arXiv subjects

Diogo Boito

Publications and source records attributed to Diogo Boito.

At least 55 records · Page 3Linked to original sources

arXiv2016

The status of the strong coupling from tau decays in 2016

While the idea of using the operator product expansion (OPE) to extract the strong coupling from hadronic $τ$ decay data is not new, there is an ongoing controversy over how to include quark-hadron "duality violations" (i.e., resonance effects) which are not described by the OPE. One approach attempts to suppress duality violations enough that they might become negligible, but pays the price of an uncontrolled OPE truncation. We critically examine a recent analysis using this approach and show that it fails to properly account for non-perturbative effects, making the resulting determination of the strong coupling unreliable. In a different approach duality violations are taken into account with a model, avoiding the OPE truncation. This second approach provides a self-consistent determination of the strong coupling from $τ$ decays.

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arXiv2016

$α_s$ in 2016 from the (revised) ALEPH data for $τ$ decay

We summarize a comparison of the two strategies which are currently available in the literature for determining the value of $α_s(m_τ)$. We will refer to these as the truncated Operator Product Expansion model and the Duality Violation model. After describing the main features of both approaches, we explain why the former fails to pass crucial tests. The latter, on the other hand, passes all the tests known up to date and, therefore, should be currently considered the only reliable method.

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arXiv2016

Scheme variations of the QCD coupling

The Quantum Chromodynamics (QCD) coupling $α_s$ is a central parameter in the Standard Model of particle physics. However, it depends on theoretical conventions related to renormalisation and hence is not an observable quantity. In order to capture this dependence in a transparent way, a novel definition of the QCD coupling, denoted by $\hat a$, is introduced, whose running is explicitly renormalisation scheme invariant. The remaining renormalisation scheme dependence is related to transformations of the QCD scale $Λ$, and can be parametrised by a single parameter $C$. Hence, we call $\hat a$ the $C$-scheme coupling. The dependence on $C$ can be exploited to study and improve perturbative predictions of physical observables. This is demonstrated for the QCD Adler function and hadronic decays of the $τ$ lepton.

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arXiv2016

The strong coupling from hadronic $τ$ decays: a critical appraisal

Several different analysis methods have been developed to determine the strong coupling via finite-energy sum-rule analyses of hadronic $τ$ decay data. While most methods agree on the existence of the well-known ambiguity in the choice of a resummation scheme due to the slow convergence of QCD perturbation theory at the $τ$ mass, there is an ongoing controversy over how to deal properly with non-perturbative effects. These are small, but not negligible, and include quark-hadron "duality violations" (i.e., resonance effects) which are not described by the operator product expansion (OPE). In one approach, an attempt is made to suppress duality violations enough that they might become negligible. The number of OPE parameters to be fit, however, then exceeds the number of available sum rules, necessitating an uncontrolled OPE truncation, in which a number of higher-dimension OPE contributions in general present in QCD are set to zero by hand. In the second approach, truncation of the OPE is avoided by construction, and duality violations are taken into account explicitly, using a physically motivated model. In this article, we provide a critical appraisal of a recent analysis employing the first approach and demonstrate that it fails to properly account for non-perturbative effects, making the resulting determination of the strong coupling unreliable. The second approach, in contrast, passes all self-consistency tests, and provides a competitive determination of the strong coupling from $τ$ decays.

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arXiv2016

Scheme variations of the QCD coupling and hadronic $τ$ decays

The Quantum Chromodynamics (QCD) coupling, $α_s$, is not a physical observable of the theory since it depends on conventions related to the renormalization procedure. We introduce a definition of the QCD coupling, denoted by $\hatα_s$, whose running is explicitly renormalization scheme invariant. The scheme dependence of the new coupling $\hatα_s$ is parameterized by a single parameter $C$, related to transformations of the QCD scale $Λ$. It is demonstrated that appropriate choices of $C$ can lead to substantial improvements in the perturbative prediction of physical observables. As phenomenological applications, we study $e^+e^-$ scattering and decays of the $τ$ lepton into hadrons, both being governed by the QCD Adler function.

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arXiv2016

$α_s$ analyses from hadronic tau decays with OPAL and ALEPH data

Recently, we extracted the strong coupling, $α_s(m_τ^2)$, from the revised ALEPH data for non-strange hadronic tau decays. Our analysis is based on a method previously used for the determination of the strong coupling from OPAL data. In our strategy, we employ different moments of the spectral functions both with and without pinching, including Duality Violations, in order to obtain fully self-consistent analyses that do not rely on untested assumptions (such as the smallness of higher dimension contributions in the OPE). Here we discuss the $α_s$ values obtained from the ALEPH and the OPAL data, the robustness of the analysis, as well as non-perturbative contributions from DVs and the OPE. We show that, although the $α_s$ determination is sound, non-perturbative effects limit the accuracy with which one can extract the strong coupling from tau decay data. Finally, we discuss the compatibility of the data sets and the possibility of a combined analysis.

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arXiv2016

The case for duality violations in the analysis of hadronic $τ$ decays

We discuss why, in the determination of $α_s(m_τ^2)$ from hadronic $τ$ decays, two important assumptions made in most of previous analyses, namely the neglect of higher-dimension condensates and of Duality Violations (DVs), have introduced uncontrolled systematic errors into this determination. Although the use of pinched weights is usually offered as a justification of these assumptions, we explain why it is not possible to simultaneously suppress these two contributions; particularly since the Operator Product Expansion is expected to be an asymptotic, rather than a convergent expansion. There is not only experimental and theoretical evidence for DVs, but they also affect the extraction of $α_s(m_τ^2)$.

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arXiv2016

Summary of the 2015 LHCb workshop on multi-body decays of D and B mesons

This document contains a summary of the LHCb workshop on multi-body decays of D and B mesons, held at CBPF, Rio de Janeiro, in July 2015. The workshop was focused on issues related to amplitude analysis of three- and four-body hadronic decays. In addition to selected LHCb results, contributions from guest theorists are included.

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arXiv2015

Anomalous dimensions of four-quark operators and renormalon structure of mesonic two-point correlators

In this work, we calculate leading-order anomalous dimension matrices for dimension-6 four-quark operators which appear in the operator product expansion of flavour non-diagonal and diagonal vector and axial-vector two-point correlation functions. The infrared renormalon structure corresponding to four-quark operators is reviewed and it is investigated how the eigenvalues of the anomalous dimension matrices influence the singular behaviour of the $u=3$ infrared renormalon pole. It is found that compared to the large-$β_0$ approximation where at most quadratic poles are present, in full QCD at $N_f=3$ the most singular pole is more than cubic with an exponent $κ\approx 3.2$.

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arXiv2015

Low-energy constants from ALEPH hadronic tau decay data

We determined the NLO chiral low-energy constant $L_{10}$, and various combinations of NNLO chiral low-energy constants employing recently revised ALEPH results for the non-strange vector (V) and axial-vector (A) hadronic tau decay distributions and recently updated RBC/UKQCD lattice data for the non-strange V-A two-point function. In this talk, we explain the ingredients of this determination. Our errors are at or below the level expected for contributions of yet higher order in the chiral expansion, suggesting that our results exhaust the possibilities of what can be meaningfully achieved in an NNLO analysis.

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arXiv2015

$α_s$ from the updated ALEPH data for hadronic $τ$ decays

We extract the strong coupling $α_s(m_τ^2)$ from the recently updated ALEPH non-strange spectral functions obtained from hadronic $τ$ decays. We apply a self-consistent analysis method, first tested in the analysis of OPAL data, to extract $α_s(m_τ^2)$ and non-perturbative contributions. The analysis yields $α_s^{\rm FO}(m_τ^2)=0.296\pm0.010 $, using Fixed Order Perturbation Theory (FOPT), and $α^{\rm CI}_s(m_τ^2)= 0.310\pm0.014$, using Contour Improved Perturbation Theory (CIPT). The weighted average of these results with those previously obtained from OPAL data give $α_s^{\rm FO}(m_τ^2)=0.303\pm 0.009$ and $α_s^{\rm CI}(m_τ^2)=0.319\pm 0.012$, which gives, after evolution to the $Z$ boson mass scale, $α^{\rm FO}_s(m_Z^2)=0.1165\pm0.0012 $ and $α_s^{\rm CI}(m_Z^2)=0.1185\pm0.0015 $, respectively. We observe that non-perturbative effects limit the accuracy with which $α_s$ can be extracted from $τ$ decay data.

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arXiv2015

Low-energy constants and condensates from ALEPH hadronic $τ$ decay data

We determine the NLO chiral low-energy constant $L_{10}^r$, and combinations $C_{12}^r\pm C_{61}^r+C_{80}^r$, $C_{13}^r-C_{62}^r+C_{81}^r$, $C_{61}^r$, and $C_{87}^r$, of the NNLO chiral low-energy constants incorporating recently revised ALEPH results for the non-strange vector ($V$) and axial-vector ($A$) hadronic $τ$ decay distributions and recently updated RBC/UKQCD lattice data for the non-strange $V-A$ two-point function. In the $\bar{\rm MS}$ scheme, at renormalization scale $μ=770\ {MeV}$, we find $L_{10}^r=-0.00350(17)$, $C_{12}^r+C_{61}^r+C_{80}^r=0.00237(16)\ {GeV}^{-2}$, $C_{12}^r-C_{61}^r+C_{80}^r=-0.00056(15)\ {GeV}^{-2}$, $C_{13}^r-C_{62}^r+C_{81}^r=0.00046(9)\ {GeV}^{-2}$, $C_{61}^r=0.00146(15)\ {GeV}^{-2}$, and $C_{87}^r=0.00510(22)\ {GeV}^{-2}$. With errors here at or below the level expected for contributions of yet higher order in the chiral expansion, the analysis exhausts the possibilities of what can be meaningfully achieved in an NNLO analysis. We also consider the dimension six and dimension eight coefficients in the operator product expansion in the $V-A$ channel.

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arXiv2015

The strong coupling from the revised ALEPH data for hadronic $τ$ decays

We apply an analysis method previously developed for the extraction of the strong coupling from the OPAL data to the recently revised ALEPH data for non-strange hadronic $τ$ decays. Our analysis yields the values $α_s(m_τ^2)=0.296\pm 0.010$ using fixed-order perturbation theory, and $α_s(m_τ^2)=0.310\pm 0.014$ using contour-improved perturbation theory. Averaging these values with our previously obtained values from the OPAL data, we find $α_s(m_τ^2)=0.303\pm 0.009$, respectively, $α_s(m_τ^2)=0.319\pm 0.012$. We present a critique of the analysis method employed previously, for example in analyses by the ALEPH and OPAL collaborations, and compare it with our own approach. Our conclusion is that non-perturbative effects limit the accuracy with which the strong coupling, an inherently perturbative quantity, can be extracted at energies as low as the $τ$ mass. Our results further indicate that systematic errors on the determination of the strong coupling from analyses of hadronic $τ$-decay data have been underestimated in much of the existing literature.

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arXiv2014

Signatures of anomalous Higgs couplings in angular asymmetries of H --> Zl+l- and e+e- --> HZ

Parametrizing beyond Standard Model physics by the SU(3) x SU(2)_L x U(1)_Y dimension-six effective Lagrangian, we study the impact of anomalous Higgs couplings in angular asymmetries of the crossing-symmetric processes H --> Zl+l- and e+e- --> HZ. In the light of present bounds on d=6 couplings, we show that some asymmetries can reveal BSM effects that would otherwise be hidden in other observables. The d=6 HZgamma couplings as well as (to a lesser extent) HZll contact interactions can generate asymmetries at the several percent level, albeit having less significant effects on the di-lepton invariant mass spectrum of the decay H --> Zl+l-. The higher di-lepton invariant mass probed in e+e- --> HZ can lead to complementary anomalous coupling searches at e+e- colliders.

hep-ph↗

arXiv2014

Anomalous Higgs couplings in angular asymmetries of H --> Zl+l- and e+e- --> HZ

We study in detail the impact of anomalous Higgs couplings in angular asymmetries of the crossing-symmetric processes H --> Zl+l- and e+e- --> HZ. Beyond Standard Model physics is parametrized in terms of the SU(3)xSU(2)_LxU(1)_Y dimension-six effective Lagrangian. In the light of present bounds on d = 6 interactions we study how angular asymmetries can reveal non-standard CP-even and CP-odd couplings. We provide approximate expressions to all observables of interest making transparent their dominant dependence on anomalous couplings. We show that some asymmetries may reveal BSM effects that are hidden in other observables. In particular, CP-even and CP-odd d = 6 HZgamma couplings as well as (to a lesser extent) HZll contact interactions can generate asymmetries at the several percent level, while having small or no effects on the di-lepton invariant mass spectrum of H --> Zl+l-. Finally, the higher di-lepton invariant mass probed in e+e- --> HZ leads to interesting differences in the asymmetries with respect to those of H --> Zl+l- that may lead to complementary anomalous coupling searches at the LHC and e+e- colliders.

hep-ph↗

arXiv2014

$α_{s}$ from the (revised) ALEPH data for $τ$ decay

We present a new analysis of $α_s$ from hadronic $τ$ decays based on the recently revised ALEPH data. The analysis is based on a strategy which we previously applied to the OPAL data. We critically compare our strategy to the one traditionally used and comment on the main differences. Our analysis yields the values $α_s(m_τ^2)=0.296\pm 0.010$ using fixed-order perturbation theory, and $α_s(m_τ^2)=0.310\pm 0.014$ using contour-improved perturbation theory. Averaging these values with our previously obtained values from the OPAL data, we find $α_s(m_τ^2)=0.303\pm 0.009$, respectively, $α_s(m_τ^2)=0.319\pm 0.012$, as the most reliable results for $α_s$ from $τ$ decays currently available.

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arXiv2013

Perturbative expansion of tau hadronic spectral function moments

In the extraction of $α_s$ from hadronic tau decay data several moments of the spectral functions have been employed. Furthermore, different renormalization group improvement (RGI) frameworks have been advocated, leading to conflicting values of $α_s$. Recently, we performed a systematic study of the perturbative behavior of these moments in the context of the two main-stream RGI frameworks: Fixed Order Perturbation Theory (FOPT) and Contour Improved Perturbation Theory (CIPT). The yet unknown higher order coefficients of the perturbative series were modelled using the available knowledge of the renormalon singularities of the QCD Adler function. We were able to show that within these RGI frameworks some of the commonly employed moments should be avoided due to their poor perturbative behavior. Furthermore, under reasonable assumptions about the higher order behavior of the perturbative series FOPT provides the preferred RGI framework.

hep-ph↗

arXiv2013

Low-energy constants and condensates from the tau hadronic spectral functions

We use results of fits to the OPAL spectral data, obtained from non-strange hadronic τdecays, to evaluate the difference between the vector and axial current correlators, Π_{V-A}(Q^2). The behavior of Π_{V-A}(Q^2) near euclidean momentum Q^2=0 is used to determine the effective low-energy constants L_10^eff and C_87^eff related to the renormalized low-energy constants L_10^r and C_87^r in the chiral lagrangian. We also investigate how well two-loop chiral perturbation theory describes Π_{V-A}(Q^2) as a function of Q^2. This is the first determination of L_10^eff and C_87^eff to employ a fully self-consistent model for the violations of quark-hadron duality in both the vector and axial channels. We also discuss the values of the coefficients C_{6,V-A} and C_{8,V-A} governing the dimension six and eight contributions to the operator product expansion representation of Π_{V-A}(Q^2).

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