arXiv subjects

Danny van Dyk

Publications and source records attributed to Danny van Dyk.

At least 55 records · Page 3Linked to original sources

arXiv2018

Status of rare exclusive $B$ meson decays in 2018

This review discusses the present experimental and theoretical status of rare flavour-changing neutral current $b$-quark decays at the begin of 2018. It includes a discussion of the experimental situation and details of the currently observed anomalies in measurements of flavour observables, including lepton flavour universality. Progress on the theory side, within and beyond the Standard Model theory is also discussed, together with potential New Physics interpretations of the present measurements.

hep-ex↗

arXiv2018

Revisiting $B \to ππ\ell ν$ at Large Dipion Masses

We revisit QCD factorization of $B\to ππ$ form factors at large dipion masses, by deriving new constraints based on the analyticity properties of these objects. We then propose a parametrization of the form factors, inspired by the leading-twist QCD factorization formula, that incorporates all known analytic properties. This parameterization is used to interpolate between the QCDF results and the constraints from the $B^*$ pole. Based on this interpolation, we predict the $B\to ππ\ellν$ decay rate in a larger phase space region than previous studies could. We obtain a partially-integrated branching ratio up to $\mathcal{B} \simeq \mathcal{O}({10^{-6}})$, which implies that a measurement of the non-resonant semileptonic decay is potentially within reach of the Belle II experiment.

hep-ph↗

arXiv2018

Testing lepton flavour universality in semileptonic $Λ_b \to Λ_c^*$ decays

Lepton Flavour Universality tests with semileptonic $Λ_b\toΛ_c^*$ decays are important to corroborate the present anomalies in the similar ratios $R_{D^{(*)}}$, and can provide complementary constraints on possible origins of these anomalies beyond the Standard Model. In this paper we provide - for the first time - all the necessary theoretical ingredients to perform and interpret measurements of $R_{Λ_c^*}$ at the LHCb experiment. For this, we revisit the heavy-quark expansion of the relevant hadronic matrix elements, and provide their expressions to order $α_s$ and $1/m$ accuracy. Moreover, we study the sensitivity to the form factor parameters given the projected size and purity of upcoming and future LHCb datasets of $Λ_b\to Λ_c^*μ\barν$ decays. We demonstrate explicitly the need to perform a simultaneous fit to both $Λ_c^*$ final states. Finally, we provide projections for the uncertainty of $R_{Λ_c^*}$ based on the form factor analysis from semimuonic decays and theoretical relations based on the heavy-quark expansion.

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arXiv2018

Prospects for disentangling long- and short-distance effects in the decays $B\to K^* μ^+μ^-$

Theory uncertainties on non-local hadronic effects limit the New Physics discovery potential of the rare decays $B\to K^*μ^+μ^-$. We investigate prospects to disentangle New Physics effects in the short-distance coefficients from these effects. Our approach makes use of an event-by-event amplitude analysis, and relies on the state of the art parametrisation of the non-local contributions. We find that non-standard effects in the short-distance coefficients can be successfully disentangled from non-local hadronic effects. The impact of the truncation on the parametrisation of non-local contributions to the Wilson coefficients are for the first time systematically examined and prospects for its precise determination are discussed. We find that physical observables are unaffected by these uncertainties. Compared to other methods, our approach provides for a more precise extraction of the angular observables from data.

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arXiv2017

Long-distance effects in $B\to K^*\ell\ell$ from Analyticity

We discuss a novel approach to systematically determine the long-distance contribution to $B\to K^*\ell\ell$ decays in the kinematic region where the dilepton invariant mass is below the open charm threshold. This approach provides the most consistent and reliable determination to date and can be used to compute Standard Model predictions for all observables of interest, including the kinematic region where the dilepton invariant mass lies between the $J/ψ$ and the $ψ(2S)$ resonances. We illustrate the power of our results by performing a New Physics fit to the Wilson coefficient $C_9$. This approach is systematically improvable from theoretical and experimental sides, and applies to other decay modes of the type $B\to V\ell\ell$, $B\to P\ell\ell$ and $B\to Vγ$.

hep-ph↗

arXiv2017

QCD Factorization for $B \to ππ\ellν$ Decays at Large Dipion Masses

We introduce a factorization formula for semi-leptonic $b \to u$ transitions in the exclusive decay mode $B^- \to π^+π^-\ell^-\barν_\ell$ in the limit of large pion energies and large dipion invariant mass. One contribution can be described in terms of a universal $B \to π$ form factor and the convolution of a short-distance kernel $T^{\rm I}$ with the respective light-cone distribution amplitudes (LCDAs) of the positively charged pion. The second contribution, at leading power, completely factorizes, with a short-distance kernel $T^{\rm II}$ convoluted with the leading-twist LCDAs for both pions and the $B$-meson. We calculate the leading contributions to the short-distance kernels $T^{\rm I}$ and $T^{\rm II}$ in fixed-order perturbation theory, and discuss the approximate relations among the resulting $B \to ππ$ partial-wave form factors. Our results provide useful theoretical constraints for phenomenological models that aim to analyze the complete $B \to ππ\ellν$ phase space.

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arXiv2016

Measuring the Breaking of Lepton Flavour Universality in $B\to K^*\ell^+\ell^-$

We propose measurements of weighted differences of the angular observables in the rare decays $B \to K^*\ell^+\ell^-$. The proposed observables are very sensitive to the difference between the Wilson coefficients $\mathcal{C}_9^{(e)}$ and $\mathcal{C}_9^{(μ)}$ for decays into electrons and muons, respectively. At the same time, the charm-induced hadronic contributions are kinematically suppressed to $\lesssim 7\% (4\%)$ in the region $1\,$GeV$^2 \leq q^2 \leq 6\,$GeV$^2$, as long as LFU breaking occurs only in $\mathcal{C}^{(\ell)}_{9}$. This suppression becomes stronger for the region of low hadronic recoil, $q^2 \geq 15\,$GeV$^2$.

hep-ph↗

arXiv2016

Using $Λ_b\to Λμ^+μ^-$ data within a Bayesian analysis of $|ΔB| = |ΔS| = 1$ decays

We study the impact of including the baryonic decay $Λ_b\to Λ(\to p π^-)μ^+μ^-$ in a Bayesian analysis of $|ΔB | = |ΔS| = 1$ transitions. We perform fits of the Wilson coefficients $C_{9}$, $C_{9'}$, $C_{10}$ and $C_{10'}$, in addition to the relevant nuisance parameters. Our analysis combines data for the differential branching fraction and three angular observables of $Λ_b\to Λ(\to p π^-)μ^+μ^-$ with data for the branching ratios of $B_s \to μ^+μ^-$ and inclusive $b \to s\ell^+\ell^-$ decays. Newly available precise lattice QCD results for the full set of $Λ_b \to Λ$ form factors are used to evaluate the observables of the baryonic decay. Our fits prefer shifts to $C_{9}$ that are opposite in sign compared to those found in global fits of only mesonic decays, and the posterior odds show no evidence of physics beyond the Standard Model. We investigate a possible hadronic origin of the observed tensions between theory and experiment.

hep-ph↗

arXiv2016

Impact of leptonic $τ$ decays on the distribution of $B \to P μ\barν$ decays

We calculate the fully-differential rate of the decays $B\to Pτ(\to μ\barνν)\barν$ where $P = D,π$, which is a background to the semimuonic decays $B\to Pμ\barν$. The decays with a $3ν$ final state can have a sizeable impact on the experimental analyses of the ratios $R_D$ and $R_π$, depending on the event selection in the analysis. We outline a strategy which permits the extraction of $R_P \mathcal{B}(τ\to μ\barνν)$ from the neutrino-inclusive rate. Our analytic results can also be used to test both existing and upcoming experimental analyses. We further provide Monte Carlo samples of the 5D rate of the neutrino-inclusive decays $B\to PμX_{\barν}$.

hep-ph↗

arXiv2016

General Analysis of B -> K^(*) l^+ l^- Decays at Low Recoil

We analyze the angular distributions of B -> K^* (-> K pi) l^+ l^- and B -> K l^+ l^- decays in the region of low hadronic recoil in a model-independent way by taking into account the complete set of dimension-six operators [sbar Gamma b][lbar Gamma' l]. We obtain several novel low-recoil observables with high sensitivity to non-standard-model Dirac structures, including CP-asymmetries which do not require flavor tagging. The transversity observables H_T^(1,3,4,5) are found to be insensitive to hadronic matrix elements and their uncertainties even when considering the complete set of operators. In the most general scenario we show that the low recoil operator product expansion can be probed at the few-percent level using the angular observable J_7. Higher sensitivities are possible assuming no tensor contributions, specifically by testing the low-recoil relation |H_T^(1)|=1. We explicitly demonstrate the gain in reach of the low-recoil observables in accessing the ratio |C_9/C_10| compared to the forward-backward asymmetry, and probing CP-violating right-handed currents Im C_10'. We give updated Standard Model predictions for key observables in B -> K^(*) l^+ l^- decays.

hep-ph↗

arXiv2016

Angular Analysis of the Decay $Λ_b \to Λ(\to N π) \ell^+\ell^-$

We study the differential decay rate for the rare $Λ_b \to Λ(\to N π)\ell^+\ell^-$ transition, including a determination of the complete angular distribution, assuming unpolarized $Λ_b$ baryons. On the basis of a properly chosen parametrization of the various helicity amplitudes, we provide expressions for the angular observables within the Standard Model and a subset of new physics models with chirality-flipped operators. Hadronic effects at low recoil are estimated by combining information from lattice QCD with (improved) form-factor relations in Heavy Quark Effective Theory. Our estimates for large hadronic recoil -- at this stage -- are still rather uncertain because the baryonic input functions are not so well known, and non-factorizable spectator effects have not been worked out systematically so far. Still, our phenomenological analysis of decay asymmetries and angular observables for $Λ_b \to Λ(\to N π)\ell^+\ell^-$ reveals that this decay mode can provide new and complementary constraints on the Wilson coefficients in radiative and semileptonic $b \to s$ transitions compared to the corresponding mesonic modes.

hep-ph↗

arXiv2015

Zero-Recoil Sum Rules for $Λ_b \to Λ_c$ Form Factors

We set up a zero recoil sum rule to constrain the form factors of the $Λ_b \to Λ_c$ transition. Our results are compared with the recent lattice calculation for these transitions. We find the same situation as in the case for $B \to D^*$: The lattice results practically saturate the sum rules, leaving basically no room for excited states.

hep-ph↗

arXiv2015

Analyzing $b\to u$ transitions in semileptonic $\bar{B}_s \to K^{*+}(\to K π)\ell^-\barν_\ell$ decays

We study the semileptonic decay $\bar{B}_s \to K^{*+} \ell^-\barν_\ell$, which is induced by $b\to u \ell^- \barν_\ell$ transitions at the quark level. We take into account the standard model (SM) operator from $W$-boson exchange as well as possible extensions from physics beyond the SM. The secondary decay $K^{*+}\to Kπ$ can be used to study a number of angular observables, which are worked out in terms of short-distance Wilson coefficients and hadronic form factors. Our analysis allows for an independent extraction of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ and for the determination of certain ratios of $\bar{B}_s\to K^*$ form factors. Moreover, a future precision measurement of the forward-backward asymmetry in the $\bar{B}_s \to K^{*+} \ell^-\barν_\ell$ decay can be used to unambiguously verify the left-handed nature of the transition operator as predicted by the SM. We provide numerical estimates for the relevant angular observables and the resulting decay distributions on the basis of available form-factor information from lattice and sum-rule estimates. In addition, we pay particular attention to suitable combinations of angular observables in the decays $\bar{B}_s \to K^{*+}(\to Kπ)\ell^-\barν_\ell$ and $\bar{B} \to K^{*0}(\to Kπ)\ell^+\ell^-$, and find that they provide complementary constraints on the relevant $b\to s$ short-distance coefficients. As a by-product, we perform a SM fit on the basis of selected experimental decay rates and hadronic input functions, which results in $|V_{ub}| = (4.07 \pm 0.20) \cdot 10^{-3}$.

hep-ph↗

arXiv2015

Extracting Angular Observables without a Likelihood and Applications to Rare Decays

Our goal is to obtain a complete set of angular observables arising in a generic multi-body process. We show how this can be achieved without the need to carry out a likelihood fit of the angular distribution to the measured events. Instead, we apply the method of moments that relies both on the orthogonality of angular functions and the estimation of integrals by Monte Carlo techniques. The big advantage of this method is that the joint distribution of all observables can be easily extracted, even for very few events. The method of moments is shown to be robust against mismodeling of the angular distribution. Our main result is an explicit algorithm that accounts for systematic uncertainties from detector-resolution and acceptance effects. Finally, we present the necessary process-dependent formulae needed for direct application of the method to several rare decays of interest.

hep-ex↗

arXiv2015

Extrapolation and unitarity bounds for the $B \to π$ form factor

We address the problem of extrapolating the vector form factor $f_{Bπ}^+$, which is relevant to $B\to π\ell ν_\ell$ decays, from the region of small to the region of large momentum transfer. As input, we use the QCD light-cone sum rule at small momentum transfer. We carry out a comprehensive Bayesian uncertainty analysis and obtain correlated uncertainties for the normalization and shape parameters of the form factor. The $z$-series parametrization for $f_{Bπ}^+$ is employed to extrapolate our results to large momentum transfer, and to compare with the lattice QCD results. To test the validity of our extrapolation we use the upper and lower bounds from the unitarity and positivity of the two-point correlator of heavy-light quark currents. This correlator is updated by including the NNLO perturbative term and the NLO correction to the quark condensate contribution. We demonstrate that an additional input including the form factor, its first and second derivative calculated at one value of momentum transfer from the light-cone sum rules, considerably improves the bounds. This only holds when the correlations between the form factor parameters are taken into account. We further combine our results with the latest experimental measurements of $B\to π\ell ν_\ell$ by the BaBar and Belle collaborations, and obtain $|V_{ub}|= (3.32^{+0.26}_{-0.22}) \cdot 10^{-3}$ from a Bayesian analysis.

hep-ph↗

arXiv2015

$1/m$ Corrections for Orbitally Excited Heavy Mesons and the $1/2$ - $3/2$ Puzzle

We re-investigate the effects of the $1/m_c$ corrections on the spectrum of the lowest orbitally excited $D$-meson states. We argue that one should expect the $1/m_c$ corrections to induce a significant mixing between the two lowest lying $1^+$ states. We discuss the implications of this mixing and compute its effect on the semileptonic decays $B \to D^{**} \ell \barν$ and the strong $D^{**}$ decays.

hep-ph↗

arXiv2014

Comprehensive Bayesian Analysis of Rare (Semi)leptonic and Radiative B Decays

The available data on $|ΔB| = |ΔS| = 1$ decays are in good agreement with the Standard Model when permitting subleading power corrections of about 15% at large hadronic recoil. Constraining new-physics effects in $\mathcal{C}_7$, $\mathcal{C}_9$, $\mathcal{C}_{10}$, the data still demand the same size of power corrections as in the Standard Model. In the presence of chirality-flipped operators, all but one of the power corrections reduce substantially. The Bayes factors are in favor of the Standard Model. Using new lattice inputs for $B\to K^*$ form factors, the evidence of models with chirality-flipped operators increases, but does not outperform the Standard Model. We use the data to further constrain the hadronic form factors in $B\to K$ and $B\to K^*$ transitions.

hep-ph↗

arXiv2014

Disentangling the Decay Observables in $B^- \to π^+π^-\ell^-\barν_\ell$

We study the semileptonic $b\to u$ transition in the decay mode $B^-\to π^+π^-\ell^-\barν_\ell$. We define $B\to ππ$ form factors in the helicity basis, and study their properties in various kinematic limits, including form factor relations in the heavy-mass and large-energy limits, the decomposition into partial waves of the dipion system, and the resonant contribution of vector and scalar mesons. We show how angular observables in $B^-\to π^+π^-\ell^-\barν_\ell$ can be used to measure dipion form factors or to perform null tests of the Standard Model.

hep-ph↗