Particle physics is a branch of physics which
studies the nature of particles that are the
constituents of what is usually referred to
as matter and radiation. In current understanding,
particles are excitations of quantum fields
and interact following their dynamics. Although
the word "particle" can be used in reference
to many objects, the term "particle physics"
usually refers to the study of "smallest"
particles and the fundamental fields that
must be defined in order to explain the observed
particles. These cannot be defined by a combination
of other fundamental fields. The current set
of fundamental fields and their dynamics are
summarized in a theory called the Standard
Model, therefore particle physics is largely
the study of the Standard Model's particle
content and its possible extensions, with
the recent finding of Higgs boson.
Subatomic particles
Modern particle physics research is focused
on subatomic particles, including atomic constituents
such as electrons, protons, and neutrons,
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 behavior
under certain experimental conditions and
wave-like behavior 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. 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 to 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",
denotes the smallest particle of a chemical
element since then, but physicists soon discovered
that atoms are not, in fact, the fundamental
particles of nature, but conglomerates of
even smaller particles, such as the electron.
The early 20th-century explorations of nuclear
physics and quantum physics culminated in
proofs of nuclear fission in 1939 by Lise
Meitner, 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 scattering experiments.
It was referred to 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 small number of 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 the gluons, W−, W+ and
Z bosons, and the photons. The Standard Model
also contains 24 fundamental 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 have found a new particle that behaves
similarly to what is expected from the Higgs
boson.
Experimental laboratories
In particle physics, the major international
laboratories are located at the:
Brookhaven National Laboratory. Its main facility
is the Relativistic Heavy Ion Collider, 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. 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,. Its main project is now the Large Hadron
Collider, 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, 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.
DESY. Its main facility is the Hadron Elektron
Ring Anlage, which collides electrons and
positrons with protons.
Fermilab,. 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.
KEK,. It is the home of a number of experiments
such as the K2K experiment, a neutrino oscillation
experiment and Belle, an experiment measuring
the CP violation of B mesons.
Many other particle accelerators do exist.
The techniques required to do 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.
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, 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".
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, hep-ph,
hep-ex, hep-lat.
Practical applications
In principle, all physics 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. Cyclotrons are used to produce medical
isotopes for research and treatment, or used
directly for certain cancer treatments. 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. Furthermore,
there may be surprises that will give us opportunities
to learn about nature.
Much of the effort to find this new physics
are focused on new collider experiments. The
Large Hadron Collider 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, 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 Long
Baseline Neutrino Experiment, among other
recommendations.
See also
References
Further reading
Introductory reading
Close, Frank. Particle Physics: A Very Short
Introduction. Oxford University Press. ISBN 0-19-280434-0. 
Close, Frank; Marten, Michael; Sutton, Christine.
The Particle Odyssey: A Journey to the Heart
of the Matter. Oxford University Press. ISBN 9780198609438. 
Ford, Kenneth W.. The Quantum World. Harvard
University Press. 
Oerter, Robert. The Theory of Almost Everything:
The Standard Model, the Unsung Triumph of
Modern Physics. Plume. 
Schumm, Bruce A.. Deep Down Things: The Breathtaking
Beauty of Particle Physics. Johns Hopkins
University Press. ISBN 0-8018-7971-X. 
Close, Frank. The New Cosmic Onion. Taylor
& Francis. ISBN 1-58488-798-2. 
Advanced reading
Robinson, Matthew B.; Bland, Karen R.; Cleaver,
Gerald. B.; Dittmann, Jay R.. "A Simple Introduction
to Particle Physics". arXiv:0810.3328 [hep-th].
Robinson, Matthew B.; Cleaver, Gerald; Cleaver,
Gerald B.. "A Simple Introduction to Particle
Physics Part II". arXiv:0908.1395 [hep-th].
Griffiths, David J.. Introduction to Elementary
Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4. 
Kane, Gordon L.. Modern Elementary Particle
Physics. Perseus Books. ISBN 0-201-11749-5. 
Perkins, Donald H.. Introduction to High Energy
Physics. Cambridge University Press. ISBN 0-521-62196-8. 
Povh, Bogdan. Particles and Nuclei: An Introduction
to the Physical Concepts. Springer-Verlag.
ISBN 0-387-59439-6. 
Boyarkin, Oleg. Advanced Particle Physics
Two-Volume Set. CRC Press. ISBN 978-1-4398-0412-4. 
External links
Symmetry magazine
Fermilab
Particle physics – it matters – the Institute
of Physics
Nobes, Matthew "Introduction to the Standard
Model of Particle Physics" on Kuro5hin: Part
1, Part 2, Part 3a, Part 3b.
CERN – European Organization for Nuclear
Research
The Particle Adventure – educational project
sponsored by the Particle Data Group of the
Lawrence Berkeley National Laboratory
