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Why the Higgs Boson Matters Why the Higgs boson matters Heather Logan Carleton University - A little about my physics trajectory - Introduction: forces and particles - The problem of mass - The Higgs mechanism - Discovery of the Higgs particle - Where we go next 1 Why the Higgs boson matters Heather Logan Carleton University - A little about my physics trajectory - Introduction: forces and particles - The problem of mass - The Higgs mechanism - Discovery of the Higgs particle - Where we go next 2 Undergrad at U. California Davis 1989{1993 Started out interested in astronomy, quickly switched to physics. Was interested in astroparticle physics. Graduate school (Ph.D.) at U. California Santa Cruz 1993{1999 Worked on Higgs physics Our mascot: the banana slug! Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 3 Postdoc at Fermilab 1999{2002 Research job! Short-term contract: 3 years. Continued learning: more Higgs, supersymmetry, B physics. Mentoring by experts in the field. Postdoc at U. Wisconsin Madison 2002{2005 Short-term contract: 3 years. Continued learning: Higgs at LHC, Little Higgs models. Mentoring by different experts. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 4 Professor at Carleton U. 2005{now from mapofnorthamerica.org Geographical roulette! Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 5 Now I am mentoring students M.Sc. students - Charged Higgs phenomenology in the lepton-specific two Higgs doublet model (2009) - Dirac neutrinos from a second Higgs doublet (2009) - Charged Higgs phenomenology in the flipped two Higgs doublet model (2010) - LHC phenomenology of a two-Higgs-doublet neutrino mass model (2010) - High-energy suppression of the Higgsstrahlung cross-section in the Minimal Composite Higgs Model (2013) Ph.D. students - Constraints on large scalar multiplets from perturbative unitarity (2012) - Constraining models with a large scalar multiplet (2013) Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 6 4th year honours projects (some projects continued during summer) - Higgs couplings in a model with triplets (2010) - Yukawa alignment from natural flavor conservation (2011) - Model-independent Higgs coupling measurements at the LHC using the h ! ZZ ! 4` lineshape (2011) - Learning what the Higgs is mixed with (2013) Postdocs - Experimental constraints on the nearly minimal supersymmetric standard model and implications for its phenomenology (2009) - Top-Higgs and Top-pion phenomenology in the Top Triangle Moose model (2011) - LHC limits on the top-Higgs in models with strong top-quark dynamics (2011) - Dilaton constraints and LHC prospects (2012) - Discovering strong top dynamics at the LHC (2012) - Can the 126 GeV boson be a pseudoscalar? (2012) Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 7 So what about the Higgs boson? ...first I need to introduce the rest of the Standard Model of particle physics. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 8 Particle physics studies the structure of matter on the smallest scales that we can probe experimentally. Image: Fermilab Image: Contemporary Physics Education Project Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 9 Matter is made up of quarks and leptons. Quarks Leptons Generation 1 ! dueν e Generation 2 ! scµν µ Generation 3 ! btτν τ electric charge: −1=3 +2=3 −1 0 Atoms: proton= u + u + d (bound state of 3 quarks) neutron= u + d + d (bound state of 3 quarks) electron= e− (a charged lepton) Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 10 Matter particles interact via the four known forces Gravity Electromagnetism Graviton (never observed; must be massless) Photon (massless) Image: European Southern Obs. Image: Wikimedia Strong interaction Weak interaction Gluon (massless) W ± and Z bosons (not massless!) + Image: Wikimedia p + p ! D + e + νe 11 Each force is carried by a \force carrier" Gravity Electromagnetism Graviton (never observed; must be massless) Photon (massless) Image: European Southern Obs. Image: Wikimedia Strong interaction Weak interaction Gluon (massless) W ± and Z bosons (NOT massless!) + Image: Wikimedia p + p ! D + e + νe 12 Electromagnetism Weak interaction \Field lines" start out like in \Field lines" spread out in space electromagnetism but quickly but continue unbroken forever. die away. drawings: Sean Carroll Weak force is \screened": range 1=r2 force law only ∼ 1=100 size of a proton! (same for gravity) (1=r2) × e−r=r0 force law Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 13 What screens the weak force? There must be a new field that: - fills all space - carries \weak charge" (meaning it feels the weak force) (this is going to be the Higgs field, but first one more problem...) Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 14 We have another problem... Quarks and leptons also have a mass problem. It starts with the spin of fermions, and what we know from experiment about how the weak interaction \talks" to them. Quarks and leptons are fermions: spin 1/2. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 15 For fast-moving particles, it's convenient to quantize spin along the direction of motion: these are called helicity states. If a particle is moving slower than the speed of light, you can \transform" a right-handed particle into a left-handed particle (from your point of view) by running faster than it: If a particle has no mass, it moves at the speed of light: you can't run faster than that, so the two helicity states are physically distinct. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 16 The problem: Weak interactions treat left-handed and right- handed particles differently! - W ± bosons couple only to left-handed fermions. - Z bosons couple with different strengths to left- and right- handed fermions. This is called parity violation: Discovered in the weak interac- tions in 1957, experiment by C.-S. Wu. Image: Smithsonian Institution This would be like the charge of the electron being different de- pending on which reference frame you look at it from|impossible, since electric charge is conserved! Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 17 The only sensible solution: all particles are massless. Then they travel at the speed of light, and left- and right-handed states are physically distinct. That is obviously wrong. The real solution: Fill the vacuum with a \sea" of weak-charged stuff. - Quarks and leptons can pick up the extra weak charge they need to flip helicity. - Weak-interaction force carriers interact with the \sea", which gives them an effective mass. This \sea" is the Higgs field | like a magnetic field but without a direction. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 18 A quasi-political explanation of the Higgs boson by David Miller (UC London), for the UK Science Minister, 1993: (cartoons: CERN) Imagine a cocktail party of political workers... These represent the Higgs field filling space. 19 A Prime Minister enters and crosses the room. Political workers cluster around her, impeding her progress. The Higgs field interacts with a particle, giving it a mass. 20 Now imagine that a rumour enters the room... A particle collision \kicks" the Higgs field, creating a vibration... 21 The rumour generates a cluster of people, which propagates across the room. The Higgs boson is a wave or vibration in the Higgs field. 22 But the universe defaults to the lowest-energy state. - A magnetic field carries energy: you need a source to produce one. So how can there be a Higgs field filling the entire universe? The solution: - The Higgs field interacts with itself. - Minimum-energy configuration has nonzero Higgs field! - It sucks itself into existence! Potential energy function: V = −aH2 + bH4 Minimum energy configuration is at a nonzero field strength. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 23 But the universe defaults to the lowest-energy state. - A magnetic field carries energy: you need a source to produce one. So how can there be a Higgs field filling the entire universe? The solution: - The Higgs field interacts with itself. - Minimum-energy configuration has nonzero Higgs field! - It sucks itself into existence! Potential energy function: V = −aH2 + bH4 Minimum energy configuration is at a nonzero field strength. Called the \Mexican hat potential" :) Image: U.S. National Park Service Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 24 A crazy idea... how do we test it? We need to \kick" the Higgs field and create a vibration. Then we can measure the properties of the vibration. We can kick up a vibration|a Higgs particle|by colliding ordi- nary particles. Kicking up a Higgs vibration requires a minimum \quantum" of energy, related to the Higgs particle mass: E = mc2. Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 25 Producing a Higgs boson To \kick" the Higgs field, you need a \foot" that interacts with the Higgs... ...one with lots of Higgs-field ?star power?. That means we need to collide very massive particles! Quarks Leptons Generation 1 ! dueν e Generation 2 ! scµν µ Generation 3 ! btτν τ electric charge: −1=3 +2=3 −1 0 But ordinary matter is made out of the lowest-mass quarks and leptons. proton= uud neutron= udd Higgs production is very rare! :( electron= e− Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 26 Producing a Higgs particle in collisions of protons: We actually kick the Higgs with a virtual top quark, W ± or Z. pp → H (NNLO+NNLL QCD + NLO EW) • Gluon fusion, gg ! H s= 7 TeV 10 LHC HIGGS XS WG 2010 H+X) [pb] ¢¡¤£ pp → → Weak boson fusion, ¥ qqH (NNLO QCD + NLO EW) • qq ! Hqq ¢¡¤£ pp 1 pp → → WH (NNLO QCD + NLO EW) ZH (NNLO QCD +NLO EW) (pp σ pp → ttH (NLO QCD) -1 • WH, ZH associated production 10 -2 • ttH associated production ¡ 10 100 200 300 400 500 1000 MH [GeV] Heather Logan Why the Higgs boson matters NSCI 1000 Fall 2013 27 Detecting a Higgs particle: The HiggsSheet1 boson quickly breaks down (\decays") into the things it interacts with most strongly.
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