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Kim Maltman

Publications and source records attributed to Kim Maltman.

At least 73 records · Page 4Linked 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

Hadronic Tau Decay Based Determinations of |V_us|

I review sum rule determinations of |V_us| employing hadronic tau decay data, taking into account recent HFAG updates of exclusive tau branching fractions and paying special attention to the impact of the slow convergence of the relevant integrated D=2 OPE series and the potential role of contributions of as-yet-unmeasured higher multiplicity modes to the strange inclusive spectral distribution. In addition to conventional flavor-breaking sum rule determinations, information obtainable from mixed tau-electroproduction sum rules having much reduced OPE uncertainties, and from sum rules based on the inclusive strange decay distribution alone, is also considered. Earlier discrepancies with the expectations of 3-family unitarity are found to be reduced, both the switch to D=2 OPE treatments favored by self-consistency tests and the increase in the strange branching fractions playing a role in this reduction.

hep-ph↗
arXiv2011

A critical look at V_us determinations from hadronic tau decay data

A critical review of hadronic tau decay data based determinations of |V_{us}| is given, focussing on the impact of the slow convergence of the integrated D=2 OPE series for the conventional flavor-breaking sum rule determination and the potential role of as-yet-unmeasured multiparticle contributions to the strange spectral distribution. Additional information obtainable from analyses of inclusive strange decay data alone, and from mixed electroproduction-tau sum rules with much reduced OPE uncertainties, is also discussed. Self-consistency tests are shown to favor determinations which reduce somewhat discrepancies with 3-family unitarity expectations.

hep-ph↗
arXiv2010

Status of the Lattice and Tau Decay Determinations of alpha_s

The two highest precision determinations of alpha_s(M_Z^2), that based on the analysis of short-distance-sensitive lattice observables, and that based on an analysis of hadronic tau decay data, have, until very recently, given results which are not in good agreement. I review new versions of these analyses which bring the two determinations into excellent agreement, and discuss prospects for additional future improvements.

hep-ph↗
arXiv2009

The strong coupling and its running to four loops in a minimal MOM scheme

We introduce the minimal momentum subtraction (MiniMOM) scheme for QCD. Its definition allows the strong coupling to be fixed solely through a determination of the gluon and ghost propagators. In Landau gauge this scheme has been implicit in the early studies of these propagators, especially in relation to their non-perturbative behaviour in the infrared and the associated infrared fixed-point. Here we concentrate on its perturbative use. We give the explicit perturbative definition of the scheme and the relation of its beta-function and running coupling to the MSbar scheme up to 4-loop order in general covariant gauges. We also demonstrate, by considering a selection of N_f=3 examples, that the apparent convergence of the relevant perturbative series can in some (though not all) cases be significantly improved by re-expanding the MSbar coupling version of this series in terms of the MiniMOM coupling, making the MiniMOM coupling also of potential interest in certain phenomenological applications.

hep-ph↗
arXiv2009

Results of the Mixed Tau-Electroproduction Sum Rule For V_us

A sum rule for determining |V_us| from a combination of hadronic tau decay and electroproduction data is discussed. Indications of problems with analogous, purely tau-decay-based analyses, most likely associated with slow convergence of the relevant D=2 OPE series, are outlined, and advantages of the mixed sum rule approach identified. With current data, one obtains |V_us|= 0.2208 (39)_exp (3)_th, a determination amenable to significant near-term reduction in experimental error, and with the smallest theoretical error of all current |V_us| determinations.

hep-ph↗
arXiv2009

alpha_s From the Lattice and Hadronic Tau Decays

Until recently, determinations of alpha_s(M_Z) from hadronic tau decays and the analysis of short-distance-sensitive lattice observables yielded results which, though precise, were not in good agreement. I review new analyses that bring these into good agreement and provide some details on the source of the main changes in the tau decay analysis.

hep-ph↗
arXiv2009

A Mixed Tau-Electroproduction Sum Rule for V_us

The interpretation of results of recent tau decay determinations of |V_us|, which yield values ~3 sigma low compared to 3-family unitarity expectations, is complicated by the slow convergence of the relevant integrated D=2 OPE series. We introduce a class of sum rules involving both electroproduction and tau decay data designed to deal with this problem by strongly suppressing D=2 OPE contributions at the correlator level. Experimental complications are briefly discussed and an example of the improved control over theoretical errors presented. The uncertainty on the resulting determination, |V_{us}| =0.2202(39), is entirely dominated by experimental errors, and should be subject to significant near-term improvement.

hep-ph↗
arXiv2008

Alpha_s from hadronic tau decay data

We discuss the extraction of alpha_s using isovector hadronic tau decay data and sum rules constructed specifically to suppress contributions associated with poorly known higher dimension condensates. We show, first, that problems with the treatment of such contributions affect earlier related analyses and, second, that these problems can be brought under good theoretical control through the use of an alternate analysis strategy. Our results, run up to the n_f=5 regime, correspond to alpha_s(M_Z^2)=0.1187(16), in excellent agreement with the recently updated global fit to electroweak data at the Z scale and other high-scale direct determinations.

hep-ph↗
arXiv2006

OPE Constraints and the Leading Order Hadronic Contribution to (g-2)_mu

OPE constraints are studied as a means of distinguishing between the versions of the I=1 vector spectral function extracted from (i) inclusive I=1 hadronic electroproduction cross-sections and (ii) inclusive I=1 hadronic tau decay data, with the goal of clarifying expectations for the leading order hadronic contribution to a_mu = (g-2)_mu /2 in the Standard Model. The constraints are shown to, at present, favor the tau decay data, and hence a Standard Model prediction for a_mu compatible with the BNL E821 experimental result. The relative role of the 2pi and 4pi contributions to the discrepancy between the integrated electroproduction results and the corresponding OPE constraints is also investigated, and the significance, in this context, of forthcoming re-measurements of the e^+e^- --> pi^+ pi^- pi^0 pi^0 cross-sections pointed out.

hep-ph↗
arXiv2006

V_us From Hadronin Tau Decays

We study the reliability of extractions of |V_{us}| based on flavor-breaking hadronic tau decay sum rules. The ``(0,0) spectral weight'', proposed previously as a favorable candidate for this extraction, is shown to produce results having poor stability with respect to s_0, the upper limit on the relevant spectral integral, suggesting theoretical errors much larger than previously anticipated. We argue that this instability is due to the poor convergence of the integrated D=2 OPE series. Alternate weight choices designed to bring this convergence under better control are shown to produce significantly improved stability, and determinations of |V_{us}| which are both mutually compatible, and consistent, within errors, with values obtained by other methods.

hep-ph↗
arXiv2005

On the Leading ORder Hadronic Contribution to (g-2)_mu

Sum rule constraints dominated by the independent high-scale input, alpha_s(M_Z), are shown to be satisfied by I=1 spectral data from hadronic tau decays, but violated by the pre-2005 electroproduction (EM) cross-section data. Determinations of the Standard Model (SM) hadronic contribution to (g-2)_mu incorporating tau decay data are thus favored over those based solely on EM data, implying a SM prediction for (g-2)_mu in agreement with current experimental results.

hep-ph↗
arXiv2005

Constraints on the I=1 hadronic tau decay and e^+e^- --> hadrons data sets and implications for (g-2)_mu

Sum rule tests are performed on the spectral data for (i) flavor 'ud' vector- current-induced hadronic tau decays and (ii) e^+ e^- hadroproduction, in the region below s~3-4 GeV^2, where discrepancies exist between the isospin- breaking-corrected charged and neutral current I=1 spectral functions. The tau data is found to be compatible with expectations based on high-scale alpha_s(M_Z) determinations, while the electroproduction data displays two problems. The results favor determinations of the leading order hadronic contribution to (g-2)_mu which incorporate hadronic tau decay data over those employing electroproduction data only, and hence a reduced discrepancy between experiment and the Standard Model prediction for (g-2)_mu.

hep-ph↗
arXiv2005

Resolving the tau versus electroproduction discrepancy for the I=1 vector spectral function and implications for the SM prediction for a_mu

Using only independent high-scale OPE input, we investigate QCD sum rule constraints on two currently incompatible versions of the I=1 vector spectral function, one obtained from electroproduction data, the other from hadronic tau decay data. Sum rules involving weighted integrals over the spectral functions, from threshold to a variable upper endpoint, s_0,are employed. It is shown that both the normalization and slope with respect to s_0 of the EM spectral integrals disagree with the corresponding OPE expectations, while both the normalization and slope are in good agreement when hadronic tau decay data is used instead. These results favor determinations of the leading order hadronic vacuum polarization contribution to a_mu obtained using the tau decay data, and hence Standard Model predictions for a_mu compatible with the current experimental determination.

hep-ph↗
arXiv2005

Isospin Breaking in the Relation Between the tau-->nu_tau pi pi and e^+e^- -->pi^+ pi^- Versions of |F_π(s)|^2$ and Implications for (g-2)_mu

We investigate two points related to existing treatments of isospin-breaking corrections to the CVC relation between the e^+e^- --> pi^+ pi^- cross-section and dGamma[tau^- --> nu_tau pi^- pi^0]/ds. Implications for the value of the hadronic contribution to a_mu =(g-2)_mu /2 based on those analyses incorporating hadronic tau decay data are also considered. We conclude that the uncertainty on the isospin-breaking correction which must be applied to the tau decay data should be significantly increased, and that the central value of the rho-omega ``mixing'' contribution to this correction may be significantly smaller than indicated by the present standard determination. Such a shift would contribute to reducing the discrepancy between the tau- and electroproduction-based determinations of the leading order hadronic contribution to a_mu.

hep-ph↗
arXiv2004

Heavy Quark Analogues of the Theta and Their Excitations

Predictions for the low-lying excitation spectrum of positive parity pentaquark systems containing one c or b antiquark and four light u,d quarks are obtained in the quark model picture for models with spin-dependent interactions given either by effective color magnetic (CM) exchange or effective Goldstone boson (GB) exchange. For the GB model, 4 excited states are predicted to lie within roughly the N-Delta splitting of the J=1/2 positive parity ground state while, for the CM model, 10 states are expected in the same range. Both the lowest excitation energy and the relative splittings are much smaller in the CM case. These predictions are on the same footing as those for the analogous splittings in the non-exotic baryon sector and, as such, provide a means of not only testing the models, but potentially ruling out either one, or both.

hep-ph↗