A post for general readers who’ve heard of quarks; if you haven’t, try reading here:
The universe has six types of quarks, some of which are found in protons and neutrons, and thus throughout all ordinary material. For no good reasons, we call them up, down, strange, charm, bottom and top. Today and tomorrow I want to show you how we know their electric charges, even though we can’t measure them directly. The only math we’ll need is addition, subtraction, and fractions.
This also intersects with my most recent post in this series on the Standard Model, which explained how we know that each type of quark comes in three “colors”, or versions — each one a type of strong nuclear charge akin to electric charge.
Today we’ll review the usual lore that you can find in any book or on any website, but we’ll see that there’s a big loophole in the lore that we need to close. Tomorrow we’ll use a clever method to close that loophole and verify the lore is really true.
The Lore for Protons and Neutrons
Physicists usually define electric charge so that
- the proton has electric charge +1
- the electron has charge -1,
- the neutron has charge 0 (i.e. electrically neutral, hence its name).
[Throughout the remainder of this post, I’ll abbreviate “electric charge” as simply “charge“.]
As for the six types of quarks, the lore is that their charges are [using notation that “Qu” means “electric charge of the u quark“]:
- Up, Charm, Top (u,c,t): Qu = Qc = Qt = 2/3
- Down, Strange, Bottom (d,s,b): Qd = Qs = Qb = -1/3
But how do we know this?
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