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Nf=12 QCD の 非摂動的 running coupling

Nf=12 QCD の 非摂動的 running coupling. 伊藤　悦子 ( 工学院大学 ) arXiv:0910.4196 and Work in progress. A01 meeting @ KEK 2010/2/15. Erek Bilgici a Antonino Flachi b Masafumi Kurachi c C. –J. David Lin d Hideo Matsufuru e Hiroshi Ohki b, f Tetsuya Onogi g Eigo Shintani g

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Nf=12 QCD の 非摂動的 running coupling

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  1. Nf=12 QCD の非摂動的running coupling 伊藤　悦子 (工学院大学) arXiv:0910.4196 and Work in progress A01 meeting @ KEK 2010/2/15

  2. Erek Bilgici a Antonino Flachi b Masafumi Kurachi c C. –J. David Lin d Hideo Matsufuru e Hiroshi Ohki b, f Tetsuya Onogi g Eigo Shintani g Takeshi Yamazaki h Collaborators Numerical simulation was carried out on the vector supercomputer NEC SX-8 in YITP, Kyoto University and RCNP, Osaka University SR and BlueGene in KEK a : University of Graz b : YITP, Kyoto University c : Tohoku University d : National Chio-Tung University, and National Center for Theoretical Science e : KEK f : Department of Physics, Kyoto University g: Osaka University h: Tsukuba University

  3. Plan to talk • Introduction • Twisted Polyakov loop scheme • Simulation (quenched QCD) • Nf=12 case (TPL scheme) • Summary and Future directions

  4. Introduction

  5. large flavor SU(3)gauge theories: Two-loop perturbative beta function: The signs of C1 and C2 depend on Nf. Gardi and Karliner Nucl.Phys.B529:383-423,1998. Ncw~12 from resummation method of 3 terms of the beta fn Miransky and Yamawaki Phys.Rev.D55:5051-5066,1997 Ncw=11.9 based on the same idea of Gardi et al. Ryttov and Sannino Phys.Rev.D78:065001,2008 Ncw=8.25 from calclulation of the anomalous dimension

  6. Previous and Recent studies in lattice QCD with Nf=12 • Appelquist, Fleming and Neil: Phys.Rev.Lett.100:171607(2008), PRD79:076010 The running coupling constant in Schrodinger functional scheme. Nf=12 : a flat region in low energy scale. Nf=8 There is no edivence of fixed point. • Deuzeman, Lombordo and Pallante arXiv:0904.4662 [hep-ph] • there is a conformal phase... • Fodor, Holland, Kuti, Nogragi, and Schroeder arXiv:0809.4890, 0911:2463 [hep-lat] • Nf=12is below the conformal window. (stout-smeared staggered fermion) • Difficulty to study low energy behavior comes from a large discretization error .(Appelquist et al. neglect O(a/L) error.) • To conclude the existence ofIR fixed point in the case of Nf=12 case, • we have to precisely estimate O(a/L) discretization error.

  7. Nonperturbative renormalized coupling constant If , the above equation gives the nonperturbative coupling constant On the lattice study, we can calculate nonperturbatively the VEV on the right hand side. In SU(3) lattice gauge theory, the VEV of operator O depends on Lattice size (L0) Lattice spacing (a) bare coupling constant a Some renormalization schemes • Schrodinger functional (SF) scheme • Wilson loop scheme • Twisted Polyakov loop scheme L0 no O(a/L) discretization error

  8. Twisted Polyakov loop scheme

  9. Twisted Polyakov loop (TPL) scheme Previous works of SU(2) gauge theory de Divitiis et al.:NPB422:382 Polyakov loop in Twisted direction To satisfy the gauge inv. and translation inv.

  10. TPL coupling: At tree level, is proportional to bare coupling. This is a lattice artifact of the TPL coupling. • In our works, • We measure the ratio of Polykov loop correlators. • We have to check the at IR fixed point which should be smaller than 32.

  11. simulation (quenched QCD)

  12. Step scaling 1. Tune the value of beta (bare coupling) for a small lattice size, which gives the renormalized coupling “u”. 2. Carry out the simulation for the twice size of lattice. 3. Take the continuum limit (energy scale ) Notation： input renormalized coupling : output renormalized coupling :

  13. quenched QCD (pure Yang-Mills theory) • pseudo-heatbath algorithm and overrelaxation (1:5) • Simulation lattice size • parameter • statistics • 200,000 – 400,000 sweeps • for each parameter • step scaling with s=1.5 input output • Interpolation using following fit fn. In practice, we use the fit function to obtain the tuned value of beta and the output values of renormalized coupling. (n=3,4)

  14. The leading order of the discretization error is O(a2).

  15. In high energy region, In low energy region, runs slower than 1-loop approximation. IR limit: TPL scheme can control both statistical and systematic errors.

  16. Comparison of Running of the renormalized coupling constants in Quenched QCD Phys. Rev. D 80, 034507 (2009) TPL scheme Wloop scheme SF scheme The value of running itself has scheme dependence. The behaviors of the coupling constant should be scheme independentexcept for lattice artifacts. Capitani et.al:NPB B544 : 669-698

  17. Nf=12 case (TPL scheme)

  18. Lattice fermion From Nielsen-Ninomiya theorem, one massless lattice fermion has twice degree of freedom for each direction. In four dimensions, there are 16 degree of freedom. • Wilson fermion • adds Wilson term to kill 15 doublers • breaks chiral symmetry • staggered fermion • 16 = 4 spinors and 4 flavors • Overlap fermion (Neuberger 1998) • Ginsparg-Wilson relation (chiral symmetry on the lattice) • Domain-Waｌl fermion (Kaplan 1992, Shamir 1993) • 5-dim. formulation • takes a limit that symmetry breaking effect goes to zero Simulation cost

  19. Nf=12 QCD • Hybrid Monte Carlo algorithm (exact algorithm) ratio of Polyakov loop Lattice size for step scaling parameter At high energy, the data values of the larger lattice size are larger than that of the small lattice . This behavior is same as one of the SF scheme. beta

  20. In low beta region(4.0 < beta < 7.0) In low beta, the SF scheme shows the inversion of the order of lattice size dependence in the fixed beta. SF scheme (Appelquist et.al.) ratio of Polyakov loop On the other hand, the TPL coupling shows an upper boundary for each lattice size. (cf. Also, in the case of Nf=16 in Wilson loop scheme: Fodor et al. arXiv:0911:2934,)

  21. The growth of renormalized couplings for each step starting point g2=0.4711 The region of these data values corresponds to 5< beta < 18. If there is a fixed point, the beta function can be approximated as a linear function of u around the fixed point . That gives We should be study the large u region (beta<5) to find the fixed point.

  22. The running coupling constant The accumulated error can be calculated by the error of scaling function. Scaling function: Error of running coupling Consistent with the two-loop result in the high energy region. We cannot find the flat region of running coupling constant yet.

  23. Nonperturbative beta function(Preliminary) We fit the scaling function sigma(u): 2-parameter fit Here, s0 and s1 can be fixed by the perturbative 2-loop beta fn. fitting results s2= 4.1(4) ×10-2 s3= -1.8(2) ×10-2 chisq/dof= 1.2 ALPHA Collaboration , Nucl. Phys. B 713:378-406,2005 Center value of beta fn. Non-perturbative beta function can be obtained iteratively the following equation. At the starting point, we use 2-loop beta function in the right hand side.

  24. Nonperturbative beta funtion (Preliminary) There are large error bars, because the errors are accumulated. example: The error of beta fn. also can be calculated by the error of the fitting parameters; s2 and s3.

  25. To reduce the magnitude of errors, we added many independent steps. scaling function # of data= 90 s2= 3.0(2) ×10-2 s3= -1.2(1)×10-2 chisq/dof= 0.85 # of data= 37 s2= 4.1(4) ×10-2 s3= -1.8(2) ×10-2 chisq/dof= 1.2 fixed point(?)

  26. the previous beta fn. the beta fn. with improved error There is a signal of fixed point (?)

  27. Summary • In low beta, the TPL coupling’s behavior is different from SF scheme, and shows the trend of reaching a plateau for each lattice size. • The TPL running coupling and the nonperturbative beta fn. in the case of Nf=12 shows a signal of IR fixed point. • In our analyses, we estimated the discretization errors in O(a2). • Now, we are working in progress to study the lower energy region. • Future directions • Numerical measurement of the anomalous dimension at the IR point • the composite operator of fermions is interesting • Study Nf dependence (for example Nf=8 or 16) • to study arbitrary Nf, we need Overlap or Domain-wall fermion • (maybe we can do in 5 years…) • The other gauge group and representation of fermions • many people are studying…(Sannino, Yamada, Ohki…) • Make some improvements to reduce the discretization errors

  28. We have to take the continuum limit of measurement values. How to take the continuum limit To take the continuum limit, we have to set the scale “ ”. It corresponds to tuning to keep a certain input physical parameter constant. Examples of input parameters: , mass, Sommer scale…. To measure the running coupling output input Notation: scaling function In our work, we measure the running coupling constant using the other scheme without O(a/L) discretization error, not SF scheme. The existence of the fixed point should be scheme independent.

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