Publications (71)68.56 Total impact
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ABSTRACT: We study the two and threedimensional Antiferromagnetic Ising Model with an imaginary magnetic field $i\theta$ at $\theta = \pi$. We use a new geometric algorithm which does not present a sign problem. This allows us to perform efficient numerical simulations of this system.12/2013;  [show abstract] [hide abstract]
ABSTRACT: In this work we study the two and threedimensional antiferromagnetic Ising model with an imaginary magnetic field $i\theta$ at $\theta=\pi$. In order to perform numerical simulations of the system we introduce a new geometric algorithm not affected by the sign problem. Our results for the $2D$ model are in agreement with the analytical solutions. We also present new results for the $3D$ model which are qualitatively in agreement with meanfield predictions.Nuclear Physics B. 12/2013;  [show abstract] [hide abstract]
ABSTRACT: We have developed two methods for handling $b$ quarks in lattice QCD. One uses NRQCD (now improved to include radiative corrections) and the other uses Highly Improved Staggered Quarks (HISQ), extrapolating to the $b$ quark from lighter masses and using multiple lattice spacings to control discretisation errors. Comparison of results for the two different methods gives confidence in estimates of lattice QCD systematic errors, since they are very different in these two cases. Here we show results for heavyonium hyperfine splittings and vector currentcurrent correlator moments using HISQ quarks, to add to earlier results testing the heavy HISQ method with pseudoscalar mesons. We also show the form factor for $B \rightarrow \pi l \nu$ decay at zero recoil using NRQCD $b$ quarks and $u/d$ quarks with physical masses. This allows us to test the soft pion theorem relation ($f_0(q^2_{max})=f_B/f_{\pi}$) accurately and we find good agreement as $M_{\pi} \rightarrow 0$. }12/2013;  [show abstract] [hide abstract]
ABSTRACT: We analyze the vacuum structure of a generalized lattice NambuJonaLasinio model with two flavors of Wilson fermions, such that its continuum action is the most general fourfermion action with 'trivial' color interactions, and having a $SU(2)_V x SU(2)_A$ symmetry in the chiral limit. The phase structure of this model in the space of the two fourfermion couplings shows, in addition to the standard Aoki phases, new phases with $<\bar\psi \gamma_5\psi> != 0$, in close analogy to similar results recently suggested by some of us for lattice QCD with two degenerate Wilson fermions. This result shows how the phase structure of an effective model for low energy QCD cannot be entirely understood from Wilson Chiral Perturbation Theory, based on the standard QCD chiral effective Lagrangian approach.Nuclear Physics B 05/2013; 875(1). · 4.33 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We present a new study of the form factors for D > K semileptonic decay from lattice QCD that allows us to compare the shape of the vector form factor to experiment and, for the first time, to extract V_cs using results from all experimental q^2 bins. The valence quarks are implemented with the Highly Improved Staggered Quark action on MILC configurations that include u, d and s sea quarks. The scalar and vector currents are nonperturbatively normalised and, using phased boundary conditions, we are able to cover the full q^2 range accessible to experiment. Our result is V_cs = 0.963(5)_{expt}(14)_{lattice}. We also demonstrate that the form factors are insensitive to whether the spectator quark is u/d or s, which has implications for other decay channels.05/2013; 
Article: Geometric representation of the 2D Antiferromagnetic Ising Model with topological term at \theta=\pi
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ABSTRACT: We study the twodimensional Antiferromagnetic Ising Model with an imaginary magnetic field i\theta at \theta=\pi. We use a new geometric algorithm which does not present a sign problem. This allows us to perform efficient numerical simulations of this system.12/2012;  [show abstract] [hide abstract]
ABSTRACT: Recently we studied the B, Bs, D and Ds meson decay constants using various treatments for the heavy quark. For B mesons, we determined fB, fBs, and fBs/fB with NRQCD bottom quarks. We then combined the ratio fBs/fB and another very precise determination from HPQCD for fBs using heavy HISQ quarks, and extracted fB with 2% total errors. We also calculated fD, fDs, and fDs/fD using HISQ charm quarks. Here we review our results and briefly discuss their implications for the determination of the CKM matrix elements Vcd and Vcs.12/2012;  [show abstract] [hide abstract]
ABSTRACT: We calculate the J/{\psi} mass, leptonic width and radiative decay rate to \gamma \eta_c from lattice QCD including u, d and s quarks in the sea for the first time. We use the Highly Improved Staggered Quark formalism and nonperturbatively normalised vector currents for the leptonic and radiative decay rates. Our results are: M_{J/\psi} M_{\eta_c} = 116.5(3.2) MeV; \Gamma(J/{\psi} to e^+e^) = 5.48(16) keV; \Gamma(J/{\psi} to \gamma \eta_c) = 2.49(19) keV. The first two are in good agreement with experiment, with \Gamma(J/{\psi} to e^+e^) providing a test of a decay matrix element in QCD, independent of CKM uncertainties, to 2%. At the same time results for the time moments of the correlation function can be compared to values from the charm contribution to \Gamma(e^+e^ to hadrons), giving a 1.5% test of QCD. Our results show that an improved experimental error would enable a similarly strong test from \Gamma(J/{\psi} to \gamma \eta_c).Physical review D: Particles and fields 08/2012; 86(9).  [show abstract] [hide abstract]
ABSTRACT: In this paper, we discuss the vacuum structure of QCD with two flavors of Wilson fermions, inside the Aoki phase. We provide numerical evidence, coming from HMC simulations in $4^4$, $6^4$ and $8^4$ lattices, supporting a vacuum structure for this model at strong coupling more complex than the one assumed in the standard wisdom, with new vacua where the expectation value of $i\bar\psi\gamma_5\psi$ can take nonzero values, and which can not be connected with the Aoki vacua by parityflavour symmetry transformations.Nuclear Physics B 08/2012; 870(1). · 4.33 Impact Factor  [show abstract] [hide abstract]
ABSTRACT: We investigate the critical behaviour at theta=pi of the twodimensional O(3) nonlinear sigma model with topological term on the lattice. Our method is based on numerical simulations at imaginary values of theta, and on scaling transformations that allow a controlled analytic continuation to real values of theta. Our results are compatible with a second order phase transition, with the critical exponent of the SU(2)_1 WessZuminoNovikovWitten model, for sufficiently small values of the coupling.Physical review D: Particles and fields 07/2012; 86(9).  [show abstract] [hide abstract]
ABSTRACT: We determine masses and decay constants of heavyheavy and heavycharm pseudoscalar mesons as a function of heavy quark mass using a fully relativistic formalism known as Highly Improved Staggered Quarks for the heavy quark. We are able to cover the region from the charm quark mass to the bottom quark mass using MILC ensembles with lattice spacing values from 0.15 fm down to 0.044 fm. We obtain f_{B_c} = 0.427(6) GeV; m_{B_c} = 6.285(10) GeV and f_{\eta_b} = 0.667(6) GeV. Our value for f_{\eta_b} is within a few percent of f_{\Upsilon} confirming that spin effects are surprisingly small for heavyonium decay constants. Our value for f_{B_c} is significantly lower than potential model values being used to estimate production rates at the LHC. We discuss the changing physical heavyquark mass dependence of decay constants from heavyheavy through heavycharm to heavystrange mesons. A comparison between the three different systems confirms that the B_c system behaves in some ways more like a heavylight system than a heavyheavy one. Finally we summarise current results on decay constants of goldplated mesons.Physical review D: Particles and fields 07/2012; 86(7). 
Article: V_cd from D Meson Leptonic Decays
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ABSTRACT: We present an update of the D meson decay constant f_D using the Highly Improved Staggered Quark (HISQ) action for valence charm and light quarks on MILC N_f = 2+1 lattices. The new determination incorporates HPQCD's improved scale r_1^{N_f = 2+1} = 0.3133(23) fm, accurately retuned bare charm quark masses and data from an ensemble that is more chiral than in our previous calculations. We find f_D = 208.3(3.4) MeV. Combining the new f_D with D > \mu \nu\ branching fraction data from CLEOc, we extract the CKM matrix element V_cd = 0.223(10)_{exp.}(4)_{lat.}. This value is in excellent agreement with V_cd from D semileptonic decays and from neutrino scattering experiments and has comparable total errors. We determine the ratio between semileptonic form factor and decay constant and find [f^{D > \pi}_+(0) / f_D ]_{lat.} = 3.20(15) GeV^{1} to be compared with the experimental value of [f^{D > \pi}_+(0) / f_D ]_{exp.} = 3.19(18) GeV^{1}. Finally, we mention recent preliminary but already more accurate D > \mu \nu\ branching fraction measurements from BES III and discuss their impact on precision V_cd determinations in the future.Physical Review D. 06/2012; 86(5).  [show abstract] [hide abstract]
ABSTRACT: We present a new determination of the Bs leptonic decay constant from lattice QCD simulations that use gluon configurations from MILC and a highly improved discretization of the relativistic quark action for both valence quarks. Our result, fBs=0.225(4) GeV, is almost 3 times more accurate than previous determinations. We analyze the dependence of the decay constant on the heavy quark’s mass and obtain the first empirical evidence for the leading 1/√mh dependence predicted by heavy quark effective theory. As a check, we use our analysis technique to calculate the mBsmηb/2 mass difference. Our result agrees with experiment to within errors of 11 MeV (better than 2%). We discuss how to extend our analysis to other quantities in Bs and B physics, making 2% precision possible for the first time.Physical review D: Particles and fields 02/2012; 85(3).  [show abstract] [hide abstract]
ABSTRACT: We present a new calculation of the D→π, lν semileptonic form factor f+D→π(q2) at q2=0 based on “highly improved staggered quark” charm and light valence quarks on MILC Nf=2+1 lattices. Using methods developed recently for HPQCD’s study of D→K, lν decays, we find f+D→π(0)=0.666(29). This signifies a better than factor 2 improvement in errors for this quantity compared to previous calculations. Combining the new result with CLEOc branching fraction data, we extract the CabibboKobayashiMaskawa matrix element Vcd=0.225(6)exp(10)lat, where the first error comes from experiment and the second from theory. With a total error of ∼5.3% the accuracy of the direct determination of Vcd from D semileptonic decays has become comparable to (and in good agreement with) that from neutrino scattering. We also check for second row unitarity using this new Vcd, HPQCD’s earlier Vcs, and Vcb from the Fermilab Lattice and MILC collaborations. We find Vcd2+Vcs2+Vcb2=0.976(50), improving on the current PDG2010 value.Physical review D: Particles and fields 12/2011; 84(11).  [show abstract] [hide abstract]
ABSTRACT: We investigate numerically the spectral flow introduced by Adams for the staggered Dirac operator on realistic gauge configurations. We study both the unimproved and the HISQ Dirac operators. We compare the spectral flow index with the index obtained by identifying lowlying modes of large chirality.11/2011;  [show abstract] [hide abstract]
ABSTRACT: We present a new and very high statistics study of D and D_s semileptonic decay form factors on the lattice. We work with MILC N_f=2+1 lattices and use the Highly Improved Staggered Action (HISQ) for both the charm and the light valence quarks. We use both scalar and vector currents to determine the form factors f_0(q^2) and f_+(q^2) for a range of D and D_s form factors including those for D to pi and D to K semileptonic decays. By using a phased boundary condition we are able to tune accurately to q^2=0. We also compare the shape in q^2 to that from experiment. We show that the form factors are very insensitive to the spectator quark: D to K and D_s to eta_s form factors are essentially the same, and the same is true for D to pi and D_s to K. This has important implications when considering the corresponding B/B_s processes.11/2011;  [show abstract] [hide abstract]
ABSTRACT: We give results for the Upsilon spectrum from lattice QCD using an improved version of the NRQCD action for b quarks which includes radiative corrections to kinetic terms at O(v^4) in the velocity expansion. We also include for the first time the effect of up, down, strange and charm quarks in the sea using 'second generation' gluon field configurations from the MILC collaboration. Using the {\Upsilon} 2S  1S splitting to determine the lattice spacing, we are able to obtain the 1P  1S splitting to 1.4% and the 3S  1S splitting to 2.4%. Our improved result for M (Upsilon)  M (eta_b) is 70(9) MeV and we predict M(Upsilon)  M(eta_b') = 35(3) MeV. We also calculate pi, K and eta_s correlators using the Highly Improved Staggered Quark action and perform a chiral and continuum extrapolation to give values for M(eta_s) (0.6893(12) GeV) and f_{eta_s} (0.1819(5) GeV) that allow us to tune the strange quark mass as well as providing an independent and consistent determination of the lattice spacing. Combining the NRQCD and HISQ analyses gives mb/ms = 54.7(2.5) and a value for the heavy quark potential parameter of r_1 = 0.3209(26) fm.Physical review D: Particles and fields 10/2011;  [show abstract] [hide abstract]
ABSTRACT: We present a new determination of the $B_s$ leptonic decay constant from lattice QCD simulations that use gluon configurations from MILC and a highly improved discretization of the relativistic quark action for both valence quarks. Our result, $f_{B_s} = 0.225(4)$\,GeV, is almost three times more accurate than previous determinations. We analyze the dependence of the decay constant on the heavy quark's mass and obtain the first empirical evidence for the leading $1/\sqrt{m_h}$ dependence predicted by Heavy Quark Effective Theory (HQET). As a check, we use our analysis technique to calculate the $m_{B_s}m_{\eta_b}/2$ mass difference. Our result agrees with experiment to within errors of $11\,\mathrm{MeV}$ (better than 2%). We discuss how to extend our analysis to other quantities in $B_s$ and $B$ physics, making 2%precision possible for the first time.10/2011;  [show abstract] [hide abstract]
ABSTRACT: We examine the taste structure of eigenvectors of the staggeredfermion Dirac operator. We derive a set of conditions on the eigenvectors of modes with small eigenvalues (nearzero modes), such that staggered fermions reproduce the 't Hooft vertex in the continuum limit. We also show that, assuming these conditions, the correlators of flavorsinglet mesons are free of contributions singular in 1/m, where m is the quark mass. This conclusion holds also when a single flavor of sea quark is represented by the fourth root of the staggeredfermion determinant. We then test numerically, using the highly improved staggeredquark action, whether these conditions hold on realistic lattice gauge fields. We find that the needed structure does indeed emerge.Physical review D: Particles and fields 06/2011; 30.  [show abstract] [hide abstract]
ABSTRACT: Using the approach developed in \cite{REFVIC2}, we succeeded in reconstructing the behaviour of the antiferromagnetic Ising model with imaginary magnetic field $i\theta$ for two and three dimensions in the low temperature regime. A meanfield calculation, expected to work well for high dimensions, is also carried out, and the meanfield results coincide qualitatively with those of the two and threedimensional Ising model. The mean field analysis reveals also a phase structure more complex than the one expected for $QCD$ with a topological $\theta$therm.Nuclear Physics B 05/2011; 851(2). · 4.33 Impact Factor
Publication Stats
509  Citations  
125  Downloads  
68.56  Total Impact Points  
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Institutions

1996–2012

University of Zaragoza
 Department of Theoretical Physics
Caesaraugusta, Aragon, Spain


2007–2008

The Ohio State University
 Department of Physics
Columbus, OH, United States


2002–2008

University of Glasgow
 School of Physics and Astronomy
Glasgow, SCT, United Kingdom
