168
R. Barrett and P. P. Delsanto
natural question that immediately follows is: “what produces this interaction?” We saw in Chap. 8 how the electromagnetic interaction is generated
by the exchange of photons. In analogy, Japanese physicist, Hideki Yukawa,
proposed in 1935 that the strong interaction was the result of the exchange
of as yet undiscovered particles between the nucleons (i.e., between neutrons
and protons).
However, a major difference exists between the electromagnetic and strong
nuclear interactions. The fact that the nuclear force is not observed in
everyday phenomena, despite the great strength it must have to hold the
nucleons together, is evidence that it has a short range. In the quantum theory
of fields, the mass of a carrier particle associated with a force is related to
the range of that force. (See Appendix 9.1.) The requirement that the new
force be of such short range, i.e. of the order of a few femtometres 1 , implies
that, unlike the photon, the carrier particle will not be massless. Indeed, its
mass could be predicted reasonably accurately. Yukawa named the hypothetical particle a “meson” (which is Greek for “intermediate”) because its mass
was expected to lie between the mass of an electron and that of a proton.
In Chap. 8 we introduced the concept of an intrinsic angular momentum,
or spin, associated with fundamental particles. As Planck found to be the
case for energy, this angular momentum is quantised, which means it comes
in multiples of an elementary basic unit. (It need not concern us here what
the actual value of this unit is 2 ; suffice it to say that it is very small.) Experimental physicists, excited by the new field opening before them, were soon
busy populating the particle zoo. The new particles they discovered could be
divided into two classes, known as bosons and fermions, differentiated by the
value of their spins. Bosons, named after Indian physicist Satyendra Nath
Bose, possess a spin of integer or zero units. On the other hand, Fermions,
named after Italian physicist Enrico Fermi, possess half odd-integer (i.e. 1/2,
3/2, 5/2, etc.) units of spin. As we shall see later, this difference results in some
important differences in the properties of these particles. Armed with this
information, experimentalists began combing cosmic ray data for evidence of
the proposed meson.
One can imagine the excitement when a new particle, dubbed the mu
meson, was discovered by Carl Anderson and Seth Neddermeyer at Caltech in
1936. (Carl Anderson also discovered the positron, so if Willis Lamb’s suggestion above had been in effect, he would have become severely impoverished.)
The elation soon turned to disappointment when the newcomer turned out
1 1 femtometre (fm) = 10 -15 metres. For comparison, the proton radius is 0.833 fm.
2 In QM, angular momentum, including spin, is quantised, and can only have distinct values in terms
of è, where è is Planck’s constant, h, divided by 2π.
R. Barrett and P. P. Delsanto
natural question that immediately follows is: “what produces this interaction?” We saw in Chap. 8 how the electromagnetic interaction is generated
by the exchange of photons. In analogy, Japanese physicist, Hideki Yukawa,
proposed in 1935 that the strong interaction was the result of the exchange
of as yet undiscovered particles between the nucleons (i.e., between neutrons
and protons).
However, a major difference exists between the electromagnetic and strong
nuclear interactions. The fact that the nuclear force is not observed in
everyday phenomena, despite the great strength it must have to hold the
nucleons together, is evidence that it has a short range. In the quantum theory
of fields, the mass of a carrier particle associated with a force is related to
the range of that force. (See Appendix 9.1.) The requirement that the new
force be of such short range, i.e. of the order of a few femtometres 1 , implies
that, unlike the photon, the carrier particle will not be massless. Indeed, its
mass could be predicted reasonably accurately. Yukawa named the hypothetical particle a “meson” (which is Greek for “intermediate”) because its mass
was expected to lie between the mass of an electron and that of a proton.
In Chap. 8 we introduced the concept of an intrinsic angular momentum,
or spin, associated with fundamental particles. As Planck found to be the
case for energy, this angular momentum is quantised, which means it comes
in multiples of an elementary basic unit. (It need not concern us here what
the actual value of this unit is 2 ; suffice it to say that it is very small.) Experimental physicists, excited by the new field opening before them, were soon
busy populating the particle zoo. The new particles they discovered could be
divided into two classes, known as bosons and fermions, differentiated by the
value of their spins. Bosons, named after Indian physicist Satyendra Nath
Bose, possess a spin of integer or zero units. On the other hand, Fermions,
named after Italian physicist Enrico Fermi, possess half odd-integer (i.e. 1/2,
3/2, 5/2, etc.) units of spin. As we shall see later, this difference results in some
important differences in the properties of these particles. Armed with this
information, experimentalists began combing cosmic ray data for evidence of
the proposed meson.
One can imagine the excitement when a new particle, dubbed the mu
meson, was discovered by Carl Anderson and Seth Neddermeyer at Caltech in
1936. (Carl Anderson also discovered the positron, so if Willis Lamb’s suggestion above had been in effect, he would have become severely impoverished.)
The elation soon turned to disappointment when the newcomer turned out
1 1 femtometre (fm) = 10 -15 metres. For comparison, the proton radius is 0.833 fm.
2 In QM, angular momentum, including spin, is quantised, and can only have distinct values in terms
of è, where è is Planck’s constant, h, divided by 2π.