Sunday July 8th
The final lecture was entitled, “Recreating the Big Bang with the LHC at CERN” with Dr David Evans.
The LHC accelerates particles to 0.99999991 of the speed of light to release the sub-atomic energies. The particles collide in four cathedral sized caverns. The circumference of the LHC is 27km. As well as the search for the Higgs the LHC is trying to create the subatomic explosions at the time of the big bang.
Why are these experiments being done? We know that atoms are the building blocks of matter as we understand it. 99.8% of the mass is found in the positive nucleus and the remainder is the cloud of electrons. If the atom was the size of a lecture hall then the nucleus would be the equivalent of a grain of sugar.
We know the nucleus is made up of protons and neutrons. A proton is made up of two up quarks and one down quark. A neutron is made up of two down quarks and an up quark. The up quark has a charge of +2/3 and the down quark has a charge of -1/3. The electron has a charge of -1. Both the electron and the quark are less than 1xE-20m across. The up quark has a mass of 0.003 of a proton; the down quark has a mass of 0.006 of a proton; the electron has a mass of 0.0005 of a proton and the electron neutrino has a mass of less than 1xE-8 of a proton. Everything is made up of these four particles. But nature supplies us with two extra families: Charm, strange, muon and muon neutrino; top; bottom, tau and tau neutrino. We don’t know why there are three families or why they have the masses that they do.
To confuse matters there are also virtual particles. The forces between fundamental particles are mediated by virtual carrier particles. For example, the electromagnetic interaction between two charged particles (two electrons for instance) is understood to be due to the exchange of virtual photons. A virtual particle is one that violates conservation of energy, but only for a short period of time (Δt < h/ΔE where h is Planck’s constant) – it “borrows” energy using the Heisenberg principle (you either know the position or momentum of the particle, not both). Virtual particles are in empty space.
The weak force is responsible for the Sun producing energy. The quarks have been stuck since the beginning of time. Gravity is actually a weak force. It’s the rapid deceleration that kills you on impact not gravity. At the moment there is no place for gravity in the standard model.
The standard model is made up of quarks, leptons and the force carriers.
The weak force is carried by the bosons. The electromagnetic force is carried by photons. The gluons carry the strong force.
Every fundamental particle has an anti particle. These have the same mass but opposite charge.
If a particle and antiparticle, each of mass M, collide they annihilate with the production of energy, E, in the form of radiation – the total mass (2M) is converted into energy.
At the start of the universe, due to the big bang, equal amounts of matter and anti-matter should have been created but we live in a universe of mainly matter. Where did the ant-matter go?
What is mass? In the world that we know it is a measure of how difficult it is to accelerate an object (F = ma). The heavier the object, the more force is needed to accelerate it. This is Newton’s second law. For subatomic particles it is more complicated. Peter Higgs, in the 1960s came up with a theory of why these particles have mass. He proposed a new heavy particle now called the Higgs which generates the Higgs field.
Particles who “feel” the Higgs field gain mass.
Dr Evans, with two volunteers, carried out a demonstration to illustrate that some particles feel the Higgs field and others don’t. It is a repeat of the demonstration of passing a powerful magnet down a cooled copper tube (the magnet takes ages to pass down the tube on the left) but there is a second tube (on the right) where a piece of steel passes down the tube falling quickly because the only force acting on it is the force due to gravity. The left tube illustrates particles feeling the Higgs field. The tube on the right illustrates particles (like a photon) who don’t feel the Higgs field.
The lecture continued with a look at dark matter. Only about 4% of the measured mass of the visible universe is made up of atoms (up and down quarks, electrons and electron neutrinos). What makes up the rest?
Dark energy is believed to be why galaxies have such a high acceleration.
How do we try to answer these questions?
By colliding particles at very high energies and studying what comes out. At the LHC the quarks (in protons) collide with energies that existed about a billionth of a second after the big bang. Birmingham University is the only British university to be working on three of the projects (ALICE, ATLAS and LHCb).
The LHC has dimensions similar to the London circle line underground. It contains superconducting magnets. Protons go round the 27km ring 11000 times a second and there are 300 trillion in a beam. The beam is about the width of a human hair and they move in a very low vacuum (1E-13 atmospheres – as low as interplanetary space). Energy of the proton beam is greater than 0.3GJ (equivalent to a freight train moving at 100mph). The temperature has to be very low (1.9K – colder than outer space) for the superconducting magnets to work. They produce huge magnetic fields (1000 times stronger than a bar magnet) to accelerate and steer the beams of particles. Energy stored in the magnets is greater than 1 GJ.
As an aside we considered whether it is possible to make an anti-matter bomb. In theory it would only take 0.5g of antimatter to produce a bomb equivalent to the bomb that fell on Hiroshima. But ……The LHC can produce 100 million anti-protons per second. A proton has a mass of 1.67xE-27kg so the mass of the anti-protons only comes 1.67xE-19 kg. It would take one hundred million years to produce 0.5g.
How are the particles detected after the collision? In the good old days there were only bubble chambers. You could only detect one event per second and the photographs had to be scanned by hand.
Modern detectors are much better.
The LHC Project is pushing out the frontiers of physics in three directions.
Matter and antimatter are not perfect mirrors of each other.
The Higgs is real. Particles appear and disappear all the time. The virtual Higgs field gives rest mass. From Einstein’s’ famous equation we know that mass is related to energy, but not all energy can become mass.
98% of the atomic mass is caused by the strong force.
The Higgs particle is so short lived it can only be identified by the particles that decay from it. For instance the Higgs can first decay into two Z bosons and then each Z boson will decay into a high energy electron and positron. It is the electron and positron that is detected.
The strong force is also known as the colour force. It is the force that binds protons and neutrons together in the nucleus. It is also the force carried by gluons that hold quarks together to form protons, neutrons and other hadron particles. Gluons are thought to interact with quarks and gluons because they all carry a type of charge called the “colour charge”. Colour charge is analogous to electromagnetic charge, but it comes in three types rather than one, and it results in a different type of force, with different rules of behaviour (quantum chromodynamics). There will also be three charges for anti matter quarks.
The weak force is just involved in creating or decaying (as in beta decay). Mathematically it is a force and at very high energies it unifies with electromagnetism. It is not a classical mechanics force.