Warped Passages (28 page)

It was clear that physicists needed to find a new type of interaction to account for nuclear processes such as beta decay, but it was not clear what this new interaction could possibly be. Before Glashow,
Weinberg, and Salam developed their theory of the weak force, Fermi made a stab at it with a theory that included new types of interaction involving four particles, such as the proton, neutron, electron, and neutrino. This
Fermi interaction
directly produced beta decay without invoking an intermediate weak gauge boson. In other words, the interaction permitted a proton to turn directly into its decay products—the neutron, electron, and neutrino.

However, it was clear, even at the time, that the Fermi theory could not be the true theory that would work at all energies. Although its predictions were correct for low energies, they were obviously completely wrong for high energies, at which particle interactions became much too strong. In fact, if you incorrectly assumed that you could apply the Fermi theory when the particles were highly energetic, you would get nonsensical predictions, such as particles that should interact with a probability greater than one. That’s impossible, since nothing can happen more often than always.

Although the theory based on the Fermi interaction was a fine effective theory for explaining interactions at low energies and between sufficiently distant particles, physicists saw that they needed a more fundamental explanation of processes such as beta decay if they were to know what happened at high energies. A theory based on forces communicated by weak gauge bosons looked as if it would work much better at high energies—but no one knew how to account for the weak force’s short range.

That short range turns out to be a consequence of nonzero masses for the weak gauge bosons. In particle physics the relationships implied by the uncertainty principle and special relativity have noticeable consequences. At the end of Chapter 6 I discussed the smallest distances at which a particle of a particular energy, such as the weak scale energy or the Planck scale energy, can be affected by forces. Because of the special relativity relation between energy and mass (
E
=
mc
²
), massive particles, such as the weak gauge bosons, automatically incorporate similar relationships between mass and distance.

In particular, the force communicated by the exchange of a particle with a given mass dies away over a larger distance when the mass is smaller. (That distance is also proportional to Planck’s constant and
inversely proportional to the speed of light.
^*
) The relationship between mass and distance given in Chapter 6 tells us that the weak gauge boson, whose mass is about 100 GeV, automatically transmits the weak force only to particles that lie within one ten thousand trillionth of a centimeter. Beyond this distance, the force conveyed by the particle becomes extremely small, too small to do anything we would ever detect.

The nonzero mass of the weak gauge boson is critical to the success of the weak force theory. The mass is the reason that the weak force acts only over very short distances and is so weak as to be almost nonexistent at longer distances. The weak gauge bosons are different in this respect from the photon and graviton, both of which are massless. Because the photon and the
graviton
, the particle that communicates the gravitational force, carry energy and momentum but have no mass, they can communicate forces across great distances.

The concept of massless particles might sound strange, but from the particle physics perspective it is nothing very remarkable. The masslessness of the particles tells us that they travel at the speed of light (after all, light is composed of massless photons), and also that energy and momentum always obey a particular relation: energy is proportional to momentum.

The carriers of the weak force, on the other hand, do have mass. And from the perspective of particle physics, a massive gauge boson—not a massless one—is the oddity. The key development that paved the way for the theory of the weak force was understanding the origin of the weak gauge boson masses, which make the distance dependence of the weak force so different from that of electromagnetism. The mechanism that gives rise to the weak gauge boson masses, known as the
Higgs mechanism
, is the subject of Chapter 10. As we will see in Chapter 12, the underlying theory—that is, the precise model that gives particles their mass—is one of the biggest puzzles facing particle
physicists today. One of the attractions of extra dimensions is that they might help solve this mystery.

Quarks and the Strong Force

A physicist friend once explained to one of my sisters that he worked on “the strong force which is called the strong force because it is so strong.” Although she did not find this particularly edifying, the strong force is in fact aptly named. It is an extremely powerful force. It binds together the constituents of the proton so powerfully that ordinarily they never separate. The strong force is only tangentially relevant to later parts of this book, but here I’ll give some basic facts about it for completeness.

The strong force, described by the theory called
quantum chromo-dynamics
(QCD), is the last of the Standard Model forces that we can explain with gauge boson exchange. It too was discovered only in the last century. The strong gauge bosons are known as gluons because they communicate the force, the “glue,” that binds strongly interacting particles together.

In the 1950s and 1960s, physicists discovered many particles in rapid succession. They gave the individual particles various Greek-letter names such as the ∏ pronounced “pion”), the (pronounced “eta”), and the ? (pronounced “Delta”—written with a capital “D” to reflect the case of the Greek letter). Collectively, these particles are called
hadrons
, after the Greek word
hadros
, which means “fat, heavy.”

Indeed, hadrons were all much more massive than the electron. They were mostly comparable in mass to the proton, which has 2,000 times the electron’s mass. The enormous multiplicity of hadrons was a mystery until the physicist Murray Gell-Mann
^*
suggested in the 1960s that the many hadrons were not fundamental particles but were instead themselves composed of particles that he named
quarks
.

Gell-Mann got the word “quark” from a poem in James Joyce’s
Finnegans Wake
: “Three quarks for Muster Mark! Sure he hasn’t got much of a bark. And sure any he has it’s all beside the mark.” This, so far as I can deduce, is pretty much unrelated to the physics of quarks except for two things: there were three of them, and they were difficult to understand.
^*

Gell-Mann proposed that there are three varieties of quark
^†
—they’re now called
up
,
down
, and
strange
—and that the numerous hadrons corresponded to the many possible combinations of quarks that could be bound together. If his proposal was correct, hadrons would have to fall neatly into predictable patterns. As was often the case when new physical principles are suggested, Gell-Mann did not actually believe in the existence of quarks when he first proposed them. Nonetheless, his proposal was quite daring since only some of the predicted hadrons had been discovered. It was therefore a major victory for him when the missing hadrons were found and the quark hypothesis was confirmed, paving the way for Gell-Mann’s 1969 Nobel Prize for Physics.

Even though physicists agreed that hadrons were made of quarks, nine years elapsed after the suggestion of quarks before hadron physics was explained in terms of the strong force. Paradoxically, the strong force was the last force to be understood, in part because of its enormous strength. We now know that the strong force is so large that the fundamental particles, such as quarks, that experience the strong force are always bound together and are difficult to isolate and therefore to study. Particles that experience the strong force are not free to roam unchaperoned.

There are three types of every quark variety. Physicists playfully label the different types with colors and sometimes call them red, green, and blue. And these colored quarks are always found with other quarks and antiquarks, bound together into
color-neutral combinations
. These are the combinations in which the strong force “charges” of the quarks and antiquarks cancel each other, analogously
to the way colors cancel in white light.
^*
There are two types of color-neutral combination. Stable hadronic configurations contain either a quark and an antiquark that team up with each other, or else three quarks (and no antiquarks) that bond among themselves. For example, a quark pairs with an antiquark in particles called pions, and three quarks bind together in the proton and the neutron.

The strong force “charge” cancels among the quarks in hadrons, much as the charge of the positively charged proton and the negatively charged electron cancel in an atom. But whereas you can readily ionize an atom, it is very difficult to pry apart the objects, such as the proton and neutron, that are bound extraordinarily tightly by the gluons of the strong force. Gluons would be more aptly named “crazygluons,”
^†
since their bonds are so difficult to break.

We are now almost ready to return to the discovery of quarks that Athena’s revisionist tale metaphorically described. The proton and neutron consist of combinations of three quarks in which the charge associated with the strong force cancels out. The proton contains two up quarks and one down quark—different types of quark with different electric charge. Because the up quark has electric charge +2/3 and the down quark has charge -1/3, the proton has electric charge +1.

A neutron, on the other hand, contains one up and two down quarks, so it has zero (the sum of -1/3, -1/3, and +2/3) electric charge.

Quarks can be thought of as hard, pointlike objects in a big, mushy proton. Quarks are embedded in a proton or neutron, like a pea buried under a mattress. But as with our bouncing princess who bruises herself on the pea, an active experimenter can shoot in a high-energy electron that emits a photon, which bounces directly off the quark. This looks very different from a photon bouncing off a big fluffy object, just as Rutherford’s alpha particle bouncing off a hard nucleus looked very different from one bouncing off more diffuse positive charge.

The Friedman-Kendall-Taylor
deep inelastic scattering
experiment,
conducted at the Stanford Linear Accelerator Center (SLAC), demonstrated the existence of quarks by registering this effect. The experiment showed how electrons behave when they scatter off protons, thereby providing the first experimental evidence that quarks really exist. For this discovery, Jerry Friedman and Henry Kendall (who were my colleagues at MIT) and Richard Taylor won the 1990 Nobel Prize for Physics.

When quarks are produced in high-energy collisions, they aren’t yet bound into hadrons, but that doesn’t mean they’re isolated—they will always have a retinue of other quarks and gluons accompanying them which make the net combination neutral under the strong force. Quarks never appear as free, unaccompanied objects but are always shielded by many other, strongly interacting particles. Instead of a single, isolated quark, a particle experiment would register a set of particles composed of quarks and gluons, going in more or less the same direction.

Collectively, the groups of particles composed of quarks and gluons that move in unison in a particular direction are known as
jets
. Once an energetic jet is formed, it is like a rope in that it will never disappear. When you cut a rope, all you do is create two new pieces of rope. Similarly, when interactions divide jets, the pieces can only form new jets: they will never separate into individual, isolated quarks and gluons. Stephen Sondheim was presumably not thinking about high-energy particle colliders when he wrote the lyrics to the Jet song from
West Side Story
, but his words apply admirably to jets of strongly interacting particles. Energetic, strongly interacting particles remain together. “They’re never alone…they’re well protected.”

The Known Fundamental Particles

This chapter has described three of the four known forces: electromagnetism, the weak force, and the strong force. Gravity, the remaining force, is so weak that it would not change particle physics predictions in an experimentally observable way.

But we have not yet finished introducing the particles of the Stan
dard Model. They are identified by their charges, and also by their handedness. As I described earlier, the left-and right-handed particles can (and do) have different weak charges.

Particle physicists categorize these particles as either quarks or
leptons
. Quarks are fundamental fermionic particles that experience the strong force. Leptons are fermionic particles that do not. Electrons and neutrinos are examples of leptons. The word “lepton” derives from the Greek
leptos
, which means “small” or “fine,” referring to the tiny mass of the electron.

The bizarre thing is that in addition to the particles that are essential to the structure of the atom, such as the electron and the up and down quarks, there are additional particles that, though heavier, have the same charges as the particles we have already introduced. All of the lightest stable quarks and leptons have heavier replicas. No one knows why they are there, or what they are good for.

When physicists first realized that the muon, a particle first seen in cosmic rays, was nothing other than a heavier version of the electron (200 times heavier), the physicist I.I. Rabi asked, “Who ordered that?” Although the muon is negatively charged, like the electron, it is heavier than the electron, into which it can decay. That is, a muon is unstable (see Figure 53) and quickly converts into an electron (and two neutrinos). So far as we know, it serves no purpose to matter here on Earth. Why does it exist? This is one of the many mysteries of the Standard Model that we hope scientific progress will solve.