The path to the Standard Model .... or how the tapestry on the previous page was woven [1]
Recall (Particles and forces) that the electron was first conceived in theoretical terms in 1892 by the Dutch physicist Hendrik Antoon Lorentz, and in 1897, Joseph John Thomson demonstrated experimentally that they really did exist. It was the first atomic particle discovered.
Then in 1928:
- Paul Dirac formulated a relativistic wave equation capable of predicting the behaviour of particles at high energies and velocities approaching the speed of light, and describing how they move.
- It made "spin" - the particle's angular momentum - a natural property of the electron, postulating "a kind of symmetry, a way of stating mathematically that a system could undergo a certain rotation". Without this modification, the suggestion has been made that Einstein’s E=mc2 said that energy was only approximately equal to mass.
- In its free form, or including electromagnetic interactions, the equation describes all spin-1/2 massive particles such as electrons for which parity is a symmetry and is consistent with both the principles of quantum mechanics and general relativity.
- It was the first theory to account fully for special relativity in a quantum mechanical context.
- It also implied the existence of a new form of matter known as antimatter, which was experimentally confirmed several years later paving the way for the eventual discovery of the positron.
The Dirac equation and its consequences, including the role played by so-called virtual particles in the context of Heisenberg's uncertainty principle, is explained on the pages Something out of nothing - again (in the segment on quantum mechanics) and E=mc² The equation's tendency to produce unwanted infinities and the mode of turning them into finite quantities by means of a process known as renormalisation are collectively considered at Feynman's sum over paths - a more detailed analysis
Discovery of the proton and neutron
In 1918, Rutherford discovered the proton and in so doing identified particles in the atom with a positive electric charge that was equal and opposite to the electron’s negative charge, and in 1932 the discovery of the neutron revealed that atomic nuclei were made of nucleons (protons and neutrons) held together by an attractive force. In the same year, using the first particle accelerator, the British physicists John Cockroft and Ernest Walton discovered the source of the energy that bound protons and neutrons (nucleons) together in the strong nuclear force.
The meson’s hypothetical role in mediating the strong nuclear force
At this time, the strong nuclear force was thought to be caused by the exchange of particles of intermediate mass between the proton (mass ~ 1000 MeV (megaelectron volts)) and the electron (mass ~ 0.5 MeV), acting also as a boson, known as mesons. Such a particle was first proposed to exist in 1935 by the Japanese physicist Hideki Yukawa as the primary force-carrying boson in nuclear interactions. It was later observed at nearly the same mass as he originally predicted (~ 100 to 200 MeV).
Discovery of the muon
In 1936 the American physicists Carl D. Anderson and Seth Neddermeyer discovered an intermediate mass object thought to be Yukawa’s meson. However, it failed to react with nuclei or other particles through the strong interaction. We now know that this particle is the muon (μ) (Who ordered that?) , a heavier version of the electron, which also comes in particle and antiparticle forms.
It is also a fermion with spin ½ and not a mediating boson. Because muons are charged, before decaying they lose energy by displacing electrons from atoms, a process known as ionisation [2]. The muon is relatively unstable, with a lifetime of only 2.2 microseconds before decaying by the weak force into an electron and two kinds of neutrinos. It never decays electromagnetically (or strongly), because there are two different types of electron and muon neutrinos and conservation laws prevent this. It is more correctly assigned to the lepton group of subatomic particles, since it does not take part in the strong interaction.[2.1]
In all these processes, classical conservation laws must be observed - conservation of energy, momentum, electric charge and angular momentum. These still apply in the realm of particle interactions and are absolute. But there are others – parity (mirror symmetry), for example - which are conserved by some of the three fundamental interactions but not all.
All particles will decay to lighter particles unless prevented from doing so by a conservation law [3], or, as the renowned British physicist Murray Gell-Mann expressed it in a pithy aphorism known as the totalitarian principle: "every process that is not forbidden must occur". in other words, any decay process which is anticipated but not observed can only be prevented from occurring by some conservation law. This approach has been fruitful in helping to determine the rules for particle decay [4].
Discovery of the meson and pion (pi meson)
In 1947 the British physicist Cecil Powell and others at the University of Bristol discovered Yukawa’s meson. It came in three versions: positively and negatively charged π ±, mass + 139.6 MeV) and neutral (π 0 mass = 135MeV): the pion or pi meson. Pions are the lightest mesons (and, more generally, the lightest hadrons), because they are composed of the lightest quarks (the u and d quarks).
They are unstable, with the charged pions π+ and π− decaying via the weak interaction 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 most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays. [5]
Enter the K meson and “strangeness” [6]
So-called V tracks were first discovered in 1932 by Patrick Blackett : a gamma ray would fly into the cloud chamber (explained Timeline - from the discovery of the electron to the Higgs boson) without leaving a track and convert into an electron and positron, which would fly apart as they went, creating a "V" shape. Then after the war, a new kind of V particle was discovered with no electric charge which then decayed to positive and negative pions, so it had to be heavy.
Some had masses greater than the proton and were called hyperons with spin ½ and others were endowed with masses between that of the pion and the proton. Having middling mass, these latter were designated kaons or K mesons with spin 0. When there was enough energy/mass and particles accelerators commenced operation in the 1950s, it became possible to produce pion beams, and to study pion-nucleon reaction rates and energy [6.1]. Kaons were ‘easily’ produced via processes such as:
pion + nucleon → K meson+ hyperon
This easy production implied that they were strongly interacting particles, but the problem was that, being the product of the strong interaction, unstable heavy particles should decay quite quickly whereas the kaon seemed to live quite long half-lives, implying that their decay was via the weak interaction, which was, quite frankly - ‘strange’.
In due course, it was realised that, while kaons were always produced in pairs, they went their own separate ways before decaying. This problem was solved by introducing another type of kaon, one with electric charge thereby importing a new quantum number into the equation: strangeness (S), conserved by the strong interaction - and a corresponding new set of rules, whereby strangeness changes from its state when the particles are produced (S = ± 1) to that when they separately decay (S = 0). In other words, if the particles decay separately, strangeness is changed by 1.
There are actually a group of four K mesons which are quite fascinating in some of their properties. They are all identical except for strangeness and the weak interaction doesn’t care. It recognises two linear combinations, with different lifetimes, decay processes and parity (mirror symmetry) and allows strangeness to change. The lifetimes involved may be short or long. One lives 100 times longer than another.
A careful analysis of particle accelerator data then revealed a whole variety of other ‘new’ particles, five, ten, twenty of them - the muon, pion and kaon apart - involving all sorts of varieties of mesons and baryons: the eta η, rho ρ, sigma Σ, xi Ξ, omega Ω (see below) and so on. A veritable “zoo” was being discovered, and as Gavin Hesketh puts it: "there was a danger of running out of Greek letters to label these things". [7].
The discovery of the omega (minus) baryon [8]
Around 1960, symmetry came to be applied to whole groups or families of particles, baryons or mesons, with common charactersistics - the same spin and parity, but different 'stangeness' (and charge/isospin) - ultimately culminating in 'particle periodic tables' containing eight particles which Murray Gell-Mann named the Eightfold Way, but the symmetry was not perfect: some members of the family had different masses. In one instance, the top particle was unknown, but the relevant group projection was able to predict its relevant properties and it was quickly discovered. [9]
The result was the omega minus baryon, an unstable hyperon heavier than the neutron and proton with a strangeness of -3! From this emerged the idea that all the known particles could be constructed from a set of more fundamental “real particles” coming in two varieties, up and down, and which, following their discovery in 1968, Murray Gell-Mann called quarks - a rather whimsical term deriving from James Joyce’s Finnegan’s Wake. It would emerge that they came in many more variations: up, down, charm, strange, beauty (or bottom) and top, arranged in generational columns according to their mass. But they had unusual charges. All this emerged from the discovery of the omega baryon. (There is now speculation that quarks may not be fundamental after all, but may themselves be composed of even smaller particles called preons).
Around 1960, symmetry came to be applied to whole groups or families of particles, baryons or mesons, with common charactersistics - the same spin and parity, but different 'stangeness' (and charge/isospin) - ultimately culminating in 'particle periodic tables' containing eight particles which Murray Gell-Mann named the Eightfold Way, but the symmetry was not perfect: some members of the family had different masses. In one instance, the top particle was unknown, but the relevant group projection was able to predict its relevant properties and it was quickly discovered. [9]
The result was the omega minus baryon, an unstable hyperon heavier than the neutron and proton with a strangeness of -3! From this emerged the idea that all the known particles could be constructed from a set of more fundamental “real particles” coming in two varieties, up and down, and which, following their discovery in 1968, Murray Gell-Mann called quarks - a rather whimsical term deriving from James Joyce’s Finnegan’s Wake. It would emerge that they came in many more variations: up, down, charm, strange, beauty (or bottom) and top, arranged in generational columns according to their mass. But they had unusual charges. All this emerged from the discovery of the omega baryon. (There is now speculation that quarks may not be fundamental after all, but may themselves be composed of even smaller particles called preons).
“The discovery of the omega (minus) baryon was a great triumph for the quark model of baryons because it was searched for and found only after its existence, mass, and decay modes were predicted by the quark model. It was discovered at Brookhaven in 1964”.
A sketch of the bubble chamber photograph in which the omega-minus baryon was discovered. Source: V. E. Barnes et al., Phys. Rev. Lett. 12, 204 (1964). The various stages of decay reactions are illustrated by the particles shooting off to the right of the main omega-minus stream: http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/omega.html
A sketch of the bubble chamber photograph in which the omega-minus baryon was discovered. Source: V. E. Barnes et al., Phys. Rev. Lett. 12, 204 (1964). The various stages of decay reactions are illustrated by the particles shooting off to the right of the main omega-minus stream: http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/omega.html
The introduction of “colour”
The omega minus baryon was composed of three strange quarks which appeared to be identical, but since quarks are fermions with spin 1/2, they must obey the Pauli exclusion principle and cannot exist in identical states. The way out of this conundrum was to introduce a fourth quantum state or number called “colour” in describing particles, because with three strange quarks the property which distinguishes them must be capable of at least three distinct values[10].
The omega minus baryon was composed of three strange quarks which appeared to be identical, but since quarks are fermions with spin 1/2, they must obey the Pauli exclusion principle and cannot exist in identical states. The way out of this conundrum was to introduce a fourth quantum state or number called “colour” in describing particles, because with three strange quarks the property which distinguishes them must be capable of at least three distinct values[10].
“Color is the strong interaction analogue to charge in the electromagnetic force. The term "color" was introduced to label a property of the quarks which allowed apparently identical quarks to reside in the same particle, for example, two "up" quarks in the proton. To allow three particles to coexist and satisfy the Pauli exclusion principle, a property with three values was needed.
“The idea of three primary colors like red, green, and blue making white light was attractive, and language about "colorless" particles sprang up. It has nothing whatever to do with real color, but provides three distinct quantum states. The property can be considered something like a "color charge" with three distinct values, with only color neutral particles allowed..... The antiquarks have anti-colors, so the mesons can be colorless by having a red and an "anti-red" quark” [11] As Gavin Hesketh points out, colour charge is different to the attraction between positive and negative charges which combine to produce zero total charge in atoms. Colour charge is “three-dimensional”, so that as well as the charge having a positive or negative sign, it can also have a direction and for convenience these different directions are labelled red, green and blue (up and down were already taken), and a particle carrying one unit of colour charge behaves like a weather vane, which can be pointing in any one of three different directions”: the red, blue or green directions - though this is just an analogy. Antiquarks carry opposite charges and antired, antiblue, or antigreen colours. These colours tend to cancel themselves out to produce particles or antiparticles which are white or colourless: red+green+blue=white or colourless, and in antiquarks, red+antired also produce no colour. In this regard, the analogy works well. Gluons also carry colour and can interact with other gluons. [11.1] |
The omega minus baryon with its quarks endowed with “colour”, to avoid violating the Pauli exclusion principle.
This model has been dubbed Quantum Chromodynamics (QCD) because of its colour dependency. But how did these coloured quarks interact? By means of the strong nuclear interaction. of course. In fact, QCD is the theory of the strong interactions, and colour is the QCD analog of the electric charge in QED, and gluons are the exchange particles or force carrier of the theory, like photons are for the electromagnetic force in quantum electrodynamics (QED) and and W and Z bosons which mediate the weak nuclear interaction) [12].
Proton and neutrons (each made up of three quarks) are stable particles because they are composed of the lightest and most stable quarks, the proton being composed of two up quarks and one down quark, and the neutron vice-versa, and the same for their antimatter counterparts. Quarks also have antimatter partners with the opposite electric charge to their matter partner.
To end up with the correct electric charge, the quarks must each carry a fractional amount: +2/3 for the up, -1/3 for the down, giving zero charge for the neutron, and +1 unit for the proton, which balances the -1 unit of the electron in the atom. Because particles with three quarks are much heavier than those with two, they were given the name collective name baryons. Those with two quarks (being lighter than baryons but heavier than the electron) and thus of intermediate mass were designated mesons. All particles made of quarks, both mesons and baryons are designated hadrons. So, according to this scheme of categorisation, just the electron and three quarks are all that is needed to explain the atom and all the particles in the zoo. [13]
We will never be able to see a quark, because isolated free quarks cannot exist in nature. Instead, they always exist in groups of three (protons and neutrons), or in pairings of a quark and anti-quark (pi-mesons), and behave as if they were joined by rubber bands [14], all joined together by the “sticky gluons” [15].
Among the data lurking inside the Large Hadron Collider (LHC) at CERN, another combination of quarks has been found: the pentaquark, a composite of four quarks and an antiquark, whose existence was predicted over 50 years ago [16]. The data fails to reveal whether all five quarks are bound tightly together or whether a baryon is loosely bound to a meson, like some kind of subatomic molecule. The revelation suggests that the particle landscape as it is currently known is far from being the complete picture.
Within the proton or neutron, in addition to the basic quark trio, there is also a sea of quarks and antiquarks popping in and out of existence. How all this works remains something of a mystery. For example, the changing numbers of gluons “flit around like fireflies, flickering in and out of existence, and pairs of quarks and antiquarks form and dissolve; the result is a ‘quantum foam’ of appearing and disappearing particles” [17]. The hypothesis is that the gluons split and keep splitting like an “out-of-control popcorn machine”. But something operates in the nature of a self-imposed limit, thought to be when the gluons become so numerous that they begin to overlap within the proton resulting in a so-called saturation state bringing the process under control [18].
In the 1980s, scientists discovered that a proton's three valance quarks (red, green, blue) account for only a fraction of the proton's overall spin. New measurements from the Relativistic Heavy Ion Collider’s PHENIX experiment now reveal that gluons (yellow corkscrews in the sketch below) contribute as much as or possibly more than the quarks in determining the intrinsic angular momentum, or spin, of these building blocks of matter[19].
The gluons again seem to be linked to the concept of colour. There are 8 of them, of which 2 are colourless, while the others are of the form green-antiblue etc. Thus a green quark may emit a green-antiblue gluon, and become a blue quark. A blue quark will then absorb this gluon and turn green [20], thus:
To end up with the correct electric charge, the quarks must each carry a fractional amount: +2/3 for the up, -1/3 for the down, giving zero charge for the neutron, and +1 unit for the proton, which balances the -1 unit of the electron in the atom. Because particles with three quarks are much heavier than those with two, they were given the name collective name baryons. Those with two quarks (being lighter than baryons but heavier than the electron) and thus of intermediate mass were designated mesons. All particles made of quarks, both mesons and baryons are designated hadrons. So, according to this scheme of categorisation, just the electron and three quarks are all that is needed to explain the atom and all the particles in the zoo. [13]
We will never be able to see a quark, because isolated free quarks cannot exist in nature. Instead, they always exist in groups of three (protons and neutrons), or in pairings of a quark and anti-quark (pi-mesons), and behave as if they were joined by rubber bands [14], all joined together by the “sticky gluons” [15].
Among the data lurking inside the Large Hadron Collider (LHC) at CERN, another combination of quarks has been found: the pentaquark, a composite of four quarks and an antiquark, whose existence was predicted over 50 years ago [16]. The data fails to reveal whether all five quarks are bound tightly together or whether a baryon is loosely bound to a meson, like some kind of subatomic molecule. The revelation suggests that the particle landscape as it is currently known is far from being the complete picture.
Within the proton or neutron, in addition to the basic quark trio, there is also a sea of quarks and antiquarks popping in and out of existence. How all this works remains something of a mystery. For example, the changing numbers of gluons “flit around like fireflies, flickering in and out of existence, and pairs of quarks and antiquarks form and dissolve; the result is a ‘quantum foam’ of appearing and disappearing particles” [17]. The hypothesis is that the gluons split and keep splitting like an “out-of-control popcorn machine”. But something operates in the nature of a self-imposed limit, thought to be when the gluons become so numerous that they begin to overlap within the proton resulting in a so-called saturation state bringing the process under control [18].
In the 1980s, scientists discovered that a proton's three valance quarks (red, green, blue) account for only a fraction of the proton's overall spin. New measurements from the Relativistic Heavy Ion Collider’s PHENIX experiment now reveal that gluons (yellow corkscrews in the sketch below) contribute as much as or possibly more than the quarks in determining the intrinsic angular momentum, or spin, of these building blocks of matter[19].
The gluons again seem to be linked to the concept of colour. There are 8 of them, of which 2 are colourless, while the others are of the form green-antiblue etc. Thus a green quark may emit a green-antiblue gluon, and become a blue quark. A blue quark will then absorb this gluon and turn green [20], thus:
The ties that bind
”An individual quark cannot be pulled free because the energy required to do it is much greater than the pair production energy of a quark-antiquark pair. If in a high energy collision, something scatters directly off one of the constituent quarks, it will give it a high energy. With an energy many times the pair production energy, it will create a jet of quark-antiquark pairs (mesons)” [21]. This has emerged from what are known as deep inelastic scattering experiments.
It is also unclear how quarks and gluons contribute to the mass of the protons when their total mass within fails to account for the proton’s total mass. How is this missing mass accounted for? Similarly, the spins of the individual quarks and gluons within the proton fail to account for the proton’s rotation as a whole [22]. Why and how? Also, how do gluons do the quark binding work in the first place and why does this binding seem to rely on a special type of ‘colour’ charge within the quarks? Thus, although the three quarks in a proton individually carry one of three, say, red green and blue colour charges, the proton itself does not have a net colour charge.
These all appear to be instances where the sum of the parts does not add up to the whole. The strong force is also peculiar in that it seems to pull on quarks more strongly the farther away they get and attract one another weakly the closer they get, perhaps because of some imaginary string: tense when far apart, slack when closer together. In short, the mystery of “how gluons glue” remains an open one [23].
How do protons and neutrons get their mass and spin? [23.1]
Gluons are massless, and the sum of the masses of the quarks inside protons and neutrons makes up roughly 2 percent of the nucleons’ total mass. So where does the rest come from?
Quarks carrying colour charge interact with one another by exchanging gluons, and gluons interact with other gluons by exchanging more gluons, and such a feedback loop of interactions, governed by the principles of QCD, is “devilishly” difficult to compute, being. Perhaps somewhere in this endless chain reaction lies the clue to the missing mass.
To get a fuller idea of how all this works and to see inside the atom’s constituent particles, scientists want to build an Electron-Ion Collider at one of two US laboratories that would smash protons and atomic nuclei with electrons to provide 3-D pictures of the inside of protons, neutrons and atomic nuclei.
The EIC will use the technique of deep inelastic scattering (DIS) by hitting nuclei with a beam of electrons at high speed. An electron exchanges a “virtual photon”, a semi-real particle that pops into and out of existence quickly, with the quarks inside a proton and neutron. By analysing the energy and recoiling angle of the electron as it bounces off, scientists learn about the object it hit. The higher the energy of the collision, the smaller the wavelength of the virtual photon, effectively creating a smaller probe that can “see” tinier scales within the nucleus. [23.2]
The EIC is one of the highest priorities of the U.S. nuclear science community.
Leptons, neutrinos and gauge bosons
As well as quarks, there are two more types of elementary particles - leptons and gauge bosons. Leptons are particles of low mass that are unaffected by the strong nuclear force but respond to the weak nuclear interaction. There are six types: the electron, muon and tau particles and their associated neutrinos: electron neutrino, muon neutrino and tau neutrino. [24]. Whereas quarks are never found alone, leptons form composites. These types or flavours are not pure states but actually contain a mix of three possible masses called mass states. Scientists do not yet know the value of each mass state, except that all three are very small. Theory suggests that there are either two exceptionally tiny masses and one slightly larger or the reverse: one extremely small mass and two slightly larger.
The first neutrinos were discovered in 1956 by Frederick Reines and Clyde Cowan. Neutrinos are believed to have come into existence in the hot and dense early universe when nuclear reactions forged helium into hydrogen, releasing a huge number of neutrinos in the process. They may also be produced in cosmic-ray collisions in the Earth’s atmosphere, and have been observed in supernova explosions[24.1], the phenomena rumoured to give rise to cosmic rays, and are also produced artificially in particle accelerators. Dark matter is shaped by neutrinos, and has subtly distorted the Cosmic Background Microwave radiation (CMBR), and by tracing these distortions, physicists may soon be able to chart dark matter’s structure [24.2].
It was once thought that neutrinos were pure objects and massless. if so, they would travel at the speed of light, and could not oscillate, or "mix": change flavour from electron to a muon or tau mid-flight. However, if they carry even the tiniest amounts of mass, they will travel at different speeds, the mass states will be able to mix and the neutrino shifts flavour as it travels. The theory of all this is amply captured by Hesketh in The Particle Zoo, together with an alluring allusion of three cars travelling on a circular track each one starting one behind the other but then travelling at slightly different speeds and in the process successively overtaking each other at different points. Positions relative to each other are changing all the time. So on successive circuits of the track, how can we predict which car will be the next to once again cross the start line?
Analogous to this is the question as to how a neutrino will behave in the next successive measurement by emitting a W boson and turning into an electron, muon or tau. The early experimental evidence disclosed a serious discrepancy between theoretical predictions and experimental observation of solar neutrinos visiting Earth, with the latter too small by a factor of around 3. This conundrum was resolved in the 1960s: provided at least two of the neutrino varieties have mass, what started out as an electron-neutrino can turn into a muon-neutrino and so forth, as they oscillate from one flavour to another as they travel - and in doing all this, the neutrino "does everything - all at once". Recent experimental observations have demonstrated that kind of mixing - a form of (quantum) interference or "beat effect" - does indeed occur, this discovery constituting the first sign of physics beyond the Standard Model, because it tells us that neutrinos must have mass of a few eV (By way of contrast, the electron's mass is 511 keV). [25].
The reason that neutrinos can change their flavour has the consequence that they cannot be massless particles traveling at light speed (as the Standard Model predicted) flows from Einstein's special theory of relativity, which tells us that time moves more slowly for an object in motion than for a stationary one. As an object's speed increases, time continues to slow until it actually stops. That is the point when the object reaches the speed of light.
If neutrinos can alter their flavour, they must undergo change and therefore experience time. They must also be traveling more slowly than light, which means they cannot be massless. Particles moving at the speed of light would have no mass, according to relativity, so if they are slower than that, they must have some mass—and the Standard Model has a problem. (This revelation, and the discovery that neutrinos oscillate, won Takaaki Kajita and Arthur B. McDonald the 2015 Nobel Prize in Physics).
So-called sterile neutrinos [25.1]
There has also been recent speculation about the existence of co-called “sterile” neutrinos which do not interact with normal matter at all: ones embracing all three neutrino varieties, without electric or colour charge, and spinning anticlockwise and so being "right handed".
All neutrinos are ghostly, but a sterile neutrino would be the ghostliest of them all. Because it does not experience the strong, weak and electromagnetic forces through which other particles interact, it would be essentially undetectable. It would only be subject to gravity. This quality would render it part of the invisible realm physicists call the dark sector, which includes the dark energy and dark matter that make up 95 percent of the energy density of the universe. Sterile neutrinos may be able to interact with dark matter through new forces of nature. They might even be dark matter. Some hypotheses suggest that sterile neutrinos make up some or even most of the invisible matter in the cosmos.
In other words, should they exist, this may go some way towards explaining the dark matter that apparently pervades the universe and exerts a gravitational pull on regular matter. The technique is to look for electronic neutrinos which may have changed flavours into a muon or a tau or be missing altogether in which case they may have turned into sterile neutrinos, but so far their existence has not been able to be confirmed experimentally. Perhaps they may pick up their mass from the Higgs field, or may be their own (Majorana) antiparticles. [25.2] The search is on for sterile neutrinos at the new Coherent CAPTAIN-Mills (CCM) experiment at Los Alamos National Laboratory. The enormous IceCube Neutrino Observatory in Antarctica is also searching for sterile neutrinos.
Other searches
An experiment known as DUNE (the Deep Underground Neutrino Experiment) is under which an accelerator will smash protons into graphite to create a beam of neutrinos which will then travel through 1300 kilometres of earth from the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois to the Sanford Underground Research Facility in South Dakota is expected to shed some light on these questions. [25.3] If all goes to plan things should be up and running in the 2020s. DUNE will be able to collect far more data at far greater levels of precision than every previous neutrino experiment. It will use a beam of neutrinos about twice as powerful as the strongest existing high-energy neutrino stream and it will blast it at a detector that is more than 100 times larger than the biggest of its kind. Physicists will be able to observe how the neutrinos oscillate between flavours over the journey.
Recall that at present scientists do not know the values of the three mass states, but theory suggests that two are lightweight and one is relatively heavy (the normal hierarchy) or that one is light and two are heavy (the inverted hierarchy). DUNE should be able to determine which hierarchy is correct. Once this is determined, scientists can then go on and determine how neutrinos get their mass. Some light may also be shed on the intriguing question of why the universe is made up of matter rather than antimatter. In this regard, DUNE will be searching for signs of CP (charge parity) violation: evidence that antineutrinos oscillate from flavour to flavour at different rates than neutrinos. DUNE will also be able to determine whether neutrinos come in only three flavours or whether there are more waiting to be discovered such as the so-called sterile neutrinos spinning anticlockwise and hence being "right handed" in lieu of the traditional left-handedness of matter neutrinos..
DUNE supersedes the NuMI Off-Axis Electron Neutrino Experiment (NOvA), operating between Fermilab and a similar but larger detector based in Minnesota, 800 kilometres away. DUNE’s additional 500 kms should be instrumental in allowing ample time for the neutrinos to display some of their trademark oscillation idiosyncrasies on the journey.
Bosons [26] differ from all of the other subatomic particles (fermions) in their “spin”. Bosons have vector wave functions with spin values that are either equal to 0 or to an integer (±1, ±2 and so on) – photons fit into this category - whereas fermions [27] (the stuff matter is composed of – quarks and leptons) have spins with half-integer values (such as ±½ or ±1½). All of the basic particles of ordinary matter – electrons, protons and neutrons – are fermions. Bosons like to congregate together and that is why lasers are possible.
A laser beam is a collection of photons in the same quantum state. Fermions, on the other hand, stay aloof – you will never find two of them in the same quantum state. This explains why atoms have electron orbitals. The basic characterisitic of all fermions is that "their fuzzy clouds don't overlap, but stack up instead like little plastic building blocks", and because electrons can't overlap, there is only room for two of them at the lowest energy level of an atom, eight at the next energy level, and so on [28]. This gives different chemical properties to different elements and structure to the world around us. [29]
Gauge bosons are “carrier” particles for the four forces. They include photons, which are carriers for the electromagnetic force; gluons – carriers for the strong nuclear force; and W and Z bosons, which mediate the weak nuclear interaction. The discovery of the gluon in August 1979 and the W boson in January 1983 were necessary precursors to the identification of the Higgs particle over three decades later. [29.1] The hypothetical graviton is thought to mediate gravity and the Higgs boson to give particles their mass, more about which also later. Today, the particle count is said to be running at something like 150 with another 100 or so suspected, and it is difficult to know what nature wants them for [30].
And after all this particularity, looking from a more elevated platform across the particle landscape, let's not forget the molecule. It constitutes the basic working model of the atom, and may be defined something like ”two or more atoms working together in a more or less stable arrangement": just add two atoms of hydrogen to one of oxygen and you have a molecule of water [31].
More quarks [32]
Remember those four leptons which came in the form of 2 pairs: the electron and its neutrino; the muon and its neutrino; and also recall that the muon is relatively unstable, with a lifetime of only 2.2 microseconds before it decays by the weak force into an electron and two kinds of neutrinos.
Notwithstanding that there are 4 leptons, there are only 3 quarks, up, down and strange, but shouldn’t there be a fourth, for symmetry’s sake? And while we're on the subject, since all “normal” matter can be built from what is known as the “first generation” - up and down quarks, and the electron and its neutrino - what is the point of all the others?
Following experiments conducted at the Stanford National Accelerator Laboratory, a new particle was discovered in 1970: the J/ψ (J/Psi) meson. This fourth quark, whose existence had been the subject of speculation by a number of authors since 1964, was labelled Charm, with a quantum number of C = 1. The J/ψ meson was in fact a “charmed particle” containing a c quark and an anti-c quark. "The discovery of the charm quark triggered the 'November Revolution' in particle physics, sweeping aside all doubts about the existence of quarks and establishing the Standard Model as the correct theory". The second generation of quarks (strange and charm) was now complete.
But a third generation still awaited discovery, and it came in the form of the discovery of the t and b quarks, variously designated top and bottom, or truth and beauty Hadrons (composite particles) with bottom quarks were found in 1977 in the form of the upsilon, but finding the top quark, whose existence was first predicted in 1973, which is very heavy, took much longer. It was eventually discovered at the Tevatron particle detector (also known as Fermilab – the Fermi National Accelerator Laboratory – about 50 miles due west of Chicago in 1995. The Tevatron was the second highest energy particle collider in the world after the Large Hadron Collider (LHC), but ceased operation on 30 September 2011 due to budget cuts. The top quark is in fact the heaviest subatomic particle ever observed, with a mass that is about as heavy as an entire atom of gold [33]. Top quarks are also among the most fleeting of particles, with a lifetime of about a trillionth of a trillionth of a second [34].
The weak interaction
With all this progress in the quark area, how good is our theory of the weak interaction, and how might we eventually be able to combine all these processes into a coherent whole?
The problem with Fermi’s theory of beta decay of a neutron [35] – that it occurred by direct coupling of a neutron with an electron, a neutrino (later determined to be an antineutrino) and a proton - was that in the case of the weak interaction there was no ‘intermediate’ particle which mediated the force, and the nature of the weak interaction which led to beta decay was unknown in Fermi's time. Theorists therefore tried to construct a model which incorporated such an intermediate particle: actually three, known as intermediate vector bosons: W+ and W- (mass ~ 80 GeV) and Z 0 (mass ~ 91 G eV). They have very short half-lives and generally decay into fermion-antifermion pairs. The full theory here actually involves 4 vector bosons because the massless photon is also involved.
Michael Box says that in a ‘higher symmetry’ world all these vector bosons are massless, but in our world, this symmetry is ‘spontaneously broken’ and 3 of them (all bar the photon) become massive. “The inherent strength of the electromagnetic and weak reactions is the same. The weak interaction only seems weaker/short range because its ‘exchange particle’ is so heavy [36].
Fleshing out the leptons
And whilst on the subject of generational change, let’s also flesh out the full range of leptons (Greek for small), those particles of low mass that remain unaffected by the strong nuclear force but respond to the weak nuclear interaction. We have already visited generations one and two: the electron and the muon and their associated neutrinos.
Then, in 1975, Stanford University Professor Emeritus Martin Perl, thinking that finding an even heavier electron might help explain the muon’s role, announced that he and his colleagues had discovered a new particle, but were not sure what it was. This new particle was 3,500 times more massive than the electron, but only lived a third of a trillionth of a second before decaying into a spray of its lighter brethren and the ghostly neutrinos [37]. It was the tau (~ 1.78 GeV, about twice the mass of the proton) which has its own neutrino (discovered in 2000) and tau-lepton number, and together they completed the third generation of leptons. These six quarks and six leptons together make up the 12 fermions which compriseup the matter particles in the Standard Model.
Why in the world around us are we only left with up and down quarks and electrons?
So, with 3 generations of quark pairs and three generations of lepton pairs making a total of 12 particles in the Standard Model, why in the world is the world made up of just the lightest, the up and down quark and the electron? The answer is, says Gavin Hesketh, because the W boson can partner any particle and all the heavier particles in the Standard Model are unstable.
“Very soon after the Big Bang, the universe was incredibly hot and dense, and must have been full of taus, top quarks, muons and so on. But within a fraction of a second all of these would have decayed away – thanks to the weak force. Only the lightest particles survived: the up and down quark and the electron – and these, along with lots of neutrinos, make up the world around us. The quarks and electrons eventually combined to form atoms, and, after a few billion years, us. We are made from the cold dust left after the Big Bang”. [38]
The simplest case in which to see this, says Hesketh, is in the decay of the (Who ordered that?) muon, which can fleetingly violate the conservation laws and borrow some energy, emit a (virtual) W and turn into a muon-electron, then decay into an electron and anti-neutrino. A few fractions of a second later, there are no more muons. And the same thing happens to the tau, which emits a W, turning into a tau-neutrino, and then the W turns into either an electron and electron-neutrino, or muon and muon-antineutrino. A few seconds later, no more taus. “Only the lightest leptons survive: the electron and lots of neutrinos – and only because there is nothing lighter they can decay into”. [39]
"Everything - all at once"
And finally, remember that when we are not actually measuring a particle - neutrinos included (weak interactions involving processes such as the emission of W and Z bosons constitute a measurement) - Hesketh's retake on Feynman's "sum over paths" approach dictates that "it will do everything - all at once". [40]
[1] There are a number of instances here which make reference to Associate Professor Michael Box’s October-November 2016 WEA Atoms course What are atoms composed of? - Session 6, What are protons composed of? (referred to herein as the Box lectures). The relevant passages have been edited, and I take full responsibility for any distortions and/or inaccuracies which may have occurred in the editing process.
[2] https://timeline.web.cern.ch/anderson-and-neddermeyer-discover-the-muon
[2.1] Physicists have recently been able to utilise muons' trajectory to peer through otherwise impenetrable substances, in much the same way an X-ray reveals a fractured bone, and by monitoring the cosmic rain on Egypt's Great Pyramid, an international research team has detected a large void hidden within 4500-year-old stone structure. Stone absorbs muons' energy in ways that pockets of air do not, and the detectors can also distinguish rock slabs from a void from water. Muon detectors have also been able to probe volcanic chambers and Fukushima's nuclear disaster zones: http://www.smh.com.au/world/cosmic-rays-reveal-mysterious-void-in-egypts-great-pyramid-20171102-gzdxqm.html
[3] The conservation laws are set out at https://www.physics.utoronto.ca/~krieger/PHY357_Lecture6.pdf
[4] On particle interactions and conservation laws, see http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/parint.html
[5] As to which see https://en.wikipedia.org/wiki/Pion
[6] The Box lectures, Gavin Hesketh, The Particle Zoo - The search for the fundamental nature of reality, Quercus, Hachette, London, 2016, 62-63, 66.
[6.1] On pion-nuclear scattering, see http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-r-131.pdf: “Low Energy Pion-nucleon scattering, by Barbara G Levi, Stanford Linear Acceleration Center, Prepared for the US Atomic Energy Commission, July 1971.
[7] Hesketh, Ibid, 69.
[8] http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/parint.html
[9] Hesketh, 69-71, The Box lectures.
[10] Source: The Box lectures; Hesketh, op cit, 59.
[11] http://hyperphysics.phy-astr.gsu.edu/hbase/Forces/color.html See also the paper by y Samia Rehman Dogar Associate Prof Federal College Of Education H-9,Islamabad, pars 66-69 at http://www.slideshare.net/samiadogar/mesons
[11.1] Hesketh, op cit, 79. Hesketh proceeds to elaborate upon the complexities of all this in the remainder of Chapter 4, pp 79ff.
[12] https://en.wikipedia.org/wiki/Quantum_chromodynamics
[13] Hesketh, op cit 71.
[14] Stephen Hawking and Leonard Mlodinow, The Grand Design, Bantam Press, 2011, 49.
[15] So described by Robert Ent, Thomas Ullrich and Raju Venugopalan, in their article “The glue that binds us”, Scientific American, May 2015, 32 at 38.
[16] "Particular joy", Scientific American, October 2015, 17. The LHC is considered on the page entitled The Large Hadron Collider See also https://www.scientificamerican.com/article/pentaquarks-make-their-debut/ also http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/hadron.html#c5
[17] Ibid, 38.
[18] Ibid.
[19] Source (and for diagram below): “Physicists zoom in on gluons' contribution to proton spin”, February 16, 2016 by Karen Mcnulty Walsh http://phys.org/news/2016-02-physicists-gluons-contribution-proton.html#jCp
[20] The Box lectures.
[21] http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/qevid.html
[22] A particle's spin is a measure of its angular momentum. It has little to do with its rate of rotation.
[23] Ent et al, op cit, 34-35, 38-39.
[23.1] This is a brief synopsis of the article by Abhay Deshpande and Rikutaro Yoshida, “The deepest recesses of the atom”, Scientific American, June 2019, 26-33.
[23.2] Ibid.
[24] The bottom quark (1977), the neutrino (1957), and the first natural high energy neutrino in 1965 were all discovered by Frederick Reines: Scientific American, February 1966, story retold Febrary 2016, p 67). The tau neutrino awaited discovery in 2000.
[24.1] Ray Jayawardhana, “Coming soon: A supernova near you”, Scientific American, December 2013, 60. Supernovas are considered later. See also "Francis Halzen, Neutrinos at the ends of the Earth", Scientific American, October 2015, 49, 51, which narrates the ongoing search for neutrinos in the Ice-cube neutrino-hunting particle receptor at the South Pole. See also Supernovas and neutrino formation
[24.2] This is the subject of further elaboration on the page Dark matter and gravitational lensing
[25] Hesketh, op cit, 201. See also the Box lectures, Session 5. segment 5.1.4; Session 6, segment 6.6.3. For the theory see Hesketh, op cit, 202-205. At 196-201, Hesketh describes a number of observations of solar and atmospheric neutrinos, including the Ray Davis experiment set up 1.5 kms underground down an old gold mine in Homestake, South Dakota from 1970 to 1974, which provided precise measurements of electron-neutrino interaction rates; another underground experiment down an old nickel mine in Sudbury, Canada - the Sudbury Neutrino Observatory (SNO), running from 1999 to 2006 - disclosing that neutrinos were in fact changing flavours during their long journey from the sun, and the Kamiokande detectors (1987; 1996) revealing much the same thing. This is also covered in Box, Session 5, segment 5.1.4.
[25.1] For this section and the preceding two paragraphs, see "The Darkest Particles" by William Charles Louis and Richard G Van de Water, Scientific American, July 2020, 42-49.
[25.2] Clara Moskowitz, “Theoretical Particles, Still Theoretical”, Scientific American, January 2015, 13. The points in the last sentence are Hesketh's, op cit, 208-210. See also Dark matter and gravitational lensing
[25.3] The story here is as recounted by Clara Moskowicz, "The Neutrino Puzzle", Scientific American October 2017, 22-29.
[26] Named after Satyendra Bose (1894 – 1974) by Paul Dirac. He provided the foundation for what is known as Bose–Einstein statistics, one of two possible ways in which a collection of non-interacting indistinguishable particles may occupy a set of available discrete energy states at thermodynamic equilibrium. Bosons obey this principle.
[27] Named after Enrico Fermi (1901-1954) for his contributions to the development of quantum theory, nuclear and particle physics, and statistical mechanics.
[28] Gavin Hesketh, The Particle Zoo - The Search for the Fundamental Nature of Reality, Quercus, Hachette, London, 2016, 8. Dana Mackenzie, The Universe in Zero Words – The Story of Mathematics, Elwin Street Productions, Sydney, 2012, 173. Best, ibid, 2. See also generally Adam Hart-Davis, Science- The definitive visual guide, DK, London, 2009, “Subatomic Particles”, 382-3
[29] Hesketh, op cit, 8.
[29.1] On the theory behind the W and Z bosons and their discovery, see Particles, fields and the four fundamental forces
[30] Bill Bryson, A Short History of Nearly Everything, Broadway Books, 2003, 164.
[31] Ibid, 133
[32] The Box lectures.
[33] https://press.cern/backgrounders/top-quark
[34] Ibid.
[35] As to which, see http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/fermi2.html
[36] The Box lectures, The four fundamental forces are considered in more detail at Particles, fields and the four fundamental forces
[37] Source: SMH obituary on Martin Perl (1927-2014) 7 October 2014. Perl and his colleagues detected the particle with the aid of a new collider called the Spear which started up on 1973, colliding electrons and their antimatter opposites, positrons, to produce tiny fireballs, little clouds of energy, which could then condense into anything including muons and their heavier brothers, if they existed. In 1995, when Perl won the Nobel Prize, he shared it with Frederick Reines, who had discovered the neutrino for their separate discoveries of “two of nature’s most remarkable sub-atomic particles”.
[38] In The Particle Zoo - The Search for the Fundamental Nature of Reality, Quercus, Hachette, London, 2016, 115.
[39] Ibid, 116-7.
[40] Ibid, 202. See also Feynman's sum over paths - overview