History of Particle physics

History of Particle Physics

Period: 1890 to 2000

Discovery of the electron

1897

Cathode ray experiment
-Thomson had applied a voltage across two plates with opposite charges inside an evacuated tube -He discovered that an electron beam had been fired from the cathode. The way the rays were deflected by magnetic and electric fields suggested they were negative. -Cathode ray particles were 1000 times smaller than the hydrogen atom.
-Different metals gave off cathode rays
Hence, Thomson concluded that atoms must have negatively charged particles within them - the ELECTRON.

Discovery of the proton

1909

Rutherford deduced that the atom must have a small concentrated positive charge and that the atom was mostly space from his experiment, where alpha particles were fired at gold foil. Rutherford also concluded that electrons were arranged in orbits.
However,there were things his model failed to explain: 1.Accelerated charged particles emit electromagnetic radiation. This means that in 16 ps an electron would spiral into the nucleus. 2.There was no mention of the arrangements of electrons.

Isotopes

1913

The Greek word 'isos' means equal whilst 'topos' means place. It was believed that the term Isotope was first used in a conversation in 1913 between Dr Margaret Todd and her famous scientist cousin Frederick Soddy. Soddy was believed to be the first to isolate the isotope. Isotopes are useful for many sectors such as medicine (bone imaging), military(nuclear weapons), archaeology (to determine age of artefacts) and is also used in smoke detectors.

Dirac predicts Antiparticles

1928

"I have trouble with Dirac. This balancing on the dizzying path between genius and madness is awful. I don't understand Dirac at all."

Albert Einstein

Dirac wrote an equation that combined quantum theory and special relativity which showed that each particle has its corresponding anti-particle.If a particle and its antiparticle collided then 'annihilation' would occur.During his Nobel prize lecture, Dirac speculated another universe identical to ours but made from antiparticles.

Discovery of Neutrons

1932

Neutrons were first theorised by Rutherford in 1920 Chadwick used a polonium source to fire alpha particles at beryllium- producing an uncharged penetrating radiation. It was then made incident on paraffin wax- ejecting protons.
In conclusion, James Chadwick deduced that the radiation made up of uncharged particles, which had had the same mass as a proton.

Discovery of the positron

1932

Using a vertical cloud chamber, Carl David Anderson to observe cosmic rays in which he found a particle that had the same mass as the electron but exhibited the opposite charge- the POSITRON. This verified Dirac's theory that anti- particles did exist. Occhialini and Blackett then came up with the concept of annihilation and pair production.
Due to this discovery, the PET scan and radiopharmaceuticals have developed to become great diagnostic tools.

Yukawa predicts the existence of Pions

1934

Yukawa predicts the existence of pions. He argued that the nuclear strong force must be carried by a force 200 times the size of an electron. Later in 1945, he received a Nobel Prize as his prediction became true.

Nobel Prize in Physics

1935

Chadwick had been held in a prison camp for 4 years during WW1. During this time, he had convinced prisons guards to supply him with a radioactive toothpaste. He also built an electroscope (using tin foil and wood) to do some simple experiments. His work allowed scientists to discover that hitting uranium with neutrons allowed for fission to occur and paved the way for the atomic bomb. During WW2, he contributed to the Manhattan Project.

Nuclear Fission

1938

Nuclear fission was discovered at the Kaiser Wilhelm Institute for Chemistry in December 1938 by Otto Hahn (taking inspiration from Fermi's experiments where he fired neutrons at a uranium). A year later, Lise Meitner and nephew Otto Frisch came up with the explanation to nuclear fission in which the uranium nucleus can be seen as charged droplet ,which after absorbing the nucleus ,oscillates and splits into 2 equal size parts whilst releasing high amounts of energy.

The Manhattan Project

1939

"Now I am become Death, the destroyer of worlds"

Robert J Oppenheimer

Killing upwards of 100,000 civilians and marking the end of the Second World War, nuclear fission's significance was indeed undeniable. Oppenheimer alongside many other scientists (including Einstein) and engineers contributed to the world's first atomic bomb- in fear of a potential Nazi bombing. This had also marked the ominous dawn of the Nuclear Age for the world.

Discovery of Mesons

1947

Predicted by Yukawa Hideki in 1935, mesons were discovered by Cecil Frank Powell's team in 1947. Mesons are subatomic particles made of a quark and an anti quark. They're classified as a boson and have a very short life span.They're responsible for the strong force which holds the nucleon together (counteracting the electrostatic force of repulsion) after an exchange of gluons has taken place A pion is the lightest meson which is responsible for most of the strong force

The Bevatron

1954

Following the discovery of the positron, scientists sought to discover the anti proton which required a minimum of 2 BeV in energy. A new machine, appropriately named the Bevatron and situated in the radiation laboratory of Berkley California, was created in pursuit of this. They were able to create both anti protons and anti neutrons as a result.

Discovery of Neutrinos

1956

Little neutral one
In 1930, Pauli hypothesised the existence of the neutrino when the momentum and energy isn't conserved during a beta decay reaction. Scientists Reines and Cowan first detected evidence of neutrinos by finding that electron neutrinos emitted by a nuclear reactor. However, there still remained many questions that are unanswered such as:
-Is the neutrino its own anti-particle?
-Are there more than 3 types of neutrinos?
-What is the absolute mass of a neutrino?

Quarks

1964

In 1961, Gell-Mann introduced a scheme 'Eight Fold Way' which classifies hadron into 2 main groups for which he received a Nobel Prize. Gell-Mann and Zweig proposed that properties of hadrons could be explained as if they were made of constituent particles, arranged in triplets
.
In 1968,after some electron proton scattering experiments and results from the bubble chamber at CERN, it was confirmed the quarks had charges of 1/3 and 2/3. Quarks are a key part of the standard model