter, Albert Einstein – building on the work of Max Planck – made the remarkable conjecture in 1905 that light is a shower of elementary particles, which later came to be known as photons.

This idea of elementary particles as the building blocks of everything – matter and forces – is one of the greatest success stories of 20th century physics. Ironically, the greatest challenge to this idea comes from the most apparent and thus commonplace of nature’s forces: gravity. Another deep mystery is the origin of the masses of the elementary particles, whose explanation is believed to lie in the still-elusive Higgs particle. These and others are challenges for the 21st century.

A Plethora of Particles

While the history of particle physics is rich and fascinating, we will touch on just a few of the highlights. For instance, one type of matter we are not aware of in our day-to-day experience is the elementary particle called the neutrino, which is actually amongst the most numerous of all particles in the universe; every cubic centimetre of space is flooded with them. Nevertheless, neutrinos hardly interact with other particles, and so rarely make their presence known. Indeed, first theorized in 1930 by Wolfgang Pauli, neutrinos were not observed until 1956, a very difficult experiment that was awarded the 1995 Nobel Prize in physics.

                         

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The particle physicist’s periodic table, showing the quarks and leptons comprising matter and the force carriers.

Where do neutrinos come from? One place is stars, like the Sun. Nuclear fusion reactions raging at the heart of the Sun produce a torrent of neutrinos blasting out into the surrounding space at nearly the speed of light. The outpouring is so intense that even here on the Earth, far from the Sun, 50 trillion solar neutrinos pass through your body every second. You don't feel them because they interact so weakly, that is rarely, with the particles in your body.  Indeed, most pass straight through the entire Earth as if it were not there! 

"There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy." – William Shakespeare

We also know now that neutrons and protons are not elementary particles after all, but in turn are composed of yet simpler building blocks – quirky particles called quarks, which come in six flavours, two of which are dubbed "up" and "down." Two up and one down quark make a proton; one up and two down quarks make a neutron. Each is a particle triplet. To sum up so far, we have quarks and electrons making up atoms, and the neutrino – technically, the electron neutrino. See the left column in the table. We also have the photon: table, top right.

To add another quirk to the whole business, each type of particle in this table has an anti-matter counterpart. In 1928, Paul Dirac brilliantly combined quantum theory with Einstein's theory of special relativity, resulting in the prediction that, associated with the electron, there should exist an antielectron. This elementary particle was first observed experimentally in 1932 and was named the positron, a discovery that was awarded the 1936 Nobel Prize. If you put an electron next to a positron (which has opposite electric charge), they will attract each other and annihilate each other. The mass of the two particles will disappear, and in its place will appear pure radiant energy in the form of two photons, a process obeying Einstein’s famous equation E = mc2. (Note that some types of particle are their own antiparticle.  For example, an antiphoton is a photon.)

Perhaps quirkier still, elementary particles cannot be thought of as ordinary particles that happen to be exceedingly tiny. For example, all of the quarks and leptons in the table are such that if you rotate one of them through 360 degrees (once around) it will not have returned to the same "state." It must be rotated through 720 degrees (twice around) to return to its original state! As bizarre as this may seem, experiments have actually verified this.

Our story so far sounds clean and simple, but this is only with the benefit of hindsight. Scientific discovery is often messy business. In the case of particle physics, one major source of messiness stemmed from the profound discovery that particles are not necessarily eternal. Amazingly, they can be transmuted from one type into another. We considered above an electron and a positron transmuting into two photons, but the phenomenon is much more general. For example, if you accelerate two electrons almost to the speed of light and smash them together in a head-on collision, out will spray a variety of particles, which might not include any electrons! The details are complicated, but the basic idea is simple: 

E = mc2 (again). In a collision, some of the kinetic energy of two fast-moving electrons can be transformed into mass. You might begin with two fast, light particles, and end up with five slower, heavier particles. Beginning in the 1950s, experiments like this, conducted in high-energy particle accelerators, produced a plethora of new particles, called the "particle zoo." Most of the Greek alphabet – and then some – was used to name all the particles, and there was great confusion about what it all meant!

                         

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Tracks of many particles emerging from the head-on collision of just two protons [more] © CERN

The Standard Model

The dust began to settle in the 1960s, with the idea of quarks. Experimental evidence for quarks came in the 1960s and ’70s, although the last quark – the top quark – wasn’t observed until 1995. Physicists eventually realized that most of the beasts in the zoo were merely composite entities, which could all be built out of the relatively small set of elementary particles comprising the first three columns in the table above. We’ve already discussed the first column. The next two are essentially carbon copies of the first column, except with progressively higher particle masses. No one knows why these exist. For instance, when the electron’s heavier cousin, the muon, was discovered, the particle theorist Isidor Rabi exclaimed, "Who ordered that?" While there is a beautiful and compelling mathematical structure underlying this "particle physicist’s periodic table," why there are three "generations" of matter remains a mystery.

The last column in our periodic table, labelled "Force Carriers", represents a remarkable shift in the way physicists think about "force." For example, when you touch a table, you are not actually "touching" the table. What’s happening is an exchange of particles – in this case, photons – between electrons in the atoms in your fingertip and those nearby in the table. The harder you push, the greater the exchange rate, but the electrons never "touch." Imagine two astronauts floating freely inside a space shuttle. If they push against each other, they fly apart. Alternatively, they could accomplish the same thing by exchanging a basketball – throwing it back and forth. In throwing the basketball, an astronaut would recoil in the opposite direction; in catching the basketball, the other astronaut would also recoil. The net effect is the same as if they had pushed on each other.

Just as photons are the force carriers of the electromagnetic force, gluons are the force carriers of the strong force (technically, the colour force), which act between quarks. For example, gluons "glue" together the triplets of quarks comprising protons and neutrons. They also keep atomic nuclei from flying apart by countering the repulsive electromagnetic force between neighbouring protons. They can even glue themselves together into "glueballs"! The W and Z force carriers are responsible for the weak force, which is how neutrinos interact with matter - weakly.

The whole picture is called the standard model of particle physics, and it is astonishingly successful. The situation is well summarized in the words of the theorist Freeman Dyson – one of the inventors of the standard model – written to an experimentalist, Gerald Gabrielse:

As one of the inventors, I remember that we thought of QED [the best-tested part of the standard model] in 1949 as a temporary and jerry-built structure, with mathematical inconsistencies and renormalized infinities swept under the rug. We did not expect it to last more than ten years before some more solidly built theory would replace it ... now, 57 years have gone by and that ramshackle structure still stands. And you still did not find the discrepancy that we hoped for. To me it remains perpetually amazing that Nature dances to the tune that we scribbled so carelessly 57 years ago. And it is amazing that you can measure her dance to one part per trillion and find her still following our beat.  (Letter from F. Dyson to G. Gabrielse, 2006 – used with permission.)

There is definitely something very right about the standard model. But just as definitely, it is not the final word. One reason is that the standard model does not predict everything. Given extremely accurate measurements of, for example, the magnetic field of the electron (besides having an electric charge, an electron also behaves like a tiny spinning bar magnet), the standard model will make a correspondingly accurate prediction of the charge of the electron. Such predictions, in turn, agree surprisingly well with other measurements, which is what "perpetually amazes" Dyson. But the standard model contains at least 18 parameters (masses of particles and strengths of forces) that can be determined only experimentally. No one knows why these parameters have the particular numerical values they do. Thus it cannot be a complete, self-contained theory of nature.

What’s Missing?

In the standard model, particles such as the electron and the quarks are inherently massless. In order for them to have mass (which they clearly do), the standard model relies on the existence of an additional particle call the Higgs particle. Roughly speaking, the idea is that all of space is filled with a "quantum fluid" called the Higgs field. The Higgs particle is to the Higgs field what the photon is to the electromagnetic field. Sort of like moving through molasses, particles acquire mass by interacting with this fluid.

The Higgs particle has not yet been directly detected. It is believed to be a very heavy particle, and so will require a very high-energy particle accelerator to create it and confirm its existence (remember E = mc2). There is great excitement and hope that the new Large Hadron Collider (LHC) at the European particle laboratory, CERN, will be the first to directly observe the elusive Higgs particle.

                        

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The ATLAS detector under construction at the LHC [more] © CERN.

If confirmed, it would take us closer to a Grand Unified Theory, in which the three fundamental forces in our periodic table above (leaving out only the fourth force, gravity) are but different aspects of a single, unified force, in the same way that electric and magnetic forces are just different aspects of the single electromagnetic force. We might also discover evidence for "supersymmetry," which suggests that for each particle in our periodic table there exists an as-yet-undetected "superpartner" particle, a crucial ingredient in superstring theory.

These questions are of great interest for cosmology. They need to be answered if we want to understand the nature of the very early universe, which involves extreme physics at incomprehensibly high particle energies and densities. Indeed, we may be able to turn this around – actually use the early cosmos as a "laboratory," and deduce from astronomical observations whether or not there is new physics beyond the standard model. This deep link between the study of the universe in the large (the cosmos) and the small (elementary particles) is extremely fertile ground for potential new discoveries. 

Already, it has been responsible for at least one huge, new development: cosmological observations indicate that a large fraction of the universe is made of so-called "dark matter." Physicists are currently investigating if this implies the existence of new particles outside of the framework of the standard model.

As indicated above, the very structure of the standard model itself is a big question. Why three generations of matter, and not one, or seventeen? Why not other forces besides the electromagnetic, weak, and strong forces (and gravity)? Why does the electron have the mass it does, and not something completely different?  In short, why is the world the way it is? There is hope that these questions may be answered by finding an all-encompassing mathematical formulation of physics beyond the standard model, such as string theory.

Finally, you might wonder why one particular interaction, the gravitational force, has not been discussed. The reason is that gravity does not fit very well into the mathematical framework of the standard model. Unlike the photon for electromagnetism, the idea of a "graviton" for gravity works only if gravity is so weak that it can essentially be neglected. Unlike the photon for electromagnetism, the idea of a "graviton" for gravity simply does not work in the standard model. Physicists have yet to find a theory of quantum gravity – perhaps the single most important problem in theoretical physics.

                          

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The ATLAS detector under construction at the LHC [more] © CERN.

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This idea of elementary particles as the building blocks of everything – matter and forces – is one of the greatest success stories of 20th century physics.