I have read these questions:

Weak Interaction and Strangeness transformation

Weak decay iff or if strangeness changes?

Understanding type of force interaction in particle decays

The decay of the strange quark via weak interactions?

As I understand, currently all experimental data support the SM as being the accepted theory. Both weak interactions and strangeness and strange quarks are part of the SM model.

As I understand, energy is always conserved in the SM, and all other forces, EM, gravity and strong all conserve both energy and strangeness.

If the SM is supported by all experimental data, and it always conserves energy, then how is it possible that some weak interactions do not conserve strangeness?

Where does the energy go? How can the energy be conserved by not strangeness?

In our modern understanding, strangeness is conserved during the strong and the electromagnetic interactions, but not during the weak interactions. Consequently, the lightest particles containing a strange quark cannot decay by the strong interaction, and must instead decay via the much slower weak interaction. In most cases these decays change the value of the strangeness by one unit. However, this doesn't necessarily hold in second-order weak reactions, where there are mixes of K0 and K0 mesons. All in all, the amount of strangeness can change in a weak interaction reaction by +1, 0 or -1 (depending on the reaction).

Consequently, the lightest particles containing a strange quark cannot decay by the strong interaction, and must instead decay via the much slower weak interaction.

It is talking about this strangeness conservation braking as if it was only for composite particles, that contain strange quarks. But what about the strange quark itself? Is the strange quark stable, or can the strange quark as elementary particle decay via weak interaction?


  1. If energy is conserved during all weak interactions, then how can strangeness not be conserved? Is strangeness equal to energy asymmetry?

  2. Can strangeness not be conserved for (interactions/decay of) non-composite particles?

  • 1
    $\begingroup$ The strangeness is not energy, it is just a quantum number. You see particles in the collider, measure their properties, incl. their mass, and their decay chains. I hope a pro will explain it in a more detailed form. My layman view is that something is "extraordinary" with the weak interaction, any time if particles change flavor, it happens due to weak interaction, or annihillation. As if the weak interaction would be like a bridge between the others. $\endgroup$
    – peterh
    Commented Apr 11, 2019 at 21:04
  • $\begingroup$ Your do appreciate quarks cannot be observed in isolation, but get dressed by gluons and quark-antiquark pairs always inside composite hadrons, no? Energy and momentum are conserved in elementary interaction vertices, but these often involve virtual particles violating the energy-momentum on-shell constraints you are used to? $\endgroup$ Commented Apr 11, 2019 at 21:24
  • $\begingroup$ This sounds a bit unclear. Surely energy isn't the exact same thing as strangeness. What do you think strangeness is? $\endgroup$
    – knzhou
    Commented Apr 12, 2019 at 14:55

1 Answer 1


Strangness is a quantum number as peterh explained in his comment. Every particle has a strangeness number $S = -(n_s - n_\overline{s})$, where $n_s$ and $n_\overline{s}$ are the number of strange quarks and anti-quarks - the minus sign in the front looks arbitrary, the important thing is that it is the difference between the particles and anti-particles. There are equivalent quantum numbers for other flavours, like Bottomness.

The CKM quark mixing matrix shows how the weak interaction mixes quarks - it arises due to the mass eigenstates not being the same as flavour eigenstates. The charged weak interaction is able to change the flavour of a quark, e.g. beta decay - $d \rightarrow W^- + u, \Delta S = 0$. In the same way, you can have a strange quark decaying into an up quark $s \rightarrow W^- + u, \Delta S = 1$ or say a top quark decaying into s, $t \rightarrow W^+ + s, \Delta S = -1$. The probability of these interactions are proportional to the square of the CKM matrix elements - $|V_{ij}|^2$.

Cosmas Zachos covers the answer about isolated quarks.

  • $\begingroup$ What is the strangeness number for the strange quark itself? Does an elementary strange quark have a strangeness number? $\endgroup$ Commented Apr 12, 2019 at 15:26
  • $\begingroup$ Yes, the strange quark carries strangeness of -1, while the anti-quark carries strangeness +1. You can check it in table 15.1. here for example pdg.lbl.gov/2018/reviews/rpp2018-rev-quark-model.pdf $\endgroup$
    – Vangi
    Commented Apr 12, 2019 at 15:48
  • $\begingroup$ How would you say to a layman, what is strangeness, is it just that some light quarks decay into heavier strange quarks inside composite particles? So basically inside a composite particle (that is made of quarks), a normal quark (up or down) will spontaneously decay into a strange quark, thereby change the strangeness of the composite particle? Why is this strange? Is there any physical law that would say they should decay differently? $\endgroup$ Commented Apr 12, 2019 at 16:03
  • $\begingroup$ I already gave examples of how a constituent quark can decay via the weak interaction producing a strange quark and the reverse process - a strange quark decaying via the weak interaction into another flavour. Strangeness is a quantity that is 0 for every other flavour of quark (see table above!), -1 for the strange quark and +1 for the strange anti-quark. It is just counting the difference (number of anti-strange quarks - number of strange quarks). The term "strangeness" was coined before the quark model to characterise unexpected long lifetimes of certain particles. The term stuck around... $\endgroup$
    – Vangi
    Commented Apr 12, 2019 at 16:16
  • $\begingroup$ You can find a more accurate history on the internet, e.g. en.wikipedia.org/wiki/Strange_quark and the references there. $\endgroup$
    – Vangi
    Commented Apr 12, 2019 at 16:19

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