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.
• The number of gauge bosons for each force is the number of generators for each symmetry group (e.g. the symmetry group for the strong nuclear force has 8 generators, hence 8 gluons).
• These gauge bosons should be massless.
• However, the W+, W-, and Z bosons (which mediate the weak nuclear force) are massive.
70 Frontiers 6 Higgs Particle
Frontiers 6 The Higgs Boson
Classical Mass
Mass in Relativity (Special and General)
Fermi Lecture 6 73 Frontiers 6 Mass and Binding Energy
-27 Proton mass, mP=1.6726x10 kg
-27 Neutron mass, mN=1.6749x10 kg
4He mass=6.6446x10-27 kg
Less than 2mP+2mN Fermi Lecture 6 74 Frontiers 6 Quark Masses
Quarks Proton Mass
The mass of a proton is much larger than the Dynamics mass of the quarks that 98% make up the proton.
Fermi Lecture 6 75 Frontiers 6 Quantum Fluctuations
The Heisenberg Uncertainty Principle states:
ΔEΔt ≥ h/2π with the Planck constant h = 6.636x10-34 m2 kg/s
This means that particles are popping in and out of existence continuously (so the vacuum is actually fluctuating).
The probability distribution of particles everywhere in space is called a field.
Fermi Lecture 6 76 Frontiers 6 The Vacuum
According to the rules of quantum field theory, the vacuum is not empty but it is actively populated by particle-antiparticle pairs that appear, annihilate, and disappear, existing fo only brief instants.
Fermi Lecture 6 77 Frontiers 6 Electroweak Symmetry Breaking
Fermi Lecture 6 78 Frontiers 6 Higgs Particle – Origin of Mass
Born in 1929, Prof. Higgs is an English theoretical physicist and Emeritus professor at the University of Edinburg. His fundamental work on the origin of mass was published in 1964.
• In the theory, all fundamental particles must be massless. • Adding particle masses to the equations “by hand” makes the theory mathematically inconsistent. • But we know from experiment that most elementary particles have mass!! • Prof. Peter Higgs was one of the first physicists to explain how particles could acquire mass. 79 Frontiers 6 The Higgs Boson
• In the “Standard Model” the origin of mass is addressed using a mechanism named after the British physicist Peter Higgs. • It predicts a new particle: the Higgs boson.
What is the Higgs boson? In 1993, the then UK Science Minister, Williamhttps://youtu.be/joTKd Waldegrave, issued a challenge to physicists to answer the questions 'What is the Higgs Fermi Lecture 6 80 boson, and why do we wanht to find it?' on one side of a single Fermi Lecture 6 81 Frontiers 6 Higgs Mechanism
• In 1964, 6 authors in 3 different papers (Brout and Englert, Higgs, and Guralnik, Hagen, and Kibble) proposed a mechanism for making the weak force carriers massive.
• Depended on having these gauge bosons couple to something called the Higgs Field, which has a fourfold symmetry.
• When the vacuum fluctuations of the Higgs field become non-zero, the symmetry is broken, which makes the W+, W-, and Z bosons massive and leaves a single massive particle called the Higgs Boson.
• Other particles (electrons, quarks, etc) can also acquire mass by coupling to the Higgs field.
Fermi Lecture 6 82 Frontiers 6 Higgs Field • There is a non-zero Higgs field that permeates all of the universe. • Similar to how there is a non-zero magnetic field around you right now, but unlike the Earth’s magnetic field, the Higgs field has no direction… and it is everywhere in the universe. • All particles start out massless, but then acquire mass by interacting with the Higgs field. • What does that mean, they “acquire” mass? • Particles feel forces when they are in an electric or magnetic field? • Some particles resist accelerating when in the Higgs field. ⇒ particles that interact with the Higgs field have mass. • Just as the forces are mediated by force carrier particles, interactions with the Higgs field is mediated by Higgs bosons.gg
Fermi Lecture 6 83 Frontiers 6 Higgs Boson • The Standard Model Higgs assumes the simplest possible form for the Higgs boson. • A single fundamental particle; • Zero intrinsic angular momentum (spin=0); • Even parity; • No electric charge; • The probability of interacting with a fermion is directly proportional to the mass of that fermion. • Precise interaction with bosons also predicted. • Basically the same as proposed by Prof. Higgs and colleagues in 1964. • There are also more complex and exotic versions. • Multiple Higgs bosons, composite Higgs made of smaller particles, … • But Occam’s razor says to start with the simplest version until there is evidence for something more complicated. Fermi Lecture 6 84 Frontiers 6 Higgs Boson • The Standard Model Higgs assumes the simplest possible form for the Higgs boson. • A single fundamental particle; • Zero intrinsic angular momentum (spin=0); • Even parity; • No electric charge; • The probability of interacting with a fermion is directly proportional to the mass of that fermion. • Precise interaction with bosons also predicted. • Basically the same as proposed by Prof. Higgs and colleagues in 1964. • There are also more complex and exotic versions. • Multiple Higgs bosons, composite Higgs made of smaller particles, … • But Occam’s razor says to start with the simplest version until there is evidence for something more complicated. Fermi Lecture 6 85 Frontiers 6 Standard Model, including the Higgs Particle Frontiers 6 Particle creation
• Mass is just a form of energy: E = mc2 • With enough energy, we can create new heavy particles.
Fermi Lecture 6 87 Frontiers 6 Particle creation
• Mass is just a form of energy: E = mc2 • With enough energy, we can create new heavy particles.
Z
H
• For example, a Z boson and a Higgs boson.
Fermi Lecture 6 88 Frontiers 6 Particle decay
• Conversely, particles with a lot of mass have a lot of energy and will decay to lighter particles.
e
e
μ • We should be able to create Higgsμ bosons in high energy collisions, which will quickly decay back to the lighter particles, which we will then measure in our detector.
89 Frontiers 6 Detecting the Higgs Boson 1) If Higgs bosons is created, what will we see? • How will Higgs boson be created and how will they decay? • How frequently do we expect to see these events? 2) What other processes can result in the same final particles? • How many of those background events do we expect? 3) How to discriminate the signal from the background. • The energies and directions of the detected particles will have different characteristics for different processes. 4) What do we observe in the data? • Do the data agree with the background-only prediction or the signal+background prediction? • How strongly do the data reject the background-only prediction? Fermi Lecture 6 90 End
Fermi Lecture 6 91