Standard model

Taking a closer look at LHC

The Standard Model of Particle Physics is the best theory that physicists currently have to describe the building blocks of the universe. It is one of the biggest scientific achievements in twentieth-century science.


The Standard Model describes the universe using 6 quarks, 6 leptons and a few force-carrying particles. There arefour known forces (or interactions), each mediated by a fundamental particle, known as a carrier particlePhotons, gravitons, and gluons have no mass, whereas weak-force carrier particles (W± and  Zº) have mass 80-90 GeV.
Gravity is only included in the Standard Model by tentative hypothesis, but gravitons have never been directly observed.



At very high energies levels and very small scales the other three forces become almost identical, but the convergence is imperfect.  

Electromagnetic & gravitational forces vary as the inverse square of distance without limit (to infinity).

But the strong and weak nuclear forces are short-range rather than inverse-square forces. Short-rangeforces only operate at very short ranges through the exchange of particles. Whereas inverse-square forces have no range-limits. It is the non-zero rest mass of the short-range force-mediating particles which causes them to decay quickly and thereby limits their range. For the strong nuclear force the exchange-particle is the gluon (nuclear "glue"). For the weak nuclear forcethe exchange-particle is W+, W- or Zº.


Particles that are affected by the strong nuclear force are called hadronswhereas leptons are not affected.Hadrons are formed by quarks, so they are not considered elementals, but leptons have no structureand they are thus truly elementary.

There are six types (also so-called flavors) for quarks and leptons.



Leptons can exist being isolated but quarks are always associated in three (baryons) or in pairs quark and antiquark (mesons). Protons and neutrons are the most known baryons and pions and kaons are the most known mesons.
Quarks only exist inside hadrons because they are confined by the strong force fields. Therefore, we cannot measure their mass by isolating them. The nature of the strong force between quarks does not permit isolated individual quarks to exist. 
This is a radically new feature of the strong force, never before encountered, but understandable in terms of the details of the strong forces characteristics.

All particles are classed as either fermions or bosons. The difference among them is due to their spin

Fermion: name for a particle that is a matter constituent, characterized by spin in odd half integer quantum units (1/2,3/2,5/2...). Named for Italian physicist Enrico Fermi. Quarks, leptons and baryons are all fermions.

Fermions, cannot occupy the same quantum state as each other. They obey the Fermi-Dirac statistics and the Pauli exclusion principle. They "resist" being placed close to each other. So, fermions possess "rigidness" and thus sometimes are considered to be "particles of matter".

The Pauli exclusion principle obeyed by fermions is responsible for the stability of the electron shells of atoms (thus for stability of atomic matter). It also is responsible for the complexity of atoms (making it impossible for all atomic electrons to occupy the same energy level), thus making complex chemistry possible. It is also responsible for the pressure within degenerate matter which largely governs the equilibrium state of white dwarfs and neutron stars.


Bosonname for any  particle with a spin of an integer number ( 0,1 or 2...) of quantum units of  angular momentum. (named for Indian physicist S.N. Bose). The carrier particles of all interactions are bosonsMesons are also bosons (named for Indian physicist and mathematician Satyendra Nath Bose, a physicist best known for his collaborations in the 1920s with Albert Einstein wich resulted in the invention of Bose-Einstein statistics).

In contrast to fermions, several bosons can occupy the same quantum stateThus, bosons with the same energy can occupy the same place in space. After the discovery of Hioggs boson, the only boson in the Standard Model that is yet to be discovered experimentally is the graviton.

When a large fraction of the bosons occupy the lowest quantum state a new state of matter is achieved, the Bose–Einstein condensate (BEC).

Superconductivity, the properties of lasers and masers, superfluid helium-4 and others Bose–Einstein condensates are all consequences of statistics of bosons.

A very important result has been presented in the beggining of 2021 at the Rencontres de Moriond EW conference and at a seminar at CERN. The LHCb Collaboration presented an updated measurement of the ratio RK, an important test of a principle of the Standard Model of particle physics known as "lepton universality". For exemple, analysing process such as B+→ (K+μ+μ-) and B+→ (K+e+e-), the expected value for RK should be very close to 1, should the SM hold. However, LHCb, has very recently measured this ratio to be minor than 0.89. The result provides "evidence" that the SM might be wrong when describing how leptons behave in nature.


Xabier Cid Vidal, PhD in experimental Particle Physics for Santiago University (USC). Research Fellow in experimental Particle Physics at CERN from January 2013 to Decembre 2015. Currently, he is in USC Particle Physics Department ("Ramon y Cajal", Spanish Postdoctoral Senior Grants).

Ramon Cid Manzano, secondary school Physics Teacher at IES de SAR (Santiago - Spain), and part-time Lecturer (Profesor Asociado) in Faculty of Education at the University of Santiago (Spain). He has a Degree in Physics and in Chemistry, and is PhD for Santiago University (USC).



CERN Directory

CERN Experimental Program

Theoretical physics (TH)

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CERN Structure

CERN and the Environment



Detector CMS

Detector ATLAS

Detector ALICE

Detector LHCb

Detector TOTEM

Detector LHCf

Detector MoEDAL



 For the bibliography used when writing this Section please go to the References Section

© Xabier Cid Vidal & Ramon Cid -  | SANTIAGO (SPAIN) |