Standard Model of Physics

Elementary particles of the Standard Model.

The Standard Model of Physics is a comprehensive theory that describes three of the four fundamental forces (electromagnetic force, strong nuclear force, weak nuclear force, but not gravity) in the universe; it attempts to classify all known elementary particles. It has its origins in the late 20th century CE, finalized ~1970s CE when quarks were confirmed by experiment. Since then, additional proofs have been discovered for the top quark (1995 CE), the tau neutrino (2000 CE), and the Higgs boson (2012 CE), all of which lend further credence to the model's accuracy.

Despite its accuracy and repeated confirmation via experiment, it fails to adequately explain all physical phenomena (and so can't be considered a unified Theory of Everything in any real sense).

Electromagnetism (Electric Charge)

Electromagnetic force (or electromagnetism) describes the interaction between particles with electric charges via electromagnetic fields; it is the dominant force in the interactions between atoms and molecules and can be thought of as a combination of electrostatics and magnetism. The electrostatic attraction between atomic nuclei and their electrons holds atoms together, and electric forces also allow different atoms to bond into molecules, including macromolecules such as proteins. Meanwhile, magnetic interactions between the spin and angular momentum magnetic moments of electrons also play a role in chemical reactivity.

Electrostatics studies slow-moving or stationary electric charges. An electric charge (q) is a fundamental physical property of matter that causes it to experience a force when placed in an electromagnetic field; it can be positive (+) or negative (-), and an object with no net charge in either direction is called electrically neutral.

Electric charge is a conserved property; the net charge of an isolated system, the quantity of positive charge minus the quantity of negative charge, cannot be altered. Electric charge is carried by many subatomic particles; in macroscopic matter, negative charge is carried by electrons, and positive charge is carried by protons in the nuclei of atoms. If there are more electrons than protons in a piece of matter, it will have a negative charge, if there are fewer it will have a positive charge, and if there are equal numbers it will be neutral.

Electric charge is quantized; it comes in integer multiples of individual small units called the elementary charge (e, ~1.602×10⁻¹⁹ C, the smallest charge that can exist freely; this is the charge of a single proton, and its negative value is the charge of a single electron). Particles called quarks have smaller charges, multiples of 1/3e, but they are found only combined in particles that have a charge that is an integer multiple of e. In the Standard Model, charge is an absolutely conserved quantum number. The proton has a charge of +e, and the electron has a charge of −e. The coulomb (C) is the unit of electric charge in the International System of Units (SI).

Electric field induced by a positive electric charge (left) and a field induced by a negative electric charge (right).

Electric current is measured in amperes (i). An ampere of current represents the passage of one coulomb of charge per second, or 6.2 × 10¹⁸ electrons per second. A current is positive when it is in the direction of the flow of positive charges; its direction is opposite to the flow of negative charges.

An ordinary flashlight battery delivers a current that provides a total charge flow of approximately 5,000 coulombs, which corresponds to more than 10²² electrons, before it is exhausted.

Electric charges produce electric fields; a changing charge also produces a magnetic field (similarly, a changing magnetic field creates an electric field). The interaction of electric charges with an electromagnetic field (a combination of an electric and a magnetic field) is the source of the electromagnetic force.

A magnetic field describes the magnetic influence on electric currents (moving electric charges), and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the magnetic field. A permanent magnet's magnetic field pulls on ferromagnetic materials such as iron and attracts or repels other magnets. In addition, a nonuniform magnetic field exerts minuscule forces on 'nonmagnetic' materials by three other effects: paramagnetism, diamagnetism, and antiferromagnetism, although these forces are usually so small they can only be detected by laboratory equipment. Magnetic fields surround magnetized materials, electric currents, and electric fields varying in time. Since both strength and direction of a magnetic field may vary with location, it is described mathematically by a function assigning a vector to each point of space, called a vector field (more precisely, a pseudovector field).

The magnetic field does not point along the direction of the source of the field; instead, it points in a perpendicular direction. In addition, the magnetic force acts in a direction that is perpendicular.

These fields are fundamental in nature and can exist in space far from the charge or current that generated them. Remarkably, electric fields can produce magnetic fields and vice versa, independent of any external charge. Electric and magnetic fields travel together through space as waves of electromagnetic radiation, with the changing fields mutually sustaining one another.

The term field denotes a property of space, so that the field quantity has a numerical value at each point of space. These values may also vary with time. The value of the electric or magnetic field is a vector (a quantity having both magnitude and direction). The value of the electric field at a point in space equals the force that would be exerted on a unit charge at that position in space.

Electromagnetic field activity absent corresponding matter can be seen with radio and television waves, microwaves, infrared rays, visible light, ultraviolet light, X-rays, and gamma rays, all of which travel at the velocity of light (roughly 300,000 kilometres, or 186,000 miles, per second). They differ from each other only in the frequency at which their electric and magnetic fields oscillate.

According to Coulomb, the electric force for charges at rest has the following properties:

1. Like charges repel each other, and unlike charges attract. Thus, two negative charges repel one another, while a positive charge attracts a negative charge.
2. The attraction or repulsion acts along the line between the two charges.
3. The size of the force varies inversely as the square of the distance between the two charges. If the distance between the two charges is doubled, the attraction or repulsion becomes weaker, decreasing to one-fourth of the original value. If the charges come 10 times closer, the size of the force increases by a factor of 100.
4. The size of the force is proportional to the value of each charge. The unit used to measure charge is the coulomb (C). If there were two positive charges, one of 0.1 coulomb and the second of 0.2 coulomb, they would repel each other with a force that depends on the product 0.2 × 0.1. If each of the charges were reduced by one-half, the repulsion would be reduced to one-quarter of its former value.

Static cling is a practical example; garments made of synthetic material collect a charge, especially in dry winter air. A plastic or rubber comb passed quickly through hair also becomes charged and will pick up bits of paper. The synthetic fabric and the comb are insulators; charge on these objects cannot move easily from one part of the object to another. Similarly, an office copy machine uses electric force to attract particles of ink to paper.

Electromagnetic force is enormously strong compared with gravitational force. The absence of one electron out of every billion molecules in two 70-kilogram people standing two metres apart would repel them with a 30,000-ton force.

Naturally occurring minerals exhibit magnetic properties and have magnetic fields, which result from the motion of electrons in the atoms of the material. They also result from a property of electrons called the magnetic dipole moment, which is related to the intrinsic spin of individual electrons. In most materials, little or no field is observed outside the matter because of the random orientation of the various constituent atoms. In some materials such as iron, atoms within certain distances tend to become aligned in one particular direction.

Weak Nuclear Force

The weak nuclear force, also known as the weak interaction, is the mechanism responsible for the radioactive decay of atoms, and it participates in both nuclear fission and fusion. The force is labeled weak because its field strength over any set distance is typically several orders of magnitude less than that of the electromagnetic force, which itself is further orders of magnitude less than the strong nuclear force. The theory describing its behavior and effects is best known as electroweak theory (EWT) because the electromagnetic and weak nuclear forces are believed to be manifestations of a unified electroweak force (both have the same intrinsic strength in high-energy early post-Big Bang conditions). At the low energies typical of the universe today, the weak force seems less potent than the electromagnetic force due to the massiveness of the W and Z bosons that mediate it.

In more precise terms, the unification of the electromagnetic and weak forces under the electroweak theory implies that at sufficiently high energies (above the electroweak scale, roughly 10² GeV), the interactions can be described by a single gauge theory. This theory predicts that the coupling strengths of the electromagnetic and weak forces 'run' with energy due to quantum effects, and converge at these high energies, indicating they are manifestations of a single force.

The weak interaction's P and CP violations are not restricted to charged particles alone; they are features of the interaction itself, affecting processes involving both neutral and charged particles. However, the manifestations of these violations are often studied and more easily observed in processes involving charged particles, such as beta decay, where a neutron decays into a proton, an electron, and an antineutrino.

The electrically charged (propagated by the W+/W-) weak interaction is unique in a number of respects:

• It is the only interaction that can change the flavor of quarks and leptons (changing one type of quark into another).
• It is the only interaction that violates parity symmetry (P), and thus is also the only one that violates charge–parity symmetry (CP). CP violation in the weak force is a subtle effect and was a groundbreaking discovery.
• Both the electrically charged and the electrically neutral interactions are propagated by force carrier particles that have significant masses, explained in the Standard Model by the Higgs mechanism.

In the weak interaction, fermions can exchange three types of force carriers, namely W⁺, W⁻, and Z bosons. It acts upon left-handed fermions - elementary particles with half-integer values of spin (or intrinsic angular momentum), and right-handed antifermions. Particles use the weak interaction to exchange force-carrier particles known as the W and Z particles. These particles are heavy, with masses about 100 times the mass of a proton; it is their heaviness that defines the extremely short-range of the force. Its effectiveness is confined to a distance range of 10⁻¹⁷ metre, about 1 percent of the diameter of a typical atomic nucleus. This range is determined by the mass of the W and Z bosons through the uncertainty principle.

Particles can be left- or right-handed, depending on the direction of their spin relative to their direction of motion. The weak interaction only affects left-handed particles and right-handed antiparticles; this is a manifestation of its violation of parity: the laws governing the weak force are not the same when you mirror-flip the system.

The weak force operates through two types of processes: charged currents and neutral currents. Charged currents involve the exchange of W⁺ or W⁻ bosons and can change the flavor of quarks (for example, turning an up quark into a down quark) and leptons (turning a neutrino into an electron, for example). This is where the weak interaction's ability to change particle types (including charge) comes into play. Neutral currents, mediated by the Z boson, do not change the type (or flavor) of the particle but can still involve interactions with left-handed particles and right-handed antiparticles.

CP violation is a more subtle phenomenon where the combined operations of charge conjugation (C), which switches particles with their antiparticles, and parity inversion do not always lead to the same physical laws. In the weak interaction, certain processes occur at different rates for particles and their CP-mirrored counterparts, leading to slight asymmetries that have profound implications for the matter-antimatter imbalance in the universe. This violation is intricately linked to the behavior of quarks within particles during certain weak decay processes and has been observed in experiments involving K mesons (kaons) and B mesons.

The lifetimes for particles that decay via the weak interaction vary from as little as 10⁻¹³ second to 896 seconds, the mean life of the free neutron. Neutrons bound in atomic nuclei can be stable, as they are when they occur in the familiar chemical elements, but they can also give rise through weak decays to the type of radioactivity known as beta decay. In this case the lifetimes of the nuclei can vary from a thousandth of a second to millions of years. Most fermions decay by a weak interaction over time. Such decay makes radiocarbon dating possible, as carbon-14 decays through the weak interaction to nitrogen-14 after the death of an organism. It can also create radioluminescence, commonly used in tritium luminescence, and in the related field of betavoltaics.

Radioactive decay, particularly that governed by the weak interaction, involves the transformation of one type of particle into another to achieve a more stable configuration. Generally, a particle will decay if there exists a more stable configuration with lower mass/energy. The mass difference between the initial particle and the decay products allows the decay to proceed according to the conservation of energy, with the excess released as kinetic energy of the decay products.

The half-life of a radioactive isotope is a measure of the time it takes for half of the atoms in a given sample to decay. This property is intrinsic to each isotope and depends on the stability of the nucleus, which is influenced by the nuclear forces and the arrangement of protons and neutrons. The half-life is a statistical average that emerges from the quantum mechanical behavior of an ensemble of nuclei and reflects the probability of decay of an individual nucleus over time. The process is random for an individual atom, but predictable when considering a large number of atoms.

The probability of decay in quantum mechanics does not mean a decay will happen precisely after one billion attempts but indicates the average likelihood across many events. For example, in a scenario where a particular weak interaction happens once in a billion tries, that ratio reflects the underlying quantum probabilities governing the interaction. It's a statistical statement about the frequency of those interactions among a vast number of opportunities, influenced by the intrinsic properties of the weak force and the particles involved.

Decay processes must conserve certain quantum numbers, such as electric charge, lepton number, and baryon number. A particle susceptible to weak decay is one that can transform into products that observe these conservation laws while releasing energy. The weak force, being responsible for processes that can change quark flavor, allows particles like neutrons (composed of quarks) to decay into protons (composed of a different combination of quarks). The neutron-to-proton transition in beta decay is a classic example where the weak interaction facilitates the decay of a less stable particle (neutron) into more stable particles (proton, electron, and antineutrino) while respecting the conservation laws.

Classes of Elemental Particles

Moving from the largest constituents to the smallest, the first division within elementary particles is between fermions and bosons.

Fermions

Fermions are particles of half-integer spin - the standard model includes 12.

Bosons

What Gaps?

• Baryon asymmetry
• gravitation
• accelerating expansion of Universe
• no dark matter / energy
• no accounting for neutrino oscillations, non-zero masses
• quantum mechanics
• hierarchy problem
• 19 parameters - origin of values?
• Strong CP Problem