Video Transcript
A phi meson is a particle that is
made up of a strange quark and a strange antiquark. What is the strangeness of a phi
meson?
Let’s say we represent this
particle using the Greek letter Φ. Now, because this particle is a
meson, we can recall that that means it’s made up of one quark and one
antiquark. And indeed we’re told that this
particle is made up of a strange quark and a strange antiquark. We then want to figure out what is
the strangeness of a phi meson.
Strangeness, we can recall, is a
property of particles as well as nuclear interactions. The strangeness of an interaction
comes from the number of strange quarks and strange antiquarks involved. The rule for determining
strangeness is, for every strange quark involved, a strangeness of negative one is
contributed. And then every strange antiquark
has a strangeness of positive one.
Now, the fact that the strangeness
of a strange quark is negative one and that of a strange antiquark is positive one
might seem a little strange, get it? But the reason for this comes down
to the historical fact that the concept of strangeness preceded the discovery of
quarks.
Anyway, we can now use these rules
to figure out the strangeness of our phi meson. Before we count up any of the
quarks involved, let’s say that our particle has an overall strangeness of zero. We can consider this our starting
value. And then as we take into account
this strange quark, our rule tells us that that contributes a strangeness of
negative one. At this point then, that’s the
strangeness of our particle overall. But then we consider the
contribution of our strange antiquark. And going back to our rule, this
contributes a strangeness of positive one. So our strangeness overall is
negative one plus one, or simply zero. And this is the strangeness of our
phi meson. The strangeness of the strange
quark and that of the strange antiquark canceled one another out.
