Encyclopedia
Particle physics is a branch of
physics that studies the elementary constituents of matter and radiation, and the interactions between them. It is also called
"high energy physics", because many elementary particles do not occur under normal circumstances in
nature, but can be created and detected during energetic
collisions of other particles, as is done in
particle accelerators.
Subatomic particles
Modern particle physics research is focused on
subatomic particles, which have less structure than
atoms. These include atomic constituents such as
electrons,
protons, and
neutrons , particles produced by
radiative and scattering processes, such as
photons,
neutrinos, and
muons, as well as a wide range of exotic particles.
Strictly speaking, the term
particle is a misnomer because the dynamics of particle physics are governed by
quantum mechanics. As such, they exhibit
wave-particle duality, displaying particle-like behavior under certain experimental conditions and
wave-like behavior in others . Following the convention of particle physicists, we will use "elementary particles" to refer to objects such as
electrons and
photons, with the understanding that these "particles" display wave-like properties as well.
All the particles and their interactions observed to date can be described by a
quantum field theory called the
Standard Model. The Standard Model has 40 species of elementary particles , which can combine to form composite particles, accounting for the hundreds of other species of particles 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.
Particle physics has had a large impact on the philosophy of science. Some particle physicists adhere to
reductionism, a point of view that has been criticized by philosophers and scientists. Part of the debate is described below.
History
The idea that all matter is composed of elementary particles dates to at least the 6th century BC. The philosophical doctrine of
atomism was studied by
ancient Greek philosophers such as
Leucippus,
Democritus, and
Epicurus. Rishi kannad of India had also predicted that all matter is made of subatomic particle called anu and parmanu. Although
Isaac Newton in the
17th century thought that matter was made up of particles, it was
John Dalton who formally stated in 1802 that everything is made from tiny
atoms.
Dmitri Mendeleev's first periodic table in 1869 helped cement the view, prevalent throughout the
19th century, that matter was made of atoms. Work by
J.J. Thomson in the late
1890s established that atoms are composed of light
electrons and massive
protons.
Ernest Rutherford established in 1911 that the protons are concentrated in a compact nucleus. The nucleus was initially thought to be composed of protons and confined electrons , but was later found to be composed of protons and
neutrons.
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 the same year. These discoveries gave rise to an active industry of generating one atom from another, even rendering possible the transmutation of
lead into gold. They also led to the development of
nuclear weapons.
Throughout the
1950s and
1960s, a bewildering variety of particles was found in scattering experiments. This was referred to as the "particle zoo". This 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.
The Standard Model
The current state of the classification of elementary particles is the
Standard Model. It describes the strong,
weak, and electromagnetic fundamental forces, using mediating gauge bosons. The species of gauge bosons are the gluons,
W- and W+ and
Z bosons, and the
photons, respectively. The model also contains 24 fundamental particles, which are the constituents of matter. Finally, it predicts the existence of a type of boson known as the Higgs boson, which has yet to be discovered.
Experiment
In particle physics, the major international collaborations are:
- CERN, located on the French-Swiss border near Geneva. Its main project is now LHC, or the Large Hadron Collider, which is currently under construction. The LHC will be in operation in 2007 and will be the world's most energetic collider upon completion. Earlier facilities include LEP
...
, the Large
Electron Positron collider, which was stopped in 2001 and which is now dismantled to give way for LHC; and SPS, or the Super
Proton Synchrotron.
- Fermilab, located near Chicago, USA. Its main facility is the Tevatron, which collides protons and antiprotons and is presently the highest energy particle collider in the world.
- KEK The High Energy Accelerator Research Organization of Japan located in Tsukuba, Japan. It is the home of a number of interesting experiments such as , a neutrino oscillation experiment and , an experiment measuring the CP-symmetry violation in the B-meson.
The
Stanford Linear Accelerator Center is a United States Department of Energy [i] N...
, located near Palo Alto, USA. Its main facility is PEP-II, which collides
electrons and
positrons.
Many other
particle accelerators exist.
The techniques required to do modern experimental particle physics are quite varied and complex, constituting a subspecialty nearly completely distinct from the theoretical side of the field. See for a partial list of the ideas required for such experiments.
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 efforts in theoretical particle physics today and each includes a range of different activities. The efforts in each area are interrelated.
One of the major activities in theoretical particle physics is the attempt 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. These efforts are made challenging by the difficult nature of calculating many quantities in quantum chromodynamics. Some theorists making these efforts refer to themselves as
phenomenologists and 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 divide of efforts in particle physics is reflected in the names of categories on the preprint archive : hep-th , hep-ph , hep-ex , hep-lat .
Reductionism
Throughout the development of particle physics, there have been many objections to the extreme reductionist approach of attempting to explain
everything in terms of elementary particles and their interaction. These objections have been raised by people from a wide array of fields, including many modern particle physicists, solid state physicists,
chemists,
biologists, and metaphysical holists. While the Standard Model itself is not challenged, it is contended that the properties of elementary particles are no more fundamental than the
emergent properties of atoms and molecules, and especially statistically large ensembles of those. Some critics of reductionism claim that even a complete knowledge of the underlying elementary particles will not lend a thorough understanding of more complicated natural processes, while others doubt that a complete knowledge of particle behavior could even be attained, thanks to
quantum indeterminacy.
Reductionists typically claim that all progress in the sciences has involved reductionism to some extent.
Public policy
Experimental results in particle physics are often obtained using enormous
particle accelerators which are very expensive and require large amounts of government funding. Because of this, particle physics research involves issues of public policy.
Many have argued that the potential advances do not justify the money spent, and that in fact particle physics takes money away from more important research and education efforts. In 1993, the US Congress stopped the
Superconducting Super Collider because of similar concerns, after US$2 billion had already been spent on its construction. Many scientists, both supporters and opponents of the SSC, believe that the decision to stop construction of the SSC was due in part to the end of the
Cold War which removed scientific competition with the
Soviet Union as a rationale for spending large amounts of money on the SSC.
Some within the scientific community believe that particle physics has also been adversely affected by the aging population. The belief is that the aging population is much more concerned with immediate issues of their health and their parents' health and that this has driven scientific funding away from physics toward the biological and health sciences. In addition, many opponents question the ability of any single country to support the expense of particle physics results and fault the SSC for not seeking greater international funding.
Proponents of particle accelerators hold that the investigation of the most basic theories deserves adequate funding, and that this funding benefits other fields of science in various ways. They point out that all accelerators today are international projects and question the claim that money not spent on accelerators would then necessarily be used for other scientific or educational purposes.
The future
Particle physicists internationally agree on the most important goals of particle physics research in the near and intermediate future. The overarching 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. Most importantly, though, there may be unexpected and unpredicted surprises which will give us the most opportunity to learn about nature.
Much of the efforts to find this new physics are focused on new collider experiments. A near term goal is the completion of the
Large Hadron Collider in 2007 which will 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. A decision for the technology of the ILC has been taken in August 2004, but the site has still to be agreed upon.
Additionally, there are important non-collider experiments which 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
life time of the proton put constraints on
Grand Unification Theories at energy scales much higher than collider experiments will be able to probe any time soon.
See also
External links
