The Sub-Atomic Zoo

Index

Definitions
Fundamental Particles
Ordinary and Strange Baryons & Mesons
Charmed and Bottom Baryons and Mesons

DEFINITIONS

Particles: Real and Virtual
- Tangible and measurable matter and energy consist of real articles.
- Virtual Particles exist because of quantum uncertainty (ΔsΔp≥h), with mass & energy in inverse proportion to the amount of time they exist (ΔEΔt≥h). This means that a large amount of energy can be borrowed for a short length of time, or a small amount of energy for a longer time, for particles to exist. Virtual massless particles (photons, gravitons) exist permanently and so give to their fields (electromagnetic, gravitational) an infinite range. Virtual particles cannot be observed but mediate all the interactions of ordinary matter and can become real by acquiring real energy, and they occur spontaneously even in empty space. Why there would not therefore be an infinite number of virtual particles everywhere all the time was then a good question for quantum mechanics. Reducing the infinities to finite numbers required a mathematical technique ("renormalization") whose validity has been questioned but which has evidently produced, in most cases, verifiable values.
Particles: Bosons & Fermions
- Bosons: virtual bosons mediate interactions of the 4 forces (gravity, electromagnetism, the weak nuclear force, and the strong nuclear force); real bosons carry energy away from interactions or carry energy to particles; all have integer spin (0 = scalar, 1 = vector, 2 = tensor) and do not obey the Pauli Exclusion Principle. Bosons with even spin mediate attractive forces (gravity); odd spin bosons mediate forces with attraction and repulsion (magnetism). Bosons are named after the Indian physicist and mathematician Satyendra Nath Bose (1894-1974), who collaborated with Albert Einstein (1879-1955) in formulating the statistics of this kind of particle (Bose-Einstein statistics).
- Fermions: comprise matter; all have half integer spin (1/2, 3/2, etc.) and obey the Pauli Exclusion Principle (no two with identical values can belong to the same quantum system) -- named for Wolfgang Pauli (1900-1958). Without the Pauli Exclusion Principle, the chemical elements would all have the same chemical properties, since all electrons could collapse to the lowest, and identical, energy states. The actual hierarchy of states for electrons not only enable atoms to interact differently, with different chemcial properties, but it even means that atoms will have different sizes -- higher energy electrons have larger orbitals. Fermions are named after the Italian physicist Enrico Fermi (1901-1954), who collaborated with Paul Dirac (1902-1984) in formulated the statistic of this kind of particle (Fermi-Dirac statistics).
Particles: Leptons & Hadrons
- Leptons: fermions that do not interact strongly, i.e. they do not "see" the strong nuclear force. They still interact in terms of mass, electromagnetism, and the weak nuclear force. The fundamental leptons (electron, muon, tau, and three kinds of neutrinos) correspond one for one with fundamental quarks.
- Hadrons: bosons & fermions that do interact strongly, i.e. they "see" the strong nuclear force. Gluons & quarks are the fundamental hadrons. Quarks constitute mesons and baryons, which are all hadrons.
Particles: Mesons & Baryons
- Mesons: bosons composed of quark and anti-quark. Gluons have a very short range and so really only mediate the strong interaction between quarks. Baryons interacting over nuclear distances will exchange mesons, which were the original theoretical mediating particles to explain the nuclear force.
- Baryons: fermions composed of three quarks or three anti-quarks. Baryons are the heavy, strongly interacting particles of the atomic nucleus. The only stable baryons in a nuclear environment are protons and neutrons. Only protons are indepenently stable. Some theories in the l970's predicted proton decay, but this has never been observed.
Particles: Matter and Anti-Matter
- Particles and anti-particles are opposite in every property except mass. Anti-matter may even be understood as ordinary particles with negative energy or as ordinary particles travelling backwards in time. When particles encounter their anti-particles, both become energy (bosons). Neutral bosons tend to be their own anti-particles (K mesons, Kaons, are an exception). Whether matter and anti-matter are equally fundamental in their opposition is an open and mysterious question, since the universe consists mostly of matter and not anti-matter -- leading to speculations about corresponding anti-matter universes. It does seem, however, that the symmetry of matter and anti-matter is not perfect: In the system of neutral Kaons, the neutral anti-Kaon (K̅⁰) preferentially becomes a neutral Kaon (K⁰), but not the other way around. There is no accepted explanation for this, but it seems to be a well established result. Note: On this webpage, anti-particles are indicated by either underlined (e.g. "e" -- the anti-electron, or positron) or overlined (e.g. "e̅") letters -- overlined text (more the convention) previously required graphic items (so underlining was used where this was more convenient) but now can be shown with Unicode overlining.
Particles: Wave/Particle Duality
- All fundamental particles (gauge bosons & quarks) are Dirac Point Particles, which means that they are not extended in space. A very simple physical consideration motivates this: If charged particles were extended, different parts of the particle would exert a repulsive force against each other, which at small distances would approach strengths sufficient to tear apart the particle. Since a geometrical point cannot spin, quantum spin (quanta of angular momentum) cannot be understood as macroscopic spinning. All particles, at the same time, can be understood as waves rather than as localized particles. Waves are extended and can be in two places at once, which often makes it seem that particles are in two places at once. There is no resolving this paradox, so far; the nature of the particles must be understood as one or the other, depending on the context, never as both at once (called Complementarity by Niels Bohr, 1885-1962). The context, however, suggests a Kantian metaphysical interpretation: Particles behave as waves when unobserved, i.e. outside of the necessary conditions of experience, but behave as particles when observed, i.e. when appearing under the formal requirements of phenomena. The clearest discussion of this duality is in The Emperor's New Mind by Roger Penrose. There is also the question about what a wave is. Quantum mechanics doesn't have to decide, since the square of the wave function gives the probability of finding the particle. The wave nature of matter, however, was taken very seriously by Louis de Broglie (1892-1987), who proposed it. The volume of atoms, and of particles like protons, indeed, is maintained by electrons, or quarks, as three dimensional standing waves (the diameter of atoms is about 10^-10 meters, which is then used as a unit of measurement, the Angstrom, ; while the size of nuclei is measured in Fermis, fm (the femtometer) = 10^-15).
Particles: Mixing or superposition of particles
- In quantum mechanics, if you can't tell the difference between two things, then they become confused -- in reality and not just in knowledge. This is especially true with neutral mesons, which are bound states of particles and anti-particles (uu̅, dd̅, ss̅, etc.), with all properties canceling each other out. There are in fact the number of particles expected, but their quark constituents are in a sense shared, mixed, or superimposed on each other. Thus the neutral pion contains both uu̅ and dd̅ combinations: π((dd̅-uu̅)/√2)⁰. A similar mixing occurs with the neutral Kaons, where the particles do not occur pure but are mixed into two forms, a short-lived K₁=(K⁰+K̅⁰)/√2 and a longer-lived K₂=(K⁰-K̅⁰)/√2. The mixing of the Kaons is especially important because of their anomalous decays. How particles can "mix" in this fashion is extremely paradoxical, unless the particles are viewed as waves which, like waves in water or other media, can be added together.

FUNDAMENTAL PARTICLES

I. Gauge (Field) Bosons
- graviton (spin 2) -- gravity [unobserved]
- γ [gamma] photon (spin 1) -- electromagnetic force
- W⁺ Z⁰ W^- (spin 1) -- weak force
  The weak force violates parity, which means that it is not symmetrical with respect to spin: only left-handed (-1/2) particles & right-handed (+1/2) anti-particles can "see" the weak force. If neutrinos are massless, they travel the speed of light and cannot change their spin; all neutrinos are left-handed and anti-neutrinos right-handed.
- G gluon (spin 1) -- strong force (manifest as "color charge"):
  
  Strong Charges r = red g = green b = blue
  
  r̅ = anti-red G_{1(rr̅/gg̅/bb̅)} G_{g-r (gr̅)} G_{b-r (br̅)}
  
  g̅ = anti-green G_{r-g (rg̅)} G_{b-g (bg̅)}
  
  b̅ = anti-blue G_{r-b (rb̅)} G_{g-b (gb̅)} G_{2 (rr̅/gg̅/bb̅)}
H⁺ H⁰ H^- Higgs boson (spin 0)

Strong Charges	r = red	g = green	b = blue
r̅ = anti-red	G_{1(rr̅/gg̅/bb̅)}	G_{g-r (gr̅)}	G_{b-r (br̅)}
g̅ = anti-green	G_{r-g (rg̅)}		G_{b-g (bg̅)}
b̅ = anti-blue	G_{r-b (rb̅)}	G_{g-b (gb̅)}	G_{2 (rr̅/gg̅/bb̅)}

II. Fundamental Fermions (spin 1/2)

Leptons	family	Quarks	family
e^-, electron	1	d^-1/3, down quark	1
_Lν^e, electron neutrino	1	u^+2/3, up quark	1
μ^-, muon	2	s^-1/3, strange quark	2
_Lμ^ν, muon neutrino	2	c^+2/3, charm quark	2
τ^-, tau	3	b^-1/3, bottom (beauty) quark	3
_Lν^τ, tau neutrino	3	t^+2/3, top (truth) quark	3
Anti-Leptons	family	Anti-Quarks	family
e̅⁺, positron	1	d̅^+1/3, anti-down quark	1
_R^e, electron anti-neutrino	1	u̅^-2/3, anti-up quark	1
μ̅ ⁺, anti-muon	2	s̅^+1/3, anti-strange quark	2
_R^μ, muon anti-neutrino	2	c̅^-2/3, anti-charm quark	2
⁺, anti-tau	3	b̅^+1/3, anti-bottom quark	3
_R^τ, tau anti-neutrino	3	t̅^-2/3, anti-top quark	3

III. Other Particles & Recent Theories

New theories, super-gravity, super-symmetry, super-strings, etc. (Grand Unified Theories, "G.U.T.'s"), predict further particles (e.g. spin 1 and spin 0 versions of the graviton -- "graviphoton" and "graviscalar"), especially whole classes of fermions that correspond to every boson (the gravitino, Wino, Zino, photino, gluino, & higgsino) and bosons that correspond to every fermion (the squarks & sleptons). There are also possible massive magnetic monopoles, long predicted.

None of these particles has ever been verifiably observed, and the principal prediction of the GUT's, the decay of the proton, after more than twenty years of observation in large scale experiments, has been, to general distress, discomfirmed. That the graviton itself has not been detected is symbolic of the incompleteness of physics, as a division remains between Einstein's successful Relativistic theory of gravity and the successful quantum theories of all the other interactions. What must be regarded as the actual failure of the unification of the strong and electroweak force puts the whole business of particle physics at the awkward moment. The String theories now seem to be the rage, but it is not clear what sorts of predictions, if any, they make -- physicists are getting positively defensive about it. It is still necessary to postulate the decay of the proton, if the predominance of matter over anti-matter in the universe is to be economically explained, but how and under what conditions this would occur is pretty much up in the air.

Much now seems to rest on the detection of the Higgs particles and their field. One of the great mysteries of particle physics is why charge and other quantum attributes occur in integer quantities while mass, the most fundamental property of all matter and energy (and, not coincidentally, the basis of gravitation), does not. The Higgs field is supposed to handle this, though it remains an obscure area little explained in population presentations. Until the String theories make some testable predictions, the key experimental test for particle physics may be the detection of a Higgs boson. After decades in which whole families of new particles spilled out of the accelerators, the well now seems to have dried up, and we await in hushed anticipation the appearance of even one....

As the Higgs Boson has now reportedly been observed in the Large Hadron Collider at CERN in Europe, I am still waiting for an intuitive explanation, or any conceptual explanation, of how the Higgs Field works and if it operates on originally integer units of mass.

ORDINARY AND STRANGE BARYONS & MESONS

Spin: The spin of baryons and mesons is determined by the addition of the spin of constituent quarks. Quarks are 1/2 spin fermions; the spin of each may reinforce or may cancel the spin of the others. Thus, three quarks may all reinforce to produce a 3/2 spin baryon, or one may cancel another out to produce a 1/2 spin baryon. A quark and an anti-quark may reinforce each other to produce a 1 spin meson, or they may cancel out to produce a 0 spin meson. These systems with different spin are different particles with different masses. Nor are the 3/2 spin particles always mere "resonances" (called "star" particles, as with the star Sigmas, ^*+, etc.) of the 1/2 spin ones -- where resonances are particles that are simply excited to a higher state by the addition of energy (various 5/2, 7/2, & higher resonances count as separate particles but are nothing more mysterious than excitations): For some 3/2 spin particles do not have 1/2 spin counterparts, and one 3/2 spin particle has two 1/2 spin counterparts. This peculiarity occurs because of the question of the identity of the quark that may be the odd one out in canceling the spin of another. A different odd quark makes for a different particle. But against this is the quantum principle that identical quarks are absolutely identical, so that if there are two or three identical quarks in a baryon, it is impossible to decide, if one quark is the odd one, which is which. This means in such a case that there cannot be different odd quarks and so there cannot be different particles. The alternatives can be displayed by examining the possible "magnetic substates" of the particles: when a particle is put in a magnetic field, its spin breaks down into a preferential orientation in the field. The number of possible magnetic orientations is simply the number of integer steps from the positive value of the spin to the negative value of the spin (or 2x+1 states, where x is the spin). Thus 1/2 spin breaks down into 2 substates (+1/2 & -1/2); 3/2 spin into 4 (+3/2, +1/2, -1/2, & -3/2); 0 spin into 1 (0); 1 spin into 3 (+1, 0, -1); 2 spin into 5 (+2, +1, 0, -1, -2), etc. The magnetic substates of excited electrons are responsible for the number of members in each grouping within the period table of chemical elements. Although all the spins reinforce in 3/2 spin baryons, nevertheless, in the +1/2 and -1/2 spin magnetic substates, there will be an odd quark (where R stands for a right-handed or +1/2 quark and L for a left-handed or -1/2 quark):

        3/2          1/2          1/2
        RRR=+3/2
        RLR=+1/2     RRL=+1/2     LRR=+1/2
        LRL=-1/2     LLR=-1/2     RLL=-1/2
        LLL=-3/2

        qqq
        qqr          qqr
        qrs          qrs          qrs

Where the baryon consists of three identical quarks (qqq), the RLR, RRL, and LRR states are indistinguishable, and so only one particle will exist, the 3/2 spin one. Where the baryon contains two identical quarks (qqr), only the RLR and LRR states are indistinguishable, so two particles are possible, one 3/2 spin and one 1/2 spin. However, where the baryon contains three different quarks (qrs), no states are indistinguishable and three different particles are possible. Hence the seemingly anomalous presence of the Lambda,