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Fermi Lecture 6 Weak Force Carriers W, Z and Origin of Mass: Higgs Particle

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Fermi Lecture 6 Weak Force Carriers W, Z and Origin of Mass: Higgs Particle Frontiers in Physics and Astrophysics Fermi Lecture 6 Weak Force carriers W, Z and Origin of Mass: Higgs Particle Barry C Barish 5-December-2019 Fermi Lecture 6 1 Enrico Fermi Fermi Lecture 6 2 Enrico Fermi Lectures 2019-2020 Frontiers of Physics and Astrophysics • Explore frontiers of Physics and Astrophysics from an Experimental Viewpoint • Some History and Background for Each Frontier • Emphasis on Large Facilities and Major Recent Discoveries • Discuss Future Directions and Initiatives ---------------------------------------------------------------------- • Thursdays 4-6 pm • Oct 10,17,24,one week break, Nov 7 • Nov 28, Dec 5,12,19 Jan 9,16,23 • Feb 27, March 5,12,19 Fermi Lecture 6 3 Frontiers Fermi Lectures 2019-2020 - Barry C Barish • Course Title: Large Scale Facilities and the Frontiers of Physics • The Course will consist of 15 Lectures, which will be held from 16:00 to 18:00 in aula Amaldi, Marconi building, according to the following schedule: • 10 October 2019 - Introduction to Physics of the Universe 17 October 2019 - Elementary Particles 24 October 2019 - Quarks 7 November 2019 – Particle Accelerators 28 November 2019 – Big Discoveries and the Standard Model 5 December 2019 – Weak Force Carriers – Z, W: and Higgs Mechanism 12 December 2019 – Higgs Discovery; Supersymmetry? Intro to Neutrinos 19 December 2019 – Neutrinos 9 January 2020 – Neutrino Oscillations and Future Perspectives (2) 16 January 2020 - Gravitational Waves (1) 23 January 2020 - Gravitational Waves (2) 27 February 2020 – Gravitational Waves – Future Perspectives 5 March 2020 – Dark Matter 12 March 2020 – Particle Astrophysics, Experimental Cosmology 19 March 2020 - The Future • All Lectures and the supporting teaching materials will be published by the Physics Department. Fermi Lecture 6 4 Frontiers 6 Constituents and Forces in the Standard Model Creation of Mass Weak Force Carriers Fermi Lecture 6 5 Frontiers 1 Particle Physics: What we Know Fermi Lecture 6 6 Frontiers 6 The Force Carriers Fermi Lecture 6 7 Frontiers 1 Particle Physics: What we Know Fermi Lecture 6 8 Frontiers 6 Particle Interactions Fermi Lecture 6 9 Weak Decay of B-Mesons Discovery, properties of Z, W Bosons Fermi Lecture 6 10 Frontiers 6 Weak Interactions of Hadrons Fermi Lecture 6 11 Frontiers 6 Lepton Quark Symmetry Fermi Lecture 6 12 Frontiers 6 Constituents and Forces in the Standard Model Fermi Lecture 6 13 Frontiers 6 Assuming Lepton-Quark Symmetry Fermi Lecture 6 14 Frontiers 6 Nicola Cabibbo Fermi Lecture 6 15 Frontiers 6 Quark Mixing – Cabibbo Angle 16 Frontiers 6 Constituents and Forces in the Standard Model Fermi Lecture 6 17 Frontiers 6 Quark Mixing – Cabbibo Angle 18 Frontiers 6 Quark Mixing – Cabibbo Angle 19 Frontiers 6 Quark Mixing – With Charm Fermi Lecture 6 20 Frontiers 6 CKM Quark Mixing Fermi Lecture 6 21 Frontiers 6 Fermi Lecture 6 22 Frontiers 6 Quark Mixing – CKM Matrix Fermi Lecture 6 23 Frontiers 6 Quark Mixing – b-quark decays Fermi Lecture 6 24 Frontiers 6 Cabibbo-Kobayashi-Maskawa CKM Matrix Fermi Lecture 6 25 Frontiers 6 Cabibbo-Kobayashi-Maskawa CKM Matrix 26 Frontiers 6 Cabibbo-Kobayashi-Maskawa CKM Matrix Fermi Lecture 6 27 Decays of t-quark and detection Fermi Lecture 6 28 Frontiers 6 Top Quark Decays 29 Frontiers 5 Silicon Vertex Detectors Enable Detection of Top M W = 77.2 ± 4.6 GeV 30 Frontiers 6 Discovery of Top Quark Fermi Lecture 6 31 Frontiers 6 Top Quark Decay Channels 32 Frontiers 6 Top Quark Detector At Fermilab 33 Frontiers 6 Top Quark Observed Channels Fermi Lecture 6 34 Frontiers 6 Top Mass Distribution and Backgrounds 35 Z and W – Weak Force Carriers Fermi Lecture 6 36 Frontiers 5 Constituents of the Standard Model Discovery announced July 2010 Fermi Lecture 6 37 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 38 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 39 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 40 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 41 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 42 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 43 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 44 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 45 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 46 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons 47 Frontiers 6 Weak Interaction Force Carriers - W, Z Bosons Fermi Lecture 6 48 Frontiers 6 Summary of W, Z Discovery 49 Break Fermi Lecture 6 50 Frontiers 6 • Many particles are too Resonances and particles short-lived to be observed directly • Observe from decay particles • e.g. pp!X+other stuff, X!µ+µ- Energy uncertainty ΔE is known as the particles width Γ. Three Uncertainty principle relates width Γ resonances and mean lifetime τ of particle ! ΔEΔt ≥ 2 Γτ = ! Γ =2ΔE Exponential decay FWHM process for particles −t /τ N = Noe 51 Frontiers 6 Particle Width/lifetime Depends on: Shorter lifetime, • Strength of interaction larger width •Strong G~10-100MeV t~10-22-10-24s •EM G~10-100keV t~10-16-10-20s •Weak G<0.01eV t~10-8-10-13s STRONG •Number of different states the particle can decay into •Can see long-lived particle through EMAG their decay distance •Can see short-lived particles through their width WEAK Flight distance Fermi Lecture 6 52 Frontiers 6 Breit-Wigner spins Energy •The total width Γ is related to the strength of the interaction ⎛ ⎞ ⎜ Γ Γ ⎟ •The particle can usually decay to a π ⎛ (2 j +1) ⎞ i f σ if = 2 ⎜ ⎟⎜ 2 ⎟ q (2s +1)(2s +1) 2 Γ number of different final states i ⎝ 1 2 ⎠⎜ (E − M ) + ⎟ ⎝ 4 ⎠ •Each individual decay mode has a width, qi = intial momentum Γi, such that sum of “partial widths”=total j = resonance spin width. s1, s2 = spins of incoming particles Γi ,Γ f = widths of initial and final states •Branching Ratio •Fraction of decays to that state • BR=Γι/Γ •Total width from width of invariant mass distribution •Mass from centre of distribution •Partial width of initial and final states from height of distribution •Shape is same for all decays Fermi Lecture 6 53 Frontiers 6 Z0 production • e+e- has two diagrams γ and Z0 exchange • Photon exchange dominates at low c.m. energies • Z0 exchange results in a resonance at Mz • At High cm energy, both photon and Z exchange– EW unification • Calculation of relative strength of cross-section 54 Frontiers 6 Extracting the Nν from ΓZ 2 ⎛ Γ Z 0 → e+e− Γ Z 0 → X ⎞ + − 12π M Z ( ) ( ) σ e e → X = ⎜ ⎟ Final states X in Z- ( ) E 2 ⎜ 2 2 2 2 ⎟ cm (Ecm − M Z )+ M Z ΓZ decays ⎝ ⎠ ⎛ Z 0 e+e− Z 0 X ⎞ ! Counting states + − 12π Γ( → )Γ( → ) σ e e → X = ⎜ ⎟ max ( ) M 2 ⎜ Γ2 ⎟ Z ⎝ Z ⎠ Γe=Γµ=Γτ ! Γl Total width from shape in any decay channel Γνe=Γνµ=Γντ ! Γν Γhad Partial widths from all seen decay channels Γ =Γ ! Γ u c u What isn’t seen is the neutrinos Γd=Γs=Γb ! Γd ΓZ=Γhad+3Γl+NνΓν ! How to get Nν Fermi Lecture 6 55 Frontiers 6 How is Z mass / lineshape reconstructed ? • Energy of electron & positron beams in accelerator adjusted • Cross-section measured at different energies • Breit-Wigner fitted to cross-sections Peak value gives mass Fermi Lecture 6 56 Frontiers 6 Aside: Measuring the LEP beamTRAINS energy •The accuracy with which the LEP beam energy is known controls the accuracy of MZ •Return current for train –TGV- changed magnet TIDE TIDE currents •Tidal effects change the shape of the ring Fermi Lecture 6 57 Frontiers 6 Counting the Number of Light Neutrinos Determination of the Z0 line- shape: Reveals the number of ‘light neutrinos’ Fantastic precision on Z0 parameters Corrections for phase of moon, water level in Lac du Geneva, passing trains,… N 2.984±0.0017 ν Existence of only 3 neutrinos MZ0 91.1852±0.0030 GeV Unless the undiscovered neutrinos have mass m >M /2 ΓZ0 2.4948 ±0.0041 GeV ν Z Fermi Lecture 6 58 Frontiers 6 Number of Neutrino Types • Mass – peak of distribution • MZ=91.1876±0.0021GeV (0.02% error!) LEP • Total width - width ΓZ=2.4952±0.0023GeV (0.1%) • Partial Widths – number events Γhad=1.741±0.006 Γl=0.0838±0.0003 • Neutrino width from theory Nν=2.97±0.07 Fermi Lecture 6 59 Frontiers 6 What Does it Mean? ⎛e ⎞⎛ µ ⎞⎛τ ⎞ ⎜ ⎟ ⎜ ⎟⎜ ⎟⎜ ⎟ • ‘Proves’ there are only three ⎝ν e ⎠⎝ν µ ⎠⎝ντ ⎠ generations of particles ⎛u ⎞⎛c ⎞⎛t ⎞ ⎜ ⎟⎜ ⎟⎜ ⎟ ⎜d ⎟⎜ s ⎟⎜b ⎟ • 3 ν families ! 3 generations ⎝ ⎠⎝ ⎠⎝ ⎠ Q 3 Q ∑ leptons + ∑ quarks • Is this really so definitive ?? leptons quarks • Assumed neutrinos massless in theory width Z0 • Neutrinos now known to have mass (more later) but very small • Mass hierarchy • We have summed Z !X+X- • If there is an extra neutrino X with mass > Z mass/2 the Z could not decay to it Fermi Lecture 6 60 Frontiers 6 Standard Model of Particle Physics Now complete, except for the ‘origin of mass.’ Fermi Lecture 6 61 Higgs Fermi Lecture 6 62 Frontiers 6 What is the origin of the particle masses? Mass t u d c e s b ν e µ ν µ τ ντ Fermi Lecture 6 63 Frontiers 6 The Standard Model Ordinary matter Frontiers 6 Matter Particles So what are the other quarks and leptons? They are just higher mass copies. Why are there three copies of each? Good question! Maybe the Higgs boson will tell us… Frontiers 6 Force Particles Strong Force " (100 times stronger than E&M) Electricity & Magnetism " Weak Force (10,000,000,000 times weaker) Gravity is not included because it’s 1036 times weaker! Frontiers 6 How do Particles cause Forces? Two matter particles are pushed apart by exchanging a force particle Fermi Lecture 6 67 Frontiers 6 How do particles cause forces? Two boats are pushed apart by exchanging a medicine ball Fermi Lecture 6 68 Frontiers 6 Gauge Bosons The force between particles is transmitted by Gauge Bosons • Electromagnetic force (carried by photons) [1] e- γ e- • Strong nuclear force (carried by gluons [8]) • Weak nuclear force (carried by W and Z particles [3]) 69 Frontiers 6 Where do Gauge Bosons come from? • Gauge bosons reflect underlying symmetries of the universe.
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