9 The Standard Model of Fundamental Particles
175
The superscripts give the electric charges of the individual particles, and we
see that the sum of the charges on the right-hand side is equal to zero, which
is the same as the charge of the neutron on the left-hand side. Similarly, the
lepton numbers, 0, 1, and −1 respectively, of the three particles on the righthand side, sum to zero, which is the lepton number of the neutron. Both
electric charge and lepton number are therefore conserved in this process.
The ability of a neutrino to change its flavour has led to the question
being asked: is a neutrino its own antiparticle? (This is not as absurd as it
may sound. Photons and gluons—to be encountered soon—are their own
antiparticles.) In this case, lepton number may not be conserved in particle
decays.
A final question springs to mind when surveying the table of leptons above:
why stop with three generations? (This is the question Alice posed to Dr
Quantum in our opening story.) How do we know that there are not more,
heavier leptons, waiting in the wings to be discovered in the future, as our
accelerators reach to ever higher energies? The same question can be asked
when we come soon to examine quarks. The answer is the same in both
cases: we don’t know. However, fourth and higher generations are considered unlikely. Their presence would result in slight changes to predictions
of electroweak theory that are not observed. Also, any extra fermions would
interact with the Higgs boson, modifying its properties so that it would not
have been detected. The observation of the Higgs boson, with the properties
it has, is evidence that there are only three generations. Statistical analyses
at CERN and the Humboldt University of Berlin exclude the presence of a
fourth generation with a 99.99999% probability [4].
Hadrons
It is now time to consider the other large family of fundamental particles,
those interacting by means of the strong interaction 6 . They are known as
hadrons, and as we have seen, have two main subgroups, called baryons and
mesons. Unlike leptons, hadrons are not truly elementary, in the sense that
they are themselves composed of smaller particles called quarks.
The name “quark” was coined by physicist, Murray Gell-mann, when he
noticed the sentence: “Three quarks for Muster Mark” in James Joyce’s book,
Finnegan’s Wake. At the time he believed that there were three quarks occurring in nature. Now it is known that there are six, and they have been given
6 They may also interact via the electroweak interaction, but not exclusively so, as is the case with
leptons.
175
The superscripts give the electric charges of the individual particles, and we
see that the sum of the charges on the right-hand side is equal to zero, which
is the same as the charge of the neutron on the left-hand side. Similarly, the
lepton numbers, 0, 1, and −1 respectively, of the three particles on the righthand side, sum to zero, which is the lepton number of the neutron. Both
electric charge and lepton number are therefore conserved in this process.
The ability of a neutrino to change its flavour has led to the question
being asked: is a neutrino its own antiparticle? (This is not as absurd as it
may sound. Photons and gluons—to be encountered soon—are their own
antiparticles.) In this case, lepton number may not be conserved in particle
decays.
A final question springs to mind when surveying the table of leptons above:
why stop with three generations? (This is the question Alice posed to Dr
Quantum in our opening story.) How do we know that there are not more,
heavier leptons, waiting in the wings to be discovered in the future, as our
accelerators reach to ever higher energies? The same question can be asked
when we come soon to examine quarks. The answer is the same in both
cases: we don’t know. However, fourth and higher generations are considered unlikely. Their presence would result in slight changes to predictions
of electroweak theory that are not observed. Also, any extra fermions would
interact with the Higgs boson, modifying its properties so that it would not
have been detected. The observation of the Higgs boson, with the properties
it has, is evidence that there are only three generations. Statistical analyses
at CERN and the Humboldt University of Berlin exclude the presence of a
fourth generation with a 99.99999% probability [4].
Hadrons
It is now time to consider the other large family of fundamental particles,
those interacting by means of the strong interaction 6 . They are known as
hadrons, and as we have seen, have two main subgroups, called baryons and
mesons. Unlike leptons, hadrons are not truly elementary, in the sense that
they are themselves composed of smaller particles called quarks.
The name “quark” was coined by physicist, Murray Gell-mann, when he
noticed the sentence: “Three quarks for Muster Mark” in James Joyce’s book,
Finnegan’s Wake. At the time he believed that there were three quarks occurring in nature. Now it is known that there are six, and they have been given
6 They may also interact via the electroweak interaction, but not exclusively so, as is the case with
leptons.