arXiv subjects

Carlos Pena

Publications and source records attributed to Carlos Pena.

46 records · Page 3Linked to original sources

arXiv2012

Contribution of the charm quark to the ΔI=1/2 rule

We report on the progress of our ongoing project to quantify the role of the charm quark in the non-leptonic decay of a kaon into two pions. The effect of its associated mass scale in the dynamics underlying the ΔI = 1/2 rule can be studied by monitoring the dependence of kaon decay amplitudes on the charm quark mass using an effective ΔS = 1 weak Hamiltonian. In contrast to commonly used approaches the charm quark is kept as an active degree of freedom. Quenched results in the GIM limit have shown that a significant part of the ΔI = 1/2 enhancement is purely due to low-energy QCD effects. Moving away from the GIM limit involves the computation of diagrams containing closed quark loops which requires new variance reduction techniques in order to determine the relevant weak effective low-energy couplings. We employ a combination of low-mode averaging and stochastic volume sources in order to compute these diagrams and observe a significant improvement in the statistical signal.

hep-lat↗

arXiv2012

N_f=2 chiral dynamics in the mixed chiral regime

We present a study of the pseudoscalar propagator in the mixed chiral regime with valence quark masses in the ε-regime and sea quark masses in the p-regime. We first show the NNLO prediction of this observable in the chiral expansion. In sectors of fixed topology, the correlator has a pole in 1/m_v^2 that can be matched to the topological zero-mode contributions to the correlator. We compute the residue of this pole in a N_f=2 mixed-action simulation and compare the results with the prediction in Chiral Perturbation Theory.

hep-lat↗

arXiv2012

Variance reduction techniques for a quantitative understanding of the ΔI = 1/2 rule

The role of the charm quark in the dynamics underlying the ΔI = 1/2 rule for kaon decays can be understood by studying the dependence of kaon decay amplitudes on the charm quark mass using an effective ΔS = 1 weak Hamiltonian in which the charm is kept as an active degree of freedom. Overlap fermions are employed in order to avoid renormalization problems, as well as to allow access to the deep chiral regime. Quenched results in the GIM limit have shown that a significant part of the enhancement is purely due to low-energy QCD effects; variance reduction techniques based on low-mode averaging were instrumental in determining the relevant weak effective lowenergy couplings in this case. Moving away from the GIM limit requires the computation of diagrams containing closed quark loops. We report on our progress to employ a combination of low-mode averaging and stochastic volume sources in order to control these contributions. Results showing a significant improvement in the statistical signal are presented.

hep-lat↗

arXiv2011

X(3872) as a D-D* molecule bound by quark exchange forces

The Bethe-Salpeter equation for the T-Matrix of D-D* scattering is solved with a meson-meson potential that results from 2nd order Born approximation of quark exchange processes. This potential turns out to be complex and energy dependent due to the pole contribution from the coupling to the intermediate J/psi-rho meson pair propagator. As a consequence, a bound state with a mass close to 3.872 GeV occurs in the J/psi-rho continuum. This result suggests that quark exchange forces may provide the solution to the puzzling question for the origin of the interaction which leads to a binding of D and D* mesons in the X(3872) state.

hep-ph↗

arXiv2011

Light quark correlators in a mixed-action setup

We report our progress in simulating Neuberger valence fermions on N_f=2 Wilson O(a)-improved sea quarks. We compute correlators with valence quark masses both in the p- and in the epsilon-regime, and we match the results with the predictions of the Chiral Effective Theory in the mixed regime. This allows us to extract the Low Energy Couplings (LECs) of the N_f=2 theory and to test the validity of the approach.

hep-lat↗

arXiv2011

Probing the chiral regime of Nf=2 QCD with mixed actions

We report on our first experiences with a mixed action setup with overlap valence quarks and non-perturbatively O(a) improved Wilson sea quarks. For the latter we employ CLS Nf=2 configurations with light sea quark masses at small lattice spacings. Exact chiral symmetry allows to consider very light valence quarks and explore the matching to (partially quenched) Chiral Perturbation Theory (ChPT) in a mixed epsilon/p-regime. We compute the topological susceptibility and the low-lying spectrum of the massless Neuberger-Dirac operator for three values of the sea quark mass, and compare the sea quark mass dependence to NLO ChPT in the mixed regime. This provides two different determinations of the chiral condensate, as well as information about some NLO low-energy couplings. Our results allow to test the consistency of the mixed-regime approach to ChPT, as well as of the mixed action framework.

hep-lat↗

arXiv2010

Exploring the chiral regime of $N_f=2$ QCD with mixed actions

We report on our simulations with Neuberger valence fermions on CLS $N_f=2$ configurations with non-perturbatively $O(a)$-improved Wilson sea quarks. We consider the matching of QCD to ChPT in the so called mixed-regime in which the sea quarks are in the $p$-regime while the valence quarks are in the $ε$-regime. From this matching, we can get information on $Σ$, $L_6$ and the combination $L_8+2L_6+2L_7$.

hep-lat↗

arXiv2007

Non-perturbative renormalization of static-light four-fermion operators in quenched lattice QCD

We perform a non-perturbative study of the scale-dependent renormalization factors of a multiplicatively renormalizable basis of $Δ{B}=2$ parity-odd four-fermion operators in quenched lattice QCD. Heavy quarks are treated in the static approximation with various lattice discretizations of the static action. Light quarks are described by non-perturbatively ${\rm O}(a)$ improved Wilson-type fermions. The renormalization group running is computed for a family of Schroedinger functional (SF) schemes through finite volume techniques in the continuum limit. We compute non-perturbatively the relation between the renormalization group invariant operators and their counterparts renormalized in the SF at a low energy scale. Furthermore, we provide non-perturbative estimates for the matching between the lattice regularized theory and all the SF schemes considered.

hep-lat↗

arXiv2006

Twisted mass QCD for weak matrix elements

I report on the application of tmQCD techniques to the computation of hadronic matrix elements of four-fermion operators. Emphasis is put on the computation of $B_K$ in quenched QCD performed by the ALPHA Collaboration. The extension of tmQCD strategies to the study of neutral $B$-meson mixing is briefly discussed. Finally, some remarks are made concerning proposals to apply tmQCD to the computation of $K\toππ$ amplitudes.

hep-lat↗

arXiv2004

NLO anomalous dimension of multiplicatively renormalizable four-fermion operators in Schroedinger Functional schemes

Renormalization constants for multiplicatively renormalizable parity-odd four-fermion operators are computed in various different Schroedinger Functional (SF) schemes and lattice regularizations with Wilson quarks at one-loop order in perturbation theory. Our results are used in the calculation of their NLO anomalous dimensions, through matching to continuum schemes. They also enable a comparison of the two-loop perturbative RG running to the previously obtained nonperturbative one in the region of small renormalized coupling.

hep-lat↗