22.4. Quarks http://www.ck12.org
electrons. At the time those particles were hypothesized, they were considered to be indivisible, having no smaller
parts. In 1964, it was first suggested that protons and neutrons do have smaller component parts, namelyquarks.
The quark model was accepted very slowly because quarks had not been observed and weren’t all observed until
- The known quarks are divided into three families, low energy, medium energy, and high energy.
TABLE22.2:
Quark Name Symbol Charge Family
Up u +^23 Low energy family
Down d −^13 Low energy family
Strange s −^13 Medium energy family
Charm c +^23 Medium energy family
Top t +^23 High energy family
Bottom b −^13 High energy familyParticles that are composed of three quarks are calledbaryons. The two best known baryons are protons and
neutrons. A proton is composed of two up quarks and one down quark (uud). The charge on the proton is the result
of adding the charges on these quarks
(
+^23 +^23 −^13 = + 1
)
. Neutrons are composed of one up quark and two down
quarks (udd). The charge on the neutron is the result of adding the charges on these quarks
(
+^23 −^13 −^13 = 0
)
.
Everyday objects are made up of quarks from the lowest energy family, namely up and down quarks. Quarks in the
middle energy family are found in cosmic rays and are produced in particle accelerators. Particles in the high energy
family are believed to have existed briefly during the earliest moments of the Big Bang and are created only in high
energy collisions.
Summary
- In 1964, it was first suggested that protons and neutrons do have smaller component parts, namely quarks.
- The known quarks are divided into three families, low energy, medium energy, and high energy.
- Particles that are composed of three quarks are called baryons.
- A proton is composed of two up quarks and one down quark (uud).
- Neutrons are composed of one up quark and two down quarks (udd).
Practice
Questions
http://www.youtube.com/watch?v=SMgi2j9Ks9k
MEDIA
Click image to the left for use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/63602Follow up questions:
- Which particle accelerator is mentioned in the video?
- How are the families of quarks referred to in the video?