Beyond the God Particle

by Leon M. Lederman

  It was a piece of cake to calculate the value of the magnetic field needed to turn the muons through 360 degrees in a reasonable time. What is a reasonable time for a muon? Well, the muons are decaying into electrons and neutrinos with a half-life of 2 microseconds. That is, half of the muons have given their all in 2 microseconds. If we turned the muons too slowly, say 1 degree per microsecond, most of the muons would have disappeared after being rotated through a few degrees and we wouldn't be able to compare the zero-degree and 180 degree yield—that is, the number of electrons emitted from the “front” of the muon as opposed to the “back,” the whole point of our experiment. If we increased the turning rate to, say, 1,000 degrees per microsecond by applying a strong magnetic field, the distribution would whiz past the detector so fast we would have a blurred-out result. We decided that the ideal rate of turning would be about 45 degrees per microsecond.

  We were able to obtain the required magnetic field by winding a few hundred turns of copper wire on a cylinder and running a current of a few amperes through the wire. We found a Lucite tube, sent Marcel to the stockroom for wire, cut the graphite stopping block down, so it could be wedged inside the cylinder, and hooked the wires to a power supply that could be controlled remotely (there was one on the shelf). In a blur of late-night activity, we had everything ready by midnight. We were in a hurry because the accelerator was always turned off at 8 a.m. on Saturday for maintenance and repairs.

  By 1 a.m. the counters were recording data; accumulation registers recorded the number of electrons emitted at various directions. But remember, with Garwin's scheme, we didn't measure these angles directly. The electron telescope remained stationary while the muons or, rather, their spin axis directions, were rotated in a magnetic field. So the electron's time of arrival now corresponded to their direction. By recording the time, we were recording the direction. Of course, we had lots of problems. We badgered the accelerator operators to give us as many protons hitting the target as possible. All the counters that registered the muons coming in and stopping had to be adjusted. The control of the small magnetic field, applied to the muons, had to be checked.

  All of this started working, and by 5 a.m. we had “20 standard deviations” of scientific proof, i.e., proof positive, that the directions in which electrons are emitted changes with the angle, relative to the muon's spin. Our muons were all right-handed. The mirror image, a left-handed version, does not exist in our laboratory, and hence, by extension, it does not exist in any laboratory. The Conservation Law of Parity was not valid for this weak force process either—the radioactive decay of the muon! We had made a profound discovery in a few days of hard work, observing parity violation in the weak decays of both pions and muons. And a few hours of data accumulation. By about 9 a.m. the word had, somehow (?), spread and we began receiving calls from physicists around the nation and, soon thereafter, from around the world. The irascible Austrian Wolfgang Pauli was soon quoted, showing his shock and disbelief: “I cannot believe that God is a weak left-hander.” Yes, fame, fortune, and promotion followed in due course. And Marcel got his Ph.D.!

  —Leon M. Lederman14

  Let's summarize. The result obtained by performing the experiment of negatively charged pion decay turns out to be shocking: the handedness of the negatively charged muon produced in decay is always L, that is, we always see events as in figure 3.5 (A), and we never see events as in figure 3.5 (B)!

  FIGURE 3.5. Parity Violation in Pion Decay. The spins of produced particles from (negatively charged) pion decays, in the weak interaction process π– → μ– + 0. In (A) the muon spin is aligned with direction of motion (right-handed muon); in (B) the muon spin is counter-aligned with direction of motion (left-handed muon). We always observe (A) in the laboratory, and we never observe (B). If we did observe (B) we could tell that we were looking at the process through a mirror.

  This indeed implies that if we ever “see” a film or a DVD of a negatively charged pion decay producing an L negatively charged muon, as in figure 3.5 (B), then we can loudly proclaim: “We are seeing an image of the process reflected in a mirror! Such a process can happen only in Alice's looking-glass house. This never happens on our side of the mirror!”15

  The mirror world with left-handed, or L, negatively charged muons coming from negative pion decay doesn't exist. (Actually, the L muon is produced instantaneously in the pion decay, but its mass flips into an R muon that balances the spin of the anti-neutrino; we'll soon have much more to say about that.) The shocking implication of the experiment is that in our world, the laws of physics contain forces and interactions that are not symmetric under parity. This happens for the class of interactions called the “weak interactions” that are producing the decay of the pion and, subsequently, the decay of the muon. Indeed, this is an example of a “broken symmetry” that occurs throughout the weak interactions, which also produce numerous other effects. The very matter out of which we are composed, hence our very existence, depends upon these feeble forces in nature, and we now learn that these forces distinguish our world from its mirror image!

  Historically, until the mid 1950s, physicists had believed that parity was an exact symmetry of physics. Thus, the looking-glass world would have been indistinguishable in any movie of any process that we might ever encounter. The question of parity (P) non-conservation in the weak interactions was first raised by two young theorists, T. D. Lee and C. N. Yang, in 1956.16 Parity symmetry was practically considered to be a bread-and-butter established fact in nature and had been used for decades in compiling data on nuclear and atomic physics. The breakthrough of Lee and Yang was the idea that the reflection symmetry—parity—could be perfectly respected in most of the interactions that physicists encountered, such as the strong force that holds the atomic nucleus together, and the electromagnetic forces together with gravity. But Lee and Yang proposed that the weak force, with its particular form of beta-decay radioactivity, might not possess this mirror symmetry.

  In 1957, parity violation was discovered experimentally, by Leon Lederman, Richard Garwin, and Marcel Weinrich, by using the charged pion decay and stopped muon decay techniques we have just described. Independently, the effect was seen by Chien-Shiung Wu, using another more complex technique. It was astounding news—the weak processes are not invariant under the parity. Parity was overthrown!

  Madame Wu observed the radioactive disintegration of cobalt 60 (60Co) at very low temperatures in a strong magnetic field.17 This experiment was a very challenging undertaking, requiring the heroic efforts of many groups with different expertise. The 60Co is a metal out of which ordinary electrons stream, coming from beta-decay processes within the material. Wu discovered that, in the strong magnetic field, the electrons were emitted in the direction of the magnetic field (this happens because the magnetic field, at low temperatures, aligns the spins of the nuclei in the cobalt, and the decay pattern is determined by the spin of the nucleus). However, her observation was enough to conclude that there was a violation of parity symmetry. The alignment of outgoing electron velocity with the magnetic field, it turns out, is the same as a handedness, and it would be reversed in a mirror. If we saw a movie or DVD showing the electrons coming out of 60Co decay counter-aligned to the magnetic field, then, again, we could announce: “This is a mirror image of the real process and does not occur in our world.”

  Parity is violated. Parity is not a symmetry. The mirror world of Alice through the looking glass is different in a fundamental way from ours. The lowly muon has led us to this. Perhaps that's why someone at the Chinese lunch table “ordered the muon” after all. There is a difference between left and right in our world.

  And this is where the story of the Higgs boson begins.

  It's 2 a.m., the early morning hours of July 4, 2012. We have congregated in our largest seminar room, One West at Fermilab, where there is standing room only. We are here to audit two talks, one each from the gigantic experimental collaborations at CERN, known as ATLAS and CMS,
talks that are being beamed in live at 9 a.m. from Geneva, Switzerland.

  As the talks begin, you can hear a pin drop. The speakers nervously yet painstakingly review, in glorious detail, the data collected by the two large detectors and the complex statistical analysis required to extract a signal of any new particles from the data. They conclude at about 4:30 a.m., Fermilab time, to a standing ovation. The two CERN experimental collaborations at the LHC have just presented the official scientific discovery of the Higgs boson to the world. It is the biggest scientific discovery in the third millennium so far. We all then partied, watched the sun come up, and the next day began poring over the data in still greater detail.

  For a moment the whole world stopped and gave heed to something abstract, something mind-bending, something ephemeral. From the Hong Kong Economic Times to the Jerusalem Post, from the Fiji Sun to the Herald Tribune from the Kane County Chronicle, to the New York Times, the story was carried on all the front pages of all the world's newspapers. But gradually, over the subsequent weeks, the daily news returned to the slowly recovering housing market, to a horrific mass murder, to the lingering unemployment rate, to an upcoming election, and to the usual rancid political squabbling in the US House of Representatives.

  But now a much larger issue looms in the distance for human knowledge: What is the Higgs boson? Why does this thing exist? Is the Higgs a loner, or is there more to come? Are the next lessons far, far out of reach, or are we about to enter an era of rich new discoveries? What are the organizing principles? What new and mysterious reality lies beyond the Higgs boson? It all revolves around one big question.

  WHAT IS MASS?

  There were lots of bad jokes out there about the Higgs boson and how it gives mass to particles. For example: “A God Particle walks into a church and the priest says, ‘What are you doing here?’ and the Higgs boson replies, ‘You can't have a mass without me.’” There was a Twitter quip by Neil deGrasse Tyson, “The Higgs discovery makes me feel heavier already. What we need instead is the anti-Higgs…a particle that takes mass away.”1 And, from our most reliable news source of all, the Onion, a more egalitarian comment: “Yeah, the Higgs boson is getting a lot of attention, but there are a lot of lower-profile bosons that are worth checking out if you get the chance.”2

  Of course, what we mean by “mass” in physics is not what is meant in other contexts, such as religion or as the term is used with “hysteria.” But after some perusing of Internet sites that try to explain what physical mass is, we concluded that you'll only get confused if you start there. In part, this reflects the general confusion throughout the history of physics at arriving at a valid definition of mass. There are in fact many definitions. So let's take a fresh look at this and adopt a simple definition of mass, one that works fairly generally, and one that can efficiently launch us into the depths of elementary particles. Following Mies van der Rohe, “Less is more.”

  MASS IS A MEASURE OF QUANTITY OF MATTER

  Yes, it's that simple. At least for the everyday objects, those that we encounter throughout our lives, mass is just a “measure of a quantity of matter.” A feather or an ant has a small mass, while an automobile or an elephant has a large mass. Perhaps it is remarkable that such a simple concept is meaningful in nature, if you pause to think about it. All forms of matter, from water to steel, from putty to peanut butter and jelly, from magma to Kool-Aid® and vodka, to the interior of the sun and the cosmic rays in the depths of space—all share a common property—we can talk about how much matter they “are” in terms of something we call mass. We needn't specify “jelly mass” to distinguish it from “Kool-Aid mass”—it's all the same—it's just mass, which is just so-and-so much matter.

  Now that's seemingly simple. But there are challenges in implementing this concept. For example, how does one measure the mass of something like a quark, which is forever trapped inside of a larger particle, together with other quarks and things called “gluons,” etc., which can push and pull and actually change the mass of a quark? Just in case you've already considered some of the litany of subtleties surrounding the concept of mass in a high school physics course, then you've probably already encountered something called “inertial mass.” Inertial mass has to do with the resistance of motion of an object to an applied force. This was defined by an equation written several centuries ago by Isaac Newton, “Force equals mass times acceleration.” This means that, for a given applied force, an object with a larger inertial mass will accelerate more slowly than an object with a smaller inertial mass.

  We would like to think the issue ends there and that “inertial mass” is all we ever have to worry about, but alas, it isn't so simple. For example, in dealing with gravitating systems, like galaxies and black holes, there are at least three types of mass, “inertial mass,” “gravitational mass,” and “passive gravitational mass.” For quarks there's “constituent mass,” “current mass,” and, more generally, “off mass-shell mass.” Then in discussions of relativity there's “transverse mass,” “longitudinal mass,” and “rest mass,” etc. These definitions are all very technical and refer to specific instances in which even the best-trained physicists can become confused about energy content, motion, interactions with other stuff, and the simple idea of plain old inertial mass just doesn't cut it.

  For now, let's be naive and simple-minded and forget these nasty complications and stick with plain and simple “mass is a measure of quantity of matter.” The jolting revelations will come later.

  MASS IS NOT WEIGHT

  One reason that mass is generally a confusing concept for many people begins immediately with a trip to the moon. Most people think that the simplest measure of the quantity of matter (usually that which is their own quantity of matter, including their left foot, their head, that slight circumferential inner tube of abdominal fat, and perhaps the Big Mac® still in their stomach that was consumed at lunch, i.e., all that faces them every morning on the bathroom scale) is actually their weight. But, no doubt, if you took Mr. Naylor's physics class in high school, you remember some admonishment such as: “Do not confuse weight with mass! Weight is not mass and mass is not weight.”

  In 1969, on the Apollo 11 lunar mission, humans first reached the nearest orb to their own world: the moon. When Neil and Buzz first jumped off that ladder on July 21, from their spaceship to the moon's surface, they were featherweights. Their bathroom scale weight on the moon would have been a mere 30 pounds if they'd weighed 180 pounds on Earth. They had experienced the multi-billion-dollar diet of departing from Planet Earth onboard a Saturn V rocket and successfully navigated to the lunar surface where the force of gravity is one-sixth of that on Earth. But—hold on—while their weight loss was a spectacular 83 percent, apart from a lean diet of mulched meat, veggies, and Tang orange liquid, their body's inertial mass was no different on the moon than on Earth. The quantity of matter that is Neil or Buzz had not changed in traversing the 200,000 miles to the moon.

  The catch, of course, is that weight is a force, and force is not mass. While the net force of gravity that an object experiences depends upon its mass, it also depends upon the strength of the gravitational field (this is Newton's way of thinking about gravity). The gravitational field at the surface of the moon is one-sixth of that on the earth, and hence the force experienced by Neil and Buzz is one-sixth of what they experience on the earth. If NASA had shipped a bathroom scale to the moon they would have read weights that were only one-sixth of what they would read on the earth. Bathroom scales measure the force of gravity acting on you, and not your mass. But the quantity of matter, and hence inertial mass, that Buzz and Neil are made of is not changed by an expensive trip to the moon.

  Incidentally, the extra cost of shipping the bathroom scale to the moon would have been about four million dollars (in 1969!).

  MEASURING MASS CAN BE TRICKY

  The best way to measure mass, in anyone's gravitational field, such as here at home on Earth, is one you have surely witnessed and
that dates to the ancients. It is to use a “balance scale.” A balance scale simply compares the mass of some object, let us a say a nugget of gold, to a standard predetermined quantity of matter. To establish standards, we could select our delegates and send them to a stuffy scientific conference somewhere in Eastern Europe, and there they agree upon a “standard of mass.” For example, we legislate that, worldwide and henceforth, a cube of water that measures 10 centimeters on each side, at standard temperature (20° C) and pressure (1 atmosphere at sea level), has a mass of matter we call “one kilogram.” We can compare any other amount of matter to our newly defined kilogram measure with our balance scale. If we place a gold nugget on one side of the balance and a 10 cm × 10 cm × 10 cm cube of water on the other side of the scale, we can immediately determine which is the greater mass: if the balance scale tips toward the gold, the nugget has more than a kilogram of mass; if the balance scale tips toward the cube of water, then the water has more mass. Voilà! We've thus made a very crude determination of the mass, or quantity of matter, that is the gold nugget.

  And we can refine this in many ways. First, it's really inconvenient to use a cube full of water in our measuring apparatus as a counterweight standard of mass, because we'll surely spill it all sooner or later, and we have to correct our measurement for the mass of the container vessel of the water. So, we make counterweights: cleverly balance little pieces of lead against a carefully measured standard kilogram of water. We add or subtract shavings of lead from the lead side of the scale until it exactly balances. We then have our local blacksmith melt the lead and pour it into a mold to make a conveniently shaped cylindrical weight, much like the weight used in a grandfather clock. We then double- (and triple-) check that the resulting weight exactly balances the one-kilogram mass of water. This might take several tries, but eventually we'll have a conveniently shaped cylindrical lead weight to use on our scale instead of the cube of water that is exactly one kilogram of mass. We ask our blacksmith to make a dozen of these. We can then ask our blacksmith to forge, with extreme care, ten smaller weights that each contain exactly one-tenth the amount of lead. We can use the balance to check that each smaller weight is the same as the others, and then use the balance to check that ten such smaller weights exactly balance the one kilogram weight we started with. In this way we develop a “one-tenth kilogram” weight, also known as a “100-gram” weight. And we can go further, down to a “one gram” and beyond to a “decigram” (tenth of a gram), a “centigram” (hundredth of a gram) until it simply becomes too difficult to make a smaller weight. We now have a wide range of weights at our disposal to use with the balance.