7.3. The Structure of Matter

Now, there a few key terms that are important.

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Charge - we are familiar with this concept, though defining what it is can be remarkably tricky. Charge is a fundamental property of matter that causes an interaction due to the electromagnetic force. It is measured in Coulombs (though in particle physics we normally talk about relative charge - relative to a proton/ electron). Particles and antiparticles always have opposite charges.

e.g. an up quark has a relative charge of +⅔. This is the same as ⅔ × 1.6×10ˉ¹⁹ = 1.07 ×10ˉ¹⁹ C.

An antiup  quark has a charge of -⅔ (which is the same as -1.07 ×10ˉ¹⁹ C)

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If multiple quarks are combined, the total charge is the sum of the individual quark charges (e.g. a proton, uud, has a charge of +⅔ +⅔ -⅓ = +1). Charge is ALWAYS conserved

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Mass - again, another fundamental property of matter. All our quarks and leptons are massive (even if some have tiny masses). Some of our bosons are massless, whereas some do have mass. If we look at the table below, our masses are given in MeV/c². This is a common way to present mass in particle physics - it is simply making use of Einstein's E = mc² equation. (note - the c² in the unit to denote that this is a mass - 2 MeV/c² is the mass equivalent of MeV of energy). Particles and antiparticles have the same mass (there is no such thing as negative mass).

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e.g. a down quark has a mass of 4.7 MeV/c². To calculate this in kilograms:

4.7 MeV/c² is the mass equivalent of 4.7 MeV of energy. First, We need to use our standard eV:  J conversion to convert this to Joules. This becomes 4.7×10⁶ × 1.6×10ˉ¹⁹  = 7.52×10ˉ¹³ J. Lastly, we can use Einstein's E= mc² (or m = E/c²) to work this out as a mass equivalent in kilograms. Dividing by c² gives us a mass for our down quark of 8.36×10ˉ³⁰ kg. The mass of an antidown quark is also 8.36×10ˉ³⁰ kg

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If multiple quarks are combined, the total mass is NOT the sum of the constituent quark masses (e.g. a proton, uud, has a greater mass than the sum of the masses of an up, an up and a down quark. This goes back to our idea of binding energy in Section 7.2. 

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Strangeness - this is a weird property, from which the strange quark is named. The strange quark has a strangeness of -1. The antistrange quark has a strangeness of +1. Everything else has a strangeness of 0. Strangeness is conserved, except through certain weak interactions.

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Spin - this is another fundamental property of the particles in the standard model. The IB does not require much understanding of particle spin. It is analogous to the rotation of macroscopic bodies, which have a certain angular momentum. Again, you don't but need to know too much about this, but all quarks and leptons have a spin of +½ - so they are given the collective term 'fermions'.