CfE HigherPhysics: Particles and Waves



-3987127591Lochgelly High SchoolCfE HigherPhysics: Particles and Waves1. The Standard Model1000000Lochgelly High SchoolCfE HigherPhysics: Particles and Waves1. The Standard ModelParticles and Waves: The Standard ModelContents TOC \o "1-3" \h \z \u Unit Specification PAGEREF _Toc391384467 \h 21. The Standard Model PAGEREF _Toc391384468 \h 2Notes PAGEREF _Toc391384469 \h 2Contexts PAGEREF _Toc391384470 \h 2Orders of Magnitude PAGEREF _Toc391384471 \h 3Questions PAGEREF _Toc391384472 \h 4The Standard Model of Fundamental Particles and Interactions. PAGEREF _Toc391384473 \h 5Historical Background PAGEREF _Toc391384474 \h 5What is the world made of? PAGEREF _Toc391384475 \h 5Elements: the simplest chemicals PAGEREF _Toc391384476 \h 5The periodic table – order out of chaos PAGEREF _Toc391384477 \h 5The discovery of the electron PAGEREF _Toc391384478 \h 5The structure of atoms PAGEREF _Toc391384479 \h 5The Rutherford scattering experiment PAGEREF _Toc391384480 \h 5The discovery of the neutron PAGEREF _Toc391384481 \h 6Matter and antimatter PAGEREF _Toc391384482 \h 6The Particle Zoo PAGEREF _Toc391384483 \h 6Quarks PAGEREF _Toc391384484 \h 7The Standard Model PAGEREF _Toc391384485 \h 7Hadrons PAGEREF _Toc391384486 \h 8Leptons PAGEREF _Toc391384487 \h 8The Four Fundamental Forces PAGEREF _Toc391384488 \h 9Force particles – The bosons PAGEREF _Toc391384489 \h 9Beta Decay and Evidence for the Neutrino PAGEREF _Toc391384490 \h 10Positron Emission Tomography (PET) Scanning PAGEREF _Toc391384491 \h 10Questions PAGEREF _Toc391384492 \h 12Unit Specification1. The Standard ModelOrders of magnitude.The Standard Model of Fundamental Particles and Interactions.NotesThe range of orders of magnitude of length from the very small (sub-nuclear) to the very large (distance to furthest known celestial objects).The evidence for the sub-nuclear particles and the existence of antimatter. Fermions, the matter particles, consist of Quarks (6 types) and Leptons (Electron, Muon and Tau, together with their neutrinos). Hadrons are composite particles made of Quarks. Baryons are made of three Quarks and Mesons are made of two Quarks. The force mediating particles are bosons (Photons, W and Z Bosons, and Gluons). Description of beta decay as the first evidence for the neutrino. ContextsThe scale of our macro world compared to astronomical and sub-nuclear scales. Gravity, electromagnetic, strong and weak forces. LHC at CERN. PET scanner. Orders of MagnitudePhysics is the most universal of all sciences. It ranges from the study of particle physics at the tiniest scales of around 10-18 m to the study of the universe itself at a scale of 1029m. Powers of 10 are referred to as orders of magnitude, i.e. something a thousand times larger is three orders of magnitude bigger. On the following page is a table giving examples of objects that range from the very small (fundamental particles) to the very large (the universe). For convenience this table starts with the electron but the neutrino comes in at 10-24 m. The smallest possible dimension is known as the Planck length and is 10-35 m (if strings exist they would be this order of magnitude).3161030110490++?+++?????BCDA00++?+++?????BCDAQuestionsThe diagram shows a simple model of the atom. Match each of the letters A, B, C and D with the correct word from the list below.Electron, neutron, nucleus, protonIn the following table the numbers or words represented by the letters A, B, C, D, E, F and G are missing.Match each letter with the correct words from the list below:diameter of nucleus diameter of protondiameter of Sundistance to nearest galaxy height of Ben Nevissize of dust particle your heightOrder of magnitude/mObject1015A1014B1010Diameter of hydrogen atom104C100D103E107Diameter of Earth109F1013Diameter of solar system1023GThe Standard Model of Fundamental Particles and Interactions.Historical BackgroundWhat is the world made of? The ancient Greeks believed the world was made of 4 elements (fire, air, earth and water). Democritus used the term ‘atom’, which means “indivisible” (cannot be divided) to describe the basic building blocks of life. Other cultures including the Chinese and the Indians had similar concepts.Elements: the simplest chemicalsIn 1789 the French chemist Lavoisier discovered through very precise measurement that the total mass in a chemical reaction stays the same. He defined an element as a material that could not be broken down further by chemical means, and classified many new elements and compounds. The periodic table – order out of chaosIn 1803 Dalton measured very precisely the proportion of elements in various materials and reactions. He discovered that they always occurred in small integer multiples. This is considered the start of modern atomic theory. In 1869 Mendeleev noticed that certain properties of chemical elements repeat themselves periodically and he organised them into the first periodic table. The discovery of the electronIn 1897 J.J. Thomson discovered the electron and the concept of the atom as a single unit ended. This marked the birth of particle physics. The structure of atomsAt the start of modern physics at the beginning of the 20th century, atoms were treated as semi-solid spheres with charge spread throughout them. This was called the Thomson model after the physicist who discovered the electron. This model fitted in well with experiments that had been done by then, but a new experiment by Ernest Rutherford in 1909 would soon change this. This was the first scattering experiment – an experiment to probe the structure of objects smaller than we can actually see by firing something at them and seeing how they deflect or reflect.The Rutherford scattering experimentRutherford directed his students Hans Geiger and Ernest Marsden to fire alpha particles at a thin gold foil. This is done in a vacuum to avoid the alpha particles being absorbed by the air.The main results of this experiment were:Most of the alpha particles passed straight through the foil, with little or no deflection, being detected between positions A and B.A few particles were deflected through large angles, e.g. to position C, and a very small number were even deflected backwards, e.g. to position D.Rutherford interpreted his results as follows:The fact that most of the particles passed straight through the foil, which was at least 100 atoms thick, suggested that the atom must be 99.99% empty space!In order to produce the large deflections at C and D, the positively charged alpha particles must be encountering something of very large mass and a positive charge: the nucleus. The nucleus is approximately 100,000 times smaller than the atom and contains a positive charge and most of the mass of the atom.No deflectionB Small deflectionA Small deflectionC Large deflectionD Deflected right backAlpha particlesNo deflectionB Small deflectionA Small deflectionC Large deflectionD Deflected right backAlpha particlesThe discovery of the neutronThe neutron was discovered by Chadwick in 1932. This explained the existence of isotopes – elements with the same number of protons but different mass.Science now had an elegant theory which explained the numerous elements using only 3 particles, the proton, neutron and electron. However this simplicity did not last long!Matter and antimatterIn 1928, Dirac found two solutions to the equations he was developing to describe electron interactions. The second solution was identical in every way apart from its charge, which was positive rather than negative. This was named the positron, and experimental proof of its existence came just 4 years later in 1932 when they were identified by tracks in a cloud chamber. (The positron is the only antiparticle with a special name – it means ‘positive electron’.)Almost everything we see in the universe appears to be made up of just ordinary protons, neutrons and electrons. However high-energy collisions revealed the existence of antimatter. So where is the antimatter from the Big Bang?Antimatter consists of particles that are identical to their counterparts in every way apart from charge, e.g. an antiproton has the same mass as a proton but a negative charge. It is believed that every particle of matter has a corresponding antiparticle. When a matter particle meets an anti-matter particle they annihilate, giving off energy. Often a pair of high energy photons (gamma rays) are produced but other particles can be created from the conversion of energy into mass (according to the relationship: E=mc2). Antimatter is the way in which hospital PET scanners work. The Particle ZooThe discovery of anti-matter was only the beginning! From the 1930s onwards the technology of particle accelerators greatly improved and nearly 200 more particles have been discovered! Colloquially this was known as the particle zoo, with more and more new species being discovered each year. A new theory was needed to explain and try to simplify what was going on. This theory is called the Standard Model. QuarksIn 1964 Murray Gell-Mann and George Zweig independently proposed that protons and neutrons consisted of three parts. Gell-Mann called these ‘quarks’. The particle zoo could now be reduced to a more manageable number of fundamental particles. The standard model was developed to include these fundamental particles of matter and also to describe how they interact with each other.Each quark has only a fraction (1/3 or 2/3) of the electron charge (1.6×10–19 C). These particles also have other properties, such as spin, colour, charm and even something called strangeness, which are not covered by this course. The existence of quarks has been confirmed by carrying out deep-inelastic scattering experiments which use high energy electrons to probe deep into the nucleus. However, they have never been observed on their own, only in combinations of two or three when they make up what are called hadrons.The Standard ModelThe standard model is the most successful theory to explain what the universe is made of. It consists of 16 particles: 12 fermions (6 quarks and 6 leptons) and 4 bosons. Fermions are the quarks and leptons that make up everyday matter. The bosons are the exchange particles for the four fundamental forces. In addition the Higgs boson has been hypothesized to give particles mass. fermionsbosonsquarksuup (?)ccharm (?)ttop (?)?photonforce carriersddown (-?)sstrange (-?)bbottom (-?)ZZ bosonLeptonseelectron (-1)?muon (-1)?tau (-1)WW boson?eelectron neutrino??muon neutrino??tau neutrinoggluonHadronsParticles which are made up of quarks are called hadrons (the word hadron means heavy particle.There are two different types of hadron, called baryons and mesons which depend on how many quarks make up the particle. Baryons are made up of 3 quarks. Examples include the proton and the neutron.The charge of the proton (and the neutral charge of the neutron) arises out of the fractional charges of the quarks. This is worked out as follows:A proton consists of 2 up quarks and a down quark. Total charge = +1 p = u u d 23+23-1 3 =+ 1.A neutron consists of 1 up quark and 2 down quarks. No charge.n = u d d 23-13-13=0Mesons are made up of 2 quarks. They always consist of a quark and an anti-quark pair.An example of a meson is a negative pion (π- = ū d). It is made up of an anti-up quark and a down quark: This gives it a charge of: -23-13= -1. Note: Quarks are never found in isolation, only in doubles or triples that give integer charge. Thus we never observe fractions of the electron charge. This means not every combination of quarks and antiquarks is possible. Using a bar above a quark means it is an antiquark e.g. ū is the anti-up quark (this is not the same as the down quark.) The negative pion only has a lifetime of around 2.6x10-8 s.LeptonsThe term ‘lepton’ (light ones) was proposed in 1948 to describe particles with similarities to electrons and neutrinos. It was later found that some of the second and third generation leptons were significantly heavier; the tau particle is actually heavier than the proton. All 3 leptons have a “ghostly” partner associated with it called the neutrino. This has no charge (its name means little neutral one). There is an electron neutrino, a muon neutrino and a tau neutrino.The Four Fundamental ForcesPhysicists currently believe that there are 4 fundamental forces. The strong nuclear force is what stops the positive nucleus from exploding. It has an extremely short range. It is only experienced by quarks and therefore by the baryons (such as protons and neutrons) and mesons that are made up from them. The weak nuclear force is involved in radioactive beta decay. It is called the weak nuclear force to distinguish it from the strong nuclear force, but it is not actually the weakest of all the fundamental forces. It is also an extremely short-range force. The electromagnetic force stops the electron from flying out of the atom. The theory of the electromagnetic force and electromagnetic waves was created by the Scottish Physicist James Clerk Maxwell in the 19th Century.The final force is gravity. Although it is one of the most familiar forces to us it is also one of the least understood. It may appear surprising that gravity is, in fact, the weakest of all the fundamental forces when we are so aware of its effect on us in everyday life. However, if the electromagnetic and strong nuclear forces were not so strong then all matter would easily be broken apart and our universe would not exist in the form it does today.Force particles – The bosonsEach force has a particle associated with it which transmits the effects of that force. The table below summarizes the current understanding of the fundamental forces.ForceExchange ParticleRange (m)Relative strengthApproximate decay time (s)Example effectsStrong nucleargluon10–15103810–23Holding protons in the nucleusWeak nuclearW and Z bosons10–18102510–10Beta decay; decay of unstable hadronsElectromagneticphoton103610–20–10–16Holding electrons in atomsGravitationalgraviton1UndiscoveredHolding matter in planets, stars and galaxiesSome theories postulate the existence of a further boson, called the Higgs boson (sometimes referred to as the ‘God particle’), which isn’t involved in forces but is what gives particles mass. Experiments are being done to verify its existence using the Large Hadron Collider at CERN and the Tevatron at Fermilab.Beta Decay and Evidence for the NeutrinoNeutrinos were first discovered in radioactive beta decay experiments. In beta decay, a neutron in the atomic nucleus decays into a proton and an electron. The electron is emitted at high speed due to the nuclear forces. This carries away kinetic energy. Precise measurement of this energy had shown that there was a continuous spread of possible values.3495675983615-1231901078865This result was unexpected because when alpha particles are created in alpha-decay they have very precise and distinct energies. This energy corresponds to the difference in the energy of the nucleus before and after the decay. The graphs below show the energy of alpha-particles emitted by alpha-decay (left) and the energy of electrons emitted by beta decay (right).It was clear that the process that creates the beta decay electrons is different from alpha-decay. The electrons in beta decay come out with a range of energies up to, but not including the expected value. In 1930 Wolfgang Pauli suggested that in addition to electrons and protons atoms also contained an extremely light neutral particle which he called the “neutron”. He suggested that this "neutron" was also emitted during beta decay (accounting for the missing energy, momentum, and angular momentum) and had simply not yet been observed. In 1931 Enrico Fermi renamed Pauli's "neutron" to neutrino, and in 1934 published a model of beta decay in which neutrinos were produced. The neutrino interaction with matter was so weak that detecting it proved a severe experimental challenge, and was not accomplished until 1956. However, the properties of neutrinos were (with a few minor modifications) as predicted by Pauli and Fermi (In fact, in beta-decay an anti-neutrino is emitted along with the electron as lepton number is conserved in particle reactions). Interesting facts:More than 50 trillion (50x 1012) solar neutrinos pass through an average human body every second while having no measurable effect. They interact so rarely with matter that massive tanks of water, deep underground are required to detect them. Positron Emission Tomography (PET) ScanningPositron emission tomography (PET) scanners use antimatter annihilation to obtain detailed 3-D scans of body function. Other imaging techniques called CT and MRI scans can give detailed pictures of the bone and tissue within the body but PET scans give a much clearer picture of how body processes are actually working.A β+ tracer with a short half-life is introduced into the body attached to compounds normally used by the body, such as glucose, water or oxygen. When this tracer emits a positron it will annihilate nearly instantaneously with an electron. This produces a pair of gamma-ray photons of specific frequency moving in approximately opposite directions to each other. (The reason it is only an approximately opposite direction is that the positron and electron are moving before the annihilation event takes place.) The gamma rays are detected by a ring of scintillators, each producing a burst of light that can be detected by photomultiplier tubes or photodiodes. Complex computer analysis traces tens of thousands of possible events each second and the positions of the original emissions are calculated. A 3-D image can then be constructed, often along with a CT or MRI scan to obtain a more accurate picture of the anatomy alongside the body function being investigated.4287520144780Tracing the use of glucose in the body can be used in oncology (the treatment of cancer) since cancer cells take up more glucose than healthy ones. This means that tumours appear bright on the PET image. Glucose is also extremely important in brain cells, which makes PET scans very useful for investigation into Alzheimer’s and other neurological disorders. If oxygen is used as the tracking molecule, PET scans can be used to look at blood flow in the heart to detect coronary heart disease and other heart problems.The detecting equipment in PET scanners has much in common with particle detectors and the latest developments in particle accelerators can be used to improve this field of medical physics.QuestionsName the particles represented by the following symbol(a) p(b)(c)e(d)(e)n(f)(g)(h)(i)(j)A particle can be represented by a symbol where M represents the mass number, A the atomic number (or charge) and X identifies the particle. For example a proton is . Give the symbols in this form for the following particles.(a)(b)e(c)(d)n(e)Copy and complete the table by placing the fermions in the list below in the correct column of the table.bottomcharmdownelectronelectron neutrinotau neutrinostrangetaumuon neutrinotopQuarksLeptonsState the difference between a hadron and a lepton in terms of the type of force experienced by each particle.Give one example of a hadron and one example of a rmation on the sign and charge relative to proton charge of six types of quarks (and their corresponding antiquarks) is shown in the table.Quark nameCharge relative to size of proton chargeAntiquark nameCharge relative to size of proton chargeup+2/3antiup–2/3charm+2/3anticharm–2/3top+2/3antitop–2/3down–1/3antidown+1/3strange–1/3antistrange+1/3bottom–1/3antibottom+1/3Calculate the charge of the following combinations of quarks:Two up quarks and one down quarkOne up quark and two down quarksTwo antiup quarks and one antidown quarkOne antiup quarks and two antidown quarksNeutrons and protons are considered to be composed of quarks.How many quarks are in each neutron and in each proton?Comment briefly on the charge and composition of the neutron and proton.Which fundamental force keeps the quarks bound together in a proton or neutron?Briefly state any differences between the ‘strong’ and ‘weak’ nuclear forces.Give an example of a particle decay associated with the weak nuclear force.Which of the two forces, strong and weak, acts over the greater distance?What are the exchange particles for each of the following forces:Electromagnetic forceWeak forceStrong forceExplain what happens in beta decay and which fundamental force is involved. ................
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