Key unit Competence 

    By the end of the lesson, I should be able to analyse the nature of particle and their interactions

    My goals

      • The key varieties of fundamental subatomic particles and how they were discovered.

      • Distinguish between fundamental particles and composite particles

      • Distinguish between particles and antiparticles

      • Describe how antimatter can be used as a source of energy

      • State some applications for elementary particles

      • Compare matter and antimatter

      • The four ways in which subatomic particles interact with each other.

      • Analyze the structure of protons, neutrons, and other particles can be explained in terms of quarks

    Introductory activity 

    Investigating the elementary particles discovery

    In the study of matter description and energy as well as their interactions; the fascinating thing of discovery is the structure of universe of unknown radius but still to know the origin of matter one need to know about small and smallest composites of matter. The smallest particle was defined to be electron, proton, and neutron. But one can ask:

      1. Are electron, proton and neutron the only particle that can define the origin of matter?

      2. What are other particles matter is composed of?

      3. Describe and discuss how particles interact with energy to form matter.


    8.1.1 Introduction 

    Activity 8.1: Investigate the presence of smaller particles

    Use internet and retrieve the definition and the information about elementary particles, and then answer to the following questions.

     1. What does elementary particle physics talk about?

     2. What are the elementary particles found through your research?

     3. Discuss and explain the use of knowledge about the elementary particles.

    Particle physics, also known as high-energy physics, is the field of natural science that pursues the ultimate structure of matter. 

    The protons and neutrons 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 . The quarks and leptons are elementary at this level (Nagashima, 2013).

    Particle physics is the study of the fundamental constituents of matter and their interactions. However, which particles are regarded as fundamental have changed with time as physicists’ knowledge has improved. Modern theory called the standard model attempts to explain all the phenomena of particle physics in terms of the properties and interactions of a small number of particles of three distinct types (see Fig.8.1):

    • Two families of fermions (of spin ½): leptons and quarks 

    • One family of bosons (of spin 1)


    I, II and III represent the first, second and the third generations. In addition, at least one spin-0 particle, called the Higgs boson, is postulated to explain the origin of mass within the theory, since without it all the particles in the model are predicted to have zero mass (see Fig.8.1).

    All the particles of the standard model are assumed to be elementary; i.e. they are treated as point particles, without internal structure or excited states. The most familiar example of a lepton is the electron (the superscript denotes the electric charge), which is bound in atoms by the electromagnetic interaction, one of the four fundamental forces of nature. A second well-known lepton is the electron neutrino, which is a light, neutral particle observed in the decay products of some unstable nuclei (the so-called β-decays). The force responsible for the β-decay of nuclei is called the weak interaction.

    8.1.2 Checking my progress

       1. Particles that make up the family of Hadrons are:

           a. Baryons and mesons

           c. Protons and electrons

           b. Leptons and baryons 

           d. Muons and Leptons
       2. Using the elementary particles, Complete the following sentences 

          I. One family of bosons of spin 1 called__________which act as ‘force carriers’ in the theory

         II. Two fermions of spin 1/2 called_________and ________ 

       3. The first antiparticle found was the

               a.  Positron              b.  Hyperons        c. Quark     d. baryon
       4. Explain what is meant by particle physics?


    Activity 8.2:  Classes of elementary particles 

    Based on the previous introduction section, reread the text and the answer to the following questions.
     1. What are the types of elementary particles?

     2. What properties are based on to classify elementary particles?

    There are three properties that describe an elementary particle ‘’mass,’’ ‘’charge’’ and ‘’spin’’. Each property is assigned as number value. These properties always stay the same for an elementary particle.

    • Mass (m): a particle has mass if it takes energy to increase its speed or to accelerate it. The values are given in MeV/C2. This comes from special relativity, which tells us that energy equals mass times the square of the speed of light. E=m×c2 All particles with mass are affected by gravity even particles with no mass like photon

    • Electriccharge (Q): particles may have positive, negative charge or none. If one particle has a negative charge and another particle has a positive, the two particles are attracted to each other. If particles have a similar charge, they repel each other. At a short distance this force is much stronger than the force of gravity which pulls all particles together. An electron has a charge -1 and a proton has a charge +1. A neutron has average charge 0. Normal quarks have charge of 2/3 or -1/3

    • Spin: the angular momentum or constant turning of particles has a particular value, called its spin number. Spin for elementary particle is 0, 1 or 1/2 . The spin property only donates the presence of angular momentum.

    8.2.1 Classification of particles by mass 

    The most basic way of classifying particles is by their mass. The heaviest particles are the hadrons and the lightest one is the leptons.


    As seen the diagram above hadrons group is divided into baryons and mesons. Baryons are the heaviest particles and are followed by mesons. 

    Hadrons are composite particles made of quarks held together by the strong force in a similar way as molecules are held together by electromagnetic force. They are subjected to the strong nuclear force and are not fundamental particles as they are made up of quarks.

                  • Baryons are composite sub-atomic particle made up of 3 quarks (tri-quarks are distinct from mesons which are composed of one quark and one antiquark). Baryon comes from Greek word which means “heavy”. The protons are only stable baryons; all other baryons eventually decay into proton.

         Ex: Protons and neutrons  

                  • Mesons are hadrons sub-atomic particles made up of one quark and one antiquark bound together by strong interaction. Ex: Pion and kaon Each pion has quark and one anti-quark therefore is a meson. It is the lightest meson and generally the lightest hadrons. They are unstable. 

    Leptons do not interact via the strong force. They carry electric charge also interact via the weak nuclear force. They include electron, muons, tau and three the types of neutrino: the electron neutrino (νE), the muon neutrino (νμ) and the tau neutrino

    In summary, leptons are subjected to the weak nuclear force and they do not feel the strong nuclear force. 

    Examples: Electron, muons and neutrino.

    8.2.2 Classification of particles by spin. 

    The spin classification determines the nature of energy distribution in a collection of particles. Particles of integer spin obey Bose-Einstein statistics whereas those of half-integer spin behave according to Fermi-Dirac statistics as shown in the following chart


    All fundamental particles are classified into fermions and bosons. Fermions have half-integer spin while bosons have full integer spin. Electrons and nucleons are fermions with spin ½. The fundamental bosons have mostly spin 1. This includes the photon. The pion has spin 0, while the graviton has spin 2.  There are also three particles, the W+, W− and Z0 bosons, which are spin 1.  They are the carriers of the weak interactions.

    Fermions are particles which have half-integer spin and therefore are constrained by the Pauli Exclusion Principle (see Section 8.4). It includes electrons, protons and neutrons.

    The fact that electrons are fermions is foundational to the buildup of the periodic table of elements since there can be only one electron for each state in an atom (only one electron for each possible set of quantum numbers). The fermion nature of electrons also governs the behavior of electrons in a metal where at low temperatures all the low energy states are filled up to a level called the Fermi energy. This filling of states is described by Fermi-Dirac statistics.

    8.2.3 Checking my progress

     1. How can elementary particles be classified?

     2. Elementary particles which are made up of one quark and one anti-quark are called

        a. Protons      . b. Neutrons     c.   Baryons         d.   Mesons     e.   Leptons

     3. The Sub-atomic particle made up of 3 quarks are called

        a. Leptons          b. Pion       c.    kaon            d.    Baryons

     4. What do you understand by the term elementary particle?


    8.3.1 Concept of particle and antiparticle

     Activity 8.3: 

    Discuss the following terms:

      3. Particle

      4. Antiparticle

    There are two important points about pair production. The first is that you need to collect energy to produce the electron-positron pair. You need the equivalent rest mass of energy that is the amount of energy contained in the both particle and antiparticle when at rest. The energy converted to mass is ‘lost’ or fully ‘’bound’’ until the particle is annihilated and the energy can be recovered. The second thing is that it needs a correct environment. The process does not occur unless certain conditions are present.

    Viewing the phenomena as a creative process we can say a threshold amount of energy is sacrificed in a correct context to manifest a pair of particle with a physical mass. It can be said something was created out of nothing. That is before the interaction, no particles with mass existed. After interaction, there were two particles with mass. Hence something was created out of nothing. But this can be said only because of the perspective taken when viewing the process. 

    For every charged particle of nature, whether it is one of the elementary particles of the standard model, or a hadron, there is an associated particle of the same mass, but opposite charge, called its antiparticle

    This result is a necessary consequence of combining special relativity with quantum mechanics. This important theoretical prediction was made by Dirac and follows from the solutions of the equation he first wrote down to describe relativistic electrons

    8.3.2 Pauli’s exclusion principle, 

    Pauli’s exclusion principle is a quantum mechanical principle which states that: “Two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state simultaneously.” 

    In case of electrons in atoms it can be stated as follows: it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers: 

    The principle quantum number No, the angular momentum quantum number (l), the magnetic quantum number (ml) and the spin quantum number (ms). For example, if two electrons reside in the same orbital and if their ms must be different and thus like electrons must have opposite half integer spin projections of 1/2 and-1/2.

    This principle was formulated by Austrian physicist Wolfgang Pauli in 1925 for electrons, and later extended to all fermions with his spin–statistics theorem of 1940. Particles with an integer spin, or bosons, are not subject to the Pauli Exclusion Principle: any number of identical bosons can occupy the same quantum state, for instance, photons produced by a laser and Bose–Einstein condensate.

    The Pauli Exclusion Principle describes the behavior of all fermions (particles with “halfinteger spin”), while bosons (particles with“ integer spin”) are subject to other principles. Fermions include elementary particles such as quarks, electrons and neutrinos. Additionally, baryons such as protons and neutrons (subatomic particles composed from three quarks) and some atoms (such as helium-3) are fermions, and are therefore described by the Pauli Exclusion Principle as well.

    8.3.3 Checking my progress

     1. What do you understand by antiparticle?

     2.  State Pauli’s exclusion principle?

     3.  Why Pauli’s exclusion Principle is known as exclusion? 


    Activity 8.4 Fundamental interaction 

    Using internet, discusses the fundamental interactions in terms of exchange particles, then find the relation between the following concepts.

     1. Gravitational forces

     2. electroweak force,

     3. Strong force and

     4. Weak forces.

    8.4.1 Forces and Interactions 

    Physicists have recognized three basic forces: 

     • The gravitational force is an inherent attraction between two masses. Gravitational force is responsible for the motion of the planets and Stars in the Universe. It is carried by Graviton. By Newton’s law of gravitation, the gravitational force is directly proportional to the product of the masses and inversely proportional to the square of the distance between them. Gravitational force is the weakest force among the fundamental forces of nature but has the greatest large−scale impact on the universe. Unlike the other forces, gravity works universally on all matter and energy, and is universally attractive 

     • The electric force is a force between charges

     • The magnetic force, which is a force between magnets or between magnetic body and ferromagnetic body.

    In the 1860s, the Scottish physicist James Clerk Maxwell developed a theory that unified the electric and magnetic forces into a single electromagnetic force. Maxwell’s electromagnetic force was soon found to be the “glue” holding atoms, molecules, and solids together. It is the force between charged particles such as the force between two electrons, or the force between two current carrying wires. It is attractive for unlike charges and repulsive for like charges. The electromagnetic force obeys inverse square law. It is very strong compared to the gravitational force. It is the combination of electrostatic and magnetic forces. 

    The discovery of the atomic nucleus, about 1910, presented difficulties that could not be explained by either gravitational or electromagnetic forces. The atomic nucleus is an unimaginably dense ball of protons and neutrons. But what holds it together against the repulsive electric forces between the protons? There must be an attractive force inside the nucleus that is stronger than the repulsive electric force. This force, called the strong force, is the force that holds the protons and neutrons together in the nucleus of an atom. It is the strongest of all the basic forces of nature. It, however, has the shortest range, of the order of 10−15 m. This force only acts on quarks. It binds quarks together to form baryons and mesons such as protons and
    neutrons. The strong force is mediated or carried by Gluons. Quarks carry electric charge so they experience electric and magnetic forces. 

    In the 1939, physicists found that thenuclear radioactivity called beta decay could not be explained by either the electromagnetic or the strong force. Careful experiments established that the decay is due to a previously undiscovered force within thenucleus. The strength of this force is less than either the strong force or the electromagnetic force, so this new force was named the weak force. Weak nuclear force is important in certain types of nuclear process such as β-decay. This force is not as weak as the gravitational force. The weak force acts on both leptons and quarks (and hence on all hadrons). The weak force is carried by W+, W- and Z. Leptons – the electrons, muons and tau – are charged so they experience electric and magnetic forces.

    Of these, our everyday world is controlled by gravity and electromagnetism. The strong force binds quarks together and holds nucleons (protons & neutrons) in nuclei. 

    The weak force is responsible for the radioactive decay of unstable nuclei and for interactions of neutrinos and other leptons with matter. 

    By 1940, the recognized forces of nature (fundamental forces)were four:

      • Gravitational forces between masses,

      • Electromagnetic forces resulting from the combination of electric and magnetic fields,

      • Strong force (nuclear force) between subatomic particles,

      • Weak forces that arise in certain radioactive decay processes. 

    By 1980, Sheldon Glashow, Abdus Salam, and Steven Berg developed a theory that unifies electromagnetism and weak force into electroweak force. Hence, our understanding of the forces of nature is in terms of three fundamental forces:

      • The gravitational force,

      • The electroweak force,

      • The strong force. 


      • W boson: short-lived elementary particle; one of the carriers of the weak nuclear force 

      • Z boson: short-lived elementary particle; one of the carriers of the weak nuclear force

      • Graviton: the hypothetical particle predicted to carry the gravitational force

    All the forces of nature should be capable of being described by single theory. But only at high energies should be the behavior of the forces combines, this is called unification.

    We can compare the relative strengths of the electromagnetic repulsion and the gravitational attraction between two protons of unit charge using the above equations.




    8.5.1 The concept of uncertainty principle

    Activity 8.5: Investigation of particle creation and position. 

    Basing on the knowledge and skills obtained from the previous sections of this unit, use internet to fid the meaning of the particle creation. 

       a. Is it possible to know the exact location of an elementary particle?

       b. Discuss and explain your fidings

    The discovery of the dual wave–particle nature of matter forces us to re-evaluate the kinematic language we use to describe the position and motion of a particle. 

    In classical Newtonian mechanics we think of a particle as a point. We can describe its location and state of motion at any instant with three spatial coordinates and three components of velocity. But because matter also has a wave aspect, when we look at the behaviour on a small enough scale comparable to the de Broglie wavelength of the particle we can no longer use the Newtonian description. Certainly no Newtonian particle would undergo diffraction like electrons do. 

    To demonstrate just how non Newtonian the behaviour of matter can be, let’s look at an experiment involving the two-slit interference of electrons (Fig.8.4). 

    We aim an electron beam at two parallel slits, just as we did for light. (The electron experiment has to be done in vacuum so that the electrons don’t collide with air molecules.) 


    What kind of pattern appears on the detector on the other side of the slits? 

    The answer is: exactly the same kind of interference pattern we saw for photons. Moreover, the principle of complementarily, tells us that we cannot apply the wave and particle models simultaneously to describe any single element of this experiment. Thus we cannot predict exactly where in the pattern (a wave phenomenon) any individual electron (a particle) will land. We can’t even ask which slit an individual electron passes through. If we tried to look at where the electrons were going by shining a light on them that is, by scattering photons off them the electrons would recoil, which would modify their motions so that the two-slit interference pattern would not appear. 

    Just as electrons and photons show the same behaviour in a two-slit interference experiment, electrons and other forms of matter obey the same Heisenberg uncertainty principles as photons do: 

    Heisenberg uncertainty principle for position and momentum is given by


    8.5.2 Checking my progress

    1. The idea of uncertainty is used in many contexts; social, economic and scientific. People often talk about uncertain times, and when you perform a measurement you should always estimate the uncertainty (sometimes called the error). In physics the Heisenberg Uncertainty relation has a very specific meaning.

      a. Write down the Heisenberg uncertainty relation for position and momentum.

      b. Explain its physical significance.

      c. Does the Heisenberg uncertainty principle need to be considered when calculating the uncertainties in a typical first year physics experiment? Why or why not?

      d. Discuss the following statement: the uncertainty principle places a limit on the accuracy with which a measurement can be made. Do you agree or disagree, and why?

     2. An electron is confined within a region of width 11 5 10 m −× (Roughly the Bohr radius)

       a.  Estimate the minimum uncertainty in the component of the electron’s momentum.

       b.  What is the kinetic energy of an electron with this magnitude of momentum? Express your    answer in both joules and electron volts.


    Activity 8.6: Describing the matter and antimatter

    Use internet to describe the following concepts:
      1. matter and give examples of  matter particles

      2. antimatter and give examples of antimatter particles

      3. Pair production

      4. Annihilation

    8.6. 1 Introduction 

    Matter is a substance that has mass and takes up a space by having a volume. This include atoms and anything made up of these but no other energy phenomena or wave such as light or sound. 

    Everything around you is made up of matter and is composed of particles including the fundamental fermions (quarks, leptons, antiquarks and antileptons) which generally are matter particles and antimatter particles. 

    Antimatter is a material composed of the antiparticle to the corresponding particle or ordinary particles. In theory a particle and its antiparticle have the same mass as one another but opposite electric charge and other differences in quantum numbers. 

    Neutrons have antineutrons, electrons have positrons and neutrons have antineutrons as their respective antimatter. It was once thought that matter would neither be created nor destroyed. We know that energy and mass are interchangeable.

    8.6.2 Pair production and annihilation 

    Pair production is a crucial example that photon energy can convert into kinetic energy as well as rest mass energy. Schematic diagram about the process of pair production is shown in Fig.8.5. The high-energy photon that has energy fh loses its entire energy when it collides with nucleus. Then, it makes pair of electron and positron and gives kinetic energy to each particle.


    Annihilation: When a particle collides with its antiparticle, the two annihilate each other with their mass being entirely converted into energy by the process called ‘’Annihilation’’ 

    These particles and anti-particles can meet each other and annihilate one another (See Fig.8.6). . In each case the particle and its antiparticle annihilate each other, releasing a pair of high energy gamma photons.


    In this example, a proton and an anti-proton meet each other and annihilate, producing high energy gamma rays in the form of photons. Rest mass, charge, momentum and energy are conserved.

    They can also be produced from a high energy photon, this is called pair production.

     8.6.3 Application of antimatter 

    Antimatter as a form of antiparticle of sub atomic particles has a variety of applications:

      • Positron emission tomography can be used to potentially treat cancer.

      • Stored antimatter can be used for interplanetary and inter stellar travel.

      • Antimatter reactions have practical applications in medical energy.

      • Antimatter has been considered as a trigger mechanism for nuclear weapons because whenever antimatter meets its corresponding matter the energy is released by annihilation.

    8.6.4 Checking my progress 

      1. Antimatter as a form of sub atomic particles

        a. Electron

        b. proton  

        c. matter 

        d. antiparticle

        e. none of them is correct

     2. The process in which a particle and antiparticle unite annihilate each other and produce one or      more photons is called………

     3. What happens when matter and antimatter collide?

     4. Compare matter and antimatter


    8.7.1 Multiple choices 

      1. The positron is called the antiparticle of electron, because it

         a. Has opposite charge and Annihilates with an electron

         b. Has the same mass

         c. Collides with an electron

         d. Annihilates with an electron

    2. Beta particles are

        e. Neutrons

        f. Protons

        g. Electrons

        h. Thermal neutrons

    3. If gravity is the weakest force, why is it the one we notice most?

       a. Our bodies are not sensitive to the other forces.

       b. The other forces act only within atoms and therefore have no effect on us.

       c. Gravity may be “very weak” but always attractive, and the Earth has enormous mass. The strong and weak nuclear forces have very short range. The electromagnetic force has a long range, but most matter is electrically neutral.

       d. At long distances, the gravitational force is actually stronger than the other forces.

       e. The other forces act only on elementary particles, not on objects our size.

    8.7.2 Structured questions 

    4. According to the classification of elementary particles by mass. Complete the following figure

        I.  State two differences between a proton and a positron. 

       II.  A narrow beam of protons and positrons travelling at the same speed enters a uniform magnetic field. The path of the positrons through the field is shown in Fig.8.7. Sketch on this figurethe path you would expect the protons to take.        


                                                           Fig.8. 5 Particles in magnetic field
                   III.  Explain why protons take a different path to that of the positrons.

       6. A positron with kinetic energy 2.2 MeV and an electron at rest annihilate each other. Calculate the average energy of each of the two gamma photons produced as a result of this annihilation. 

    8.7.3 Essay question 

      7. Describe briefly the following particle-terms terms: π -meson, muon, neutrino, antiparticle,      hadrons and lepton.