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G. Martinelli

Publications and source records attributed to G. Martinelli.

At least 109 records · Page 6Linked to original sources
arXiv2004

Semileptonic Hyperon Decays on the Lattice: an Exploratory Study

We present preliminary results of an exploratory lattice study of the vector form factor $f_1(q^2 = 0)$ relevant for the semileptonic hyperon decay $Σ^{-} --> n l ν$. This study is based on the same method used for the extraction of $f^+(0)$ for the decay $K^0 --> π^{-} l ν$. The main purpose of this study is to test the method for hyperon form factors in order to estimate the precision that can be reached and the importance of SU(3)-breaking effects.

hep-lat↗
arXiv2004

Renormalization Constants of Quark Operators for the Non-Perturbatively Improved Wilson Action

We present the results of an extensive lattice calculation of the renormalization constants of bilinear and four-quark operators for the non-perturbatively O(a)-improved Wilson action. The results are obtained in the quenched approximation at four values of the lattice coupling by using the non-perturbative RI/MOM renormalization method. Several sources of systematic uncertainties, including discretization errors and final volume effects, are examined. The contribution of the Goldstone pole, which in some cases may affect the extrapolation of the renormalization constants to the chiral limit, is non-perturbatively subtracted. The scale independent renormalization constants of bilinear quark operators have been also computed by using the lattice chiral Ward identities approach and compared with those obtained with the RI-MOM method. For those renormalization constants the non-perturbative estimates of which have been already presented in the literature we find an agreement which is typically at the level of 1%.

hep-lat↗
arXiv2004

CKM-Triangle analysis: Updates and Novelties for Summer 2004

Using the most recent determinations of several theoretical and experimental parameters, we update the Unitarity Triangle analysis in the Standard Model. The basic experimental constraints come from the measurements of epsilon_k, Vub/Vcb and Delta m_d, the limit on Delta m_s, and the measurement of the CP asymmetry in the B sector through the J/psi K^0 channel. In addition we also include in our analysis the direct determination of sin 2 alpha, gamma and sin(2 beta + gamma) from the measurements of new CP-violating quantities, recently coming from the B-Factories. We also discuss the opportunities offered by improving the precision of the measurements of the various physical quantities entering in the determination of the Unitarity Triangle parameters. The results and the plots presented in this paper can be also found at http://www.utfit.org, where they are continuously updated with the newest experimental and theoretical results.

hep-ph↗
arXiv2003

Non perturbative renormalization in coordinate space

We present an exploratory study of a gauge-invariant non-perturbative renormalization technique. The renormalization conditions are imposed on correlation functions of composite operators in coordinate space on the lattice. Numerical results for bilinears obtained with overlap and O(a)-improved Wilson fermions are presented. The measurement of the quark condensate is also discussed.

hep-lat↗
arXiv2003

Matrix elements of DI=3/2 K-->pi pi decays

We present a numerical computation of matrix elements of DI=3/2 K-->pi pi decays by using Wilson fermions. In order to extrapolate to the physical point we work at unphysical kinematics and we resort to Chiral Perturbation Theory at the next-to-leading order. In particular we explain the case of the electroweak penguins O_{7,8} which can contribute significantly in the theoretical prediction of epsilon'/epsilon. The study is done at beta=6.0 on a 24^3x64 lattice.

hep-lat↗
arXiv2003

Finite-Volume Partially-Quenched Two-Pion Amplitudes in the I=0 Channel

We present a study of the finite-volume two-pion matrix elements and correlation functions of the I=0 scalar operator, in full and partially quenched QCD, at one-loop order in chiral perturbation theory. In partially quenched QCD, when the sea and valence light quark masses are not equal, the lack of unitarity leads to the same inconsistencies as in quenched QCD and the matrix elements cannot be determined. It is possible, however, to overcome this problem by requiring the masses of the valence and sea quarks to be equal for the u and d quarks while keeping the strange quark (s) quenched (or partially quenched), but only in the kinematic region where the two-pion energy is below the two-kaon threshold. Although our results are obtained at NLO in chiral perturbation theory, they are more general and are also valid for non-leptonic kaon decays (we also study the matrix elements of (8,1) operators, such as the QCD penguin operator Q6). We point out that even in full QCD, where any problems caused by the lack of unitarity are clearly absent, there are practical difficulties in general, caused by the fact that finite-volume energy eigenstates are linear combination of two-pion, two-kaon and two-eta states. Our work implies that extracting Delta I=1/2, K-->pipi decay amplitudes from simulations with m_s=m_{d,u} is not possible in partially quenched QCD (and is very difficult in full QCD).

hep-lat↗
arXiv2003

Crystal Undulator As A Novel Compact Source Of Radiation

A crystalline undulator (CU) with periodically deformed crystallographic planes is capable of deflecting charged particles with the same strength as an equivalent magnetic field of 1000 T and could provide quite a short period L in the sub-millimeter range. We present an idea for creation of a CU and report its first realization. One face of a silicon crystal was given periodic micro-scratches (grooves), with a period of 1 mm, by means of a diamond blade. The X-ray tests of the crystal deformation have shown that a sinusoidal-like shape of crystalline planes goes through the bulk of the crystal. This opens up the possibility for experiments with high-energy particles channeled in CU, a novel compact source of radiation. The first experiment on photon emission in CU has been started at LNF with 800 MeV positrons aiming to produce 50 keV undulator photons.

physics.acc-ph↗
arXiv2003

Highly efficient crystal deflector for channeling extraction of a proton beam from accelerators

The design and performance of a novel crystal deflector for proton beams are reported. A silicon crystal was used to channel and extract 70 GeV protons from the U-70 accelerator in Protvino with an efficiency of 85%, as measured for a beam of ~1e12 protons directed towards crystals of ~2 mm length in spills of ~2 s duration. Experimental data agree with the theoretically predicted Monte Carlo results for channeling. The technique allows one to manufacture a very short deflector along the beam direction (2 mm). Consequently, multiple encounters of circulating particles with the crystal are possible with little probability of multiple scattering and nuclear interactions per encounter. Thus, drastic increase in efficiency for particle extraction out of the accelerator was attained. We show the characteristics of the crystal- deflector and the technology behind it. Such an achievement is important in devising a more efficient use of the U-70 accelerator and provides crucial support for implementing crystal-assisted slow extraction and collimation in other machines, such as the Tevatron, RHIC, the AGS, the SNS, COSY, and the LHC.

physics.acc-ph↗
arXiv2003

Neutron Electric Dipole Moment on the Lattice: a Theoretical Reappraisal

We present a strategy for a lattice evaluation of the neutron electric dipole moment induced by the strong CP violating term of the QCD Lagrangian. Our strategy is based on the standard definition of the electric dipole moment, involving the charge density operator J0, in case of three flavors with non-degenerate masses. We present a complete diagrammatic analysis showing how the axial chiral Ward identities can be used to replace the opological charge operator with the flavor-singlet pseudoscalar density PS. Our final result is characterized only by disconnected diagrams, where the disconnected part can be either the single insertion of PS or the separate insertions of both PS and J0. The applicability of our strategy to the case of lattice formulations that explicitly break chiral symmetry, like the Wilson and Clover actions, is discussed.

hep-lat↗
arXiv2003

Finite-Volume Two-Pion Amplitudes in the I=0 Channel

We perform a calculation in one-loop chiral perturbation theory of the two-pion matrix elements and correlation functions of an I=0 scalar operator, in finite and infinite volumes for both full and quenched QCD. We show that major difficulties arise in the quenched theory due to the lack of unitarity. Similar problems are expected for quenched lattice calculations of $K \to ππ$ amplitudes with $ΔI=1/2$. Our results raise the important question of whether it is consistent to study $K\toππ$ amplitudes beyond leading order in chiral perturbation theory in quenched or partially quenched QCD.

hep-lat↗
arXiv2002

K^+ -> pi^+ pi^0 Decays on Finite Volumes and at Next-to-Leading Order in the Chiral Expansion

We present the ingredients necessary for the determination of physical K->pi pi decay amplitudes for Delta I=3/2 transitions, from lattice simulations at unphysical kinematics and the use of chiral perturbation theory at next-to-leading order. In particular we derive the expressions for the matrix elements , where O_W is one of the operators appearing in the Delta S=1 weak Hamiltonian, in terms of low-energy constants at next-to-leading order in the chiral expansion. The one-loop chiral corrections are evaluated for arbitrary masses and momenta, both in full QCD and in the quenched approximation. We also investigate the finite-volume effects in this procedure.

hep-lat↗
arXiv2002

Non-perturbative renormalization constants and light quark masses

We present the results of an extensive non-perturbative calculation of the renormalization constants of bilinear quark operators for the non-perturbatively O(a)-improved Wilson action. The results are obtained at four values of the lattice coupling, by using the RI/MOM and the Ward identities methods. A new non-perturbative renormalization technique, which is based on the study of the lattice correlation functions at short distance in x-space, is also numerically investigated. We then use our non-perturbative determination of the quark mass renormalization constants to compute the values of the strange and the average up/down quark masses. After performing an extrapolation to the continuum limit, we obtain ms^msbar(2 GeV) = (106 +- 2 +- 8) MeV and ml^msbar(2 GeV)=(4.4 +- 0.1 +- 0.4) MeV.

hep-lat↗
arXiv2002

Kaon Weak Matrix Elements with Wilson Fermions

We present results of several numerical studies with Wilson fermions relevant for kaon physics. We compute the B_K parameter by using two different methods and extrapolate to the continuum limit. Our preliminary result is B_K(2 GeV)=0.66(7). Delta I=3/2 K->pi pi matrix elements are obtained by using the next-to-leading order expressions derived in chiral perturbation theory in which the low energy constants are determined by the lattice results computed at unphysical kinematics. From the simulation at beta=6.0 our (preliminary) results read: _{I=2}=0.14(1)(1) GeV^3 and _{I=2}=0.69(6)(6) GeV^3.

hep-lat↗
arXiv2002

Effects of Quenching in ΔI=1/2 Kaon Decays

We present the inconsistencies which arise in quenched QCD in $ΔI=1/2$ non-leptonic kaon decays using chiral perturbation theory (ChPT) to one loop. In particular we discuss how the lack of unitarity of the quenched theory invalidates the usual methods for the extraction of matrix elements from correlation functions.

hep-lat↗
arXiv2002

Charming Penguins Saga

We briefly recall the main formulae for computing the B -> K pi branching ratios within the "charming penguin" approach, present an updated fit to the data, and explain why we believe that, in general, these fits can hardly be used to extract gamma.

hep-ph↗
arXiv2002

$K^{+}\toπ^{+}π^{0}$ decays at next-to-leading order in the chiral expansion on finite volumes

We present the ingredients for determining $K^{+}\toπ^{+}π^{0}$ matrix elements via the combination of lattice QCD and chiral perturbation theory ($χ$PT). By simulating these matrix elements at unphysical kinematics, it is possible to determine all the low-energy constants (LECs) for constructing the physical $K^{+}\toπ^{+}π^{0}$ amplitudes at next-to-leading order (NLO) in the chiral expansion. In this work, the one-loop chiral corrections are calculated for arbitrary meson four-momenta, in both $χ$PT and quenched $χ$PT (q$χ$PT), and the finite-volume effects are studied.

hep-lat↗