accelerator
A "machine" used to accelerate particles to high speeds, and thus high energy compared to their
rest mass energy. They are used for Nuclear and Subnuclear
research, for Structure of matter research, for medical therapies and
some industrial
applications.
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alpha rays
The ambient natural radioactivity consists of alpha, beta
gamma rays. Alpha rays are the least
penetrating and may be stopped by a thin paper sheet. Alpha rays consist
of helium
nuclei (with
positive charge) made of two protons and two
neutrons (He-4). Alpha rays are not dangerous for living organisms,
unless the emitting source is very strong and is inhaled, eaten, or comes
into contact with eyes.
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annihilation
Process in which a particle meets
its corresponding antiparticle, and both
disappear. The total energy of the initial pair changes into the
rest mass of other particles and antiparticles and their kinetic
energy.
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antiparticle
In particle physics every fermion has a
corresponding antiparticle (antifermion). A particle and its
antiparticle have identical
mass but opposite
electric charge and fermionic
(barionic or leptonic) quantum number. The
interactions among antiparticles are
essentially identical to those among the corresponding particles.
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atom
The smallest part of a simple substance (element), which maintains
unaltered the properties of the simple substance. It is usually pictured
as a miniature solar system with a positive nucleus surrounded by
electrons. Atom dimensions are
~ 10 -10 m. In nature there are 92
different atoms.
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atomic orbital
The region in space where the probability
of finding an electron
of an atom at a given position is higher. It is computed using the mathematical tools of
quantum mechanics.
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atomic nucleus
The central
part of an atom. It is made of protons and
neutrons. Almost all
the mass of the atom is concentrated in the nucleus.
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baryon
Particle (hadron) with a
spin of half-integer number of quantum units; it is
made of 3
quarks, each of a different
color (strong).
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beta rays
Beta rays are electrons (beta-) or
positrons (beta+)
emitted by a radioactive atomic
nucleus or by a neutron during a
transformation process. Beta rays are more penetrating than
alpha rays, but less than X or gamma
rays.
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Big Bang (Theory)
It is the best known and more accredited theory which describes the
evolution of the Universe at the very begining. The Universe was born
with the Big
Bang. Initially, all matter and energy were contained in a point
(singularity) which then has grown and reached the dimensions of the
present Universe.
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Big Bang
The super powerful "explosion"
that gave birth to the Universe; immediately after the Big Bang the Universe had very
small dimensions and very high density, pressure and temperature. The Big
Bang occurred about 15 billion years ago.
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boson
The elementary particles are classified as bosons and
fermions. Bosons have integer
spin (0, 1, 2, ... in units of
).
The
"force particles" mediators of the
fundamental interactions (gamma,
W+/-,
Z0, 8
gluons), are fundamental bosons. Also the particles
(hadrons) made of an even integer number of quanta
(like the mesons made by a quark-antiquark
pair) are bosons.
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charge
A fundamental physical quantity carried by a particle that
determines its participation in an
interaction process. A particle with
electric charge has
electromagnetic interactions; one with
color charge (or strong charge) has strong
interactions, etc.
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collider
An accelerator in which beams of particles,
travelling in opposite directions, are made to collide head on.
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color charge
The
charge associated with strong
interactions. Quarks and
gluons have color charges and consequently
participate in strong interactions. Leptons,
photons and
W and Z0
bosons do not have color charge and therefore do not
participate in the strong interaction. There are three
color-charges (usually called red, green, blue) for quarks, and there are three
anticolor-charges (anti-red, anti-green, anti-blue) for antiquarks.
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cosmic radiation (cosmic rays)
It is made of protons, helium
nuclei and some heavier nuclei of high energy,
which reach the Earth from the outer space. Cosmic rays are the only
example of particles with mass which reach us from outside the solar
system. The mechanisms which accelerate some cosmic rays to extreme
energies are unknown. Primary cosmic rays interact with the nuclei (N, O)
of the molecules which make up the higher atmosphere of the Earth. In
these collisions many new particles are produced, most of which are
unstable. The particles which reach the surface (secondary cosmic rays)
are positive and negative muons, eletcrons
and
positrons.
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CP violation
C, P mean charge conjugation and parity. C, P and CP are
quantum mechanical
symmetry operations. Some interactions violate CP conservation (very
small effect); it is believed that this violation lead to different
evolution of matter and animatter in the Universe.
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decay
A process in which a particle disappears and in its place two or
more different particles appear. The total mass of the produced particles
is lower than the mass of the initial particle. In atomic
nuclei: a process in which a massive nucleus breaks apart into less-massive nucleus
plus alpha, or beta,
or gamma ray emission.
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detector
Any device sensitive to the passage of an elementary particle or nucleus.
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electric charge
Property,
quantum number of a
particle which determines its participation in the electromagnetic
interaction.
The electric charge is always conserved.
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electromagnetic interaction
This interaction originates from
electric charges and is mediated by
photons. The electromagnetic
interaction binds
electrons and a
nucleus to make an
atom. The force, which
binds atoms to make the molecules, is a "residual" electromagnetic force
shielded by the atomic electrons.
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electromagnetic waves (e.m.)
Visible light, ultraviolet rays, infrared rays, radio and TV waves,
microwaves are examples of
electromagnetic waves.
Also X rays and
gamma rays or high energy
photons are electromagnetic waves.
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electromagnetic radiation
Energy in transit in the form of electromagnetic
waves (photons).
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electron
A fundamental particle, negatively charged with a
mass of 0.51 MeV. It is the lowest mass charged
lepton and it is a stable particle.
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ElectronVolt
Kinetic energy acquired by an
electron subjected to a voltage difference of 1
V. One uses multiples (eg. MeV = one million ElectronVolts,
GeV = one billion ElectronVolts, ect). Particle masses may be
expressed in eV/c2
(remember Einstein's relation E= mc2).
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elementary particle
The term usually denotes the fundamental constituents (quarks and
leptons), and the fundamental bosons,
but also the "composite" particles (like protons, neutrons
and all other hadrons).
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fermion
Every particle with half integer spin (1/2, 3/2, ... in units
of
).
All "matter particles" (quarks and
leptons)
are fermions.
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flavour
Name used to indicate different types of quarks
("strong" flavour: up, down, strange, charm, bottom, top) and
leptons ("weak" flavour : electron,
muon, tau, neutrinoe, neutrinomu,
neutrinotau). In other words the flavour is a
quantum number which distinguishes different quarks
and leptons.
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fundamental family (generation)
There are three fundamental families. The first family includes the
quarks u, d, the electron and the electron neutrino. The second
family includes the quarks c, s, the muon
and the muon neutrino. The third family: t, b, the
tau and the tau neutrino.
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fundamental interaction
There are four fundamental interactions:
gravitational,
electromagnetic,
weak and strong. At
the submicroscopic level the gravitational interaction is negligible
(while it plays a fundamental role at the macroscopic level). At very
high energies (> 100
GeV) one has the unification of the
electromagnetic interactions with the weak interactions into electroweak
interactions.
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fundamental particle
A particle with no internal substructure. In the
Standard Model of the microcosm,
quarks and leptons are fundamental fermions,
photons, gluons,
W and Z bosons
are fundamental bosons. All other objects are made from these
particles.
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gamma rays
They are electromagnetic waves of very short
wavelengths. It is important to consider the corpuscolar aspect:
a gamma ray is a photon of high
energy. Gamma rays are emitted by radioactive
nuclei, and in high energy collisions. Gamma
rays are penetrating radiations, like
X rays (but with shorter wavelengths).
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gluon (quantum of strong interaction)
Gluons mediate the strong
interactions. There are 8 gluons, all with
spin 1 and zero mass. A gluon carries a color charge
and an anticolor charge.
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gravitational interaction
Interaction between bodies due to their mass/energy. The graviton is the
mediator of the gravitational interaction.
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graviton
It is the mediator of the gravitational interaction. The graviton has not yet
been observed directly.
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hadron
Every particle subjected to strong
interaction. Hadrons are classified in baryons
(composed of 3 quarks) like neutron and proton, with
half integer
spin and mesons (composed of a
quark and an antiquark) having integer spin. Hadrons have "strong color charge"
equal to zero, but are subject to the residual strong interaction
(nuclear interaction).
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heavy ion
It is made of a heavy nucleus (like gold) and a
number of
electrons smaller or larger (usually smaller)
than that of a standard atom. In some cases all electrons may be missing
totally (totally ionized atom).
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Higgs boson
Theoretical particle responsible for the mass of every
other particle and of the "breaking" of the electroweak symmetry
(electroweak force
---> electromagnetic and weak forces). It has not yet been seen
experimentally.
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last (fundamental) constituents
The 6
quarks and 6
leptons. They are the smallest "objects" known:
their dimensions are smaller than 10-18 m. They have
spin 1/2
.
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LEAR
Ring with low energy antiprotons at CERN, where
the first anti-hydrogen atoms have been observed.
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lepton
Fundamental constituent which does not participate in the strong
interaction. The lepton family includes the electron
(e-), the muon
(mu-), the tau
(tau-) and the corresponding
neutrinos (nue,
numu, nutau). The antileptons are e+,
mu+, tau+, anti(nue), anti(numu),
anti(nutau).
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meson
A hadron with integer
spin composed of a quark and an
antiquark of opposite "color".
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microwaves
Electromagnetic waves with frequencies of 109
-
3 x 1011 Hz. (cycles per second).
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muon
Fundamental lepton with negative
electric charge (mu-), it is
similar to the electron, but it has a mass 200
times larger. It is abundant in the secondary cosmic rays. The
corresponding antiparticle is the mu+.
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neutrini
Fundamental neutral leptons with mass zero or very
small. Three types exist: electron neutrino (nue), muon
neutrino (numu)
and tau neutrino(nutau).
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neutron
Neutral hadron with
spin 1/2
;
it is made of three
quarks: n = udd. Neutrons are found
inside atomic nuclei. The free neutron is
unstable,
with a life time of approximately 900 s.
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photon (quantum of the electromagnetic field)
Fundamental neutral particle of spin 1,
which mediates the electromagnetic interaction.
According to its energy it is denoted radiowave, infrared ray, visible
light, ultraviolet ray, X ray,
gamma ray.
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Planck (constant)
Fundamental physical constant: the elementary quantum
of action, indicated by h = 6.62620 x 10-34 joule·second.
Note that
= h / (2xpigreco).
For photon it is the ratio of its energy and its
frequency h = E/ f.
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positron
Antiparticle of the electron. It has the same
mass as
the electron, but opposite charge (positive).
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proton
The proton is the positive nucleus of the hydrogen
atom; it is made of three
quarks: p =
uud of three different "strong colors", for ex. ured
ugreen
dblu.
Together with the neutron it
forms the atomic nuclei.
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quantum
The smallest discrete amount of a physical quantity. The quantum of
electricity is the charge of the electron. The
quantum of light is the photon.
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quantum number
Number which characterizes the state of a physical system described by quantum mechanichs. For example for the complete
description of an atomic orbital are need 4 quantum numbers which
define the energy, the form, the orientation of the orbital in space and
the spin of the electron.
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quantum mechanics
Quantum mechanics is the theory which is valid at the atomic scale and
at the smallest dimensions of the fundamental particles.
In an atom the energy, the momentum, the angular
momentum, assume discrete values called
quanta.
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quark
Fundamental fermion constituent which participates
in the strong interaction. It has
spin 1/2 and fractional electric charge
(-1/3 or +2/3). A free quark has never been observed. There are six types
of quark of different "strong flavour": d, s, b
with electric charge -1/3; u, c, t with charge +2/3.
Ordinary matter is composed only of quark u, d. Every
quark appears in one of three
"colors": red, blu,
green. The
"color" charge is the equivalent for the
strong interaction as is the electric charge for the electromagnetic
interaction.
An antiquark has an electric charge and a color charge opposite to those of
the corresponding quark.
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radioactive nucleus
Unstable nucleus which emits alpha, and/or
beta and/or
gamma.
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rest mass
The rest mass of a particle is defined as the ratio between the rest
energy of an isolated free particle and the square of the speed of light (m=E/c2).
Normally, when we speak of mass we are referring
to the rest mass.
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Special Relativity
Theory developed by Einstein, based on two hypotheses: that the speed of
light in vacuum, c, is constant and that the laws of physics are
equal for observers in relative motion with constant speed. Special
Relativity has a new concept of space-time and the possibility to convert
energy into mass (and viceversa mass into energy). All systems with
velocities close to the speed of light must be analyzed with relativistic
formule, not with those of classic mechanics.
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spin
Intrinsic angular momentum of a particle in units of
.
The spin of bosons is an integer, for
fermions half integer.
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Standard Model (SM)
Describes the properties and behaviour of the fundamental particles: the
fermions (leptons and quarks)
and the bosons (photons,
gluons,
W e Z) which mediate three of the
fundamental interactions (forces) (strong,electromagnetic
and weak).
At high energies the electromagnetic and weak interaction are unified in
the electroweak interaction.
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strong interaction
It is the interaction (force) which originates from the "color
charges" of
quarks and is mediated by
gluons. The strong interaction binds three quarks
to make a proton,
a quark and a antiquark to make a
meson. The force which binds protons and
neutrons to make atomic nuclei is a
"residual" atomic force. The strong force, mediated by 8 gluons, is a
short range force, approximately 10-15 m; in this range it is
100 times stronger than electromagnetic force.
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tau
The heaviest fundamental lepton with negative
electric charge (tau-).
The other charged leptons are the electron and
the muon.
The corresponding antiparticle is tau+.
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uncertainty principle
The precision with which we may measure the position (Delta(x))
and the moment (Delta(px)) of a particle is limited by Delta(x)·Delta(px) >
/2.
The same applies for the y and z components and for the time and energy components: Delta(t)·Delta(E) >
/2.
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unstable particle
Particle which spontaneously decays into other particles.
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virtual particle
Particle exchanged in an interaction according
to the laws of quantum mechanics. For a
virtual particle the relation E=mc2 is not valid. Also if it
is not a "free" object, a virtual particle influences physical states up
to distances of 1 fermi=10-15
m.
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W (particle)
Carrier particle of the
charged current weak interaction. There are two types of W
boson with electric charges +1 (W+) and -1 (W-).
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wave-particle dualism
Possession by physical entities (such as light and
electrons) of both wave-like and particle-like characteristics. Like waves
they are caracterized by a wavelength lambda and a
frequency ni, like particles by an energy E= h
ni = hc/ lambda where h is Plank's
costant. At low energies the wave characteristic is
dominant, at high energy particle characteristic is dominant.
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weak interaction
Is responsible for the radioactive decays
beta, where heavy quarks and
leptons decay in lighter particles; it is also responsible of the first
nuclear reaction of the cicle inside the sun. The weak interaction has a
short range and it is mediated by the massive bosons
W+, W-,
Z0.
At ordinary energies the weak interaction is much weaker than the
electromagnetic
and strong interactions.
For energies larger than 100 GeV the weak interaction becomes unified
with the electromagnetic interaction into the electroweak interaction.
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X rays
They are electromagnetic waves with wavelengths
between 10 and 0.01 nm; they are due to the slowing down of
electrons in matter.
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Z (particle)
Neutral
boson (Z0) responsible for the neutral current
weak interaction. Its mass is 91.1 GeV.
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