The heavy-light sector of N f = twisted mass lattice QCD

Transcription

1 The heavy-light sector of N f = twisted mass lattice QCD Marc Wagner Humboldt-Universität zu Berlin, Institut für Physik mcwagner@physik.hu-berlin.de mcwagner/ September 24, 2009

2 Introduction Heavy-light sector: mesons made from a heavy s or c quark and a light u or d quark. Mainly K (e.g. su, J P = 0 ) and D (e.g. cu, J P = 0 ). Problems arise in particular, when considering charmed mesons (the D), because of twisted mass flavor breaking.

3 Outline N f = tm lattice QCD. m(k) and m(d). A. Generalized eigenvalue problem. B. Fitting exponentials. C. Parity and flavor restoration Optimization of operators. Decay constants f K and f D. Vector mesons K and D. Status at Lattice Mixed action OS setup. Conclusions.

4 N f = tm lattice QCD (1) Dirac operator of the mass degenerate light doublet: Q (χ(l) ) = γ µ D µ + m + iµγ 5 τ 3 a 2. Dirac operator of the mass split heavy doublet: Q (χ(h) ) = γ µ D µ + m + iµ σ γ 5 τ 1 + τ 3 µ δ a 2. Because of the Wilson term (a/2), parity is not a symmetry and the heavy flavors cannot be diagonalized instead of the four sectors (s, ), (s, +), (c, ), (c, +) there is only a single combined sector (s/c, /+) in twisted mass lattice QCD.

5 N f = tm lattice QCD (2) Twist rotation (in the continuum): ( ψ (u) ψ (d) ( ψ (s) ψ (c) ) ) ( ) ( χ = exp iγ 5 τ 3 ω l /2 (u) χ (d) ( ) ( χ = exp iγ 5 τ 1 ω h /2 (s) χ (c) ) ). Typically we use the twisted basis meson creation operators { χ (d) γ 5 χ (s), χ (d) γ 5 χ (c), χ (d) χ (s), χ (d) χ (c)}, O j to access the J = 0 (s/c, /+) sector (the K and the D).

6 N f = tm lattice QCD (3) Twist rotation of these operators (at finite lattice spacing): ψ (d) γ 5 ψ (s) ψ (d) γ 5 ψ (c) ψ (d) ψ (s) ψ (d) ψ (c) = Z S = c l c h s l s h is l c h +ic l s h s l s h c l c h +ic l s h is l c h is l c h +ic l s h c l c h s l s h +ic l s h is l c h s l s h c l c h (Z P /Z S )c l c h (Z P /Z S )s l s h is l c h +ic l s h (Z P /Z S )s l s h (Z P /Z S )c l c h +ic l s h is l c h i(z P /Z S )s l c h +i(z P /Z S )c l s h c l c h s l s h +i(z P /Z S )c l s h i(z P /Z S )s l c h s l s h c l c h } {{ } =M tw.rot. (ω l,ω h,z P /Z S ) Z P χ (d) γ 5 χ (s) Z P χ (d) γ 5 χ (c) Z S χ (d) χ (s) Z S χ (d) χ (c) where c x = cos(ω x /2) and s x = sin(ω x /2) and Z P and Z S are operator dependent renormalization constants. χ (d) γ 5 χ (s) χ (d) γ 5 χ (c) χ (d) χ (s) χ (d) χ (c) =,

7 m(k) and m(d) (1) Starting point: correlation matrix ( Ω C jk (t) = Ω O j (t) O k (0)) { χ (d) γ 5 χ (s), χ (d) γ 5 χ (c), χ (d) χ (s), χ (d) χ (c)}. O j Three (slightly) different methods: A. Generalized eigenvalue problem. B. Fitting exponentials. C. Parity and flavor restoration. Most results presented in the following correspond to these two ensembles: β L 3 T µ κ µ σ µ δ

8 A. Generalized eigenvalue problem (1) Generalized eigenvalue problem (GEP): C jk (t)v (n) j (t, t 0 ) = C jk (t 0 )v (n) j (t, t 0 )λ (n) (t, t 0 ). Effective meson masses: m (n) effective (t, t 0) = ln ( λ (n) (t, t 0 ) λ (n) (t + a, t 0 ) Fitting constants to effective mass plateaus at t 1 yields meson masses. From the eigenvectors v (n) one can read off the quantum numbers flavor and parity, i.e. (s, ), (s, +), (c, ) or (c, +). ).

9 A. Generalized eigenvalue problem (2) For t 1 GEP yields the lowest four states in the combined (s/c, /+) sector; the D is not among them. m(k ± ) = 494 MeV, m(k 0 ) = 498 MeV (J P = 0 ). m(k 0(800)) = 672(40) MeV (J P = 0 + ). m(k 0 (1430)) = 1425(50) MeV (J P = 0 + ). m(k(1460)) = 1400 MeV MeV (J P = 0 ).... m(k + π). m(k + 2 π).... m(d 0 ) = 1865 MeV, m(k ± ) = 1870 MeV (J P = 0 ).

10 A. Generalized eigenvalue problem (3) Why can we still expect to get an estimate for m(d) from GEP? In the continuum an exact diagonalization of C jk is possible yielding one correlator for each of the four sectors (s, ), (s, +), (c, ), (c, +) GEP would not yield the four lowest masses but m(k), m(s, +), m(d) and m(c, +). At finite lattice spacing corrections are O(a); at not too large temporal separations one of the four effective masses should be dominated by the D. β = 1.90, µ = β = 1.95, µ = effective masses from a generalized eigenvalue problem (charm,+) (charm, ) (strange,+) (strange, ) effective masses from a generalized eigenvalue problem (charm,+) (charm, ) (strange,+) (strange, ) m effective m effective Marc Wagner, t The heavy-light sector of N f = twisted mass lattice QCD, September 24, t 2009

11 A. Generalized eigenvalue problem (4) Identification of quantum numbers (s, ), (s, +), (c, ) or (c, +) by rotating the eigenvectors v (n) (t, t 0 ) to the pseudo physical basis. β = 1.95, µ = flavor and parity content of the (strange, )-state 1 flavor and parity content of the (strange,+)-state c (flavor,parity) (strange, ) (charm, ) (strange,+) (charm,+) c (flavor,parity) (strange, ) (charm, ) (strange,+) (charm,+) t t 1 flavor and parity content of the (charm, )-state 1 flavor and parity content of the (charm,+)-state c (flavor,parity) (strange, ) (charm, ) (strange,+) (charm,+) c (flavor,parity) (strange, ) (charm, ) (strange,+) (charm,+) t t

12 A. Generalized eigenvalue problem (5) With larger matrices (which are able to resolve all single and multi particle strange states below the D) GEP becomes exact (not feasible at currently available statistics, extremely complicated).

13 B. Fitting exponentials (1) Perform a χ 2 minimizing fit of C jk (t) = N ( n=1 a (n) j ) a (n) k ( ) exp m (n) t (N is an arbitrary number of exponentials) to the computed correlation matrix C jk (t), t min t t max. Similar to GEP, but less obvious, why it works: Exponentials can end up in the same sector. All exponentials will end up in the strange sectors, if large t values are considered in the fit. Results might depend on the initial fitting parameters.

14 B. Fitting exponentials (2) Left plot: effective mass values for K, K 0/K + π and D obtained by considering different fitting ranges t min t t max. Right plots: histograms of effective mass values for m(k) and m(d); the widths of these histograms are taken as the combined statistical and systematical errors for m(k) and m(d). β = 1.95, µ = Effective K-D-meson masses Distribution of fitted K-mass values (χ 2 < 5) Distribution of fitted D-mass values (χ 2 < 5) x 64 lattice, N f =2+1+1 Iwasaki gauge action β = 1.95, κ = µ l = , µ h = / x 64 lattices, N f =2+1+1 Iwasaki action, β = 1.95, κ = µ l = , µ h = / Range: [1 -> 25] = [ > ] am K = (63) x 64 lattices, N f =2+1+1 Iwasaki action, β = 1.95, κ = µ l = , µ h = / Range: [1 -> 25] = [ > ] am D = 0.823(15) 10 5 am K,D h(n) 8 6 h(n) t *(t 2 -t 1 ) am am

15 B. Fitting exponentials (3) With larger matrices (which are able to resolve all single and multi particle strange states below the D) fitting exponentials becomes exact (not feasible at currently available statistics, extremely complicated).

16 C. Parity and flavor restoration (1) Express the correlation matrix C jk (t) in the physical basis in terms of the twist angles ω l and ω h and Z P /Z S : C (physical) jk (t; ω l, ω h, Z P /Z S ) = (. = M tw.rot. (ω l, ω h, Z P /Z S )C jk (t) M tw.rot. (ω l, ω h, Z P /Z S )) Determine ω l, ω h and Z P /Z S by requiring C (physical) jk (t; ω l, ω h, Z P /Z S ) = 0. j k At finite lattice spacing and small t this cannot be achieved exactly (excited states, O(a) effects). At large t (when only the K survives) it can be achieved. This corresponds to removing any K contribution from the diagonal correlators C (physical) jj, j (s, ).

17 C. Parity and flavor restoration (2) Analyze the diagonal correlators C (physical) jj separately; one correlator for each of the four sectors (s, ), (s, +), (c, ), (c, +). ω l, ω h and Z P /Z S are determined from parity odd/flavor non-diagonal matrix elements, which are not O(a) improved. The D correlator C (physical) jj, j = (c, ) is contaminated by O(a) contributions from lighter states (not K, but K0, K + π, excited K,...) at large t the effective mass will break down to the mass value of the lightest of these states.

18 C. Parity and flavor restoration (3) β = 1.90, µ = β = 1.95, µ = A: Kaon Effective Mass B: Kaon Effective Mass local smeared 0.24 local smeared M eff (x 0 ) M eff (x 0 ) x x A: D Meson Effective Mass 1.2 B: D Meson Effective Mass M eff (x 0 ) 0.8 M eff (x 0 ) Marc Wagner, x 0 The heavy-light sector of N f = twisted mass lattice QCD, Septemberx24,

19 m(k) and m(d) (2) Results obtained with these three methods agree within statistical errors. Rather precise results for m(k). Statistical error 0.2%. All three methods require assumptions for m(d), i.e. there is a systematical error involved. Way of estimating the systematical error is somewhat arbitrary. Combined statistical and systematical error < 5%. Systematical error larger than statistical error.

20 Optimization of operators (1) Mesons are characterized by quantum numbers flavor and J P. Many different meson creation operators with the same quantum numbers. Width of the operator and the corresponding trial state. Smearing method and parameters (we use Gaussian smearing). Ground state overlaps are significantly larger. Optimized smearing essential for m(d). β = 1.9, L T = , µ = , κ = , µ σ = 0.15, µ δ = local quark operators (charm,+) (charm, ) (strange,+) (strange, ) smeared quark operators (charm,+) (charm, ) (strange,+) (strange, ) m effective m effective T T

21 Optimization of operators (2) Inclusion of γ 0, i.e. operators { χ (d) γ 0 γ 5 χ (s), χ (d) γ 0 γ 5 χ (c), χ (d) γ 0 χ (s), χ (d) γ 0 χ (c)}. O j... Ground state overlaps are slightly smaller.

22 Decay constants f K and f D Decay constants f K and f D can be obtained via f K = µ + µ σ (Z P /Z S )µ δ m(k) 3/2 Ω 1 ( P K + P D i Z ) S (S K S D ) K 2 Z P f D = µ + µ σ + (Z P /Z S )µ δ m ( D) 3/2 Ω 1 ( P K + P D + i Z ) S (S K S D ) D 2 Z P P K/D = ( χ (d) γ 5 χ (s/c)) (, S K/D = χ (d) χ (s/c)), where K K = 1 and D D = 1. Z P /Z S is determined from parity odd/flavor non-diagonal matrix elements, which are not O(a) improved (cf. method C.). For f D similar problems as for m(d).

23 Vector mesons K and D (1) Vector mesons (J P = 1 ): m(k (892)) = 892 MeV. m(d (2007) 0 ) = 2007 MeV, m(d (2010) ± ) = 2010 MeV. Meson creation operators: { χ (d) γ j χ (s), χ (d) γ j χ (c), χ (d) γ j γ 5 χ (s), χ (d) γ j γ 5 χ (c)}. O j effective masses (vector mesons) (charm,+) (charm, ) (strange,+) (strange, ) m effective t

24 Vector mesons K and D (2) Preliminary results (β = 1.9, L 3 T = , κ = , µ = , µ σ = 0.15, µ δ = 0.19): m(k )/m(k) = 1.77(5) (PDG: m(k )/m(k) 1.80). m(d )/m(d) = 1.08(4) (PDG: m(d )/m(d) 1.08). K problematic, because it is a resonance: K can decay to K + π.

25 Status at Lattice 2009 (2m(K) 2 m 2 PS )r2 0 as a function of (m PSr 0 ) 2 (m(k)/m(k )) 2 as a function of (m PS r 0 ) 2 m(d)r 0 as a function of (m PS r 0 ) m(d) r β = 1.90, L/a = 24 β = 1.90, L/a = 32 β = 1.95, L/a = 32 physical D meson Marc Wagner, The heavy-light sector of N f = 2 + (m1 + 1 twisted mass lattice QCD, September 24, 2009 PS r 0 ) 2

26 Mixed action OS setup (1) Heavy Dirac operator in the valence sector: Q (χ(h),valence) = γ µ D µ + m + iγ 5 ( +µ1 0 0 µ 2 ) a 2. Both heavy and light flavors are diagonal two distinct sectors (s, /+) and (c, /+) instead of four only problems with parity determination of m(d) becomes easy (the D is the lightest state in the charm sector). Formulation cannot be used during simulations (sign problem).

27 Mixed action OS setup (2) Matching of valence and sea quark masses: Method 1: µ 1,2 = µ σ Z P Z S µ δ (Z P /Z S has not been determined in a satisfactory way yet). Method 2: tune µ 1 and µ 2 such that m(k) and m(d) are the same both in the unitary and in the mixed action setup (what we do at the moment).

28 Conclusions Unitary setup: Precise results in the strange sector (m(k),...). Problems in the charm sector (m(d),...), due to twisted mass flavor breaking. Mixed action OS setup: A promising method to do charm physics. Unitary setup (at the moment) necessary, to achieve a matching of quark masses.