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Rainer Sommer

Publications and source records attributed to Rainer Sommer.

At least 73 records · Page 4Linked to original sources
arXiv2013

A one-loop study of matching conditions for static-light flavor currents

Heavy Quark Effective Theory (HQET) computations of semi-leptonic decays, e.g. B -> pi l nu, require the knowledge of the parameters in the effective theory for all components of the heavy-light flavor currents. So far non-perturbative matching conditions have been employed only for the time component of the axial current. Here we perform a check of matching conditions for the time component of the vector current and the spatial component of the axial vector current up to one-loop order of perturbation theory and to lowest order of the 1/m-expansion. We find that the proposed observables have small higher order terms in the 1/m-series and are thus excellent candidates for a non-perturbative matching procedure.

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arXiv2012

B $\to$ $π$ form factor with 2 flavours of $O(a)$ improved Wilson quarks

The determinations of $|V_{\rm ub}|$ from the exclusive branching ratios of $B\to τν$ and $B \to πl ν$ tend to show a tension at the level of $3σ$ \cite{Beringer:1900zz}. On the theoretical side they depend on the lattice computation of the hadronic matrix elements $f_{\rm B}$ and the $B\to π$ form factor $f_+(q^2)$. To understand the tension, improved precision and a careful analysis of the systematics involved are necessary. Working towards this goal, we present preliminary lattice results of the ALPHA collaboration for the $B\to π$ form factor $f_+(q^2)$ with $N_{\rm f}=2$ flavours of $O(a)$-improved Wilson fermions. Our computation uses HQET in the static limit, pion masses ranging down to $\sim250$ MeV, large volumes with $m_πL >4$, three lattice spacings, and non-perturbative renormalization. We describe the techniques adopted to reduce the statistical noise (stochastic all-to-all with full time dilution) and the contamination from excited states (smearing for the B and the pion). We estimate the size of the chiral and continuum extrapolations. We discuss the impact our result could have to clarify the above mentioned discrepancy in the determination of $|V_{\rm ub}|$.

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arXiv2012

The strange quark mass and Lambda parameter of two flavor QCD

We complete the non-perturbative calculations of the strange quark mass and the Lambda parameter in two flavor QCD by the ALPHA collaboration. The missing lattice scale is determined via the kaon decay constant, for whose chiral extrapolation complementary strategies are compared. We also give a value for the scale r_0 in physical units as well as an improved determination of the renormalization constant Z_A.

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arXiv2012

Parameters of Heavy Quark Effective Theory from Nf=2 lattice QCD

We report on a non-perturbative determination of the parameters of the lattice Heavy Quark Effective Theory (HQET) Lagrangian and of the time component of the heavy-light axial-vector current with Nf=2 flavors of massless dynamical quarks. The effective theory is considered at the 1/mh order, and the heavy mass mh covers a range from slightly above the charm to beyond the beauty region. These HQET parameters are needed to compute, for example, the b-quark mass, the heavy-light spectrum and decay constants in the static approximation and to order 1/mh in HQET. The determination of the parameters is done non-perturbatively. The computation reported in this paper uses the plaquette gauge action and two different static actions for the heavy quark described by HQET. For the light-quark action we choose non-perturbatively O(a)-improved Wilson fermions.

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arXiv2011

M_b and f_B from non-perturbatively renormalized HQET with Nf=2 light quarks

We present an updated analysis of the non-perturbatively renormalized b-quark mass and B meson decay constant based on CLS lattices with two dynamical non-perturbatively improved Wilson quarks. This update incorporates additional light quark masses and lattice spacings in large physical volume to improve chiral extrapolations and to reach the continuum limit. We use Heavy Quark Effective Theory (HQET) including 1/m_b terms with non-perturbative coefficients based on the matching of QCD and HQET developed by the ALPHA collaboration during the past years.

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arXiv2011

Strange quark mass and Lambda parameter by the ALPHA collaboration

We determine f_K for lattice QCD in the two flavor approximation with non-perturbatively improved Wilson fermions. The result is used to set the scale for dimensionful quantities in CLS/ALPHA simulations. To control its dependence on the light quark mass, two different strategies for the chiral extrapolation are applied. Combining f_K and the bare strange quark mass with non-perturbative renormalization factors and step scaling functions computed in the Schroedinger Functional, we determine the RGI strange quark mass and the Lambda parameter in units of f_K.

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arXiv2011

Determination of the Wilson ChPT low energy constant c_2

Following a suggestion by Aoki and Bär, c2 can be extracted by analyzing volume effects in 2-pion states. To this end we consider renormalized ratios of four point to two point correlation functions. We present the results from various CLS lattices, with pion masses ranging from 280 to 450 MeV and lattice spacings of 0.07 fm and 0.08 fm. This low energy constant is useful to understand discretization effects in chiral extrapolations with Wilson fermions, especially for quantities which vanish in the chiral limit, like the pion mass. Since our procedure is computationally cheap and straightforward, it is recommended as a routine study for any Wilson-type simulation, as a check of discretization effects.

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arXiv2011

One-loop lattice artifacts of a dynamical charm quark

For a few observables in O(a) improved lattice QCD, we compute discretization effects arising from the vacuum polarization of a heavy quark at one-loop order. In particular, the force between static quarks, the running coupling in the Schrodinger functional and a related quantity, {\bar \upsilon}, are considered. Results show that the cutoff effects of a dynamical charm quark are typically smaller than those present in the pure gauge theory. This perturbative result is a good indication that dynamical charm quarks are feasible already now.

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arXiv2011

On the computation of hadron-to-hadron transition matrix elements in lattice QCD

We discuss the accurate determination of matrix elements < f| h_w | i > where neither |i> nor |f> is the vacuum state and h_w is some operator. Using solutions of the Generalized Eigenvalue Problem (GEVP) we construct estimators for matrix elements which converge rapidly as a function of the Euclidean time separations involved. |i> and |f> may be either the ground state in a given hadron channel or an excited state. Apart from a model calculation, the estimators are demonstrated to work well for the computation of the B*B pi-coupling in the quenched approximation. They are also compared to a standard ratio as well as to the "summed ratio method" of [1,2,3]. In the model, we also illustrate the ordinary use of the GEVP for energy levels.

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arXiv2011

Determination of the Static Potential with Dynamical Fermions

We present in detail a technique to extract the potential between a static quark and anti-quark pair from Wilson loops measured on dynamical configurations. This technique is based on HYP smearing and leads to an exponential improvement of the noise-to-signal ratio of Wilson loops. We explain why the correct continuum potential is obtained and show numerical evidence that the cut-off effects are small. We present precise results for the non-perturbative potential. As applications, we determine the scale r_0/a and study the shape of the static potential in the range of distances around r_0, where it can be compared with phenomenological potential models.

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arXiv2010

B meson spectrum and decay constant from Nf=2 simulations

We report on the status of an ALPHA Collaboration project to extract quantities for B physics phenomenology from Nf=2 lattice simulations. The framework is Heavy Quark Effective Theory (HQET) expanded up to the first order of the inverse b-quark mass. The couplings of the effective theory are determined by imposing matching conditions of observables computed in HQET with their counterpart computed in QCD. That program, based on Nf=2 simulations in a small physical volume with SF boundary conditions, is now almost finished. On the other side the analysis of configurations selected from the CLS ensembles, in order to measure HQET hadronic matrix elements, has just started recently so that only results obtained at a single lattice spacing, a=0.07 fm, will be discussed. We give our first results for the b-quark mass and for the B meson decay constant.

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arXiv2010

The B*Bpi Coupling in the Static Limit

We study an accurate method for the lattice calculation of the B*Bpi Coupling in the static limit, paying particular attention to excited state contamination. As this coupling is a parameter of the heavy meson chiral Lagrangian, it is useful for constraining the chiral behaviour of various observables in B-physics. We present a precise study of the continuum limit in the quenched approximation and preliminary results with 2 flavours of improved Wilson quarks (using CLS lattices) for pion masses down to around 250 MeV. With dynamical quarks both the lattice spacing and the light quark mass dependences are found to be very weak. We can quote g = 0.51(2) for the continuum value in the chiral limit, where the error will be reduced when a more complete analysis has been performed.

hep-lat↗
arXiv2010

Running of the SF-coupling with four massless flavours

We discuss the status of different determinations of alpha_s, motivating a precise and reliable computation from lattice QCD. In order to suppress perturbative errors, the non-perturbative computation has to reach high energy scales mu. Such results already exist in the SF-scheme for N_f=0,2 and N_f=3. We recently added the running with four massless flavours in a range of alpha from about 0.07 to 0.3 . It is based on our recent determination of the Sheikholeslami Wohlert coefficient in the four-flavour theory.

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arXiv2010

HQET at order 1/m: III. Decay constants in the quenched approximation

We report on the computation of the $B_s$ meson decay constant in Heavy Quark Effective Theory on the lattice. The next to leading order corrections in the HQET expansion are included non-perturbatively. We estimate higher order contributions to be very small. The results are extrapolated to the continuum limit, the main systematic error affecting the computation is therefore the quenched approximation used here. The Generalized Eigenvalue Problem and the use of all-to-all propagators are important technical ingredients of our approach that allow to keep statistical and systematic errors under control. We also report on the decay constant $f_{B'_s}$ of the first radially excited state in the $B_s$ sector, computed in the static limit.

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arXiv2010

Critical slowing down and error analysis in lattice QCD simulations

We study the critical slowing down towards the continuum limit of lattice QCD simulations with Hybrid Monte Carlo type algorithms. In particular for the squared topological charge we find it to be very severe with an effective dynamical critical exponent of about 5 in pure gauge theory. We also consider Wilson loops which we can demonstrate to decouple from the modes which slow down the topological charge. Quenched observables are studied and a comparison to simulations of full QCD is made. In order to deal with the slow modes in the simulation, we propose a method to incorporate the information from slow observables into the error analysis of physical observables and arrive at safer error estimates.

hep-lat↗
arXiv2010

HQET at order $1/m$: II. Spectroscopy in the quenched approximation

Using Heavy Quark Effective Theory with non-perturbatively determined parameters in a quenched lattice calculation, we evaluate the splittings between the ground state and the first two radially excited states of the $B_s$ system at static order. We also determine the splitting between first excited and ground state, and between the $B_s^*$ and $B_s$ ground states to order $1/m_b$. The Generalized Eigenvalue Problem and the use of all-to-all propagators are important ingredients of our approach.

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arXiv2010

HQET at order 1/m: I. Non-perturbative parameters in the quenched approximation

We determine non-perturbatively the parameters of the lattice HQET Lagrangian and those of the time component of the heavy-light axial-vector current in the quenched approximation. The HQET expansion includes terms of order 1/m. Our results allow to compute, for example, the heavy-light spectrum and B-meson decay constants in the static approximation and to order 1/m in HQET. The determination of the parameters is separated into universal and non-universal parts. The universal results can be used to determine the parameters for various discretizations. The computation reported in this paper uses the plaquette gauge action and the "HYP1/2" action for the b-quark described by HQET. The parameters of the current also depend on the light-quark action, for which we choose non-perturbatively O(a)-improved Wilson fermions.

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