UNIT 8: PROPERTIES AND BASIC PRINCIPLES OF QUARKS
Key unit competence: Organize the properties and basic principles of quarks.
My goals• List types of quarks, identify quarks, antiquarks and hadrons (baryonsINTRODUCTORY ACTIVITY
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.
8.1 INTRODUCTION
ACTIVITY 8.1: 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 about elementary 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 beenrepeated 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 ofα-particles striking a golden foil (See Fig 9.1).
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 amongthem.
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
The quarks
and leptons are elementary at this level (Nagashima, 2013). Some compositeparticles as stated by (Hirsch, 2002) are summarized in the tables below
8.1. Checking my progress
1. Each hadron consists of a proper combination of a few elementary
components called
a. Photons. c. Quarks.
b. Vector bosons. d. Meson-baryon pairs.
2. Which of the following is not conserved in a nuclear reaction?
a. Nucleon number. c. Charge
b. Baryon number. d. All of the above are
c. Conserved.
3. The first antiparticle found was the
a. Positron. c. Quark.
b. Hyperon. d. Baryon.
8.2 TYPES OF QUARKS
ACTIVITY 9.2: Investigating Quark particles
Use search internet and find the explanation about quarks and types ofquarks
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, and bottom (see Fig. 8.2). Up and down quarks have the lowestmasses of all quarks.
The heavier quarks rapidly change into up and down quarks through a process
of particle 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).
8.2.1 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 c. Neutrons
b. Leptons d. Bosons
3. Particle which explains about mass of matter is called
a. Higgs boson c. Leptons
b. Protons d. Neutrons
4. Describe the types and the characteristics of the quarks as well as their
interaction properties.
8.3 BARYON NUMBER, LEPTON NUMBER AND THEIR LAWS OFCONSERVATION
ACTIVITY 8.3: Investigating about particle numbers
Use search internet and retrieve the meaning of the following property
of elementary particles.• Baryon numbers and• Lepton numbers
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 tohold 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 reactionshave never been found to occur:
Even though charge, energy, and so on are conserved
means an antiproton and 
means the reaction does not occur). To understand why such a reaction does
not occur, physicists hypothesized a new conservation law, the conservationof 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 mustequal 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
, and all antinucleons (antiprotons, antineutrons) have 
. All
other types of particles, such as photons, mesons, and electrons and otherleptons have
The reaction (9.01) shown above does not conserve baryon number since the
.
left side B = +1+1 = +2 , and the right-hand side has B = +1+1−1 = +1 On
the other hand, the following reaction does conserve B and does occur if theincoming proton has sufficient energy
As indicated,
on both sides of this equation. From these and other
reactions, the conservation of baryon number has been established as basicprinciple of physics.
Also useful are the conservation laws of the three lepton numbers, associated
with weak interactions including decays, in ordinary
decay, an electron or
positron is emitted along with a neutrino or antineutrino. In a similar type of
decay, a particle known as or mu meson, or muo, can be emitted instead of
an electron. The muon seems to be much like an electron, except its mass is
207 times larger
The neutrino
that accompanies an emitted
electron is found to be different from the neutrino
that accompanies anemitted muon. Each of these neutrinos has an antiparticle
The law of conservation of electron-lepton number states that: “The sum of the
electron-lepton numbers before reaction or decay must equal the sum of the
electron-lepton numbers after the reaction or decay.”In ordinary decay we have for example,, a second quantum number, muon
lepton number, is conserved. The and are assigned and and havewhereas other
particles have, too is conserved in interaction and decays. Similarly assignment
can be made for the tau lepton number associated with the Lepton and itsneutrino,
Keep in mind that antiparticles have not only opposite electric charge fromtheir particles, but also opposite
For example, neutrino has B = 1 an antineutrion has B = −1while all the
The particle predicted by Yukawa was discovered in cosmic rays by C.F Powell
and G. Ochialini in 1947, and is called the π or pi meson, or simple called pion.
The incident proton from the accelerator must have sufficient energy to produce
the additional mass of the free pion. Baryon number conservation keeps the
proton stable, since it forbids the decay of the proton to e.g. a 0 π and a + π eachof which have baryon number of zero.
8.3.1 Checking my progress
8.4 SPIN STRUCTURES OF HADRONS (HADRONS AND MESONS)
ACTIVITY 8.4: Investigating the structure of elementary particles1. Use search internet and find the structure of elementary particles:
Hadrons and mesons.
2. Discuss and explain your findings in a brief summary aboutstructure 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 trulyelementary particles, like leptons. The three quarks originally proposed we
labeled u, d, s and have the names up, down and strange. The theory today hassix 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.8.2). 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), andwhich (like strangeness) are conserved in strong, but not weak, interactions.
Table 8. 1 Properties of Quarks (Antiquarks have opposite sign Q, B. S, c, b and t
All hadrons are considered to be made up of combinations of quarks, and their
properties are described by looking at their quark content. Mesons consist of
quark-antiquark pair (See Table 8.2).For example, a + π meson is a ud combination: note that for the ud pair,
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 ofconstituents.
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”. The others 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 sixquarks and six antiquarks.
Note that the quarks and leptons are arranged into three generations each
8.4.1 Checking my progress
8.5 COLOR IN FORMING OF BOUND STATES OF QUARKS
ACTIVITY 8.5: 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 andproton and bound state of elementary particles.
8.5.1 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 protonis heavy while the electron is light (See Fig.8.3)
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, asspouses 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.8.4)
inside are rushing around as fast as possible, at nearly the speed of light(Strassler, 2011).
Fig.8. 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 quarksin a proton.
8.5.2 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 toas 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 anti color. Both baryons and mesons are thuscolorless 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 electrons, protons and neutrons, are called fermions. Those that don’t are
called bosons. These two categories are distinguished also in their spin: bosons
have integer spin (0, 1, etc) whereas fermions have half-integer spin, usual
as for electrons and nucleons, but other fermions have spin
Matter is made up mainly of fermions, but the carriers of forces ( and
gluons) are all bosons. Quarks are fernions they have spin 2
1 and therefore
should obey the exclusion principle. Yet for three particular baryons (uuu, ddd,
and sss), all three quarks would have the same quantum numbers, and at least
two quarks have their spin in the same quantum numbers, and at least twoquarks have their spin in the same direction (since there are only two choices,
This would seem to violate the exclusion principle; but if quarks have an
additional quantum number (color), which is different for each quark, it would
serve to distinguish them and allow the exclusion principle to hold. Although
quark color, and the resulting threefold increase in the number of quarks, was
originally an adhoc idea, it also served to bring the theory into better agreement
with experiment, such as predicting the correct lifetime of the π 0 meson. The
idea of color soon became a central feature of the theory as determining theforce binding quarks together in hadron.
8.5.3 Colour as component of quarks and gluons
ACTIVITY 8.6: 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 consequences3. 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 theold Yukawa theory (Nagashima, 2013).
Particles were classified into two categories:
Quarks and leptons have an intrinsic angular momentum called spin, equal to
a half-integer (
) 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, soSimilar processes occur in mesons such as pions:
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 areexchanged.
• The quark–antiquark pairs of mesons have canceling color andThe gluon is then absorbed by the antiblue antiquark, converting it to an antired
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–antiredvirtual gluon.
antiquark (Fig. 8.8). 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 quantumstates:
• Blue–antiblue,In terms of quarks and gluons, these mediating virtual mesons are quark–
• Green–antigreen, and• Red–antired.
antiquark systems bound together by the exchange of gluons.
Fig.8. 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, andblue–antiblue pairs.
8.5.4 Checking my progess
1. Label the illustration below and analyze the interaction between itsparticles
Define and describe the following key concept:
I. Color charge:
II. Gluons
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. Mesonc. Baryon
END UNIT ASSESSMENT 8
A. 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
a. Protons c. Neutrons
b. Teptons d. Bosons
3. Particle which explains about mass of matter is calleda. Higgs boson c. Protons4. A conservation law that is not universal but applies only to certain
b. Leptons d. Neutrons
kinds of interactions is conservation of:a. Lepton number d. Charge5. In quantum electrodynamics (QED), electromagnetic forces are
b. Baryon number e. Strangeness
c. Spin
mediated bya. the interaction of electrons.
b. hadrons.
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.7. The conservation law violated by the reaction
b. electric charge.
c. baryon and lepton numbers.d. linear and angular momentum.
is the
conservation ofa. Charge.8. Particles that participate in the strong nuclear interaction are called
b. Energy.
c. Linear momentum.
d. Lepton number and baryon number.
e. Angular momentum.a. NeutrinosB. Structured questions
b. Hadrons
c. Leptons
d. Electrons
e. Photons
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?
b. There are four compound particles here
I. Which combination has the right charge to be a proton?II. Which combination has the right charge to be a neutron?
III. There is a particle called the
which has a charge of –1e. Whichquark combination could be the
IV. There is a particle called the ∆++ which has a charge of + 2e. Which
quark combination could be the ∆− ?
V. A neutron can be changed to a proton if one quark changes ‘flavour’.
What change is needed? What charge must be carried away if thishappens?
c. Making mesons
Other, lighter ‘middle-weight’ particles called mesons can be made from
pairs of quarks. But they have to be made from a special combination: a
quark and an antiquark. There are now four particles to play with: Up
quark u: charge +2/3 e, Down quark d: charge –1/3 e, Antiup quark
charge –2/3 e. Antidown quark
: charge + 1/3 e.
