Particle Physics: (More) Fact and Fantasy Professor Ken Peach
Professor Peach started his lecture by outlining what he said at last year’s lecture which happened before the famous announcement about the Higgs Boson.
The above picture shows two things. The left hand side shows the equations of the standard model prior to the discovery of the Higgs (thankfully not on any A level syllabus) and summarises the discoveries of the latter part of the 19th century and all of the 20th century. The right hand side is a Feynman diagram showing that when a positron (e+) and an electron (e-) meet they annihilate each other to produce a photon of electromagnetic radiation, which can in turn reform back into a positron and an electron. is a probability process and C. of M. Energy means the total energy of the system as calculated in the frame of reference where there is zero total momentum. C. of M. stands for centre of mass (or possibly centre of momentum).
In particle physics, colour charge is a property of quarks and gluons that is related to the particles’ strong interactions in the theory of quantum chromodynamics (QCD). Colour charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD, there are many technical differences. The “colour” of quarks and gluons is completely unrelated to visual perception of colour. A quark is a fundamental particle that is found in all hadrons such as protons and neutrons. A gluon is a “messenger” particle of the strong nuclear force, which binds quarks together.
In the picture above left the colours and lines are probabilities. GeV and TeV are units of energy. Thickness indicates prediction/precision. In the picture above right the top line is the sum of the others.
The Sun is a big ball of plasma containing quarks and gluons.
X = fraction of the momentum; xf(x) = probability distribution function; Q^2 = 10GeV^2 is a measure of how hard the partons have been “hit”.
Dick Roberts’ paper published in 2003/2004 is the most cited theoretical particle physics paper (so far). Calculated from experimental observations. Probe proton structure with muons. Fire point probes at the protons. These are reflected off the “pips” (i.e. the quarks).
Bremsstrahlung radiation occurs. Bremsstrahlung (from bremsen “to brake” and Strahlung “radiation”, i.e. “braking radiation” or “deceleration radiation”) is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the accelerated particles increases.
Energy is transferred to the partons.
At A level students learn that protons are made up of two up quarks and one down quark but this is not a realistic picture. Protons actually contain odd things such as antiquarks and C quarks. You can visualise the protons as a bath or sea of quark and antiquarks pairs that are continuously being made and destroyed by gluons. Overall however there are always two up quarks and one down quark.
Before the LHC the Tevatron was used to search for the Higgs Boson. The Higgs particle is very heavy because of quantum mechanics.
The masses of subatomic particle do not emerge alone from the SM.
According to the Standard Model, the vacuum is filled with a condensate of Higgs particles: quarks, leptons, W and Z bosons continuously collide with these Higgs particles as they travel through the “vacuum”. The Higgs condensate acts like molasses and slows down anything that interacts with it. The stronger the interactions between the particles and the Higgs condensate are, the heavier the particles become. In other words: the coupling to the Higgs boson is proportional to the mass.
The top quark is 35 times heavier than the bottom quark.
If we have the Higgs particle the two photons (the wiggly lines) shown in the above picture on the left must add up to a mass of 125 GeV/c^2.
The above picture on the right might show evidence for supersymmetry but more data refuted it. Errors came from background “noise” (3 statistical error).
Monte Carlo method allows a simulation to be made that can show all outcomes.
The above picture on the right is about the decay of the Higgs particle into two photons.
One of the main objectives for lead-ion running is to produce tiny quantities of such matter, which is known as quark-gluon plasma, and to study its evolution into the kind of matter that makes up the Universe today. This exploration will shed further light on the properties of the strong interaction, which binds quarks, into bigger objects, such as protons and neutrons. Heavy-ion collisions provide a unique micro-laboratory for studying very hot, dense matter. This part of the LHC programme will be probing matter as it would have been in the first instants of the Universe’s existence.