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Matthias Jamin

Publications and source records attributed to Matthias Jamin.

At least 37 records · Page 2Linked to original sources
arXiv2011

On the Extraction of the Strong Coupling from Hadronic Tau Decay

The finite energy sum rule extraction of the strong coupling α_s from hadronic τ decay data provides one of its most precise experimental determinations. As precision improves, small non-perturbative effects become increasingly relevant to both the central value and error. Here we present a new framework for this extraction employing a physically motivated model to accommodate violations of quark-hadron duality and enforcing a consistent treatment of higher-dimension operator product expansion contributions. Using 1998 OPAL data for the non-strange vector and axial-vector spectral functions, we find the n_f=3 values for α_s(m_τ^2) of 0.307(19) for fixed-order perturbation theory and 0.322(26) for contour-improved perturbation theory, corresponding to n_f=5 values for α_s(M_Z^2) of 0.1169(25) and 0.1187(32), respectively.

hep-lat↗
arXiv2011

A new determination of α_s from hadronic τ decays

We present a new framework for the extraction of the strong coupling from hadronic τdecays through finite-energy sum rules. Our focus is on the small, but still significant non-perturbative effects that, in principle, affect both the central value and the systematic error. We employ a quantitative model in order to accommodate violations of quark-hadron duality, and enforce a consistent treatment of the higher-dimensional contributions of the Operator Product Expansion to our sum rules. Using 1998 OPAL data for the non-strange isovector vector and axial-vector spectral functions, we find the n_f=3 values α_s(m_τ^2)=0.307+-0.019 in fixed-order perturbation theory, and 0.322+-0.026 in contour-improved perturbation theory. For comparison, the original OPAL analysis of the same data led to the values 0.324+-0.014 (fixed-order) and 0.348+-0.021 (contour-improved).

hep-ph↗
arXiv2011

What two models may teach us about duality violations in QCD

Though the operator product expansion is applicable in the calculation of current correlation functions in the Euclidean region, when approaching the Minkowskian domain, violations of quark-hadron duality are expected to occur, due to the presence of bound-state or resonance poles. In QCD finite-energy sum rules, contour integrals in the complex energy plane down to the Minkowskian axis have to be performed, and thus the question arises what the impact of duality violations may be. The structure and possible relevance of duality violations is investigated on the basis of two models: the Coulomb system and a model for light-quark correlators which has already been studied previously. As might yet be naively expected, duality violations are in some sense "maximal" for zero-width bound states and they become weaker for broader resonances whose poles lie further away from the physical axis. Furthermore, to a certain extent, they can be suppressed by choosing appropriate weight functions in the finite-energy sum rules. A simplified Ansatz for including effects of duality violations in phenomenological QCD sum rule analyses is discussed as well.

hep-ph↗
arXiv2011

The Kpi vector form factor and constraints from Kl3 decays

The slope and curvature parameters of the $Kπ$ vector form factor, $F_+^{Kπ}$, are fitted to the data on $\tauKpi$ and $K_{l3}$ decays yielding $λ_+'=(25.49 \pm 0.31) \times 10^{-3}$ and $λ_+"= (12.22 \pm 0.14) \times 10^{-4}$. The pole position of the $K^*(892)^\pm$ is found to be at $m_{K^*(892)^\pm}= 892.0\pm 0.5$ MeV and $Γ_{K^*(892)^\pm}= 46.5 \pm1.1$ MeV. The phase-space integrals relevant for $K_{l3}$ analyses and the $P$-wave isospin-1/2 $Kπ$ phase-shift threshold parameters are also calculated.

hep-ph↗
arXiv2011

Recent progress in hadronic tau decays

The determination of alpha_s from hadronic tau decays is impeded by the fact that two choices for the renormalisation group resummation, namely fixed-order (FOPT) and contour-improved perturbation theory (CIPT), yield systematically differing results. On the basis of a model for higher-order terms in the perturbative series, which incorporates well-known structure from renormalons, it is found that FOPT smoothly approaches the Borel sum for the tau hadronic width, while CIPT is unable to account for the resummed series. An example for the behaviour of QCD spectral function moments, displaying a similar behaviour, is presented as well.

hep-ph↗
arXiv2010

Improving the Kpi vector form factor through Kl3 constraints

The $Kπ$ vector form factor, $F_+^{Kπ}$, used to reproduce the Belle spectrum of $\tauKpi$ decays is described by means of a three-times subtracted dispersion relation also incorporating constraints from $K_{l3}$ decays. The slope and curvature of $F_+^{Kπ}$ are fitted to the data yielding $λ_+'=(25.49 \pm 0.31) \times 10^{-3}$ and $λ_+"= (12.22 \pm 0.14) \times 10^{-4}$. The pole parameters of the $K^*(892)^\pm$ are found to be $m_{K^*(892)^\pm}= 892.0\pm 0.5$ MeV and $Γ_{K^*(892)^\pm}= 46.5 \pm1.1$ MeV. The phase-space integrals relevant for $K_{l3}$ analyses and the $P$-wave isospin-1/2 $Kπ$ phase-shift threshold parameters are also calculated.

hep-ph↗
arXiv2010

Constraining the Kpi vector form factor by tau---> K pi nu_tau and K_l3 decay data

A subtracted dispersive representation of the $Kπ$ vector form factor, $F_+^{Kπ}$, is used to fit the Belle spectrum of $\tauKpi$ decays incorporating constraints from results on $K_{l3}$ decays. Through the use of three subtractions, the slope and curvature of $F_+^{Kπ}$ are obtained directly from the data yielding $λ_+'=(25.49 \pm 0.31) \times 10^{-3}$ and $λ_+"= (12.22 \pm 0.14) \times 10^{-4}$. The phase-space integrals relevant for $K_{l3}$ analyses are calculated. Additionally, from the pole position on the second Riemann sheet the mass and width of the $K^*(892)^\pm$ are found to be $m_{K^*(892)^\pm}= 892.0\pm 0.5$ MeV and $Γ_{K^*(892)^\pm}= 46.5 \pm 1.1$ MeV. Finally, we study the $P$-wave isospin-1/2 $Kπ$ phase-shift and its threshold parameters.

hep-ph↗
arXiv2009

Dispersive representation of the K pi vector form factor and fits to tau -> K pi nu(tau) and Ke3 data

Recently, we introduced several dispersive representations for the vector $Kπ$ form factor and fitted them to the Belle spectrum of $τ\to K πν_τ$. Here, we briefly present the model and discuss the results for the slope and curvature of $F_+(s)$ arising from the best fit. Furthermore, we compare the pole position of the charged $K^*(892)$ computed from our model with other results in the literature. Finally, we discuss the prospects of a simultaneous fit to $τ\to K πν_τ$ and $K_{e3}$ spectra.

hep-ph↗
arXiv2009

alpha_s and the tau hadronic width

Different choices exist for the renormalisation group resummation in the determination of $α_s$ from hadronic $τ$ decays: namely fixed-order (FOPT) and contour-improved perturbation theory (CIPT). The two approaches lead to systematic differences in the resulting $α_s$. On the basis of a model for higher-order terms in the perturbative series, which incorporates well-known structure from renormalons, it is found that while CIPT is unable to account for the fully resummed series, FOPT smoothly approaches the Borel sum. Employing the model to determine $α_s$ yields $α_s(M_τ)=0.316 \pm 0.006$, which after evolution leads to $α_s(M_Z) = 0.1180 \pm 0.0008$.

hep-ph↗
arXiv2008

K pi vector form factor, dispersive constraints and tau -> nu_tau K pi decays

Recent experimental data for the differential decay distribution of the decay $τ^-\toν_τK_Sπ^-$ by the Belle collaboration are described by a theoretical model which is composed of the contributing vector and scalar form factors $F_+^{Kπ}(s)$ and $F_0^{Kπ}(s)$. Both form factors are constructed such that they fulfil constraints posed by analyticity and unitarity. A good description of the experimental measurement is achieved by incorporating two vector resonances and working with a three-times subtracted dispersion relation in order to suppress higher-energy contributions. The resonance parameters of the charged $K^*(892)$ meson, defined as the pole of $F_+^{Kπ}(s)$ in the complex $s$-plane, can be extracted, with the result $M_{K^*}=892.0 \pm 0.9 $MeV and $Γ_{K^*}=46.2 \pm 0.4 $MeV. Finally, employing the three-subtracted dispersion relation allows to determine the slope and curvature parameters $λ_+^{'}=(24.7\pm 0.8)\cdot 10^{-3}$ and $λ_+^{''}=(12.0\pm 0.2)\cdot 10^{-4}$ of the vector form factor $F_+^{Kπ}(s)$ directly from the data.

hep-ph↗
arXiv2008

alpha_s and the tau hadronic width: fixed-order, contour-improved and higher-order perturbation theory

The determination of $α_s$ from hadronic $τ$ decays is revisited, with a special emphasis on the question of higher-order perturbative corrections and different possibilities of resumming the perturbative series with the renormalisation group: fixed-order (FOPT) vs. contour-improved perturbation theory (CIPT). The difference between these approaches has evolved into a systematic effect that does not go away as higher orders in the perturbative expansion are added. We attempt to clarify under which circumstances one or the other approach provides a better approximation to the true result. To this end, we propose to describe the Adler function series by a model that includes the exactly known coefficients and theoretical constraints on the large-order behaviour originating from the operator product expansion and the renormalisation group. Within this framework we find that while CIPT is unable to account for the fully resummed series, FOPT smoothly approaches the Borel sum, before the expected divergent behaviour sets in at even higher orders. Employing FOPT up to the fifth order to determine $α_s$ in the $\MSb$ scheme, we obtain $α_s(M_τ)=0.320 {}^{+0.012}_{-0.007}$, corresponding to $α_s(M_Z) = 0.1185 {}^{+0.0014}_{-0.0009}$. Improving this result by including yet higher orders from our model yields $α_s(M_τ)=0.316 \pm 0.006$, which after evolution leads to $α_s(M_Z) = 0.1180 \pm 0.0008$. Our results are lower than previous values obtained from $τ$ decays.

hep-ph↗
arXiv2008

OPE-RchiT matching at order alpha_s: hard gluonic corrections to three-point Green functions

In this work we push the matching between the QCD operator product expansion (OPE) and resonance chiral theory (RchiT) to order alpha_s. To this end we compute two- and three-point QCD Green functions (GFs) in both theories and compare the results. GFs which are order parameters of chiral symmetry breaking make this matching more transparent and thus we concentrate on those. On the OPE side one needs to calculate the hard-gluon virtual corrections to the quark condensate, and in particular for three-point GFs this computation was hitherto missing. We also discuss the need for including the infinite tower of hadronic states in the hadronic representation of the GF when non-analytic terms such as logarithms are present in the OPE Wilson coefficients.

hep-ph↗
arXiv2008

What can be learned from the Belle spectrum for the decay tau- -> nu_tau K_S pi-

A theoretical description of the differential decay spectrum for the decay tau- -> nu_tau K_S pi-, which is based on the contributing K pi vector and scalar form factors F_+^{K pi}(s) and F_0^{K pi}(s) being calculated in the framework of resonance chiral theory (R$χ$T), additionally imposing constraints from dispersion relations as well as short distance QCD, provides a good representation of a recent measurement of the spectrum by the Belle collaboration. Our fit allows to deduce the total branching fraction B[tau- -> nu_tau K_S pi-] = 0.427 +- 0.024 % by integrating the spectrum, as well as the K^* resonance parameters M_{K^*} = 895.3 +- 0.2 MeV and Gamma_{K^*} = 47.5 +- 0.4 MeV, where the last two errors are statistical only. From our fits, we confirm that the scalar form factor F_0^{K pi}(s) is required to provide a good description, but we were unable to further constrain this contribution. Finally, from our results for the vector form factor F_+^{K pi}(s), we update the corresponding slope and curvature parameters lambda'_+ = (25.2 +- 0.3)*10^{-3} and lambda''_+ = (12.9 +- 0.3)*10^{-4}, respectively.

hep-ph↗
arXiv2007

Theoretical progress on the V_us determination from tau decays

A very precise determination of V_us can be obtained from the semi-inclusive hadronic decay width of the tau lepton into final states with strangeness. The ratio of the Cabibbo-suppressed and Cabibbo-allowed tau decay widths directly measures (V_us/V_ud)^2, up to very small SU(3)-breaking corrections which can be theoretically estimated with the needed accuracy. Together with previous LEP and CLEO data, the recent measurements by Babar and Belle of some Cabibbo-suppressed tau decays imply V_us= 0.2165 +- 0.0026_exp +- 0.0005_th, which is already competitive with the standard extraction from K_l3 decays. The uncertainty is largely dominated by experimental errors and should be easily reduced with the high statistics of the B factories, providing the most accurate determination of this parameter. A 1% experimental precision on the Cabibbo-suppressed tau decay width would translate into a 0.6% uncertainty on V_us.

hep-ph↗
arXiv2006

|V_us| and m_s from hadronic tau decays

Recent progress in the determination of |V_us| employing strange hadronic tau-decay data are reported. This includes using the recent OPAL update of the strange spectral function, as well as augmenting the dimension-two perturbative contribution with the recently calculated order alpha_s^3 term on the theory side. These updates result in |V_us| = 0.2220 +- 0.0033, with the uncertainty presently being dominated by experiment, and already being competitive with the standard extraction from K_e3 decays and other new proposals to determine |V_us|. In view of the ongoing work to analyse tau-decay data at the B-factories BaBar and Belle, as well as future results from BESIII, the error on |V_us| from tau decays is expected to be much reduced in the near future.

hep-ph↗
arXiv2006

|V_us| from Strange Hadronic Tau Data

We report on recent work to determine the CKM matrix element |V_{us}| using strange hadronic τdecay data. We use the recent OPAL update of the strange spectral function, while on the theory side we update the dimension-two perturbative contribution including the recently calculated α_s^3 terms. Our result |V_{us}|=0.2220 \pm 0.0033 is already competitive with the standard extraction from K_{e3} decays and other new proposals to determine |V_{us}|. The actual uncertainty on |V_{us}| from τdata is dominated largely by experiment and will eventually be much reduced by B-factories and future τ-charm factory data, providing one of the most accurate determinations of this Standard Model parameter.

hep-ph↗
arXiv2005

Contour-improved versus fixed-order perturbation theory in hadronic tau decays

The hadronic decay rate of the tau lepton serves as one of the most precise determinations of the QCD coupling alpha_s. The dominant theoretical source of uncertainty at present resides in the seeming disparity of two approaches to improving the perturbative expansion with the help of the renormalisation group, namely fixed-order and contour-improved perturbation theory. In this work it is demonstrated that in fact both approaches yield compatible results. However, the fixed-order series is found to oscillate around the contour-improved result with an oscillation frequency of approximately six perturbative orders, approaching it until about the 30th order, after which the expansion reveals its asymptotic nature. Additionally, the renormalisation scale and scheme dependencies of the perturbative series for the tau hadronic width are investigated in detail.

hep-ph↗