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Today I was reading "Fundamentals of Physics" Resnick, Halliday and Wheeler. There they said that kaon and sigma were always produced together in an equation where pion and proton reacts. Then scientists came up with this new thing called strangeness which needs to be conserved. But I didn't understand why is the strangeness of kaon +1 and sigma -1?

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    $\begingroup$ By definition , so that the sum adds up to zero :input pion +proton =0 strangeness, output sigma+kaon +0 strangeness, The choice of who is + and who is - is arbitrary to start with, but has to be consistent in all interactions. $\endgroup$ – anna v Mar 31 at 15:05
  • $\begingroup$ Now we understand this, I think, as being because a strange quark-antiquark pair can get created when a pion and a proton collide, and one goes into the kaon and the other into the sigma. So strangness is just keeping track of the number of strange quarks or antiquarks. $\endgroup$ – G. Smith Apr 1 at 0:39
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As an addition to the comments of anna v and G. Smith [I can't reply there yet] strangeness is (only) conserved in strong and EM interactions, but not necessarily in weak interactions, e.g. a constituent strange quark decays $s \rightarrow W^- + u$, where you can see there is a strange quark on the LHS and none on the RHS.

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