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Quantum Chromodynamics Quantum Chromodynamics Caterina Doglioni - Lund University Note on references: all the images from experiments are taken from the experiment websites unless otherwise noted + diagrams and tables taken from Cahn&Goldhaber / Halzen & Martin Introduction About me Master’s thesis in Rome (Italy) Summer Student at CERN (Switzerland) PhD in Oxford (UK) Post-doc in Geneva (Switzerland) Lecturer in Lund since April 2015 Member of the ATLAS Collaboration Dark Matter, jet and HASCO enthusiast This week: found either here or work-lurking on a desk outside Always found at [email protected] Wikipedia/NASA What I do: http://www.hep.lu.se/staff/doglioni/ @catdoglund 2 Introduction About Quantum ChromoDynamics Large Hadron Collider: Quark and gluon (→ jet) factory Force describing interactions of quarks and gluons: QCD Measure QCD processes: Precision measurements of the Standard Model Search for deviations from QCD: look for new particles decaying intoWikipedia/NASA quarks and gluons 3 Introduction A warning: not all of QCD is in these lectures Goal: brief introduction to basic concepts of QCD in chunks of ~ 25’ each, 5’ Q&A / activity after each chunk, then 10’ break More detailed references and slides can be found as links Wikipedia/NASA on the slides or at the end of the lectures 4 Introduction Outline of the QCD lectures •Lecture 1: Quarks gluons and strong interactions •1.1 From the hadron zoo to the quark model •1.2 Experimental proof of the quark model •1.3 Strong force: confinement and asymptotic freedom •Lecture 2 (Tuesday): Experimental aspects of QCD •How to see quarks and gluons: jets •Jet substructure •Measurements of QCD at the LHC Wikipedia/NASA•Looking for bumps above QCD 5 1.1 From the hadron zoo to the quark model Quark models and spectroscopy Evidence of quarks The strong force The most elementary particles (so far) (at least we hope) today/tomorrow Wikipedia/NASA http://arxiv.org/abs/1311.1769 7 Quark models and spectroscopy Evidence of quarks The strong force Particle situation as of the 1960s Particle Adventure website Wikipedia/NASA Is there an order in this apparent chaos? This is how the Standard Model was born! 8 Quark models and spectroscopy Evidence of quarks The strong force A necessity: better microscopes Cosmotron - BNL - 1952-1966 Bevatron - LBNL - 1954-1993 First accelerator beyond the GeV scale Billions of electron volts (6.5 GeV/beam) https://www.bnl.gov/about/history/accelerators.php interactions.org Experimental study of the spectroscopy of zoo of composite particles! 9 Quark models and spectroscopy Evidence of quarks The strong force What is a symmetry? Symmetry Nature likes symmetry! (our eye does too) Conservation law Quantum number https://it.wikipedia.org/wiki/Emmy_Noether 10 Quark models and spectroscopy Evidence of quarks The strong force An approximate symmetry: isospin (1930s) proton neutron mass = 938.272 MeV mass = 939.565 MeV Same properties, same reactions to strong force slightly different mass, different electric charge 1/2 Isospin multiplet: = I3= -1/2 ( ) ( ) 11 Quark models and spectroscopy Evidence of quarks The strong force Gell-Mann Nishijima relationship baryon number quark quantum numbers 12 Quark models and spectroscopy Evidence of quarks The strong force What does it mean for p,n contents? 13 Quark models and spectroscopy Evidence of quarks The strong force Thinking ahead: quarks Note that in the 1960s quarks had not been discovered! Antiquark: opposite quantum numbers 14 Quark models and spectroscopy Evidence of quarks The strong force Let’s try to classify these mesons 0 + 0 0 + ⇡ ,⇡ ,⇡−,K , K¯ ,K ,K− Antiquark: opposite quantum numbers 15 Quark models and spectroscopy Evidence of quarks The strong force Some order: meson nonets spin-parity 0−: pseudoscalar meson nonet spin-parity 1−: vector meson nonet. 16 Quark models and spectroscopy Evidence of quarks The strong force An analogy… 17 Quark models and spectroscopy Evidence of quarks The strong force More order: baryons spin-parity 1+: octet of light baryons spin-parity 3+: baryon decuplet 18 QuarkFeynman models diagrams and spectroscopyTracking detectorsEvidence Muon of detectorsquarks CalorimetersThe strong forceEvents Magnets! Wikipedia/NASA 19 Quark models and spectroscopy Evidence of quarks The strong force Baryon spectroscopy: a summary tableFor further explanation, see pp 1-4 of: http://www.curtismeyer.com/material/lecture.pdf Charge conjugation HistoryWikipedia/NASA of Principal Quantum Number: see http://ptp.oxfordjournals.org/ content/32/5/861.full.pdf+html 20 Quark models and spectroscopy Evidence of quarks The strong force Resonances in CMS and LHCb Wikipedia/NASA 21 Quark models and spectroscopy Evidence of quarks The strong force Resonances in CMS and LHCb Wikipedia/NASA 22 Quark models and spectroscopy Evidence of quarks The strong force What about the Δ and Pauli’s principle? Quarks: udd Parity: + Charge: 2 Spin: 3/2 -> spins are aligned Isospin: Q+Y/2 = 3/2 Completely symmetric wavefunction! But: three fermions should not all have the same state, according to Pauli’s principle! 23 Quark models and spectroscopy Evidence of quarks The strong force Everyone needs more color Solution to the Δ puzzle: add another degree of freedom that can be anti-symmetric color is the charge of quantum chromodynamics Hyperphysics Confinement ‘rule’: Baryons and mesons are colourless 24 Quark models and spectroscopy Evidence of quarks The strong force Evidence of color 25 Quark models and spectroscopy Evidence of quarks The strong force Octets and quarks “Three quarks for muster Mark” (J. Joyce, Finnegans Wake) Gell-Mann / Zweig: • hadrons are built by three constituent quarks, u d and s • mesons are composed by quark/antiquark pairs Wikipedia/NASA 26 Quark models and spectroscopy Evidence of quarks The strong force The “eightfold way” comes true http://www.quantumdiaries.org/2011/01/29/mysteries-of-mesons-part-1-the-eightfold-way/ 27 Quark models and spectroscopy Evidence of quarks The strong force Fast-forward: a pentaquark! http://arxiv.org/pdf/1507.03414v1.pdf 28 Quark models and spectroscopy Evidence of quarks The strong force Not a tetraquark? https://indico.cern.ch/event/509820/ news.fnal.gov 29 Conclusions Concepts for part 1.1 •Part 1.1: Quarks gluons and strong interactions •From the hadron zoo to the quark model • Protons and neutrons: isospin multiplet •Characterizing hadrons: the eightfold way, color • Classification by isospin / hypercharge • Where there’s structure there is substructure: baryons and mesons contain quarks • Color is theorised and experimentally verified Wikipedia/NASA 30 1.1.A A quick quiz about quarks https://www.mentimeter.com/s/2e80ff5e957166b493780b5a9959e7c5/e87a95d56892 1.1.B Q&A / 10’ Break 1.2 The discovery of quarks and gluons Quark models and spectroscopy Evidence of quarks The strong force The smallest constituents of matter (so far) Wikipedia/NASA 35 Quark models and spectroscopy Evidence of quarks The strong force Deep inelastic scattering proton Increase of probe energy Wikipedia/NASA 36 Quark models and spectroscopy Evidence of quarks The strong force Elastic scattering Remember the Rutherford experiment How would you modify this for electrons to quarks (elastic scattering)? Wikipedia/NASA 37 Quark models and spectroscopy Evidence of quarks The strong force Elastic scattering Remember the Rutherford experiment How would you modify this for electrons to quarks (elastic scattering)? Wikipedia/NASA 38 Quark models and spectroscopy Evidence of quarks The strong force Elastic scattering (for electron) Looking at entire solid angle: Wikipedia/NASA 39 Quark models and spectroscopy Evidence of quarks The strong force More elastic scattering & form factors Point charge Diffuse charge Form factor Form factor Can measure the charge radius of the proton Hofstadter & McAllister, 1960 Wikipedia/NASA assume fixed charge distribution 40 Quark models and spectroscopy Evidence of quarks The strong force More elastic scattering & form factors Point charge photon exchange Diffuse charge Form factor Adding magnetic moment as well: two form factors G1 and G2 Wikipedia/NASA 41 Quark models and spectroscopy Evidence of quarks The strong force The proton is not point-like! If the proton had been point-like, the form factors would have been=1 Wikipedia/NASA 42 Quark models and spectroscopy Evidence of quarks The strong force Elastic vs inelastic scattering Elastic Inelastic Dominates at high energies 43 Quark models and spectroscopy Evidence of quarks The strong force Inelastic scattering Cross-section for inelastic scattering W1 and W2 are form factors depending on: - the momentum transfer Q2=-q2 - the energy lost by the electron (lab) Bjorken (1967): If the proton is made by point-like constituents, 2 then W2 does not fall with Q2 but scales with ω = 2Mν/Q 44 Quark models and spectroscopy Evidence of quarks The strong force Inelastic scattering Cross-section for inelastic scattering W1 and W2 are form factors depending on: - the momentum transfer Q2=-q2 - the energy lost by the electron (lab) Bjorken (1967): If the proton is made by point-like constituents, 2 then W2 does not fall with Q2 but scales with ω = 2Mν/Q Feynman (1967): The proton is composed of point-like partons, each taking a fraction of the original proton momentum 45 Quark models and spectroscopy Evidence of quarks The strong force Inelastic scattering and structure functions Wikipedia/NASA Bjorken x = Feynman’s ‘fraction’ Proton structure functions Callan-Gross relations Scaling invariance at high Q^2 -> expect ~flat structure
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