Beyond the God Particle (41 page)

The remaining particles comprising the vast list of strongly interacting particles were called the “hadrons” or “strong ones.” All strongly interacting particles were found to have a finite nonzero “size” of about a hundredth of a thousandth of a billionth of a centimeter (0.2 × 10
^–13
cm). Various patterns began to emerge within that class of particles. Indeed, the patterns began to hint that hadrons are actually composed of smaller, more elementary objects deep down within another stratum that could not yet be resolved with the existing accelerator probes.

For a long time there was considerable resistance to the idea of any further substructure within the hadrons. No matter how high an energy probe struck the proton, it was not possible to “smash it into smithereens.” All that happened was that other short-lived hadrons were produced in these collisions, and you ended up back with the original proton (or a neutron or pions) you started with. Evidently Democritus's idea of fundamental underlying “atoms” was breaking down with the discovery and properties of the hadrons. Very novel and Zen ideas emerged—perhaps hadrons are composed of
each other
in such a way that none is truly fundamental and yet all are? It was as if the world of hadrons were an Escher staircase, eternally going uphill, only to return again to the first step.

Connected to this idea was the notion that hadrons are not made of point-like objects but are more like the consistency of putty—deformable
and malleable rather than point-like and hard. One of the most intriguing patterns among these objects could be explained if it was assumed that, as the putty rotates rapidly, it becomes drawn out into a kind of putty “string.” Various quantum modes of motion of this “string” were studied, and it seemed to make sense—all of the hadrons could be explained as putty strings, and many of their properties were predicted and emergent from the idea. Thus was born, in attempting to explain hadrons, a new type of dynamical quantum theory, the string theory.

THE STRATUM OF QUARKS AND LEPTONS

But the long list of “too many strongly interacting particles” led some physicists, most notably Murray Gell-Mann and George Zweig of the California Institute of Technology, to assert that these were not fundamental. The long list of hadrons had certain patterns, like the recurring chemical properties of atoms, and hinted at the existence of yet another layer of the physics onion. Yet there was a serious problem with the idea of another stratum of nature—whatever comprised the strongly interacting particles could never be set free from the particles they composed by any experiment. Even the most powerful of particle accelerators, producing the most violent collisions, never liberated any of the hadronic innards, and instead simply produced more and more of the unstable hadrons.

Nonetheless, for a particular theoretical next layer of constituency of matter, whether real or purely mathematical, Gell-Mann introduced the term “quark.”
¹
In the early 1970s, through the theoretical insights of James Bjorken,
²
the first “photograph” of the inner world of the proton was taken at the Stanford Linear Accelerator by scattering very energetic electrons off of protons, a process known as “deep inelastic scattering.” For the first time, the constituents of hadrons—the quarks—were seen. It was also observed that half of the constituents of the hadrons were something else—a mysterious electrically neutral component of these particles was detected. Could this be the “glue” that holds the quarks inside?

Initially, almost comically, the theoretical force carriers that bind quarks within hadrons were dubbed “gluons.” Soon, however, by making a profound analogy with electric and magnetic forces generated by photons,
a real theory of quarks and gluons, called “quantum chromodynamics” (QCD for short; QCD is a Yang–Mills gauge theory) took hold. Gluons joined the panoply of elementary particles and entered the list of bosons, like the photon and graviton. Gluons, indeed, generate the force that holds the quarks inside the strongly interacting particles.
³

FIGURE A.35. Table of Quarks and Leptons.
This exhibits the “generation structure” of the matter particles, by which a pair of “up”-type and “down”-type colored quarks fit together with a pair of “electron” and “neutrino”-type leptons.
⁴
In addition, there are the antiparticles, required by special relativity. Antiparticles have opposite electric charges and anti-colors, hence the blue quark has an antiparticle that is “anti-blue,” which acts like a combination of red and yellow. The neutrinos have extremely tiny masses, expected to be less than about 2 electron volts.

As of today we have built many particle accelerators, some so powerful that we can clearly see the quarks and gluons deep inside the hadrons, like the nucleus inside the atom or the DNA inside of a living cell. The gluon force is not, however, like anything we have seen before. Unlike familiar electromagnetism, the gluonic force doesn't fall off like the inverse square law between two separated electric charges but is rather a constant force as we try to separate the quarks. This behavior ends up forbidding us from ever isolating the quarks. Quarks are confined forever inside of hadrons. In fact, the gluon force, when we rapidly rotate a hadron, becomes the putty-like string.

TODAY: THE PATTERNS OF QUARKS, LEPTONS, AND BOSONS

The elementary constituents of the hadrons are the quarks and gluons. Quarks and gluons are real, and their properties are measured, but they can never be set free from the prisons of the hadrons that they comprise. With quarks and gluons a more Democritus-styled explanation of the hadrons took hold, and this is the view that we have of them today. Quarks, like their sisters the leptons, are point-like and structureless matter particles.

We often refer to the quarks and leptons as “the matter particles.” Each of these particles is a tiny gyroscope, each has spin 1/2 (see “
Spin
” below), in accordance with the rules of quantum mechanics. All the everyday matter in our world is essentially composed of the two quarks, the
up
and
down
(and
gluons
), and the one lepton, the
electron
. These quarks are distinguished by their electric charges and their masses. We always define the electron to have an electric charge of –1. In these units, the up quark (
u
) has an electric charge of +2/3, and the down quark (
d
) an electric charge of –1/3. The proton is therefore not an elementary particle but is rather a composite particle, built of three quarks in the pattern
u
+
u
+
d
(or
uud
). Adding up the electric charges of the constituent quarks, we see that the proton charge is +2/3 + 2/3 – 1/3 = +1. Similarly, the neutron is composed of
u
+
d
+
d
, and the corresponding electric charge combination is +2/3 – 1/3 – 1/3 = 0.

Every particle in nature has a corresponding antiparticle. This was Dirac's famous discovery based upon unifying quantum theory with special relativity. The antielectron is the
positron
and has electric charge +1 and the
same mass and spin as the electron. The antiquarks likewise have the opposite electric charges to their quark counterparts. We designate the anti-up quark as
, and it has an electric charge –2/3, while the anti-down is
, with electric charge +1/3.

The pions are composed of combinations of a quark and antiquark. We easily see that there are four possible quark-antiquark combinations involving
u
,
d
,
, and
, which are
d
(–1),
u
(0),
u
(+1),
d
(0). In quantum mechanics, neutral particle states often become “blended” (added together in particular ways), and the resulting composite particles are