Lecture 39, 40 Supplement: Particle physics in the LHC era

Transcription

1 Lecture 39, 40 Supplement: Particle physics in the LHC era

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3 The Matter Particles (Fermions) plus their antiparticles...

4 What is measured? quarks confined into hadrons A zoo of strongly interacting particles... Their periodic table : quark model of Gell-Mann, Zweig (1960 s)

5 The Forces and Force Carriers (Bosons)

6 The mysterious Higgs... Proposed spin 0, electrically neutral fundamental particle P. Higgs Its interactions are responsible for masses of all known particles...

7 The Standard Model 3 generations of matter (fermions) electroweak+strong force mediated by γ, W, Z, g Higgs boson (gives particles masses) required! Describes physics from early universe (BBN) to smallest laboratory scales! Glashow, Weinberg, Salam Nobel Prize 79

8 Problems (I) Naturalness: the mysterious Higgs sector... QM virtual corrections make the Higgs mass too large! tree loops 2 m h ~ (200 GeV) 2 top gauge higgs Aesthetics: complicated and arbitrary why do the forces have the observed features? why 3 families? what is the origin of the quark and lepton mass patterns?...

9 Cosmology: Problems (II) What makes up the mass of the universe? No dark matter candidate No understanding of dark energy No explanation of the matter-antimatter asymmetry No predictive quantized gravity

10 These issues suggest new physics at the TeV scale! can test and answer at the Large Hadron Collider!

11 The LHC Era 14 TeV pp collider with ATLAS and CMS general-purpose detectors Enormous event rates: over 10 7 W, Z, t t events per year Discovery reach to 5 6 TeV and precision physics

12 Popular guesses for new TeV physics... (extended) Higgs sectors? extra dimensions/strings? supersymmetry? Stringy resonance in at the LHC

13 Excavating the signal H SM!! Simulated 2! mass plot for 10 5 pb -1 m H =130 GeV in the lead tungstate calorimeter New physics yields new signals: but there are lots of backgrounds! Events / 500 MeV for 10 5 pb CMS, 10 5 pb -1 Higgs signal To understand them: must understand QCD, which binds quarks and gluons into hadrons m!! (GeV) D_D_1055c World experts at UW-Madison in collider phenomenology and QCD

14 The Standard Model and Beyond Why not just wait for the LHC to tell us what lies beyond the SM? Historically, successful models almost always guessed. Nearly impossible for data alone to reveal the principles which govern the theory which subsumes the Standard Model! Usual approach...exploit the power of symmetry.

15 The Fundamental Role of Symmetry in Physics Noether s theorem: Symmetries time translations spatial translations rotations Conservation laws energy linear momentum angular momentum Symmetries in particle physics: gauge symmetries (electroweak, strong interactions) global symmetries (baryon number, lepton number,...)

16 Hidden Symmetries (I) A toy example: xy = 4 x y symmetry Solution 1: x = 2, y = 2 (symmetry preserving) Solution II: x = 1, y = 4 (symmetry violating) Perturb: x x 0 + δx, y y 0 + δy Solution I1: 0 = δx + 4δy + δxδy not δx δy symmetric, but original symmetry is hidden! δx 4δy

17 Hidden Symmetries (II) This is what happens in the Standard Model! Higgs mechanism: V (φ) = m 2 φ 2 + λ φ 4 φ = 0 (symm preserving) φ 0 (symm violating) We say the symmetry is spontaneously broken. Other Nobel-worthy hidden symmetries in superconductivity theory, effective theory of mesons,...

18 Beyond the Standard Model: Supersymmetry (SUSY): proposed symmetry between bosons and fermions Q B = F Q F = B Ramond 71; Golfand & Likhtman 71; Wess, Zumino unique extension of special relativity the ultimate symmetry!

19 Supersymmetrizing the Standard Model SM fermions sfermions (spin 0) SM gauge bosons gauginos (spin 1/2) Higgs bosons higgsinos (spin 1/2) Must be a hidden symmetry! (otherwise particles and their partners degenerate in mass) The big questions: is it there? how does it hide?