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Elementary Particle Physics Glossary
Course organiser: Dr Marcella Bona
February 9, 2016
1
Contents
1 Terms A-C
1.1 Accelerator . . . . .
1.2 Annihilation . . . . .
1.3 Antimatter . . . . .
1.4 Antiparticle . . . . .
1.5 Antiquark . . . . . .
1.6 Astrophysics . . . .
1.7 B-Factory . . . . . .
1.8 Baryon . . . . . . . .
1.9 Beam . . . . . . . .
1.10 Big Bang Theory . .
1.11 Boson . . . . . . . .
1.12 Bottom Quark . . .
1.13 CERN . . . . . . . .
1.14 Charge . . . . . . . .
1.15 Charge Conservation
1.16 Charm Quark . . . .
1.17 Collider . . . . . . .
1.18 Colour Charge . . .
1.19 Confinement . . . . .
1.20 Conservation . . . .
1.21 Cosmology . . . . .
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2 Terms D-F
2.1 Dark Matter . . . . . . . .
2.2 Decay . . . . . . . . . . . .
2.3 Down Quark . . . . . . . .
2.4 Electric Charge . . . . . . .
2.5 Electromagnetic Interaction
2.6 Electron . . . . . . . . . . .
2.7 Electroweak Interaction . .
2.8 Event . . . . . . . . . . . .
2.9 Fermilab . . . . . . . . . . .
2.10 Fermion . . . . . . . . . . .
2.11 Fixed-target Experiment . .
2.12 Flavour . . . . . . . . . . .
2.13 Fundamental Interaction . .
2.14 Fundamental Particle . . . .
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7
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3 Terms G-I
3.1 Generation . . . . . . . .
3.2 GeV . . . . . . . . . . . .
3.3 Gluon . . . . . . . . . . .
3.4 Gravitational Interaction .
3.5 Graviton . . . . . . . . . .
3.6 Hadron . . . . . . . . . .
3.7 Interaction . . . . . . . .
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2
4 Terms J-L
4.1 Kaon .
4.2 Lepton
4.3 LHC .
4.4 Linacs
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10
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5 Terms M-O
5.1 Mass . . . . . .
5.2 Meson . . . . .
5.3 Muon . . . . .
5.4 Muon Chamber
5.5 Neutral . . . .
5.6 Neutrino . . . .
5.7 Neutron (n) . .
5.8 Nucleus . . . .
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6 Terms P-R
6.1 Particle . . . . . . . . . .
6.2 Pauli Exclusion Principle
6.3 Photon . . . . . . . . . . .
6.4 Pion . . . . . . . . . . . .
6.5 Positron (e+ ) . . . . . . .
6.6 Proton (p) . . . . . . . . .
6.7 Quantum . . . . . . . . .
6.8 Quantum Mechanics . . .
6.9 Quark (q) . . . . . . . . .
6.10 Residual Interaction . . .
6.11 Rest Mass . . . . . . . . .
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7 Terms S-U
7.1 SLAC . . . . . . . . .
7.2 Spin . . . . . . . . . .
7.3 Stable . . . . . . . . .
7.4 Standard Model . . . .
7.5 Strange Quark (s) . .
7.6 Strong Interaction . .
7.7 Subatomic Particle . .
7.8 Synchrotron . . . . . .
7.9 Tau . . . . . . . . . .
7.10 TeV . . . . . . . . . .
7.11 Top Quark (t) . . . . .
7.12 Track . . . . . . . . .
7.13 Tracking . . . . . . . .
7.14 Uncertainty Principle .
7.15 Up Quark . . . . . . .
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8 Terms V-X
14
8.1 Virtual Particle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.2 W + , W − Boson . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8.3 Weak Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3
9 Terms Y-Z
14
9.1 Z Boson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4
1
1.1
Terms A-C
Accelerator
A machine used to accelerate particles to high speeds, and thus high energy
compared to their rest-mass energy.
1.2
Annihilation
A process in which a particle meets its corresponding antiparticle and both disappear. The energy appears in some other form, perhaps as a different particle
and its antiparticle (and their energy), perhaps as many mesons, perhaps as a
single neutral boson. The produced particles may be any combination allowed
by conservation of energy and momentum and of all the charge types.
1.3
Antimatter
Material made from antifermions. We define the fermions that are common in
our universe as matter and their antiparticles as antimatter. In the particle
theory there is no a priori distinction between matter and antimatter. The
asymmetry of the universe between these two classes of particles is a deep puzzle
for which we are not yet completely sure of an explanation.
1.4
Antiparticle
For most particle types (and every fermion type) there is another particle type
that has exactly the same mass but the opposite value of all other charges
(quantum numbers). This is called the antiparticle. For example, the antiparticle of an electron is a particle of positive electric charge called the positron.
Most boson types also have antiparticles except for those that have zero value
for all charges, such as a photon or a composite boson made from a quark and
its corresponding antiquark. In these cases there is no distinction between the
particle and the antiparticle; they are the same object.
1.5
Antiquark
The antiparticle of a quark.
1.6
Astrophysics
The physics of astronomical objects such as stars and galaxies.
1.7
B-Factory
The B-Factory is an accelerator at SLAC designed to maximize the production
of B mesons. The properties of the B meson are then studied with special
detectors.
5
1.8
Baryon
A hadron made from three quarks. The proton (up-up-down) and the neutron
(up-down-down) are both baryons. They may also contain additional quarkantiquark pairs.
1.9
Beam
The particle stream injected into an accelerator. These particles travel in clusters.
1.10
Big Bang Theory
The theory that the expanding universe began as an infinitely dense and hot
medium. The initial instant is called the Big Bang.
1.11
Boson
A particle that has integer intrinsic angular momentum (spin) measured in units
of h-bar (spin =0, 1, 2, ...). All particles are either fermions or bosons. The
particles associated with all the fundamental interactions (forces) and composite
particles with even numbers of fermion constituents (quarks) are bosons.
1.12
Bottom Quark
The fifth flavour of quark (in order of increasing mass), with electric charge of
-1/3.
1.13
CERN
CERN (European Laboratory for Particle Physics) is the major European international accelerator laboratory located near Geneva, Switzerland.
1.14
Charge
A quantum number carried by a particle. Determines whether the particle
can participate in an interaction process. A particle with electric charge has
electrical interactions; one with strong charge has strong interactions, etc.
1.15
Charge Conservation
The observation that electric charge is conserved in any process of transformation of one group of particles into another.
1.16
Charm Quark
Charm (c) is the fourth quark (in order of increasing mass), with an electric
charge +2/3.
6
1.17
Collider
A collider is an accelerator in which two beams travelling in opposite directions
are steered together to provide high-energy collisions between the particles in
one beam and those in the other.
1.18
Colour Charge
The quantum number that determines participation in strong interactions. Quarks
and gluons carry non-zero colour charges.
1.19
Confinement
The property of the strong interaction that quarks or gluons are never found
separately but only inside colour-neutral composite objects.
1.20
Conservation
When a quantity is always the same before and after a particle reaction, it
is said to be conserved. Such quantities include electric charge, energy, and
momentum.
1.21
Cosmology
The study of the history of the universe.
2
2.1
Terms D-F
Dark Matter
Matter that is in space but is not visible to us because it emits no radiation
that we can observe. The nature of the motion of stars around the centres of
their galaxies implies that about 90% of the matter in a typical galaxy is dark.
Physicists speculate that there is also dark matter between the galaxies, but
this will be much harder to verify.
2.2
Decay
A process in which a particle disappears and in its place different particles
appear. The sum of the masses of the produced particles is always less than the
mass of the original particle.
2.3
Down Quark
The down quark (d) is the second flavour of quark (in order of increasing mass),
with an electric charge -1/3.
2.4
Electric Charge
The quantum number that determines participation in electromagnetic interactions.
7
2.5
Electromagnetic Interaction
The interaction due to electric charge; this includes magnetic interactions.
2.6
Electron
The least massive electrically-charged particle, hence absolutely stable. It is the
most common lepton, with electric charge -1.
2.7
Electroweak Interaction
In the Standard Model, electromagnetic and weak interactions are related (unified); physicists use the term electroweak to encompass both of them.
2.8
Event
What occurs when two particles collide or a single particle decays. Particle
theories predict the probabilities of various possible events occurring when many
similar collisions or decays are studied. They cannot predict the outcome for
any single event.
2.9
Fermilab
Fermi National Accelerator Laboratory in Batavia, Illinois (near Chicago). Named
for particle physics pioneer Enrico Fermi.
2.10
Fermion
Any particle that has odd-half-integer (1/2, 3/2, ...) intrinsic angular momentum (spin), measured in units of h-bar. All particles are either fermions or
bosons. Fermions obey a rule called the Pauli Exclusion Principle, which states
that no two fermions can exist in the same state at the same time. Many of
the properties of ordinary matter arise because of this rule. Electrons, protons,
and neutrons are all fermions, as are all the fundamental matter particles, both
quarks and leptons.
2.11
Fixed-target Experiment
An experiment in which the beam of particles from an accelerator is directed
at a stationary (or nearly stationary) target. The target may be a solid, a tank
containing liquid or gas, or a gas jet.
2.12
Flavour
The name used for the different quarks types (up, down, strange, charm, bottom, top) and for the different lepton types (electron, muon, tau). For each
charged lepton flavour there is a corresponding neutrino flavour. In other words,
flavour is the quantum number that distinguishes the different quark/lepton
types. Each flavour of quark and lepton has a different mass.
8
2.13
Fundamental Interaction
In the Standard Model the fundamental interactions are the strong, electromagnetic, weak, and gravitational interactions. Four interaction types are all that
are needed to explain all observed physical phenomena. The theory proposes
at least one more fundamental interaction that is responsible for fundamental
particle masses.
2.14
Fundamental Particle
A particle with no internal substructure. In the Standard Model the quarks, leptons, photons, gluons, W + and W − bosons, and the Z bosons are fundamental.
All other objects are made from these.
3
3.1
Terms G-I
Generation
A set of one of each charge type of quark and lepton, grouped by mass. The
first generation contains the up and down quarks, the electron and the electron
neutrino. There are three generations of matter.
3.2
GeV
1 billion electron Volts. (109 eV )
3.3
Gluon
The carrier particle of the strong interactions.
3.4
Gravitational Interaction
The interaction of particles due to their mass/energy.
3.5
Graviton
The carrier particle of the gravitational interactions; not yet directly observed.
3.6
Hadron
A particle made of strongly-interacting constituents (quarks and/or gluons).
These include the meson and baryons. Such particles participate in residual
strong interactions.
3.7
Interaction
A process in which a particle decays or it responds to a force due to the presence
of another particle (as in a collision). The four fundamental interactions are
gravitational, electromagnetic, strong, and weak.
9
4
4.1
Terms J-L
Kaon
A meson containing a strange quark and an anti-up (or anti-down) quark, or an
anti-strange quark and an up (or down) quark.
4.2
Lepton
A fundamental fermion that does not participate in strong interactions. The
electrically-charged leptons are the electron (e− ), the muon (µ), the tau (τ),
and their antiparticles. Electrically-neutral leptons are called neutrinos (ν).
4.3
LHC
The Large Hadron Collider at the CERN laboratory in Geneva, Switzerland.
LHC will collide protons into protons at a center-of-mass energy of about 14
TeV. When completed in the year 2004, it will be the most powerful particle
accelerator in the world. It is hoped that it will unlock many of the secrets of
particle physics.
4.4
Linacs
An abbreviation for linear accelerator, that is, an accelerator that has no bends
in it.
5
5.1
Terms M-O
Mass
See rest mass.
5.2
Meson
A hadron made from an even number of quark-antiquark constituents. The
basic structure of most meson is one quark and one antiquark.
5.3
Muon
The second flavour of charged leptons (in order of increasing mass), with electric
charge -1.
5.4
Muon Chamber
The outer layers of a particle detector capable of registering tracks of charged
particles. Except for the chargeless neutrinos, only muons reach this layer from
the collision point.
10
5.5
Neutral
Having a net charge equal to zero. Unless otherwise specified, it usually refers
to electric charge.
5.6
Neutrino
A lepton with no electric charge. Neutrinos participate only in weak and gravitational interactions and are therefore very difficult to detect. There are three
known types of neutrinos, all of which have very little mass.
5.7
Neutron (n)
A baryon with electric charge zero; it is a fermion with a basic structure of
two down quarks and one up quark (held together by gluons). The neutral
component of an atomic nucleus is made from neutrons. Different isotopes of
the same element are distinguished by having different numbers of neutrons in
their nucleus.
5.8
Nucleus
A collection of neutrons and protons that forms the core of an atom (plural:
nuclei).
6
6.1
Terms P-R
Particle
A subatomic object with a definite mass and charge.
6.2
Pauli Exclusion Principle
The principle that no two particles in the same quantum state may exist in
the same place at the same time. Particles that obey this principle are called
fermions; particles that do not are called bosons.
6.3
Photon
The force carrier particle of electromagnetic interactions.
6.4
Pion
The least massive type of meson, pions can have electric charges of +1, -1, or 0.
6.5
Positron (e+ )
The antiparticle of the electron.
11
6.6
Proton (p)
The most common hadron, a baryon with electric charge +1 equal and opposite
to that of the electron. Protons have a basic structure of two up quarks and
one down quark (bound together by gluons). The nucleus of a hydrogen atom
is a proton. A nucleus with electric charge Z contains Z protons, which is why
the number of protons is what distinguishes the different chemical elements.
6.7
Quantum
The smallest discrete amount of any quantity (plural: quanta).
6.8
Quantum Mechanics
The laws of physics that apply on very small scales. The essential feature is
that electric charge, momentum, and angular momentum, as well as charges,
come in discrete amounts called quanta.
6.9
Quark (q)
A fundamental fermion that has strong interactions. Quarks have electric charge
of either +2/3 (up, charm, top) or -1/3 (down, strange, bottom) in units where
the proton charge is 1.
6.10
Residual Interaction
An interaction between objects that do not carry a charge but do contain constituents that have that charge. Although some chemical substances involve
electrically-charged ions, much of chemistry is due to residual electromagnetic
interactions between electrically-neutral atoms. The residual strong interaction between protons and neutrons, due to the strong charges of their quark
constituents, is responsible for the binding of the nucleus.
6.11
Rest Mass
The rest mass (m) of a particle is the mass defined by the energy of the isolated
(free) particle at rest, divided by the speed of light squared. When particle
physicists use the word ”mass,” they always mean the ”rest mass” (m) of the
object in question.
7
7.1
Terms S-U
SLAC
The Stanford Linear Accelerator Center in Stanford, California.
7.2
Spin
A quantum particle property of intrinsic angular momentum.
12
7.3
Stable
Does not decay. A particle is stable if there exist no processes in which a particle
disappears and in its place different particles appear.
7.4
Standard Model
Physicists have developed a theory of fundamental particles and interactions
called the Standard Model. This site describes various aspects of this model.
7.5
Strange Quark (s)
The third flavour of quark (in order of increasing mass), with electric charge
-1/3.
7.6
Strong Interaction
The interaction responsible for binding quarks, antiquarks, and gluons to make
hadrons. Residual strong interactions provide the nuclear binding force.
7.7
Subatomic Particle
Any particle that is small compared to the size of the atom.
7.8
Synchrotron
A type of circular accelerator in which the particles travel in synchronized
bunches at fixed radius.
7.9
Tau
The third flavour of charged lepton (in order of increasing mass), with electric
charge -1.
7.10
TeV
1 trillion electron Volts. (1012 eV )
7.11
Top Quark (t)
The sixth flavour of quark (in order of increasing mass), with electric charge
2/3. Its mass is much greater than any other quark or lepton.
7.12
Track
The record of the path of a particle traversing a detector.
7.13
Tracking
The reconstruction of a ”track” left in a detector by the passage of a particle
through the detector.
13
7.14
Uncertainty Principle
The quantum principle, first formulated by Heisenberg, that states that is is not
possible to know exactly both the position x and the momentum p of an object
at the same time. The same is true with energy and time (see virtual particle).
7.15
Up Quark
The least massive flavour of quark, with electric charge 2/3.
8
8.1
Terms V-X
Virtual Particle
A particle that exists only for an extremely brief instant in an intermediary
process. Then the Heisenberg Uncertainty Principle allows an apparent violation
of the conservation of energy. However, if one sees only the initial decaying
particle and the final decay products, one observes that the energy is conserved.
8.2
W + , W − Boson
A carrier particle of the weak interactions. It is involved in all electric-chargechanging weak processes.
8.3
Weak Interaction
The interaction responsible for all processes in which flavour changes, hence for
the instability of heavy quarks and leptons, and particles that contain them.
Weak interactions that do not change flavour (or charge) have also been observed.
9
9.1
Terms Y-Z
Z Boson
A carrier particle of the weak interactions. It is involved in all weak processes
that do not change flavour.
14