4 Order in the Zoo and Quarks
31
number not changing is called “baryon number conservation.” Thus, the
discovery of baryon number non-conservation, which will break this rule, will
be a discovery with fundamental significance for physics with far reaching
consequences for the fate of the universe. As we will see below, there are
theoretical reasons to suspect that the baryon number may indeed be broken
very weakly. Many experiments have been carried out in the past 30 years to
search for the decay of the proton and oscillation of the neutron to an antineutron, which are both processes that break the baryon number. So far, these
experiments have gone without success. However, there is a pervading belief in
the theoretical physics community that the baryon number is indeed broken,
but it happens so very weakly that we do not yet see it.
Incidentally, using Tables 4.1 and 4.2 we see that if we give quarks a baryon
number (B) of 1/3 and anti-quarks a B = −1/3, then baryons which contain
three quarks have baryon number 1 and mesons, which consist of a quark
and anti-quark have baryon number zero in accord with the observations that
baryons are stable objects and mesons are not stable objects. Mesons can decay
to photons or leptons (electrons and anti-neutrinos, see the next section).
Just like the proton, neutrons and hyperons and their baryon number zero
cousins, i.e. the pi and K-mesons, could all be understood in terms of a
simple quark picture. The next natural question is: does the electron have a
complex family (called leptons) and a simpler set of fundamental constituents
to explain them? Until the late sixties and seventies, the only family members
with similarity to the electron was the muon and a second neutrino known
as the muon neutrino. In the late 1970s, a third pair, the tau lepton, and
its accompanying neutrino were discovered. The number of similar particles
however did not proliferate enough, like the baryon and meson family did,
to require a more fundamental constituent description. So far, there is no
evidence for such substructure for electrons or muons or neutrinos. Similarly,
there is no evidence for any substructure for quarks either. May be we have
reached the end of the line as far as a constituent picture is concerned !
4.3 Leptons Have Their Own Marker Too: The
Lepton Number (L)
Particles such as electrons, muons, and neutrinos are different from the baryons
and mesons and were called leptons. An important observation about leptons
is that just as the baryons have their own marker—the baryon number—so do
the leptons. They have something called a lepton number which guarantees
31
number not changing is called “baryon number conservation.” Thus, the
discovery of baryon number non-conservation, which will break this rule, will
be a discovery with fundamental significance for physics with far reaching
consequences for the fate of the universe. As we will see below, there are
theoretical reasons to suspect that the baryon number may indeed be broken
very weakly. Many experiments have been carried out in the past 30 years to
search for the decay of the proton and oscillation of the neutron to an antineutron, which are both processes that break the baryon number. So far, these
experiments have gone without success. However, there is a pervading belief in
the theoretical physics community that the baryon number is indeed broken,
but it happens so very weakly that we do not yet see it.
Incidentally, using Tables 4.1 and 4.2 we see that if we give quarks a baryon
number (B) of 1/3 and anti-quarks a B = −1/3, then baryons which contain
three quarks have baryon number 1 and mesons, which consist of a quark
and anti-quark have baryon number zero in accord with the observations that
baryons are stable objects and mesons are not stable objects. Mesons can decay
to photons or leptons (electrons and anti-neutrinos, see the next section).
Just like the proton, neutrons and hyperons and their baryon number zero
cousins, i.e. the pi and K-mesons, could all be understood in terms of a
simple quark picture. The next natural question is: does the electron have a
complex family (called leptons) and a simpler set of fundamental constituents
to explain them? Until the late sixties and seventies, the only family members
with similarity to the electron was the muon and a second neutrino known
as the muon neutrino. In the late 1970s, a third pair, the tau lepton, and
its accompanying neutrino were discovered. The number of similar particles
however did not proliferate enough, like the baryon and meson family did,
to require a more fundamental constituent description. So far, there is no
evidence for such substructure for electrons or muons or neutrinos. Similarly,
there is no evidence for any substructure for quarks either. May be we have
reached the end of the line as far as a constituent picture is concerned !
4.3 Leptons Have Their Own Marker Too: The
Lepton Number (L)
Particles such as electrons, muons, and neutrinos are different from the baryons
and mesons and were called leptons. An important observation about leptons
is that just as the baryons have their own marker—the baryon number—so do
the leptons. They have something called a lepton number which guarantees