A Palette of Particles

Jeremy Bernstein. American Scientist. Volume 100, Issue 2. Mar/Apr 2012.

The physics of elementary particles in the 20th century was distinguished by the observation of particles whose existence had been predicted by theorists sometimes decades earlier. There were also particles no one had predicted that just appeared. Five of them are of interest to me here. In order of increasing modernity, they are the neutrino, the pi meson, the antiproton, the quark and the Higgs boson. Let’s begin with the neutrino.

On the 4th of December 1930, Wolfgang Pauli sent a letter to a group of colleagues who were attending a physics conference in Tübingen. He addressed them as “Dear Radioactive Ladies and Gentlemen.” Pauli apologized for not being able to attend the conference. A very good dancer who had a fondness for women, he explained that he wanted to attend a ball in Zurich instead. The letter is one of the most remarkable documents in 20th-century physics.

Pauli’s concern was an anomaly that had occurred in experiments on what was known as beta decay. The great New Zealand-born physicist Ernest Rutherford, who had made an extended study of radioactivity, had identified three types of decays, which he called alpha, beta and gamma. Heavy nuclei such as plutonium can in decaying produce an alpha particle that was identified as a nucleus of helium. Many other nuclei can decay producing a gamma ray, which is a very energetic electromagnetic quantum. Some nuclei produce a beta particle, which is just an ordinary electron. It was in these last decays that the anomaly manifested itself.

The obvious scenario was that a parent nucleus decayed into a daughter nucleus and an electron. If energy and momentum are conserved in this decay, then the electron must emerge with one and only one energy. The problem was that experiment snowed that the emerging electron had a spectrum of energies. This was such a puzzle that Niels Bohr even proposed that energy and momentum were not conserved in the decay. Pauli thought that this idea was nonsense, and in his letter he made a counterproposal. He suggested that an invisible third particle was emitted with the other two and that this particle carried off some of the energy and momentum. The particle, he concluded, was invisible since it was electrically neutral and interacted very weakly with anything. It simply departed from the scene of the decay.

I have no idea what the “radioactive ladies and gentlemen” made of this suggestion. How seriously Pauli took his suggestion is unclear. He never published it. But Enrico Fermi in Rome took it seriously and created the first real theory of beta decay. Pauli called the particle the neutron, but in 1932 James Chadwick discovered what we call the neutron—the electrically neutral component of the atomic nucleus. Fermi noted that neutrone means “big neutral one” in Italian and since this particle, if it existed, had a small mass, he called it the neutrino, “the little neutral one”. The name stuck.

When I first learned about it in the early 1950s, the neutrino had an odd role in nuclear physics, like that of a sort of crazy uncle who was not all there. This changed thanks to the nuclear reactors that Fermi had created during the war. These reactors are factories for producing radioactive fission fragments that beta decay and produce an almost unbelievable flux of neutrinos. In 1956 Los Alamos physicists Clyde Cowan and Fred Reines observed a flux of more than 10,000 billion neutrinos per square centimeter per second at the Savannah River plant in South Carolina. Pauli was still around, and one can imagine his feelings. Now neutrino experiments are commonplace. And we know that there are three distinct kinds and that they are all massive. This means that they move at speeds close to that of light. Recent claims, much disputed, say that they actually move faster than light, which contradicts Einstein’s relativity. “Dear Radioactive Ladies and Gentlemen” indeed.

The Pi Meson

In 1909 Ernest Rutherford and two young colleagues in Manchester discovered the atomic nucleus in which most of the mass of the atom was concentrated. This led naturally to the question of how it was composed and what held its parts together. It was clear that there must be positive charges in the nucleus. This was because the atom, as one usually encountered it, was electrically neutral. Negatively charged electrons were somehow distributed with the nuclear material and these charges must be balanced for the atom to be electrically neutral. But it became clear that the positively charged objects—protons they were called—could not be the whole story. Electrically neutral objects were needed to account for the mass. Rutherford made the sensible suggestion that these must be electrons and protons bound together, but by 1930 Pauli and others argued that such a composite did not fit the atomic spectra data. The matter was resolved in 1932 when Chadwick discovered the neutron. But what held the nucleus together?

The nucleus is tens of thousands of times smaller then the average distance to the closest electrons. Electrons engage in the chemical business of the atoms while the nucleus is a bystander. This configuration shows incidentally that the force of attraction between the electrons and the protons in the nucleus must act over a long range. But the protons repel each other electrically. If there were not a counteracting force the nuclei would come apart. This counteracting force must be much stronger than the electric force and must be very short range. This was the puzzle. Enter Hideki Yukawa.

Yukawa was born in Tokyo in 1907. His father was a geology professor at the Imperial University in Kyoto where Yukawa studied. He finished his Ph.D. there in 1929, just as the depression began in Japan, and became an unpaid assistant to a professor of theoretical physics while continuing to live with his family. Journals allowed him to keep up with the rapidly developing quantum theory. Werner Heisenberg in 1932 wrote three papers on nuclear forces. Like Chadwick, Heisenberg believed that the neutron was a bound electron and proton, meaning that there would be electrons in the nucleus. Yukawa believed that the neutrons were elementary particles and he used this idea to create his own theory of nuclear forces.

The clearest description of what he did can be understood with help from diagrams invented by Richard Feynman and shown here on page 150. I begin with the force that causes two electrons to interact. In the quantum electrodynamic picture this force is caused by the exchange of photons where two electrons exchange a “virtual” photon. It is virtual in the sense that it is not detected directly in this process.

The mathematics associated with Figure 4 leads to the same force between the two electrons that the 18th-century French physicist Charles-Augustin de Coulomb claimed acted between charged bodies. In this case, the strength of the force declines as the square of the distance between the two charged bodies grows. This change is related to the fact that the particle being exchanged—the photon—has zero mass. For many reasons this force cannot be what holds the nucleus together. It is too weak and is the reason the protons repel each other. Yukawa replaced the electrons with what he called a “heavy quantum.” That freed him to take the coupling of the heavy quantum to the neutrons and protons at any strength he liked so he could choose it to overpower the electric force. Moreover he could choose any mass he wanted. Quantum mechanics tells us that the range of the force decreases the larger the mass of this quantum. Knowing the size of the nuclei enabled Yukawa to guess that the mass of the heavy quantum had to be about 200 times that of the electron. In 1934 he wrote a paper in excellent English, which was published in a distinguished Japanese journal the next year and then was totally ignored.

The Yukawa particle was found in cosmic rays in 1947, and it became known as the pi meson. It came in three varieties with positive, negative and zero charges. It had about the mass that Yukawa had predicted. The charged varieties decayed rapidly into mu mesons, heavy electrons and neutrinos. In 1949, Yukawa became the first Japanese man to win the Nobel Prize. Edward Teller, who is not generally known for his light verse, nicely summed up the sense of surprise at the discovery in this bit of doggerel.

There are mesons pi, and there
are mesons mu.
The former ones serve us as
nuclear glue.
There are mesons tau—or so we
suspect –
And many more mesons which
we can’t yet detect.
Can’t you see them at all?
Well, hardly at all,
For their lifetimes are short
And their ranges are small.
The mass may be small, and the
mass may be large.
We may find a positive or
negative charge.
And some mesons will never
show on a plate,
For their charge is zero, though
their mass is quite great.
What, no charge at all?
No, no charge at all.
Or if Blackett is right,
It’s exceedingly small.

The Antiproton

In 1928 Paul Dirac produced an equation for the electron that united the quantum theory with Einstein’s relativity. Some people think it is the most beautiful equation in physics. I’m not sure I would go that far but it is a very beautiful equation.

Dirac at once recognized there were problems with its solutions. For each value of the momentum of the electron there were four solutions and only two made sense. The other two gave the electron a negative energy, which was impossible. This led to a couple of years of what Pauli called “desperation physics.” If the electron had a negative charge and was represented by two of the positive energy solutions, then the other two solutions represented a particle identical to the electron but with the opposite charge. This introduced the physics of antimatter. But at the time that this solution was arrived at, no such antielectron was known. The matter was resolved when Caltech physicist Carl Anderson found one in cosmic rays in 1933. It became known as the positron.

Once the positron was found, the whole subject of antimatter was taken seriously: Every particle had an antiparticle. In a few cases such as the photon and the electrically neutral pi meson, the particles and antiparticles are identical. Neutrinos have no charge, but it is an open question as to whether they are identical to their antiparticles. But if a particle had a charge e then the antiparticle would have a charge -e. If a particle had a mass m, then the antiparticle would have the same mass. In particular the proton had a mass about 2,000 times that of the electron and a charge of e. Thus there had to be an antiproton with the same mass and the opposite charge. How to find it?

Cosmic rays were not promising because the antiproton annihilates when it comes in contact with ordinary matter and what we see is the detritus. A sufficiently powerful accelerator was needed to produce the antiproton. The idea was to generate an energetic beam of protons in the accelerator that would reach a target such as liquid hydrogen, whose nucleus is just one proton. In the collision, an antiproton would be produced. But how much energy would this machine need? Here was a complication You began with two protons—one in the beam and one in the target. Combined, these had two units of positive charge. You wanted to end up with an antiproton, which has one unit of negative charge. To balance the charges you need a final state with three protons and one antproton. If I call the antiproton jf and the proton ? then the reaction in question is: p+p->p+p+p+ff. There is no simpler reaction. But what is the rrvinimum proton beam energy needed? A straightforward calculation shows that this energy is about six times the mass of the proton. When this calculation was made there was no accelerator in the world that had beams of this energy.

Planning for such an accelerator—called the “Bevatron” for the energy it was to produce—began in the late 1940s at the University of California at Berkeley, home to Ernest Lawrence, the inventor of the cyclotron. Not only was Lawrence an excellent physicist, he was a genius for raising the kind of money needed to build such a machine. In 1955 the team of experimenters announced that the antiproton had been found with the accelerator. I don’t think that this came as a surprise to anyone, but it opened up a good deal of popular fantasy about antimatter.

This was neatly summarized by a 1956 poem in the New Yorker which was only signed by the initials H.P.F. I knew that the initials stood for Harold Fürth, who was a physicist and a classmate of mine from Harvard where he wrote for the Lampoon. The poem:

The Perils of Modern Living

Well up above the tropostrata
There is a region stark and
stellar Where, on a streak of
Lived Dr. Edward Anti-Teller.
Remote from Fusion’s origin,
He lived unguessed and
With all his antikith and kin,
And kept macassars on his
One morning, idling by the sea,
He spied a tin of monstrous girth
That bore three letters: A. E. C.
Out stepped a visitor from Earth.
Then, shouting gladly o’er the
Met two who in their alien ways
Were like as lentils.
Their right hands
Clasped, and the rest was
gamma rays.

I do not know where the “tropostrata” is, but I do know that you can find a profusion of antiprotons in the inner Van Allen Belt. The Van Allen Belts are regions above the Earth where charged particles are trapped in the Earth’s magnetic field. The inner belt extends from about 60 to about 6,000 miles above the Earth’s surface. Very high-energy protons get trapped there, and they can make antiprotons using the same reaction that the Bevatron did. There are not enough of them to constitute a menace to space travel.

The Quark

Three quarks for Muster Mark!
Sure he hasn’t got much of a
And sure any he has it’s all
beside the mark.
(James Joyce, Finnegans Wake)

In March of 1963 I attended a lecture that Murray Gell-Mann gave at Columbia University on a system he had recently invented for classifying elementary particles. It was somewhat playfully called The Eight-Fold Way. The Buddha had taught that a noble eightfold path would lead to a cessation of suffering. The path included “right speech,” “right intention” as well as “right concentration.” Gell-Mann’s route to the cessation of suffering was the adoption of a model for classifying the bewildering shower of elementary particles that no one had predicted or understood. The number eight played an essential role. Despite their apparent differences the particles seemed to be classifiable into multiplets, subsets of particles that had predictable characteristics. One of the first multiplets to be identified was an octet of mesons that included the pi mesons with new mesons that had recently been discovered.

At the lecture, Robert Serber, a very profound but generally quite reserved senior physicist at Columbia, asked a question. Serber had studied Gell-Mann’s paper on The Eight-Fold Way and had noticed that the simplest representation that arose from the theory was a multiplet that consisted of just three particles. Had Gell-Mann considered that? He had, but it led to a conundrum. Gell-Mann said something about it and then adjourned with some of us for a Chinese lunch where there was a furious and very fast-paced discussion.

Let us suppose that we have such a triplet, the conversation went. The object would be to use the three particles as building blocks out of which to construct all the elementary particles. Such an activity had already been tried. Fermi, for example, imagined that the pi mesons were composites of the neutron and proton and their antiparticles. The negatively charged pi meson was to be a composite of a neutron and an antiproton. Once the strange particles began appearing it was clear that this model had to be generalized.

The Japanese physicist Soichi Sakata had introduced a model in which all the particles were to be made up of three particles that were thought somehow to be more elementary. These were the neutron, the proton and a newly discovered strange particle that was called the ?0. Although it was possible to make all the known particles out of these three, the scheme was ultimately unsatisfactory. It was not clear why these three chosen particles were any more elementary than anything else. Further, the model predicted new particles that were never found. And it was not compatible with The Eight-Fold Way, which seemed to work. Hence Gell-Mann had to start from scratch.

There was no reason The Eight-Fold Way triplet had to be any of the known particles. Indeed, as the Sakata model showed, it was better if they were not. So Gell-Mann introduced three hypothetical particles that he called up (u), doom (d) and strange (s). Now we can begin constructing the known particles. The proton is for example uud while the neutron is udd. The Λ0 is uds. One can at once see a problem or at least a puzzle. What electric charges should these particles have? All elementary particles that had ever been observed had charges that are in magnitude integer multiples of the charge of the electron. For the proton the integer is 1 while for the neutron it is 0. If you try to do this for the various composites made out of these hypothetical building blocks, it does not work. Suppose for example you give the u a unit positive charge. Then to get the proton charge from uud you must give the d a negative 1 charge. But that gives the neutron udd the wrong charge. Integer charges simply don’t work. What to do?

Gell-Mann was driven to try something that was either crazy or very audacious. He assigned these particles fractional charges! The u was assigned a charge of 2/3, the d, minus 1/3 and the s, minus 1/3. The model worked but at the cost of introducing a type of particle that had never been observed. And where were the particles? They should stand out like a sore thumb. Soon after, a worldwide search began.

In the meantime Gell-Mann selected a name. While first working on these objects, he came up with a sound related to his avid bird watching. It was something like “quack.” He also had a considerable interest in linguistics and the kind of wordplay found in Finnegans Wake. While trolling through the book he hit on the sentence “Three quarks for Muster Mark,” quacks became “quarks” and a new term was introduced. (“Quark” it turns out is also the name for a rather smelly cheese made from sour milk.)

It fairly rapidly became clear to physicists that free quarks were not going to be found. Making a virtue out of necessity, they invented a dynamics that would permanently confine quarks within particles. In this scenario quarks exchange particles called gluons. This gluon dynamics has a very special characteristic. I pointed out previously that the forces that we are familiar with—the electric, the strong and even the gravitational—fall off with the distance that separates objects. In the case of the strong pion force, this decline is very rapid indeed. But the gluon dynamics produce a force that has just the opposite property. It is more like stretching a rubber band where the restoring force becomes greater the more you stretch. Of course you can stretch a rubber band enough so that it breaks. Not quarks. The confining force simply gets stronger. There is no escape. Quarks are imprisoned forever.

For some, this raised the question: In what sense can one say that quarks “exist?” When I heard this discussion I was immediately reminded of similar discussions at the end of the 19th and beginning of the 20th century as to whether atoms existed. They had not been observed either. Einstein, who was a witness to all of this, put the matter very well. There were two kinds of atoms—the physicist’s atom and the chemist’s atom, he said. The chemist’s atom is a visualizing symbol. You can make a little diagram for H2O without knowing anything about the masses or sizes of the hydrogen and oxygen atoms. The diagram is a kind of bookkeeping device. But physicists wanted to know the precise masses and sizes of atoms. A chemist might well argue that atoms “existed” while a physicist might have reservations. With quarks it was worse. The free quark was in principle unobservable.

In the 1960s a new accelerator, the Stanford Linear Collider, was built. Unlike a cyclotron in which particles are circulated in circles, in the new machine, electrons were accelerated to an enormous energy down a two-mile straight path. Electrons collided with protons. Most physicists would have guessed that the electrons would go more or less straight through the protons. But this is not what happened. The results of the experiments showed that the electrons had struck hard objects inside the proton. Moreover these objects had the quark charges. It had become hard to argue that quarks do not exist.

I would be remiss if I did not fill in some of what has happened since this discovery. We now know that there are six different “flavors” of quarks that have been observed: up, down, strange, charmed, bottom and top. They all are fractionally charged and have masses that range from a tiny fraction of the proton mass to nearly 200 times that mass. But the situation is still more complex. It was understood from quantum mechanics that you could not have, say, two identical quarks in the makeup of a particle. This is an example of what is called the Pauli exclusion principle. The proton is made of the uud quarks. This is all right because the two us can be put in different angular momentum states. But in 1964 a wonderful particle called the O- was discovered at the Brookhaven National Laboratory. It had been predicted as a completion of an Eight-Fold Way decuplet. But its quark composition was three s quarks, and this Pauli did not allow. The theorists invented a new property that is usually called color to make distinct types of quarks. Color is really a metaphor for some generic distinction. The colors that are chosen usually reflect the nationality of the physicist in question. American physicists like red, white and blue. Hence when you write that the O- consists of three s quarks you assume that you have chosen them to have different colors. If you think all of this is complicated you should look at a list of the elementary particles. That they are all composed of such a limited number of objects is truly remarkable.

The Higgs Boson

It was understood from the beginning that The Eight-Fold Way was only an approximate symmetry. This is evident from the masses of the particles. The pi mesons are placed in an octet, which also includes the strange K mesons. The mass of the K meson is more than three times the pi’s. If the symmetry was exact all the particles in the octet would have the same mass. Indeed when you look at the properties of these particles it is not clear at all that they have anything to do with each other. That is why finding the symmetry that unites them was so difficult.

The kind of symmetry breaking that is manifested in The Eight-Fold Way is as old as the quantum theory itself. It was brought into the theory by the mathematician Hermann Weyl and the physicist Eugene Wigner. Wigner applied it to atomic spectra. The electrons outside the nucleus of atoms are located on “orbits.” The quantization of these orbits was the great work of Niels Bohr. If the atom is not disturbed the electrons find their way to the orbits of the least energy—the “ground states.” But if heat or an electric shock excites the atom, the electrons jump to higher orbits. Once the disturbance is over they go back to the ground state. But as they do radiation is emitted. This emission produces beautiful and characteristic atomic spectra. The atomic spectrum is like a fingerprint that identifies an element. That is how helium was first discovered in the atmosphere of the Sun. These lines reflect the symmetry of the theory that describes these energy levels. But putting the atoms in a magnetic field can compromise this symmetry, breaking it in a well-defined way. As a consequence, what was a single spectral line becomes a multiplet as Figure 7 illustrates.

The number and intensity of these lines can be predicted. They reflect the symmetry that was broken. This kind of Wigner-Weyl symmetry breaking is at play in The Eight-Fold Way. One adds a perturbation that breaks the symmetry so the multiplet masses get split. If one does this with tact then the resulting masses are related. These relations hold empirically, which produces further confidence in the scheme.

I learned about the Wigner-Weyl symmetry breaking in the late 1950s, while studying quantum mechanics. But in 1961 Yoichiro Nambu and his student Giovanni Jona-Lasinio published two papers that changed everything. This was an entirely different kind of symmetry breaking. In his 2008 Nobel Lecture Nambu gave an example.

Suppose you have a straight rod that is elastic and you stand it vertically. It looks the same from any horizontal direction. The symmetry is perfect. But if you squeeze it down, it will bend in some direction and the symmetry is gone. An example that I like is to take the same rod and balance it on a point. Quantum mechanics tells us that this balance can never be perfect. There is always some uncertainty in the angle it makes with the ground. Hence it will tip over but equally likely fall in any direction. Once it does this, the symmetry is gone. Putting the matter somewhat more abstractly, equations can show symmetries but one is not forced to choose solutions that respect these symmetries. This kind of symmetry breaking is called “spontaneous” symmetry breaking to distinguish it from the Wigner-Weyl type where a symmetry-breaking term is added to the equations. Nothing here is added.

Nambu has wide influence in physics. He has applied his idea to magnets, crystals and superconductors as well as to the theory of elementary particles. When he studied a model of the latter, an uninvited guest appeared. In the mid- 1920s an Indian physicist named Satyendra Nath Bose (pronounced “bosh”) introduced a kind of statistics that applied to particles such as the pi meson and the K meson. Bose applied it to the photon. He sent his paper to Einstein who made some corrections and had it published. Particles of this kind are called bosons. The other class of particles, which include the electron, the proton and the neutrino, are called fermions, after Fermi. The uninvited guest in Nambu’s model, was a boson of zero mass. At the time, it was known that the photon had zero mass and it was thought that neutrinos had zero mass, which they don’t. But no boson of the Nambu type was known and is not known to this day. At first it was thought that this might be an artifact of Nambu’s model but in 1961 the British-born physicist Jeffrey Goldstone found another model with particles that are now called Nambu-Goldstone bosons. The feeling grew that these particles were a blight that came with this spontaneous symmetry breaking and hence it might not have much relevance to the world of elementary particle physics.

The breakthrough came in 1964 from Walter Gilbert at Harvard University, then an associate professor of biophysics. He cofounded the biotechnology innovator Biogen and in 1980 won the Nobel Prize in Chemistry—a remarkable career. Goldstone’s and Nambu’s arguments were correct but they are not general enough. Gilbert analyzed the assumptions and noted that they did not apply to electrodynamics. This was a wonderfully liberating discovery and it was first fully exploited for elementary particles, at least in print, by the English physicist Peter Higgs. He considered a theory that began with a massless photon and some bosons that coupled to each other and the photon. This theory enjoys an apparent symmetry that is broken spontaneously. Something miraculous then happens. The massless photon acquires a mass and the theory acquires a massive boson, which is now universally called the Higgs boson. You have to see the mathematics to believe it.

The first person to take advantage of all of this in a substantial way was Steve Weinberg. To explain what he did, I’ll need to back up some. Looked at naively, there are four basic forces. The strong force is what holds the quarks in their particle confinements. It is mediated by the exchange of gluons. Next in strength is the electromagnetic, which is mediated by the exchange of photons. I will skip over the weak force for the moment and mention the weakest of them all, the gravitational. It is mediated by the exchange of gravitons. They have never been observed as free particles and some investigators have conjectured that they never can be. It may seem odd to think of gravitation as being the weakest since it is the force we deal with on a daily basis. That is because gravity acts between any masses and adds up. Thus the Earth holds us in place, which is just as well.

The weak force is responsible for reactions such as beta decay. Fermi tried to model it on electrodynamics with a big modification. In this theory the interaction was mediated in effect by a particle whose mass was infinite. The force had zero range, meaning that the particles involved had to be on top of each other. The theory explained a lot but seemed out of joint with the other forces that were mediated by particles with reasonable masses. Hence the physicists invented particles to mediate this interaction. They were supposed to be very heavy but with finite masses. There were three: two charged particles that were called W mesons—the “W” was for “weak”—and there was a neutral one that was called the Z for reasons unclear to me. Various properties were conjectured. Remarkably they were both observed in 1983 and found to have masses that were something like 90 times the proton mass. Physicists had assumed they existed and constructed theories with them for many years. But these theories had an issue. They produced infinities that could not be gotten rid of using the technique that had tamed the infinities in quantum electrodynanties—“renormalization” as it was called. This produced a good deal of desperation physics too. But then came Weinberg.

In Weinberg’s electroweak theory you begin with a symmetric situation in which both the photons and the weak mesons have no mass. They interact with bosons along the lines of the Higgs model. The symmetry is spontaneously broken and the same miracle happens. The weak mesons acquire a mass and the photon does not. The Higgs boson acquires a mass. It all fits together like a Swiss watch. Moreover, the dreadful infinities are tamed. It is all quite wonderful but it leaves open a question: Where is the Higgs boson?

Never have so many physicists spent so much time and money in such a quest. We have come a long way since Rutherford, who discovered the atomic nucleus in an experiment that more or less fit on a tabletop. Three accelerators have been in the hunt—two of them at the European Organization for Nuclear Research (CERN). The oldest of these CERN accelerators—the LEP—is decommissioned. Prior to 2000 it had shown that the Higgs could not have a mass less than about a 120 proton masses. It also found no credible evidence for the particle. Experiments at the Tevatron at the Fermi Lab near Chicago, since decommissioned due to lack of funds, pushed up the limit by a few percent. Hence we know how tight it can be. Theorists have presented convincing arguments as to how heavy it can be. If all of this is correct, we have the Higgs confined to a very narrow range of mass. This mass is accessible to the Large Hadron Collider at CERN. There are hints but we await the results.

My feelings are mixed. If they find the Higgs boson, it will end a chapter in physics. I am reminded of a French colleague. He had had a “parameter” named after him, much discussed because theory explaining weak interactions hinged on it. Finally it was measured and the theory was confirmed. When I went to congratulate him, I found him unhappy “because now they will speak of it no more.” If they don’t find the Higgs, this will be exciting because new physics is needed.

I doubt many of my colleagues feel this way. They remind me of the story about Einstein after he had received a telegram with the news that the eclipse expeditions had confirmed his general relativity prediction about the Sun bending starlight. He was very pleased and showed the telegram to one of his students, Ilse Rosenthal Schneider. She asked what he would have done if the telegram had carried the opposite news. “Da könnt’ mir halt der lieber Gott leid tun-die Theorie stimmt doch,” he replied. Or, in English: “Then I would have been sorry for the dear Lord. The theory is right.”