Warped Passages (29 page)
Figure 53.
In muon decay, the muon turns into a muon neutrino and a virtual W
-
gauge boson, which then converts to an electron and an electron antineutrino.
In fact, there are three copies of the full set of particles with the same Standard Model charges (see Figure 52). Each of these copies is called a
generation
, or sometimes a
family
. The first generation of
particles contains a left-and a right-handed electron, a left-and a right-handed up quark, a left-and a right-handed down quark, and a left-handed neutrino. This first generation contains all the stable stuff of which atoms, and therefore all stable matter, is composed.
The second and third generations contain particles that decay and are not present in “normal” known matter. They are not exact copies of the first generation; they have charges identical to those of their first generation counterparts but are heavier. They were discovered only when they were produced at high-energy particle colliders, and their purpose remains obscure. The second generation consists of a left-and a right-handed muon, a left-and a right-handed
charm quark
, and a left-and a right-handed
strange quark
, as well as a stable left-handed
muon neutrino
.
*
The third generation contains a left-and a right-handed
tau
, a left-and a right-handed
top quark
, a left-and a right-handed
bottom quark
, and a left-handed
tau neutrino
. The identical copies of a particular particle with the same charge assignments, each a member of a different generation, are often called
flavors
of the particle type.
From Figure 52 you can see that although there were only three known flavors of quark when Gell-Mann first proposed them, we now know of six: three “up types” and three “down types”—one in each generation. In addition to the up quark itself, there are two identically charged up-type quarks—the charm and the top. Similarly, the down, strange, and bottom quarks are different flavors of down-type quark. And the muon and tau leptons are heavier versions of the electron.
Physicists are still trying to understand the reason for three generations and why particles have their particular masses. These are major questions about the Standard Model that fuel the research being conducted today. Along with many others, I’ve worked on these problems throughout my career, but we’re still searching for the answers.
The heavier flavors are significantly heavier than the lighter ones. Although the next heaviest quark, the bottom, was discovered in 1977, the very heavy top quark eluded discovery until 1995. Two
particle experiments, including the remarkable one that discovered the top quark, are the subject of the following chapter.
What to Remember
- The Standard Model consists of the nongravitational forces and the particles that experience those forces. In addition to the well-known force of electromagnetism, there are two forces that act within a nucleus: the
strong force
and the
weak force
. - The weak force poses the most important remaining mystery about the Standard Model. Whereas the other two forces are communicated by massless particles, the gauge bosons that communicate the weak force have mass.
- In addition to the particles that communicate forces, the Standard Model contains particles that experience those forces. These particles are divided into two categories:
quarks
, which experience the strong force, and
leptons
, which do not. - The light quarks and leptons found in matter (the up quark, the down quark, and the electron) are not the only known particles. Heavier quarks and leptons also exist: the up quark, the down quark, and the electron each have two heavier versions.
- These heavy particles are unstable, which means that they decay to lighter quarks and leptons. But experiments at particle accelerators have produced them and shown that these heavier particles experience the same forces as the familiar light, stable particles.
- Each of the groups of particles that include a charged lepton, an up-type quark, and a down-type quark is known as a
generation
. There are three generations, each of which contains successively heavier versions of each particle type. These particle varieties are known as
flavors
. There are three up-type quark flavors, three down-type quark flavors, three charged lepton flavors, and three neutrino flavors. - I won’t use the details or names of any particular quark or lepton later on. However, you will need to know about flavors and generations because of the strong constraints on the particles’ properties, which give us vital clues and constraints on the physics that lies beyond the Standard Model.
- Chief among these constraints is that different flavors of quarks and leptons with the same charges rarely, if ever, turn into one another. Theories in which particles readily change flavor are ruled out. We will see later that this poses a big challenge for models of broken supersymmetry and other proposed extensions of the Standard Model.
Experimental Interlude: Verifying the Standard Model
One way, or another
I’m gonna find you…
Blondie
Ike once again dreamed he met the quantum detective. This time, the sleuth knew what he was after—and he had a pretty good idea where it should be. All he had to do was wait—sooner or later, if he wasn’t mistaken, his quarry would appear.
Finding heavy particles is not easy. Yet that’s what we must do if we are to discover the structure underlying the Standard Model and, ultimately, the physical makeup of the universe. Most of what we know about particle physics comes from high-energy
particle accelerator
experiments, which first accelerate a rapidly moving beam of particles and then smash them into other matter.
In a
high-energy particle collider
, the accelerated beam of particles actually collides with an accelerated beam of antiparticles so that they meet in a small collision region containing a huge amount of energy. This energy is then sometimes converted into heavy particles not readily found in nature. High-energy particle colliders are the only place where the heaviest known particles have appeared since the time of the Big Bang, when the much hotter universe contained all particles in abundance. Colliders can create pairs of any kind of particle and antiparticle, in principle, as long as they have enough energy for that particular pair, the energy given by Einstein’s
E
=
mc
2
.
But the goal of high-energy physics is not merely to find new
particles. Experiments at high-energy colliders will tell us about fundamental laws of nature that cannot be observed in any other way—laws that operate at too close a range to be visible more directly. High-energy experiments are the only way to probe any short-distance interactions that operate at extremely tiny distance scales.
This chapter is about two of the collider experiments that were important in confirming the predictions of the Standard Model and constraining what physical theories might lie beyond. These experiments are both impressive in their own right. But they should also give you a sense of what physicists will be up against when they will search for new phenomena, such as extra dimensions, in the future.
The Top Quark Discovery
The search for the top quark beautifully illustrates the difficulties of finding a particle at a collider when the collider’s energy is barely adequate to produce it, and the ingenuity with which experimenters can rise to this challenge. Although the top quark is not part of any atom or known matter, the Standard Model would be inconsistent without it, so most physicists had been confident of its existence since the 1970s. Yet as recently as 1995, no one had ever detected one.
At that time, experiments had been looking for the top quark in vain for many years. The bottom quark, the next-heaviest Standard Model particle, which weighs in at five times the mass of a proton, was discovered in 1977. But although physicists back then thought the top quark would soon show up, and experimenters raced to find it and claim the glory, to everyone’s surprise experiment after experiment failed. It wasn’t found at colliders that operated at 40 times, or 60 times, or even 100 times the energy required to produce a proton. The top quark was evidently heavy—remarkably heavy compared with the other quarks, all of which had been detected. When it finally made its appearance after twenty years of searching, it turned out to have a mass almost 200 times that of the proton.
Because the top quark is so heavy, the relations of special relativity tell us that only a collider that operated at extremely high energy
could produce it. High energy always requires a very large accelerator, which is technically difficult to design and expensive to construct.
The collider that eventually produced the top quark was the Tevatron in Batavia, Illinois, thirty miles west of Chicago. The collider at Fermilab was initially designed with far too low an energy to produce a top quark, but engineers and physicists had made many changes that improved its potential enormously. By 1995 the Tevatron, the culmination of these improvements, operated at far higher energy and produced many more collisions than the original machine could have managed.
The Tevatron, which is still in operation, is located at Fermilab, an accelerator center that was commissioned in 1972 and named after the physicist Enrico Fermi. I was very amused when I first visited Fermilab and found there were wild corn, geese, and for some strange reason, buffalo on the site. Buffalo aside, the region is fairly flat and boring. The movie
Wayne’s World
was set in Aurora, about five miles south of Fermilab, and if you are familiar with this movie, you might have some idea of the Fermilab surroundings. Fortunately, the physics there is exciting enough to keep people happy anyway.
The Tevatron gets its name because it accelerates both protons and antiprotons to an energy of a TeV (pronounced T-e-V, although the “Tev” in “Tevatron” rhymes with “Bev”), which is the same as 1,000 GeV, the highest energy that has been achieved so far at any accelerator. The energetic beams of protons and antiprotons that the Tevatron produces circulate in a ring and smash together every 3.5 microseconds at two collision points.
Two separate experimental collaborations set up detectors at each of the two collision points, where the beams of particles and antiparticles cross paths and the interesting physical processes can happen. One of these experiments was named CDF (Collider Detector at Fermilab) and the other was called D0, the designation of the collision point between protons and antiprotons at which the detector was located. The two experiments searched extensively for new physical particles and processes, but in the early 1990s the top quark was their Holy Grail. Each experimental collaboration wanted to be the first to find it.
Many heavy particles are unstable and decay almost immediately.
When that is the case, experiments search for visible evidence of a particle’s decay products, rather than the particle itself. The top quark, for example, decays into a bottom quark and a W (the charged gauge boson that communicates the weak force). And the W also decays, either into leptons or quarks. So experiments seeking the top quark look for the bottom quark in conjunction with other quarks or leptons.
Particles do not come with nametags, however, so detectors have to identify them by their distinguishing properties, such as their electric charge or the interactions in which they participate, and separate components of the detectors are needed to record these properties. The two detectors at CDF and D0 are each segmented into several pieces, each of which records different characteristics. One piece is a
tracker
, which detects charged particles by the electrons from ionized atoms that they leave in their wake. Another piece, called a
calorimeter
, measures the energy that particles deliver as they pass through. The detectors have other components which can identify particles with other specific distinguishing properties, such as a bottom quark, which lasts longer than most other particles before it decays.
Once a detector registers a signal, it transmits the signal through an extensive array of wires and amplifiers, and records resulting data. However, not everything that is detected is worth recording. When a proton and an antiproton collide, the interesting particles such as the top and antitop quarks are only rarely produced. Much more often, collisions produce only lighter quarks and gluons, and more often still, nothing of real interest. In fact, for every top quark that was produced at Fermilab, there were ten trillion collision events that didn’t contain a top quark.