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MASS CHARGE GROUPING &
in MeV —e 0 +e STRANGENESS
1400p= Y[>AJtT- YỊ3A}LT? VỈ›AẰI s=-2
T395
=_- =0 S=-2
1300 1319 TBIT
1200 _— >° >+ s=-llố
1196 1191 "T189 È
Ạ9 S=_-1|m
1100 1115
—n - mi S=0
839 938
s00 023m S=0
Đ—OT‡T 271m7 p°©X‡T S=0
500 _KC KT gr s=ml| 2
494 498 494 õ
TT— r0 ii S=0
T39.6 Tâ5Ø 139.6
_H—— œ
100 T0B.6 z
0 B5 ~8—
--- Trang 66 ---
mass, within one or two percent. Each particle in a multiplet has the same
strangeness. The first multiplet is the proton-neutron doublet, and then there is
a singlet (the lambda) then the sigma triplet, and ñnally the xi doublet. Very
recenfly, in 1961, even a few more particles were found. Ôr are they particles?
They live so short a time, they disintegrate almost instantaneouslÌy, as soon as
they are formed, that we do not know whether they should be considered as new
particles, or some kind of “resonance” interaction of a certain definite energy
between the Á and z produects into which they disintegrate.
In addition to the baryons the other particles which are involved in the nuclear
interaction are called rmesons. “Thore are first the pions, which come in three
varleties, positive, negative, and neutral; they form another multiplet. We have
also found some new things called K-mesons, and they occur as a doublet, KT
and K0. Also, every particle has its antiparticle, unless a particle is is ơun
antiparticle. Eor example, the x— and the z? are antiparticles, but the #2 is
its own antiparticle. The K~ and KT are antiparticles, and the KU and KD.
In addition, in 1961 we also found some more mesons or ?nø;/be mmesons which
disintegrate almost immediately. A thing called œ¡ which goes into three pions
has a mass 780 on this scale, and somewhat less certain is an object which
disintegrates into two pions. These particles, called mesons and baryons, and
the antiparticles of the mesons are on the same chart, but the antiparticles of
the baryons must be put on another chart, “reflected” through the charge-zero
column.
Just as Mendeleev's chart was very good, except for the fact that there were
a number oŸ rare earth elements which were hanging out loose from it, so we have
a number of things hanging out loose from this chart—particles which do not
interact strongly in nuclei, have nothing to do with a nuclear interaction, and do
not have a strong interaction (I mean the powerful kind of interaction of nuclear
energy). These are called leptons, and they are the following: there is the electron,
which has a very small mass on this scale, only 0.510 MeV. Then there is that
other, the /-meson, the muon, which has a mass mụch higher, 206 times as heavy
as an electron. So far as we can tell, by all experiments so far, the diference
bebween the electron and the muon is nothing but the mass. Everything works
exactly the same for the muon as for the electron, except that one is heavier than
the other. Why is there another one heavier; what is the use for it? We do not
know. In addition, there is a lepton which is neutral, called a neutrino, and this
particle has zero mass. In fact, it is now known that there are £#o diferent kinds
of neutrinos, one related to electrons and the other related to muons.
--- Trang 67 ---
Pinally, we have two other particles which do not interact strongly with the
nuclear ones: one is a photon, and perhaps, If the field of gravity also has a
quantum-mechanical analog (a quantum theory of gravitation has not yet been
worked out), then there will be a particle, a graviton, which will have zero mass.
What is this “zero mass”? "he masses given here are the masses of the
particles ø‡ resf. The fact that a particle has zero mass means, in a way, that it
cannot be at resf. Á photon is never at rest, i is always moving at 186,000 miles
a second. We will understand more what mass means when we understand the
theory of relativity, which will come in due time.
Thus we are confronted with a large number of particles, which together seem
to be the fundamental constituents of matter. Fortunately, these particles are
not all diferent in their zn#eraclions with one another. In fact, there seem to be
Just ƒour kinds of interaction between particles which, in the order of decreasing
strength, are the nuclear force, electrical interactions, the beta-decay interaction,
and gravity. The photon is coupled to all charged particles and the strength of the
interaction is measured by some number, which is 1/137. The detailed law of this
coupling is known, that is quantum electrodynamics. Gravity is coupled to all
cnergu, but its coupling is extremely weak, much weaker than that of electricity.
This law is also known. Then there are the so-called weak decays——beta. decay,
which causes the neutron to disintegrate into proton, electron, and neutrino,
relatively slowly. This law is only partly known. The so-called strong interaction,
the meson-baryon interaction, has a strength of 1 in this scale, and the law 1s
completely unknown, although there are a number of known rules, such as that
the number of baryons does not change in any reaction.
Table 2-3. Elementary Interactions
Coupling Strength” Law
Photon to charged particles ~ 1072 Law known
Gravity to all energy ~ 10? Law known
'Weak decays ~10" Law partly known
Mesons to baryons ~1 Law unknown (some rules known)
” The “strength” is a dimensionless measure of the coupling constant involved
in each interaction (~ means “of the order”).
--- Trang 68 ---
This then, is the horrible condition of our physics today. To summarize it,
I would say this: outside the nucleus, we seem to know all; inside it, quantum
mmechanics is valid——the principles of quantum mechanies have not been found to
fail. The stage on which we put all of our knowledge, we would say, is relativistic
space-time; perhaps gravity is involved in space-time. We do not know how the
universe got started, and we have never made experiments which check our ideas
OŸ space and time accurately, below some tỉny distance, so we only knou that our