Subatomic particle

From Wikipedia, the free encyclopedia

In the physical sciences, subatomic particles are particles much smaller than atoms.^[1] There are two types of subatomic particles: elementary particles, which according to current theories are not made of other particles; and composite particles.^[2] Particle physics and nuclear physics study these particles and how they interact.^[3]
In particle physics, the concept of a particle is one of several concepts inherited from classical physics. But it also reflects the modern understanding that at the quantum scale matter and energy behave very differently from what much of everyday experience would lead us to expect.

The idea of a particle underwent serious rethinking when experiments showed that light could behave like a stream of particles (called photons) as well as exhibit wave-like properties. This led to the new concept of wave–particle duality to reflect that quantum-scale "particles" behave like both particles and waves (also known as wavicles). Another new concept, the uncertainty principle, states that some of their properties taken together, such as their simultaneous position and momentum, cannot be measured exactly.^[4] In more recent times, wave–particle duality has been shown to apply not only to photons but to increasingly massive particles as well.^[5]

Interactions of particles in the framework of quantum field theory are understood as creation and annihilation of quanta of corresponding fundamental interactions. This blends particle physics with field theory.

Classification

By statistics

The Standard Model classification of particles

Any subatomic particle, like any particle in the 3-dimensional space that obeys laws of quantum mechanics, can be either a boson (an integer spin) or a fermion (a half-integer spin).

By composition

The elementary particles of the Standard Model include:^[6]

• Six "flavors" of quarks: up, down, bottom, top, strange, and charm;
• Six types of leptons: electron, electron neutrino, muon, muon neutrino, tau, tau neutrino;
• Twelve gauge bosons (force carriers): the photon of electromagnetism, the three W and Z bosons of the weak force, and the eight gluons of the strong force;
• The Higgs boson.

Various extensions of the Standard Model predict the existence of an elementary graviton particle and many other elementary particles.

Composite subatomic particles (such as protons or atomic nuclei) are bound states of two or more elementary particles. For example, a proton is made of two up quarks and one down quark, while the atomic nucleus of helium-4 is composed of two protons and two neutrons. Composite particles include all hadrons: these include baryons (such as protons and neutrons) and mesons (such as pions and kaons).

By mass

In special relativity, the energy of a particle at rest equals its mass times the speed of light squared (). That is, mass can be expressed in terms of energy and vice versa. If a particle has a frame of reference where it lies at rest, then it has a positive rest mass and is referred to as massive.

All composite particles are massive. Baryons (meaning "heavy") tend to have greater mass than mesons (meaning "intermediate"), which in turn tend to be heavier than leptons (meaning "lightweight"), but the heaviest lepton (the tau particle) is heavier than the two lightest flavours of baryons (nucleons). It is also certain that any particle with an electric charge is massive.

All massless particles (particles whose invariant mass is zero) are elementary. These include the photon and gluon, although the latter cannot be isolated.

The question of the masses of neutrinos is uncertain.

Other properties

Through the work of Albert Einstein, Louis de Broglie, and many others, current scientific theory holds that all particles also have a wave nature.^[7] This has been verified not only for elementary particles but also for compound particles like atoms and even molecules. In fact, according to traditional formulations of non-relativistic quantum mechanics, wave–particle duality applies to all objects, even macroscopic ones; although the wave properties of macroscopic objects cannot be detected due to their small wavelengths.^[8]

Interactions between particles have been scrutinized for many centuries, and a few simple laws underpin how particles behave in collisions and interactions. The most fundamental of these are the laws of conservation of energy and conservation of momentum, which let us make calculations of particle interactions on scales of magnitude that range from stars to quarks.^[9] These are the prerequisite basics of Newtonian mechanics, a series of statements and equations in Philosophiae Naturalis Principia Mathematica, originally published in 1687.

Dividing an atom

The negatively-charged electron has a mass equal to ¹⁄₁₈₃₆ of that of a hydrogen atom. The remainder of the hydrogen atom's mass comes from the positively charged proton. The atomic number of an element is the number of protons in its nucleus. Neutrons are neutral particles having a mass slightly greater than that of the proton. Different isotopes of the same element contain the same number of protons but differing numbers of neutrons. The mass number of an isotope is the total number of nucleons (neutrons and protons collectively).

Chemistry concerns itself with how electron sharing binds atoms into structures such as crystals and molecules. Nuclear physics deals with how protons and neutrons arrange themselves in nuclei. The study of subatomic particles, atoms and molecules, and their structure and interactions, requires quantum mechanics. Analyzing processes that change the numbers and types of particles requires quantum field theory. The study of subatomic particles per se is called particle physics. The term high-energy physics is nearly synonymous to "particle physics" since creation of particles requires high energies: it occurs only as a result of cosmic rays, or in particle accelerators.
Particle phenomenology systematizes the knowledge about subatomic particles obtained from these experiments.

History

The term "subatomic particle" is largely a retronym of 1960s made to distinguish a big number of baryons and mesons (that comprise hadrons) from particles that are now thought to be truly elementary. Before that hadrons were usually classified as "elementary" because their composition was unknown.
A list of important discoveries follows:

Particle	Composition	Theorized	Discovered	Comments
Electron e−	elementary (lepton)	G. Johnstone Stoney (1874)	J. J. Thomson (1897)	Minimum unit of electrical charge, for which Stoney suggested the name in 1891.[10]
alpha particle α	composite (atomic nucleus)	never	Ernest Rutherford (1899)	Proven by Rutherford and Thomas Royds in 1907 to be helium nuclei.
Photon γ	elementary (quantum)	Max Planck (1900)	Albert Einstein (1905) or Ernest Rutherford (1899) as γ rays	Necessary to solve the problem of black body radiation in thermodynamics.
Proton p	composite (baryon)	Long ago	Ernest Rutherford (1919, named 1920)	The nucleus of 1H.
Neutron n	composite (baryon)	Ernest Rutherford (c.1918)	James Chadwick (1932)	The second nucleon.
Antiparticles		Paul Dirac (1928)	Carl D. Anderson (e+, 1932)	Now explained with CPT symmetry.
Pions π	composite (mesons)	Hideki Yukawa (1935)	César Lattes, Giuseppe Occhialini (1947) and Cecil Powell	Explains the nuclear force between nucleons. The first meson (by modern definition) to be discovered.
Muon μ−	elementary (lepton)	never	Carl D. Anderson (1936)	The first named meson; today considered a lepton.
Kaons K	composite (mesons)	never	1947	Discovered in cosmic rays. The first strange particle.
Lambda baryons Λ	composite (baryons)	never	University of Melbourne (Λ0, 1950)[11]	The first hyperon discovered.
Neutrino ν	elementary (lepton)	Wolfgang Pauli (1930), named by Enrico Fermi	Clyde Cowan, Frederick Reines (ν e, 1956)	Solved the problem of energy spectrum of beta decay.
Quarks (u, d, s)	elementary	Murray Gell-Mann, George Zweig (1964)	No particular confirmation event for the quark model.
charm quark c	elementary (quark)	1970	1974
bottom quark b	elementary (quark)	1973	1977
Weak gauge bosons	elementary (quantum)	Glashow, Weinberg, Salam (1968)	CERN (1983)	Properties verified through the 1990s.
top quark t	elementary (quark)	1973	1995	Does not hadronize, but is necessary to complete the Standard Model.
Higgs boson	elementary (quantum)	Peter Higgs et al. (1964)	CERN (2012)	Thought to be confirmed in 2013. More evidence found in 2014.[12]
Tetraquark	composite	?	Zc(3900), 2013, to be confirmed as a tetraquark	A new class of hadrons.
Graviton	elementary (quantum)	Albert Einstein (1916)	Not discovered	Interpretation of a gravitational wave (also hypothetical) as a particle is controversial.
Magnetic monopole	elementary (unclassified)	Paul Dirac (1931)	Not discovered

7 questions with John Christy and Roy Spencer: Climate change skeptics for 25 years

Original link:  http://www.al.com/news/huntsville/index.ssf/2015/04/7_questions_with_john_christy.html

A chart put together by John Christy, director of the Earth System
Science Center at the University of Alabama in Huntsville, that
reflects how the temperature satellite data (the green line) contrasts
with temperature models.Paul Gattis | pgattis@al.com 

The silver anniversary of Roy Spencer's career-defining moment arrived with no expectation in March. He didn't realize it until someone mentioned it to him.

For John Christy, he had no idea that a discovery announced in 1990 would not only still resonate 25 years later but would be at the center of a raging debate.

The date was March 29, 1990. That was the day - though unbeknownst to either Christy or Spencer - they publicly became climate change skeptics.

The scientists at the University of Alabama in Huntsville are known throughout the environmental community as being skeptical that climate change (or global warming) will have a catastrophic effect on the earth. The crux of the matter is that their research, using satellite data to measure temperatures in the atmosphere, disagrees with climate models they say that overstates the earth's warming.

"We are in the minority, there's no question about that," Christy said.

Yes, they agree, that there is climate change. Yes, they agree, humans play a role in that climate change. No, they agree, it's not a catastrophic event.

"We had no clue at that time, 25 years ago, we would be in the center of a huge controversy almost 25 years to the day with congressional investigations, the secretary of state, the vice president telling us we don't even believe in gravity," Christy said. "Who would have thought that 25 years ago?"

Still, they carry on - comfortable in their research and data that has remained true to their findings 25 years ago.

What Christy and Spencer (who then worked for NASA at Marshall Space Flight Center just down the street from UAH) announced at that press conference on March 29, 1990, was that their study of temperature data from satellites indicated the world was not warming as much as was believed.

These days, such an opinion is ridiculed from President Obama on down.

"I think we knew it was going to be an important new way of monitoring the climate," Spencer said. "But you just never know if something like that is going to have legs scientifically. Whether somebody will come up with a new way of doing it better in two years.

"Looking back, I'm kind of surprised this is still the leading way of doing this. Really our only competitors in the field have the same answer we do, very close to the same answer."

AL.com recently sat down with Christy and Spencer for extended interviews as the anniversary approached. Here are excerpts of those conversations:

AL.com: So how did this research get started?

Spencer: John came here to work on a different project. It wasn't too long after he came here that we were at a meeting -- I think it was in New Hampshire -- and we were discussing things over lunch. And the subject came up, Hey, don't we have satellites? Jim Hansen (a climate scientist who sounded perhaps the first alarm about climate change in the 1980s) had just done his testimony for Al Gore in Congress. That's sort of when global warming became public knowledge, when Hansen testified. We were discussing, Don't we have something better than the thermometer data to monitor global temperatures? (UAH scientist) Dick McNider said, 'What about the microwave sounders we have on the weather satellites? We got back to Huntsville and we started looking at how we could get all that data."

AL.com: President Obama recently said that Republicans are going to have to change their opinions on the dangers of climate change. Is this a partisan issue?

Christy: Numbers are numbers. That's what we produced. Those aren't Republican numbers or Democratic numbers. Those are numbers. Those are observations from real satellites. Roy and I were the pioneers. We discovered how to do this with satellites before anyone else did. You can see this very strongly in the administration. Secretary of State John Kerry comes out and says it's like denying gravity. The attack on skeptics was ramped up in the past month. It was a very orchestrated plan having the congressional investigation (by U.S. Rep. Raul Grijalva, D-Arizona).

AL.com: How do you respond to the perception that 97 percent of scientists agree on climate change? (The Wall Street Journal in 2013 reported on the "myth" of the 97 percent).
 
Christy: The impression people make with that statement is that 97 percent of scientists agree with my view of climate change, which typically is one of catastrophic change. So if a Senate hearing or the president or vice president says 97 percent of the scientists agree with me, that's not true. The American Meteorological Society did their survey and they specifically asked the question, Is man the dominate controller of climate over the last 50 years? Only 52 percent said yes. That is not a consensus at all in science.

Then when you look at the core of that question, the core is do you believe that man has some influence on the climate. I don't know anyone who would say no to that. Who are the 3 percent who didn't agree with that? Roy and I have both made the statement that we are in the 97 percent because we believe in some (man-made) effect. It wasn't quantified and it wasn't this dangerous thing. That wasn't part of the question.

Spencer: Whoever came up with that, it was very powerful. It was a good idea. It was very misleading, but it was a good idea. There are different ways people handle that. I use the angle that based on the way they come up with the 97 percent, John and I could be considered part of the 97 percent. This is where things get all muddy. They call us global warming deniers. It's a great soundbite except what do we exactly deny? Or the science is settled. OK, what science is settled? You never hear the specifics.

"That's the great thing about politics. People throw out these platitudes and you could read into them whatever you want. It's so generic or non-specific in the thing that they're saying that you can interpret it anyway you want. You turn it into your own thing because you fill in the details. So being a global warming denier, the truth is we don't know global warming. The science is settled? Well, some of it is. Adding CO2 to the atmosphere probably adds some warming. The science on that is pretty solid. But then the devil's in the details. How much warming does it actually cause? It makes a huge difference.

AL.com: When you hear about the catastrophic effects of climate change, data from reputable organizations such as National Oceanic and Atmospheric Administration (NOAA) or NASA is frequently cited. How do you respond to that?

Christy: NASA, NOAA, EPA, DOE, those are agencies. Agency leaders are appointed by the government, by the current administration. They do not represent objective independent scientific organizations. They can't. They are appointed by the head. They try. People who come out with different views in their organizations are found to be squashed. There is an agenda in those agencies, so it does not surprise me when they go full bore on something like climate change. They are marching to the drum of the administration. It's always been that way. But this administration has been extremely opaque. When you try to go provide information to EPA like these pictures, they will just dismiss it. They will come up with their findings and will not provide you with background for information so that you will know they made a scientific finding.

There are skeptics in NASA and NOAA, a good number. But they are quiet. They know in this administration, they don't speak out.

Spencer: I know that they're not unbiased. Most of them probably really do believe we're destroying the earth. When I talk to scientists who should be objective over a beer at the end of the day, I will argue with them and their final position will always be, 'Yeah, but we need to get away from fossil fuels anyway.' Where did that come from? Are you an expert in alternative energy sources and what they cost? How many poor people are you going to hurt? How many more people are you going to make poor through energy poverty because they are paying five to 10 times as much for their energy?

These guys in government are not unbiased and they have pressures from above. Those organizations, NASA and NOAA, they are part of the executive branch. So the White House has some influence over what direction they go. The heads are political appointees so you have political influence from the top down on scientists. And that's a problem.

AL.com: What about the value of renewable energy sources?

Christy: I am for any energy source that is affordable and doesn't destroy the environment. If carbon dioxide was a poisonous gas, I'd be against it. Carbon dioxide makes things grow. The world used to have five times as much carbon dioxide as it does now. Plants love this stuff. It creates more food. CO2 is not the problem.

Why would you go to wind energy that is so much more expensive or solar? The point here is that if it's not economically sustainable, it's not sustainable. And you have to have an energy plan that's sustainable, that people can afford. Right now, wind and solar, you can't afford. And so the only wind and solar that exists is because of huge government subsidies.  I'm not against renewables or any other source of energy. Can we afford it? Solar and wind are so low density that it takes acres and square miles to do anything. And that's not environmentally smart.

There is absolutely no question that carbon energy provides with longer and better lives. There is no question about that. Anyway scientific way you toss that. Any scientific way you toss that question, you come out that people live better and longer lives. And to suppress, to me, is immoral.

AL.com: Why is your research using satellite data a more effective way of measuring climate change than surface temperature? After all, humans live on the surface, not in the upper atmosphere.

Paul Gattis | pgattis@al.com 

Christy: Carbon dioxide is a greenhouse gas. When you put more of it in the atmosphere, the radiation budget will respond appropriately. It's just that what we found with the real data is that the way the earth responds is to shed a lot of that heat, not keep it in, which climate models do. So I'd rather base policy on observations than on climate models.

Where is the biggest response to greenhouse gases? It's in the atmosphere, not on the surface. So if you want to measure the response and say that's the greenhouse gas response, you would look in the atmosphere. That's precisely where satellites measure it. So the scientific to how does the world respond is found here. The response of the climate system is stronger in the atmosphere than on the surface.

AL.com: What's it like to be labeled as a "denier" of the dangers of climate change?

Spencer: I don't mind being defined that way. I've done other research that I've published and I always get a little pushback on, Why are you dabbling in that? You're supposed to be in global temperature monitoring. Because of us doing the satellite temperature dataset, people expect that's all we can do. And they pigeonhole you and expect you to stay in that.

I've come to terms with and accepted that this is the most important thing probably I'll be known for. I have to keep doing it, which I don't mind. On the positive side, I could have just been an average researcher who never did anything of note. So I feel blessed to be in the position that I'm in.

Quark model

From Wikipedia, the free encyclopedia

Figure 1: The pseudoscalar meson nonet. Members of the octet are shown in green, the singlet in magenta. The name of the Eightfold Way derives from this classification.

In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the Eightfold Way, the successful classification scheme organizing the large number of lighter hadrons that were being discovered starting in the 1950s and continuing through the 1960s. It received experimental verification beginning in the late 1960s and is a valid effective classification of them to date. The quark model was independently proposed by physicists Murray Gell-Mann,^[1] and George Zweig^[2][3] (also see ^[4]) in 1964. Today, the model has essentially been absorbed as a component of the established quantum field theory of strong and electroweak particle interactions, dubbed the Standard Model.

Hadrons are not really "elementary", and can be regarded as bound states of their "valence quarks" and antiquarks, which give rise to the quantum numbers of the hadrons. These quantum numbers are labels identifying the hadrons, and are of two kinds. One set comes from the Poincaré symmetry—J^PC, where J, P and C stand for the total angular momentum, P-symmetry, and C-symmetry, respectively.

The remaining are flavour quantum numbers such as the isospin, strangeness, charm, and so on. The strong interactions binding the quarks together are insensitive to these quantum numbers, so variation of them leads to systematic mass and coupling relationships among the hadrons in the same flavor multiplet.

All quarks are assigned a baryon number of ⅓. Up, charm and top quarks have an electric charge of +⅔, while the down, strange, and bottom quarks have an electric charge of −⅓. Antiquarks have the opposite quantum numbers. Quarks are spin-½ particles, and thus fermions. Each quark or antiquark obeys the Gell-Mann−Nishijima formula individually, so any additive assembly of them will as well.

Mesons are made of a valence quark−antiquark pair (thus have a baryon number of 0), while baryons are made of three quarks (thus have a baryon number of 1). This article discusses the quark model for the up, down, and strange flavors of quark (which form an approximate flavor SU(3) symmetry). There are generalizations to larger number of flavors.

History

Developing classification schemes for hadrons became a timely question after new experimental techniques uncovered so many of them, that it became clear that they could not all be elementary. These discoveries led Wolfgang Pauli to exclaim "Had I foreseen that, I would have gone into botany," and Enrico Fermi to advise his student Leon Lederman: "Young man, if I could remember the names of these particles, I would have been a botanist." These new schemes earned Nobel prizes for experimental particle physicists, including Luis Alvarez, who was at the forefront of many of these developments. Constructing hadrons as bound states of fewer constituents would thus organize the "zoo" at hand. Several early proposals, such as the ones by Enrico Fermi and Chen-Ning Yang (1949), and by Shoichi Sakata (1956), ended up satisfactorily covering the mesons, but failed with baryons, and so were unable to explain all the data.

The Gell-Mann–Nishijima formula, developed by Murray Gell-Mann and Kazuhiko Nishijima, led to the Eightfold way classification, invented by Gell-Mann, with important independent contributions from Yuval Ne'eman, in 1961. The hadrons were organized into SU(3) representation multiplets, octets and decuplets, of roughly the same mass, due to the strong interactions; and smaller mass differences linked to the flavor quantum numbers, invisible to the strong interactions. The Gell-Mann–Okubo mass formula systematized the quantification of these small mass differences among members of a hadronic multiplet, controlled by the explicit symmetry breaking of SU(3).

The spin-³⁄₂ Ω− baryon, a member of the ground-state decuplet, was a crucial prediction of that classification. After it was discovered in an experiment at Brookhaven National Laboratory, Gell-Mann received a Nobel prize in physics for his work on the Eightfold Way, in 1964.

Finally, in 1964, Gell-Mann, and, independently, George Zweig, discerned what the Eightfold Way picture encodes. They posited elementary fermionic constituents, unobserved, and possibly unobservable in a free form, underlying and elegantly encoding the Eightfold Way classification, in an economical, tight structure, resulting in further simplicity. hadronic mass differences were now linked to the different masses of the constituent quarks.

It would take about a decade for the unexpected nature−−and physical reality−−of these quarks to be appreciated more fully (See Quarks). Counter-intuitively, they cannot ever be observed in isolation (color confinement), but instead always combine with other quarks to form full hadrons, which then furnish ample indirect information on the trapped quarks themselves. Conversely, the quarks serve in the definition of Quantum chromodynamics, the fundamental theory fully describing the strong interactions; and the Eightfold Way is now understood to be a consequence of the flavor symmetry structure of the lightest three of them. To date, no Nobel prize has been awarded to Gell-Mann and Zweig for this discovery.

Mesons

Figure 2: Pseudoscalar mesons of spin 0 form a nonet

Figure 3: Mesons of spin 1 form a nonet

The Eightfold Way classification is named after the following fact. If we take three flavors of quarks, then the quarks lie in the fundamental representation, 3 (called the triplet) of flavor SU(3). The antiquarks lie in the complex conjugate representation 3. The nine states (nonet) made out of a pair can be decomposed into the trivial representation, 1 (called the singlet), and the adjoint representation, 8 (called the octet). The notation for this decomposition is

.

Figure 1 shows the application of this decomposition to the mesons. If the flavor symmetry were exact (as in the limit that only the strong interactions operate, but the electroweak interactions are notionally switched off), then all nine mesons would have the same mass. However, the physical content of the full theory includes consideration of the symmetry breaking induced by the quark mass differences, and considerations of mixing between various multiplets (such as the octet and the singlet).

N.B. Nevertheless, the mass splitting between the η and the η′ is larger than the quark model can accommodate, and this "η–η′ puzzle" has its origin in topological peculiarities of the strong interaction vacuum, such as instanton configurations.

Mesons are hadrons with zero baryon number. If the quark–antiquark pair are in an orbital angular momentum L state, and have spin S, then

• |L − S| ≤ J ≤ L + S, where S = 0 or 1,
• P = (−1)^{L + 1}, where the 1 in the exponent arises from the intrinsic parity of the quark–antiquark pair.
• C = (−1)^{L + S} for mesons which have no flavor. Flavored mesons have indefinite value of C.
• For isospin I = 1 and 0 states, one can define a new multiplicative quantum number called the G-parity such that G = (−1)^{I + L + S}.

If P = (−1)^J, then it follows that S = 1, thus PC= 1. States with these quantum numbers are called natural parity states; while all other quantum numbers are thus called exotic (for example the state J^PC = 0⁻⁻).

Baryons

Figure 4. The S = ¹⁄₂ ground state baryon octet

Figure 5. The S = ³⁄₂ baryon decuplet

Since quarks are fermions, the spin-statistics theorem implies that the wavefunction of a baryon must be antisymmetric under exchange of any two quarks. This antisymmetric wavefunction is obtained by making it fully antisymmetric in color, discussed below, and symmetric in flavor, spin and space put together. With three flavors, the decomposition in flavor is

.

The decuplet is symmetric in flavor, the singlet antisymmetric and the two octets have mixed symmetry. The space and spin parts of the states are thereby fixed once the orbital angular momentum is given.

It is sometimes useful to think of the basis states of quarks as the six states of three flavors and two spins per flavor. This approximate symmetry is called spin-flavor SU(6). In terms of this, the decomposition is

The 56 states with symmetric combination of spin and flavour decompose under flavor SU(3) into

where the superscript denotes the spin, S, of the baryon. Since these states are symmetric in spin and flavor, they should also be symmetric in space—a condition that is easily satisfied by making the orbital angular momentum L = 0. These are the ground state baryons.
The S = ¹⁄₂ octet baryons are the two nucleons (p+, n0), the three Sigmas (Σ+, Σ0, Σ−), the two Xis (Ξ0, Ξ−), and the Lambda (Λ0). The S = ³⁄₂ decuplet baryons are the four Deltas (Δ++, Δ+, Δ0, Δ−), three Sigmas (Σ∗+, Σ∗0, Σ∗−), two Xis (Ξ∗0, Ξ∗−), and the Omega (Ω−).

Mixing of baryons, mass splittings within and between multiplets, and magnetic moments are some of the other questions that the model predicts successfully.

The discovery of color

Color quantum numbers are the characteristic charges of the strong force, and are completely uninvolved in electroweak interactions. They were discovered as a consequence of the quark model classification, when it was appreciated that the spin S = ³⁄₂ baryon, the Δ++, required three up quarks with parallel spins and vanishing orbital angular momentum. Therefore, it could not have an antisymmetric wave function, (due to the Pauli exclusion principle), unless there were a hidden quantum number. Oscar Greenberg noted this problem in 1964, suggesting that quarks should be para-fermions.^[5]
Instead, six months later, Moo-Young Han and Yoichiro Nambu suggested the existence of three triplets of quarks to solve this problem, but flavor and color intertwined in that model--- they did not commute.^[6]

The modern concept of color completely commuting with all other charges and providing the strong force charge was articulated in 1973, by William Bardeen, Harald Fritzsch, and Murray Gell-Mann.^[7][8]

States outside the quark model

While the quark model is derivable from the theory of quantum chromodynamics, the structure of hadrons is more complicated than this model allows. The full quantum mechanical wave function of any hadron must include virtual quark pairs as well as virtual gluons, and allows for a variety of mixings. There may be hadrons which lie outside the quark model. Among these are the glueballs (which contain only valence gluons), hybrids (which contain valence quarks as well as gluons) and "exotic hadrons" (such as tetraquarks or pentaquarks).

Pion

From Wikipedia, the free encyclopedia

Pion

The quark structure of the pion.
Composition	π+: ud π0: uu or dd π−: du
Statistics	Bosonic
Interactions	Strong
Symbol	π+, π0, and π−
Theorized	Hideki Yukawa (1935)
Discovered	César Lattes, Giuseppe Occhialini (1947) and Cecil Powell
Types	3
Mass	π±: 139.57018(35) MeV/c2 π0: 134.9766(6) MeV/c2
Electric charge	π+: +1 e π0: 0 e π−: −1 e
Spin	0
Parity	−1

In particle physics, a pion (short for pi meson, denoted with the Greek letter pi: π) is any of three subatomic particles: π0, π+, and π−. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and are unstable, with the charged pions π+ and π− decaying with a mean lifetime of 26 nanoseconds (2.6×10⁻⁸ seconds), and the neutral pion π0 decaying with a much shorter lifetime of 8.4×10⁻¹⁷ seconds. Charged pions usually decay into muons and muon neutrinos, and neutral pions into gamma rays.
The exchange of virtual pions, along with the vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but are produced commonly in high energy accelerators in collisions between hadrons. All types of pions are also produced in natural processes when high energy cosmic ray protons and other hadronic cosmic ray components interact with matter in the Earth's atmosphere. Recently, detection of characteristic gamma rays originating from decay of neutral pions in two supernova remnant stars has shown that pions are produced copiously in supernovas, most probably in conjunction with production of high energy protons that are detected on Earth as cosmic rays.^[1]

The concept of mesons as the carrier particles of the nuclear force was first proposed in 1935 by Hideki Yukawa. While the muon was first proposed to be this particle after its discovery in 1936, later work found that it did not participate in the strong nuclear interaction. The pions, which turned out to be examples of Yukawa's proposed mesons, were discovered later: the charged pions in 1947, and the neutral pion in 1950.

History

An animation of the nuclear force (or residual strong force) interaction. The small colored double disks are gluons. Anticolors are shown as per this diagram (larger version).

The same process as in the animation with the individual quark constituents shown, to illustrate how the fundamental strong interaction gives rise to the nuclear force. Straight lines are quarks, while multi-colored loops are gluons (the carriers of the fundamental force). Other gluons, which bind together the proton, neutron, and pion "in-flight," are not shown.

Theoretical work by Hideki Yukawa in 1935 had predicted the existence of mesons as the carrier particles of the strong nuclear force. From the range of the strong nuclear force (inferred from the radius of the atomic nucleus), Yukawa predicted the existence of a particle having a mass of about 100 MeV. Initially after its discovery in 1936, the muon (initially called the "mu meson") was thought to be this particle, since it has a mass of 106 MeV. However, later particle physics experiments showed that the muon did not participate in the strong nuclear interaction. In modern terminology, this makes the muon a lepton, and not a true meson. However, some communities of nuclear physicists, continue to call the muon a "mu-meson."

In 1947, the first true mesons, the charged pions, were found by the collaboration of Cecil Powell, César Lattes, Giuseppe Occhialini, et al., at the University of Bristol, in England. Since the advent of particle accelerators had not yet come, high-energy subatomic particles were only obtainable from atmospheric cosmic rays. Photographic emulsions, which used the gelatin-silver process, were placed for long periods of time in sites located at high altitude mountains, first at Pic du Midi de Bigorre in the Pyrenees, and later at Chacaltaya in the Andes Mountains, where they were impacted by cosmic rays.

After the development of the photographic plates, microscopic inspection of the emulsions revealed the tracks of charged subatomic particles. Pions were first identified by their unusual "double meson" tracks, which were left by their decay into another "meson". (It was actually the muon, which is not classified as a meson in modern particle physics.) In 1948, Lattes, Eugene Gardner, and their team first artificially produced pions at the University of California's cyclotron in Berkeley, California, by bombarding carbon atoms with high-speed alpha particles. Further advanced theoretical work was carried out by Riazuddin, who in 1959, used the dispersion relation for Compton scattering of virtual photons on pions to analyze their charge radius.^[2]

Nobel Prizes in Physics were awarded to Yukawa in 1949 for his theoretical prediction of the existence of mesons, and to Cecil Powell in 1950 for developing and applying the technique of particle detection using photographic emulsions.

Since the neutral pion is not electrically charged, it is more difficult to detect and observe than the charged pions are. Neutral pions do not leave tracks in photographic emulsions, and neither do they in Wilson cloud chambers. The existence of the neutral pion was inferred from observing its decay products from cosmic rays, a so-called "soft component" of slow electrons with photons. The π0 was identified definitively at the University of California's cyclotron in 1950 by observing its decay into two photons.^[3] Later in the same year, they were also observed in cosmic-ray balloon experiments at Bristol University.

The pion also plays a crucial role in cosmology, by imposing an upper limit on the energies of cosmic rays surviving collisions with the cosmic microwave background, through the Greisen–Zatsepin–Kuzmin limit.

In the standard understanding of the strong force interaction (called QCD, "quantum chromodynamics"), pions are understood to be the pseudo-Nambu-Goldstone bosons of spontaneously broken chiral symmetry. This explains why the three kinds of pions' masses are considerably less than the masses of the other mesons, such as the scalar or vector mesons. If their current quarks were massless particles, hypothetically, making the chiral symmetry exact, then the Goldstone theorem would dictate that all pions have zero masses. In reality, since the light quarks actually have minuscule nonzero masses, the pions also have nonzero rest masses, albeit almost an order of magnitude smaller than that of the nucleons, roughly^[4] m_π ≈ √v m_q / f_π ≈ √m_q 45 MeV, where m are the relevant current quark masses in MeV, 5−10 MeVs.

The use of pions in medical radiation therapy, such as for cancer, was explored at a number of research institutions, including the Los Alamos National Laboratory's Meson Physics Facility, which treated 228 patients between 1974 and 1981 in New Mexico,^[5] and the TRIUMF laboratory in Vancouver, British Columbia.

Theoretical overview

The pion can be thought of as one of the particles that mediate the interaction between a pair of nucleons. This interaction is attractive: it pulls the nucleons together. Written in a non-relativistic form, it is called the Yukawa potential. The pion, being spinless, has kinematics described by the Klein–Gordon equation. In the terms of quantum field theory, the effective field theory Lagrangian describing the pion-nucleon interaction is called the Yukawa interaction.

The nearly identical masses of π± and π0 imply that there must be a symmetry at play; this symmetry is called the SU(2) flavour symmetry or isospin. The reason that there are three pions, π+, π− and π0, is that these are understood to belong to the triplet representation or the adjoint representation 3 of SU(2). By contrast, the up and down quarks transform according to the fundamental representation 2 of SU(2), whereas the anti-quarks transform according to the conjugate representation 2*.

With the addition of the strange quark, one can say that the pions participate in an SU(3) flavour symmetry, belonging to the adjoint representation 8 of SU(3). The other members of this octet are the four kaons and the eta meson.

Pions are pseudoscalars under a parity transformation. Pion currents thus couple to the axial vector current and pions participate in the chiral anomaly.

Basic properties

Pions are mesons with zero spin, and they are composed of first-generation quarks. In the quark model, an up quark and an anti-down quark make up a π+, whereas a down quark and an anti-up quark make up the π−, and these are the antiparticles of one another. The neutral pion π0 is a combination of an up quark with an anti-up quark or a down quark with an anti-down quark. The two combinations have identical quantum numbers, and hence they are only found in superpositions. The lowest-energy superposition of these is the π0, which is its own antiparticle. Together, the pions form a triplet of isospin. Each pion has isospin (I = 1) and third-component isospin equal to its charge (I_z = +1, 0 or −1).

Charged pion decays

Feynman diagram of the dominating leptonic pion decay.

The π± mesons have a mass of 139.6 MeV/c² and a mean lifetime of 2.6×10⁻⁸ s. They decay due to the weak interaction. The primary decay mode of a pion, with probability 0.999877, is a purely leptonic decay into an anti-muon and a muon neutrino:

π+	→	μ+	+	ν μ
π−	→	μ−	+	ν μ

The second most common decay mode of a pion, with probability 0.000123, is also a leptonic decay into an electron and the corresponding electron antineutrino. This "electronic mode" was discovered at CERN in 1958:^[6]

π+	→	e+	+	ν e
π−	→	e−	+	ν e

The suppression of the electronic mode, with respect to the muonic one, is given approximately (to within radiative corrections) by the ratio of the half-widths of the pion–electron and the pion–muon decay reactions:

and is a spin effect known as the helicity suppression. Its mechanism is as follows: The negative pion has spin zero, therefore the lepton and antineutrino must be emitted with opposite spins (and opposite linear momenta) to preserve net zero spin (and conserve linear momentum). However, the antineutrino, due to very high speed, is always right-handed, so this implies that the lepton must be emitted with spin in the direction of its linear momentum (i.e., also right-handed). If, however, leptons were massless, they would only exist in the left-handed form, just as the neutrino does (due to parity violation), and this decay mode would be prohibited. Therefore, suppression of the electron decay channel comes from the fact that the electron's mass is much smaller than the muon's. The electron is thus relatively massless compared with the muon, and thus the electronic mode is almost prohibited.^[7]

Hence, electronic mode decay favors the left-handed symmetry and inhibits this decay channel. Measurements of the above ratio have been considered for decades to be tests of the V − A structure (vector minus axial vector or left-handed lagrangian) of the charged weak current and of lepton universality. Experimentally this ratio is 1.230(4)×10⁻⁴.^[8]

Besides the purely leptonic decays of pions, some structure-dependent radiative leptonic decays (that is, decay to the usual leptons plus a gamma ray) have also been observed.

Also observed, for charged pions only, is the very rare "pion beta decay" (with probability of about 10⁻⁸) into a neutral pion plus an electron and electron antineutrino (or for positive pions, a neutral pion, positron, and electron neutrino).

π−	→	π0	+	e−	+	ν e
π+	→	π0	+	e+	+	ν e

The rate at which pions decay is a prominent quantity in many sub-fields of particle physics, such as chiral perturbation theory. This rate is parametrized by the pion decay constant (ƒ_π), related to the wave function overlap of the quark and antiquark, which is about 130 MeV.^[9]

Neutral pion decays

The π0 meson has a slightly smaller mass of 135.0 MeV/c² and a much shorter mean lifetime of 8.4×10⁻¹⁷ s, in comparison to the charged pion. It decays in an electromagnetic force process. The main decay mode, with a branching ratio BR=0.98823, is into two photons:

π0

→

2 γ.

Its second largest decay mode (BR=0.01174) is the Dalitz decay (named after Richard Dalitz), which is a two-photon decay with an internal photon conversion resulting a photon and an electron-positron pair in the final state:

π0

→

γ

+

e−

+

e+.

The third largest established decay mode (BR=3.34×10⁻⁵) is the double Dalitz decay, with both photons undergoing internal conversion which leads to further suppression of the rate:

π0

→

e−

+

e+

+

e−

+

e+.

The fourth largest established decay mode is the loop-induced and therefore suppressed (and additionally helicity-suppressed) leptonic decay mode (BR=6.46×10⁻⁸):

π0

→

e−

+

e+.

The neutral pion has also been observed to decay into positronium with a branching fraction of the order of 10⁻⁹. No other decays modes are experimentally established. The braching fractions above are the PDG central values, and their uncertainties are not quoted.

Pions

Particle name	Particle symbol	Antiparticle symbol	Quark content[10]	Rest mass (MeV/c2)	IG	JPC	S	C	B'	Mean lifetime (s)	Commonly decays to (>5% of decays)
Pion[8]	π+	π−	ud	139.570 18 ± 0.000 35	1−	0−	0	0	0	2.6033 ± 0.0005 × 10−8	μ+ + ν μ
Pion[11]	π0	Self	[a]	134.976 6 ± 0.000 6	1−	0−+	0	0	0	8.4 ± 0.6 × 10−17	γ + γ

^[a] ^ Make-up inexact due to non-zero quark masses.^[12]