# Baryon number

The baryon the particle , a quantum number of the elementary particles is defined as the difference in the number of quark and the number of anti quark divided by 3: ${\ displaystyle B \! \,}$

${\ displaystyle B = {\ frac {n_ {q} -n _ {\ overline {q}}} {3}}}$.

So it is:

• / 0+1 for baryons like the proton and the neutron (each composed of 3 quarks)
• +1/3 for quarks
• + / 00 for leptons (such as the electron ) and for mesons
• −1/3 for antiquarks and
• / 0−1 for antibaryons (each composed of 3 antiquarks).

## Baryon number as a conserved quantity

Experience has shown that the number of baryons in a closed system always remains constant, so it is an absolute conservation quantity . This knowledge - a basic component of the standard model of elementary particle physics - makes the stability of matter understandable. Since a spontaneous decay can only ever lead to lighter particles because of the conservation of energy, the lightest baryon, the proton, is stable.

In many theories going beyond the standard model, such as B. According to the great unified theory (GUT) the baryon number is not an exact conserved quantity, so that protons decay over time , but with a very long half-life .

The currently assumed mechanisms of baryogenesis , the emergence of the imbalance between matter and antimatter in the early universe fractions of a second after the Big Bang , assume that the baryon number is not maintained.

In most versions of the GUT, however, at least the difference BL of baryons and leptons is strictly preserved.