Inside matter, from nuclear reactions to particle discoveries

Try the physics behind the evidence

Each investigation uses a stated teaching model. Change the apparatus or selection, then follow what happens to the tracks, counts or spectrum. The model notes explain the approximations and identify simulated data.

1. Follow a decay chain

2. Probe a charge distribution

3. Fill nuclear shells

4. Find a common mass

5. Scatter inside a proton

6. Reverse nuclear spin

7. Follow an unstable mixture

8. Keep an accelerating ion in step

The nucleus and its reactions

Start with a uranium salt that exposes a plate in the dark. Follow neutral projectiles, repeated acceleration and scattering into the arrangement of nuclear matter and the making of elements.

1. How an unexposed plate led to radioactivity

   How did a dark photographic plate lead from uranium rays to the discovery of new elements?

2. The recoiling atoms that revealed the neutron

   How could physicists identify a particle that left no charged track of its own?

3. Splitting lithium with a laboratory proton beam

   How did flashes from a lithium target show that an accelerator could change an atomic nucleus?

4. How a cyclotron gives the same ion another push

   How can a modest alternating voltage accelerate an ion to much more energy than one crossing would supply?

5. Why slowing neutrons made some reactions stronger

   Why did surrounding a neutron source with wax make some irradiated samples more radioactive?

6. What makes a star a steady nuclear engine

   Why can hydrogen fusion power a star for billions of years instead of making it flash once?

7. Why some nuclei close a shell

   Why are nuclei with certain numbers of protons or neutrons unusually hard to change?

8. How a nucleus can turn as a whole

   How did nuclear spectra reveal collective rotation alongside the motion of individual protons and neutrons?

9. Measuring the proton with scattered electrons

   How can the directions of scattered electrons reveal structure inside a proton?

10. Making the elements one measured reaction at a time

    How did measurements in a laboratory help explain which elements stars produce?

How a particle leaves something we can read

Gas, droplets, photographic grains, bubbles and wires turn invisible motion into evidence. Each detector answers a different question about charge, direction, speed or identity.

1. Lenard lets cathode rays out of the tube

   How could a ray made in a nearly empty tube be studied in ordinary air?

2. Thomson finds a common particle in cathode rays

   How did a glowing discharge tube reveal a particle smaller than an atom?

3. Wilson turns ionisation into a visible track

   Why does a passing particle leave a thin trail of cloud?

4. Bothe and Geiger ask whether two signals belong together

   When two counters respond, how can we tell whether they recorded the same event?

5. Blackett and Occhialini let a particle take its own photograph

   How did two counters turn rare cloud-chamber tracks into a dependable stream of photographs?

6. Cherenkov follows the blue light to a moving charge

   How can a particle outrun light in water without outrunning light in vacuum?

7. The pion leaves a second track in photographic emulsion

   How did microscopic silver grains reveal that two different particles had passed through a plate?

8. Glaser makes a particle trail start the boiling

   Why can a hot liquid remain clear, then grow bubbles along a particle’s path?

9. Alvarez’s group finds short-lived particles in combinations of tracks

   How can a particle be discovered when it decays before it can leave a visible track?

10. Charpak turns a particle’s position into an electrical signal

    How did a plane of fine wires make particle tracks readable without developing a photograph?

Particles, forces and symmetry

Begin with a mirror test of weak decay, then compare classification schemes with the particles and distributions that put them to the test. Theory and experiment take turns asking the next question.

1. The lead plate that revealed a positron

   How did Anderson tell a positive electron from an ordinary electron moving the other way?

2. Why Yukawa expected a particle between the electron and proton

   How can the short reach of the nuclear force suggest the mass of a new particle?

3. What symmetry tells us about a particle

   Why do mass and spin label particles seen from many different viewpoints?

4. QED turned small atomic discrepancies into precision predictions

   How did a theory troubled by infinities predict tiny changes in an electron’s magnetic behaviour?

5. Finding the antiproton among thousands of pions

   How did the Berkeley team identify a rare negative particle as an antiproton?

6. Wu’s cobalt experiment showed a preference for one direction

   What did unequal electron counts have to do with the symmetry of a mirror?

7. A symmetric law can have an asymmetric ground state

   How can a system acquire a preferred state without changing the symmetry of its equations?

8. The rare kaon decay that broke CP symmetry

   Why did a small number of two-pion decays matter in a long-lived kaon beam?

9. How quarks brought order to the particle families

   Why did combining a few fractionally charged constituents explain so many particles?

10. How electromagnetism and weak decay came to share one theory

    How can a long-range electric force and short-range weak decay come from one framework?

11. Electrons found compact constituents inside the proton

    What in the SLAC scattering data suggested that a proton contains smaller charged objects?

12. How quantum corrections made electroweak theory precise

    Why did a theory that predicted W and Z still need a new way to calculate?

13. Why a third quark generation opened a way to CP violation

    What changed when quark mixing had three generations instead of two?

14. Why quarks look almost free in a hard collision

    How can quarks be tightly bound in a proton yet respond almost independently at short distances?

15. Two experiments met at the same narrow particle peak

    Why did the J and psi observations in 1974 change particle physics so quickly?

16. The tau appeared through an electron, a muon and missing energy

    How could two visible particles reveal a short-lived particle that escaped direct observation?

17. W and Z particles emerged from the collider debris

    How did UA1 and UA2 recognize heavy force carriers that decayed before leaving a track?

18. Two mass peaks brought the Higgs field within reach

    What did the 2012 particle discovery reveal about the mechanism behind W and Z masses?

Follow the discoveries in time

These are the experimental or theoretical milestones discussed in the articles. Prize years appear separately in the contribution map.

1. 1894Lenard lets cathode rays out of the tube
2. 1896How an unexposed plate led to radioactivity
3. 1897Thomson finds a common particle in cathode rays
4. 1911Wilson turns ionisation into a visible track
5. 1925Bothe and Geiger ask whether two signals belong together
6. 1932The lead plate that revealed a positron
7. 1932Blackett and Occhialini let a particle take its own photograph
8. 1932The recoiling atoms that revealed the neutron
9. 1932Splitting lithium with a laboratory proton beam
10. 1932How a cyclotron gives the same ion another push
11. 1934Cherenkov follows the blue light to a moving charge
12. 1934Why slowing neutrons made some reactions stronger
13. 1935Why Yukawa expected a particle between the electron and proton
14. 1939What makes a star a steady nuclear engine
15. 1939What symmetry tells us about a particle
16. 1947The pion leaves a second track in photographic emulsion
17. 1948QED turned small atomic discrepancies into precision predictions
18. 1949Why some nuclei close a shell
19. 1952Glaser makes a particle trail start the boiling
20. 1953How a nucleus can turn as a whole
21. 1955Finding the antiproton among thousands of pions
22. 1955Measuring the proton with scattered electrons
23. 1957Making the elements one measured reaction at a time
24. 1957Wu’s cobalt experiment showed a preference for one direction
25. 1960Alvarez’s group finds short-lived particles in combinations of tracks
26. 1961A symmetric law can have an asymmetric ground state
27. 1964The rare kaon decay that broke CP symmetry
28. 1964How quarks brought order to the particle families
29. 1967How electromagnetism and weak decay came to share one theory
30. 1968Charpak turns a particle’s position into an electrical signal
31. 1969Electrons found compact constituents inside the proton
32. 1972How quantum corrections made electroweak theory precise
33. 1973Why a third quark generation opened a way to CP violation
34. 1973Why quarks look almost free in a hard collision
35. 1974Two experiments met at the same narrow particle peak
36. 1975The tau appeared through an electron, a muon and missing energy
37. 1983W and Z particles emerged from the collider debris
38. 2012Two mass peaks brought the Higgs field within reach

From Nobel contributions to the underlying physics

38 bilingual articles cover 39 contribution entries.