3. PARTICLE ZOO AND THE STANDARD MODEL OF PARTICLE PHYSICS
At the beginning of the 1900s, it became clear that all known matter is made of atoms. In turn, an atom is made up of three particles: the electron, the proton, and the neutron. In the 1930s other particles were discovered, and by the 1960s, hundreds of new particles were discovered—a new particle was discovered every week. There was mass confusion until some theorists realized that there is a simple mathematical structure—first proposed in 1961 by the American physicist Murray Gell-Mann and the Israeli physicist Yuval Neʾeman—that explained all of this. Now we know there are a handful of truly fundamental particles, called quarks, which all fit together in a nice and neat pattern. Gell-Mann introduced the concept of quarks as the physical basis for a classification system called the Eightfold Way (in analogy with the Eightfold Path of Buddhism because of the centrality of the number eight). One of the early triumphs of the Eightfold Way was the prediction of the existence of a heavy subatomic particle1, similar to the way Dimitri Mendeleev predicted properties of undiscovered elements in his Periodic Table from the gaps he found in their arrangement.
The belief that all matter is composed of tiny, indivisible particles originated thousands of years ago in the in Greek and Indian philosophical writings. Instead of examining the ideas chronologically, we will start our story of the Standard Model by introducing the quark and then revisit some of its unusual characteristics in a later section. We will first see how protons and neutrons, collectively called nucleons, that form the nucleus of an atom, are constructed with quarks.
To make up everyday matter, one only needs an electron (e), an up quark (u) and a down quark (d). A proton and a neutron are made up of three quarks:
With electrons, protons and neutrons, any atom can be constructed. The following are the salient features of an atom:
- Protons and neutrons reside inside the central nucleus while the electrons revolve around the nucleus.
- Electrons can only occupy certain orbits (usually referred to as energy levels or shells) around the nucleus. Only a certain number of electrons could fit in each energy level.
- According to the Quantum Model of the atom, it isn’t possible to tell exactly where an electron is at a given moment, although we can calculate the probability that an electron will be found in a given volume of space (this isn’t the same as knowing where that electron is).
- Atoms are mostly empty space.
- Protons and neutrons are more-or-less identical particles except that the proton has a charge of +1 and the neutron a charge of zero.
- The charge on the electron is -1.
- The atom is electrically neutral which means that there are the same number of protons and electrons in an atom.
- An element is defined by its atomic number, (Z), which is the number of protons in a nucleus.
- The mass number of an element (A) is the number of nucleons (protons and neutrons) in a nucleus. The number of neutrons in a nucleus is A-Z.
- Atoms of elements may contain different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties. These elements are known as isotopes. A particular isotope of an element is defined by giving the name of the element and the mass number of the isotope. An example of this would be uranium-235.
We now know that quarks and the electron (and a few other particles) are truly fundamental in the sense they do not have internal structure or, in other words, cannot be subdivided into smaller constituent particles just the way protons and neutrons can.
The fundamental constituents of matter, as we know it, come in pairs or “doublets” of elementary particles which are very similar to each other except that their charges differ by one unit. For example, let’s consider the up and the down quarks:
Charge of the down quark:
Charge difference =
In addition to these light, stable quarks, there are two more doublets which have larger mass, and are unstable, eventually decaying into up and down quarks. These are called charm (c, +2/3) and strange (s, -1/3), top (t, +2/3) and bottom (b, -1/3).
The odd thing about quarks is they carry fractional electric charge. They exhibit other fascinating characteristics as well. We will talk about them later in this article but first it is necessary to introduce the leptons and a few other particles called force carriers followed by discussions of some essential ideas in particle physics in order to put everything in the proper context.
The electron belongs to a family of particles called leptons and, like the quarks, there are 6 leptons consisting of three charged leptons – electron (e), muon (μ) and tau (τ), each associated with a very light charge-less neutrino, separated by one unit of charge.
Leptons are point-like particles without internal structure. The electron () is well known while the other two charged leptons are the muon (µ) and the tau (τ), which are charged like electrons but more massive. A muon has lifetime of only 2.2 microseconds before it decays into an electron and two kinds of neutrinos. The tau (τ) is the heaviest of the leptons, having a mass of 1776.82 MeV/c2 (compared to 105.7 MeV/c2 for muons and 0.511 MeV/c2 for electrons) and a lifetime of sec.
The other leptons are the three types of neutrinos (ν). They have no electrical charge, very little mass, and they are very hard to find. Neutrinos are fascinating particles in their own right. Scientists think that these illusive particles hold the key to some of the unsolved mysteries of our universe. They very rarely interact with matter, and – according to the Standard Model of particle physics – are massless. Only in the late 1990s were they shown to have a small mass.
Neutrinos are undoubtedly the “superheroes of the particle world”. If you are interested to know about them, visit my blog post entitled “Neutrinos”.
3.3 STANDARD MODEL TABLE OF PARTICLES
Thus the Standard Model of particle physics has three generations of fundamental particles organized in two groups, quarks and leptons, shown in Figure 3-2.For each particle, the mass, charge, and the spin is shown in the table. Like mass and charge, spin of a particle is also an important characteristic of the particle. Spin will be discussed later when we talk about particle classification.
Why are there only three generations in the Standard Model? We do not know the answer. You will later see in this article how the existence of six quarks was theoretically predicted by two Japanese physicists, Makoto Kobayashi and Toshihide Maskawa, to explain a phenomenon called charge-mirror symmetry violation or CP violation. But why is the six quark model so special? We do not know. In fact, it is possible to demonstrate the violation of CP symmetry in a system that has many elementary particles, not just six. But subsequent experiments have always supported the six-quark model. Standard Model cannot tell if there are more generations of quarks. The same is true for leptons. So far only three generations of leptons have been observed.
The British physicist Paul Dirac was the first to introduce antiparticles in physics.
In 1928 Dirac published his famous equation to describe the behavior of electrons moving at relativistic speeds. The Dirac equation as it came to be known, combined quantum theory and Einstein’s special relativity. Curiously, it allowed two solutions, one for an electron with positive energy, and the other for an electron with negative energy. But, how can a particle’s energy be a negative number?
Dirac’s interpretation of the solutions of his equation is a landmark achievement in particle physics that inspired generations of physicists. He suggested for every particle there exists a corresponding antiparticle, exactly matching the particle but with opposite charge. For the electron there should be an “antielectron” identical in every way but with a positive electric charge.
In 1932, Carl Anderson discovered electron’s antiparticle, the positron, using a detector called a cloud chamber. Dirac won the Nobel Prize in physics in 1933 for this ground-breaking work and so did Anderson for discovering the first established antiparticle in 1936.
Antiparticles have captured the public’s attention following the publication of Dan Brown’s fictional novel Angels and Demons. The following are some important characteristics of particles and antiparticles:
- Particle-antiparticle pairs can annihilate each other, producing photons; the total charge is conserved since the charges of the particle and antiparticle are opposite.
- Electrically neutral particles are not always identical to their antiparticles. For example, the neutron is made of quarks, the antineutron from antiquarks. They are distinguishable from one another because neutrons and antineutrons annihilate each other upon contact.
- Other neutral particles are their own antiparticles, such as photons, the hypothetical gravitons, and some WIMPs. We will talk more about photons, gravitons, and WIMPs later in this article.
With the introduction of antiparticles, note that the particle table doubled in size (see Figure 3-3).
3.5 HADRONS AND MESONS
By the start of the 1930’s, physicists believed that an atom consisting of the electron, proton and neutron were the only building blocks of matter. Quarks or leptons were unknown at the time. Scientists thought that the atom’s inner structure had been discovered and all that remained was the task for formulating a theoretical framework to explain their behavior.
Then came the muon—a surprisingly heavy cousin of the electron with no apparent purpose. The muon was so unexpected that, regarding its discovery, Nobel laureate Isidor Isaac Rabi famously quipped, “Who ordered that?”
As particle accelerators became more powerful, more and more particles began to emerge. In 1964, Murray Gell-Mann and George Zweig independently realized that quarks are not the constituents of just protons and neutrons. In fact, they make up a group of particles called hadrons. Protons and neutrons fall into this category. Hadrons were discovered in in large numbers during 1950 and 60’s with the emergence of accelerator technology.
Mesons were predicted theoretically in 1935 by the Japanese physicist Hideki Yukawa. Their existence was confirmed in 1947 by a team led by the English physicist Cecil Frank Powell with the discovery of the pi-meson (pion) in cosmic-ray particle interactions. Today, we know a pion (or a pi meson, denoted with the Greek letter pi: π) is any of three subatomic particles: , , and .
Since then more than 200 mesons have been produced and characterized, most in high-energy particle-accelerator experiments.
Hadrons include mesons and baryons. Baryons are massive particles which are made up of three quarks including the proton and neutron and other short-lived particles such as lambda, sigma, xi, and omega particles. Baryons are distinct from mesons in that mesons are composed of only two quarks—a quark-antiquark pair. For example, see Table 1. The is made of an up quark and a down anitiquark. The antiparticle, , just has its quark and antiquark switched, so is made of a down quark and an up antiquark.
Because a meson consists of a particle and an antiparticle, it is very unstable. The K meson lives much longer than most mesons, which is why it was called “strange” and gave this name to the strange quark, one of its components.
If you are confused with the sudden explosion of the number of particles, you are not alone. Scientists in the 1960s were also confused and did not understand how these particles were related or if there is a hidden pattern that explained their abundance. It took the genius of Murray Gell-Mann and others to eventually find the pattern. Their work lead to the development of Quantum Chromodynamics, the highly successful theory of strong interactions. Strong interaction is a fundamental force describing the interactions between quarks and gluons which make up hadrons.