Particle Physics
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Pauli exclusion principle
no two identical fermions (particles with halfinteger spin) may occupy the same quantum
state simultaneously.
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Bosons
Unlike fermions, bosons do not follow the Pauli exclusion principle, which makes
them perfect for the job as force carriers.
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bosons
What is the Pauli
Exclusion principle
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Standard Model
Fermions
Bosons
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Standard Model
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The Standard Model of particle physics, which
was developed throughout the mid to late 20th
century, is a theory concerning the
electromagnetic, weak, and strong nuclear
interactions, which mediate the dynamics of the
known subatomic particles. Since then,
discoveries of the bottom quark (1977), the top
quark (1995) and the tau neutrino (2000) have
given further evidence that the Standard Model
is accurate theory.
However, the Standard Model falls short of being a complete theory of fundamental
interactions because it does not incorporate the physics of dark energy nor of the
full theory of gravitation as described by general relativity. The theory does not
contain any viable dark matter particle that possesses all of the required properties
deduced from observational cosmology. It also does not correctly account for
neutrino oscillations (and their non-zero masses).
W± bosons
Higgs boson
Z boson
Graviton
Higgs Boson
The Higgs boson is a hypothetical elementary particle
predicted by the Standard Model of particle physics.
The Standard Model predicts the existence of a field (called the Higgs field) which has a
non-zero amplitude in its ground state. The field can be pictured as a pool of molasses
where Higgs bosons "stick" to the otherwise massless fundamental particles that travel
through the field, converting them into particles with mass that form the components of
atoms.
As of December 2011, the Higgs boson has yet to be confirmed experimentally,
despite large efforts invested in accelerator experiments at CERN and Fermilab. Like
other massive particles (e.g. the top quark and W and Z bosons), Higgs bosons
created in particle accelerators decay long before they reach any of the detectors.
However, the Standard Model precisely predicts the possible modes of decay and
their probabilities. This allows events in which a Higgs was created to be identified by
examining the decay products.
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Weak Bosons
The W and Z bosons (together known as the weak
bosons) are the elementary particles that mediate the
weak interaction; their symbols are W+, W−, and Z.
These bosons are among the heavyweights of the elementary particles. With masses of
80.4 GeV/c2 and 91.2 GeV/c2, respectively, the W and Z bosons are almost 100 times as
massive as the proton - heavier, even, than entire atoms of iron. (This does not mean
they are bigger than iron atoms. They still have zero size, but with lots of mass.)
The masses of these bosons are significant because they act as the force carriers of the
weak force, and the high mass thus limits the range. By way of contrast, the
electromagnetic force has an infinite range because its force carrier, the photon, has zero
rest mass; and the same is supposed of the hypothetical graviton.
The emission of a W+ or W− boson either raises or lowers the electric charge of the
emitting particle by one unit, and also alters the spin by one unit. The neutral Z boson
obviously cannot change the electric charge of any particle, nor can it change any other of
the so-called "charges" (such as strangeness, color, charm, etc.). The emission or
absorption of a Z boson can only change the spin, momentum, and energy of the other
particle.
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The photon is a quantum of light and all other forms of EM radiation,
and the force carrier for the EM force.
The photon is currently understood to be strictly massless, but if the
photon is not a massless particle, it would not move at the exact
speed of light in vacuum, c. Its speed would be lower and depend on
its frequency. Relativity would be unaffected by this; the so-called
speed of light, c, would then not be the actual speed at which light
moves, but a constant of nature which is the maximum speed that
any object could theoretically attain in space-time. Thus, it would
still be the speed of space-time ripples (gravitational waves and
gravitons), but it would not be the speed of photons.
Because the photon is thought to have zero rest mass, the electromagnetic force has an
infinite range
Speed = c
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+
Gluons are elementary particles which act as the exchange particles for the strong force
between quarks.
There are eight independent types of gluons. Quarks carry three types of color charge;
antiquarks carry three types of anticolor charge. Gluons may be thought of as carrying
both color and anticolor. QCD considers there to be eight gluons of the possible nine
color-anticolor combinations. The expulsion or absorption of gluons causes a color
change in quarks.
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The Mysterious: Graviton
In physics, the graviton is a hypothetical elementary particle that mediates the force
of gravitation in the framework of quantum field theory. If it exists, the graviton
must be massless, because the gravitational force has unlimited range, and must be
a spin 2 boson.
Attempts to extend the Standard Model or other quantum field theories by adding
gravitons run into serious theoretical difficulties at high energies. When
calculating the probability that a particle will emit or absorb a graviton, the
solutions give nonsensical answers. Since classical general relativity and quantum
mechanics seem to be incompatible at such energies, from a theoretical point of
gravity cannot be explained. One possible solution is to replace particles by
strings. However, string theories cannot be disproved at the current time, and
therefore may not count as “real” theories.
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A fermion can be an elementary particle, such as the electron; or it can be a
composite particle, such as the proton
In contrast to bosons, only one fermion can occupy a particular quantum state at
any given time (they follow the Pauli exclusion principle). If more than one fermion
occupies the same physical space, at least one property of each fermion, such as its
spin, must be different. Fermions are usually associated with matter.
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Leptons
Hadrons
Lepton’s differ from their fermion
brothers (the quarks) because leptons are
not involved in the strong force
electron neutrino
electron
tau neutrino
muon
tau
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Electron
eThe electron is a subatomic particle with a negative elementary electric charge. Electrons
have the lowest mass of any charged lepton (or electrically charged particle of any type)
and belong to the first-generation of fundamental particles. The second and third
generation contain charged leptons, the muon and the tau, which are identical to the
electron in charge, spin and interactions, but are more massive.
Unlike the muon and tau, the electron is thought to be stable on theoretical grounds: the
electron is the least massive particle with non-zero electric charge, so its decay would
violate charge conservation. If it did decay, the experimental lower bound for the
electron's mean lifetime is 4.6×1026 years
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Muon
µ
The muon is an unstable subatomic particle with a mean lifetime of 2.2 µs. This lifetime is
comparatively long (the second longest known) and is due to being mediated by the weak
interaction.
Muons have a mass of 105.7 MeV/c2, which is about 200 times the mass of an electron.
Since the muon's interactions are very similar to those of the electron, a muon can be
thought of as a much heavier version of the electron. Due to their greater mass, muons
are not as sharply accelerated when they encounter electromagnetic fields, and do not
emit as much radiation.
The muon was the first elementary particle discovered that does not appear in ordinary
atoms. Muons can, however, form muonic atoms (also called mu-mesic atoms), by
replacing an electron in ordinary atoms
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Tau
Tau leptons have a lifetime of 2.9×10−13 s and a mass of
1,777 MeV/c2
The tau was detected in a series of experiments
between 1974 and 1977 by Martin Lewis Perl with his
colleagues at the SLAC-LBL group.
The tau is the only lepton that can decay into
hadrons—the other leptons do not have the
necessary mass.
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The Neutrinos
The neutrino (meaning "small neutral one" in Italian) is denoted by the Greek letter ν.
All evidence suggests that neutrinos have mass but that their mass is tiny even by the
standards of subatomic particles. Their mass has never been measured accurately.
Most neutrinos passing through the Earth emanate from the Sun. About 65 billion
(6.5×1010) solar neutrinos per second pass through every square centimeter.
Neutrinos cannot be detected directly, because they do not ionize the materials they
are passing through because they do not carry an electric charge. Because they are
neutral particles, antineutrinos and neutrinos may actually be the same particle.
So far, there is no detection method for low energy neutrinos that can be uniquely
distinguished from other causes. Neutrino detectors are often built underground in
order to isolate the detector from cosmic ray and other background radiation.
Hadrons
Made out of 100% quality quarks and antiquarks
A hadron is a composite particle made of quarks and held together by the strong
force. Hadrons are categorized into two families: baryons (made of three quarks)
and mesons (made of one quark and one antiquark).
Quarks ->
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Quarks
Due to a phenomenon known as color confinement, quarks are never directly observed
or found in isolation; they can only be found within baryons or mesons.
Quarks have fractional electric charge values — either 1⁄3 or 2⁄3 times the elementary
charge, depending on flavor. Since the electric charge of a hadron is the sum of the
charges of the constituent quarks, all hadrons must have integer charges.
A quark of one flavor can transform
into a quark of another flavor only
through the weak interaction, one
of the four fundamental
interactions in particle physics. By
absorbing or emitting a W boson.
Quarks possess a property called
color charge. Check out color
charge!
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Pick a flavor
Up Quark
The up quark is the lightest of all quarks. It, along with the down
quark, forms the neutrons (one up quark, two down quarks) and
protons (two up quarks, one down quark) of atomic nuclei. It
was named an up quark because of the its spin.
Down Quark
The up quark is the second lightest of all quarks. It, along with
the up quark, forms the neutrons (one up quark, two down
quarks) and protons (two up quarks, one down quark) of atomic
nuclei. It was named a down quark because of the its spin.
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Charm Quark
The charm quark is the third most massive of all quarks, a type
of elementary particle. They are named charm quarks, because
the researchers were charmed to find them as they fit perfectly
into the Standard Model.
Strange Quark
The strange quark is the third lightest of all quarks, a type of
elementary particle. They are named strange quarks, because
the researchers found the particles in cosmic rays and thought
their long decay lifetime was strange.
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Top Quark
The top quark was found in 1995 and is the most massive
observed elementary particle. It has a mass of
172.9±1.5 GeV/c2 which is about the same mass as an atom of
tungsten. It almost exclusively decays to a W boson and a
bottom quark, but it can also sometimes decay into a strange
quark, and on the rarest of occasions, into a down quark.
Bottom Quark
The bottom quark also known as the beauty quark. The bottom
quark was discovered in 1977 by Fermilab. On its discovery,
there were efforts to name the bottom quark "beauty", but
"bottom" became the predominant usage.
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Color Charge
Quarks possess a property called color charge. There are three types of color
charge, arbitrarily labeled blue, green, and red. Each of them is complemented
by an anticolor—antiblue, antigreen, and antired. Every quark carries a color,
while every antiquark carries an anticolor.
The color charge of a hadron should be white (R+B+G). However, hadrons are
not always made of the right combination of color charged quarks. Therefore,
the color charge of quarks can be changed by emitting or absorbing one of the
eight gluons, which contain both a color and an anticolor charge. The continual
passing of gluons is what creates the strong force.