A machine used to accelerate particles to high speeds (and thus high
energy compared to their rest mass-energy).
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 such as a
The produced particles may be any combination allowed
by conservation of energy and momentum and of all the charge types
and other rules.
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.
For every fermion type there is another fermion 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. Bosons also have antiparticles, except for those that have
zero value for all charges, for example, a photon or a
composite boson made from a quark and its corresponding antiquark. In
this case there is no distinction between the particle and the
antiparticle, they are the same object.
The antiparticle of a quark. An antiquark is denoted by putting a bar over
the corresponding quark
ATLAS is a particle physics experiment in the Large Hadron Collider at the CERN laboratory. It will explore the fundamental nature of matter and the basic forces that shape our Universe.
A hadron made from three quarks.
The proton (uud) and the neutron (udd) are both baryons.
They may also contain additional quark-antiquark pairs.
The observation that the Universe contains many baryons but few
antibaryons; a fact that needs explanation.
The particle stream produced by an accelerator usually clustered in bunches.
A particle that has integer intrinsic angular momentum (spin) measured in
units of (spin = 0, 1, 2,...).
All particles are either fermions or bosons. The particles associated with
all the fundamental interactions (forces) are bosons. Composite
particles with even numbers of fermion constituents (quarks) are also bosons.
bottom quark (b):
The fifth flavor of quark (in order of increasing mass), with electric
A device that can measure the energy deposited in it. (originally
devices to measure heat energy deposited, using change of temperature;
particle physicists use the word for any energy measuring device.)
CERN is the European Organization for Nuclear Research, the world's largest particle physics centre. It sits astride the French-Swiss border near Geneva.
CERN is a laboratory where scientists unite to study the building blocks of matter and the forces that hold them together. CERN exists primarily to provide them with the necessary tools. These are accelerators, which accelerate particles to almost the speed of light and detectors to make the particles visible.
Founded in 1954, the laboratory was one of Europe's first joint ventures and includes now 20 Member States.
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.
The observation that electric charge is conserved in any process of
transformation of one group of particle into another.
charm quark (c):
The fourth flavor of quark (in order of increasing mass),
with electric charge +2/3.
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.
Experiments done at colliders.
The quantum number that determines participation in strong interactions,
quarks and gluons carry non-zero color charges.
An object with no net color charge. For composites made of color charged
particles the rules of neutralization are complex. Three quarks
(baryon) or a
quark plus an antiquark (meson) can both form color-neutral combinations.
The property of the strong interactions that quarks or gluons are never
found separately but only inside color-neutral composite objects.
When a quantity (e.g.-
electric charge, energy or momentum)
is conserved, it is the same after a reaction
between particles as it was before.
A process in which a particle disappears and in its place two or
more different particles appear. The sum of the masses of the
produced particles is always less than the mass of the original
Any device used to sense the passage of a particle.
Also a collection of such devices designed so that each serves a
particular purpose in allowing physicists to reconstruct particle
down quark (d):
The second flavor of quark (in order of increasing mass), with electric
The quantum number that determines participation in electromagnetic
The interaction due to electric charge; this includes magnetic effects
which have to do with moving electric charges.
electron [e-LEC-tron] (e):
The least massive electrically charged particle, hence absolutely stable.
It is the most common lepton, with electric charge -1.
In the Standard Model, electromagnetic and weak interactions are
related (unified), physicists use the term
electroweak to encompass both of them.
The energy gained by an electron which accelerates through
a potential difference of one volt.
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.
Fermi National Accelerator Laboratory in Batavia, Illinois (near
Chicago). Named for particle physics pioneer Enrico Fermi.
Any particle that has odd-half-integer (1/2, 3/2, ...) intrinsic
angular momentum (spin), measured in units of
. As a
consequence of this peculiar angular momentum, fermions obey a rule
called the Pauli Exclusion Principle, which states that no two
fermions can exist in the same state at the same place and 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.
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.
The name used for the different quark types (up, down, strange,
charm, bottom, top) and for the different lepton types (electron,
muon, tau). For each charged lepton flavor there is a
corresponding neutrino flavor.
In other words, flavor is the quantum number that distinguishes the
different quark/lepton types. Each flavor of quark and charged
lepton has a different mass.
For neutrinos we do not yet know if they have a mass or what the
In the Standard Model the fundamental interactions are the strong,
electromagnetic, weak and gravitational interactions. There is
at least one more
fundamental interaction in the theory that is responsible for
fundamental particle masses. Five interaction types are all that are
needed to explain all observed physical phenomena.
A particle with no internal substructure. In the Standard Model the
quarks, leptons, photons, gluons,
and Z0 bosons are fundamental.
All other objects are made from these.
1 billion electron Volts. (109 eV)
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.
gluon [GLUE-on] (g):
The carrier particle of strong interactions.
grand unified theory:
Any of a class of theories which contain the Standard Model but go
beyond it to predict further types of interactions mediated by
particles with masses of order
1015 GeV/c2. At energies large
compared to this mass (times c2) the strong, electromagnetic
and weak interactions are seen as different aspects of one unified
The interaction of particles due to their mass-energy.
The carrier particle of the gravitational interactions;
not yet directly observed.
A particle made of strongly-interacting constituents (quarks and/or
gluons). These include the mesons and baryons. Such particles
participate in residual strong interactions.
The carrier particle or quantum excitation of the additional force needed
to introduce particle masses in the Standard Model. Not yet observed.
A process in which a particle decays or it responds to a force due to
the presence of another particle (as in a collision).
Also used to mean the
underlying property of the theory that causes such effects.
Depending on their energy, the quarks and gluons emerging
from a collision will materialize into 5-30 particles (mostly mesons
and baryons). At high momentum, these particles will appear in
clusters called ``jets,'' that is, in groups of particles moving in
roughly the same direction, centered about the original quark or
A meson containing a strange quark and an anti-up
(or an anti-down) quark, or
an anti-strange quark and an up (or down) quark.
A fundamental fermion that does not participate in strong interactions.
The electrically-charged leptons are the electron (e), the
and their antiparticles.
Electrically-neutral leptons are called neutrinos
The Large Hadron Collider at the CERN laboratory in Geneva,
Switzerland. LHC will collide protons into protons at a
center-of-mass energy that will eventually reach 14 TeV. It is currently the most powerful particle accelerator in the
world. It is hoped that it will unlock many of the remaining
secrets of particle physics.
An abbreviation for linear accelerator, that is an accelerator
that is has no bends in it.
The number of particles per square-centimeter per second generated in the beams of high energy particle experiments. The higher the luminosity, the greater the number of events produced for study.
see rest mass.
A hadron made from an even number of quark constituents.
The basic structure of most mesons is one quark and one antiquark.
One million electron-volts, where one electron-volt is the energy
gained by an electron which accelerates through a potential difference
of one volt.
An electromagnetic wave with wavelength in the micron range.
The second flavor of charged lepton (in order of increasing mass),
with electric charge -1.
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.
Having a net charge equal to zero. Unless specified otherwise, it
usually refers to electric charge.
A lepton with no electric charge. Neutrinos participate only in weak
and gravitational interactions and therefore are very difficult to detect.
There are three known types of neutrino all of which are very light and
could possibly even have zero mass.
neutron [new-TRON] (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.
A proton or a neutron; that is, one of the particles that makes up a
A collection of neutrons and protons that forms the core of an atom
A subatomic object with a definite mass and charge.
The carrier particle of electromagnetic interactions.
The least massive type of meson, pions can have
electric charges ± 1 or 0.
A gas of charged particles.
positron [PAUSE-i-tron] (e+):
The antiparticle of the electron.
proton [PRO-tahn] (p):
The most common hadron, a baryon with electric charge (+1) equal and
opposite to that of the electron (-1). 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; therefore the number of protons is what distinguishes the
different chemical elements.
The smallest discrete amount of any quantity (plural: quanta).
The laws of physics that apply on very small scales. The essential
feature is that energy, momentum, and angular momentum as well as
charges come in discrete amounts called quanta.
The theory which describes laws of physics that apply on very small scales. The essential
feature is that energy, momentum, and angular momentum as well as
charges come in discrete amounts called quanta.
quark [KWORK] (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.
Interaction between objects that do not carry a charge but do
contain constituents that have 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.
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.
The Stanford Linear Accelerator Center in Stanford, California.
Intrinsic angular momentum of a particle, given
in units of
the quantum unit of angular momentum,
= 6.58 x 10-34 Js.
Does not decay. A particle is stable if there exist no processes
in which a particle disappears and in its place two or
more different particles appear.
Physicists' name for the theory of fundamental particles and
their interactions. It is widely tested
and is accepted as correct by particle physicists.
strange quark (s):
The third flavor of quark (in order of increasing mass), with electric
The interaction responsible for binding quarks, antiquarks, and gluons to make
hadrons. Residual strong interactions provide the nuclear binding force.
Any particle that is small compared to the size of the atom.
A type of circular accelerator in which the particles
travel in synchronized bunches at fixed radius.
tau [TAOW] lepton:
The third flavor of charged lepton (in order of increasing mass),
with electric charge -1.
1 trillion electron Volts. (1012 eV)
The sixth flavor of quark (in order of increasing mass), with electric
charge 2/3. Its mass is much greater than any other quark or
The record of the path of a particle traversing a detector.
The reconstruction of a ``track'' left in a detector by the
passage of a particle through the detector.
The quantum principle, first formulated
by Heisenberg, that states that it is not possible to know
exactly both the position x and the momentum p of an object at the
It can be written as
E means the uncertainty in energy and
uncertainty in lifetime of a state (see
The least massive flavor of quark, with electric charge 2/3.
A detector placed very close to the collision point
in a colliding beam experiment so that tracks coming from the decay
of a short-lived particle produced in the collision can be accurately
reconstructed and seen to emerge from a `vertex' point that is different
from the collision point.
A particle that exists only for an extremely brief instant as an
intermediary in a process. The intermediate or virtual particle
stages of a process cannot be directly observed. If they were
observed, we might think that conservation of energy was violated.
However, the Heisenberg Uncertainty Principle (which can be written
Δ t >
/2) allows an apparent violation
of the conservation of energy. If one sees only the initial
decaying particle (such as a meson with the c quark) and the final
decay products (such as
one observes that energy
is conserved. The ``virtual'' particle
(such as the W±)
exists for such a short time that it can never be observed.
A carrier particle of the weak interactions.
It is involved in all electric-charge-changing weak processes.
The interaction responsible for all processes in which flavor changes,
hence for the instability of heavy quarks and leptons, and particles
that contain them. Weak interactions that do not change
flavor (or charge) have also been observed.
A carrier particle of weak interactions.
It is involved in all weak processes that do not change flavor.