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O. Pene

Publications and source records attributed to O. Pene.

At least 37 records · Page 2Linked to original sources
arXiv2003

Phenomenological discussion of $B\to P V$ decays in QCD improved factorization approach

Trying a global fit of the experimental branching ratios and CP-asymmetries of the charmless $B\to PV$ decays according to QCD factorization, we find it impossible to reach a satisfactory agreement, the confidence level (CL) of the best fit is smaller than .1 %. This failure reflects the difficulty to accommodate several large experimental branching ratios of the strange channels. Furthermore, experiment was not able to exclude a large direct CP asymmetry in $\bar {B}^0\toρ^+ π^-$, which is predicted very small by QCD factorization. Proposing a fit with QCD factorization complemented by a charming-penguin inspired model we reach a best fit which is not excluded by experiment (CL of about 8 %) but is not fully convincing. These negative results must be tempered by the remark that some of the experimental data used are recent and might still evolve significantly.

hep-ph↗
arXiv2002

A lattice estimate of the g_{D^* D pi} coupling

We present the results of the first direct determination of the g_{D^* D pi} coupling using lattice QCD. From our simulations in the quenched approximation, we obtain g_{D^* D pi} = 18.8 +/- 2.3^{+1.1}_{-2.0} and hat(g) = 0.67 +/- 0.08^{+0.04}_{-0.06}. It is in agreement with a recent experimental result from CLEO.

hep-lat↗
arXiv2002

First lattice QCD estimate of the g_{D^* D pi} coupling

We present the results of the first lattice QCD study of the strong coupling g_{D^* D pi}. From our simulations in the quenched approximation, we obtain g_{D^* D pi} = 18.8 +/- 2.3^{+1.1}_{-2.0} and hat(g)_c = 0.67 +/- 0.08^{+0.04}_{-0.06}. Whereas previous theoretical studies gave different predictions, our result favours a large value for hat(g)_c. It agrees very well with the recent experimental value by CLEO. hat(g) varies very little with the heavy mass and we find in the infinite mass limit hat(g)_infinity = 0.69(18).

hep-ph↗
arXiv2001

Remarks on sum rules in the heavy quark limit of QCD

We underline a problem existing in the heavy quark limit of QCD concerning the rates of semileptonic B decays into P-wave $D_J(j)$ mesons, where $j = {1 \over 2}$ (wide states) or $j = {3 \over 2}$ (narrow states). The leading order sum rules of Bjorken and Uraltsev suggest $Γ[ \bar{B} \to D_{0,1} ({1 \over 2}) \ell ν] \ll Γ[ \bar{B} \to D_{1,2} ({3 \over 2}) \ell ν]$, in contradiction with experiment. The same trend follows also from a sum rule for the subleading $1/m_Q$ curent matrix element correction $ξ_3(1)$. The problem is made explicit in relativistic quarks models à la Bakamjian and Thomas, that give a transparent physical interpretation of the latter as due, not to a $L \cdot S$ force, but to the Wigner rotation of the light quark spin. We point out moreover that the selection rule for decay constants of $j = {3 \over 2}$ states, $f_{3/2} = 0$, predicts, assuming the model of factorization, the opposite hierarchy $Γ[ \bar{B} \to \bar{D}_{s_{1,2}} ({3 \over 2}) D^{(*)} ] \ll Γ[ \bar{B} \to \bar{D}_{s_{0,1}} ({1 \over 2}) D^{(*)} ]$.

hep-ph↗
arXiv2001

The APENEXT project

APENEXT is a new generation APE processor, optimized for LGT simulations. The project follows the basic ideas of previous APE machines and develops simple and cheap parallel systems with multi T-Flops processing power. This paper describes the main features of this new development.

hep-lat↗
arXiv2001

The Pion Light-Cone Wave Function Phi_pi on the lattice: a partonic signal?

We determine the conditions required to study the pion light-cone wave function Phi_pi with a new method: a direct display of the partons constituting the pion. We present the preliminary results of a lattice computation of Phi_pi following this direction. An auxiliary scalar-quark is introduced. The spectroscopy of its bound states is studied. We observe some indications of a partonic behavior of the system of this scalar-quark and the anti-quark.

hep-lat↗
arXiv2001

Status of APEmille

This paper presents the status of the APEmille project, which is essentially completed, as far as machine development and construction is concerned. Several large installations of APEmille are in use for physics production runs leading to many new results presented at this conference. This paper briefly summarizes the APEmille architecture, reviews the status of the installations and presents some performance figures for physics codes.

hep-lat↗
arXiv2001

Unquenched calculation of alpha_s from Green functions: progress report

We present preliminary results on the computation of the QCD running coupling constant in the MOM_tilde scheme and Landau gauge with two flavours of dynamical Wilson quarks. Gluon momenta range up to about 7 GeV (beta=5.6, 5.8 and 6.0) with a constant dynamical quark mass. This range already allows to exhibit some evidence for a sizeable 1/mu^2 correction to the asymptotic behaviour, as in the quenched approximation. We find Lambda_MS^(N_f=2) = 264(27) MeV x [a^{-1}(5.6,0.1560)/(2.19 GeV)], which leads to alpha_s(M_Z) = 0.113(3)(4). In view of the systematics error to be controlled, this encouraging result is more a preliminary indication than a real prediction.

hep-ph↗
arXiv2001

Duality in semileptonic inclusive B-decays in potential models: regular versus singular potentials

Making use of the nonrelativistic potential model for the description of mesons, and working in the Shifman-Voloshin limit, we compare the integrated rate $Γ(B\to X_clν)$ calculated as a sum of the individual decay rates to the quantum-mechanical analog of the OPE. In the case of a potential regular at the origin, we find a well-defined duality violation, which is however exponentially small. It corresponds to the charm resonances kinematically forbidden in the decay process, but apparently picked up by the OPE. For singular potentials, we do not obtain a full OPE series, but only a limited Taylor expansion, since the cofficients become infinite beyond some order. In this case, we do not find an indication of duality violation: the difference is smaller than the last term of the limited expansion. This emphasizes that the case of singular potentials, which may be relevant for QCD, deserves further study.

hep-ph↗
arXiv2001

Asymptotic behaviour of the gluon propagator from lattice QCD

We study the flavorless gluon propagator in the Landau gauge from high statistics lattice calculations. Hypercubic artifacts are efficiently eliminated by taking the $\sum p_μ^4 \to 0$ limit. The propagator is fitted to the three-loops perturbative formula in an energy window ranging form $\sim$ 2.5 GeV up to $\sim$ 5.5 GeV. $α_s$ is extracted from the best fit in a continuous set of renormalisation schemes. The fits are very good, with a $χ^2$ per d.o.f smaller than 1. We propose a more stringent test of asymptotic scaling based on scheme independence of the resulting $Λ_{\bar MS}$. This method shows that asymptotic scaling at three loops is not reached by the gluon propagator although we use rather large energies. We are only able to obtain an effective flavorless three-loops estimate $Λ_{\bar MS}^{(3)}= 353 \pm 2 ^{+25}_{-10} $ MeV. We argue that the real asymptotic value for $Λ_{\bar MS}$ should plausibly be smaller.

hep-ph↗
arXiv2000

Semileptonic inclusive heavy meson decay: duality in a nonrelativistic potential model in the Shifman-Voloshin limit

Quark-hadron duality in the inclusive semileptonic decay $B\to X_c lν$ in the Shifman-Voloshin limit $Λ\ll δm=m_b - m_c \ll m_b, m_c$ is studied within a nonrelativistic potential model. The integrated semileptonic decay rate is calculated in two ways: first, by constructing the Operator Product Expansion, and second by a direct summation of the exclusive channels. Sum rules (Bjorken, Voloshin, etc.) for the potential model are derived, providing a possibility to compare the two representations for $Γ(B\to X_c lν)$. An explicit difference between them referred to as duality-violation effect is found. The origin of this effect is related to higher charm resonances which are kinematically forbidden in the decay process but are nevertheless picked up by the OPE. Within the considered $1/m_c^2$ order the OPE and the sum over exclusive channels match each other, up to the contributions of higher resonances, by virtue of the sum rules. In particular this is true for the terms of order $δm^2/m_c^2$ and $Λδm/m_c^2$ which are present in each of the decay channels and cancel in the sum of these channels due to the Bjorken and Voloshin sum rules, respectively. The size of the duality violation effects is estimated to be of the order $O(Λ^{2+b}/m_c^2δm^b)$ with $b>0$ depending on the details of the potential. Constraints for a better accuracy are discussed.

hep-ph↗
arXiv1999

Asymptotic scaling of the gluon propagtor on the lattice

We pursue the study of the high energy behaviour of the gluon propagator on the lattice in the Landau gauge in the flavorless case (n_f=0). It was shown in a precedin g paper that the gluon propagator did not reach three-loop asymptotic scaling at an energy scale as high as 5 GeV. Our present high statistics analysis includes also a simulation at $β=6.8$ ($a\simeq 0.03$ fm), which allows to reach $μ\simeq 10$ GeV. Special care has been devoted to the finite lattice-spacing artifacts as well as to the finite volume effects, the latter being acute at $β=6.8$ where the volume is bounded by technical limits. Our main conclusion is a strong evidence that the gluon propagator has reached three-loop asymptotic scaling, at $μ$ ranging from 5.6 GeV to 9.5 GeV. We buttress up this conclusion on several demanding criteria of asymptoticity, including scheme independence. Our fit in the 5.6 GeV to 9.5 GeV window yields $Λ^{\bar{\rm MS}} = 319 \pm 14 ^{+10}_{-20}$ MeV, in good agreement with our previous result, $Λ^{\bar{\rm MS}} = 295 \pm 20 $ MeV, obtained from the three gluon vertex, but it is significantly above the Schrödinger functional method estimate : $238 \pm 19$ MeV. The latter difference is not understood. Confirming our previous paper, we show that a fourth loop is necessary to fit the whole ($2.8 ÷9.5$) GeV energy window.

hep-ph↗
arXiv1999

Gluon propagator, triple gluon vertex and the QCD coupling constant

We study the UV-scaling of the flavorless gluon propagator in the Landau gauge in an energy window up to 9 GeV. Dominant hypercubic lattice artifacts are eliminated. A large set of renormalization schemes is used to test asymptotic scaling. We compare with our results obtained directly from the triple gluon vertex. We end-up with Λ_{\bar{\rm{MS}}} = 318(12)(5) MeV and 292(5)(15) MeV respectively for these two methods, compatible which each other but significantly above the Schrodinger method estimate.

hep-lat↗
arXiv1999

Inclusive decay of B mesons into $D_s$ or $D^*_s$

We compute the inclusive decay rates $b \to D_s^-(D^{*-}_s) c$ including lowest order QCD corrections on the quark legs, and compare with existing data. Unlike the short distance QCD corrections, that are of higher order, these corrections are of order $α_s$. In the on-shell renormalization scheme and for $α_s(m_b) \cong 0.2$ we find a correction of -10% to the inclusive rate computed using factorization. This gives a total rate $BR(b \to D_s^-(D^{*-}_s)c) \cong 8%$ consistent within $1σ$ with the measured value $BR(B \to D_s^{\pm}X) = (10.0 \pm 2.5)%$. The general formulae given here include the case of vanishing mass for the final quark $b \to D_s^-(D_s^{*-})u$. The radiative correction to this rate is -17%. We show in another place that this process can be useful for the measurement of the CKM matrix element $V_{ub}$. We also give the renormalized vertex at the interesting values $q^2 = 0$ and $q^2 = q_{max}^2$, and compare with existing literature.

hep-ph↗
arXiv1999

Measuring $|V_{ub}|$ with B$\to D_s^+ X_u$ transitions

We propose the determination of the CKM matrix element $|V_{ub}|$ by the measurement of the spectrum of $B \to D_s^+ X_u$, dominated by the spectator quark model mechanism $\bar{b} \to D_s^{(*)+} \bar{u}$. The interest of considering $B \to D_s^+X_u$ versus the semileptonic decay is that more than 50 % of the spectrum for $B \to D_s^+ X_u$ occurs above the kinematical limit for $B \to D_s^+ X_c$, while most of the spectrum $B \to l νX_u$ occurs below the $B \to l νX_c$ one. Furthermore, the measure of the hadronic mass $M_X$ is easier in the presence of an identified $D_s$ than when a $ν$ has been produced. As a consistency check, we point out that the rate $\bar{b} \to D_s^{(*)+} \bar{c}$ (including QCD corrections that we present elsewhere) is consistent with the measured $BR (B \to D_s^{\pm} X)$. Although the hadronic complications may be more severe in the mode that we propose than in the semileptonic inclusive decay, the end of the spectrum in $B \to l νX_u$ is not well understood on theoretical grounds. We argue that, in our case, the excited $D_s^{**}$, decaying into $D K$, do not contribute and, if there is tagging of the $B$ meson, the other mechanisms to produce a $D_s$ of the right sign are presumably small, of $O(10^{-2})$ relative to the spectator amplitude, or can be controlled by kinematical cuts. In the absence of tagging, other hadronic backgrounds deserve careful study. We present a feasability study with the BaBar detector.

hep-ph↗
arXiv1999

Three-loop beta function and non-perturbative $α_s$ in asymmetric momentum scheme

We determine the three-loop coefficient of the beta function in the asymmetric momentum subtraction scheme in Landau gauge. This scheme is convenient for lattice studies of α_s, the running coupling constant of QCD. We present high statistics lattice results for α_s in the SU(3) Yang-Mills theory without quark, compare with the three-loop running and extract the value of the corresponding Λ_\msbar parameter. We estimate the systematic error coming from four-loop terms. We obtain the result: Λ_\msbar = 295 (5) (15) {a^{-1}(β=6.0) \over 1.97 GeV} MeV.

hep-ph↗
arXiv1998

Addendum to Finite-size effects on multibody neutrino exchange

The interaction energy of the neutrons due to massless neutrino exchange in a neutron star has recently been proved, using an effective theory, to be extremely small and infrared-safe. Our comment here is of conceptual order: two approaches to compute the total interaction energy density have recently been proposed. Here, we study the connection between these two approaches. From CP invariance, we argue that the resulting interaction energy has to be even in the parameter $b=-G_F n_n /\sqrt{2}$, which expresses the static neutrino potential created by a neutron medium of density $n_n$.

hep-ph↗