There are things that are not directly observable
[like quantized fields]
in terms of which we shall have a more fundamental understanding. Julian Schwinger (1918-1994;
Nobel 1965)
The Lamb shift. The original motivation for renormalization.
Quantum Field theory was originally developped in Germany and Switzerland by
the likes of Pauli, Heisenberg and Wigner... Two main textbooks were published
which Freeman Dyson read
just before coming to America
(as he was taking courses given by
Nicholas Kemmer):
The Quantum Theory of Radiation (1936) by
Walter Heitler (1904-1981).
Zur Quantentheorie der Wellenfelder by
Gregor Wentzel
(of WKB fame).
(2018-06-17) The Lamb shift
The key measurement which motivated renormalized calculations.
Hans Bethe understood physically that
.../...
Bethe was first to publish a rough result agreeing with experiment and assigned
two of his graduate students the task of making more careful calculations:
Dick Scalettar
for the relevant spin-½ electron (he stalled).
Freeman Dyson for an unphysical spin-0 particle.
A training exercise handed to Dyson as his very first research problem.
In 1947, Polykarp Busch made a precise determination of the
magnetic moment of the electron expressed in Bohr magnetons, namely:
1.00119
Julian Schwinger (1918-1994)
was the first to make a fully-relativistic
theoretical computation consistent with the experimental results, giving center
stage to Green's function
(it seems Schwinger was the first to promote that name). Few people besides Schwinger's own
students took the time and effort to understand what he was really doing
(with the notable exception of Freeman Dyson).
Shortly thereafter, Richard Feynman
also got the right answer with the help of his own newly-introduced Feynman diagrams,
which made the subject far more accessible. It was later revealed that
Tomonaga had reached the
same conclusions by himself in war-torn Japan.
It is Freeman Dyson who showed that the approaches
of those three people were equivalent
(thus Dyson was instrumental in the joint award of the
1965 Nobel Prize for that work
to Tomonaga, Schwinger and Feynman).
Dyson hinted that the theory could be carried to higher orders using perturbation theory
but he carefully stated that he reserved judgement concerning the ultimate convergence of the method.
This was wise: Although QED gives incredibly precise result at low orders
(because the single dimensionless coupling constant
of QED happens to be so small) the method rapidly becomes intractable and ultimately
was shown not to converge at high orders.
Megaelectronvolt of mass. 1 MeV/c2 =
1.732 661 758(46) 10-30 kg
Elementary Fermions (matter) :
At first, nobody knew why there should be three generations
of fermions. Ordinary matter is entirely made from fermions
of the first generation; the elctron and two (composite) nucleons
(a proton consists of two up-quarks and a down-quark, a neutron
consists of one up-quark and two down-quarks).
When the muon was first identified as a heavy version of the electron.
I.I. Rabi (1898-1988)
famously exclaimed: "Who ordered that?"
Toshihide Maskawa (1940-) and
Makoto Kobayashi (1944-)
split half of the
2008 Nobel Prize for finding
"the origin of the broken symmetry which predicts the existence of at least three families of quarks in nature".
They did so in 1973, when the existence of a third generation hadn't yet been observed
(the bottom quark was only discovered in
1977, by a Fermilab team led by Leon Lederman, 1922-2018).
So far, the existence of a fourth generation hasn't been quite ruled out experimentally,
but nearly so.
Each generation can be identified by its charged lepton, but this only commonly
done for neutrinos. So, commonly talk of the electron neutrino or the muon antineutrino which are
not known by any other name.
The use of ordinary numerals for generation numbers
is occasionally encountered instead of traditional names or unnumbered abbreviations.
For quarks, u1, d1, u2, d2, u3 and d3 stand for u, d, c, s, t and b.
The electron (e-) was discovered by
Jean Perrin in 1895 and by
J.J. Thomson in 1897.
The positron
(e+) is the antiparticle of the electron.
It was discovered in 1932 by
Carl Anderson (1905-1991).
The muon
(m-)
was discovered in 1936 by Anderson and his very first doctoral student,
Seth Neddermeyer (1907-1988).
The tauon
(t-) was discovered in 1975 by
Martin Perl (1927-2014).
In addition to the classification in three generations, a practical distinction
is also made between light quarks (u,d,s, generically q)
and heavy quarks (c,b,t, generically Q). The elusive tetraquarks
are generally thought to be:
qqQQ.
Quarks are normally not observed alone but in composite particles known
as hadrons of which there are two broad types,
mesons and baryons :
A meson is a boson consisting of a quark
and its antiquark.
A baryon is a fermion combining 3 quarks
in a color-neutral way.
All interactions between fermions are mediated by at a boson, although that boson may be
so short-lived that the thing may look like a contact interaction.
By analogy with the well-studied case of neutral kaons, it would seem that neutral pions are only observed
in the superposition of up and down quark-antiquark pair stated third (all others being probably too short lived or forbiden).
I was unable to find a source confirming this.
The first charged kaon was spotted in 1944 at the laboratory of Polytechnique
by Louis Leprince-Ringuet (1901-2000)
who also coined the word hyperon in 1953 to denote any
baryon with at least one strange quark in combination with up or down quarks (only).
It's often heard that "there are more than 200 distinct baryons".
That statement is probably based on the fact that the 6 quarks and their 6 antiparticles
form 12 distinct particles. There are
220 = C(12,3) combinations of 3 of these,
not accounting for possible mixing...
(2012-07-22) The Road to the Renormalization Group.
Beyond Ward's identities...
The central idea behind renormalization is the fact that couplings depend on the scale
under consideration (in termes of either momentum or distance).
Yet some relations stay the same.
The experimental discovery of a Higgs-like particle was ceremoniously announced by CERN
on 4 July 2012.
It was obvious to everybody that this was worth a Nobel prize.
A few months later, the Nobel committee decided to wait a little
and awarded instead a share of the 2012 prize to my former coach,
Serge Haroche for research unrelated to high-energy physics...
The media was unprepared for this.
On 14 March 2013, CERN confirmed that the
newly-discovered particle has zero spin and even parity (two key property of the predicted Higgs boson)
duly paving the way for the award of the 2013 Nobel prize to Higgs and Englert
(which stirred up a controversy about the other aforementioned physicists who were left out).
(2020-05-05) Yukawa Interaction
Tools intended for the old pion-mediated nuclear forces
can now be applied to Higgs interactions.
In simpler times, it was thought that all nucear interactions could
be mediated by a single spinless particle which was hastlily identified with the
pion (pseudoscalar meson) recently discover by Hideki Yukawa.
That got Yuakwa the Nobel prize (1949).
The mass of the neutral pion had roughly the correct mass to explain the short range of nuclear interactions.
