Beyond the God Particle (9 page)

Almost all pure physics research was interrupted by World War II, as the world's scientists were redirected to serve military needs. The quest for Yukawa's pions could resume only after the war. In the meantime humans had conquered the atomic nucleus, with its strong force—and unleashed its fury.
¹⁵

In 1947, the pions, predicted by Hideki Yukawa to explain the strong nuclear force, were finally discovered by using cosmic rays. This vindicated Yukawa's ingenious theory, for which he later won the Nobel Prize in 1949.

The pions arrived in three types, distinguished by their electric charges,
π
⁺
,
π
^–
, and
π
⁰
, where the superscript refers to the particle's electric charge. We often refer to
π
⁺
and
π
^–
as the “charged pions” and
π
⁰
as the “neutral pion.”
¹
The strong force, which welds the protons and neutrons together to build the atomic nucleus, arises as Yukawa had theorized through the “exchange of pions,” hopping back and forth between protons and neutrons as quantum fluctuations. The picture of the atom and its nucleus was now complete.

Soon there would be particle accelerators, and numerous “elementary particles” emerged from experiments. Most of the multitude of new objects were strongly interacting, that is, they “felt” the strong force, and they interacted strongly with the pions, protons, and neutrons. It was also discovered that the proton, the neutron, the pions, and the long list of new strongly interacting particles, were not point-like objects but actually had finite sizes of about a hundredth of a trillionth of a centimeter. In the extreme cases some new particles were discovered that had lifetimes as short as the time it took light to transit their finite diameters. The nascent world of particle physics was never more confusing and chaotic than in the 1950s as the first higher-energy particle accelerators came online.

Throughout this time, the poor muon seemed to be an oddball, an almost uninvited guest at the dinner table.
²
The muon has a mass about 200 times that of the electron. It “decays” (through the weak interaction) into an electron and two very difficult to observe particles called neutrinos, living a mere two millionths of a second when at rest in the laboratory. Otherwise, the muon seemed to play no particular role in anything else, pointless in the fabric of nature. Its serendipitous and seemingly random appearance had elicited the famous quip by I. I. Rabi, “Who ordered that?”
³

Figure 3.3. The Atom, Pion Exchange, and the Atomic Nucleus.
An atom consists of the cloudlike motion of electrons about a dense nucleus containing protons and neutrons. The nucleus is held together by the exchange of pions that hop back and forth between the neutrons and protons.

We could retrace the long and winding road taken by particle physics from 1947 onward. There followed the era of the 1950s and 1960s when powerful new accelerators and various national laboratories came along. At one time there was a new energy frontier particle accelerator every few years or so, and a plethora of new “particles” and particle phenomena were discovered. Yes, we could stroll down memory lane and recount all of the history and structure of the Standard Model. Your eyes might glaze over, your eyelids becoming heavy. Rather, we'd like to veer off that traditional litany and do something a little different. We want to hop, skip, and jump to the Higgs boson as quickly as we can, to actually delve in and try to explain it to you in a way that is as close as we can get to how physicists understand it.

Indeed, by the time physicists understood the details of the forces of nature, in particular the “radioactive” transmutations between these particles that involve the “weak force,” it was soon realized that some kind of “Higgs boson” was a necessity. This realization mainly came from the work of one of the architects of the Standard Model,
⁴
a theorist named Steven Weinberg (see chap. 1,
note 14
). There was no other way to make particles behave the way they do, and simultaneously to have mass, without something like a “Higgs mechanism.” Remarkably, one of the key ingredients to this revelation, the ingredient that mandated the theoretical existence of a Higgs boson, was revealed by the lowly muon (in concert with the charged pion, which also decays through the weak force into a muon and a neutrino). The pion and muon decays provided the major clue about the weak forces of elementary particles that would lead directly to the Higgs boson. It was in an almost incidental way that the muon revealed the essential aspect of the weak force that ultimately legislates the Higgs boson into existence. The unexpected and uninvited guest at our table, the muon, was actually a gift—perhaps this is the answer to Rabi's question as to why the muon was “ordered.”

We also think that in the not-too-distant future humans will use the muon as a powerful practical tool, much like we use everything we discover in nature. In fact, muons are already providing themselves as new diagnostic tools that scientists use to study nuclear and atomic processes. In some quarters there is a fervent albeit long-shot hope that the muon might ultimately provide the catalyst needed to unleash the ultimate energy source—nuclear
fusion—through a process known as “muon-catalyzed fusion.”
⁵
And we believe our favorite laboratory, Fermilab (or some other laboratory in another country, if the US government doesn't get its act together), will someday build a new type of high-energy particle collider—one that will literally rank as the most sophisticated
thing
humans ever built—a machine that collides muons—
the
Muon Collider
.
⁶
There are many reasons why this is very good ideas—so let's now pop open some champagne and celebrate…

THE LOWLY MUON

As we've seen, people were expecting to observe the pion, with a mass of about 100 MeV, to confirm Yukawa's theory that explained the strong force between the neutron and proton and that holds together the atomic nucleus. Instead, Carl Anderson and Seth Neddermeyer, at Caltech in 1936, found the muon in the debris of cosmic rays that bombard the surface of the earth.
⁷
Muons are produced in the upper atmosphere when primary, very energetic cosmic rays from outer space collide with nuclei of atoms in the thin air. But even that story has a peculiar twist.

How are muons produced in these collisions? Remarkably, we know
today
that pions are, indeed, immediately produced by the cosmic rays hitting atomic nuclei in atoms of nitrogen and oxygen in the earth's upper atmosphere. This happens because the pions are
strongly coupled
to nuclei (they are the glue that holds nuclei together, after all), and the cosmic rays are essentially protons colliding with other protons and neutrons in the nuclei of atoms in the atmosphere—at the high energies of cosmic rays, this process readily ejects pions. The pions then rapidly decay into muons (and neutrinos) within about a hundredth of a millionth of a second.

Now, think of what a bizarre situation that is—Yukawa had theoretically predicted the pion, but no one had seen the pion, and no one would see it until 1947. Anderson and Neddermeyer found the surprising new particle—the muon—coming from the cosmic rays, and it had almost the exact mass that Yukawa predicted for the pion. The muon confused the heck out of everyone, as it turned out not to be the pion. But the muons Anderson and Neddermeyer had observed, unbeknownst to them,
were the by-product of pions that are readily produced (and rapidly decay into muons) at very high altitudes
in the atmosphere!
⁸

Now, the very fact that a muon can make it to the surface of the earth is, in part, a miracle of Albert Einstein's theory of relativity. We know that a muon is unstable, and it almost always decays within about two millionths of second (into an electron plus two neutrinos). This is actually, approximately, its “half-life”—after about two millionths of a second, there'll be about half as many muons as you started with, then in another two millionths of a second, a quarter as many, then an eighth, and so on.
⁹
Muons are produced about ten to twenty miles up in the atmosphere in cosmic ray collisions. So a simple calculation shows that if a muon traveled as fast as possible, at the speed of light c, then in t = 2 millionths of second, it would only travel about ct = 0.6 kilometers, less than a half mile. So, we wouldn't expect many muons to make it to the surface of the earth before they had decayed.

But Einstein told us that, for particles approaching the speed of light,
time slows down
. The slowing of time is observed by we who are sitting at rest on the ground watching the high-speed muons. The amount of slowing down of time that we observe is the amount by which the lifetime of a muon will be lengthened due its traveling near the speed of light. This effect, called “time dilation,” is easily computed: we take the energy of the muon and divide by its mass (times the speed of light squared, that is, we divide by mc
²
). So, if a muon has an energy that is 20 times its own rest mass energy, mc
²
, then it will have its lifetime extended by a factor of 20. With enough “lifetime-extending energy,” the high-energy muons can easily reach the surface of the earth ten miles below. The arrival of muons at the earth's surface is one of the many stunning confirmations of Einstein's theory of relativity. Kooks and others who want to challenge and demolish Einstein's theory of relativity, please take note: Relativity is a prime example of a “theory” that has become “fact”!

Alas, the pion cannot travel far enough to arrive at the surface of the earth to be detected. It too is unstable, but it decays in a mere one hundredth of a millionth of a second—that's a hundred times shorter lifetime than that of a muon. We would need super-energetic pions to be produced with sufficient energy to lengthen their lifetimes by 2,000 times to get them to the surface of the earth—the effects of relativity simply aren't enough to help cosmic ray pions get down to the earth's surface.

And, there's another big difference between pions and the muons that prohibits the former from making it down to the earth's surface. For the very reason that it holds the atomic nucleus together, a pion interacts very strongly with protons and neutrons. This means that when a cosmic ray makes a pion in the thin upper atmosphere, the pion is quickly reabsorbed by protons and neutrons in further collisions with atoms of nitrogen or oxygen. This, more than anything, cuts off the number of pions in cosmic rays observed at sea level. You have to go way up into the atmosphere to detect them.

Nonetheless, in 1947 the charged pions, produced by cosmic rays, were finally found by a collaboration of scientists at the University of Bristol in England.
¹⁰
Photographic plates were placed for long periods of time at high altitudes on mountains, first at Pic du Midi de Bigorre in the Pyrenees and later at Chacaltaya in the Andes Mountains. Here the photographic plates were directly hit by the primary cosmic rays, and after development, the plates were inspected under microscopes. This revealed the tracks of electrically charged particles. Pions were first identified by unusual double tracks, where one incoming track would suddenly shift direction into another outgoing track. The scientists were actually seeing the charged pion as it decayed into the muon, plus an invisible neutrino.