Skip to Content
OpenStax Logo
Buy book
  1. Preface
  2. Unit 1. Optics
    1. 1 The Nature of Light
      1. Introduction
      2. 1.1 The Propagation of Light
      3. 1.2 The Law of Reflection
      4. 1.3 Refraction
      5. 1.4 Total Internal Reflection
      6. 1.5 Dispersion
      7. 1.6 Huygens’s Principle
      8. 1.7 Polarization
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    2. 2 Geometric Optics and Image Formation
      1. Introduction
      2. 2.1 Images Formed by Plane Mirrors
      3. 2.2 Spherical Mirrors
      4. 2.3 Images Formed by Refraction
      5. 2.4 Thin Lenses
      6. 2.5 The Eye
      7. 2.6 The Camera
      8. 2.7 The Simple Magnifier
      9. 2.8 Microscopes and Telescopes
      10. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
    3. 3 Interference
      1. Introduction
      2. 3.1 Young's Double-Slit Interference
      3. 3.2 Mathematics of Interference
      4. 3.3 Multiple-Slit Interference
      5. 3.4 Interference in Thin Films
      6. 3.5 The Michelson Interferometer
      7. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    4. 4 Diffraction
      1. Introduction
      2. 4.1 Single-Slit Diffraction
      3. 4.2 Intensity in Single-Slit Diffraction
      4. 4.3 Double-Slit Diffraction
      5. 4.4 Diffraction Gratings
      6. 4.5 Circular Apertures and Resolution
      7. 4.6 X-Ray Diffraction
      8. 4.7 Holography
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
  3. Unit 2. Modern Physics
    1. 5 Relativity
      1. Introduction
      2. 5.1 Invariance of Physical Laws
      3. 5.2 Relativity of Simultaneity
      4. 5.3 Time Dilation
      5. 5.4 Length Contraction
      6. 5.5 The Lorentz Transformation
      7. 5.6 Relativistic Velocity Transformation
      8. 5.7 Doppler Effect for Light
      9. 5.8 Relativistic Momentum
      10. 5.9 Relativistic Energy
      11. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
    2. 6 Photons and Matter Waves
      1. Introduction
      2. 6.1 Blackbody Radiation
      3. 6.2 Photoelectric Effect
      4. 6.3 The Compton Effect
      5. 6.4 Bohr’s Model of the Hydrogen Atom
      6. 6.5 De Broglie’s Matter Waves
      7. 6.6 Wave-Particle Duality
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
    3. 7 Quantum Mechanics
      1. Introduction
      2. 7.1 Wave Functions
      3. 7.2 The Heisenberg Uncertainty Principle
      4. 7.3 The Schrӧdinger Equation
      5. 7.4 The Quantum Particle in a Box
      6. 7.5 The Quantum Harmonic Oscillator
      7. 7.6 The Quantum Tunneling of Particles through Potential Barriers
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    4. 8 Atomic Structure
      1. Introduction
      2. 8.1 The Hydrogen Atom
      3. 8.2 Orbital Magnetic Dipole Moment of the Electron
      4. 8.3 Electron Spin
      5. 8.4 The Exclusion Principle and the Periodic Table
      6. 8.5 Atomic Spectra and X-rays
      7. 8.6 Lasers
      8. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
    5. 9 Condensed Matter Physics
      1. Introduction
      2. 9.1 Types of Molecular Bonds
      3. 9.2 Molecular Spectra
      4. 9.3 Bonding in Crystalline Solids
      5. 9.4 Free Electron Model of Metals
      6. 9.5 Band Theory of Solids
      7. 9.6 Semiconductors and Doping
      8. 9.7 Semiconductor Devices
      9. 9.8 Superconductivity
      10. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    6. 10 Nuclear Physics
      1. Introduction
      2. 10.1 Properties of Nuclei
      3. 10.2 Nuclear Binding Energy
      4. 10.3 Radioactive Decay
      5. 10.4 Nuclear Reactions
      6. 10.5 Fission
      7. 10.6 Nuclear Fusion
      8. 10.7 Medical Applications and Biological Effects of Nuclear Radiation
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
    7. 11 Particle Physics and Cosmology
      1. Introduction
      2. 11.1 Introduction to Particle Physics
      3. 11.2 Particle Conservation Laws
      4. 11.3 Quarks
      5. 11.4 Particle Accelerators and Detectors
      6. 11.5 The Standard Model
      7. 11.6 The Big Bang
      8. 11.7 Evolution of the Early Universe
      9. Chapter Review
        1. Key Terms
        2. Key Equations
        3. Summary
        4. Conceptual Questions
        5. Problems
        6. Additional Problems
        7. Challenge Problems
  4. A | Units
  5. B | Conversion Factors
  6. C | Fundamental Constants
  7. D | Astronomical Data
  8. E | Mathematical Formulas
  9. F | Chemistry
  10. G | The Greek Alphabet
  11. Answer Key
    1. Chapter 1
    2. Chapter 2
    3. Chapter 3
    4. Chapter 4
    5. Chapter 5
    6. Chapter 6
    7. Chapter 7
    8. Chapter 8
    9. Chapter 9
    10. Chapter 10
    11. Chapter 11
  12. Index

Learning Objectives

By the end of this section, you will be able to:
  • Compare and contrast the six known quarks
  • Use quark composition of hadrons to determine the total charge of these particles
  • Explain the primary evidence for the existence of quarks

In the 1960s, particle physicists began to realize that hadrons are not elementary particles but are made of particles called quarks. (The name ‘quark’ was coined by the physicist Murray Gell-Mann, from a phrase in the James Joyce novel Finnegans Wake.) Initially, it was believed there were only three types of quarks, called up (u), down (d), and strange (s). However, this number soon grew to six—interestingly, the same as the number of leptons—to include charmed (c), bottom (b), and top (t).

All quarks are spin-half fermions (s=1/2),(s=1/2), have a fractional charge (1/3or2/3e)(1/3or2/3e), and have baryon number B=1/3.B=1/3. Each quark has an antiquark with the same mass but opposite charge and baryon number. The names and properties of the six quarks are listed in Table 11.3.

Quark Charge (units of e) Spin (s) Baryon number Strangeness number
Down (d) −1/3−1/3 1/2 1/3 0
Up (u) +2/3+2/3 1/2 1/3 0
Strange (s) −1/3−1/3 1/2 1/3 −1−1
Charm (c) +2/3+2/3 1/2 1/3 0
Bottom (b) −1/3−1/3 1/2 1/3 0
Top (t) +2/3+2/3 1/2 1/3 0
Table 11.3 Quarks

Quark Combinations

As mentioned earlier, quarks bind together in groups of two or three to form hadrons. Baryons are formed from three quarks. Sample baryons, including quark content and properties, are given in Table 11.4. Interestingly, the delta plus (Δ+Δ+) baryon is formed from the same three quarks as the proton, but the total spin of the particle is 3/2 rather than 1/2. Similarly, the mass of Δ+Δ+ with spin 3/2 is 1.3 times the mass of the proton, and the delta zero (Δ0Δ0) baryon with a spin 3/2 is 1.3 times the neutron mass. Evidently, the energy associated with the spin (or angular momentum) of the particle contributes to its mass energy. It is also interesting that no baryons are believed to exist with top quarks, because top quarks decay too quickly to bind to the other quarks in their production.

Name Symbol Quarks Charge (unit of e) Spin (s) Mass (GeV/c2GeV/c2)
Proton p u u d 1 1/2 0.938
Neutron n u d d 0 1/2 0.940
Delta plus plus ++ u u u 2 3/2 1.232
Delta plus + u u d 1 3/2 1.232
Delta zero 0 u d d 0 3/2 1.232
Delta minus d d d 11 3/2 1.232
Lambda zero Λ0Λ0 u d s 0 1/2 1.116
Positive sigma Σ+Σ+ u u s 1 1/2 1.189
Neutral sigma Σ0Σ0 u d s 0 1/2 1.192
Negative xi ΞΞ s d s −1−1 1/2 1.321
Neutral xi Ξ0Ξ0 s u s 0 1/2 1.315
Omega minus ΩΩ s s s −1−1 3/2 1.672
Charmed lambda ΛC+ΛC+ u d c 1 1/2 2.281
Charmed bottom Λb0Λb0 u d b 0 1/2 5.619
Table 11.4 Baryon Quarks

Mesons are formed by two quarks—a quark-antiquark pair. Sample mesons, including quark content and properties, are given in Table 11.5. Consider the formation of the pion (π+=udπ+=ud). Based on its quark content, the charge of the pion is

23e+13e=e.23e+13e=e.

Both quarks are spin-half (s=½s=½), so the resultant spin is either 0 or 1. The spin of the π+π+ meson is 0. The same quark-antiquark combination gives the rho (ρρ) meson with spin 1. This meson has a mass approximately 5.5 times that of the π+π+ meson.

Example 11.4

Quark Structure Show that the quark composition given in Table 11.5 for Ξ0Ξ0 is consistent with the known charge, spin, and strangeness of this baryon.

Strategy Ξ0Ξ0 is composed of two strange quarks and an up quark (s u s). We can add together the properties of quarks to predict the resulting properties of the Ξ0Ξ0 baryon.

Solution The charge of the s quark is e/3e/3 and the charge of the u quark is 2e/3. Thus, the combination (s u s) has no net charge, in agreement with the known charge of Ξ0Ξ0. Since three spin 1/21/2 quarks can combine to produce a particle with spin of either 1/2 or 3/2, the quark composition is consistent with the known spin (s=1/2s=1/2) of Ξ0Ξ0. Finally, the net strangeness of the (s u s) combination is (1)+0+(1)=2,(1)+0+(1)=2, which also agrees with experiment.

Significance The charge, spin, and strangeness of the Ξ0Ξ0 particle can be determined from the properties of its constituent quarks. The great diversity of baryons and mesons can be traced to the properties of just six quarks: up, down, charge, strange, top, and bottom.

Check Your Understanding 11.4

What is the baryon number of a pion?

Name Symbol Quarks Charge (e) Spin Mass (GeV/c2)(GeV/c2)
Positive pion π+π+ udud 1 0 0.140
Positive rho ρ+ρ+ udud 1 1 0.768
Negative pion ππ udud −1−1 0 0.140
Negative rho ρρ udud −1−1 1 0.768
Neutral Pion π0π0 uuuu or dddd 0 0 0.135
Neutral eta η0η0 uuuu, dddd or ssss 0 0 0.547
Positive kaon K+K+ usus 1 0 0.494
Neutral kaon K0K0 dsds 0 0 0.498
Negative kaon KK usus −1−1 0 0.494
J/Psi J/ψJ/ψ cccc 0 1 3.10
Charmed eta η0η0 cccc 0 0 2.98
Neutral D D0D0 ucuc 0 0 1.86
Neutral D D*0D*0 ucuc 0 1 2.01
Positive D D+D+ dcdc 1 0 1.87
Neutral B B0B0 dbdb 0 0 5.26
Upsilon ϒϒ bbbb 0 1 9.46
Table 11.5 Meson Quarks

Color

Quarks are fermions that obey Pauli’s exclusion principle, so it might be surprising to learn that three quarks can bind together within a nucleus. For example, how can two up quarks exist in the same small region of space within a proton? The solution is to invent a third new property to distinguish them. This property is called color, and it plays the same role in the strong nuclear interaction as charge does in electromagnetic interactions. For this reason, quark color is sometimes called “strong charge.”

Quarks come in three colors: red, green, and blue. (These are just labels—quarks are not actually colored.) Each type of quark (u,d,c,s,b,t)(u,d,c,s,b,t) can possess any other colors. For example, three strange quarks exist: a red strange quark, a green strange quark, and a blue strange quark. Antiquarks have anticolor. Quarks that bind together to form hadrons (baryons and mesons) must be color neutral, colorless, or “white.” Thus, a baryon must contain a red, blue, and green quark. Likewise, a meson contains either a red-antired, blue-antiblue, or green-antigreen quark pair. Thus, two quarks can be found in the same spin state in a hadron, without violating Pauli’s exclusion principle, because their colors are different.

Quark Confinement

The first strong evidence for the existence of quarks came from a series of experiments performed at the Stanford Linear Accelerator Center (SLAC) and at CERN around 1970. This experiment was designed to probe the structure of the proton, much like Rutherford studied structure inside the atom with his αα-particle scattering experiments. Electrons were collided with protons with energy in excess of 20 GeV. At this energy, EpcEpc, so the de Broglie wavelength of an electron is

λ=hp=hcE6×10−17m.λ=hp=hcE6×10−17m.
(11.1)

The wavelength of the electron is much smaller than the diameter of the proton (about 10−15m).10−15m). Thus, like an automobile traveling through a rocky mountain range, electrons can be used to probe the structure of the nucleus.

The SLAC experiments found that some electrons were deflected at very large angles, indicating small scattering centers within the proton. The scattering distribution was consistent with electrons being scattered from sites with spin 1/2, the spin of quarks. The experiments at CERN used neutrinos instead of electrons. This experiment also found evidence for the tiny scattering centers. In both experiments, the results suggested that the charges of the scattering particles were either +2/3e+2/3e or 1/3e1/3e, in agreement with the quark model.

Interactive

Watch this video to learn more about quarks.

The quark model has been extremely successful in organizing the complex world of subatomic particles. Interestingly, however, no experiment has ever produced an isolated quark. All quarks have fractional charge and should therefore be easily distinguishable from the known elementary particles, whose charges are all an integer multiple of e. Why are isolated quarks not observed? In current models of particle interactions, the answer is expressed in terms of quark confinement. Quark confinement refers to the confinement of quarks in groups of two or three in a small region of space. The quarks are completely free to move about in this space, and send and receive gluons (the carriers of the strong force). However, if these quarks stray too far from one another, the strong force pulls them back it. This action is likened to a bola, a weapon used for hunting (Figure 11.5). The stones are tied to a central point by a string, so none of the rocks can move too far from the others. The bola corresponds to a baryon, the stones correspond to quarks, and the string corresponds to the gluons that hold the system together.

Three strings are tied together at one end. A weight is attached to the other end of each. The strings are labeled quark confinement. The weights are labeled quarks.
Figure 11.5 A baryon is analogous to a bola, a weapon used for hunting. The rocks in this image correspond to the baryon quarks. The quarks are free to move about but must remain close to the other quarks.
Citation/Attribution

Want to cite, share, or modify this book? This book is Creative Commons Attribution License 4.0 and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at https://openstax.org/books/university-physics-volume-3/pages/1-introduction
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at https://openstax.org/books/university-physics-volume-3/pages/1-introduction
Citation information

© Sep 29, 2016 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License 4.0 license. The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.