Particle physics (also known as high energy
physics) is a branch of physics that studies
the nature of the particles that constitute
matter and radiation.
Although the word particle can refer to various
types of very small objects (e.g. protons,
gas particles, or even household dust), particle
physics usually investigates the irreducibly
smallest detectable particles and the fundamental
interactions necessary to explain their behaviour.
By our current understanding, these elementary
particles are excitations of the quantum fields
that also govern their interactions.
The currently dominant theory explaining these
fundamental particles and fields, along with
their dynamics, is called the Standard Model.
Thus, modern particle physics generally investigates
the Standard Model and its various possible
extensions, e.g. to the newest "known" particle,
the Higgs boson, or even to the oldest known
force field, gravity.
== Subatomic particles ==
Modern particle physics research is focused
on subatomic particles, including atomic constituents
such as electrons, protons, and neutrons (protons
and neutrons are composite particles called
baryons, made of quarks), produced by radioactive
and scattering processes, such as photons,
neutrinos, and muons, as well as a wide range
of exotic particles.
Dynamics of particles is also governed by
quantum mechanics; they exhibit wave–particle
duality, displaying particle-like behaviour
under certain experimental conditions and
wave-like behaviour in others.
In more technical terms, they are described
by quantum state vectors in a Hilbert space,
which is also treated in quantum field theory.
Following the convention of particle physicists,
the term elementary particles is applied to
those particles that are, according to current
understanding, presumed to be indivisible
and not composed of other particles.
All particles and their interactions observed
to date can be described almost entirely by
a quantum field theory called the Standard
Model.
The Standard Model, as currently formulated,
has 61 elementary particles.
Those elementary particles can combine to
form composite particles, accounting for the
hundreds of other species of particles that
have been discovered since the 1960s.
The Standard Model has been found to agree
with almost all the experimental tests conducted
to date.
However, most particle physicists believe
that it is an incomplete description of nature
and that a more fundamental theory awaits
discovery (See Theory of Everything).
In recent years, measurements of neutrino
mass have provided the first experimental
deviations from the Standard Model.
== History ==
The idea that all matter is composed of elementary
particles dates from at least the 6th century
BC.
In the 19th century, John Dalton, through
his work on stoichiometry, concluded that
each element of nature was composed of a single,
unique type of particle.
The word atom, after the Greek word atomos
meaning "indivisible", has since then denoted
the smallest particle of a chemical element,
but physicists soon discovered that atoms
are not, in fact, the fundamental particles
of nature, but are conglomerates of even smaller
particles, such as the electron.
The early 20th century explorations of nuclear
physics and quantum physics led to 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; both
discoveries also led to the development of
nuclear weapons.
Throughout the 1950s and 1960s, a bewildering
variety of particles were found in collisions
of particles from increasingly high-energy
beams.
It was referred to informally as the "particle
zoo".
That term was deprecated after the formulation
of the Standard Model during the 1970s, in
which the large number of particles was explained
as combinations of a (relatively) small number
of more fundamental particles.
== Standard Model ==
The current state of the classification of
all elementary particles is explained by the
Standard Model.
It describes the strong, weak, and electromagnetic
fundamental interactions, using mediating
gauge bosons.
The species of gauge bosons are eight gluons,
W−, W+ and Z bosons, and the photon.
The Standard Model also contains 24 fundamental
fermions (12 particles and their associated
anti-particles), which are the constituents
of all matter.
Finally, the Standard Model also predicted
the existence of a type of boson known as
the Higgs boson.
Early in the morning on 4 July 2012, physicists
with the Large Hadron Collider at CERN announced
they had found a new particle that behaves
similarly to what is expected from the Higgs
boson.
== Experimental laboratories ==
The world's major particle physics laboratories
are:
Brookhaven National Laboratory (Long Island,
United States).
Its main facility is the Relativistic Heavy
Ion Collider (RHIC), which collides heavy
ions such as gold ions and polarized protons.
It is the world's first heavy ion collider,
and the world's only polarized proton collider.
Budker Institute of Nuclear Physics (Novosibirsk,
Russia).
Its main projects are now the electron-positron
colliders VEPP-2000, operated since 2006,
and VEPP-4, started experiments in 1994.
Earlier facilities include the first electron-electron
beam-beam collider VEP-1, which conducted
experiments from 1964 to 1968; the electron-positron
colliders VEPP-2, operated from 1965 to 1974;
and, its successor VEPP-2M, performed experiments
from 1974 to 2000.
CERN (European Organization for Nuclear Research)
(Franco-Swiss border, near Geneva).
Its main project is now the Large Hadron Collider
(LHC), which had its first beam circulation
on 10 September 2008, and is now the world's
most energetic collider of protons.
It also became the most energetic collider
of heavy ions after it began colliding lead
ions.
Earlier facilities include the Large Electron–Positron
Collider (LEP), which was stopped on 2 November
2000 and then dismantled to give way for LHC;
and the Super Proton Synchrotron, which is
being reused as a pre-accelerator for the
LHC and for fixed-target experiments.
DESY (Deutsches Elektronen-Synchrotron) (Hamburg,
Germany).
Its main facility was the Hadron Elektron
Ring Anlage (HERA), which collided electrons
and positrons with protons.
The accelerator complex is now focused on
the production of synchrotron radiation with
PETRA III, FLASH and the European XFEL.
Fermi National Accelerator Laboratory (Fermilab)
(Batavia, United States).
Its main facility until 2011 was the Tevatron,
which collided protons and antiprotons and
was the highest-energy particle collider on
earth until the Large Hadron Collider surpassed
it on 29 November 2009.
Institute of High Energy Physics (IHEP) (Beijing,
China).
IHEP manages a number of China's major particle
physics facilities, including the Beijing
Electron Positron Collider (BEPC), the Beijing
Spectrometer (BES), the Beijing Synchrotron
Radiation Facility (BSRF), the International
Cosmic-Ray Observatory at Yangbajing in Tibet,
the Daya Bay Reactor Neutrino Experiment,
the China Spallation Neutron Source, the Hard
X-ray Modulation Telescope (HXMT), and the
Accelerator-driven Sub-critical System (ADS)
as well as the Jiangmen Underground Neutrino
Observatory (JUNO).
KEK (Tsukuba, Japan).
It is the home of a number of experiments
such as the K2K experiment, a neutrino oscillation
experiment and Belle II, an experiment measuring
the CP violation of B mesons.
SLAC National Accelerator Laboratory (Menlo
Park, United States).
Its 2-mile-long linear particle accelerator
began operating in 1962 and was the basis
for numerous electron and positron collision
experiments until 2008.
Since then the linear accelerator is being
used for the Linac Coherent Light Source X-ray
laser as well as advanced accelerator design
research.
SLAC staff continue to participate in developing
and building many particle detectors around
the world.Many other particle accelerators
also exist.
The techniques required for modern experimental
particle physics are quite varied and complex,
constituting a sub-specialty nearly completely
distinct from the theoretical side of the
field.
== Theory ==
Theoretical particle physics attempts to develop
the models, theoretical framework, and mathematical
tools to understand current experiments and
make predictions for future experiments.
See also theoretical physics.
There are several major interrelated efforts
being made in theoretical particle physics
today.
One important branch attempts to better understand
the Standard Model and its tests.
By extracting the parameters of the Standard
Model, from experiments with less uncertainty,
this work probes the limits of the Standard
Model and therefore expands our understanding
of nature's building blocks.
Those efforts are made challenging by the
difficulty of calculating quantities in quantum
chromodynamics.
Some theorists working in this area refer
to themselves as phenomenologists and they
may use the tools of quantum field theory
and effective field theory.
Others make use of lattice field theory and
call themselves lattice theorists.
Another major effort is in model building
where model builders develop ideas for what
physics may lie beyond the Standard Model
(at higher energies or smaller distances).
This work is often motivated by the hierarchy
problem and is constrained by existing experimental
data.
It may involve work on supersymmetry, alternatives
to the Higgs mechanism, extra spatial dimensions
(such as the Randall-Sundrum models), Preon
theory, combinations of these, or other ideas.
A third major effort in theoretical particle
physics is string theory.
String theorists attempt to construct a unified
description of quantum mechanics and general
relativity by building a theory based on small
strings, and branes rather than particles.
If the theory is successful, it may be considered
a "Theory of Everything", or "TOE".
There are also other areas of work in theoretical
particle physics ranging from particle cosmology
to loop quantum gravity.
This division of efforts in particle physics
is reflected in the names of categories on
the arXiv, a preprint archive: hep-th (theory),
hep-ph (phenomenology), hep-ex (experiments),
hep-lat (lattice gauge theory).
== Practical applications ==
In principle, all physics (and practical applications
developed therefrom) can be derived from the
study of fundamental particles.
In practice, even if "particle physics" is
taken to mean only "high-energy atom smashers",
many technologies have been developed during
these pioneering investigations that later
find wide uses in society.
Particle accelerators are used to produce
medical isotopes for research and treatment
(for example, isotopes used in PET imaging),
or used directly in external beam radiotherapy.
The development of superconductors has been
pushed forward by their use in particle physics.
The World Wide Web and touchscreen technology
were initially developed at CERN.
Additional applications are found in medicine,
national security, industry, computing, science,
and workforce development, illustrating a
long and growing list of beneficial practical
applications with contributions from particle
physics.
== Future ==
The primary goal, which is pursued in several
distinct ways, is to find and understand what
physics may lie beyond the standard model.
There are several powerful experimental reasons
to expect new physics, including dark matter
and neutrino mass.
There are also theoretical hints that this
new physics should be found at accessible
energy scales.
Much of the effort to find this new physics
are focused on new collider experiments.
The Large Hadron Collider (LHC) was completed
in 2008 to help continue the search for the
Higgs boson, supersymmetric particles, and
other new physics.
An intermediate goal is the construction of
the International Linear Collider (ILC), which
will complement the LHC by allowing more precise
measurements of the properties of newly found
particles.
In August 2004, a decision for the technology
of the ILC was taken but the site has still
to be agreed upon.
In addition, there are important non-collider
experiments that also attempt to find and
understand physics beyond the Standard Model.
One important non-collider effort is the determination
of the neutrino masses, since these masses
may arise from neutrinos mixing with very
heavy particles.
In addition, cosmological observations provide
many useful constraints on the dark matter,
although it may be impossible to determine
the exact nature of the dark matter without
the colliders.
Finally, lower bounds on the very long lifetime
of the proton put constraints on Grand Unified
Theories at energy scales much higher than
collider experiments will be able to probe
any time soon.
In May 2014, the Particle Physics Project
Prioritization Panel released its report on
particle physics funding priorities for the
United States over the next decade.
This report emphasized continued U.S. participation
in the LHC and ILC, and expansion of the Deep
Underground Neutrino Experiment, among other
recommendations.
== High energy physics compared to low energy
physics ==
The term high energy physics requires elaboration.
Intuitively, it might seem incorrect to associate
"high energy" with the physics of very small,
low mass objects, like subatomic particles.
By comparison, an example of a macroscopic
system, one gram of hydrogen, has ~ 6×1023
times the mass of a single proton.
Even an entire beam of protons circulated
in the LHC contains ~ 3.23×1014 protons,
each with 6.5×1012 eV of energy, for a total
beam energy of ~ 2.1×1027 eV or ~ 336.4 MJ,
which is still ~ 2.7×105 times lower than
the mass-energy of a single gram of hydrogen.
Yet, the macroscopic realm is "low energy
physics", while that of quantum particles
is "high energy physics".
The interactions studied in other fields of
physics and science have comparatively very
low energy.
For example, the photon energy of visible
light is about 1.8 to 3.1 eV.
Similarly, the bond-dissociation energy of
a carbon–carbon bond is about 3.6 eV.
Other chemical reactions typically involve
similar amounts of energy.
Even photons with far higher energy, gamma
rays of the kind produced in radioactive decay,
mostly have photon energy between 105 eV and
107 eV – still two orders of magnitude lower
than the mass of a single proton.
Radioactive decay gamma rays are considered
as part of nuclear physics, rather than high
energy physics.
The proton has a mass of around 9.4×108 eV;
some other massive quantum particles, both
elementary and hadronic, have yet higher masses.
Due to these very high energies at the single
particle level, particle physics is, in fact,
high-energy physics.
== See also
