In Aristotle’s philosophy there were four elements The concept of elements In 1808 Dalton listed many elements we recognise today
The periodic table In 1860’s Mendeleev arranged the elements by property into the periodic table
The periodic table Not only was this a beautiful pattern it was also predictive Some elements were missing and their properties could be predicted All were later discovered
Turn of the 20 th century Thus by the end of the 19th century the idea of elements was well developed The smallest piece of an element was known as an atom with atoms imagined as small spheres All matter in the Universe could be described by around 100 different atoms – not too bad!
Enter the electron In 1897 J J Thompson discovered the electron It soon became clear that it not only played an important role in electricity but was also contained inside atoms Atoms have sub-structure!
The plum pudding model One of the first models of the atom to include electrons Thompson imagined the electrons, with their negative charge, were stuck in a blob of positively charged material
The structure of atoms In 1912 the first particle physics experiment was carried out Fired radioactive particles at gold foil Found most of the particles went straight through But occasionally some did scatter back….. This was totally unexpected!
Rutherford scattering This showed that the atom has a dense positively charged nucleus surrounded by a cloud of electrons Rutherford said “It was if someone had told me that having fired a pistol at a sheet of paper, the bullet had bounced back!” The plum pudding model predicts that the average electric field is zero – no scattering The dense positively charged nucleus leads to scattering from a ‘point like’ object whose size could be worked out
A new picture of matter So in the 1930’s the Universe was a simple place All matter was made of atoms Atoms had a nucleus made from protons and neutrons surrounded by a cloud of electrons All the known matter in the Universe could be described by three particles
Finding patterns Like Mendeleev, group particles with similar properties together Patterns Sub-structure In 1964 Murray Gell-Mann suggested that the many particles found could be made from just three quarks He called them up , down and strange But no free quarks seen……………….
Man-made cosmic rays By the 1960’s particle accelerators were operating in America (Berkeley - West coast, Brookhaven - Long Island NY and SLAC – Stanford California) and Europe (CERN – Geneva) Length 0.5m Energy of electron beam 20kV Length 3200m Energy of electron beam million times greater
Enter the quark In 1967 used SLAC to scatter electrons off protons Still the Rutherford scattering experiment, on a bigger scale! Results showed that proton had sub-structure Made up of three point like objects - quarks
The particles of matter Model of atom today Quarks and electrons are fundamental As far as we can tell no further sub-structure Proton – up up down (uud) Neutron – down down up (ddu) All ordinary matter in the Universe is made up from these three particles
So that’s that? Not quite! We can describe ordinary matter with three particles – two quarks (u and d) and the electron Remember to describe all the particles that were found using cosmic rays we needed a third quark – strange (s) There was also another particle the muon – just like the electron but heavier Where do these particles fit?
Back to the particle accelerators By the 1970’s large circular accelerators being built
Creating New Particles positron (e + ) electron (e - ) muon ( - ) antimuon ( + ) E=mc 2 !
Back to the particle accelerators One such accelerator was SPEAR a ring which collided electrons and positrons together In 1974 evidence for a fourth quark – charm (c) was seen at the SPEAR In 1975 evidence for a particle like the electron and the muon but much heavier – the tau ( )
Even more quarks! As accelerator energies increased still further yet another quark was discovered in 1977 – bottom (b) By now theorists were convinced that a pattern was beginning to emerge with families consisting of pairs of quarks and their matching electron like particles WHERE IS TOP (t)?
Will it ever end? Whilst the Americans built an accelerator to find the top quark At CERN LEP was built – a huge accelerator to collide beams of electrons and positrons
Inside the LEP tunnel LEP was 27000m in circumference Four bunches of electrons and positrons circulated inside the vacuum pipe One ten thousandth of a second for a complete circuit About one electron-positron collision per second Energy of electron beam ten million times greater than TV
Will it ever end? In 1991 experiments at LEP proved that there are only three families of quarks and their associated electron like particles or leptons Found no evidence for quark sub-structure What about the sixth quark? Top quark discovered in 1995 at Fermilab in USA Number of different neutrinos = 2.994 ± 0.011 20 000 000 Rate
The matter particles First Generation Ordinary Matter Second Generation Cosmic Rays Third Generation Accelerators
How can we test the mechanism for the origin of mass?
How do the fundamental particles get their mass? New concept needed: Higgs mechanism
The Higgs mechanism New field postulated that fills all space: the Higgs field All fundamental particles obtain their masses from interacting with the Higgs field The Higgs boson is the field quantum of the Higgs field (like the photon is the quantum of the e.m. field)
Example from LHC: starting from this event… … we are looking for this “signature” Selectivity: 1 in 10 13 Like looking for 1 person in a thousand world populations Or for a needle in 20 million haystacks!