Video Transcript
Which of the particles shown in the
diagram are mesons? The quarks shown in the diagram are
colored according to their electric charge.
Okay, we see these six particles
marked (a), (b) (c), (d), (e), and (f). And we want to figure out which of
them are mesons. We’re told that the quark shown in
this diagram are colored according to their electric charge. Positively charged quarks, like the
up quarks in particle (a), are colored red and negatively charged quarks, like the
down quark here, are colored blue. Now mesons are a class of particle
that are defined by two conditions. First, they’re made up of one quark
and one antiquark. And second, the total relative
charge of a meson is an integer value. Considering this first condition
that mesons are made of one quark and one antiquark, we see right away that three of
these particles don’t meet that condition.
Particles (a), (d), and (f) are all
made of three quarks and therefore can’t be mesons, leaving us with particles (b),
(c), and (e). Looking at these three, we see that
each one is indeed made up of one quark and one antiquark. So all that remains to be seen is
if each one has a total relative charge of an integer value. Looking first at particle (b), we
can recall that an up quark has a relative electric charge of positive two-thirds,
where this is positive two-thirds times the charge of a single proton, while a down
antiquark has a relative electric charge of positive one-third. Two-thirds plus one-third equals
one, which is an integer. So particle (b) satisfies both of
our conditions and therefore is a meson.
Considering next particle (c), we
know that a strange quark has a relative electric charge of negative one-third times
the charge of a proton, while an up antiquark has a relative charge of negative
two-thirds. Adding these two charges together,
we find a result of negative one, which also is an integer. So particle (c) is a meson as
well. The charm quark in particle (e) has
a relative charge of positive two-thirds and the down antiquark has a relative
charge of positive one-third. Combining these charges gives an
overall charge of one. So this particle too is a
meson. In this diagram then, particles
(b), (c), and (e) are all mesons.
