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Introduction to the Standard Model and QCD
Morgan Gholson
November 10, 2016
Introduction
In Chapter Five Serway proposed an interesting question ”Do you think that most major experimental
discoveries are made by careful planning or by accident?”
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The overwhelming consensus among the class
was that it was not just one or the other, but both. Another question to further this thought; what do we
do when the unexpected happens? Do we start from scratch, and erase everything we know? We certainly
cannot disregard the data to continue believing in our beloved models that are proven wrong. So we must
adapt, analyze, and try to create an understanding of why these results have come about, unless the data is
wrong like it was in several instances of this particular work.
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In the early 1900s physicists were trying to gain an understanding of what exactly happens at the
subatomic level. They used cosmic rays to study penetrating radiation. They used superaccelerators,
smashing atoms together. All of this was in an attempt to observe the forces at work at these very small
distances. However, what was observed instead, was a multitude of particles, all with different masses. These
smaller particles did not work with any model at the time, these were commonly referred to as the ”strange
particles.”
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In 1964 Gell-Mann came forward with the concept of ”quarks.” These sub-entities called quarks make
up what are classified as Hadrons, the massive particles. Apart from hadrons, there are Leptons. Leptons
are the much, much smaller particles such as electrons and muons, that are not made of quarks. Together,
quarks and leptons account for all matter in the current Standard Model.
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Furthermore, The inner workings
of these hadrons do not necessarily obey the same rules as macroscopic, or even as microscopic systems, but
instead operate with their own set of rules, defined by Quantum Chromodynamics.
Quarks and Leptons
Also presented independently by George Zweig around the same time frame, called ”Aces” 1 , Gell-Mann
presented his idea of quarks and their interaction and was subsequently awarded the Nobel Prize in Physics
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in 1969 ”for his contributions and discoveries concerning the classification of elementary particles and their
interactions.”
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Since, the concept of quarks have been expanded to six ”flavors” that include up (u), down
(d), strange (s), charmed (c), top (t), and bottom (b). These different type of quarks combine to form
ordinary matter. For example, a proton is made up of two up quarks and one down quark. An up quark has
a charge of +2e/3 and a down quarks has a charge of −e/3, which one can see combine to give the familiar
proton charge +e.
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Furthermore, the strange, charm, bottom, and top quarks each have their own individual properties,
such as Strangeness, and odd processes take place involving these properties. For example, in strangeness,
never can just one strange particle be produced, and strangeness is not conserved for weak processes.
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The
masses of these quarks are some what more difficult to ascertain due to a property called confinement of
quarks. Due a strong force, which will be discussed shortly, an individual quark has never been observed.
Apart from flavors, quarks also have ”colors.” The colors used are red, blue, green for a total of three colors.
These quarks are not actually observed as a color, but instead used as a label. The concept of quark color
was developed to satisfy the exclusion principle and explain certain experimental results.
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Leptons exist independent of quarks, and currently, it is believed that there are only six of them.These
include the electron, the muon, the tau, and a neutrino associated with each. These particles have rest
energies of 511 KeV, 105.7 MeV, and 1784 MeV respectively.
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For comparison, the rest energy of an Up
Quark is supposedly around 360 MeV, which is to be taken with a bit of healthy skepticism in its true value
do to quark’s confinement property. So one can see that leptons, named for so for such reasons, are much
smaller when compared to Hadrons.
Quantum Chromodynamics
Quantum Chromodynamics, or QCD, is the theory of how quarks interact with each other. There are a
few key points to grasp and understand that is the foundation of QCD.The strong force, also know as the
color force, is the force between quarks. Nuclear reactions are controlled by a massless field of particles,
similar to a gravitational field and gravitons, called Gluons. This nuclear force is a secondary effect of the
strong force. Interestingly, a quark is capable of absorbing or emitting a gluon, and by this process, change
color. For example, a blue quark that emits a gluon may become a red quark, and vice versa when it a red
emits one. Serway uses an analogy about the electric force to relate the topic back to more rudimentary
knowledge. Like the electric force, where like charges repel and opposite charges attract, the color force
repels for like colors and attracts for unlike colors. However, unlike the electric force where there are two
charges, positive and negative, the color has three. When two of opposite color combine, they form a Meson,
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and when three quarks of different color combine a Baryon. Both are under the Hadron category.
The residual force between two baryons, say a proton and a neutron, is the force that binds together that
nucleus. Again using the electric force analogy, one can imagine two dipoles. ”Each dipole is electrically
neutral. An electric field surrounds the dipoles, however, because of the separation of the positive and
negative charges. As a result, an electric interaction occurs between the dipoles that is weaker than the force
between single charges.”
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So these things have internal forces that bind the individual particles existence
together, but at the same time its residual effects help form atoms.
One last key element of the strong force is its range. Unlike weaker forces, the strong force only acts at
very short distances. Griffiths compares this to the range of a boxer due to the length of his arm, in that
the force dies of exponentially once outside its range.
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This range of the strong force can also attribute to
why atoms only become so large, meaning the can only fit so many baryons, before becoming unstable.
Symmetry of Matter and The Standard Model
The symmetry of matter can be expressed in a rather simple statement, for every particle, there is an
antiparticle that is equal and opposite. So, revisiting the statement earlier about there being six type of
quarks and three colors, this is not exactly true. Following this symmetry logic, there are six antiquarks and
three anticolors for each antiquark. Considering this would lead one to the assumption that larger particles
made of quarks each have their own counter particle. For example, a more familiar particle and antiparticle
combination would be an electron and a positron.
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Now that basics of QCD have been covered and the
multitude of particles have been somewhat reviewed, the Standard Model can be discussed.
As one can see, in the figure above, that standard model builds off quarks and QCD to form a well rounded
explanation for the current observations. One thing not noted in the figure are the forces. Quarks participate
in all fundamental forces, and leptons in all but the strong force.
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It divides matter into subcategories in
which everything, at the present moment, is accounted for. Another strength of it is that it seems difficult
to provide unfavorable evidence that would disavow its legitimacy, in that it is very adaptive and is subject
to change.
Conclusion
Conceptualizing quarks and constructing a model to classify these particles not only provided clarification
of the present discoveries, but gave a framework to physicists to search for new particles as well. Interestingly
enough ”Gell-Mann boldly predicted that such a particle would be found, and told the experimentalists
exactly how to produce it. Moreover, he calculated its mass... and sure enough, in 1964 the famous omegaminus particle was discovered, precisely as Gell-Mann had predicted.” 1 It is a nice sentiment that everything
can be classified so neatly, but it is not a complete victory, there are still many unknowns in particle physics.
To return to our question presented at the beginning, it would seem that in this instance it was again
both. One could argue that it took a considerable amount of careful planning to set up these experiments
with the resources available and get the results they did, but at the same time, the results were themselves,
at times, complete accidents, and in some early cases developed becouse of ”malfunctioning” equipment.
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What is most important issue above this is to continue asking the correct questions, and not turn our back
to the answers; to always look for the next step forward in developing an understanding of our existence.
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References
1
Griffiths, David J. Introduction to Elementary Particles. Weinheim: Wiley-VCH, 2008. Print.
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Richards, James A., Francis W. Sears, M. Russell Wehr, and Mark W. Zemansky. Modern University
Physics. Massachusetts, U.S.A.: Addison-Wesley, 1960. Print.
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Serway, Raymond A., Robert J. Beichner, and John W. Jewett. Physics: For Scientist and Engineers with
Modern Physics. Philadelphia: Saunders College, 2000. Print.
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Serway, Raymond A., Clement J. Moses, and Curt A. Moyer. Modern Physics. Belmont, CA: Thomson
Brooks/Cole, 2005. Print.
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”The Nobel Prize in Physics 1969”. Nobelprize.org. Nobel Media AB 2014. Web. 7 Nov 2016.
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TEDtalksDirector. ”Murray Gell-Mann: Beauty and Truth in Physics.” YouTube. YouTube, 07 Dec.
2007. Web. 09 Nov. 2016
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