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Lighting up the Glue in the Proton

CCSU Physics Club Lecture Series, Apr. 5, 2010. Lighting up the Glue in the Proton. Prof. Richard Jones, UConn. Physics: uncovering hidden structure. at large scales the solar system the galaxy the visible universe the “dark” universe ?. Physics: uncovering hidden structure.

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Lighting up the Glue in the Proton

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  1. CCSU Physics Club Lecture Series, Apr. 5, 2010 Lighting up the Glue in the Proton Prof. Richard Jones, UConn

  2. Physics: uncovering hidden structure • at large scales • the solar system • the galaxy • the visible universe • the “dark” universe • ? CCSU Physics Club Lecture, April 5, 2010

  3. Physics: uncovering hidden structure • at small scales • bulk matter • atoms • electrons + nuclei • neutrons + protons • quarks Elementary particles: electrons quarks neutron proton CCSU Physics Club Lecture, April 5, 2010

  4. Nuclear Physics: uncovering hidden structure • Hideki Yukawa proposed a theory of the nuclear force (1935) • mediated by exchanges of particles called pmesons • mass of p meson about 250 times that of the electron • p meson discovered (12 years after it was predicted!) (Lattes, Muirhead, Occhialini, Powell, 1947) time CCSU Physics Club Lecture, April 5, 2010

  5. Nuclear Physics: uncovering hidden structure • Experiments soon revealed many more new particles involved in strong interactions! • protons and neutrons lightest particles in a large spectrum of baryons • pions lightest member of equally numerous sequence of mesons many more… CCSU Physics Club Lecture, April 5, 2010

  6. Nuclear Physics: uncovering hidden structure • pattern suggests substructure • Murray Gell-Mann  quarks • George Zweig  aces • quarks: • fractional electric charge! • spin 1/2 • come in flavors (up, down, strange) • baryons = three quarks • mesons = quark-antiquark pair Gell-Mann Zweig +2/3e –1/3e CCSU Physics Club Lecture, April 5, 2010

  7. Nuclear Physics: uncovering hidden structure • experiments at Stanford Linear Accelerator Center (Friedman, Kendall and Taylor, 1968) • like the Rutherford experiment • scattered electrons off protons • looked at large momentum transfers • found point-like charges inside proton • new charges initially called partons, but • fractional charges confirmed • scattering consistent with massless quarks CCSU Physics Club Lecture, April 5, 2010

  8. Nuclear Physics: uncovering hidden structure • discovery of J/Y mesonin November 1974 (BNL, SLAC) • interpreted as bound state of new flavor of quark called charm • predicted as weak partner of strange quarks • discovery of U meson in August, 1977 (Fermilab) • interpreted as bound state of new flavor called bottom • new partner predicted at higher mass, to be called top • ultra-heavy quark finally observed in 1995 (Fermilab) • weak interaction comparable with strong at 180 GeV/c2! • no more quarks expected below mass scale ~1 TeV/c2 CCSU Physics Club Lecture, April 5, 2010

  9. Nuclear Physics: uncovering hidden structure … and yet, • not a single solitary quark was ever seen in a detector • heavy quarks decay to light quarks via weak interactions • light quarks “cloak” themselves with anti-quarks to form mesons • mesons are seen in detectors • What kind of theory might explain this? confinement CCSU Physics Club Lecture, April 5, 2010

  10. Nuclear Physics: uncovering hidden structure • Quantum Chromodynamics (QCD) (Gross, Politzer, Wilczek, 1974) • modeled after QED • incorporates quarks with an arbitrary number of flavors • in place of the EM field (photon), proposes a new force field that binds to quarks: glue (gluon) + time gluon-gluon interactions! quark-gluon interactions CCSU Physics Club Lecture, April 5, 2010

  11. consider the hydrogen atom where a=1/137, weak coupling Þno confinement atom can be ionized with energy E0 isolated electrons exist as physical states Nuclear Physics: uncovering hidden structure V r n=2 n=1 CCSU Physics Club Lecture, April 5, 2010

  12. Explore: Confinement in atomic physics • Note the energy scale: • What happens if a ~ 1 or greater? • <T> grows to the same size as mass-energy mc2 • <U> is of same order as mc2 • special relativity changes things • How might we study these effects? • consider Z > 1 • for Z = 140, a = 1.02 CCSU Physics Club Lecture, April 5, 2010

  13. Explore: Confinement in atomic physics • Warning! • relativistic corrections to the Hamiltonian shift the g.s. energy E1 from this simple extrapolation of E0 • the Dirac equation must be solved • Qualitative results • something new happens when E1 > 2mc2 • the bare nucleus spontaneously grows an electron in its g.s. • a positron (anti-electron) simultaneously flies off • process continues until ionization energy of atom < 2mc2 • The Z=180 nucleus is confined to the neighborhood of its electrons – i.e. physical states must have Q < 180 ! CCSU Physics Club Lecture, April 5, 2010

  14. Explore: Confinement in atomic physics • Can this effect be observed in experiment? • nuclei with Z >100 are increasingly unstable and radioactive • compound nuclei can be created in A+A collisions with a lifetime of order 10-21 s • lifetime is too short to do atomic spectroscopy • Experiment with heavy ion collider was performed at G.S.I. in Darmstadt, Germany • positron emission rate was monitored vs. Z of beams • some excess yield was seen for Z > 160 • Is there some other system for which a ~ 1 for which real spectroscopy is possible? quarks! CCSU Physics Club Lecture, April 5, 2010

  15. Explore: Confinement in nuclear physics • this atomic physics analogy is imperfect • only one of the two charges is large • for true a ~ 1 BOTH charges must grow • new things happen • when B.E. > 2mc2 • new matter-antimatter pairs spontaneously created • vacuum is unstable! • a new phase is formed to replace the ordinary vacuum • “empty space” becomes full of particles • the Dirac equation is of little use • field theory is the only approach CCSU Physics Club Lecture, April 5, 2010

  16. The underlying theories are formally almost identical – The Gauge Principle Nuclear Physics: uncovering hidden structure Summary Comparison QEDQCD 1 kind of charge (q) 3 kinds of charge (r,g,b) force mediated by photons force mediated by gluons photons are neutral gluons are charged (eg. rg, bb, gb) a is nearly constant as strongly depends on distance confinement reinterpreted CCSU Physics Club Lecture, April 5, 2010

  17. Explore: the static quark potential • V(r<<r0) ~ 1/r • 1-gluon exchange • asymptotic freedom • V(r>>r0) ~ r • like electrodynamics in 1d • confinement CCSU Physics Club Lecture, April 5, 2010

  18. Explore: Lattice Gauge Field Theory quarks • hypercubic space-time lattice • quarks reside on sites, gluons reside on links between sites • lattice excludes short wavelengths from theory (regulator) • regulator removed using standard renormalization • systematic errors • discretization • finite volume gluons CCSU Physics Club Lecture, April 5, 2010

  19. Explore: how well does LGFT do? • best test is with heavy quarkonium (quenched approx.) • as ~ 0.2 • reveals static Vqq(r) • contains effects of strong coupling at large distances • shows confinement! • good agreement with experimental spectrum CCSU Physics Club Lecture, April 5, 2010

  20. LQCD: what is a meson? • Intuitive picture within Born-Oppenheimer approximation • quarks are massive – slow degrees of freedom • gluons aremassless – generate effective potential • Glue can be excited ground-state flux-tube m=0 excited flux-tube m=1 a new kind of meson (Hybrid) is predicted CCSU Physics Club Lecture, April 5, 2010

  21. Can we actually “see” elementary particles? • not exactly… CCSU Physics Club Lecture, April 5, 2010

  22. Thomas Jefferson National Accelerator • racetrack accelerator • accelerates electrons to 6 GeV • upgrading to 12 GeV • experiments CCSU Physics Club Lecture, April 5, 2010

  23. Explore: Why are quarks difficult to see? • How were electrons first seen? • make a glass vacuum tube • apply force using electric potential of several kV • electrons rip free from atoms • electric current flows The world’s first particle accelerator CCSU Physics Club Lecture, April 5, 2010

  24. Explore: What makes seeing quarks interesting? • J.J Thompson: "Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?" • Can we try it again with quarks? • a vastly smaller scale means a much smaller wavelength is needed CCSU Physics Club Lecture, April 5, 2010

  25. Explore: What makes seeing quarks interesting? • 99% of the mass of ordinary matter is locked up in particles composed of quarks • mass is energy – E = mc2 • all of this energy is stored safely in the nuclei of the ordinary elements • can additional energy be pumped in and converted to more mass? new exotic particles CCSU Physics Club Lecture, April 5, 2010

  26. Explore: Two quarks on a string • So what happens when you pull on a quark inside a proton? • N. Isgur, 1988: What happens if you stretch the string, and then pluck it? • the quark begins to move • a glue string forms • the string stretches • the quark slows down • the quark snaps back – denied! theoretical simulation courtesy of D. Leinweber CCSU Physics Club Lecture, April 5, 2010

  27. The GlueX experiment CCSU Physics Club Lecture, April 5, 2010

  28. Explore: The GlueX experiment • Time line for experiment • 1997 – first meeting • 1999 – initial proposal • 2002 – mature proposal • 2003 – project adopted by DOE • 2006 – mature design • 2009 – construction started • 2014 – commissioning • 2015 – first results! UConn responsibility CCSU Physics Club Lecture, April 5, 2010

  29. The GlueX experiment Live webcam feed of Hall D site Ground breaking in April 2009 Current plans call for the first beam in HallD/GlueX in late 2014. CCSU Physics Club Lecture, April 5, 2010

  30. The GlueX experiment CCSU Physics Club Lecture, April 5, 2010

  31. Prototyping detectors CCSU Physics Club Lecture, April 5, 2010

  32. The Competition • China – Beijing Electron-Positron Collider • BES experiment • Europe – FAIR Antiproton Accelerator • PANDA experiment • Japan – JPARC Proton Accelerator • several multi-GeV beam lines • proposals in preparation CCSU Physics Club Lecture, April 5, 2010

  33. Summary • $30M funding is approved for GlueX by DOE • Construction is now underway • Excellent opportunities for graduate studies • Exciting discoveries await! CCSU Physics Club Lecture, April 5, 2010

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