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The section on particle interactions in my revision guide says that only the weak interaction can change quark types, e.g. when a neutron changes to a proton the down quarks in the neutron are changed to up quarks.

So I was wondering: If it is only the weak interaction that can change quark type, that means that quarks can't be changed in a strong interaction. This means that the quark types have to balance on both sides of the equation in a strong interaction.

My real question: Is this therefore a valid conservation law for particle interactions?

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Yes, there are the quantum numbers Charm, Strangeness, Topness and Bottomness, which are conserved by strong and electromagnetic interactions, but not by weak interactions. Upness and Downness are simply the Isospin, which is also preserved for strong interactions, when the quark masses can be neglected, which is usually a very good approximation as $m_u,m_d\ll \Lambda_{QCD}$. This $SU(2)$ isospin symmetry essentially breaks into $U(1)^2$, upness and downness, when the light quark masses become relevant.

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Is there a quark conservation law?

No. In proton-antiproton annihilation (see Wikipedia) the quarks are destroyed. See this section:

"...when a proton encounters an antiproton, one of its constituent valence quarks may annihilate with an antiquark, while the remaining quarks and antiquarks will undergo rearrangement into a number of mesons (mostly pions and kaons), which will fly away from the annihilation point. The newly created mesons are unstable, and will decay in a series of reactions that ultimately produce nothing but gamma rays, electrons, positrons, and neutrinos."

One can always talk about some interactions conserving quarks and/or certain quantum numbers, but IMHO it's going too far to start talking about quark conservation. Energy is conserved, matter isn't.

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    $\begingroup$ -1 This does not at all break the conservation of the number of quarks. An antiquark is considered equal to -1 quark. $\endgroup$
    – Poseidaan
    Commented Jan 9, 2021 at 11:08
  • $\begingroup$ @Martin van IJcken : So how many quarks are there in a meson? By your logic there are none. $\endgroup$ Commented Jan 19, 2021 at 20:56
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    $\begingroup$ Yes, a meson has net zero quarks. Just like how a hydrogen atom has net zero electric charge. We wouldn't say the electron positron anihilation violates charge conservation just because the number of electrically charged particles changes. $\endgroup$
    – Poseidaan
    Commented Jan 20, 2021 at 13:14
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    $\begingroup$ I know that the concept of the dirac sea is outdated, but it is sufficient for this situation: the antiquark could be seen not so much as a quark but as a hole in an endless sea of quarks, quark anti-quark anihilation has a quark fall into that hole, so the number of particles+holes decreases but the number of quarks remains constant. $\endgroup$
    – Poseidaan
    Commented Jan 20, 2021 at 13:22

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