Subatomic Physics

310 The Electromagnetic Interaction
Table 10.1: Vector Mesons. ηP is the parity and ηc the charge parity
of the vector mesons.
Rest Dominant
Energy Width Decay
Meson I J ηP ηc Y (MeV) (MeV) Mode
ρ 0 1 1 −1 −1 0 770 153 ππ
ω 0 0 1 −1 −1 0 783 10 π + π − π 0
φ 0 0 1 −1 −1 0 1020 4 KK
peak was identified with the rho meson. Why does the rho turn up here? Before
answering this question, two more experiments will be discussed to provide additional
information. In Fig. 10.16, the cross section for the process e + e − → K + K −
is shown as a function of the total energy 2E0 at energies near 1 GeV. Again a
resonance peak appears but this time with a peak energy of about 1020 MeV and a
width of about 4 MeV. The φ 0 meson has these two properties. Observation of the
reaction e + e − → π + π − π 0 yields a peak at about 780 MeV (see inset of Fig. 10.15)
with a width of about 10 MeV. These values point to the ω 0 . The virtual photon
in the reaction e + e − → hadrons produces resonances at the positions of the ρ 0 ,
the ω 0 ,andtheφ 0 . To see what these three mesons have in common, we list their
properties in Table 10.1.
The three mesons in Table
10.1 satisfy the conditions
set out above: They have
spin J = 1, negative parity,
negative charge parity, and
strangeness 0. Since a vector
has negative parity and
the same number of independent
components as a spin-1
particle, the mesons are called
vector mesons. The rho has
isospin1andisanisovector,
whereas the two others are
isoscalars.
Figure 10.17: The transformation of a virtual photon into
a vector meson gives rise to the resonances and their decays
observed in colliding beam experiments.
As pointed out in Section 8.6, after Eq. (8.30), the electric charge operator is
composed of an isoscalar and the third component of an isovector. The photon, as
carrier of the electromagnetic force, should have the same transformation properties,
and it matches the vector mesons in their isospin properties. The diagrams for the
production of the three vector mesons listed in Table 10.1 are given in Fig. 10.17.