Particle physics or high-energy physics is the study of fundamental particles and forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the scale of protons and neutrons, while the study of combinations of protons and neutrons is called nuclear physics.
The fundamental particles in the universe are classified in the Standard Model as (matter particles) and (force-carrying particles). There are three generations of fermions, although ordinary matter is made only from the first fermion generation. The first generation consists of Up quark and which form and , and and electron neutrinos. The three fundamental interactions known to be mediated by bosons are electromagnetism, the weak interaction, and the strong interaction.
Quark cannot exist on their own but form . Hadrons that contain an odd number of quarks are called and those that contain an even number are called . Two baryons, the proton and the neutron, make up most of the mass of ordinary matter. Mesons are unstable and the longest-lived last for only a few hundredths of a microsecond. They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in . Mesons are also produced in or other particle accelerators.
Particles have corresponding with the same mass but with opposite . For example, the antiparticle of the electron is the positron. The electron has a negative electric charge, the positron has a positive charge. These antiparticles can theoretically form a corresponding form of matter called antimatter. Some particles, such as the photon, are their own antiparticle.
These elementary particles are excitations of the quantum fields that also govern their interactions. The dominant theory explaining these fundamental particles and fields, along with their dynamics, is called the Standard Model. The Quantum gravity to the current particle physics theory is not solved; many theories have addressed this problem, such as loop quantum gravity, string theory and Supersymmetry.
Experimental particle physics is the study of these particles in radioactive processes and in particle accelerators such as the Large Hadron Collider. Theoretical particle physics is the study of these particles in the context of cosmology and quantum theory. The two are closely interrelated: the Higgs boson was postulated theoretically before being confirmed by experiments.
History
The idea that all
matter is fundamentally 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
meaning "indivisible", has since then denoted the smallest particle of a
chemical element, but physicists later 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
. Bethe's 1947 calculation of the
Lamb shift is credited with having "opened the way to the modern era of particle physics".
Throughout the 1950s and 1960s, a bewildering variety of particles was found in collisions of particles from beams of increasingly high energy. It was referred to informally as the "particle zoo". Important discoveries such as the CP violation by James Cronin and Val Fitch brought new questions to Baryon asymmetry.
Standard Model
The current state of the classification of all elementary particles is explained by the
Standard Model, which gained widespread acceptance in the mid-1970s after experimental confirmation of the existence of
. It describes the strong,
weak interaction, and
electromagnetism fundamental interactions, using mediating
. The species of gauge bosons are eight
, , and bosons, and the
photon.
The Standard Model also contains 24 fundamental
(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. 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.
The Standard Model, as currently formulated, has 61 elementary particles.
Subatomic particles
Modern particle physics research is focused on subatomic particles, including atomic constituents, such as
,
, and
(protons and neutrons are composite particles called
, made of
), that are produced by radioactive and
scattering processes; such particles are
,
, and
, as well as a wide range of
.
All particles and their interactions observed to date can be described almost entirely by the Standard Model.
Dynamics of particles are 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.
Quarks and leptons
Ordinary
matter is made from first-generation quarks (
Up quark,
Down quark) and leptons (
electron, electron neutrino).
Collectively, quarks and leptons are called
, because they have a
quantum spin of
(−1/2, 1/2, 3/2, etc.). This causes the fermions to obey the Pauli exclusion principle, where no two particles may occupy the same
quantum state.
Quarks have fractional elementary electric charge (−1/3 or 2/3)
and leptons have whole-numbered electric charge (0 or -1).
Quarks also have
color charge, which is labeled arbitrarily with no correlation to actual light
color as red, green and blue.
Because the interactions between the quarks store energy which can convert to other particles when the quarks are far apart enough, quarks cannot be observed independently. This is called color confinement.
There are three known generations of quarks (up and down, Strange quark and Charm quark, Top quark and Bottom quark) and leptons (electron and its neutrino, muon and Muon neutrino, tau and Tau neutrino), with strong indirect evidence that a fourth generation of fermions does not exist.
Bosons
Bosons are the
Force carrier of fundamental interactions, such as
electromagnetism, the
weak interaction, and the strong interaction.
Electromagnetism is mediated by the
photon, the
Quantum of
light.
["Role as gauge boson and polarization" §5.1 in ] The weak interaction is mediated by the W and Z bosons.
The strong interaction is mediated by the
gluon, which can link quarks together to form composite particles.
Due to the aforementioned color confinement, gluons are never observed independently.
The
Higgs boson gives mass to the W and Z bosons via the
Higgs mechanism – the gluon and photon are expected to be massless.
All bosons have an integer quantum spin (0 and 1) and can have the same
quantum state.
Antiparticles and color charge
Most aforementioned particles have corresponding
, which compose
antimatter. Normal particles have positive
Lepton number or
baryon number, and antiparticles have these numbers negative.
Most properties of corresponding antiparticles and particles are the same, with a few gets reversed; the electron's antiparticle, positron, has an opposite charge. To differentiate between antiparticles and particles, a plus or negative sign is added in
superscript. For example, the electron and the positron are denoted and .
However, in the case that the particle has a charge of 0 (equal to that of the antiparticle), the antiparticle is denoted with a line above the symbol. As such, an electron neutrino is , whereas its antineutrino is . When a particle and an antiparticle interact with each other, they are
Annihilation and convert to other particles.
Some particles, such as the photon or gluon, have no antiparticles.
Quarks and gluons additionally have color charges, which influences the strong interaction. Quark's color charges are called red, green and blue (though the particle itself have no physical color), and in antiquarks are called antired, antigreen and antiblue.[Part III of ]
Composite
The
and
in the
Atomic nucleus are
– the neutron is composed of two down quarks and one up quark, and the proton is composed of two up quarks and one down quark.
A baryon is composed of three quarks, and a
meson is composed of two quarks (one normal, one anti). Baryons and mesons are collectively called
. Quarks inside hadrons are governed by the strong interaction, thus are subjected to quantum chromodynamics (color charges). The
Bound state quarks must have their color charge to be neutral, or "white" for analogy with
Additive color.
More
can have other types, arrangement or number of quarks (
tetraquark,
pentaquark).
An atom is made from protons, neutrons and electrons.[§1.8, Constituents of Matter: Atoms, Molecules, Nuclei and Particles, Ludwig Bergmann, Clemens Schaefer, and Wilhelm Raith, Berlin, Germany: Walter de Gruyter, 1997, .] A simple example would be the hydrogen-4.1, which has one of its electrons replaced with a muon.
Hypothetical
The
graviton is a hypothetical particle that can mediate the gravitational interaction, but it has not been detected or completely reconciled with current theories.
Many other hypothetical particles have been proposed to address the limitations of the Standard Model. Notably,
Supersymmetry particles aim to solve the hierarchy problem,
address the strong CP problem, and various other particles are proposed to explain the origins of
dark matter and
dark energy.
Experimental laboratories
The world's major particle physics laboratories are:
-
Brookhaven National Laboratory (Long Island, New York, 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 VEPP-2000,
-
detector for LHC]]CERN (European Organization for Nuclear Research) (France-Switzerland border, near Geneva, Switzerland). 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.
-
Fermilab (Batavia, Illinois, 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 II(BEPC II), 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 and its successor T2K experiment, a neutrino oscillation experiment and Belle II, an experiment measuring the CP violation of .
-
SLAC National Accelerator Laboratory (Menlo Park, California, 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.
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. Theorists make quantitative predictions of observables at collider and astronomical experiments, which along with experimental measurements is used to extract the parameters of the Standard Model with less uncertainty. This work probes the limits of the Standard Model and therefore expands scientific understanding of nature's building blocks. Those efforts are made challenging by the difficulty of calculating high precision quantities in quantum chromodynamics. Some theorists working in this area use the tools of perturbative quantum field theory and effective field theory, referring to themselves as phenomenologists. 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.
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 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.
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
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
Major efforts to look for physics beyond the Standard Model include the Future Circular Collider proposed for CERN
and the Particle Physics Project Prioritization Panel (P5) in the US that will update the 2014 P5 study that recommended the Deep Underground Neutrino Experiment, among other experiments.
See also
External links
