Key unit Competence
By the end of this unit, I should be able to organize the properties and basic principles of quarks.
• List types of quarks, identify quarks, antiquarks and hadrons (baryons and mesons)
• Define baryon number and state the law of conservation of baryon number.
• Interpret the baryon number and apply the law of conservation of baryon number
• State colors of quarks and gluons.
• Explain how color forms bound states of quarks.
• Formulate the spin structure of hadrons (baryon and mesons)
In the study of matter description and energy as well as their interactions; the fascinating thing of discovery is the structure of universe of infinite size but still there is a taskto know the origin of matter. The smallest particle was defined to be electron, proton, and neutron. But one can ask:
1. What particles are components of matter?
2. Describe and discuss how particles interact with energy to form matter.
Activity 9.1: Investigating about elementary particles
Considering the knowledge and skills obtained from unit 8, about the study of elementary particles, discuss and explain the following questions:
1. Discuss the major groups of elementary particles
2. Explain and analyze the family of quarks and their interactions.
3. Why should we learn aboutelementary particles?
Particle physics is the field of natural science that pursues the ultimate structure of matter. This is possible in two ways. One is to look for elementary particles, the ultimate constituents of matter at their smallest scale, and the other is to clarify what interactions are acting among them to construct matter as we see them. The exploitable size of microscopic objects becomes smaller as technology develops. What was regarded as an elementary particle at one time is recognized as a structured object and relinquishes the title of “elementary particle” to more fundamental particles in the next era. This process has been repeated many times throughout the history of science (Nagashima, 2013).
In the 19th century, when modern atomic theory was established, the exploitable size of the microscopic object was and the atom was “the elementary particle”. Then it was recognized as a structured object when J.J. Thomson extracted electrons in 1897 from matter in the form of cathode rays. Its real structure (the Rutherford model) was clarified by investigating the scattering pattern of α-particles striking a golden foil (See Fig.9.1).
Fig.9. 1: Scattering pattern of α-particles striking a golden foil. In 1932, Chadwick discovered that the nucleus, the core of the atom, consisted of protons and neutrons. In the same year, Lawrence constructed the first cyclotron. In 1934 Fermi proposed a theory of weak interactions. In 1935 Yukawa proposed the meson theory to explain the nuclear force acting among them.
It is probably fair to say that the modern history of elementary particles began around this time. The protons and neutrons together with their companion pions, which are collectively called hadrons, were considered as elementary particles until 1960. We now know that they are composed of more fundamental particles, the quarks. Electrons remain elementary to this day.
Muons and τ-leptons, which were found later, are nothing but heavy electrons, as far as the present technology can tell, and they are collectively dubbed leptons. Quarks and leptons are the fundamental building blocks of matter. The microscopic size that can be explored by modern technology is nearing 10−19m .
The quarks and leptons are elementary at this level (Nagashima, 2013). Some composite particles as stated by (Hirsch, 2002) are summarized in the tables below
9.1.2 Checking my progress
1. Each hadron consists of a proper combination of a few elementary components called
a. Photons. b. Vector bosons. c. Quarks. d. Meson-baryon pairs.
2. Which of the following is not conserved in a nuclear reaction?
a. Nucleon number. b. Baryon number. c. Charge d. All of the above are c. Conserved.
3. The first antiparticle found was the
a. Positron. b. Hyperon. c. Quark. d. Baryon.
4. The proton, neutron, electron, and the photon are called
a. Secondary particles. b. Fundamental particles c. Basic particles. d. Initial particles.
5. The exchange particle of the electromagnetic force is the
a. Gluon. b. Muon. c. Proton. d. Photon.
6. Particles that are bound by the strong force are called
a. leptons. b. hadrons. c. muons. d. electrons
7. At the present time, the elementary particles are considered to be the
a. Photons and baryons. b. Baryons and quarks. c. Leptons, quarks and bosons d. Baryons and leptons.
8. The electron and muon are both
a. Jadrons. b. Leptons. c. Baryons. d. Mesons.
9. Particles that make up the family of hadrons are
a. Baryons and mesons. b. Leptons and baryons. c. Protons and electrons. d. Muons and leptons.
10. Is it possible for a particle to be both:
a. A lepton and a baryon? b. A baryon and hadron? c. A meson and a quark? d. A hadron and a lepton?
11. Distinguish between
a. fermions and bosons
b. leptons and hadrons
c. mesons and baryon number
12. Which of the four interactions (strong, electromagnetic, weak, gravitational) does an electron, neutrino, proton take part in?
9.2 TYPES OF QUARKS
Activity 9.2: Investigating quark particles
Use search internet and find the explanation about quarks and types of quarks
Aquark is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within hadrons, such as baryons (of which protons and neutrons are examples) and mesons. For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves (Douglass, PHYSICS, Principles with applications., 2014).
Quarks have various intrinsic properties, including electric charge, mass, color charge, and spin. Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction (see section 8.5), as well as the only known particles whose electric charges are not integer multiples of the elementary charge.
There are six types of quarks, known as flavors: up, down, strange, charm, top, andbottom (see Fig. 9.2). Up and down quarks have the lowest masses of all quarks. The heavier quarks rapidly change into up and down quarks through a process ofparticle decay (the transformation from a higher mass state to a lower mass state). Because of this, up and down quarks are generally stable and the most common in the universe, whereas strange, charm, bottom, and top quarks can only be produced in high energy collisions (such as those involving cosmic rays and in particle accelerators). For every quark flavor there is a corresponding type of antiparticle, known as an antiquark, that differs from the quark only in that some of its properties have equal magnitude but opposite sign (Nagashima, 2013).
Checking my progress
1. A proton is made up of
a. One up quark and two down quarks
b. An up quark and down antiquark
c. Two up quarks and a down quark
d. Strange quark and an anti-strange quark
2. Particles that are un affected by strong nuclear force are
a. Protons b. Leptons c. Neutrons d. Bosons
3. Particle which explains about mass of matter is called
a. Higgs boson b. Protons c. Leptons d. Neutrons
4. Describe the types and the characteristics of the quarks as well as their interaction properties.
9.3 BARYON NUMBER, LEPTON NUMBER AND THEIR LAWS OF CONSERVATION
Activity 9.3: Investigating about particle numbers.
Use search internet and retrieve the meaning of the following property of elementary particles.
• Baryon numbers and
One of the important uses of high energy accelerators is to study the interactions of elementary particles with each other. As a means of ordering this sub-nuclear world, the conservation laws are indispensable. The law of conservation of energy, of momentum, of angular momentum, and of electric charge is found to hold precisely in all particle interactions.
A study of particle interactions has revealed a number of new conservation laws which (just like the old ones) are ordering principles. They help to explain why some reactions occur and others do not. For example, the following reactions have never been found to occur:
Even though charge, energy, and so on are conserved (p with bar over means an antiproton and allow means the reaction does not occur). To understand why such a reaction does not occur, physicists hypothesized a new conservation law, the conservation of baryon number.
Thus the law of conservation of baryon number states that: “Whenever a nuclear reaction or decay occurs, the sum of baryon numbers before the process must equal the sum of the baryon numbers after the process.”
Baryon number is a generalization of nucleon number, which is conserved in nuclear reaction and decays. All nucleons are defined to have baryon number B=+1 , and all antinucleons (antiprotons, antineutrons) have . All other types of particles, such as photons, mesons, and electrons and other leptons have B=0.
9.4 SPIN STRUCTURES OF HADRONS (HADRONS AND MESONS)
Activity 9.5: Investigating the structure of elementary particles
Use search internet and find the structure of elementary particles: Hadrons and mesons. Discuss and explain your findings in a brief summary about structure of hadrons.
There are hundreds of hadrons, on the other hand, and experiments indicate they do have an internal structure. In 1963, M. Gell-Mann and G. Zweig proposed that none of the hadrons, not even the proton and neutron, are more fundamental, point like entities called, somewhat whimsically, quarks. Today, the quark theory is well accepted, and quarks are considered the truly elementary particles, like leptons. The three quarks originally proposed we labeled u, d, s and have the names up, down and strange. The theory today has six quarks, just as there are six leptons based on presumed symmetry in nature,
The other three quarks are called charmed, bottom and top (see Fig.9.2). The theory names apply also to new properties of each (quantum numbers c, t, b) that distinguish the new quarks from the old quarks (see Table 9.1below), and which (like strangeness) are conserved in strong, but not weak, interactions. All quarks have spin 1/2 and an electric charge of either+2/3e or -1/2e (that is, a fraction of the previously thought smallest charge e). Antiquarks have opposite sign of electric charge Q, baryon number B, strangeness S, charm c, bottomness b, and topness t.
Table 9. 3 Properties of some baryons
After the quark theory was proposed, physicists began looking for those fractionally charged particles, but direct detection has not been successful. Current models suggest that quarks may be so tightly bound together that they may not ever exist singly in the free State.
But observations of very high energy electrons scattered off protons suggest that protons are indeed made up of constituents.
Today, the truly elementary particles are considered to be the six quarks, the six leptons and the gauge bosons that carry the fundamental forces. See Table 9.4 where the quarks and leptons are arranged in three “generations.” Ordinary matter-atoms made of protons, neutrons, and electrons are contained in the “first generation”. Theothers are thought to have existed in the very early universe, but are seen by us today at powerful accelerators or in cosmic rays. All of the hundreds of hadrons can be accounted for by combinations of the six quarks and six antiquarks.
Note that the quarks and leptons are arranged into three generations each.
Example 9.1 1.
Find the baryon number, charge, and strangeness for the following quark combinations, and identify the hadrons particle that is made up of these quark combinations:
9.5 COLOR IN FORMING OF BOUND STATES OF QUARKS.
Activity 9.6: Investigating the bound state of an atom.
Take the case of an electronic configuration of hydrogen atom. Make the illustration and then contrast the interaction between electron and proton and bound state of elementary particles.
9.6.1 Bound state of quarks
In the hydrogen atom configuration, the proton is located at centre while electron moves around it at a speed of about 1% the speed of light. The proton is heavy while the electron is light (See Fig.9.3)
This is the simplest example of what physicists call a “bound state”. The word “state” basically just meaning a thing that hangs around for a while, and the word “bound” meaning that it has components that are bound to each other, as spouses are bound in marriage.
The inside of the proton itself is more like a commune packed full of single adults and children: pure chaos. It too is a bound state, but what it binds is not something as simple as a proton and an electron, as in hydrogen, or even a few dozen electrons to an atomic nucleus, as in more complicated atoms such as gold, but zillions (meaning “too many and too changeable to count usefully”) of lightweight particles called quarks, antiquarks and gluons. It is impossible to describe the proton’s structure simply, or draw simple pictures, because it’s highly disorganized. All the quarks and antiquarks and gluons (see Fig.9.4) inside are rushing around as fast as possible, at nearly the speed of light (Strassler, 2011).
Fig.9. 4 Snapshot of a proton: Imagine all of the quarks (up, down, and strange: u, d, s), antiquarks (u, d, s with a bar on top), and gluons (g) zipping around near the speed of light, banging into each other, and appearing and disappearing (Strassler, 2011)
You may have heard that a proton is made from three quarks but this is not true. In fact there are billions of gluons, antiquarks, and quarks in a proton.
9.6.2 Color in forming of bound states of quarks.
In the standard model of Quantum Chromodynamics (QCD) and the electroweak theory (Giancoli D. C., Physics: principals with application, 2005), not long after the quark theory was proposed, it was suggested that quarks have another property (or quality) called color, or ‘color charge’ (analogous to electric charge). The distinction between the six quarks (u, d, s, c, b, t) was referred to as flavors.
According to the theory, each of the flavors of quark can have three colors, usually designated red, green and blue. These are the three primary colors which, when added together in equal amounts, as on a TV screen, produce white. Note that the names ‘color’ and ‘flavor’ have nothing to do with our sense, but are purely whimsical as are other names, such as charm, in this new field.The antiquarks are colored antired, antigreen and antiblue. Baryons are made up of three quarks, one of each color. Mesons consist of quark-antiquark pair of a particular color and its anticolor. Both baryons and mesons are thus colorless or white.
Originally, the idea of quark color was proposed to preserve the Pauli exclusion principle. Not all particles obey the exclusion principle. Those that do, such as
6.3 Colour as component of quarks and gluons
Activity 9.7: Investigating the origin of color
When a metal like iron is heated red-hot, one can observe the change in color. As the energy increases, as the color changes. Use the same experiment and discuss on the following questions
1. As the color changes in the metal, what are the scientific reasons behind that?
2. Explain the matter−energy interaction and their consequences
3. What is color in the field of elementary particles?
The attractive interactions among quarks are mediated by massless spin bosons called gluons in much the same way that photons mediate the electromagnetic interaction or that pions mediated the nucleon–nucleon force in the old Yukawa theory (Nagashima, 2013).
Particles were classified into two categories:
Quarks and leptons have an intrinsic angular momentum called spin, equal to a halfinteger (1/2) of the basic unit and are labeled as fermions. Fermions obey the exclusion
principle on which the Fermi-Dirac distribution function is based. This would seem to forbid a baryon having two or three quarks with the same flavor and same spin component. To avoid this difficulty, it is assumed that each quark comes in three varieties, which are called color: red, green, and blue. The exclusion principle applies separately to each color. Particles that have zero or integer spin are called bosons. Bosons do not obey the exclusion principle and have a different distribution function, the Bose-Einstein distribution.
• A baryon always contains one red, one green, and one blue quark, so the baryon itself has no net color.
• Each gluon has a color–anticolor combination (for example, blue–antired) that allows it to transmit color when exchanged, and color is conserved during emission and absorption of a gluon by a quark.
• The gluon-exchange process changes the colors of the quarks in such a way that there is always one quark of each color in every baryon. The color of an individual quark changes continually as gluons are exchanged
Similar processes occur in mesons such as pions:
• The quark–antiquark pairs of mesons have canceling color and anticolor (for example, blue and antiblue), so mesons also have no net color. Suppose a pion initially consists of a blue quark and an antiblue antiquark.
• The blue quark can become a red quark by emitting a blue–antired virtual gluon.
The gluon is then absorbed by the antiblue antiquark, converting it to an antired antiquark (Fig. 9.8). Color is conserved in each emission and absorption, but a blue– antiblue pair has become a red–antired pair. Such changes occur continually, so we
have to think of a pion as a superposition of three quantum states:
• Green–antigreen, and
In terms of quarks and gluons, these mediating virtual mesons are quark–antiquark systems bound together by the exchange of gluons.
Fig.9. 5 (a) A pion containing a blue quark and an antiblue antiquark. (b) The blue quark emits a blue–antired gluon, changing to a red quark. (c) The gluon is absorbed by the antiblue antiquark, which becomes an antired antiquark. The pion now consists of a red–antired quark–antiquark pair. The actual quantum state of the pion is an equal superposition of red–antired, green antigreen, and blue–antiblue pairs.
9.6.4 Checking my understanding
1. Label the illustration below and analyze the interaction between its particles
Fig.9. 6 Illustration diagram
1. Define and describe the following key concept:
I. Color charge:
III. Quantum chromodynamics
2. Which one of the following sets of color combinations is added in color vision in TV’?
a. Red, green and blue c. White. red and yellow b. Orange, back and violet d. Yellow, green and blue
3. What are the color composition of a. Gluons b. Meson c. Baryon
9.6 END UNIT ASSEMENT
9.7.1 Multiple choices
1. A proton is made up of
a. One up quark and two down quarks
b. An up quark and down antiquark
c. Two up quarks and a down quark
d. Strange quark and an antistrange quark
2. Particles that are unaffected by strong nuclear force are
3. Particle which explains about mass of matter is called
a. Higgs boson b. Leptons c. Protons d. Neutrons
4. A conservation law that is not universal but applies only to certain kinds of interactions is conservation of:
a. Lepton number b. Baryon number c. Spin d. Charge e. Strangeness
5. In quantum electrodynamics (QED), electromagnetic forces are mediated by
a. the interaction of electrons.
c. D. the weak nuclear interaction.
d. action at a distance.
e. E. the exchange of virtual photons.
6. Conservation laws that describe events involving the elementary particles include the conservation of energy.
a. All of these are correct.
b. electric charge.
c. baryon and lepton numbers.
d. linear and angular momentum.
7. The conservation law violated by the reaction is the
c. Linear momentum.
d. Lepton number and baryon number.
e. Angular momentum.
8. Particles that participate in the strong nuclear interaction are called
9.7.2 Structured questions
9. In the table cross-word below, find at least fifteen names associated to elementary particles. Among them, select ones that represents quarks, leptons or radiations.
10.a. Making massive particles: Relatively massive particles like the proton and neutron are made of combinations of three quarks.
I. What is the charge on the combination uuu?
II. What is the charge on the combination uud?
III. What is the charge on the combination udd?
IV. What is the charge on the combination ddd