1897
•
Joseph John Thomson discovers negatively charged cathode
rays ("corpuscles"), by measuring the charge/mass ratio through
deflection in electric & magnetic fields
•
"birth of subatomic physics"
•
term "electron" coined by George Johnstone Stoney (1891),
as the fundamental unit of electric charge
(Stoney was a distant relative of Alan Turing)
1900
•
to explain spectrum of blackbody radiation, Max Planck
postulates that electromagnetic radiation is quantized
1904
•
J.J. Thomson proposes the "plum pudding" model of the atom,
in which the atom is composed of electrons surrounded by
a soup of positive charge, to balance the electron's charge,
like plums surrounded by pudding
(a.k.a. "chocolate chip cookie model")
1909
•
Hans Geiger and Ernest Marsden (in Ernest Rutherford's lab
at U. Manchester) scatter alpha particles (4He nuclei)
from a gold foil, and notice that on rare occasions
(about 1 in 8000) the projectile comes right back!
•
according to Rutherford, "It was almost as incredible as
if you fired a fifteen-inch shell at a piece of tissue paper
and it came back and hit you"
•
leads to the downfall of J.J. Thompson's "plum pudding"
model of the atom
1911
•
Ernest Rutherford ("father of nuclear physics", also quoted as
saying "In science there is only physics; all the rest is stamp
collecting") interprets the experiment as indicating the
atom as a negatively charged electrons surrounding a small,
dense, positively charged nucleus
•
"Rutherford model" has an atom made up of a positively charged
nucleus (composed of protons), with embedded electrons, surrounded
by a cloud of orbiting electrons (to balance charge), as in a
"planetary model" with electrons orbiting a sun-like nucleus
•
e.g. 4He has 4 p + 2 e in the
nucleus (neutrons weren't discovered yet!), with 2 e
in orbit around nucleus;
14N has 14 p + 7 e
in the nucleus plus a cloud of 7 e
•
but an accelerating electric charge would emit synchrotron
radiation, so that the electron would lose energy and spiral into
nucleus!
1913
•
Neils Bohr postulates the quantum model of the atom, in which
orbiting electrons exist in discrete energy levels
1924
•
birth of "spin": Pauli introduces a "two-valued quantum
degree of freedom" in order to account for the emission spectrum
of alkali metals
•
Ralph Kronig (1925) suggested this was produced by the
self-rotation of the electron, which Pauli criticized severely,
noting that this would violate the theory of relativity --
consequently Kronig decided not to publish
•
George Uhlenbeck and Samuel Goudsmit (working under Paul
Ehrenfest) do publish a similar idea!
•
later Pauli (1927) does embrace the idea, using quantum
mechanics
1925
•
protons and electrons discovered to have spin 1/2
1928
•
P.A.M. Dirac postulates the existence of a positively charged
electron, as a consequence of the Dirac equation
1929
•
Franco Rasetti (Caltech) finds 14N has spin 1
•
problem for the Rutherford model, in which 14 p + 7 e
nucleus would have spin 1/2 (14 p + 6 e ⇒ spin 0,
final e gives spin 1/2)
1930
•
Wolfgang Pauli suggests the presence of a third, neutral subatomic
particle, the neutrino (term actually coined by Enrico Fermi in
1931), in the nucleus (very light, non-interacting, spin 1/2)
•
the neutrino is also introduced to explain the energy spectrum of
beta decays of nuclei
(14C → 14N + e + νe),
in which the undetected particle carries away the difference
between the energy and angular momentum of the initial and
final particles
•
because of their very weak interactions with matter,
neutrinos weren't detected until mid-1950s
1930
•
Walther Bothe and Herbert Becker (Germany) discover that
alpha-A (alpha ≡ 4He; A = Be, B, Li) collisions
produce an unusually penetrating radiation, thought at first
to be a form of γ radiation
•
Irene Joliot-Curie and Frederik Joliot (1932, Paris) find
that if this (unknown) radiation falls on hydrogen-containing
compounds, it ejects high energy protons
1932
•
James Chadwick (Cambridge) shows that the gamma ray hypothesis
is untenable, and suggests that the new radiation consists
of uncharged particles ("neutrons") with mass similar
to the proton mass
•
Dmitri Iwanenko suggested that neutrons were spin-1/2 particles
and that nucleus contained neutrons rather than electrons
•
Francis Perrin suggested that neutrinos were not nuclear particles
but were created during β decay (β ≡ e-)
•
Heisenberg proposed that proton and neutron are two states
of same particle, the "nucleon", with mass difference arising
from electromagnetic interactions (⇒ "isotopic spin" or
"isospin")
1932
•
Carl Anderson (Caltech) discovers "positron" in cosmic rays,
which have the same mass as the electron but opposite charge
1935
•
Hideki Yukawa's theory of the strong force: nucleus held
together by protons and neutrons exchanging massive "mesons"
(or "mesotron")
•
from the range of the nuclear force (inferred from the radius
of the nucleus), mass of predicted meson ~ 100 MeV
1936
•
Carl Anderson and Seth Neddermeyer discover muon,
with mass ~ 106 MeV, initially thought to be pion, but did not
interact strongly with matter (hence was only a massive version
of electron)
•
puzzled as to how the unexpected discovery could fit into
any logical scheme of particle physics, I. I. Rabi famously
asked "Who ordered that?"
1938
•
Stueckelberg introduces concept of baryon number conservation
(actually, Pais in 1953 invented the term "baryon")
1947
•
Cecil Powell, Cesar Lattes and Giuseppe Occhialini (Bristol)
discovered pion, with mass ~ 140 MeV, in atmospheric cosmic rays
1947
•
Rochester & Butler discover strange, "V" shaped tracks in
cloud chambers,
K0 → π+ π- decay
(K0 was first known as
V0, then as a θ0)
1949
•
Powell discovers charged kaon,
K+
→ π+ + π+ + π-
(K+ was first known as the τ+)
1950
•
Carl Anderson's group discovers Λ → p π- decay;
Λ as the first "hyperon"
1951
•
Enrico Fermi discovers Δ(1232) resonance in π p scattering
1953
•
Konopinski and Mahmoud introduce lepton number conservation:
leptons created in particle-antiparticle pairs
1953
•
Clyde Cowan Jr. and Frederick Reines observe "inverse β-decay"
process,
ν—e + p
→ p + e+,
confirming the existence of the neutrino
[Nobel Prize 1995]
•
Raymond Davis Jr. (1955) does not observe reaction
ν—e
+ n(37Cl)
→ p(37Ar) + e-;
null result implies that ν and ν—
are different particles
[Nobel Prize 2002]
•
Leon Lederman, Melvin Schwartz and Jack Steinberger (1962)
demonstrate existence of muon-neutrino, in addition to
electron-neutrino, through absence of
ν—μ + p
→ e+ + n
reaction
[Nobel Prize 1988]
1950s
•
explosion of newly discovered particles
(Δ, Σ, Ξ, ...), especially with BNL's first particle
accelerator, Cosmotron (1952)
•
Emilio Segre and Owen Chamberlain discover antiproton (1955)
•
Willis Lamb in his 1955 Nobel Prize speech claimed that
he had heard it said at one point that "the finder of a new
elementary particle used to be rewarded by a Nobel Prize, but
such a discovery now ought to be punished by a $10,000 fine"
•
Enrico Fermi to Leon Lederman: "Young man, if I could remember
the names of these particles, I would have been a botanist!"
•
tidy garden of 1947 had grown to chaotic jungle by 1960,
akin to chemistry before the Periodic Table
1953
•
Gell-Mann and Nishijima introduce "strangeness"
quantum number, conserved in strong interactions, but not in weak
1956
•
T.D. Lee and C.N. Yang propose parity violation in weak
interactions, to solve the "theta-tau" puzzle; confirmed
experimentally shortly after by Mme. Wu
[Nobel Prize 1957]
1961
•
Murray Gell-Mann, Yuval Ne'eman: "Eightfold Way",
classification of spin-1/2 and spin-3/2 baryons into
SU(3)F octet and decuplet;
spin-0 and spin-1 mesons into SU(3)F octets
•
prediction of S = -3 baryon
1964
•
Ω (sss) discovered, confirming flavor symmetric
classifications of the Eightfold Way
1964
•
Murray Gell-Mann and George Zweig independently postulate
fractionally charged constituents ("quarks" or "aces")
to systematically account for quantum numbers of baryons ( qqq )
and mesons ( q q—)
•
"Three quarks for Muster Mark", Finnegan's Wake by James Joyce
•
Gell-Mann did not believe these were actually physical,
but more like book-keeping devices
•
Richard Dalitz (Oxford) took quarks as physical entities
seriously, and developed quark model of hadrons
•
Beg, Lee, Pais compute magnetic moment ratios, with
μn / μp = 2/3, which agrees to
3% with experiment
1964
•
B. Sakita: problem with statistics for Δ++ (uuu):
how can one have antisymmetric wave function with all 3 quarks in an
S-wave, when the Pauli Exclusion Principle forbids two fermions
to occupy the same quantum mechanical state?
•
Oscar Greenberg (1964): "paraquark" model, quarks obey
parastatistics of order 3; paraquarks obey both commutation
and anticommutation relations
•
Moo-Young Han & Yoichiro Nambu (1965): three triplets of
quarks with integer charges
[ Re+e- = 4,
1/2 < Rnp < 2 ]
•
William Bardeen, Harald Fritzsch & Murray Gell-Mann (1972,
hep-ph/0211388): quarks have internal "color"
degree of freedom
[ Re+e- = 3,
1/4 < Rnp < 4 ]
1964
•
Cronin & Fitch discover CP violation in
K0 - K—0
system
1969
•
deep inelastic scattering experiments at SLAC (Jerome
Friedman, Henry Kendall, Richard Taylor) see scaling, or Q2
independence, of inclusive cross section, or structure function,
confirming existence of point-like constituents (or "partons",
as Feynman refered to them; "put-ons" according to Gell-Mann)
inside nucleon
[Nobel Prize 1990]
•
partons identified with quarks (Bjorken, Feynman)
1973
•
David Gross, Frank Wilczek, David Politzer: β function in
non-Abelian SU(3) is negative; led to asymptotic freedom,
and Quantum Chromodynamics (QCD)
[Nobel Prize 2004]
1974
•
charm quark & J/ψ
( c c—)
discovered, by C.C. Ting (BNL, "J")
and Burton Richter (SLAC, "ψ");
confirmation of quark model
[Nobel Prize 1976]
1975
•
charmed baryon, Λc ( udc ),
and charmed mesons, D+
( c d—)
& D0
( c u—), discovered
1975
•
τ lepton discovered by Marton Perl (SLAC)
[Nobel Prize 1995]
1977
•
bottom (b) quark and ϒ
( b b—)
discovered by Leon Lederman (FNAL);
initial claim in 1976 was erroneous, statistical
fluctuation, named the "Oops-Leon"
1995
•
top (t) quark discovered by D0 and CDF Collaborations
(FNAL), completing the 3 families of quarks; as heavy as gold
atom!