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Proposing New Particles

lpetrich

lpetrich

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Outlook for Supersymmetry physicist John Ellis states on the last page "If you have a problem, postulate a new particle:" then making a table of them. I have made some revisions and additions.

[table="class:grid"]
[tr][td]Problem[/td][td]Particle[/td][td]Proposed[/td][td]Detected[/td][/tr]
[tr][td]Unification of electric and magnetic fields[/td][td]Electromagnetic waves[/td][td]1865[/td][td]prehistoric, 1887[/td][/tr]
[tr][td]QM and special relativity[/td][td]Antimatter[/td][td]1928[/td][td]1932[/td][/tr]
[tr][td]Nuclear spectra[/td][td]Neutron[/td][td]1920[/td][td]1932[/td][/tr]
[tr][td]Continuous spectrum in β decay[/td][td]Neutrino[/td][td]1930[/td][td]1956[/td][/tr]
[tr][td]Nucleon-nucleon interactions[/td][td]Pion[/td][td]1935[/td][td]1947[/td][/tr]
[tr][td]Absence of lepton-number violation[/td][td]Second neutrino[/td][td]Late 1940's[/td][td]1962[/td][/tr]
[tr][td]Flavour SU(3)[/td][td]Ω baryon[/td][td]1962[/td][td]1964[/td][/tr]
[tr][td]Flavour SU(3)[/td][td]Quarks[/td][td]1962[/td][td]1968 - early 1970's[/td][/tr]
[tr][td]Flavor-changing neutral currents[/td][td]Charm quark[/td][td]1970[/td][td]1974[/td][/tr]
[tr][td]CP violation[/td][td]Third generation of quarks[/td][td]1973[/td][td]1977 (b), 1995 (t)[/td][/tr]
[tr][td]Strong dynamics[/td][td]Gluons, QCD[/td][td]1972[/td][td]1979[/td][/tr]
[tr][td]Weak interactions[/td][td]W±, Z0[/td][td]1968[/td][td]1983[/td][/tr]
[tr][td]Electroweak renormalizability[/td][td]Higgs particle[/td][td]1964[/td][td]2012[/td][/tr]
[tr][td]Space-time as a dynamical entity[/td][td]Gravitational waves[/td][td]1916[/td][td]1981 (indirect), 2015[/td][/tr]
[/table]

Here are some still-undetected particles.
[table="class:grid"]
[tr][td]Problem[/td][td]Particle[/td][td]Proposed[/td][/tr]
[tr][td]Naturalness[/td][td]Supersymmetry partners[/td][td]1971[/td][/tr]
[tr][td]Strong CP[/td][td]Axion[/td][td]1977[/td][/tr]
[tr][td]Dark matter[/td][td]WIMP[/td][td]1980's?[/td][/tr]
[tr][td]Cosmic inflation[/td][td]Inflaton[/td][td]1980[/td][/tr]
[/table]
 
Isn't the inflationary model now included in the current (although as far as I know, still unconfirmed) particle model?

I admit it's been a decade or so since I've looked.
 
Worldtraveller said:
Isn't the inflationary model now included in the current (although as far as I know, still unconfirmed) particle model?
Click to expand...
In the Standard Model? No. The inflaton (no second i) is most likely a non-Standard-Model particle.

[0710.3755] The Standard Model Higgs boson as the inflaton
We argue that the Higgs boson of the Standard Model can lead to inflation and produce cosmological perturbations in accordance with observations. An essential requirement is the non-minimal coupling of the Higgs scalar field to gravity; no new particle besides already present in the electroweak theory is required.
Click to expand...
An extra interaction that would make the SM Higgs particle change the effective gravitational constant, something like the extra field in the Generalized Brans-Dicke theory. So it would require a departure from general relativity.

In any case, the energy scale of the inflationary phase is something like 10^(15) GeV, not much less than that of gravity, 10^(19) GeV.
 
I should add some more:

[TABLE="class: grid"]
[TR]
[TD]Problem
[/TD]
[TD]Particle
[/TD]
[TD]Proposed
[/TD]
[/TR]
[TR]
[TD]Neutrinos' low but nonzero masses
[/TD]
[TD]Very massive neutrinos
[/TD]
[TD]1977
[/TD]
[/TR]
[TR]
[TD]Standard-Model particle patterns
[/TD]
[TD]GUT particles
[/TD]
[TD]1974
[/TD]
[/TR]
[/TABLE]

These particles, like the inflaton, are expected to be very massive, roughly around 10^(15) GeV.
 
The history behind these discoveries is interesting.

Electromagnetic waves first. In the early 19th cy, research into electric and magnetic fields had yielded several equations:

e0*(D.E) = den -- Coulomb, Laplace, Poisson, Gauss
D.B = 0 -- Faraday (Gauss)
DxB = m0*(curr) -- Ampere, Biot-Savart
DxE = - dB/dt -- Faraday

(D = space derivative, E = electric field, B = magnetic field, e0 and m0 = constants relating charges/currents and fields)

In the 1850's, James Clerk Maxwell put them together and concluded that there is a missing piece. If one takes D.(third equation), one finds D.curr = 0

Integrating over some volume gives integral of curr.d(surface area) = 0

But there is a BIG problem. This result is only correct for constant electric charge. The more general equation for electric-charge conservation is
d(den)/dt + D.curr = 0

or d(electric charge in volume)/dt + integral of curr.d(surface area) = 0
Change of electric charge in a volume = amount of electric current going in

So Maxwell decided that the third equation was incomplete. He added a term that he called a "displacement current":
DxB = m0*(curr + e0*(dE/dt))

One can now calculate D.(third equation) and insert the value of D.E from the first equation. That gives the proper charge-conservation equation.
 
The next thing that one can do is to work out equations in only one field. Ignoring charges/currents, one finds

D^2(E,B) = (e0*m0)*(d^2(E,B)/dt^2)

It's a wave equation for a wave with speed c given by 1/sqrt(e0*m0).

JCM noticed that this was close to the then-measured speed of light in a vacuum, and visible-light polarization was consistent with it being an electromagnetic wave. This why I listed electromagnetic waves as "prehistoric", though that was long before anyone knew what visible light is. The second date is for Heinrich Hertz's generation of radio waves with electric currents.

Some relatively recent technology provides a very direct demonstration of the electromagnetic nature of visible light: synchrotron light sources. They make radio waves, infrared light, visible light, ultraviolet light, and X-rays. All from making electrons swerve with a magnetic field. What is a Light Source? – Lightsources.org

-

Advancing to the next one, by the 1920's, special relativity was considered well-established. That raised the issue of how one constructs a relativity-friendly version of Schroedinger's wavefunction equation.

The first attempt was made by Oskar Klein and Walter Gordon, Vladimir Fock, Johann Kudar, Théophile de Donder and Frans-H. van den Dungen, and Louis de Broglie, all in 1926. Named after the first two, the Klein-Gordon equation is a relativistic wave equation for a massive particle.

If one works out a version for particles with electric charge q, one finds solutions with charges +q and -q.

In 1928, Paul Adrien Maurice Dirac worked out an equation suitable for spin-1/2 particles like electrons, an equation that was more-or-less the square root of the KG equations. For charge q, he found solutions for charges +q and -q there also.

So in addition to ordinary electrons, there must exist positively-charged electrons. They were indeed discovered in cosmic-ray experiments in 1932. They were the first antimatter ever observed. Antimatter versions of numerous other elementary particles have since been made, and elementary-particle experiments routinely make it alongside ordinary matter. This doing so is an excellent illustration of Einstein's famous equation: E = m*c^2.

Antimatter is not some kind of bizarro matter. Instead it is just like ordinary matter with some properties' signs reversed. Macroscopic antimatter will behave just like macroscopic ordinary matter, though it is difficult to make antimatter in macroscopic quantities, and to store it once it is made. But this extreme similarity is well-tested at the level of elementary particles.
 
The "nuclear spectra" bit is a story that starts in the early 19th cy.

In 1808, John Dalton published a table of atomic weights, done by assuming small-number proportions in compounds. Thus, he believed that water is HO, though we now know it to be H2O. In 1815 and 1816, William Prout noticed that several atomic weights were close to integer multiples of that of hydrogen. But that regularity was not perfect. For instance, chlorine has 35.45.

A further difficulty was discovered by radiochemist Frederick Soddy. He discovered that some "radioelements" could not be chemically separated, and that there are some 40 radielements between uranium and lead, and only 11 periodic-table slots between them. Also, alpha decay reduced the atomic number by 2 and the atomic weight by 4, and beta decay increased the atomic number by 1 and left the atomic weight with very little change. Thus, an alpha and two betas would produce an element with the same atomic number and atomic weight lower by 4. Thus, isotopes.

Variations in the atomic weight of lead suggested origins from different radioactive elements.

While that was going, J.J. Thomson discovered that the element neon has two isotopes, Ne-20 and Ne-22. Then F.W. Aston discovered numerous other isotopes. For instance, chlorine is 76% Cl-35 and 24% Cl-37, thus accounting for that odd atomic-weight value.

He also discovered that many elements' isotopes have atomic weights that are approximately integer multiples of the H-1 atomic weight, the "whole number rule", a modification of Prout's hypothesis.


Then Ernest Rutherford started doing nuclear reactions, attacking nitrogen nuclei with alpha particles. Protons would often emerge, and ER became convinced that atomic nuclei contain them. But they also have multiples of the H-1 atomic weight that cannot be accounted for, and ER decided that that was due to some particle that he called a "neutron", from it being electrically neutral.

It could not be some nucleus-sized bound state of an electron, because that would bump up its mass to well in excess of what was observed.

This particle was discovered by James Chadwick in 1932, and it fit perfectly.
 
Thanks, lpetrich. Wish I had a better mathematical mind and could have continued my Physics studies. US education in Maths is better than what we have in India. but yes. we produce some excellent stuff off and on.
 
Now to the prediction of the pion. By the 1920's, it was evident that atomic nuclei were composite entities, with the exception of the hydrogen-1 one, the proton. But whatever holds them together must be much stronger than the electromagnetic force. It must also be a very short-range force. A nucleus can be approximated with a constant-density fluid of nuclear matter, and one can derive a successful "semi-empirical math formula" with that approximation. Each nucleon interacts with its neighbors, but at the surface, a nucleon has neighbors over only some directions. That makes it bound less than interior nucleons. The SEMF has a volume term, from each nucleon only interacting with its neighbors and an area term, for the partial presence of neighbors for surface nucleons.

In 1935, Hideki Yukawa proposed that nucleons interact by making virtual particles between them, in the fashion of electromagnetic interactions being virtual photons. If the particle has a mass, then the interaction has an exponential falloff with the rate being the mass in reciprocal-length quantum-mechanical units: m*c/hbar (mass m, speed of light c, hbar = (Planck's constant) / (2*pi)).

A year later, a particle was discovered in cosmic rays that had close to the right mass, about 106 MeV or 206 electron masses.

But that particle, now called a muon, was not strongly interacting, and if anything, it interacted just like an electron. So the muon wasn't it.

But in 1947, a kind of particle was discovered in cosmic-ray experiments that fit the bill: the pion. A charged one has mass 140 MeV and a neutral one 135 MeV. That's about right for Yukawa's predictions.

It turns out that the strong interaction is more complicated than that, but exchange of pions seems to be a reasonable long-distance approximation.
 
Now to the continuous spectrum of electron energies in beta decay. Electrons don't come off at one energy but over a full range of energies from zero to some maximum. This means that some energy must be missing.

Angular momentum has a similar story here. From the quantum-mechanical theory of angular momentum, two angular momenta j1 and j2 combine to make a total angular momentum between |j2 - j1| and (j1 + j2) inclusive, in integer steps between those bounds.

The initial AM of a beta decay is that of a neutron, 1/2, and the final AM is that of a proton, 1/2 and an electron, 1/2, combining to make 0 or 1. So there is some missing angular momentum there also, (integer) + 1/2.

In 1930, Wolfgang Pauli proposed that the missing energy and angular momentum were going into a particle that he called a neutron, but that name soon became used for the neutral relative of the proton. Enrico Fermi introduced "neutrino" for this energy and angular-momentum sink, and that name stuck.

From the distribution of beta-decay energies, the neutrino's mass had to be very small, and from initial and final angular momenta, its spin had to be 1/2 + (some integer), and likely 1/2 itself.

Neutrinos from a nuclear reactor were detected in 1956, though strictly speaking, they were antineutrinos and not ordinary ones. They had small mass and spin 1/2.

They were produced by beta decays of fission products in that reactor. Light nuclides, like C-12, N-14, and O-16, tend to be about half-half protons and neutrons, but heavy ones have more neutrons than protons, because the protons electrostatically repel each other and the neutrons don't. Uranium-238 has 92 protons and 146 neutrons, and U-235 92 p's and 143 n's. That gives fission products an excess of neutrons, and that makes them radioactive. The excess of neutrons causes a lot of beta decays:
n -> p + e + nu* (antineutrino)

Some of these antineutrinos nu* are energetic enough to induce this reaction in protons in some nuclei:
p + nu* -> n + e+ (positron)

The positrons then find electrons and disappear with those electrons into gamma rays, usually 2 or 3 of them.

Not many, but enough to observe.
 
Returning to muons, they decay into electrons, but here also, their decays have missing energy.
Muon -> electron + photon (gamma ray) + (nothing else)

is *very* rare, with the most recent upper limit being less than one in a trillion decays.

This means that muons also decay into some difficult-to-detect particles. These particles turned out to be neutrinos, as was discovered in 1962. So there must be two flavors of neutrinos, just as there are two flavors of electrons, ordinary electrons and muons.

A third electron flavor, the tau lepton, was discovered in 1975, and its neutrino was observed in 2000.


Turning to neutrinos' masses, they turn out to be misaligned with their weak-interaction flavors, and this misalignment produces neutrino-flavor oscillations. These oscillations account for how solar neutrinos are detected at 1/3 of their expected amount. En route, they oscillate between electron, muon, and tau flavors, and they are left at only about 1/3 electron flavor.

The more massive mass states have masses something like 0.03 eV, less than 10^(-7) of an electron's mass.
 
Turning to hadrons, the first ones discovered were the proton and the neutron. In 1932, Werner Heisenberg noted that they have two nearly equal masses and that the strong interactions between them treated them as different but otherwise identical. He proposed that they have an approximate symmetry, "isotopic spin" or "isobaric spin", or "isospin" for short, from a mathematical analogy between protons and neutrons as nucleon flavors, and up and down spin-1/2 states.

When pions were discovered, their strong interactions were also discovered to respect isospin symmetry.

Some particles do not last long enough to travel across detectors. They are apparent as spikes in reaction-product quantities, and for that reason, are often called "resonances". One of them is the delta baryon, a spin-3/2, nucleon-like particle.

Isospin multiplets:
  • Nucleon: 1/2 -- proton 1/2, neutron -1/2
  • Pion: 1 -- pi+ 1, pi0 0, pi- -1
  • Delta: 3/2 -- D++ 3/2, D+ 1/2, D0 -1/2, D- -3/2
By the 1950's, a pattern was evident:
  • The strong interaction respects both flavor and isospin
  • Electromagnetism respects flavor but breaks isospin
  • The weak interaction breaks both flavor and isospin
  • Particle masses respect flavor but weakly break isospin
In the early 1950's, a particle called the kaon was discovered that had a strange property. It could easily be produced with the strong interaction, but it lasted too long to have a strong-interaction decay. Instead, it has a weak-interaction decay. Some other particles were discovered to have this property, and that led to the recognition of a new quantum number: strangeness.

The strong and electromagnetic interactions respect strangeness, but the weak interaction doesn't.

How does strangeness combine with isospin? Surprisingly well.

Isospin has symmetry group SU(2), and combined with strangeness, the two have symmetry group SU(3). It has eight generators, thus the term "Eightfold Way" for it. This symmetry group was proposed in 1962 by Murray Gell-Mann and Yuval Ne'eman.

The spin-1/2 light baryons by isospin (total, projected) and strangeness:
  • S = 0 -- nucleon+ (proton) (1/2,1/2), nucleon0 (neutron) (1/2,-1/2)
  • S = -1 -- sigma+ (1,1), lambda0 (0,0), sigma0 (1,0), sigma- (1,-1)
  • S = -2 -- xi0 (1/2,1/2), xi- (1/2,-1/2)
The spin-3/2 light baryons:
  • S = 0 -- delta++ (3/2,3/2), delta+ (3/2,1/2), delta0 (3/2,-1/2), delta- (3/2,-3/2)
  • S = -1 -- sigma-x+ (1,1), sigma-x0 (1,0), sigma-x- (1,-1)
  • S = -2 -- xi-x0 (1/2,1/2), xi-x- (1/2,-1/2)
  • S = -3 -- omega- (0,0)
But in 1962, the omega baryon was a prediction of this flavor-symmetry model. It was discovered in 1964.

What might underlie this flavor symmetry? Some new elementary particle? It got called a quark from "Three quarks for Muster Mark", though another physicist proposed "ace" for them.

But in this model, a quark had a super weird property. While it was expected to have spin 1/2, its spin-flavor combinations were what one would expect from an integer-spin particle (Bose-Einstein statistics), rather than from a half-odd-spin particle (Fermi-Dirac statistics).
 
The quarks by quantum numbers (electric charge, isospin, i.s.-projection, strangeness):
  • up: +2/3, 1/2, +1/2, 0
  • down: -1/3, 1/2, -1/2, 0
  • strange: -1/3, 1/2, -1/2, -1
The light baryons get some nice patterns:
  • Spin 1/2: uud, udd - uus, uds, dds - uss, dss
  • Spin 3/2: uuu, uud, udd, ddd - uus, uds, dds - uss, dss - sss
Some mesons' quark contents:
  • pion: pi+: ud*, pi0: (uu*-dd*)/sqrt(2), pi-: du*
  • kaon: K+: us*, K0: ds*, K0*: sd*, K-: su*
The neutral pion is a mixed state, a mixture of up-antiup and down-antidown states.

What sort of physical reality did quarks have? An answer came from a rather unexpected source. In the late 1960's, experiments started at the Stanford Linear Accelerator Center to probe the structure of protons with accelerated electrons. It turned out that protons are composite, composed of "partons" as they became named. The partons that interacted with electrons fitted the expected quark content of protons. So quarks were like other elementary particles, and not just SU(3) flavor symmetry.
 
Returning to the weak interaction, Nicola Cabibbo proposed in 1963 that up quarks map onto a mixture of down and strange quarks, the mixture given by the "Cabibbo angle", an angle of about 13d - mostly down and some strange. So both the down and the strange quarks can decay into the up quark, and vice versa.

Expanding on the weak interactions, neutral kaons have a weird property. K0 and K0* are a mixture of long-lived and short-lived states, so K0 and K0* particles must interconvert, an effect called a "flavor-changing neutral current". K0 is ds* and K0* is d*s, so one must do d <-> u <-> s and s* <-> u* <-> d* together. But for this to work properly, there must be some additional up-like quark alongside the up quark to serve as an intermediate. This prediction was made in 1970, and particles with the appropriate quark were found in 1974, and this quark was named the charm quark. Like the up quark, the weak interaction maps it onto a mixture of the down and strange quarks, though mostly the strange one.

Returning to neutral kaons, their intermixture produces an additional weird effect, "CP violation". It makes some decays happen faster than their antiparticle-counterpart decays. This could be accommodated by the quark model by adding an additional "generation" of quarks, an additional up-like quark and down-like quark. The quarks' "mixing matrix" has a "complex phase" that cannot be redefined out of existence by redefining the particle fields, as one can do with two generations. Thus, one can get CP violation out of three generations of quarks but not two generations.

This prediction was made by Makoto Kobayashi and Toshihide Maskawa in 1973, and the bottom quark, a down-like quark, was found in 1977, and the top quark, an up-like quark, was found in 1995.

The mixing matrix for (up, charm, top) <-> (down, strange, bottom) is called the Cabibbo–Kobayashi–Maskawa (CKM) matrix. It has three mixing angles and one complex phase.

For the first two generations, (up, charm) <-> (down, strange), the mixing matrix is the Cabibbo matrix, with one mixing angle, the Cabibbo angle, and no complex phases.

For n generations, there are (1/2)*n*(n-1) mixing angles and (1/2)*(n-1)*(n-2) complex phases.


There is a counterpart for leptons, the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix. It relates neutrino and charged-lepton mass states, and its being non-diagonal makes neutrino oscillations.
 
Turning to the strong interaction, it resisted physicists' efforts to find a good theory of it until the early 1970's. A curious direction was the bootstrap model by Geoffrey Chew and others. It pictured all hadrons as somehow coequal, and its supporters tried to find interaction rates by using self-consistency and some simple hypotheses about them. That was never very successful, though an early version of string theory emerged from these efforts.

But by the early 1970's, they finally succeeded with a theory that they called quantum chromodynamics (QCD). I will now try to motivate it.

Consider the conundrum about quarks that they had spin 1/2 but that their combined spin-flavor states are symmetric under interchange of two quarks. This is obvious for delta++ (uuu), delta- (ddd), and omega- (sss), though less obvious for the other baryons. For a proton, one finds (flavors u,d; spin states +,-):

( -2*(u+,u+,d-) + (u+,u-,d+) + (u-,u+,d+) - 2*(u+,d-,u+) + (u+,d+,u-) + (u-,d+,u+) - 2*(d-,u+,u+) + (d+,u+,u-) + (d+,u-,u+) )/sqrt(18)

Quantum mechanics is full of weirdness, and a big bit of QM weirdness is the spin-statistics theorem:
  • Bose-Einstein statistics (bosons) -- integer spin -- symmetric on exchange -- state occupation number 0, 1, 2, 3, ...
  • Fermi-Dirac statistics (fermions) -- half-odd spin -- antisymmetric on exchange -- state occupation number 0, 1
  • BE: Symmetric on exchange: X(x2,x1) = + X(x1,x2)
  • FD: Antisymmetric on exchange: X(x2,x1) = - X(x1,x2)
Since quarks have spin 1/2, they must be antisymmetric on exchange, not symmetric.

If quarks have an additional degree of freedom, then quarks in a baryon can be antisymmetric on exchange. That degree of freedom is called "color", because it has three possible values, corresponding to the primary colors of human vision: red, green, blue. One can create an antisymmetric combination of three quarks with these "colors":
(rgb + gbr + brg - rbg - grb - bgr)/sqrt(6)

Combining this antisymmetry with the spin-flavor symmetry gives overall antisymmetry. Quark FD statistics thus rescued.


Since all three colors look alike from the outside, the system must be symmetric under exchanging colors and reversing their signs. If one does them partially, using complex numbers for partial sign reversals, one gets a rather complicated symmetry group. It reduces into two parts, changing the colors all together by the same amount, and changing them relative to each other. The former part is giving all of them the same complex phase, while the second one is rather complicated. It's SU(3), complete with 8 generators, though a separate SU(3) from up-down-strange flavor SU(3).
 
Now for the dynamics part. We start with something called "gauge symmetry".

It was first found for electromagnetism. Consider multiplying particles' wavefunctions by a complex phase: exp(i*q*a) for electric charge q and constant a. By conservation of charge, these phases cancel out and the equations of motion look the same as before.

For a constant, this is a "global symmetry". But one can make it a "local symmetry" by making a variable. But when one does so, one gets some extra terms in the particles' equations of motion. One can cancel them out by adding appropriate terms to the electromagnetic-field potential. When one calculates the field values from the potential values, those terms drop out.

Electromagnetism has a simple sort of symmetry group, U(1), but QCD has a much more complicated one for its colors: SU(3). Its eight generators interact with each other, a big complication. It's much like 3D rotations around different axes - what results one gets depends on the order that one does the rotations in. Also, groups like SU(3) may be interpreted as groups of complex-valued rotations.

This means that QCD has 8 color states of its counterpart of the photon, the "gluon". The states are color-anticolor states, and they can be expressed as rg*, rb*, gr*, gb*, br*, bg*, (rr* - gg*)/sqrt(2), and (rr* + gg* - 2*bb*)/sqrt(6). The remaining one is (rr* + gg* + bb*)/sqrt(3) -- since it is colorless, it is not a part of QCD.

Since the generators of the color-symmetry group interact with each other, the color states of the gluon also interact with each other. That seems like a big mess, and it took a lot of work to work out how to handle that kind of theory, a "Yang-Mills theory". But it was done, and some weird aspects of QCD soon appeared.

For instance, the effective gluon "charge" varies with distance. This is something found with the quantum-mechanical theory of electromagnetism, quantum electrodynamics (QED). An electric field tries to pull particle-antiparticle pairs out of the vacuum, polarizing it. This makes the effective electric charge slowly increase as one goes farther and farther inward. The effect sets in for distances smaller than the distance corresponding to the electron's mass, its reduced Compton wavelength hbar/(m*c), and it increases as one gets closer to that value for each particle. In theoretical work with relativity, one often sets c = 1, and with QM, one often sets hbar = 1, so that distance becomes 1/m.

But with QCD, the effective charge slowly decreases with decreasing distance. It becomes close to 1 for distances corresponding to the sizes of light hadrons like the nucleons. That's what inspired the name "gluon", since this particle seems gluey at the sizes of light hadrons. That size is much smaller than the Compton wavelengths of the up and down quarks, so they are strongly relativistic inside of them. So most of the masses of the nucleons is quark and gluon kinetic and interaction energies, and not the quark rest masses.

That also means that QCD interactions weaken enough at small distances to allow particles to behave like approximately free ones. But when they get far enough apart, they make a string of gluon that repeatedly breaks, making jets of particles.

This weakening permits a test of QCD with sufficiently energetic collisions, above a few GeV energy or therabouts. One can do e+e- collisions, and compare the results one would expect for quarks with what one finds for muons. The collision makes a virtual photon, and then some particle-antiparticle pair. One can naively extrapolate from muons to quarks, and one finds that the observed rate is 3 times larger. That is a result of making red-antired, green-antigreen, and blue-antiblue quark pairs.
 
Turning to the weak interactions, the first theory of them was a proton-neutron current and an electron-neutrino current that come into contact with each other, where the "current" is one kind of particle becoming another.

Thus, electron capture is a proton becoming a neutron (proton-neutron current) and an electron becoming a neutrino (electron-neutrino current).

There is a very nice feature about relativistic quantum field theory. Different kinds of interaction are related very closely. Consider a charged particle interacting with a photon. That interaction is essentially the same as the interaction that creates that particle and its antiparticle from a photon (pair production), and makes that particle and its antiparticle become a photon (annihilation).

Positive beta decay is a proton becoming a neutron (proton-neutron current) and a positron and a neutrino appearing (electron-neutrino current).

Ordinary beta decay is a neutron becoming a proton (proton-neutron current) and an electron and an antineutrino (electron-neutrino current).

Note the proton-neutron current going backward -- that's completely legitimate.

When the muon was discovered, this meant the occurrence of a muon-neutrino current.

How do these currents interact? The first proposed interaction was a contact interaction. But at about a TeV of energy (about 1100 proton rest masses), the interaction strength becomes impossibly high. So there has to exist some particle that couples the two currents in a weak-interaction event. A particle soon named the W. Since it transfers charge from one current to the other, it must be charged: W+ and W-.

It also has problems with making excessively high interaction strengths at high energies, but the work on self-interacting gauge theories that yielded QCD was also work that helped here. In the late 1960's, Steven Weinberg, Abdus Salam, and Sheldon Glashow worked out a unification of the electromagnetic and weak interactions: the "electroweak interaction", a unification that was well-behaved at high energies.

In addition to the photon, W+, and W-, it has a massive neutral interaction carrier: the Z. It couples neutral currents, unlike the charged currents associated with the W+-, and its currents don't change flavor, unlike the charged ones. Evidence for the Z was detected in 1973 in the Gargamelle bubble chamber at CERN. The experiment involved sending neutrinos into a tank filled with liquid hydrogen. Every so often, a neutrino would make a Z that would kick one of the protons or electrons, and that effect was observed.

But it was in 1983 that the W+- and the Z were produced as "real" particles and not just virtual ones in other particles' interactions.
 
A big problem with the theory of electroweak interactions is how to make the particles' masses in some well-behaved way, one that keeps the underlying gauge symmetries of the theory. Doing so naively will break those symmetries. The solution to this problem is the Higgs particle, one that respects those underlying symmetries while having a nonzero ground-state (vacuum) value. This nonzero value gives masses to most of the other Standard-Model particles, by making the Higgs particle always around, as it were.

This particle was detected in 2012, made by the Large Hadron Collider. So far, its interactions are close to what one would expect of the Higgs particle as a mass maker, though only the interactions with the more massive particles have been tested: the bottom and top quarks, the tau lepton, and the W and Z. The muon is likely to be the next particle tested, with the upcoming high-luminosity upgrade of the LHC.

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I finally turn to the most recent predicted particle successfully detected: gravitational waves.

A consequence of general relativity is that space-time is a dynamic entity, with wave solutions possible. But those solutions were a controversial part of GR for a long time, though by the 1960's, it was pretty much settled.

There was a purported detection of G-waves around 1970, by Joseph Weber from our Galaxy's center. But the energy flux from there was almost impossibly great, by the mid-1970's, other researchers failed to find any signals, and by the late 1970's, G-wave researchers decided that JW's claimed observations were spurious.

But in 1974, Russell Alan Hulse and Joseph Hooton Taylor discovered a binary pulsar, and over the next decade, the two stars' inspiraling was measured. It was discovered to agree with the expected rate from the G-wave energy loss predicted from GR.

In 2015, the LIGO G-wave detector detected a signal, GW150914. It fit the expected signal from two black holes spiraling into each other, merging, and then ringing afterward. Its distance is estimated to be about 410 megaparsecs or 1.34 billion light years, and the two parent black holes had masses of about 36 and 31 solar masses. They merged to make a single BH with 64 solar masses. Some 3 solar masses of energy was radiated away as G-waves, and the merger's peak luminosity was comparable to the combined luminosity of all the stars in the observable Universe.
 
lpetrich said:
In 2015, the LIGO G-wave detector detected a signal, GW150914. It fit the expected signal from two black holes spiraling into each other, merging, and then ringing afterward. Its distance is estimated to be about 410 megaparsecs or 1.34 billion light years, and the two parent black holes had masses of about 36 and 31 solar masses. They merged to make a single BH with 64 solar masses. Some 3 solar masses of energy was radiated away as G-waves, and the merger's peak luminosity was comparable to the combined luminosity of all the stars in the observable Universe.
Click to expand...

Events that huge and barely 1 billion ly distant, are rarish, but by coincidence they detected one shortly after first enabling detection.

What's the best Youtube for learning about the marvelous LIGO detector? I like [YOUTUBE]iphcyNWFD10[/YOUTUBE]
"The Absurdity of Detecting Gravitational Waves." It does seem absurd: A precise laser of huge power; the smoothest mirrors ever manufactured; vacuum; detectors in both Louisiana and Washington to rule out local noise; etc. — all to detect a change in distance smaller than the width of a proton.

Nitpick: Isn't "Luminosity" defined to be EMF waves exclusively, not gravitational?
 
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