Overview. The quest of Particle Physics research is to understand the fundamental particles of nature and their interactions.

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2 Overview The quest of Particle Physics research is to understand the fundamental particles of nature and their interactions. Our understanding is about to take a giant leap.. the Large Hadron Collider has started. These lectures are aimed to explain the background to our current understanding and the challenges involved in bringing the LHC to fruition. along with the fruits of the first data I hope you enjoy them! Lecture 1 : Matter and Forces Lecture 2 : Electroweak Unification Lecture 3 : Beyond the Frontier.. The LHC accelerator Lecture 4 : The LHC Experiments Lecture 5 : First Results and Expectations 2

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4 The Atom Photon Nucleus and electrons bound by electromagnetic force m

5 blow atom up to size of Millennium Dome... nucleus is now 3 mm across! Atoms are % empty space

6 The Atom Photon Nucleus and electrons bound by electromagnetic force m Gluon Protons and neutrons in nucleus bound by strong nuclear force m Quarks bound by the strong force. <10-15 m

7 γ e - ATOM Electrons bound to atom by electromagnetic force Binding energy 10 ev Size: Atom ~10-10 m, e - < m Charge: Atom is neutral, electron e Mass: Atom mass ~ in nucleus, m e = MeV/c 2 Chemical properties depend on Z. π NUCLEUS Nuclei held together by strong nuclear force Size: Nucleus (medium A) ~ 5fm Binding energy 0.1 MeV 1fm = m NUCLEON Protons and neutrons held together by the strong force Size: p, n ~ 1fm Charge: p +e n 0 Mass : p, n = MeV/c 2 ~ 1836 m e Binding energy 10 GeV 7

8 Energy and Mass Common practise in Particle Physics NOT to use SI units. Energies are measured in units of ev: 1 ev = Energy an electron acquires when it is accelerated through a potential difference of 1V. Common to use: KeV (10 3 ev) MeV (10 6 ev) GeV (10 9 ev) TeV (10 12 ev) Masses quoted in units of MeV/c 2 or GeV/c 2 e.g. Electron mass m e = kg = ( )( ) = ev c 2 = MeV c 2 8

9 Matter We now know that all matter is made of two types of elementary particles (spin ½ fermions): Electron e - LEPTONS: e.g. e -, ν e QUARKS: e.g. up quark (u), down quark (d) proton (uud) Quark u Proton uud And.. Antimatter 9

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11 Antimatter Every matter particle has a partner which has exactly the same properties except a charge which is opposite in sign. Electron e - Positron e + Quark u Proton uud Antiquark Antiproton uud u e - E=mc 2 e + Exploit matter-antimatter annihilation to produce new particles. Energy can be transformed back into any type of mass.

12 Matter: 1 st Generation Almost all phenomena you will have encountered can be described by the interactions of FOUR spin ½ particles: THE FIRST GENERATION Particle Symbol Type Charge Units of e Electron e - Lepton -1 Neutrino ν e Lepton 0 Up Quark u Quark +2/3 Down Quark d Quark -1/3 The proton and neutron are the lowest energy states of the combination of 3 quarks: p u u d n d d u 12

13 Matter: 3 Generations Nature is not quite so simple. There are THREE generations of fundamental fermions: 1 st Generation 2 nd Generation 3 rd Generation Electron e - Muon µ - Tau τ Electron Neutrino ν e Muon Neutrino Up quark u Charm quark Down quark d Strange quark ν µ Tau Neutrino ν τ c Top quark t s Bottom quark b Each generation e.g. (µ, ν µ, c, s) is an exact copy of (e, ν e, u, d) The only difference is the mass of the particles: the 1 st generation are the lightest and the 3 rd generation are heaviest. Clear symmetry origin of 3 generations is NOT UNDERSTOOD. 13

14 3 Generations Top t Bottom b Neutrino ν τ Tau τ The heavier particles decay to lighter ones Charm c Strange s Neutrino ν µ Muon µ There are two other families of quarks and leptons, which are heavier Up u Down d Quarks Neutrino ν e Electron e Leptons All normal matter consists of u and d quarks and electrons

15 Quarks Quarks experience ALL the forces (electromagnetic, strong, weak) Spin ½ fermions Fractional charge 6 distinct flavours Quarks come in 3 colours Red, Green, Blue Quarks are confined within HADRONS e.g. u u d u d Gen. 1 st 2 nd 3 rd Flavour Charge (e) Approx. Mass (GeV/c 2 ) u +2/ d -1/ c +2/3 1.5 s -1/3 0.5 t +2/3 171 b -1/ antiquarks u,d, COLOUR is a label for the charge of the strong interaction. Unlike the electric charge of an electron (-e), the strong charge comes in 15 3 orthogonal colours RGB.

16 Leptons Particles which DO NO INTERACT via the STRONG interaction. Spin ½ fermions 6 distinct FLAVOURS 3 charged leptons: e -, µ -, τ - µ and τ unstable 3 neutral leptons: ν e, ν µ, ν τ Neutrinos are stable and (almost?) massless ν e mass < 3 ev/c 2 ν µ mass < 0.17 MeV/c 2 Gen. 1 st 2 nd 3 rd Flavour Charge (e) Approx. Mass e (MeV/c 2 ) ν e 0 Massless? µ ν µ 0 Massless? τ ν τ 0 Massless? ν τ mass < 18.2 MeV/c 2 +antimatter partners, e +, ν e Charged leptons only experience the electromagnetic and weak forces Neutrinos only experience the weak force 16

17 Hadrons Single free quarks are NEVER observed, but are always CONFINED in bound states, called HADRONS. Macroscopically hadrons behave as point-like COMPOSITE particles. Hadrons are of two types: MESONS (qq) Bound states of a QUARK and an ANTIQUARK All have INTEGER spin 0, 1, 2, Bosons e.g. π + ( ud) charge = +2/3e + 1/3e = +1e π ud charge = -2/3e 1/3e = -1e BARYONS (qqq) Bound states of 3 QUARKS All have HALF-INTEGER spin 1/2, 3/2, Fermions e.g. ( ) ( ) n ( udd) p uud q q q q q PLUS ANTIBARYONS (qqq) e.g. p ( uud) n ( udd) 17

18 Periodic Table of the Elements Only three elements are formed in the Big Bang Differences between materials are due simply to the number of protons and electrons in their atoms.

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20 Forces Classical Picture: A force is something which pushes matter around and causes objects to change their motion (Newtons II). e.g. Electromagnetic forces arise via the action at a distance of the electric and magnetic fields. Newton: that a body can act upon another at a distance, through a vacuum, without the mediation of anything else,, is to me a great absurdity 20

21 Forces Quantum Mechanically: Forces arise due to exchange of VIRTUAL FIELD QUANTA (Gauge Bosons): second quantization. Field strength at any point is uncertain Number of quanta emitted and absorbed Massless particle e.g. photon 21

22 Strong Forces Weak e Polonium Astatine Electromagnetism Gravity

23 Gauge Bosons GAUGE BOSONS mediate the fundamental forces Spin 1 particles (i.e. Vector Bosons) No generations The manner in which the Gauge Bosons interact with the leptons and quarks determines the nature of the fundamental forces. Force Boson Spin Strength Mass (GeV/c 2 ) Strong Gluon g 1 1 Massless Electromagnetic Photon γ Massless Weak W and Z W ±, Z , 91 Gravity Graviton? Massless 23

24 Range of Forces The range of a force is directly related to the mass of the exchanged bosons. Force Strong Range ~ 1 mass Strong (Nuclear) Electromagnetic Range (m) Weak Gravity Force (GeV/fm) Weak Force Gravitational Force Electromagnetic Force Strong Force (hadrons) Strong Force (quarks) Distance (fm) Due to quark confinement, nucleons start to experience the strong interaction at ~ 2 fm 24

25 The Standard Model Spin ½ Fermions LEPTONS QUARKS Charge (units of e) /3-1/3 PLUS antileptons and antiquarks. Spin 1 Bosons Mass (GeV/c 2 ) Gluon g 0 STRONG Photon γ 0 EM W and Z Bosons W ±, Z /80.3 WEAK The Standard Model also predicts the existence of a spin 0 HIGGS BOSON which gives all particles their masses via its interactions. 25

26 The Standard Model of Particle Physics Explains all the data we have so far. but there are many unanswered questions...

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28 Theoretical Framework Macroscopic Microscopic Slow Classical Mechanics Quantum Mechanics Fast Special Relativity Quantum Field Theory The Standard Model is a collection of related QUANTUM FIELD THEORIES that describe particle interactions. ELECTROMAGNETISM: QUANTUM ELECTRODYNAMICS (QED) 1948 Feynman, Schwinger, Tomonaga (1965 Nobel Prize) ELECTROMAGNETISM: ELECTROWEAK UNIFICATION +WEAK 1968 Glashow, Weinberg, Salam (1979 Nobel Prize) STRONG: QUANTUM CHROMODYNAMICS (QCD) 1974 Politzer, Wilczek, Gross (2004 Nobel Prize) 28

29 Feynman Diagrams Richard Feynman devised a pictorial method for calculating the interaction between fundamental particles ELECTROMAGNETIC e - e - q STRONG q Quark-antiquark annihilation p p u u n d d d u Electron-proton scattering g W WEAK Neutron decay e - p 29

30 Summary of Standard Model Vertices ELECTROMAGNETIC STRONG WEAK CC WEAK NC (QED) (QCD) γ W Z 0 γ g W Z0 +antiparticles 30

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32 QED Quantum Electrodynamics is the theory of the electromagnetic interaction Some QED processes : Compton Scattering γ e - γ e - γ γ e e e Pair production γ e + e - Nucleus e + e Annihilation 32

33 Discovery of Quarks Virtual γ carries energy and momentum e - e - Large momentum small wavelength Large energy high frequency Photon oscillates rapidly in space and time probes short distances and short time. p p Small E,p Rutherford Scattering E, p increases Excited states Large E,p Elastic scattering from quarks in proton dσ dω λ<< size of proton SLAC e - scattering (1972) Expected Rutherford scattering E = 8 GeV Angle (radians) 33

34 Rutherford 1911 SLAC

35 Experimental Tests of QED QED is an extremely successful theory tested to very high precision. Example: Magnetic moments of e ±, µ ± : For a point-like spin ½ particle: Dirac Equation However, higher order terms introduce an anomalous magnetic moment i.e. g not quite 2. v v O(1) O(α) O(α 4 ) diagrams 35

36 O(α 3 ) g e 2 2 g e 2 2 = ( ± 0.041) = ( ± 0.3) Experiment Theory Agreement at the level of 1 in QED provides a remarkable precise description of the electromagnetic interaction! 36

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38 QCD QED: is the quantum theory of the electromagnetic interaction. mediated by massless photons photon couples to electric charge strength of interaction: α = g 2 = e 2 ~ QCD: is the quantum theory of the strong interaction. mediated by massless gluons gluon couples to strong charge, called COLOUR only quarks have non-zero COLOUR, therefore only quarks feel the strong interaction. Quarks carry COLOUR (red, green, blue) Antiquarks carry ANTI-COLOUR (anti-red, anti-green, anti-blue) strength of interaction: α s = g s 2 ~ 1 38

39 Gluons QCD looks like a stronger version of QED. However, there is one BIG difference and that is GLUONS also carry COLOUR. GLUONS CAN INTERACT WITH OTHER GLUONS 3 GLUON VERTEX 4 GLUON VERTEX Example: Gluon-gluon scattering gg gg 39

40 Confinement NEVER OBSERVE single FREE quarks or gluons. Quarks are always confined within hadrons This is a consequence of the strong interaction of gluons. Qualitatively, compare QCD with QED: QCD Colour field QED Electric field Self interactions of the gluons squeeze the lines of force into a narrow tube or STRING. The string has a tension and as the quarks separate the string stores potential energy. Energy stored per unit length in field ~ constant Energy required to separate two quarks is infinite. Quarks always come in 40 combinations with zero net colour charge CONFINEMENT.

41 Jets Consider the quark, anti-quark pair produced in e + e - annihilation: As the quarks separate, the energy in the colour field ( string ) starts to increase linearly with separation. When the energy stored exceeds 2m q, new quark, anti-quark pairs can be created. As energy decreases hadrons (mainly mesons) freeze out 41

42 As quarks separate, more quark, anti-quark pairs are produced. This process is called HADRONIZATION. Start out with quarks and end up with narrowly collimated JETS of HADRONS. JET JET Typical event The hadrons in a quark (antiquark) jet follow the direction of the original quark (antiquark). Consequently, is observed as a pair of back-to-back jets. 42

43 Evidence for Gluons In QED, electrons can radiate photons. In QCD, quarks can radiate gluons. Example: In QED we can detect the photons. In QCD, we never see free gluons due to confinement. Experimentally, detect gluons as an additional jet: 3-JET events. 43

44 Discovery of gluons (1978) LEP Event (1990) 44

45 Summary So far we have discussed the fundamental particles (quarks and leptons) the forces of nature (electromagnetism, strong, weak and gravity) the force carriers (photon, gluon, W and Z and the graviton) the theory of electromagnetism (Quantum Electrodynamics) the theory of the strong interaction (Quantum Chromodynamics) Next lecture we will discuss The Weak Force and Electroweak Unification 45