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Quantum Chromodynamics : The Origin of Mass as We Know it

Quantum Chromodynamics : The Origin of Mass as We Know it

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Quantum Chromodynamics : The Origin of Mass as We Know it

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  1. Quantum Chromodynamics:The Origin of Mass as We Know it Craig D. RobertsPhysics DivisionArgonne National Laboratory & School of PhysicsPeking University Transition Region

  2. Argonne National Laboratory Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  3. Argonne National Laboratory Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Physics Division • ATLAS Tandem Linac: International User Facility for Low Energy Nuclear Physics • 37 PhD Scientific Staff • Annual Budget: $27million

  4. Length-Scales of Physics Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  5. Physics Division • Research sponsored primarily • by Department of Energy: • Office of Nuclear Physics • Nuclear • Hadron • Tests of Standard Model Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  6. Physics Division • Research sponsored primarily • by Department of Energy: • Office of Nuclear Physics • Nuclear • HADRON • Tests of Standard Model Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  7. Hadron Physics “Hadron physics is unique at the cutting edge of modern science because Nature has provided us with just one instance of a fundamental strongly-interacting theory; i.e., Quantum Chromodynamics (QCD). The community of science has never before confronted such a challenge as solving this theory.” Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  8. NSACLong Range Plan Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It “A central goal of (DOE Office of ) Nuclear Physics is to understand the structure and properties of protons and neutrons, and ultimately atomic nuclei, in terms of the quarks and gluons of QCD.”

  9. Quarks and Nuclear Physics • Standard Model of Particle Physics: • Six quark flavours Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  10. Quarks and Nuclear Physics • Standard Model of Particle Physics: • Six quark flavours • Real World • Normal matter – only two • light-quark flavours are active Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  11. Quarks and Nuclear Physics • Standard Model of Particle Physics: • Six quark flavours • Real World • Normal matter – only two • light-quark flavours are active • Or, perhaps, three Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  12. Quarks and Nuclear Physics • Standard Model of Particle Physics: • Six quark flavours • Real World • Normal matter – only two • light-quark flavours are active • Or, perhaps, three • For numerous good reasons, • much research also focuses on • accessible heavy-quarks • Nevertheless, I will focus on • the light-quarks; i.e., u & d. Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  13. What is QCD? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  14. What is QCD? • Relativistic Quantum Gauge Theory: • Interactions mediated by vector boson exchange • Vector bosons are perturbatively-massless Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  15. What is QCD? • Relativistic Quantum Gauge Theory: • Interactions mediated by vector boson exchange • Vector bosons are perturbatively-massless • Similar interaction in QED Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  16. What is QCD? • Relativistic Quantum Gauge Theory: • Interactions mediated by vector boson exchange • Vector bosons are perturbatively-massless • Similar interaction in QED • Special feature of QCD – gluon self-interactions, which completely change the character of the theory 3-gluon vertex 4-gluon vertex Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  17. QED cf. QCD? Running coupling Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  18. QED cf. QCD? Running coupling Add 3-gluon self-interaction Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  19. QED cf. QCD? gluon antiscreening fermion screening Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  20. QED cf. QCD? gluon antiscreening fermion screening Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • 2004 Nobel Prize in Physics : Gross, Politzer and Wilczek

  21. Simple picture- Proton Three quantum-mechanical constituent-quarks interacting via a potential, derived from one constituent-gluon exchange Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  22. Simple picture- Pion Two quantum-mechanical constituent-quarks - particle+antiparticle -interacting via a potential, derived from one constituent-gluon exchange Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  23. Modern Miraclesin Hadron Physics Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • proton = three constituent quarks • Mproton ≈ 1GeV • Therefore guess Mconstituent−quark ≈ ⅓ × GeV ≈ 350MeV

  24. Modern Miraclesin Hadron Physics Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • proton = three constituent quarks • Mproton ≈ 1GeV • Therefore guess Mconstituent−quark ≈ ⅓ × GeV ≈ 350MeV • pion = constituent quark + constituent antiquark • Guess Mpion ≈ ⅔ × Mproton≈ 700MeV

  25. Modern Miraclesin Hadron Physics Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • proton = three constituent quarks • Mproton ≈ 1GeV • Therefore guess Mconstituent−quark ≈ ⅓ × GeV ≈ 350MeV • pion = constituent quark + constituent antiquark • Guess Mpion ≈ ⅔ × Mproton≈ 700MeV • WRONG . . . . . . . . . . . . . . . . . . . . . . Mpion = 140MeV

  26. Modern Miraclesin Hadron Physics Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • proton = three constituent quarks • Mproton ≈ 1GeV • Therefore guess Mconstituent−quark ≈ ⅓ × GeV ≈ 350MeV • pion = constituent quark + constituent antiquark • Guess Mpion ≈ ⅔ × Mproton≈ 700MeV • WRONG . . . . . . . . . . . . . . . . . . . . . . Mpion = 140MeV • Rho-meson • Also constituent quark + constituent antiquark – just pion with spin of one constituent flipped • Mrho ≈ 770MeV ≈ 2 × Mconstituent−quark

  27. Modern Miraclesin Hadron Physics Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • proton = three constituent quarks • Mproton ≈ 1GeV • Therefore guess Mconstituent−quark ≈ ⅓ × GeV ≈ 350MeV • pion = constituent quark + constituent antiquark • Guess Mpion ≈ ⅔ × Mproton≈ 700MeV • WRONG . . . . . . . . . . . . . . . . . . . . . . Mpion = 140MeV • Rho-meson • Also constituent quark + constituent antiquark – just pion with spin of one constituent flipped • Mrho ≈ 770MeV ≈ 2 × Mconstituent−quark What is “wrong” with the pion?

  28. Dichotomy of the pion Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • How does one make an almost massless particle from two massive constituent-quarks? • Naturally, one could always tune a potential in quantum mechanics so that the ground-state is massless • However: current-algebra (1968) • This is impossible in quantum mechanics, for which one always finds:

  29. NSACLong Range Plan? • What is a constituent quark, a constituent-gluon? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  30. NSACLong Range Plan? • What is a constituent quark, a constituent-gluon? • Do they – can they – correspond to well-defined quasi-particle degrees-of-freedom? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  31. NSACLong Range Plan? • What is a constituent quark, a constituent-gluon? • Do they – can they – correspond to well-defined quasi-particle degrees-of-freedom? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • If not, with what should they be replaced?

  32. NSACLong Range Plan? • What is a constituent quark, a constituent-gluon? • Do they – can they – correspond to well-defined quasi-particle degrees-of-freedom? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • If not, with what should they be replaced? • What is the meaning of the NSAC Challenge?

  33. What is themeaning of all this? If mπ=mρ, then repulsive and attractive forces in the Nucleon-Nucleon potential have the SAME range and there is NO intermediate range attraction. Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  34. What is themeaning of all this? If mπ=mρ, then repulsive and attractive forces in the Nucleon-Nucleon potential have the SAME range and there is NO intermediate range attraction. Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It Under these circumstances: • Can 12C be stable? • Is the deuteron stable; can Big-Bang Nucleosynthesis occur? • Many more existential questions …

  35. What is themeaning of all this? If mπ=mρ, then repulsive and attractive forces in the Nucleon-Nucleon potential have the SAME range and there is NO intermediate range attraction. Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It Under these circumstances: • Can 12C be stable? • Is the deuteron stable; can Big-Bang Nucleosynthesis occur? (Many more existential questions …) Probably not … but it wouldn’t matter because we wouldn’t be around to worry about it.

  36. QCD’s Challenges • Quark and Gluon Confinement • No matter how hard one strikes the proton, • one cannot liberate an individual quark or gluon Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  37. QCD’s Challenges • Quark and Gluon Confinement • No matter how hard one strikes the proton, • one cannot liberate an individual quark or gluon Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Dynamical Chiral Symmetry Breaking Very unnatural pattern of bound state masses; e.g., Lagrangian (pQCD) quark mass is small but . . . no degeneracy between JP=+ and JP=− (parity partners)

  38. QCD’s Challenges • Quark and Gluon Confinement • No matter how hard one strikes the proton, • one cannot liberate an individual quark or gluon Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Dynamical Chiral Symmetry Breaking Very unnatural pattern of bound state masses; e.g., Lagrangian (pQCD) quark mass is small but . . . no degeneracy between JP=+ and JP=− (parity partners) • Neither of these phenomena is apparent in QCD’s LagrangianYetthey are the dominant determiningcharacteristics of real-world QCD.

  39. QCD’s ChallengesUnderstand emergent phenomena • Quark and Gluon Confinement • No matter how hard one strikes the proton, • one cannot liberate an individual quark or gluon Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Dynamical Chiral Symmetry Breaking Very unnatural pattern of bound state masses; e.g., Lagrangian (pQCD) quark mass is small but . . . no degeneracy between JP=+ and JP=− (parity partners) • Neither of these phenomena is apparent in QCD’s LagrangianYetthey are the dominant determiningcharacteristics of real-world QCD. • QCD – Complex behaviour arises from apparently simple rules.

  40. Why don’t we juststop talking & solve the problem? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Emergent phenomena can’t be studied using perturbation theory

  41. Why don’t we juststop talking & solve the problem? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Emergent phenomena can’t be studied using perturbation theory • So what? Same is true of bound-state problems in quantum mechanics!

  42. Why don’t we juststop talking & solve the problem? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Emergent phenomena can’t be studied using perturbation theory • So what? Same is true of bound-state problems in quantum mechanics! • Differences: • Here relativistic effects are crucial – virtual particles Quintessence of Relativistic Quantum Field Theory • Interaction between quarks – the Interquark Potential – Unknown throughout > 98% of the pion’s/proton’s volume!

  43. Why don’t we juststop talking & solve the problem? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Emergent phenomena can’t be studied using perturbation theory • So what? Same is true of bound-state problems in quantum mechanics! • Differences: • Here relativistic effects are crucial – virtual particles Quintessence of Relativistic Quantum Field Theory • Interaction between quarks – the Interquark Potential – Unknown throughout > 98% of the pion’s/proton’s volume! • Understanding requires ab initio nonperturbative solution of fully-fledged interacting relativistic quantum field theory

  44. Universal Truths Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Spectrum of hadrons (ground, excited and exotic states), and hadron elastic and transition form factors provide unique information about long-range interaction between light-quarks and distribution of hadron'scharacterising properties amongst its QCD constituents.

  45. Universal Truths Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Spectrum of hadrons (ground, excited and exotic states), and hadron elastic and transition form factors provide unique information about long-range interaction between light-quarks and distribution of hadron'scharacterising properties amongst its QCD constituents. • Dynamical Chiral Symmetry Breaking (DCSB) is most important mass generating mechanism for visible matter in the Universe. Higgs mechanism is (almost) irrelevant to light-quarks.

  46. Universal Truths Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Spectrum of hadrons (ground, excited and exotic states), and hadron elastic and transition form factors provide unique information about long-range interaction between light-quarks and distribution of hadron'scharacterising properties amongst its QCD constituents. • Dynamical Chiral Symmetry Breaking (DCSB) is most important mass generating mechanism for visible matter in the Universe. Higgs mechanism is (almost) irrelevant to light-quarks. • Running of quark mass entails that calculations at even modest Q2 require a Poincaré-covariant approach. Covariance requires existence of quark orbital angular momentum in hadron's rest-frame wave function.

  47. Universal Truths Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It • Spectrum of hadrons (ground, excited and exotic states), and hadron elastic and transition form factors provide unique information about long-range interaction between light-quarks and distribution of hadron'scharacterising properties amongst its QCD constituents. • Dynamical Chiral Symmetry Breaking (DCSB) is most important mass generating mechanism for visible matter in the Universe. Higgs mechanism is (almost) irrelevant to light-quarks. • Running of quark mass entails that calculations at even modest Q2 require a Poincaré-covariant approach. Covariance requires existence of quark orbital angular momentum in hadron's rest-frame wave function. • Confinement is expressed through a violent change of the propagators for coloured particles & can almost be read from a plot of a states’ dressed-propagator. It is intimately connected with DCSB.

  48. How can we tackle the SM’sStrongly-interacting piece? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It

  49. How can we tackle the SM’sStrongly-interacting piece? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It The Traditional Approach – Modelling

  50. How can we tackle the SM’sStrongly-interacting piece? Craig Roberts, Physics Division: QCD - Origin of Mass as We Know It The Traditional Approach – Modelling – has its problems.