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1.6 More Quarks
``Strange'' mesons and baryons were discovered in the late 1940s, soon after the discovery of the pions. It is apparent that as well as the u and d quarks there exists a so-called strange quark s, and strange particles contain one or more s quarks. An s quark can replace a u or d quark in any baryon or meson to make the strange baryons and strange mesons. The electric charges show that the s quark, like the d, has charge -e/3, and the spectra can be understood if the s is assigned isospin I = 0.
The lowest mass strange mesons are the I = 1/2 doublet,
K- (s, mass
494 MeV) and K0 (d
, mass 498 MeV). Their antiparticles make up
another doublet, the K+(u
) and
0(
s).
The effect of quark replacement on the meson spectrum is illustrated
in Fig. 1.4. Each level in the
spectrum of Fig. 1.4(b) has a member
(d) with charge
-e. Fig. 1.4(c) shows the
spectrum of strange (s
)
mesons. There is a correspondence in angular momentum and parity
between states in the two spectra. The energy differences are a
consequence of the s quark having a much larger mass, of the order of 200 MeV.
The excess of mass of the s quark over the u and d quarks makes the s quark in any strange particle unstable to decay by the weak interaction.
Besides the u, d and s quarks there are considerably heavier quarks:
the charmed quark c (mass 1.3 GeV / c2, charge 2e/3), the
bottom quark
b (mass
4.3 GeV /
c2, charge -e/3), and the top quark t
(mass
180
GeV / c, charge 2e/3). The quark masses are most
remarkable, being even
more disparate than the lepton masses. The experimental investigation
of the elusive top quark is still in its infancy, but it seems that
three quarks of any of the six known flavours can be bound to form a
system of states of a baryon (or three antiquarks to form antibaryon
states), and any quark-antiquark pair can bind into mesonic states.
The c and b quarks were discovered in e+e-
colliding beam machines.
Very prominent narrow resonances were observed in the
e+e-
annihilation cross-sections. Their widths, of less than 15 MeV,
distinguished the meson states responsible from those made up of u, d
or s quarks. There are two groups of resonant states. The group at
around 3 GeV centre of mass energy are known as J / resonances, and
are interpreted as charmonium c
states. Another group around 10 GeV,
the
(upsilon) resonances,
are interpreted as bottomonium b
states. The current state of knowledge of the c
and b
energy levels
is displayed in Fig. 1.5. We shall discuss
these systems in Chapter 17.
The existence of the top quark was established in 1995 at Fermilab,
in p collisions.