From Thomson's electron to the Higgs — a hundred and twenty-five years of finding pieces of matter that refused to break further.
Cathode-ray experiments at Cambridge reveal a particle ~1/1836 the mass of hydrogen — the electron. The atom is no longer indivisible.
Alpha particles fired at gold foil bounce backward. Atoms are mostly empty space with a tiny dense nucleus. Plum-pudding is dead.
The neutron is found. The positron arrives — Dirac's prediction of antimatter is real.
Cosmic-ray cloud chambers and early accelerators produce muons, pions, kaons, lambdas. Dozens of "elementary" particles. Nobody is happy about this.
Gluons bind quarks into protons, neutrons. Confined to ~10-15 m. Strongest, shortest range.
Photons. Infinite range. Light, chemistry, electricity, magnetism — all one thing since Maxwell.
W±, Z bosons. Beta decay. Massive carriers — that's why it's "weak" and short-range (~10-18 m).
Hypothetical graviton. Infinite range, absurdly weak (10-39 of EM). Not in the Standard Model.
The Standard Model unifies the first three. The fourth still refuses to join.
The stuff that makes things. Obey Pauli exclusion — no two in the same quantum state. That's why atoms have shells, why solids are solid.
Mediators. Pile up freely in the same state — that's why lasers and superconductors work. They exchange between fermions to make forces happen.
Murray Gell-Mann (and George Zweig, independently) propose: the particle zoo isn't a zoo of elementaries. The hadrons are composite, built from a small set of fractionally-charged constituents.
Three flavors at first — u, d, s — later expanded to six. Combinations:
"Three quarks for Muster Mark." — Joyce, via Gell-Mann.
Leptons don't feel the strong force. Three charged leptons, three neutrinos — arranged in three generations, each heavier than the last.
Mass 0.511 MeV. Stable. Builds every atom.
νe — electron neutrino. Nearly massless.
Mass 106 MeV. Lifetime 2.2 μs.
νμ — muon neutrino.
Mass 1.78 GeV. Heavier than a proton.
ντ — tau neutrino.
Rabi on the muon's discovery: "Who ordered that?"
1968. Glashow, Weinberg, Salam unify electromagnetism and the weak force into electroweak theory. The theory predicts three new heavy bosons: W⁺, W⁻, Z⁰.
1983. Carlo Rubbia and Simon van der Meer at CERN's Super Proton Synchrotron see them at exactly the predicted masses.
A theory makes a number. A machine measures the number. They agree to four decimals. That's the Standard Model working.
Gauge symmetry says W, Z, and fermions should be massless. They aren't. W weighs as much as a silver atom.
Englert · Brout · Higgs propose a scalar field filling all space. Particles drag through it; the drag is mass. The field's quantum is a new boson.
ATLAS and CMS at the LHC see a bump at 125 GeV, decaying as the theory predicted. 5σ announced 4 July 2012.
Nobel: Englert & Higgs, 2013.
Galaxies rotate as if there's ~5× more matter than we see. Standard Model has no candidate. WIMPs, axions, sterile neutrinos — none confirmed.
Neutrinos oscillate between flavors — therefore have mass. The Standard Model says they shouldn't. Where does the mass come from? Majorana? See-saw?
Why is the Higgs 125 GeV and not 1019 GeV (the Planck scale)? Quantum corrections should drag it up. They don't. Why?
The Big Bang should have made equal amounts. We see a universe of matter. CP violation in the SM is ~10⁹ too small to explain it.
The expansion is accelerating. About 68% of the universe is some unknown vacuum-energy-like thing. The SM offers nothing.
Just absent. General relativity is classical; the SM is quantum. They don't combine without infinities.
27 km ring under France/Switzerland. Proton-proton at 13.6 TeV. Found the Higgs. Currently Run 3.
General-purpose detector at LHC. 7000 tonnes, 100M readout channels. One of two that found the Higgs.
Compact Muon Solenoid. The other Higgs-discovery detector. Different design, same answer — that's how you cross-check.
Illinois. Tevatron found the top quark (1995). Now hosts g-2 muon experiments — possible hints of new physics.
A cubic kilometer of Antarctic ice instrumented for neutrinos. First detection of astrophysical neutrinos, 2013.
Neutrino mass and oscillation experiments. Patient, precise, low-background. The opposite of a hadron collider in style.
String theory and loop quantum gravity remain mathematically rich, experimentally untested. Energies of 1019 GeV are roughly 1015× beyond the LHC.
Predicted superpartners would solve the hierarchy problem and offer a dark-matter candidate. The LHC has not found them at expected masses. Minimal SUSY is in trouble; weaker variants survive.
Three serious proposals: FCC-ee/hh (CERN, 91 km), CEPC (China, ~100 km), muon collider (Fermilab concept). Decades and tens of billions either way.
Muon g-2 tension, lepton-flavor universality hints in B mesons, the W-mass measurement controversy. Each is small. Each could be the crack.
"The history of physics is a history of finding the next layer down. There is no reason to think we're near the bottom."