Physics SME

     Key Unit Competence: 

      Classify the nature of particles and their interactions

     Introductory activity

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. 
a) Basing on the text above, what are the smallest particles in the universe 
    you know as of now?
b) Are those particles mentioned in a) above, have the same mass? If no, 
     what causes the difference in masses?
c) Basing on your submission in a) above do those particles interact? If not, 
     why not? If yes, how do they interact?

d) Scientifically, how would you classify these particles?

      4.1 FUNDAMENTAL PARTICLES 

               Activity 4.1

a) From your knowledge and understanding about atom in chemistry, can 
      you define an atom?
b) State all the particles you think make up an atom
c) Can you suggest a scientific name that may be used to describe these 

     particles? 

The idea that the world is made of fundamental particles has a long history. In 

about 400 B.C. The Greek philosophers Democritus and Leucippus suggested

that matter is made of indivisible particles that they called atoms, a word 
derived from a (not) and tomos (cut or divided). At that time atoms were thought to 
be indivisible constituents of matter. They were regarded as elementaryparticle. 
This idea lay dormant until about 1804, when the English scientist John Dalton 
(1766–1844), often called the father of Modern Chemistry, discovered that many 
chemical phenomena could be explained if atoms of each element are the basic, 
indivisible building blocks of matter. 

Although Dalton had postulated that atoms were indivisible particles, experiments 
conducted around the beginning of the last century showed that atoms themselves 
consist of particles. These experiments showed that an atom consists of two kinds 
of particles: a nucleus, the atom’s central core, which is positively charged and 
contains most of the atom’s mass, and one or more electrons. 

         4.1.1 The Electron and the Proton
Some of the earliest evidence about atomic structure was supplied in the early 
1800s by the English chemist Humphry Davy (1778–1829). He found that when 
he passed electric current through some substances, the substances decomposed. 
He therefore suggested that the elements of a chemical compound are held together 
by electrical forces. In 1832–1833, Michael Faraday (1791–1867), Davy’s 
student, determined the quantitative relationship between the amount of electricity 
used in electrolysis and the amount of chemical reaction that occurs. Studies of 
Faraday’s work by George Stoney (1826–1911) led him to suggest in 1874 that 
units of electric charge are associated with atoms. In 1891, he suggested that they 
be named electrons.

Rutherford’s experiments in 1910–1911 revealed that atoms consist of mostly 
empty space with electrons surrounding a dense central nucleus made up of 
protons and neutrons.
 
        4.1.2 The Neutron
In 1930 the German physicists Walther Bothe and Herbert Becker and Irene 
Joliot-Curie (1897–1956) observed that when beryllium, boron, or lithium was 
bombarded by alpha particles, the target material emitted a radiation that had much 
greater penetrating power than the original alpha particles.

Experiments done by the English physicist James Chadwick (1891–1974) in 1932 
showed that the emitted particles were electrically neutral, with mass approximately 
equal to that of the proton. Chadwick christened these particles neutrons(symbol 
                        
Elementary particles are usually detected by their electromagnetic effect for 
instance, by the ionization that they cause when they pass through matter. (This 
is the principle of the cloud chamber). Because neutrons have no charge, they 
interact hardly at all with electrons and produce little ionization when they pass 
through matter and so are difficult to detect directly. 

However, neutrons can be slowed down by scattering from nuclei, and they can 
penetrate a nucleus. Hence slow neutrons can be detected by means of a nuclear 

reaction in which a neutron is absorbed and an alpha particle is emitted. An example 

                              

     The ejected alpha particle is easy to detect because it is charged. Later experiments

       

      4.1.3 The Photon

Einstein explained the photoelectric effect in 1905 by assuming that the energy 
of electromagnetic waves is quantized; that is, it comes in little bundles called 

photons with energy 

         

        4.1.4 The Neutrino
The Sun emits neutrinos copiously from the nuclear furnace at its core, and at night 
these messengers from the center of the Sun come up at us from below, Earth 
being almost totally transparent to them. 

         4.1.5 The Positron and other antiparticles

         

 Experiment and theory tell us that the masses of the positron and electron are 
identical, and that their charges are equal in magnitude but opposite in sign. We 
use the term antiparticle for a particle that is related to another particle as the 
positron is to the electron. The positron is said to be the antiparticle to the 

electron.

In 1955 the antiparticle to the proton, the antiproton    which carries a negative 
charge, was discovered at the University of California, Berkeley, by Emilio Segrè 
(1905–1989) and Owen Chamberlain (1920–2006). A bar, such as over the p, 

is used to indicate the antiparticle. 

Each kind of particle has a corresponding antiparticle. But a few, like the photon, 
the and the Higgs, do not have distinct antiparticles we say that they are their 
own antiparticles.

By the 1930s, it was accepted that all atoms can be considered to be made up 
of neutrons, protons, and electrons. The basic constituents of the universe were 
no longer considered to be atoms (as they had been for 2000 years) but rather 
the proton, neutron, and electron. Besides these three “elementary particles,” 
several others were also known by the 1950s and 1960: the positron (a positive 
electron), the neutrino, and the γ particle (or photon), for a total of six elementary 
particles.

     4.1.6 Mesons and Beginning of Elementary Particle Physics
Elementary particle physics might be said to have begun in 1935 when the 
Japanese physicist Hideki Yukawa (1907–1981) predicted the existence of a 
new particle that would mediate the strong nuclear force the force that holds 
nucleons together in the nucleus. Yukawa called this predicted particle meson

(meaning medium mass).

       

Electrons contain no discernible structure; they cannot be reduced or separated into 
smaller components. It is therefore reasonable to call them “elementary” particles, 
a name that in the past was mistakenly given to particles such as the proton, which 
is in fact a complex particle that contains quarks. The term subatomic particle
refers both to the true elementary particles, such as quarks and electrons, and to 

the larger particles that quarks form.

By the term fundamental particle, we mean a particle that is so simple, so basic; 
that it has no internal structure (is not made up of smaller subunits). 
The science of the study of the particle is called Particle Physics, 

Elementary Particle Physics or sometimes High Energy Physics (HEP).

           Application activity 4.1 

1. The positron is called the antiparticle of electron, because it
       A. Has opposite charge               C. Collides with an electron
      B. Has the same mass                  D. Annihilates with an electron
2. Beta particles are
         A. Neutrons                               C. Electrons
        B. Protons                                  D. Thermal neutrons
3. The proton, neutron, electron, and the photon are called
      A. secondary particles                                  C. basic particles
      B. fundamental particles                             D. initial particles
4. Particles that are unaffected by strong nuclear force are
      A. protons                                         C. neutrons
      B. leptons                                         D. bosons
5. The first antiparticle found was the
      A. positron.                                  C. quark. 
     B. hyperon.                                   D. baryon.
6. The exchange particle of the electromagnetic force is the
     A. Gluon.                                     C. proton. 

     B. Muon.                                     D. photon.

           4.2 CLASSIFICATION OF PARTICLES

                      Activity 4.2

a) Basing on what so-far you have studied, classify these elementary 
     particles?
b) Explain what you have based on to classify them.

c) State the common characteristics/features for each group.

Today there are several hundreds of known particles. Naming them has strained the 
resources of the Greek alphabet, and most are known only by an assigned number 
in a periodically issued compilation. To make sense of this array of particles, we 
look for simple physical criteria by which we can place the particles in categories. 
The result is known as the Standard Model of particles. Although this model is 
continuously challenged by theorists, it remains our best scheme of understanding 
all the particles discovered to date. 

To explore the Standard Model, we make the following three rough families of 
the known particles: the photon: fermion or boson, hadron or lepton, particle or 

antiparticle?

As more and more particles were discovered, it became clear that they were not all
elementary particles (fundamental or basic particles). The suggestion was 
made that the hadrons are made up of smaller, more elementary particles called 
quarks. They are three families of quarks and three corresponding antiquarks, and 
hadrons are constructed from combinations of these. Thus the quarks are elevated 
to the status of elementary particles for the elementary particles for the family of 
hadrons. The particles in the photon and lepton families are considered to be 
elementary, and such they are not composed of quarks.

The fundamental particles were classified into two categories according to their 

spin: Fermions and Bosons 

          4.2.1 Fermions
Particles with half-integer spin quantum numbers (like electrons) are called 
fermions, after Fermi, who (simultaneously with Paul Dirac) discovered the 
statistical laws that govern their behavior. 

They are two families of fermions (of spin ½): leptons and quarks

      a. Leptons
Leptons (from the Greek leptos meaning small or light) are a group of particles 
that participate in the weak nuclear force, they can exert gravitational, and they are 
charged particles hence exert electromagnetic force on other particles. All leptons 
have spin  and thus are fermions.
  Three pairs or families of leptons and their anti-particles exist as listed in the 

    table 4.1. 

        

The six leptons are considered to be truly fundamental particles because they do 

not show any internal structure, and have no measurable size. 

      b. Quarks
They are six quarks or flavors of quarks: up (u), down (d), strange (s), charm 
(c),bottom (b)and top (t) quarks and they each have their partner anti-quarks

(designated by a line over the letter symbol). 

        

          

Quarks combine to form hadrons or meson. The hadrons are a composite particle 
made of quarks (u, d, c, s, t, b) held together by the strong nuclear force. Hadrons 
can also interact by weak nuclear force, gravitational force and electromagnetic 

forces but at short distances

Hadrons are categorized into families distinguished by their masses and spins:

Hadrons and baryons

        a. Mesons

        

         b. Baryons

            

      The table below show gauge bosons

         

        In summary, the following diagram shows some classes of elementary particles

         

                Application activity 4.2

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 unaffected by strong nuclear force are
      A. protons                           C. neutrons
      B. leptons                           D. bosons
3. Particle which explains about mass of matter is called
     A. Higgs boson                   C. protons
     B. Leptons                            D. neutrons
4. Each hadron consists of a proper combination of a few elementary 
        components called
      A. Photons                                C. quarks
      B. Vector bosons                    D. meson-baryon pairs.
5. The proton, neutron, electron, and the photon are called
       A. secondary particles                          C. basic particles
       B. fundamental particles                    D. initial particles
6. The exchange particle of the electromagnetic force is the
       A. gluon.                           C. proton. 
       B. muon.                          D. photon.
7. Particles that interact by the strong force are called
     A. leptons                    C. muons
     B. hadrons                  D. electrons
8. At the present time, the elementary particles are considered to be 
        the
     A. Photons and baryons. C. Baryons and quarks. 
     B. Leptons and quarks. D. Baryons and leptons.
9. The electron and muon are both
      A. Hadrons.           C. Baryons.
      B. Leptons.            C. Mesons.
10. 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.
11. Is it possible for a particle to be both:
       A. A lepton and a baryon?         C. A meson and a quark?
       B. A baryon and hadron?           D. A hadron and a lepton?
12. Distinguish between (a) fermions and bosons, (b) leptons and 
       hadrons and (c) mesons and baryon number
13. Which of the four interactions (strong, electromagnetic, weak, and 
      gravitational) does an electron take part in? A neutrino? A proton?
14. Describe the types and the characteristics of the quarks as well as 

       their interaction properties.

        4.3 FUNDAMENTAL FORCES AND INTERACTIONS

                Activity 4.3

From the previous lessons, you have learned that the particles have different 
charges and masses. Basing on that, explain the magnitude of force that may 
rise between these elementary particles depending on:
a) Masses (You can apply newton’s law of gravitation)

b) Charges (You can use coulombs law of charges)

       4.3.1 Antiparticle and antimatter
Antiparticles are produced in nuclear reactions when there is sufficient energy 
available to produce the required mass, and they do not live very long in the 
presence of matter. 

Antimatter is a term referring to material that would be made up of “antiatoms” in 
which antiprotons and antineutrons would form the nucleus around which positrons 
(antielectrons) would move. The term is also used for antiparticles in general.                                                                                           Anti matter is a material composed of anti-particles. 

We use the term antiparticle for a particle that is related to another particle as the 
positron is to the electron. Each kind of particle has a corresponding antiparticle. 
For a few kinds of particles (necessarily all neutral) the particle and antiparticle are 
identical, and we can say that they are their own antiparticles. The photon is an 
example; there is no way to distinguish a photon from an anti photon. 

Pair production and pair annihilation
Positrons do not occur in ordinary matter. Electron–positron pairs are produced 
during high-energy collisions of charged particles or γ -rays with matter. 

 This process is called  pair production.

                                        

When a particle meets its antiparticle, the two can annihilate each other and 
release a large amount of energy. That is, the particle and antiparticle disappear 
and their combined energies reappear in other forms. For an electron annihilating 

with a positron, this energy reappears as two gamma-ray photons:

                                     

      4.3.2 Fundamental Interactions and Force Mediators

In nature they are two types of forces, fundamental and non-fundamental forces. 
Fundamental (basic) forces are the ones that are truly unique, in the sense that all 
other forces can be explained in terms of them. 
By 1940, Physicists have long recognized for forces of nature (fundamental 
forces): 

• The gravitational force 
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 but its existence has not been detected and it 
may not be detectable. 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 electromagnetic interaction
In Classical Physics we describe the interaction of charged particles in terms of 
electric and magnetic forces. The electric force is a force between charges.
The magnetic force is a force between magnets or between magnetic body and 

ferromagnetic body.

In quantum mechanics we can describe this interaction in terms of emission and 
absorption of photons. Two electrons repel each other as one emits a photon and 
the other absorbs it, just as two skaters can push each other apart by tossing a 

heavy ball back and forth between them (Fig. 4.5). 

               

 For an electron and a proton, in which the charges are opposite and the force 
is attractive, we imagine the skaters trying to grab the ball away from each other 
(Fig. 4.6). The electromagnetic interaction between two charged particles is 

mediated or transmitted by photons

                               

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.

                     

    • Strong nuclear force
In 1935 the Japanese physicist Hideki Yukawa suggested that a hypothetical 
particle that he called a meson might mediate the strong nuclear force. The 
strong nuclear force is the force that holds the protons and neutrons together 
in the nucleus of an atom. It plays a primary role in stability of the nucleus of the 
atoms. It is the strongest of all the basic forces of nature. It, however, has the 
shortest range, of the order of 10−15 m (the size of the nucleus). This force only acts 
on quarks. Quarks carry electric charge so they experience electric and magnetic 
forces. It binds quarks together to form baryons and mesons such as protons and 
neutrons. The strong force is mediated by Gluons. However, the force between 

nucleons is more easily described in terms of mesons as the mediating particles.

                     

            • The weak force

In the 1939, Physicists found that the nuclear radioactivity called beta decay could 
not be explained by either the electromagnetic or the strong force. 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). 
 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 weak force is responsible for the radioactive decay of 

unstable nuclei and for interactions of neutrinos and other leptons with matter. 

By 1980, Scientists developed a theory that unifies electromagnetism and weak 
force into electroweak force mediated by particles, the 
Hence, our understanding of the forces of nature is in terms of three fundamental 

forces: the gravitational force, the electroweak force, and the strong force. 

The table below summaries the fundamental forces: 

                   

The intrinsic strengths of the forces can be compared relative to the strong 
force, here considered to have unit strength(i.e., = 1). In these terms, the 
electro magnetic force has an intrinsic strength of (1/137). The weak force
is a billion times weaker than the strong force. The weakest of them all is the 
gravitational force. This may seem strange, since it is strong enough to hold 
the massive Earth and planets in orbit around the Sun! But we know that that the 
gravitational force between two bodies a distance r apart is proportional to the 
product of the two masses (M and m) and inversely proportional to the distance r 

squared:

                                     

We see now what is meant by intrinsic strength. It is given by the magnitude 
of the universal force constant, in this case, G, independent of the masses or 

distances involved. 

In similar terms, the electromagnetic force between two particles is proportional to 

the product of the two charges (Q and q) and inversely to the distance r squared:

            here the universal constant, k, gives the intrinsic strength.

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

equations.

              

Example 4.1

The 2 up quarks (u) in a proton are separated by   These quarks 
each have an electric charge  so they repel each other through an electric
force obeying Coulomb’s law. They also attract each other through a gravitational 
force. What is the ratio of the magnitude of the electric and gravitational forces 

between these 2 quarks?

     Answer

        

         Application activity 4.3

1. If gravity is the weakest force, why is it the one we notice most?
    Choose the letter correspond to the correct answer
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.
2. The two basic interaction forces that have finite ranges are:
     A. electromagnetic and gravitational            D. gravitational and weak
     B. electromagnetic and strong                         E. weak and strong
     C. electromagnetic and weak
3. Name the four fundamental interactions and the particles that 
     mediate each interaction.
4. State two differences between a proton and a positron.
5. Copy figure below into your notebook. Complete the diagram by 
     filling in the names of the fundamental forces and the names of the 

     unification theories.

            

                                    Skills Lab 4

In this activity, you make a comprehensive research about nature of particles 
and their interactions.

Materials needed: Computer set, internet and reference books.

What to do.
a) Using internet, make general studies about the following
      • Classification of particles and antiparticles.
      • Properties of fundamental particles (Charges, spin, quark contents)
b) Compile your findings and make a report. Relate your findings to what 
     you have studied in this unit.
c) You can compare your report to your friends’ report and check whether 
     the information you have is related to your friends work.
d) You and your classmates may prepare a session of presentation so 
      that you can harmonize what you got in your research. Present using 
      PowerPoint presentations.
e) In case you find new points from other presentations, include it to your 

     report.

          End of unit 4 assessment

1. Which of the following are today considered fundamental particles 
(that is, not composed of smaller components)? Choose as many 
as apply.
      A. Atoms.             C. Protons.         E. Quarks.         G. Higgs boson
      B. Electrons.       D. Neutrons.     F. Photon
2. The electron’s antiparticle is called the positron. Which of the 
     following properties, if any, are the same for electrons and positrons?
      A. Mass.                                      C. Lepton number
     B. Charge.                                  D. None of the above.
3. The strong nuclear force between a neutron and a proton is due to
      A. The exchange of π mesons between the neutron and the proton.
      B. The conservation of baryon number.
      C. The beta decay of the neutron into the proton.
      D. The exchange of gluons between the quarks within the neutron and 
            the proton.
E. Both (A) and (D) at different scales.
4. Which of the following will interact via the weak nuclear force only?
      A. Quarks.                    C. Neutrons        E. Electrons      G. Higgs boson.
      B. Gluons.                    D. Neutrinos.     F. Muons. 
5. Messenger particles of the weak interaction are called:
     A. gluons                                           D. gravitons
     B. photons                                        E. pions
     C. W and Z
6. Messenger particles of the electromagnetic interaction are called:
      A. gluons                           D. gravitons
      B. photons                       E. pions
      C. W and Z
7. The pair annihilation of an electron and a positron has been 
     investigated for many years at CERN in Switzerland. Two gamma-ray 

     photons are produced during this annihilation. What is a positron?

8. True or false: if the statement is true, explain why it is true. If it is 
           false, give a counterexample.
       (a) leptons are fermions
       (b) all baryons are hadrons
       (c) all hadrons are baryons
(d) mesons are spin ½ particles
       (e) leptons consist of three quarks
       (f) the times for decays via the weak interaction are typically much 
             longer than those for decays via the strong interaction

      (g) the electron interacts with the proton via the strong interaction