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Physics for the 21st Century

The Basic Building Blocks of Matter

An exploration of particle physics

In this unit, we shall explore particle physics, the study of the fundamental constituents of matter. These basic building blocks lay the foundation for all of the ambitious projects detailed throughout this course. Dramatic discoveries over the last century have completely changed our view of the structure of matter, as physicists have delved into the atom and deeper to discover the quarks and gluons inside the proton, have observed neutrino oscillations, and have carried out precise studies of the subtle asymmetry between matter and antimatter. The research has led to a detailed, if still incomplete, understanding of the most basic constituents of our universe.

Unit Glossary

alpha rays
Alpha particles, also known as alpha rays, consist of two protons and two neutrons bound together into a particle identical to the nucleus of a helium atom. Alpha particles are emitted when certain radioactive atoms decay, and typically have an energy of about 5 MeV.

Antimatter is a type of matter predicted by Paul Dirac when he attempted to write down a version of quantum mechanics that incorporated Einstein’s theory of special relativity. In the Standard Model, every particle has a corresponding antiparticle that has the same mass but opposite electric charge, baryon number, and strangeness. When a particle meets its antiparticle counterpart, the pair annihilates: they disappear, and their total energy is converted into other particles.

An antiquark is the antimatter counterpart of a quark. See: antimatter, quark.

atomic number
The atomic number of an atom, denoted by Z, is the number of protons in its nucleus. The atomic number of an atom determines its place in the periodic table, and thus which chemical element it is.

B factory
A B factory is a particle physics apparatus designed to create B mesons, which are mesons that contain one bottom antiquark and one quark of a different flavor. In a B factory, electrons and positrons from an accelerator collide, producing B mesons and anti-B mesons in equal amounts. The science goal of the B factories is to study CP violation in B meson decay, which may shed light on why the universe contains more matter than antimatter.

The term “baryon” refers to any particle in the Standard Model that is made of three quarks. Murray Gell-Mann arranged the baryons into a periodic table-like structure according to their baryon number and strangeness (see Unit 1, Fig. 1). Protons and neutrons are the most familiar baryons.

baryon number
Every particle composed of quarks is assigned a baryon number. A particle’s baryon number is one-third the number of quarks minus the number of antiquarks in the particle. Baryon number is conserved in collisions, which means that the total baryon number of incoming particles is the same as the total baryon number of the particles produced in the collision.

beta rays
Beta particles, also known as beta rays, are the electrons emitted when a neutron in the nucleus of a radioactive atom decays into a proton. Beta particles typically have an energy of up to 2.5 MeV, sharing the total energy released in the radioactive decay with a neutrino that is produced at the same time.

The Bevatron is a particle accelerator operated at the Lawrence Berkeley National Laboratory from 1954 to 1993. It was designed to test the hypothesis that every particle has a corresponding antiparticle, and accelerated protons to high enough energies (6.2 GeV) that antiprotons might be produced in a collision with a fixed target. The Bevatron successfully produced antiprotons, and remained a productive research facility through several upgrades until its decommissioning in 1993.

bubble chamber
Bubble chambers were the primary detectors used to study particles produced in particle accelerator collisions throughout the 1960s and 1970s. A bubble chamber is a vessel filled with superheated liquid, usually hydrogen, which will immediately boil when disturbed. When a charged particle passes through a bubble chamber, a trail of bubbles traces out its path through the liquid. Photographs of these trails allowed particle physicists to figure out what particles were produced in high energy collisions. Bubble chambers supplanted cloud chambers because they are easier to build in large sizes and the higher density in the liquid gave better resolution in the detector.

charge conjugation
Charge conjugation is an operation that changes a particle into its antiparticle.

cloud chamber
Cloud chambers are one of the earliest types of detectors used to study particles in cosmic rays and those produced in particle accelerator collisions. A cloud chamber is an airtight box filled with supersaturated water vapor. When a charged particle passes through a cloud chamber, liquid water droplets condense out of the vapor along the particle’s path, leaving a visible trail.

CP violation
The CP operation is a combination of charge conjugation (C) and parity (P). In most interactions, CP is conserved, which means that the interaction proceeds exactly the same way if the CP operation is performed on the interacting particles. If CP is conserved, particles with opposite charge and parity will interact in the same way as the original particles. CP violation occurs when an interaction proceeds differently when the CP operation is performed—particles with opposite charge and parity interact differently than the original particles. CP violation was first observed in neutral kaon systems.

A cyclotron is a type of particle accelerator, first developed in the 1930s, consisting of two D-shaped cavities in a constant magnetic field. There is a gap between the cavities, so they form a circle with a missing stripe in the middle. A radioactive source placed in the center of the cyclotron—in the gap between the cavities—provides particles that will be accelerated. When a charged particle is emitted by the radioactive source, it is accelerated by a voltage placed across the gap. The magnetic field bends the particle’s path so it travels in a circle. When the particle circles back to the gap, it is traveling in the opposite direction. At that point, the voltage across the gap is reversed so the particle is accelerated further. The voltage is alternated so that the particle is accelerated each time it crosses the gap. As the particle speeds up, its path is bent less by the magnetic field and it travels in an increasingly larger circle. Eventually, it spirals out of the cyclotron moving at a high speed. The largest cyclotron currently in use is TRIUMF at the University of British Columbia in Vancouver, Canada.

dark energy
Dark energy is the general term for the substance that causes the universe to expand at an accelerated rate. Although dark energy is believed to be 74 percent of the total energy in the universe, we know very few of its properties. One active area of research is to determine whether dark energy behaves like the cosmological constant or changes over time.

dark matter
Dark matter is a form of matter unlike the ordinary matter that is described by the Standard Model. It accounts for most of the mass in the universe, but only has been observed indirectly through its gravitational influence on ordinary matter. Dark matter is believed to account for 23 percent of the total energy in the universe.

electron accelerator
An electron accelerator is a particle accelerator designed to accelerate electrons. See: particle accelerator, SLAC.

gamma rays
Gamma rays are high-energy photons that are sometimes emitted from the nucleus of an atom that has just decayed by emitting an alpha or a beta particle. Gamma ray photons typically have energies greater than 1 MeV. They are on the high-energy end of the electromagnetic spectrum.

Gluons are particles in the Standard Model that mediate strong interactions. Because gluons carry color charge, they can participate in the strong interaction in addition to mediating it. The term “gluon” comes directly from the word glue, because gluons bind together into mesons.

The term hadron refers to the Standard Model particle made of quarks. Mesons and baryons are classified as hadrons.

Higgs boson
The Higgs boson is a Standard Model particle thought to give particles their mass. Light particles interact less strongly with the Higgs than heavy particles. As of 2010, it had not yet been discovered. If the Higgs exists, experiments at LEP and the Tevatron have determined that its mass cannot be smaller than 110 GeV, and cannot lie between 163 and 166 GeV.

The term kaon refers to any one of four mesons with nonzero strangeness. The positively charged K+ is composed of an up quark and an anti-strange quark. Its antiparticle is the negatively charged K– , which is composed of an anti-up quark and a strange quark. The two neutral kaons, and are made of down, anti-down, strange, and anti-strange quarks. CP violation was first observed in the neutral kaon system.

Large Hadron Collider (LHC)
The Large Hadron Collider (LHC) is a particle accelerator operated at CERN on the outskirts of Geneva, Switzerland. The LHC accelerates two counter-propagating beams of protons in the 27 km synchrotron beam tube formerly occupied by Large Electron-Positron Collider (LEP). It is the largest and brightest accelerator in the world, capable of producing proton-proton collisions with a total energy of 14 TeV. Commissioned in 2008–09, the LHC is expected to find the Higgs boson, the last undiscovered particle in the Standard Model, as well as probe physics beyond the Standard Model.

The Large Electron-Positron Collider (LEP) is a particle accelerator that was operated at CERN on the outskirts of Geneva, Switzerland, from 1989 to 2000. LEP accelerated counterpropagating beams of electrons and positrons in a 27 km diameter synchrotron ring. With a total collision energy of 209 GeV, LEP was the most powerful electron-positron collider ever built. Notably, LEP enabled a precision measurement of the mass of W and Z bosons, which provided solid experimental support for the Standard Model. In 2000, LEP was dismantled to make space for the LHC, which was built in its place.

lepton number
Every lepton in the Standard Model is assigned a lepton number. The electron, muon, and tau and their corresponding neutrinos have a lepton number of 1. Their antiparticle partners have a lepton number of -1. In the Standard Model, lepton number is conserved in all interactions, except in the case of neutrino oscillation.

The leptons are a family of fundamental particles in the Standard Model. The lepton family has three generations, shown in Unit 1, Fig. 1: the electron and electron neutrino, the muon and muon neutrino, and the tau and tau neutrino.

The term linac is a shortened version of “linear accelerator.” A linac is a particle accelerator that accelerates charged particles in a straight line. Charged particles enter the accelerator at one end and are accelerated as they pass through a series of voltages placed along the beam path. Because the path the particles follow is shorter and they pass through fewer accelerating voltages, linacs cannot accelerate particles as much as circular accelerators can. However, linacs are easier to build and run, and they are often used to create beams of particles to be injected into a synchrotron. SLAC is a linac, as were J.J. Thomson’s cathode ray tubes.

The term meson refers to any particle in the Standard Model that is made of one quark and one anti-quark. Murray Gell-Mann arranged the leptons into a periodic-table-like structure according to their electric charge and strangeness (see Unit 1, Fig. 1). Examples of mesons are pions and kaons.

The muon is a fundamental particle in the Standard Model. It is a member of the second generation of leptons. The muon is negatively charged, heavier than the electron, and lighter than the tau.

Neutrinos are fundamental particles in the lepton family of the Standard Model. Each generation of the lepton family includes a neutrino (see Unit 1, Fig. 18). Neutrinos are electrically neutral and nearly massless. When neutrinos are classified according to their lepton family generation, the three different types of neutrinos (electron, muon, and tau) are referred to as “neutrino flavors.” While neutrinos are created as a well-defined flavor, the three different flavors mix together as the neutrinos travel through space, a phenomenon referred to as “flavor oscillation.” Determining the exact neutrino masses and oscillation parameters is still an active area of research.

Parity is an operation that turns a particle or system of particles into its mirror image, reversing their direction of travel and physical positions.

particle accelerator
Particle accelerators are the primary experimental tool used in particle physics experiments. They accelerate beams of charged particles—such as protons, electrons, and ions—to very high speeds. In a particle physics experiment, the fast-moving beams are steered into a collision either with a stationary target or a beam traveling in the opposite direction. These collisions release a tremendous amount of energy that can create new particles. Much of the Standard Model was developed by studying the particles produced in such collisions. See: cyclotron, linac, synchrotron.

The term pion refers to any one of three mesons containing up and down quarks and their antiparticles. The positively charged is composed of an up quark and an anti-down quark. Its antiparticle is the negatively charged , which is composed of an anti-up quark and a down quark. The neutral pion, , is made of down, anti-down, up, and anti-up quarks. Pions are the lightest mesons , and play a role in strong interactions in the nuclei of atoms.

The positron is the antimatter counterpart to the electron. It has an electric charge of +1 and the same mass as an electron.

The quarks are a family of fundamental particles in the Standard Model. The quark family has three generations, shown in Unit 1, Fig. 1: up and down quarks, the charm and strange quarks, and top and bottom quarks. Individual, isolated quarks are never observed in nature. Instead, we observe bound groups of quarks as baryons and mesons.

The results of particle physics experiments are often expressed as the probability of particles interacting in the detector depending on how much energy the particles have. As the particle energy increases, the interaction probability changes slowly and smoothly, except at certain special energies called “resonances,” which appear as bumps on the graph of probability versus energy. At a resonance, the probability of the particles interacting increases significantly. For example, the J/psi meson was discovered when the electron-positron collision energy just equaled the mass of the charm quark and anti-charm quark, creating a huge spike in particle production. Finding resonances is the primary way new particles are discovered in particle accelerator experiments. The mass of the new particle is the resonance energy.

The Stanford Linear Accelerator Center (SLAC) is a linear particle accelerator (linac) operated by Stanford University. SLAC is the longest linear accelerator in the world, accelerating electrons or positrons for two miles. The collision energy when the accelerated particles hit a fixed target is 50 GeV. Since the SLAC began operation in 1966, its results have led to Nobel Prizes for the discovery of the J/Psi particle, which provided evidence for the existence of the charm quark, the discovery of structure inside protons and neutrons indicating that they are made of quarks, and the discovery of the tau lepton. SLAC is now used in a wide variety of projects that range from astrophysics to biology, chemistry, and materials science.

Standard Model
The Standard Model is the name given to the current theory of fundamental particles and how they interact. It includes three generations of quarks and leptons interacting via the strong, weak, and electromagnetic forces. The Standard Model does not include gravity.

Strangeness is a number assigned to Standard Model particles made of quarks. It is defined as the number of strange quarks minus the number of anti-strange quarks in the particle. Strangeness is useful in arranging baryons into a “periodic table,” and is conserved in strong and electromagnetic interactions, but not in weak interactions.

strong interaction
The strong interaction, or strong nuclear force, is one of the four fundamental forces of nature. It acts on quarks, binding them together into mesons. Unlike the other forces, the strong force between two particles remains constant as the distance between them grows, but actually gets weaker when the particles get close enough together. This unique feature ensures that single quarks are not found in nature. True to its name, the strong force is a few orders of magnitude stronger than the electromagnetic and weak interactions, and many orders of magnitude stronger than gravity.

A synchrotron is a type of circular particle accelerator similar to the cyclotron, but capable of accelerating particles to much higher energies. It consists of a toroidal tube in a magnetic field. Charged particles are injected into the synchrotron’s beam tube from a preliminary accelerator, usually a linac. Once in the synchrotron, the magnetic field directs the particles in a circular path. A voltage is placed across short sections of the beam tube along the path the particles follow. The particles are accelerated each time they pass through one of these voltages. As the particles speed up, the strength of the magnetic field is increased to keep the particles traveling in the same circular path. After circling through the accelerator many times, the particles are traveling at nearly the speed of light. The Bevatron, Tevatron, and LHC are all synchrotron accelerators.

The tau, also called the tauon, is a fundamental particle in the Standard Model. It is a member of the third generation of leptons. The tau is negatively charged, and is heavier than the electron and muon.

The Tevatron is a particle accelerator operated at the Fermi National Accelerator Laboratory in Batavia, Illinois. Since the completion of construction in 1983, the Tevatron has accelerated counterpropagating beams of protons and antiprotons in a 6.28 kN diameter synchrotron ring. Many important aspects of the Standard Model were supported by Tevatron experiments. Notably, the top quark was first discovered in Tevatron collisions. The Tevatron is the most powerful proton-antiproton collider in the world, with collision energies of up to 2 TeV. Only the LHC, a proton-proton collider, is capable of creating higher energy collisions.

weak interaction
The weak interaction, or weak force, is one of the four fundamental forces of nature. It is called “weak” because it is significantly weaker than both the strong force and the electromagnetic force; however, it is still much stronger than gravity. The weak changes one flavor of quark into another, and is responsible for radioactive decay.

Z boson
Z bosons are electrically neutral particles in the Standard Model that, along with the electrically charged W+ and W– bosons, mediate weak interactions.

Content Developer: Natalie Roe

Content Developer: Natialia Roe

Natalie Roe is a senior scientist in the physics division at the Lawrence Berkeley National Laboratory, a Fellow of the American Physical Society, and former chair of the APS Division of Particles and Fields. Roe has worked on a variety of particle physics experiments at CERN, Fermilab, and SLAC. In her most recent work in particle physics, she led the design and construction of the silicon vertex detector for the BaBar experiment and used it to study CP violation, the subtle asymmetry between matter and antimatter. She is now working in particle cosmology and is the instrument scientist for BOSS, the Baryon Oscillation Spectroscopic Survey. The BOSS spectrograph upgrade was installed at the Sloan Digital Sky Survey 2.5 meter telescope in Apache Point, New Mexico. BOSS will study the large-scale structure of the universe through precise redshift measurements of 1.5 million galaxies and in the absorption spectra of 150,000 quasars.

Featured Scientist: Bonnie Fleming

Featured Scientist: Bonnie Fleming

Bonnie Fleming is the Horace D. Taft Associate Professor of Physics at Yale University. She earned her Ph.D. in physics at Columbia University in 2002. She heads the Yale High Energy Neutrino Physics Group, an experimental group focusing on new physics in the neutrino sector. The group is currently working on the MiniBooNE, ArgoNeuT, and MicroBooNE experiments at Fermilab while doing research and development on the new generation of accelerator neutrino detectors—Liquid Argon Time Projection Chambers.


Featured Scientist: Mark C. Kruse

Featured Scientist: Mark C. Kruse

Mark C. Kruse is an associate professor at Duke University and specializes in experimental high energy physics. He is primarily interested in searches for the Higgs boson, production of vector boson pairs, and model-independent analysis techniques for new particle searches. Kruse led the CDF Higgs discovery group at Fermilab from January 2007 to January 2009 and continues to play an active role in Higgs searches. In addition, he has developed a global analysis to search for new physics using events containing a high-energy electron and muon. These programs of research are being conducted with the CDF detector at Fermilab and the ATLAS detector at the Large Hadron Collider (LHC) which started colliding beams of protons in late 2009. Kruse is also interested in silicon detector design for high-energy particle physics experiments and is part of a group developing the next generation of silicon detectors for the ATLAS experiment.


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Physics for the 21st Century


Produced by the Harvard-Smithsonian Center for Astrophysics Science Media Group in association with the Harvard University Department of Physics. 2010.
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  • ISBN: 1-57680-891-2