A Level Physics Practice Exam

What is the baryon number of an up quark?

1. 1/3

2. 1

3. 0

4. -1/3

The correct answer is: 1/3

The baryon number is a fundamental property in particle physics that helps classify baryons and non-baryons. Baryons, such as protons and neutrons, have a baryon number of +1, while antibaryons have a baryon number of -1. Non-baryonic particles, such as leptons and quarks, do not constitute baryons, so they have a baryon number of 0. In the case of quarks, each individual quark contributes a fractional baryon number to the overall baryon number of a baryon. Specifically, a single up quark contributes a baryon number of +1/3. When three up quarks combine, as they do in a proton, their baryon numbers add up to give the proton a total baryon number of +1. Thus, the baryon number of an up quark is correctly identified as +1/3. This distinction helps illustrate how quarks combine in groups of three or two (in the case of mesons) to form particles that adhere to the baryon number conservation law in particle reactions.