Environment, Magnetism, Quantum mechanics, Subatomic particles

Standard Model of particle physics

The Standard Model was presented in 1970. It’s the result of many researches and discoveries in particle physics area since 1930.

Division of fundamental particles

The Standard Model has 17 elementary particles, i.e., can’t be divided into smaller particles.

The compositions of proton and neutron. Source: Quora.
Standard Model
In the Standard Model, elementary particles are divided into two groups: fermions and bosons. Fermions are divided into quarks and leptons, the others are bosons. Source: A ciência divulgada.

Fermions on Standard Model

These elementary particles form the matter, follow the Pauli exclusion principle, and spin value is a half integer multiple (1/2, 3/2, etc.) of . What is spin? It is the particle’s intrinsic angular momentum and doesn’t depend on its motion, spin has a quantized value, i.e., can only adopt discrete values, which are a positive or negative multiple of . Where ℏ is the reduced Planck constant, whose value and equation are below. All observed fermions until now have a spin of 1/2ℏ or -1/2ℏ.

reduced Planck constant
Planck constant (h) is 6,626\cdot 10^{-34}J\cdot s. Source: Todo estudo.
electron spin number
Spin indicates the rotation direction of electrons and other particles. Source: Un-Understand Physics.

The Pauli exclusion principle claims that two or more particles can’t have the same quantum state, i.e., can’t have all quantum numbers exactly equal when they are in the same confinement region.

An example of the Pauli exclusion principle, two electrons that occupy the same electronic layer (main quantum number), same energy sublevel (azimuth quantum number), and same orbital (magnetic quantum number), must have opposite spins. Source: expii.

Pauli exclusion principle makes the atoms stable.


Quarks are always in groups of two (meson) or three (baryon) because the strong nuclear force attracts them.

Sets of quarks form the hadrons. Source: Pinterest.

In addition to quarks being divided into six “flavors” (up, down, top, bottom, charm, and strange), they also have a property called “cor”, which can be red, green, or blue. They aren’t actually colors, just arbitrary labels to explain why two or three identical quarks can occupy the same quantum state, which is an apparent violation of the Pauli exclusion principle. This concept gave origin to the field of quantum chromodynamics (QCD).


Leptons aren’t affected by strong nuclear force. But they interact with electromagnetic and weak nuclear forces. The muon is 207 times heavier than the electron (e^{-}) and was discovered during an observation of cosmic rays, in 1936. It’s an unstable particle, that lasts only 2.2 microseconds before decay to lighter particles. Below are the decay reactions of muon (\mu^{-}) and antimuon (\mu^{+}) respectively.

\mu^{-}\rightarrow e^{-}+\nu _{\mu}+\bar{\nu}_{e}

\mu^{+}\rightarrow e^{+}+\bar{\nu _{\mu}}+\nu_{e}


  • The e^{+} is the positron, electron’s antiparticle.
  • \nu_{e} and \bar{\nu}_{e} are electron’s neutrino and antineutrino, respectively.
  • \nu _{\mu} and \bar{\nu _{\mu}} are muon’s neutrino and antineutrino, respectively.

Tau is heavier than muon, it’s also found in cosmic rays and lasts 2.9\cdot 10^{-13} seconds.

Tau decay and Standard Model particles.
The tau can decay in many particle sets, each set has a probability. It’s the only lepton that can decay in particles with quarks, such as pions. Source: ClearlyExplained.

When particles travel at a speed close to the light, they can last longer.

Every fermion has its antimatter counterpart or antiparticle and antiquarks have anticolors. However, antimatter interacts with gravity in the same way as common matter. The neutrinos will be topic of another post.

Bosons on Standard Model

These particles carry energy and exert the universe’s fundamental forces. They have a spin integer value and many of these particles can occupy the same quantum state.

Gluons and photons

Gluons transmit the strong nuclear force, which keeps the quarks united to form protons, neutrons, and other hadrons. Thanks to the strong nuclear force, protons with positive charge can be united to form the atomic nucleus with neutrons. Photons are light particles, which exert electromagnetic force, transmitting energy to electrically charged particles such as electrons.

Z and W bosons

These particles mediate the weak nuclear force and are too heavy, limiting the influence of weak nuclear force, that’s why has this name. This force is responsible for beta decay, which is when a neutron becomes a proton and when an unstable isotope of an element decays to one more stable.

beta decay
Beta decay can emit an electron or a positron, depending on the involved elements. Source: NASAPhysics.

It’s also responsible for changing particles’ identity when they emit or absorb one of these bosons. Below are diagrams that show some examples of interactions between bosons and fermions.

Boson W interactions with other particles on Standard Model.
The boson W is electrically charged with positive or negative charge. Source: KeyStageWiki.
β is another symbol for an electron. Source: KeyStageWiki.
Boson Z interactions with other particles on Standard Model.
While the Z boson is electrically neutral. Source: Quantum Physics.

The Standard Model is incomplete because it predicts the existence of graviton, the still not observed particle responsible for gravity, which is another nature’s fundamental force. CERN discovered the Boson of Higgs in 2012, this particle will be subject for a future post.

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About Pedro Ney Stroski

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