The idea that matter consists of smaller particles
and that there exists a limited number of
sorts of primary, smallest particles in nature
has existed in natural philosophy at least
since the 6th century BC. Such ideas gained
physical credibility beginning in the 19th
century, but the concept of "elementary particle"
underwent some changes in its meaning: notably,
modern physics no longer deems elementary
particles indestructible. Even elementary
particles can decay or collide destructively;
they can cease to exist and create (other)
particles in result.
Increasingly small particles have been discovered
and researched: they include molecules, which
are constructed of atoms, that in turn consist
of subatomic particles, namely atomic nuclei
and electrons. Many more types of subatomic
particles have been found. Most such particles
(but not electrons) were eventually found
to be composed of even smaller particles such
as quarks. Particle physics studies these
smallest particles and their behaviour under
high energies, whereas nuclear physics studies
atomic nuclei and their (immediate) constituents:
protons and neutrons.
== Early development ==
The idea that all matter is composed of elementary
particles dates to at least the 6th century
BC. The philosophical doctrine of atomism
and the nature of elementary particles were
studied by ancient Greek philosophers such
as Leucippus, Democritus, and Epicurus; ancient
Indian philosophers such as Kanada, Dignāga,
and Dharmakirti; Muslim scientists such as
Ibn al-Haytham, Ibn Sina, and Mohammad al-Ghazali;
and in early modern Europe by physicists such
as Pierre Gassendi, Robert Boyle, and Isaac
Newton. The particle theory of light was also
proposed by Ibn al-Haytham, Ibn Sina, Gassendi,
and Newton.
Those early ideas were founded through abstract,
philosophical reasoning rather than experimentation
and empirical observation and represented
only one line of thought among many. In contrast,
certain ideas of Gottfried Wilhelm Leibniz
(see Monadology) contradict to almost everything
known in modern physics.
In the 19th century, John Dalton, through
his work on stoichiometry, concluded that
each chemical element was composed of a single,
unique type of particle. Dalton and his contemporaries
believed those were the fundamental particles
of nature and thus named them atoms, after
the Greek word atomos, meaning "indivisible"
or "uncut".
== From atoms to nucleons ==
=== 
First subatomic particles ===
However, near the end of 19th century, physicists
discovered that Dalton's atoms are not, in
fact, the fundamental particles of nature,
but conglomerates of even smaller particles.
Electron was discovered between 1879 and 1897
in works of William Crookes, Arthur Schuster,
J. J. Thomson, and other physicists; its charge
was carefully measured by Robert Andrews Millikan
and Harvey Fletcher in their oil drop experiment
of 1909. Physicists theorized that negatively
charged electrons are constituent part of
"atoms", along with some (yet unknown) positively
charged substance, and it was later confirmed.
Electron became the first elementary, truly
fundamental particle discovered.
Studies of the "radioactivity", that soon
revealed the phenomenon of radioactive decay,
provided another argument against considering
chemical elements as fundamental nature's
elements. Despite these discoveries, the term
atom stuck to Dalton's (chemical) atoms and
now denotes the smallest particle of a chemical
element, not something really indivisible.
=== Researching particles' interaction ===
Early 20th-century physicists knew only two
fundamental forces: electromagnetism and gravitation,
where the latter could not explain the structure
of atoms. So, it was obvious to assume that
unknown positively charged substance attracts
electrons by Coulomb force.
In 1909 Ernest Rutherford and Thomas Royds
demonstrated that an alpha particle combines
with two electrons and forms a helium atom.
In modern terms, alpha particles are doubly
ionized helium (more precisely, 4He) atoms.
Speculation about the structure of atoms was
severely constrained by Rutherford's 1907
gold foil experiment, showing that the atom
is mainly empty space, with almost all its
mass concentrated in a tiny atomic nucleus.
=== Inside the atom ===
By 1914, experiments by Ernest Rutherford,
Henry Moseley, James Franck and Gustav Hertz
had largely established the structure of an
atom as a dense nucleus of positive charge
surrounded by lower-mass electrons.
These discoveries shed a light to the nature
of radioactive decay and other forms of transmutation
of elements, as well as of elements themselves.
It appeared that atomic number is nothing
else than (positive) electric charge of the
atomic nucleus of a particular atom. Chemical
transformations, governed by electromagnetic
interactions, do not change nuclei – that's
why elements are chemically indestructible.
But when the nucleus change its charge and/or
mass (by emitting or capturing a particle),
the atom can become the one of another element.
Special relativity explained how the mass
defect is related to the energy produced or
consumed in reactions. The branch of physics
that studies transformations and the structure
of nuclei is now called nuclear physics, contrasted
to atomic physics that studies the structure
and properties of atoms ignoring most nuclear
aspects. The development in the nascent quantum
physics, such as Bohr model, led to the understanding
of chemistry in terms of the arrangement of
electrons in the mostly empty volume of atoms.
In 1918, Rutherford confirmed that the hydrogen
nucleus was a particle with a positive charge,
which he named the proton. By then, Frederick
Soddy's researches of radioactive elements,
and experiments of J. J. Thomson and F.W.
Aston conclusively demonstrated existence
of isotopes, whose nuclei have different masses
in spite of identical atomic numbers. It prompted
Rutherford to conjecture that all nuclei other
than hydrogen contain chargeless particles,
which he named the neutron.
Evidences that atomic nuclei consist of some
smaller particles (now called nucleons) grew;
it became obvious that, while protons repulse
each other electrostatically, nucleons attract
each other by some new force (nuclear force).
It culminated in proofs of nuclear fission
in 1939 by Lise Meitner (based on experiments
by Otto Hahn), and nuclear fusion by Hans
Bethe in that same year. Those discoveries
gave rise to an active industry of generating
one atom from another, even rendering possible
(although it will probably never be profitable)
the transmutation of lead into gold; and,
those same discoveries also led to the development
of nuclear weapons.
== Revelations of quantum mechanics ==
Further understanding of atomic and nuclear
structures became impossible without improving
the knowledge about the essence of particles.
Experiments and improved theories (such as
Erwin Schrödinger's "electron waves") gradually
revealed that there is no fundamental difference
between particles and waves. For example,
electromagnetic waves were reformulated in
terms of particles called photons. It also
revealed that physical objects do not change
their parameters, such as total energy, position
and momentum, as continuous functions of time,
as it was thought of in classical physics:
see atomic electron transition for example.
Another crucial discovery was identical particles
or, more generally, quantum particle statistics.
It was established that all electrons are
identical: although two or more electrons
can exist simultaneously that have different
parameters, but they do not keep separate,
distinguishable histories. This also applies
to protons, neutrons, and (with certain differences)
to photons as well. It suggested that there
is a limited number of sorts of smallest particles
in the universe.
The spin–statistics theorem established
that any particle in our spacetime may be
either a boson (that means its statistics
is Bose–Einstein) or a fermion (that means
its statistics is Fermi–Dirac). It was later
found that all fundamental bosons transmit
forces, like the photon that transmits light.
Some of non-fundamental bosons (namely, mesons)
also may transmit forces (see below), although
non-fundamental ones. Fermions are particles
"like electrons and nucleons" and generally
comprise the matter. Note that any subatomic
or atomic particle composed of even total
number of fermions (such as protons, neutrons,
and electrons) is a boson, so a boson is not
necessarily a force transmitter and perfectly
can be an ordinary material particle.
The spin is the quantity that distinguishes
bosons and fermions. Practically it appears
as an intrinsic angular momentum of a particle,
that is unrelated to its motion but is linked
with some other features like a magnetic dipole.
Theoretically it is explained from different
types representations of symmetry groups,
namely tensor representations (including vectors
and scalars) for bosons with their integer
(in ħ) spins, and spinor representations
for fermions with their half-integer spins.
This culminated in the formulation of ideas
of a quantum field theory. The first (and
the only mathematically complete) of these
theories, quantum electrodynamics, allowed
to explain thoroughly the structure of atoms,
including the Periodic Table and atomic spectra.
Ideas of quantum mechanics and quantum field
theory were applied to nuclear physics too.
For example, α decay was explained as a quantum
tunneling through nuclear potential, nucleons'
fermionic statistics explained the nucleon
pairing, and Hideki Yukawa proposed certain
virtual particles (now knows as π-mesons)
as an explanation of the nuclear force.
== Inventory ==
== 
Modern nuclear physics ==
Development of nuclear models (such as the
liquid-drop model and nuclear shell model)
made prediction of properties of nuclides
possible. No existing model of nucleon–nucleon
interaction can analytically compute something
more complex than 4He based on principles
of quantum mechanics, though (note that complete
computation of electron shells in atoms is
also impossible yet).
The most developed branch of nuclear physics
in 1940s was studies related to nuclear fission
due to its military significance. The main
focus of fission-related problems is interaction
of atomic nuclei with neutrons: a process
that occurs in a fission bomb and a nuclear
fission reactor. It gradually drifted away
from the rest of subatomic physics and virtually
became the nuclear engineering. First synthesised
transuranium elements were also obtained in
this context, through neutron capture and
subsequent β− decay.
The elements beyond fermium cannot be produced
in this way. To make a nuclide with more than
100 protons per nucleus one has to use an
inventory and methods of particle physics
(see details below), namely to accelerate
and collide atomic nuclei. Production of progressively
heavier synthetic elements continued into
21st century as a branch of nuclear physics,
but only for scientific purposes.
The third important stream in nuclear physics
are researches related to nuclear fusion.
This is related to thermonuclear weapons (and
conceived peaceful thermonuclear energy),
as well as to astrophysical researches, such
as stellar nucleosynthesis and Big Bang nucleosynthesis.
== Physics goes to high energies ==
=== 
Strange particles and mysteries of the weak
interaction ===
In the 1950s, with development of particle
accelerators and studies of cosmic rays, inelastic
scattering experiments on protons (and other
atomic nuclei) with energies about hundreds
of MeVs became affordable. They created some
short-lived resonance "particles", but also
hyperons and K-mesons with unusually long
lifetime. The cause of the latter was found
in a new quasi-conserved quantity, named strangeness,
that is conserved in all circumstances except
for the weak interaction. The strangeness
of heavy particles and the μ-lepton were
first two signs of what is now known as the
second generation of fundamental particles.
The weak interaction revealed soon yet another
mystery. In 1957 it was found that it does
not conserve parity. In other words, the mirror
symmetry was disproved as a fundamental symmetry
law.
Throughout the 1950s and 1960s, improvements
in particle accelerators and particle detectors
led to a bewildering variety of particles
found in high-energy experiments. The term
elementary particle came to refer to dozens
of particles, most of them unstable. It prompted
Wolfgang Pauli's remark: "Had I foreseen this,
I would have gone into botany". The entire
collection was nicknamed the "particle zoo".
It became evident that some smaller constituents,
yet invisible, form mesons and baryons that
counted most of then-known particles.
=== Deeper constituents of matter ===
The interaction of these particles by scattering
and decay provided a key to new fundamental
quantum theories. Murray Gell-Mann and Yuval
Ne'eman brought some order to mesons and baryons,
the most numerous classes of particles, by
classifying them according to certain qualities.
It began with what Gell-Mann referred to as
the "Eightfold Way", but proceeding into several
different "octets" and "decuplets" which could
predict new particles, most famously the Ω−,
which was detected at Brookhaven National
Laboratory in 1964, and which gave rise to
the quark model of hadron composition. While
the quark model at first seemed inadequate
to describe strong nuclear forces, allowing
the temporary rise of competing theories such
as the S-matrix theory, the establishment
of quantum chromodynamics in the 1970s finalized
a set of fundamental and exchange particles
(Kragh 1999). It postulated the fundamental
strong interaction, experienced by quarks
and mediated by gluons. These particles were
proposed as a building material for hadrons
(see hadronization). This theory is unusual
because individual (free) quarks cannot be
observed (see color confinement), unlike the
situation with composite atoms where electrons
and nuclei can be isolated by transferring
ionization energy to the atom.
Then, the old, broad denotation of the term
elementary particle was deprecated and a replacement
term subatomic particle covered all the "zoo",
with its hyponym "hadron" referring to composite
particles directly explained by the quark
model. The designation of an "elementary"
(or "fundamental") particle was reserved for
leptons, quarks, their antiparticles, and
quanta of fundamental interactions (see below)
only.
=== Quarks, leptons, and four fundamental
forces ===
Because the quantum field theory (see above)
postulates no difference between particles
and interactions, classification of elementary
particles allowed also to classify interactions
and fields.
Now a large number of particles and (non-fundamental)
interactions is explained as combinations
of a (relatively) small number of fundamental
substances, thought to be fundamental interactions
(incarnated in fundamental bosons), quarks
(including antiparticles), and leptons (including
antiparticles). As the theory distinguished
several fundamental interactions, it became
possible to see which elementary particles
participate in which interaction. Namely:
All particles participate in gravitation.
All charged elementary particles participate
in electromagnetic interaction.
As a consequence, neutron participate in it
with its magnetic dipole in spite of zero
electric charge. This is because it is composed
of charged quarks whose charges sum to zero.
All fermions participate in the weak interaction.
Quarks participate in the strong interaction,
along gluons (its own quanta), but not leptons
nor any fundamental bosons other than gluons.The
next step was a reduction in number of fundamental
interactions, envisaged by early 20th century
physicists as the "united field theory". The
first successful modern unified theory was
the electroweak theory, developed by Abdus
Salam, Steven Weinberg and, subsequently,
Sheldon Glashow. This development culminated
in the completion of the theory called the
Standard Model in the 1970s, that included
also the strong interaction, thus covering
three fundamental forces. After the discovery,
made at CERN, of the existence of neutral
weak currents, mediated by the Z boson foreseen
in the standard model, the physicists Salam,
Glashow and Weinberg received the 1979 Nobel
Prize in Physics for their electroweak theory.
The discovery of the weak gauge bosons (quanta
of the weak interaction) through the 1980s,
and the verification of their properties through
the 1990s is considered to be an age of consolidation
in particle physics.
While accelerators have confirmed most aspects
of the Standard Model by detecting expected
particle interactions at various collision
energies, no theory reconciling general relativity
with the Standard Model has yet been found,
although supersymmetry and string theory were
believed by many theorists to be a promising
avenue forward. The Large Hadron Collider,
however, which began operating in 2008, has
failed to find any evidence whatsoever that
is supportive of supersymmetry and string
theory, and appears unlikely to do so, meaning
"the current situation in fundamental theory
is one of a serious lack of any new ideas
at all." This state of affairs should not
be viewed as a crisis in physics, but rather,
as David Gross has said, "the kind of acceptable
scientific confusion that discovery eventually
transcends."The fourth fundamental force,
gravitation, is not yet integrated into particle
physics in a consistent way.
=== Higgs boson ===
As of 2011, the Higgs boson, the quantum of
a field that is thought to provide particles
with rest masses, remained the only particle
of the Standard Model to be verified.
On July 4, 2012, physicists working at CERN's
Large Hadron Collider announced that they
had discovered a new subatomic particle greatly
resembling the Higgs boson, a potential key
to an understanding of why elementary particles
have masses and indeed to the existence of
diversity and life in the universe. Rolf-Dieter
Heuer, the director general of CERN, said
that it was too soon to know for sure whether
it is an entirely new particle, which weighs
in at 125 billion electron volts – one of
the heaviest subatomic particles yet – or,
indeed, the elusive particle predicted by
the Standard Model, the theory that has ruled
physics for the last half-century. It is unknown
if this particle is an impostor, a single
particle or even the first of many particles
yet to be discovered. The latter possibilities
are particularly exciting to physicists since
they could point the way to new deeper ideas,
beyond the Standard Model, about the nature
of reality. For now, some physicists are calling
it a "Higgslike" particle. Joe Incandela,
of the University of California, Santa Barbara,
said, "It's something that may, in the end,
be one of the biggest observations of any
new phenomena in our field in the last 30
or 40 years, going way back to the discovery
of quarks, for example." The groups operating
the large detectors in the collider said that
the likelihood that their signal was a result
of a chance fluctuation was less than one
chance in 3.5 million, so-called "five sigma,"
which is the gold standard in physics for
a discovery. Michael Turner, a cosmologist
at the University of Chicago and the chairman
of the physics center board, said
This is a big moment for particle physics
and a crossroads — will this be the high
water mark or will it be the first of many
discoveries that point us toward solving the
really big questions that we have posed?
Confirmation of the Higgs boson or something
very much like it would constitute a rendezvous
with destiny for a generation of physicists
who have believed the boson existed for half
a century without ever seeing it. Further,
it affirms a grand view of a universe ruled
by simple and elegant and symmetrical laws,
but in which everything interesting in it
being a result of flaws or breaks in that
symmetry. According to the Standard Model,
the Higgs boson is the only visible and particular
manifestation of an invisible force field
that permeates space and imbues elementary
particles that would otherwise be massless
with mass. Without this Higgs field, or something
like it, physicists say all the elementary
forms of matter would zoom around at the speed
of light; there would be neither atoms nor
life. The Higgs boson achieved a notoriety
rare for abstract physics. To the eternal
dismay of his colleagues, Leon Lederman, the
former director of Fermilab, called it the
"God particle" in his book of the same name,
later quipping that he had wanted to call
it "the goddamn particle". Professor Incandela
also stated,
This boson is a very profound thing we have
found. We're reaching into the fabric of the
universe at a level we've never done before.
We've kind of completed one particle's story
[...] We're on the frontier now, on the edge
of a new exploration. This could be the only
part of the story that's left, or we could
open a whole new realm of discovery.
In quantum theory, which is the language of
particle physicists, elementary particles
are divided into two rough categories: fermions,
which are bits of matter like electrons, and
bosons, which are bits of energy and can transmit
forces, like the photon that transmits light.
Dr. Peter Higgs was one of six physicists,
working in three independent groups, who in
1964 invented the notion of the cosmic molasses,
or Higgs field. The others were Tom Kibble
of Imperial College, London; Carl Hagen of
the University of Rochester; Gerald Guralnik
of Brown University; and François Englert
and Robert Brout, both of Université Libre
de Bruxelles. One implication of their theory
was that this Higgs field, normally invisible
and, of course, odorless, would produce its
own quantum particle if hit hard enough, by
the right amount of energy. The particle would
be fragile and fall apart within a millionth
of a second in a dozen different ways depending
upon its own mass. Unfortunately, the theory
did not say how much this particle should
weigh, which is what made it so difficult
to find. The particle eluded researchers at
a succession of particle accelerators, including
the Large Electron–Positron Collider at
CERN, which closed down in 2000, and the Tevatron
at the Fermi National Accelerator Laboratory,
or Fermilab, in Batavia, Ill., which shut
down in 2011.Further experiments continued
and in March 2013 it was tentatively confirmed
that the newly discovered particle was a Higgs
Boson.
Although they have never been seen, Higgslike
fields play an important role in theories
of the universe and in string theory. Under
certain conditions, according to the strange
accounting of Einsteinian physics, they can
become suffused with energy that exerts an
antigravitational force. Such fields have
been proposed as the source of an enormous
burst of expansion, known as inflation, early
in the universe and, possibly, as the secret
of the dark energy that now seems to be speeding
up the expansion of the universe.
=== Further theoretical development ===
Modern theoretical development includes refining
of the Standard Model, researching in its
foundations such as the Yang–Mills theory,
and researches in computational methods such
as the lattice QCD.
A long-standing problem is quantum gravitation.
No solution that is useful for particle physics
has been achieved.
=== Further experimental development ===
There are researches about quark–gluon plasma,
a new (hypothetical) state of matter. There
are also some recent experimental evidences
that tetraquarks and glueballs exist.
The proton decay is not observed (or, generally,
non-conservation of the baryon number), but
predicted by the Standard Model, so there
are searches for it.
== See also ==
Timeline of atomic and subatomic physics
Golden age of physics
Subatomic particle#History, authors and dates
of important discoveries
History of string theory
== Notes
