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Option D: Relativity and particle physics
D5 Quarks
D.5.1 List the six types of quark.
D.5.2 State the content, in terms of quarks and
antiquarks, of hadrons (that is, baryons and
mesons).
D.5.3 State the quark content of the proton and
the neutron.
D.5.4 Define baryon number and apply the
conservation of baryon number.
D.5.5 Deduce the spin structure of hadrons (that
is, baryons and mesons). Only an elementary
discussion in terms of ‘spin up’ and ‘spin
down’ is required. See D.4.4 above for
details.
Option D: Relativity and particle physics
D5 Quarks
List the six types of quark.
You have already been introduced to the six
quarks:
Flavor
Up
Down
Strange
Charm
Bottom
Top
Quark
Symbol
u
d
s
c
b
t
Charge
+ 2/3
- 1/3
- 1/3
+ 2/3
- 1/3
+ 2/3
Antiquark
Symbol
Charge
u
- 2/3
d
+ 1/3
s
+ 1/3
c
- 2/3
b
+ 1/3
t
- 2/3
Quarks participate in the strong force whose
carrier is the gluon.
Option D: Relativity and particle physics
D5 Quarks
State the content, in terms of quarks and
antiquarks, of hadrons (that is, baryons and
mesons).
A hadron is a particle that participates in the
strong force.
Since quarks participate in the strong force, and
since baryons and mesons are made of quarks,
baryons and mesons are hadrons.
A baryon is made of three quarks (qqq). An
antibaryon is made of three antiquarks (qqq).
A meson is made up of a quark and an antiquark
(qq):
FYI
A single quark cannot be isolated. We will talk
about quark confinement later. Basically,
confinement states that you cannot separate a
single quark from a hadron.
Option D: Relativity and particle physics
D5 Quarks
State the quark content of the proton and the
neutron.
A proton is a baryon made up of two up quarks and
a down quark. p = (uud).
A neutron is a baryon made up of one up quark and
two down quarks. n = (udd).
EXAMPLE:
Show that the charge of a proton is +1, and that
the charge of a neutron is 0.
SOLUTION:
The charge of an up quark is +2/3.
The charge of a down quark is -1/3.
Thus
Proton = uud : +2/3 + +2/3 + -1/3 = +1.
Neutron = udd : +2/3 + -1/3 + -1/3 = 0.
Option D: Relativity and particle physics
D5 Quarks
Define baryon number and apply the conservation
of baryon number.
The baryon number of a quark is +1/3. The baryon
number of an antiquark is -1/3.
Quark: B = +1/3
quark (q) or antiquark (q)
Antiquark: B = 1/3
baryon number B
PRACTICE: What is the baryon number of a proton
and an antiproton?
SOLUTION:
Proton = uud : +1/3 + +1/3 + +1/3 = +1.
Antiproton = uud : -1/3 + -1/3 + -1/3 = -1.
PRACTICE: What is the baryon number of a meson?
SOLUTION: A meson has the quark makeup (qq) so
that it has a baryon number of +1/3 + -1/3 = 0.
FYI
Baryon number is conserved in all reactions.
Option D: Relativity and particle physics
D5 Quarks
Deduce the spin structure of hadrons (that is,
baryons and mesons).
The spin of a quark (or antiquark) is spin up (
= 1/2) or spin down ( = -1/2).
Spin up
+1/2 or 
quark (q) or
Spin down -1/2 or 
antiquark (q) spin
PRACTICE: Deduce the possible spin structures of
a baryon:
SOLUTION: A baryon is (qqq):
Possibility 1: +1/2 + +1/2 + +1/2 = +3/2.
Possibility 2: +1/2 + +1/2 + -1/2 = +1/2.
Possibility 3: +1/2 + -1/2 + -1/2 = -1/2.
Possibility 4: -1/2 + -1/2 + -1/2 = -3/2.
FYI
Recall that fermions have odd multiples of ½ as
their spin and obey the Pauli exclusion
principle.
Option D: Relativity and particle physics
D5 Quarks
Deduce the spin structure of hadrons (that is,
baryons and mesons).
The spin of a quark (or antiquark) is spin up (
= 1/2) or spin down ( = -1/2).
Spin up
+1/2 or 
quark (q) or
Spin down -1/2 or 
antiquark (q) spin
PRACTICE: Deduce the possible
a meson:
SOLUTION: A meson is (qq):
Possibility 1: +1/2 + +1/2
Possibility 2: +1/2 + -1/2
Possibility 3: -1/2 + -1/2
spin structures of
= +1.
= 0.
= -1.
FYI
Recall that bosons have even multiples of ½ as
their spin (whole number spins) and DO NOT obey
the Pauli exclusion principle.
Option D: Relativity and particle physics
D5 Quarks
Deduce the spin structure of hadrons (that is,
baryons and mesons).
Pauli says they can’t have identical
quantum numbers. Their colors differ.
A proton has a quark structure of uud.
Each quark has a spin of  ½.
If two of the three quarks has a spin of
+ ½ and the other a spin of – ½ the proton
has a spin of ½.
Option D: Relativity and particle physics
D5 Quarks
D.5.6 Explain the need for color in forming bound
states of quarks.
D.5.7 State the color of quarks and gluons.
D.5.8 Outline the concept of strangeness.
D.5.9 Discuss quark confinement.
D.5.10 Discuss the interaction that binds
nucleons in terms of the color force between
quarks.
Option D: Relativity and particle physics
D5 Quarks
Explain the need for color in forming bound
states of quarks.
Two electrons repel, yet an electron and a
positron (or proton) attract. To explain such
interactions we use the model of “charge,” and
give charges either a (+) or a (–) value.
Quarks, on the other hand, seem to show three
types of charge, rather than two. A model using
(+) and (-) numbers fails.
Instead, physicists use the color
charge model because of the three
primary colors red, green, and blue.
FYI
Perhaps you recall from an art class
that adding red, green and blue light
produces white light. The color charge model
requires all particles to be “white.”
Option D: Relativity and particle physics
D5 Quarks
State the color of quarks and gluons.
Just as (+) and (-) are used to explain the
electromagnetic force, the three colors are used
to explain the strong force.
Quarks come in red, green, and blue.
Antiquarks come in anti-red, anti-green, and
anti-blue.
FYI
Note that, for example, anti-red is
the green-blue combo (cyan).
Note that since all particles must
be white, a baryon must have the
color combo (qqq) and an antibaryon
must have the color combo (qqq).
A meson must have the color combo
(qq), (qq), (qq).
The color charge of a particle is zero (white).
Option D: Relativity and particle physics
D5 Quarks
State the color of quarks and gluons.
Gluons, the strong force carrier, have two colors
and are not white (and thus have a non-zero color
charge).
This gives gluons the ability to carry the strong
force.
The complete theory of the color force is called
quantum chromo dynamics (QCD).
quark
quark
quark
Option D: Relativity and particle physics
D5 Quarks
Outline the concept of strangeness.
The strangeness number S of a baryon is related
to the number of strange quarks the particle has
and is found using the formula
S = #Antistrange quarks
strangeness S
- #strange quarks
EXAMPLE: The lambda zero particle (0) is a
baryon having the quark combo of (uds). What is
its charge? What is its strangeness?
SOLUTION:
From a previous table the charges are u = +2/3,
d = -1/3 and s = -1/3 so that the total charge is
0.
From the above formula
S = #Antistrange quarks - #strange quarks
= 0 – 1 = -1.
Option D: Relativity and particle physics
D5 Quarks
Outline the concept of strangeness.
The – is a hadron because it is composed
of quarks.
Option D: Relativity and particle physics
D5 Quarks
Outline the concept of strangeness.
The proton is composed of uud.
Option D: Relativity and particle physics
D5 Quarks
Outline the concept of strangeness.
If X is sss, then the reaction can be
written in terms of quarks as follows:
su + uud  ds + us + sss
The left has an s, u, and d left.
The right also has an s, u, and d left.
The quarks are balanced on each side.
Option D: Relativity and particle physics
D5 Quarks
Discuss quark confinement.
Quark confinement means
that we cannot ever
separate a single quark
from a baryon or a
meson.
Because of the nature
of the strong force
holding the quarks
together we need to
provide an energy that
is proportional to the
separation.
Eventually, that energy is so vast that a new
quark-antiquark pair forms and all we have is a
meson, instead of an isolated quark!
Option D: Relativity and particle physics
D5 Quarks
Discuss quark confinement.
Option D: Relativity and particle physics
D5 Quarks
Discuss the interaction that binds nucleons in
terms of the color force between quarks.
The residual color force of the gluons that hold
the quarks together is what is known as the
strong force.
Thus nuclear
reactors and
bombs represent
only the
residual energy
in the gluon
interaction
between quarks.