In the case of strange quarks we formed strange baryons and
mesons which were a few hundred MeV more massive than their up
and down flavoured counterparts. A similar story happens with the
charm quark, but due to its greater mass, the analogous charmed
mesons and baryons weigh in correspondingly heavier, the lightest
being found around ∼1900 MeV or nearly 2 GeV. In part as a result
of this greater mass, they are not easily produced in cosmic rays,
and it was only with the advent of dedicated experiments at high-
energy particle accelerators that the existence of charmed particles,
and the charm quark, became known in the final quarter of the
20th century.
Charm quarks can link in threes with any combination of up, down,
or strange quarks to make baryons with charm, or even with both
charm and strangeness. A few examples have even been seen where
two charmed quarks have joined with an up, down, or strange
quark. We expect that three charmed quarks can join to make a
baryon with three units of charm, but clear evidence for its
existence is still awaited.
A charmed quark can link with a single antiquark that can be any of
(anti)- up, down, or strange. The most celebrated examples, though,
are where a charmed quark joins with a charmed antiquark, cc
¯
,
leading to yet another electrically neutral partner, adding to the
pion and etas, made from uu¯; dd
¯
or ss
¯
that we already met. The
resulting ‘eta-c’, written η
c
, has a mass of just below 3,000 MeV,
3 GeV, and as such is the lightest example of a whole spectroscopy
known as ‘charmonium’.
It was through charmonium that the charm property was first
discovered. The η
c
is formed when the c and c
¯
, each having
spin 1/2, couple their spins to a total of zero (see Figure 26).
They can also couple their spins to give a total value of one; this
forms a slightly heavier state at 3.1 GeV known as the psi: ψ. When
an electron and a positron meet and annihilate, they do so most
readily when their spins are correlated to make spin one. In such a
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Particle Physics