The Physics of Hadrons: Standard Model, Quantum Chromodynamics, and Open Questions

1. The Physics of Hadrons Published collaborations in 2010/2011 Adnan BASHIR (U Michoacan); Stan BRODSKY (SLAC); Lei CHANG (ANL & PKU); Huan CHEN (BIHEP); Ian CLOËT (UW); Bruno EL-BENNICH (Sao Paulo); Xiomara GUTIERREZ-GUERRERO (U Michoacan); Roy HOLT (ANL); Mikhail IVANOV (Dubna); Yu-xin LIU (PKU); Trang NGUYEN (KSU); Si-xue QIN (PKU); Hannes ROBERTS (ANL, FZJ, UBerkeley); Robert SHROCK (Stony Brook); Peter TANDY (KSU); David WILSON (ANL) Students Early-career scientists Craig Roberts Physics Division www.phy.anl.gov/theory/staff/cdr.html

2. Standard Model of Particle Physics Craig Roberts: The Physics of Hadrons

3. Standard Model- History (a part) • With the advent of cosmic ray science and particle accelerators, numerous additional particles were discovered: • muon (1937), pion (1947), kaon (1947), Roper resonance (1963), … • By the mid-1960s, it was apparent that not all the particles could be fundamental. • A new paradigm was necessary. • Gell-Mann's and Zweig's constituent-quark theory (1964) was a critical step forward. • Gell-Mann, Nobel Prize 1969: "for his contributions and discoveries concerning the classification of elementary particles and their interactions". • Over the more than forty intervening years, the theory now called the Standard Model of Particle Physics has passed almost all tests. Craig Roberts: The Physics of Hadrons In the early 20th Century, the only matter particles known to exist were the proton, neutron, and electron.

4. Standard Model- The Heavy Piece • Politzer, Gross and Wilczek – 1973-1974 • Perturbative Quantum Chromodynamics – QCD • Nobel Prize (2004): • "for the discovery of asymptotic freedom in the theory of the strong interaction". • NB. • Worth noting that the character of 96% of the matter in the Universe is completely unknown Craig Roberts: The Physics of Hadrons • Strong interaction • Existence and composition of the vast bulk of visible matter in the Universe: • proton, neutron • the forces that form and bind them to form nuclei • responsible for more than 98% of the visible matter in the Universe

5. Simple picture- Proton Three quantum-mechanical constituent-quarks interacting via a potential, derived from one constituent-gluon exchange Craig Roberts: The Physics of Hadrons

6. Simple picture- Pion Two quantum-mechanical constituent-quarks - particle+antiparticle -interacting via a potential, derived from one constituent-gluon exchange Craig Roberts: The Physics of Hadrons

7. Top Open Questions in Physics Craig Roberts: The Physics of Hadrons

8. Excerpts from the top-10, or top-24, or … Saul Perlmutter, Brian P. Schmidt, Adam G. Riess, Nobel Prize 2011: for the discovery of the accelerating expansion of the Universe through observations of distant supernovae. Craig Roberts: The Physics of Hadrons • What is dark energy? • 1998: A group of scientists had recorded several dozen supernovae, including some so distant that their light had started to travel toward Earth when the universe was only a fraction of its present age. • Contrary to their expectation, the scientists found that the expansion of the universe is not slowing, but accelerating.

9. Excerpts from the top-10, or top-24, or … Craig Roberts: The Physics of Hadrons • Can we quantitatively understand quark and gluon confinement in quantum chromodynamics and the existence of a mass gap? • Quantum chromodynamics, or QCD, is the theory describing the strong nuclear force. • Carried by gluons, it binds quarks into particles like protons and neutrons. • Apparently, the tiny subparticles are permanently confined: one can't pull a quark or a gluon from a proton because the strong force gets stronger with distance and snaps them right back inside.

10. Quantum Chromodynamics Craig Roberts: The Physics of Hadrons

11. cf.Quantum Electrodynamics Craig Roberts: The Physics of Hadrons QED is the archetypal gauge field theory Perturbatively simple but nonperturbatively undefined Chracteristic feature: Light-by-light scattering; i.e., photon-photon interaction – leading-order contribution takes place at order α4. Extremely small probability because α4 ≈10-9 !

12. What is QCD? • Relativistic Quantum Gauge Field Theory: • Interactions mediated by vector boson exchange • Vector bosons are perturbatively-massless • Similar interaction in QED • Special feature of QCD – gluon self-interactions 3-gluon vertex 4-gluon vertex Craig Roberts: The Physics of Hadrons

13. What is QCD? 3-gluon vertex 4-gluon vertex Craig Roberts: The Physics of Hadrons • Novel feature of QCD • Tree-level interactions between gauge-bosons • O(αs) cross-section cf. O(αem4) in QED • One might guess that this is going to have a big impact • Elucidating part of that impact is the origin of the 2004 Nobel Prize to Politzer, and Gross & Wilczek

14. Running couplings Craig Roberts: The Physics of Hadrons • Quantum gauge-field theories are all typified by the feature that Nothing is Constant • Distribution of charge and mass, the number of particles, etc., indeed, all the things that quantum mechanics holds fixed, depend upon the wavelength of the tool being used to measure them • particle number is not conserved in quantum field theory • Couplings and masses are renormalised via processes involving virtual-particles. Such effects make these quantities depend on the energy scale at which one observes them

15. QED cf. QCD? 5 x10-5 Add 3-gluon self-interaction gluon antiscreening fermion screening Craig Roberts: The Physics of Hadrons • 2004 Nobel Prize in Physics : Gross, Politzer and Wilczek

16. What is QCD? 0.5 0.4 ↔ 0.3 αs(r) 0.2 0.1 0.002fm 0.02fm 0.2fm Craig Roberts: The Physics of Hadrons This momentum-dependent coupling translates into a coupling that depends strongly on separation. Namely, the interaction between quarks, between gluons, and between quarks and gluons grows rapidly with separation Coupling is hugeat separations r = 0.2fm ≈ ⅟₄ rproton

17. 0.5 Confinement in QCD 0.4 0.3 αs(r) 0.2 0.1 0.002fm 0.02fm 0.2fm • The Confinement Hypothesis: • Colour-charged particles cannot be isolated and therefore cannot be directly observed. They clump together in colour-neutral bound-states • This is hitherto an empirical fact. Craig Roberts: The Physics of Hadrons • A peculiar circumstance; viz., an interaction that becomes stronger as the participants try to separate • If coupling grows so strongly with separation, then • perhaps it is unbounded? • perhaps it would require an infinite amount of energy in order to extract a quark or gluon from the interior of a hadron?

18. Millennium prize of $1,000,000 for proving that SUc(3) gauge theory is mathematically well-defined, which will necessarily prove or disprove the confinement conjecture, but in the absence of dynamical quarks Confinement? Craig Roberts: The Physics of Hadrons

19. Strong-interaction: QCD • Nature’sonly example of truly nonperturbative, • fundamental theory • A-priori, no idea as to what such a theory • can produce Craig Roberts: The Physics of Hadrons • Asymptotically free • Perturbation theory is valid and accurate tool at large-Q2 • Hence chiral limit (massless theory) is defined • Essentiallynonperturbative for Q2 < 2 GeV2

20. Perhaps?! • What we know unambiguously … • Is that we know too little! The Problem with QCD What is the interaction throughout more than 98% of the proton’s volume? Craig Roberts: The Physics of Hadrons

21. The study of nonperturbative QCD is the puriew of … Hadron Physics Craig Roberts: The Physics of Hadrons

22. Hadron: Any of a class of subatomic particles that are composed of quarks and/or gluons and take part in the strong interaction.  Examples: proton, neutron, & pion. International Scientific Vocabulary: hadr- thick, heavy (from Greek hadros thick) + 2on First Known Use: 1962 Baryon: hadron with half-integer-spin Meson: hadron with integer-spin Hadrons Craig Roberts: The Physics of Hadrons

23. Nuclear Science Advisory Council 2007 – Long Range Plan • Internationally, this is an approximately $1-billion/year effort in experiment and theory, with approximately $375-million/year in the USA. • Roughly 90% of these funds are spent on experiment • $1-billion/year is the order of the operating budget of CERN Craig Roberts: The Physics of Hadrons “A central goal of (the 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.”

24. Facilities Craig Roberts: The Physics of Hadrons

25. FacilitiesQCD Machines A three dimensional view of the calculated particle paths resulting from collisions occurring within RHIC's STAR detector Craig Roberts: The Physics of Hadrons • USA • Thomas Jefferson National Accelerator Facility, Newport News, Virginia Nature of cold hadronic matter Upgrade underway Construction cost $310-million New generation experiments in 2016 • Relativistic Heavy Ion Collider, Brookhaven National Laboratory, Long Island, New York Strong phase transition, 10μs after Big Bang

26. proton pion The structure of matter Hadron Theory Craig Roberts: The Physics of Hadrons

27. Nature’s strong messenger – Pion Craig Roberts: The Physics of Hadrons • 1947 – Pion discovered by Cecil Frank Powell The beginning of Particle Physics • Then came • Disentanglement of confusion between (1937) muon and pion – similar masses • Discovery of particles with “strangeness” (e.g., kaon1947-1953) • Subsequently, a complete spectrum of mesons and baryons with mass below ≈1 GeV • 28 states • Became clear that pion is “too light” - hadrons supposed to be heavy, yet …

28. Simple picture- Pion • Gell-Mann and Ne’eman: • Eightfold way(1961) – a picture based • on group theory: SU(3) • Subsequently, quark model – • where the u-, d-, s-quarks • became the basis vectors in the • fundamental representation • of SU(3) • Pion = • Two quantum-mechanical constituent-quarks - particle+antiparticle - • interacting via a potential Craig Roberts: The Physics of Hadrons

29. Some of the Light Mesons IG(JPC) 140 MeV 780 MeV Craig Roberts: The Physics of Hadrons

30. Modern Miraclesin Hadron Physics Craig Roberts: The Physics of Hadrons • 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?

31. Dichotomy of the pion Craig Roberts: The Physics of Hadrons • 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 – but some are still making this mistake • However: current-algebra (1968) • This is impossible in quantum mechanics, for which one always finds:

32. Dichotomy of the pionGoldstone mode and bound-state HIGHLY NONTRIVIAL Impossible in quantum mechanics Only possible in asymptotically-free gauge theories Craig Roberts: The Physics of Hadrons • The correct understanding of pion observables; e.g. mass, decay constant and form factors, requires an approach to contain a • well-defined and validchiral limit; • and an accurate realisation of dynamical chiral symmetry breaking.

33. Chiral Symmetry Craig Roberts: The Physics of Hadrons • Interacting gauge theories, in which it makes sense to speak of massless fermions, have a nonperturbativechiral symmetry • It is realised in the theory’s spectrum via the appearance of degenerate parity partners • Perturbative QCD: u- & d- quarks are very light mu /md≈ 0.5 & md≈ 4MeV H. Leutwyler, 0911.1416 [hep-ph] • However, splitting between parity partners is greater-than 100-times this mass-scale; e.g.,

34. Dynamical Chiral Symmetry Breaking Craig D Roberts John D Roberts Craig Roberts: The Physics of Hadrons • Something is happening in QCD • some inherent dynamical effect is dramatically changing the pattern by which the Lagrangian’schiral symmetry is expressed • Qualitatively different from spontaneous symmetry breaking aka the Higgs mechanism • Nothing is added to the theory • Have only fermions & gauge-bosons Yet, the mass-operator generated by the theory produces a spectrum with no sign of chiral symmetry

35. QCD’s Challenges Understand emergent phenomena • Quark and Gluon Confinement • No matter how hard one strikes the proton, • one cannot liberate an individual quark or gluon Craig Roberts: The Physics of Hadrons • 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.

36. Hadron Physics Craig Roberts: The Physics of Hadrons

37. Nucleon … Two Key HadronsProton and Neutron Friedman, Kendall, Taylor, Nobel Prize (1990): "for their pioneering investigations concerning deep inelastic scattering of electrons on protons and bound neutrons, which have been of essential importance for the development of the quark model in particle physics" Craig Roberts: The Physics of Hadrons • Fermions – two static properties: proton electric charge = +1; and magnetic moment, μp • Magnetic Moment discovered by Otto Stern and collaborators in 1933; Stern awarded Nobel Prize (1943): "for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton". • Dirac (1928) – pointlikefermion: • Stern (1933) – • Big Hint that Proton is not a point particle • Proton has constituents • These are Quarks and Gluons • Quark discovery via e-p-scattering at SLAC in 1968 • the elementary quanta of QCD

38. Nucleon StructureProbed in scattering experiments Structurelessfermion, or simply structured fermion, F1=1 & F2=0, so that GE=GM and hence distribution of charge and magnetisation within this fermion are identical F1 = Dirac form factor F2 = Pauli form factor GM = Sachs Magntic form factor If a nonrelativistic limit exists, this relates to the magnetisation density GE = Sachs Electric form factor If a nonrelativistic limit exists, this relates to the charge density Craig Roberts: The Physics of Hadrons Electron is a good probe because it is structureless Electron’s relativistic current is Proton’s electromagnetic current

39. Which is correct? How is the difference to be explained? Craig Roberts: The Physics of Hadrons • Data before 1999 • Looks like the structure of the proton is simple • The properties of JLab (high luminosity) enabled a new technique to be employed. • First data released in 1999 and paint a VERY DIFFERENT PICTURE

40. We were all agog Craig Roberts: The Physics of Hadrons

41. Nuclear Science Advisory Council 2007 – Long Range Plan “A central goal of (the 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.” Craig Roberts: The Physics of Hadrons • So, what’re the holdups? They are legion … • Confinement • Dynamical chiral symmetry breaking • A fundamental theory of unprecedented complexity • QCD defines the difference between nuclear and particle physicists: • Nuclear physicists try to solve this theory • Particle physicists run away to a place where tree-level computations are all that’re necessary – perturbation theory, the last refuge of a scoundrel

42. Understanding NSAC’sLong Range Plan • What are the quarks and gluons of QCD? • Is there such a thing as a constituent quark, a constituent-gluon? • After all, these are the concepts for which Gell-Mann won the Nobel Prize. Craig Roberts: The Physics of Hadrons • Do they – can they – correspond to well-defined quasi-particle degrees-of-freedom? • If not, with what should they be replaced? What is the meaning of the NSAC Challenge?

43. What is themeaning of all this? Suppose QCD behaved reasonably →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: The Physics of Hadrons Under these circumstances: • Can 12C be produced, can it 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.

44. Why don’t we just stop talking and solve the problem? Craig Roberts: The Physics of Hadrons

45. Just get on with it! Craig Roberts: The Physics of Hadrons • But … QCD’s 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, something which Mathematics and Theoretical Physics are a long way from achieving.

46. How can we tackle the SM’sStrongly-interacting piece? Craig Roberts: The Physics of Hadrons The Traditional Approach – Modelling – has its problems.

47. How can we tackle the SM’sStrongly-interacting piece? – Spacetime becomes an hypercubic lattice – Computational challenge, many millions of degrees of freedom Craig Roberts: The Physics of Hadrons Lattice-QCD

48. How can we tackle the SM’sStrongly-interacting piece? – Spacetime becomes an hypercubic lattice – Computational challenge, many millions of degrees of freedom – Approximately 500 people worldwide & 20-30 people per collaboration. Craig Roberts: The Physics of Hadrons Lattice-QCD –

49. A Compromise?Dyson-Schwinger Equations Craig Roberts: The Physics of Hadrons

50. A Compromise?Dyson-Schwinger Equations Craig Roberts: The Physics of Hadrons • 1994 . . . “As computer technology continues to improve, lattice gauge theory [LGT] will become an increasingly useful means of studying hadronic physics through investigations of discretised quantum chromodynamics [QCD]. . . . .”