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D-Branes and Topological Charge in Lattice QCD

D-Branes and Topological Charge in Lattice QCD. Hank Thacker University of Virginia. INT String/Gauge Workshop, Seattle, May, 2008. References: QCD Results: I. Horvath, et al. Phys.Rev. D68:114505(2003);Phys.Lett. B612: 21(2005);B617:49(2005).

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D-Branes and Topological Charge in Lattice QCD

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  1. D-Branes and Topological Charge in Lattice QCD Hank Thacker University of Virginia INT String/Gauge Workshop, Seattle, May, 2008 References: QCD Results: I. Horvath, et al. Phys.Rev. D68:114505(2003);Phys.Lett. B612: 21(2005);B617:49(2005). P.Keith-Hynes and HT, Phys. Rev. D75:085001 (2007). Ilgenfritz, et al. Nuc. Phys Proc. Suppl. (2006). MILC Collaboration, Nuc. Phys. Proc. Suppl. (2005). CPN-1 Results: J. Lenaghan, S. Ahmad, and HT Phys.Rev. D72:114511 (2005) Y. Lian and HT, Phys. Rev. D75:065031 (2007), P. Keith-Hynes and HT, arXiv:0804.1534 [hep-lat] .

  2. Topological Charge Studies in Lattice QCD: • A variety of lattice calculations have provided an increasingly clear picture of the structure and role of topological charge fluctuations in QCD (quenched chiral logs, eta-prime mass insertion, topological susceptibility, direct studies of local q(x) distributions in MC configurations). • The advent of exactly chiral lattice fermions (ala Ginsparg-Wilson) has provided a new and improved “fermionic” definition of topological charge on the lattice. This has revealed long range coherent structure (Horvath et al, Phys Lett. B, 2003) without the use of invasive “cooling” procedures. • The observed structure consists of extended, thin, codimension 1 surfaces of coherent (same sign) TC in a laminated, alternating sign “topological sandwich.” Analogous codimension 1 structures are observed in 2D CPN models. • The lattice topological charge sheets have a natural holographic interpretation as wrapped D6 branes of IIA string theory = domain walls between k-vacua with .

  3. Direct measurement of hairpin diagram (Bardeen, et al, Phys. Rev. D 2004) In large N chiral Lagrangian (Witten-Venziano) description, quenched hairpin propagator should look like: H(p) = = Results:

  4. Exactly chiral Dirac operators: A new method for studying topological charge on the lattice: Discovery of exactly chiral Ginsparg-Wilson fermions provided a new definition of topological charge on the lattice: -- Definition of q(x) in terms of overlap Dirac operator provides an effective tool for studying local topological charge structure without modifying (e.g cooling) the gauge field. (Horvath et al, PRD (2003) ) -- Overlap expression for q derived from structure of the chiral anomaly on the lattice. Chiral symmetry and non-ultralocality of D lead to a smoothing of short-range fluctuations allowing the possibility of observing long-range coherent structure.

  5. Results of first study of overlap q(x) distribution in 4D QCD (Horvath, et al, Phys. Rev. D (2003)): Extended coherent 3-dimensional sheets in 4-D space !! Results: -- Only small 4D coherent structures found with sizes of O(a) and integrated q(x)<<1. (No instantons.) -- Large coherent structures are observed which are locally 3-D sheets in 4-D space (surfaces of codimension 1), typically only ~1 or 2 lattice spacings thick in transverse direction. -- In each gauge configuration, two extended sheets of opposite charge are found, which are everywhere close to each other - - Possibly a single membrane with a dipole layer of topological charge Short range, negative TCh correlator (required by spectral arguments). (Note: <q(x)q(0)> correlator must be )

  6. 2D slice of Q(x) distribution for 4D QCD Note: Topological charge distributed more-or-less uniformly throughout membrane, not concentrated in localized lumps. (Horvath, et al, Phys.Lett. B(2005) )

  7. CP(N-1) models on the lattice(Lenaghan, Ahmad, and HT, PRD 2005) Here z = N-component scalar, and U = U(1) gauge field As in QCD, we study the topological charge distribution using q(x) constructed from overlap Dirac operator. To exhibit coherent structure, look for nearest-neighbor-connected structures. Plot largest simply connected structure. For best visualization plot 1 for sites on structure, 0 otherwise. To normalize expectations, first look for connected structures on random q(x) distributions, then compare with q(x) distributions in CPN-1 Monte Carlo configurations:

  8. Largest coherent structure from a random TC distribution:

  9. Another random TC distribution:

  10. Another CP(3) configuration:

  11. Still another CP(3) configuration:

  12. Plot sign(q(x)) for CP3 config: “Backbone” of coherent 1D regions is only 1 to 2 sites thick (~range of nonultralocality). Positive and negative regions everywhere close.

  13. Topological charge correlator for CP(3) meson corr length changes by factor of 5 over this range

  14. QCD

  15. large Nc instantons Theta dependence, phase transitions, and domain walls: Witten (1979): Large-Nc arguments require a phase transition (cusp) at Contradicts instanton expansion, which gives smooth - dependence. . = (free energy) Large Nc behavior conjectured from chiral Lagrangian arguments (Witten, 1979) Confirmed by AdS/CFT duality (Witten, 1998)

  16. Wilson loop = -vacuum Wilson loops in 2D and Wilson membranes (“bags”) in 4D: On an open 2D surface with boundary, a theta term is equivalent to a Wilson loop of charge around the boundary For CPN-1 can be obtained from area law for fractionally charged Wilson loops (P. Keith-Hynes and HT, arXiv:0804.1534 [hep-lat] .)

  17. To calculate theta-vacuum energy density, measure slope of linear potential: CP5for loop charge = 0.3 : V(R) R

  18. CP1 CP5 CP9 large N Instanton gas

  19. Digression on “melting” instantons(Y. Lian and HT, hep-lat/0607026): • CP1 and CP2 are dominated by small instantons with radii of order a (so correlator remains negative for nonzero separation in continuum limit). Small instantons are easily seen with overlap q(x). • CP3 is on the edge of the instanton melting point – has some instantons but mostly coherent line excitations. • CP4 and higher have no instantons – only line excitations. Crude estimate (lower bound) for instanton melting point = “tipping point” of integral over instanton size in semiclassical calculation (Luscher, 82): for CPN-1, Ncrit=2 for QCD, Ncrit=12/11 So SU(3) QCD should be well above instanton melting point, and topological charge should come in the form of membranes or domain walls. (Figuratively speaking, instantons become arbitrarily large and hollow like soap bubbles. No force between walls of bubble because vacuum inside and vacuum outside differ by .) (c.f. quantization of D6 brane RR charge ala Polchinski (Witten, 98)– discussed later)

  20. CPN-1 instantons from overlap topological charge: CP2, beta=1.8, Q = 1

  21. CP1, beta=1.6, Q = 1

  22. CP1, beta=1.6, Q = -2

  23. CP9, beta=0.9, Q = -1

  24. Plot integrated q(x) in highest structure (within 2 sites of highest peak) for all configs with Q = +-1 cp1 cp2 cp3 cp5 cp9 (End of digression on melting instantons.)

  25. Summary of Monte Carlo results: In both 4D QCD and 2D CPN-1 (for N>3), topological charge comes in the form of extended membranes of codimension 1, with opposite sign sheets (or lines in CPN-1) juxtaposed in dipole layers. (Exception -- CP1 and CP2 models are dominated by small instantons) The QCD vacuum is a “topological sandwich’’ of alternating sign membranes. This topological structure of the gauge fields presumably induces chiral symmetry breaking via surface near-zero modes with qL and qR attached to + and – topological charge membranes. (MILC collaboration: evidence from IPR of quark near-zero modes gives effective dimensionality of , indicating that modes are delocalized along 2+1 D membrane-like surfaces.)

  26. AdS/CFT Holography and QCD topological charge: • 4D QCD IIA String Theory in a black hole metric. Among other things,ADS/CFT confirms Witten’s (1979) large-Nc view of topological charge--Instantons “melt” and are replaced by (Witten, PRL 98): • Multiple vacuum states (“k-vacua”) with • Local k-vacua separated by domain wall = membrane • Domain wall = fundamental D6-brane of IIA string wrapped around S4 • = Wilson line around D=disk with BH singularity at center ~ Aharonov-Bohm phase around “Dirac string” • k=integer is a Dirac quantization of 6-brane RR charge • TC is dual to Ramond-Ramond charge in string theory. 5D Chern-Simons term  4D term

  27. Holographic view of domain wall between k-vacua: Dirac string monopole In Witten’s brane construction, 4D Yang-Mills is viewed from 6 dimensions = R4 x D, where D = S1 x radial coordinate of black-hole metric. Radius of S1 is an ultraviolet cutoff, analogous to lattice spacing. Analog in (1+1)-D is (3+1)-dimensional solid cylinder. (1+1)-D U(1) case is equivalent to Laughlin’s gedankenexperiment for topological understanding of integer quantum Hall effect. (Corbino disk) x-axis domain wall Longitudinal component of monopole field (B) is dual to topological charge (= longitudinal E field) in CP(N-1) model.

  28. What are coherent sheets of TC in QCD? Are they D-branes? The ADS/CFT holographic view of topological charge in the QCD vacuum has an analog in 2D U(1) theories: --Multiple discrete k-vacua characterized by an effective value of which differs from the in the action by integer multiples of . -- Interpretation of effective similar to Coleman’s discussion of 2D massive Schwinger model (Luscher (1978), Witten (1979,1998)), where background E field. In 2D U(1) models (CP(N-1) or Schwinger model): Domain walls between k-vacua are world lines of charged particles:

  29. Precise analogy between CPN-1 models in 2D and QCD in 4D (Luscher, 1978): • Identify Chern-Simons currents for the two theories. In both cases, CS current correlator has massless pole ~1/q2 This analogy suggests that the coherent 1D structures in CPN-1 are charged particle world lines, and the 3D coherent structures in QCD are Wilson bags=excitation of Chern-Simons tensor on a 3-surface.

  30. Quarks and D8-branes: -- In Witten’s construction of 4D YM, gluons are world volume gauge fields of Nc D4-branes -- Topological charge membranes are D6-D6bar brane pairs (= Luscher’s Wilson bags). -- Introduce quarks in the “probe” (~ quenched) approximation, via Nf D8-D8bar brane pairs. (Sakai & Sugimoto; Antonyin, Harvey, Jensen, Kutasov). Angular coordinates around black hole space-time disk S4 0 1 2 3 4 5 6 7 8 9 D4 branes x x x x x - - - - - D6 branes x x x - - - x x x x D8 branes x x x x - x x x x x the 5 dimensions transverse to D4 branes.

  31. -- In D4/D8 model, tachyonic mode of D8-D8bar string (and/or gravitational effects induced by D4 branes) leads to chiral symmetry breaking = joining together of D8 and D8bar into a single brane 4-5 disk • Interplay between topological charge and chiral symmetry breaking (work in progress with D. Vaman and E. Barnes): . • Tachyon effective field on D8-D8bar branes reduces to the NfxNf chiral field of QCD. Tachyon effective potential related to chiral Lagrangian. • D8 brane tachyon theory has solitonic vortex solutions which are D6 branes (A. Sen, 2004). The topological charge lamination of the QCD vacuum can be related to tachyonic instability of D8-D8bar pairs, thereby to SXSB.

  32. Conclusions: • The large Nc holographic view of topological charge structure in QCD is strongly supported by lattice results. Witten’s D6 branes (= Luscher’s Wilson bags) provide a plausible model for observed coherent sheets of topological charge. • Further detailed studies of relation between topological charge sheets and low Dirac eigenmodes should clarify their role in chiral symmetry breaking. Lattice results on spacetime structure of low eigenmodes suggest they are surface modes on the sheets. (A membrane version of the Dyakanov-Petrov instanton-driven chiral condensate, but fundamentally different because surface modes are delocalized along membranes, as compared to localized ‘tHooft zero modes in instanton model.)

  33. The emerging picture -- A “laminated” vacuum: • Alternating sign sheets (or lines) of topological charge: D6-D6bar strings + - + - + - + - • Possible dynamics of vacuum lamination in CPN-1: • Spectrum consists of nonsinglet and singlet z-zbar pairs. • Msinglet > Mnonsinglet due to annihilation diagrams: • singlet pairs pop out of vacuum, but they can propagate farther by forming nonsinglet pairs with members of neighboring singlet pairs: + - + - + - + - - + - + - + - + Two degenerate vacua with topological order (ala Wen and Zee in quantum hall eff.) or

  34. Note that singlet pairs must all polarize in the same direction to form nonsinglet mesons with nearest neighbors. A “dimerization” (lamination) of the vacuum. Similar to Peierls transition in one-dimensional chain of atoms . Like antiferromagnetic order, but not tied to even-odd sublattice (hence topological) This mechanism is also compatible with long range behavior of large N solution : Vacuum polarization tensor + In large N, singlet pairs are treated individually, but with massive z-propagators. spontaneous generation of constituent z-mass nonzero top. susc.

  35. Conformal field theory between the branes: CPN-1 in Lorentz gauge Thickness of coherent lines of q(x) is of order a in continuum. Consider an idealized brane vacuum configuration where q(x) is confined to one-dimensional subspaces (or zero-dimensional for small instantons). In voids between branes, so can be written as the real part of a holomorphic fcn of z=x1+ix2 -- has branch cuts at branes (and/or poles at small instantons) ? string coordinate for flux tube between charged particles (= Dbranes = string endpoints)

  36. CPN Topological Charge Correlators from CFT Static dilute brane approximation M2 >> q2 (in QCD, large ps glueball mass) • Effective theory with z’s integrated out: • OPE for Chern-Simons current correlator: Form of OPE coefficients completely determined up to 2 overall constants by CFT arguments: Result for TC correlator is sum of contact terms: Gives a good fit to all lattice correlators using With lattice spacings, essentially independent of beta.

  37. Fit correlator to

  38. SUSY Relics in QCD (Aharony, Shifman, and Veneziano (2002)): • (P. Keith-Hynes and HT, Phys. Rev. D, 2006) • Holographic projection of orientifold compactification of string theory predicts “planar (large N) equivalence” between SUSY YM and ordinary 1-flavor QCD. • A “SUSY relic” prediction that can be tested by Monte Carlo: Degeneracy between the scalar and pseudoscalar mesons in 1-flavor QCD (they belong to the same WZ multipltet in SUSY YM chiral Lagrangian (Veneziano, Yankielowicz)). • Prediction is tested using MC results for scalar and pseudoscalar valence and hairpin diagrams. (see poster). • D-branes of topological charge in ordinary QCD may be SUSY relics – related to domain walls between discrete chiral condensates in SUSY. • Role of vacuum Dbranes in breaking conformal symmetry points to a connection between chiral anomaly and conformal anomaly (which also belong to same WZ multiplet in SUSY YM).

  39. Mass2 (MeV)2 Mass2 (MeV)2 Pseudoscalar Pseudoscalar (+515+12-13 MeV)2 (+515+12-13 MeV)2 (+427+249-756 MeV)2 (+427+249-756 MeV)2 Scalar Scalar Figure 2: Scalar and Pseudoscalar Mass2 from valence diagrams and hairpin mass shifts. Figure 2: Scalar and Pseudoscalar Mass2 from valence diagrams and hairpin mass shifts. Test of scalar-pseudoscalar degeneracy in 1-flavor QCD: (with Patrick Keith-Hynes) (valence mass)2 (hairpin mass shift)2 (total mass)2 +[315(6)]2 +[407(11)]2 +[515(13)]2 +[1416(14)]2 - [1350(90)]2 +[427+249-756]2 Mass (GeV) 1.0 Pseudoscalar mass Scalar mass 0.5

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