Quarks, Particle Physics | Physics

Learning objectives

Section Key Terms

annihilation	antimatter	baryon	bottom quark	charmed quark
color	down quark	flavor	gluon	hadron
Higgs boson	Higgs field	lepton	meson	pair production
positron	quantum chromodynamics	quark	Standard Model	strange quark
top quark	up quark

Quarks

“The first principles of the universe are atoms and empty space. Everything else is merely thought to exist…”

“… Further, the atoms are unlimited in size and number, and they are borne along with the whole universe in a vortex, and thereby generate all composite things—fire, water, air, earth. For even these are conglomerations of given atoms. And it because of their solidity that these atoms are impassive and unalterable.”

Diogenes Laertius (summarizing the views of Democritus, circa 460–370 B.C.)

The search for fundamental particles is nothing new. Atomists of the Greek and Indian empires, like Democritus of fifth century B.C., openly wondered about the most finite components of our universe. Though dormant for centuries, curiosity about the atomic nature of matter was reinvigorated by Rutherford’s gold foil experiment and the discovery of the nucleus. By the early 1930s, scientists believed they had fully determined the tiniest constituents of matter—in the form of the proton, neutron, and electron.

This would be only partially true. At present, scientists know that there are hundreds of particles not unlike our electron and nucleons, all making up what some have termed the particle zoo. While we are confident that the electron remains fundamental, it is surrounded by a plethora of similar sounding terms, like leptons, hadrons, baryons, and mesons. Even though not every particle is considered fundamental, they all play a vital role in understanding the intricate structure of our universe.

A fundamental particle is defined as a particle with no substructure and no finite size. According to the Standard Model, there are three types of fundamental particles: leptons, quarks, and carrier particles. As you may recall, carrier particles are responsible for transmitting fundamental forces between their interacting masses. Leptons are a group of six particles not bound by the strong nuclear force, of which the electron is one. As for quarks, they are the fundamental building blocks of a group of particles called hadrons, a group that includes both the proton and the neutron.

Now for a brief history of quarks. Quarks were first proposed independently by American physicists Murray Gell-Mann and George Zweig in 1963. Originally, three quark types—or flavors—were proposed with the names up (u), down (d), and strange (s).

At first, physicists expected that, with sufficient energy, we should be able to free quarks and observe them directly. However, this has not proved possible, as the current understanding is that the force holding quarks together is incredibly great and, much like a spring, increases in magnitude as the quarks are separated. As a result, when large energies are put into collisions, other particles are created—but no quarks emerge. With that in mind, there is compelling evidence for the existence of quarks. By 1967, experiments at the SLAC National Accelerator Laboratory scattering 20-GeV electrons from protons produced results like Rutherford had obtained for the nucleus nearly 60 years earlier. The SLAC scattering experiments showed unambiguously that there were three point-like (meaning they had sizes considerably smaller than the probe’s wavelength) charges inside the proton as seen in Figure_23_02_SLAC. This evidence made all but the most skeptical admit that there was validity to the quark substructure of hadrons.

View original figure (external; image not reproduced in this text edition)

Original figure description: The image shows a large blue sphere representing a proton with three small spheres within it. The three small spheres are labeled ‘u’, ‘u’, and ‘d’, and are colored blue, green, and red, respectively. These three smaller spheres represent up and down quarks. From the left to the right of the image are arrows representing the trajectory of electrons. Some of the electrons pass through the proton, while others are shown striking the quarks and scattering away.

Scattering of high-energy electrons from protons at facilities like SLAC produces evidence of three point-like charges consistent with proposed quark properties. This experiment is analogous to Rutherford’s discovery of the small size of the nucleus by scattering α particles. High-energy electrons are used so that the probe wavelength is small enough to see details smaller than the proton.

The inclusion of the strange quark with Zweig and Gell-Mann’s model concerned physicists. While the up and down quarks demonstrated fairly clear symmetry and were present in common fundamental particles like protons and neutrons, the strange quark did not have a counterpart of its own. This thought, coupled with the four known leptons at the time, caused scientists to predict that a fourth quark, yet to be found, also existed.

In 1974, two groups of physicists independently discovered a particle with this new quark, labeled charmed. This completed the second exotic quark pair, strange (s) and charmed (c). A final pair of quarks was proposed when a third pair of leptons was discovered in 1975. The existence of the bottom (b) quark and the top (t) quark was verified through experimentation in 1976 and 1995, respectively. While it may seem odd that so much time would elapse between the original quark discovery in 1967 and the verification of the top quark in 1995, keep in mind that each quark discovered had a progressively larger mass. As a result, each new quark has required more energy to discover.

One of the most confounding traits of quarks is their electric charge. Long assumed to be discrete, and specifically a multiple of the elementary charge of the electron, the electric charge of an individual quark is fractional and thus seems to violate a presumed tenet of particle physics. The fractional charge of quarks, which are $\pm (\frac{2}{3}) q_{e}$ and $\pm (\frac{1}{3}) q_{e}$ , are the only structures found in nature with a nonintegral number of charge $q$ . However, note that despite this odd construction, the fractional value of the quark does not violate the quantum nature of the charge. After all, free quarks cannot be found in nature, and all quarks are bound into arrangements in which an integer number of charge is constructed. Table_23_03 shows the six known quarks, in addition to their antiquark components, as will be discussed later in this section.

Quarks and Antiquarks
Flavor	Symbol	Antiparticle	Charge^[1][2]
^[1]The lower of the $\pm$ symbols are the values for antiquarks.
^[2]There are further qualities that differentiate between quarks. However, they are beyond the discussion in this text.
Up	$u$	$\bar{u}$	$\pm \frac{2}{3} q_{e}$
Down	$d$	$\bar{d}$	$\mp \frac{1}{3} q_{e}$
Strange	$s$	$\bar{s}$	$\mp \frac{1}{3} q_{e}$
Charmed	$c$	$\bar{c}$	$\pm \frac{2}{3} q_{e}$
Bottom	$b$	$\bar{b}$	$\mp \frac{1}{3} q_{e}$
Top	$t$	$\bar{t}$	$\pm \frac{2}{3} q_{e}$

While the term flavor is used to differentiate between types of quarks, the concept of color is more analogous to the electric charge in that it is primarily responsible for the force interactions between quarks. Note—Take a moment to think about the electrostatic force. It is the electric charge that causes attraction and repulsion. It is the same case here but with a color charge. The three colors available to a quark are red, green, and blue, with antiquarks having colors of anti-red (or cyan), anti-green (or magenta), and anti-blue (or yellow).

Why use colors when discussing quarks? After all, the quarks are not actually colored with visible light. The reason colors are used is because the properties of a quark are analogous to the three primary and secondary colors mentioned above. Just as different colors of light can be combined to create white, different colors of quark may be combined to construct a particle like a proton or neutron. In fact, for each hadron, the quarks must combine such that their color sums to white! Recall that two up quarks and one down quark construct a proton, as seen in Figure_23_02_SLAC. The sum of the three quarks’ colors—red, green, and blue—yields the color white. This theory of color interaction within particles is called quantum chromodynamics, or QCD. As part of QCD, the strong nuclear force can be explained using color. In fact, some scientists refer to the color force, not the strong force, as one of the four fundamental forces. Figure_23_02_Color is a Feynman diagram showing the interaction between two quarks by using the transmission of a colored gluon. Note that the gluon is also considered the charge carrier for the strong nuclear force.

View original figure (external; image not reproduced in this text edition)

Original figure description: The diagram shows two quarks, a down quark and a strange quark, traveling toward each other. As the down quark expels a gluon, it changes color from red to green. As the strange quark receives the gluon, it changes color from green to red. The gluon is colored red and magenta and is labeled ‘red-antigreen’.

The exchange of gluons between quarks carries the strong force and may change the color of the interacting quarks. While the colors of the individual quarks change, their flavors do not.

Note that quark flavor may have any color. For instance, in Figure_23_02_Color, the down quark has a red color and a green color. In other words, colors are not specific to a particle quark flavor.

Hadrons and Leptons

Particles can be revealingly grouped according to what forces they feel between them. All particles (even those that are massless) are affected by gravity since gravity affects the space and time in which particles exist. All charged particles are affected by the electromagnetic force, as are neutral particles that have an internal distribution of charge (such as the neutron with its magnetic moment). Special names are given to particles that feel the strong and weak nuclear forces. Hadrons are particles that feel the strong nuclear force, whereas leptons are particles that do not. All particles feel the weak nuclear force. This means that hadrons are distinguished by being able to feel both the strong and weak nuclear forces. Leptons and hadrons are distinguished in other ways as well. Leptons are fundamental particles that have no measurable size, while hadrons are composed of quarks and have a diameter on the order of 10^–15 m. Six particles, including the electron and neutrino, make up the list of known leptons. There are hundreds of complex particles in the hadron class, a few of which (including the proton and neutron) are listed in Table_23_04.

List of Leptons and Hadrons.
Category	Particle Name	Symbol	Antiparticle	Rest Mass $(M e V / c^{2})$	Mean Lifetime (s)
^[1]Neutrino masses may be zero. Experimental upper limits are given in parentheses.
^[2]Many other mesons known
^[3]Many other baryons known
Leptons	Electron	$e^{-}$	$e^{+}$	0.511	Stable
	Neutrino (e)	$v_{e}$	${\bar{v}}_{e}$	$0 (7.0 eV)$ ^[1]	Stable
	Muon	$μ^{-}$	$μ^{+}$	105.7	$2.20 \times 10^{- 6}$
	Neutrino (μ)	$v_{μ}$	${\bar{v}}_{μ}$	$0 (< 0.27)$ ^[1]	Stable
	Tau	$τ^{-}$	$τ^{+}$	1,777	$2.91 \times 10^{−6}$
	Neutrino (τ)	$v_{τ}$	${\bar{v}}_{τ}$	$0 (< 31)$ ^[1]	Stable
Hadrons – Mesons^[2]	Pion	$π^{+}$	$π^{-}$	139.6	$2.60 \times 10^{−8}$
	Pion	$π^{0}$	Self	135.0	$8.40 \times 10^{−17}$
	Kaon	$K^{+}$	$K^{-}$	493.7	$1.24 \times 10^{−8}$
	Kaon	$K^{0}$	$K^{0}$	497.6	$0.90 \times 10^{−10}$
	Eta	$η^{0}$	Self	547.9	$2.53 \times 10^{−19}$
Hadrons – Baryons^[3]	Proton	p	$\bar{p}$	938.3	Stable
	Neutron	n	$\bar{n}$	939.6	882
	Lambda	$Λ^{0}$	${\bar{Λ}}^{0}$	1,115.7	$2.63 \times 10^{- 10}$
	Omega	$Ω^{-}$	$Ω^{+}$	1,672.5	$0.82 \times 10^{−10}$

There are many more leptons, mesons, and baryons yet to be discovered and measured. The purpose of trying to uncover the smallest indivisible things in existence is to explain the world around us through forces and the interactions between particles, galaxies and objects. This is why a handful of scientists devote their life’s work to smashing together small particles.

What internal structure makes a proton so different from an electron? The proton, like all hadrons, is made up of quarks. A few examples of hadron quark composition can be seen in Figure_23_02_Hadron. As shown, each hadron is constructed of multiple quarks. As mentioned previously, the fractional quark charge in all four hadrons sums to the particle’s integral value. Also, notice that the color composition for each of the four particles adds to white. Each of the particles shown is constructed of up, down, and their antiquarks. This is not surprising, as the quarks strange, charmed, top, and bottom are found in only our most exotic particles.

View original figure (external; image not reproduced in this text edition)

Original figure description: There are four pictures in this figure, each showing the structure of a hadron. The proton on the left is composed of three quarks (two up and one down). The colors of the quarks are blue, green, and red. The second picture is of a neutron, which is composed of three quarks (one up and two down). The colors of its quarks are green, blue, and red. The third image is of a positive pion, which is composed of two quarks (up and anti-down). The colors of its quarks are red and anti-red. The final picture is of a negative pion, which is composed of two quarks (anti-up and down). The colors of its quarks are green and anti-green. Underneath each quark is a summation of the quarks fractional charge. The summation of quark charges for both the proton and positive pion is +1. The summation of quark charges for the neutron is zero. The summation of quark charges for the negative pion is -1.

All baryons, such as the proton and neutron shown here, are composed of three quarks. All mesons, such as the pions shown here, are composed of a quarkantiquark pair. Arrows represent the spins of the quarks. The colors are such that they need to add to white for any possible combination of quarks.

You may have noticed that while the proton and neutron in Figure_23_02_Hadron are composed of three quarks, both pions are comprised of only two quarks. This refers to a final delineation in particle structure. Particles with three quarks are called baryons. These are heavy particles that can decay into another baryon. Particles with only two quarks—a-quarkanti-quark pair—are called mesons. These are particles of moderate mass that cannot decay into the more massive baryons.

Before continuing, take a moment to view Figure_23_02_Strong. In this figure, you can see the strong force reimagined as a color force. The particles interacting in this figure are the proton and neutron, just as they were in Figure_23_01_FeynYuk. This reenvisioning of the strong force as an interaction between colored quarks is the critical concept behind quantum chromodynamics.

View original figure (external; image not reproduced in this text edition)

Original figure description: This is a complex picture of the relationship between individually colored quarks in the strong nuclear force, during the interaction between a proton and a neutron.

This Feynman diagram shows the interaction between a proton and a neutron, corresponding to the interaction shown in Figure_23_01_FeynYuk. This diagram, however, shows the quark and gluon details of the strong nuclear force interaction.

Matter and Antimatter

Antimatter was first discovered in the form of the positron, the positively charged electron. In 1932, American physicist Carl Anderson discovered the positron in cosmic ray studies. Through a cloud chamber modified to curve the trajectories of cosmic rays, Anderson noticed that the curves of some particles followed that of a negative charge, while others curved like a positive charge. However, the positive curve showed not the mass of a proton but the mass of an electron. This outcome is shown in Figure_23_02_Positron and suggests the existence of a positively charged version of the electron, created by the destruction of solar photons.

View original figure (external; image not reproduced in this text edition)

Original figure description: The image shows two spirals originating from the same spot, with one directed upward and one directed downward. The spot from which the spirals originate is labeled ‘incident photon.’ The upward spiral is the positron, while the downward spiral is labeled ‘electron.’

The image above is from the Fermilab 15 foot bubble chamber and shows the production of an electron and positron (or antielectron) from an incident photon. This event is titled pair production and provides evidence of antimatter, as the two repel each other.

Antimatter is considered the opposite of matter. For most antiparticles, this means that they share the same properties as their original particles with the exception of their charge. This is why the positron can be considered a positive electron while the antiproton is considered a negative proton. The idea of an opposite charge for neutral particles (like the neutron) can be confusing, but it makes sense when considered from the quark perspective. Just as the neutron is composed of one up quark and two down quarks (of charge $+ \frac{2}{3}$ and $- \frac{1}{3}$ , respectively), the antineutron is composed of one antiup quark and two antidown quarks (of charge $- \frac{2}{3}$ and $+ \frac{1}{3}$ , respectively). While the overall charge of the neutron remains the same, its constituent particles do not!

A word about antiparticles: Like regular particles, antiparticles could function just fine on their own. In fact, a universe made up of antimatter may operate just as our own matter-based universe does. However, we do not know fully whether this is the case. The reason for this is annihilation. Annihilation is the process of destruction that occurs when a particle and its antiparticle interact. As soon as two particles (like a positron and an electron) coincide, they convert their masses to energy through the equation $E = m c^{2}$ . This mass-to-energy conversion, which typically results in photon release, happens instantaneously and makes it very difficult for scientists to study antimatter. That said, scientists have had success creating antimatter through high-energy particle collisions. Both antineutrons and antiprotons were created through accelerator experiments in 1956, and an antihydrogen atom was even created at CERN in 1995! As referenced in Figure_22_05_PET, the annihilation of antiparticles is currently used in medical studies to determine the location of radioisotopes.

Completing the Standard Model of the Atom

The Standard Model of the atom refers to the current scientific view of the fundamental components and interacting forces of matter. The Standard Model (Figure_23_02_Standard) shows the six quarks that bind to form all hadrons, the six lepton particles already considered fundamental, the four carrier particles (or gauge bosons) that transmit forces between the leptons and quarks, and the recently added Higgs boson (which will be discussed shortly). This totals 17 fundamental particles, combinations of which are responsible for all known matter in our entire universe! When adding the antiquarks and antileptons, 31 components make up the Standard Model.

View original figure (external; image not reproduced in this text edition)

Original figure description: The image is of seventeen boxes, sixteen of which are in a four-by-four grid. (The seventeenth box, in yellow and off to the side, is of the Higgs boson.) Spheres in purple are quarks, spheres in green are leptons, and spheres in red are gauge bosons.

The Standard Model of elementary particles shows an organized view of all fundamental particles, as currently known: six quarks, six leptons, and four gauge bosons (or carrier particles). The Higgs boson, first observed in 2012, is a new addition to the Standard Model.

Figure_23_02_Standard shows all particles within the Standard Model of the atom. Not only does this chart divide all known particles by color-coded group, but it also provides information on particle stability. Note that the color-coding system in this chart is separate from the red, green, and blue color labeling system of quarks. The first three columns represent the three families of matter. The first column, considered Family 1, represents particles that make up normal matter, constructing the protons, neutrons, and electrons that make up the common world. Family 2, represented from the charm quark to the muon neutrino, is comprised of particles that are more massive. The leptons in this group are less stable and more likely to decay. Family 3, represented by the third column, are more massive still and decay more quickly. The order of these families also conveniently represents the order in which these particles were discovered.

The Standard Model also summarizes the fundamental forces that exist as particles interact. A closer look at the Standard Model, as shown in Figure_23_02_Revised, reveals that the arrangement of carrier particles describes these interactions.

View original figure (external; image not reproduced in this text edition)

Original figure description: The sixteen boxes shown before are now shaded due to force. A red shade extends from the gluon and colors all purple quarks. A blue shade extends from the photon and colors all quarks and the first row of leptons (the electron, muon, and tau). A green shade extends from the Z and W bosons and colors all quarks and leptons. As the background colors overlap, the red shade appears brown, and the blue shade appears dark green.

The revised Standard Model shows the interaction between gauge bosons and other fundamental particles. These interactions are responsible for the fundamental forces, three of which are described through the chart’s shaded areas.

Each of the shaded areas represents a fundamental force and its constituent particles. The red shaded area shows all particles involved in the strong nuclear force, which we now know is due to quantum chromodynamics. The blue shaded area corresponds to the electromagnetic force, while the green shaded area corresponds to the weak nuclear force, which affects all quarks and leptons. The electromagnetic force and weak nuclear force are considered united by the electroweak force within the Standard Model. Also, because definitive evidence of the graviton is yet to be found, it is not included in the Standard Model.

The Higgs Boson

One interesting feature of the Standard Model shown in Figure_23_02_Revised is that, while the gluon and photon have no mass, the Z and W bosons are very massive. What supplies these quickly moving particles with mass and not the gluons and photons? Furthermore, what causes some quarks to have more mass than others?

In the 1960s, British physicist Peter Higgs and others speculated that the W and Z bosons were actually just as massless as the gluon and photon. However, as the W and Z bosons traveled from one particle to another, they were slowed down by the presence of a Higgs field, much like a fish swimming through water. The thinking was that the existence of the Higgs field would slow down the bosons, causing them to decrease in energy and thereby transfer this energy to mass. Under this theory, all particles pass through the Higgs field, which exists throughout the universe. The gluon and photon travel through this field as well but are able to do so unaffected.

The presence of a force from the Higgs field suggests the existence of its own carrier particle, the Higgs boson. This theorized boson interacts with all particles but gluons and photons, transferring force from the Higgs field. Particles with large mass (like the top quark) are more likely to receive force from the Higgs boson.

While it is difficult to examine a field, it is somewhat simpler to find evidence of its carrier. On July 4, 2012, two groups of scientists at the LHC independently confirmed the existence of a Higgs-like particle. By examining trillions of proton–proton collisions at energies of 7 to 8 TeV, LHC scientists were able to determine the constituent particles that created the protons. In this data, scientists found a particle with similar mass, spin, parity, and interactions with other particles that matched the Higgs boson predicted decades prior. On March 13, 2013, the existence of the Higgs boson was tentatively confirmed by CERN. Peter Higgs and Francois Englert received the Nobel Prize in 2013 for the “theoretical discovery of a mechanism that contributes to our understanding of the origin and mass of subatomic particles.”

Particle Physicist

If you have an innate desire to unravel life’s great mysteries and further understand the nature of the physical world, a career in particle physics may be for you!

Particle physicists have played a critical role in much of society’s technological progress. From lasers to computers, televisions to space missions, splitting the atom to understanding the DNA molecule to MRIs and PET scans, much of our modern society is based on the work done by particle physicists.

While many particle physicists focus on specialized tasks in the fields of astronomy and medicine, the main goal of particle physics is to further scientists’ understanding of the Standard Model. This may mean work in government, industry, or academics. Within the government, jobs in particle physics can be found within the National Institute for Standards and Technology, Department of Energy, NASA, and Department of Defense. Both the electronics and computer industries rely on the expertise of particle physicists. College teaching and research positions can also be potential career opportunities for particle physicists, though they often require some postgraduate work as a prerequisite. In addition, many particle physicists are employed to work on high-energy colliders. Domestic collider labs include the Brookhaven National Laboratory in New York, the Fermi National Accelerator Laboratory near Chicago, and the SLAC National Accelerator Laboratory operated by Stanford University. For those who like to travel, work at international collider labs can be found at the CERN facility in Switzerland in addition to institutes like the Budker Institute of Nuclear Physics in Russia, DESY in Germany, and KEK in Japan.

Shirley Jackson became the first African American woman to earn a Ph.D. from MIT back in 1973, and she went on to lead a highly successful career in the field of particle physics. Like Dr. Jackson, successful students of particle physics grow up with a strong curiosity in the world around them and a drive to continually learn more. If you are interested in exploring a career in particle physics, work to achieve good grades and SAT scores, and find time to read popular books on physics topics that interest you. While some math may be challenging, recognize that this is only a tool of physics and should not be considered prohibitive to the field. High-level work in particle physics often requires a Ph.D.; however, it is possible to find work with a master’s degree. Additionally, jobs in industry and teaching can be achieved with solely an undergraduate degree.

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Check Your Understanding

In what particle were quarks originally discovered?

the electron
the neutron
the proton
the photon

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Does antimatter exist?

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What fundamental property is provided by particle interaction with the Higgs boson?

charge
mass
spin
color

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What particles were launched into the proton during the original discovery of the quark?

bosons
electrons
neutrons
photons

Section Summary

There are three types of fundamental particles—leptons, quarks, and carrier particles.
Quarks come in six flavors and three colors and occur only in combinations that produce white.
Hadrons are thought to be composed of quarks, with baryons having three quarks and mesons having a quark and an antiquark.
Known particles can be divided into three major groups—leptons, hadrons, and carrier particles (gauge bosons).
All particles of matter have an antimatter counterpart that has the opposite charge and certain other quantum numbers. These matterantimatter pairs are otherwise very similar but will annihilate when brought together.
The strong force is carried by eight proposed particles called gluons, which are intimately connected to a quantum number called color—their governing theory is thus called quantum chromodynamics (QCD). Taken together, QCD and the electroweak theory are widely accepted as the Standard Model of particle physics.

Concept Items

How does the charge of an individual quark determine hadron structure?

Since the hadron must have an integral value, the individual quarks must be combined such that the average of their charges results in the value of a quark.
Since the hadron must have an integral value, the individual atoms must be combined such that the sum of their charges is less than zero.
The individual quarks must be combined such that the product of their charges is equal to the total charge of the hadron structure.
Since the hadron must have an integral value of charge, the individual quarks must be combined such that the sum of their charges results in an integral value.

Why do leptons not feel the strong nuclear force?

Gluons are the carriers of the strong nuclear force that interacts between quarks through color interactions, but leptons are constructed of quarks that do not have gluons.
Gluons are the carriers of the strong nuclear force that interacts between quarks through mass interactions, but leptons are not constructed of quarks and are not massive.
Gluons are the carriers of the strong nuclear force that interacts between quarks through mass interactions, but leptons are constructed of the quarks that are not massive.
Gluons are the carriers of the strong nuclear force that interacts between quarks through color interactions, but leptons are not constructed of quarks, nor do they have color constituents.

What property commonly distinguishes antimatter from its matter analogue?

mass
charge
energy
speed

Can the Standard Model change as new information is gathered?

What is the relationship between the Higgs field and the Higgs boson?

The Higgs boson is the carrier that transfers force for the Higgs field.
The Higgs field is the time duration over which the Higgs particles transfer force to the other particles.
The Higgs field is the magnitude of momentum transferred by the Higgs particles to the other particles.
The Higgs field is the magnitude of torque transfers by the Higgs particles on the other particles.

What were the original three flavors of quarks discovered?

up, down, and charm
up, down, and bottom
up, down, and strange
up, down, and top

Protons are more massive than electrons. The three quarks in the proton account for only a small amount of this mass difference. What accounts for the remaining excess mass in protons compared to electrons?

The highly energetic gluons connecting the quarks account for the remaining excess mass in protons compared to electrons.
The highly energetic photons connecting the quarks account for the remaining excess mass in protons compared to electrons.
The antiparallel orientation of the quarks present in a proton accounts for the remaining excess mass in protons compared to electrons.
The parallel orientation of the quarks present in a proton accounts for the remaining excess mass in protons compared to electrons.

Critical Thinking

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Why are mesons considered unstable?

Mesons are composites of two antiparticles that quickly annihilate each other.
Mesons are composites of two particles that quickly annihilate each other.
Mesons are composites of a particle and antiparticle that quickly annihilate each other.
Mesons are composites of two particles and one antiparticle that quickly annihilate each other.

Does antimatter have a negative mass?

No, antimatter does not have a negative mass.
Yes, antimatter does have a negative mass.

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How were particle collisions used to provide evidence of the Higgs boson?

Because some particles do not contain the Higgs boson, the collisions of such particles will cause their destruction.
Because only the charged particles contain the Higgs boson, the collisions of such particles will cause their destruction and will expel the Higgs boson.
Because all particles with mass contain the Higgs boson, the collisions of such particles will cause their destruction and will absorb the Higgs boson.
Because all particles with mass contain the Higgs boson, the collisions of such particles will cause their destruction and will expel the Higgs boson.

Explain how the combination of a quark and antiquark can result in the creation of a hadron.

The combination of a quark and antiquark can result in a particle with an integer charge and color of white, therefore satisfying the properties for a hadron.
The combination of a quark and antiquark must result in a particle with a negative charge and color of white, therefore satisfying the properties for a hadron.
The combination of a quark and antiquark can result in a particle with an integer charge and color that is not white, therefore satisfying the properties for a hadron.
The combination of a quark and antiquark can result in particle with a fractional charge and color that is not white, therefore satisfying the properties for a hadron.

Test Prep Multiple Choice

To what color must quarks combine for a particle to be constructed?

black
green
red
white

What type of hadron is always constructed partially of an antiquark?

baryon
lepton
meson
photno

What particle is typically released when two particles annihilate?

graviton
antimatter
pion
photno

Which of the following categories is not one of the three main categories of the Standard Model?

gauge bosons
hadrons
leptons
quarks

Analysis of what particles began the search for the Higgs boson?

W and Z bosons
up and down quarks
mesons and baryons
neutrinos and photons

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Test Prep Short Answer

What two features of quarks determine the structure of a particle?

the color and charge of individual quarks
the color and size of individual quarks
the charge and size of individual quarks
the charge and mass of individual quarks

What fundamental force does quantum chromodynamics describe?

the weak nuclear force
the strong nuclear force
the electromagnetic force
the gravitational force

Is it possible for a baryon to be constructed of two quarks and an antiquark?

Yes, the color of the three particles would be able to sum to white.
No, the color of the three particles would not be able to sum to white.

Can baryons be more massive than mesons?

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Does a neutron have an antimatter counterpart?

No, the antineutron does not exist.
Yes, the antineutron does exist.

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Which particles in the Standard Model account for the majority of matter with which we are familiar?

particles in fourth column of the Standard Model
particles in third column of the Standard Model
particles in the second column of the Standard Model
particles in the first column of the Standard Model

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Test Prep Extended Response

In this image, how does the emission of the gluon cause the down quark to change from a red color to a green color?

View original figure (external; image not reproduced in this text edition)

Original figure description: The figure has two equations R right arrow G plus R G and R G plus G right arrow R. A vertical time axis and horizontal x-axis or distance is in the bottom left hand corner of the schematic relating to these equations. Inside of the axis are five spheres, three in a diagonal pattern are red with two green ones on either side of the filled diagonal. The two equations are showing the process by which the R, red d sphere, and G, green s sphere, react with a creation of the R G, red g sphere, by the R, red d sphere, to create G, green d sphere. The other initial G, green s sphere, reacts with the newly created R G sphere to create R, red s sphere.

The emitted red gluon is made up of a green and a red color. As a result, the down quark changes from a red color to a green color.
The emitted red gluon is made up of an anti-green and an anti-red color. As a result, the down quark changes from a red color to a green color.
The emitted red gluon is made up of a green and an anti-red color. As a result, the down quark changes from a red color to a green color.
The emitted red gluon is made up of an anti-green and a red color. As a result, the down quark changes from a red color to a green color.

Neutrinos are much more difficult for scientists to find when compared to other hadrons and leptons. Why is this?

Neutrinos are hadrons, and they lack charge.
Neutrinos are not hadrons, and they lack charge.
Neutrinos are hadrons, and they have positive charge.
Neutrinos are not hadrons, and they have a positive charge.

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The term force carrier particle is shorthand for the scientific term vector gauge boson. From that perspective, can the Higgs boson truly be considered a force carrier particle?

No, the mass quality provided by the Higgs boson is a scalar quantity.
Yes, the mass quality provided by the Higgs boson results in a change of particles direction.

Glossary

annihilation: the process of destruction that occurs when a particle and antiparticle interact
antimatter: matter constructed of antiparticles; antimatter shares most of the same properties of regular matter, with charge being the only difference between many particles and their antiparticle analogues
baryon: hadrons that always decay to another baryon
bottom quark: a quark flavor
charmed quark: a quark flavor, which is the counterpart of the strange quark
color: a property of quarks the relates to their interactions through the strong force
down quark: the second lightest of all quarks
flavor: quark type
gluons: exchange particles of the nuclear strong force
hadron: particles composed of quarks that feel the strong and weak nuclear force
Higgs boson: a massive particle that provides mass to the weak bosons and provides validity to the theory that carrier particles are identical under certain circumstances
Higgs field: the field through which all fundamental particles travel that provides them varying mass through the transport of the Higgs boson
lepton: fundamental particles that do not feel the nuclear strong force
meson: hadrons that can decay to leptons and leave no hadrons
pair production: the creation of a particle and antiparticle, commonly an electron and positron, due to the annihilation of a photon
positron: a particle of antimatter that has the properties of a positively charged electron
quantum chromodynamics: the theory of color interaction between quarks that leads to understanding of the nuclear strong force
quark: an elementary particle and fundamental constituent of matter that is a substructure of hadrons
Standard Model: an organization of fundamental particles and forces that is a result of quantum chromodynamics and electroweak theory
strange quark: the third lightest of all quarks
top quark: a quark flavor
up quark: the lightest of all quarks

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