Decay Form Factors Research Articles

Radiative semileptonic overline{B} decays

We consider the form factors for the radiative semileptonic decays overline{B} (v) → D(*)(v′)ℓ overline{nu} ℓγ in the kinematic region where the photon momentum, k, is small enough that heavy quark symmetry (HQS) can be applied without the radiated photon changing the heavy quark velocity (i.e., v(′) ∙ k < m(b,c)). We find that HQS is remarkably powerful, leaving only four new undetermined form factors at leading order in 1/m(b,c). In addition, one of them is fixed in terms of the leading order Isgur-Wise function in the kinematic region, v(′) ∙ k < ΛQCD.

Hadronic currents and form factors in three-body semileptonic tau decays

Three-body semileptonic tau -decays offer a path to understand the properties of light hadronic systems and CP symmetry violations through searches for electric dipole moments. In studies of electro-weak physics, the hadronic part of the final states has traditionally been described using the language of form factors. Spectroscopic information, resolved in terms of orbital angular momentum quantum-numbers, is best being derived from an explicit decomposition of the hadronic current in the orbital angular momentum basis. Motivated by the upcoming large data samples from mathrm {B} factories, we present the full description of the hadronic currents decomposed into quantum numbers of the hadronic final state using the isobar picture. We present formulas for orbital angular momenta up to three and apply the rules derived from hadron spectroscopy to formulate the decay chain of hadronic three-body systems of arbitrary mass. We also translate this formalism to the language of form factors and thereby correct insufficiencies found in previous analyses of three-body hadronic final states.

Precision calculations of the double radiative bottom-meson decays in soft-collinear effective theory

Employing the systematic framework of soft-collinear effective theory (SCET) we perform an improved calculation of the leading-power contributions to the double radiative Bd,s-meson decay amplitudes in the heavy quark expansion by including the perturbative resummation of enhanced logarithms of mb/ΛQCD at the next-to-leading-logarithmic accuracy. We then construct the QCD factorization formulae for the subleading power contributions arising from the energetic photon radiation off the constituent light-flavour quark of the bottom meson at tree level. Furthermore, we explore the factorization properties of the subleading power correction from the effective SCET current at mathcal{O}left({alpha}_s^0right) by virtue of the operator identities due to the classical equations of motion. The higher-twist contributions to the Bd,s→ γγ helicity form factors from the two-particle and three-particle bottom-meson distribution amplitudes are evaluated with the perturbative factorization technique, up to the twist-six accuracy. In addition, the subleading power weak-annihilation contributions from both the current-current and QCD penguin operators are taken into account at the one-loop accuracy. We proceed to apply the operator-production-expansion-controlled dispersion relation for estimating the power-suppressed soft contributions to the double radiative Bd,s-meson decay form factors, which cannot be factorized into the light-cone distribution amplitudes of the heavy-meson and the resolved photon as well as the hard-scattering kernel calculable in perturbation theory canonically. Phenomenological explorations of the radiative Bd,s→ γγ decay observables in the presence of the neutral-meson mixing, including the CP-averaged branching fractions, the polarization fractions and the time-dependent CP asymmetries, are carried out subsequently with an emphasis on the numerical impacts of the newly computed ingredients together with the theory uncertainties from the shape parameters of the HQET bottom-meson distribution amplitudes.

The weak decay B_c to Z(3930) and X(4160) by Bethe\u2013Salpeter method

Considering Z(3930) and X(4160) as chi _{c2}(2P) and chi _{c2}(3P) states, the semileptonic and nonleptonic of B_c decays to Z(3930) and X(4160) are studied by the improved Bethe–Salpeter (B–S) Method. The form factors of decay are calculated through the overlap integrals of the meson wave functions in the whole accessible kinematical range. The influence of relativistic corrections are considered in the exclusive decays. Branching ratios of B_c weak decays to Z(3930) and X(4160) are predicted. Some of the branching ratios are: Br(B_c^+rightarrow Z(3930)e^+nu _e)=(3.03^{+0.09}_{-0.16})times 10^{-4} and Br(B_c^+rightarrow X(4160)e^+nu _e)=(3.55^{+0.83}_{-0.35})times 10^{-6}. These results may provide useful information to discover Z(3930) and X(4160) and the necessary information for the phenomenological study of B_c physics.

QCD Sum Rules Analysis of Weak Decays of Doubly-Heavy Baryons

We calculate the weak decay form factors of doubly-heavy baryons using three-point QCD sum rules. The Cutkosky rules are used to derive the double dispersion relations. We include perturbative contributions and condensation contributions up to dimension five, and point out that the perturbative contributions and condensates with lowest dimensions dominate. An estimate of a part of the gluon–gluon condensates show that it plays a less important role. With these form factors at hand, we present a phenomenological study of semileptonic decays. The future experimental facilities can test these predictions, and deepen our understanding of the dynamics in the decays of doubly-heavy baryons.

Form factors for B→j1j2 decays into two currents in QCD

We discuss the general properties of the $B$-meson decay form factors $F({q}^{2},{q}^{\ensuremath{'}2})$, describing $B$-meson decays induced by two currents, e.g., $B$ decays into four leptons in the final state. We study the analytic properties of these complicated objects and identify those regions of ${q}^{2}$ and ${q}^{\ensuremath{'}2}$, where perturbative QCD can be applied for obtaining predictions for the physical form factors.

Towards a heavy diquark effective theory for weak decays of doubly heavy baryons

We construct a leading-order effective field theory for both scalar and axial-vector heavy diquarks, and consider its power expansion in the heavy diquark limit. By assuming the transition from QCD to diquark effective theory, we derive the most general form for the effective diquark transition currents based on the heavy diquark symmetry. The short-distance coefficients between QCD and heavy diquark effective field theory are also obtained by a tree level matching. With the effective currents in the heavy diquark limit, we perform a reduction of the form factors for semi-leptonic decays of doubly heavy baryons, and find that only one nonperturbative function is remaining. It is shown that this soft function can be related to the Isgur–Wise function in heavy meson transitions. As a phenomenological application, we take a single pole structure for the reduced form factor, and use it to calculate the semi-leptonic decay widths of doubly heavy baryons. The obtained results are consistent with others given in the literature, and can be tested in the future.

QCD calculations of radiative heavy meson decays with subleading power corrections

We revisit QCD calculations of radiative heavy meson decay form factors by including the subleading power corrections from the twist-two photon distribution amplitude at next-to-leading-order in αs with the method of the light-cone sum rules (LCSR). The desired hard-collinear factorization formula for the vacuum-to-photon correlation function with the interpolating currents for two heavy mesons is constructed with the operator- product-expansion technique in the presence of evanescent operators. Applying the back- ground field approach, the higher twist corrections from both the two-particle and three- particle photon distribution amplitudes are further computed in the LCSR framework at leading-order in QCD, up to the twist-four accuracy. Combining the leading power “point- like” photon contribution at tree level and the subleading power resolved photon corrections from the newly derived LCSR, we update theory predictions for the nonperturbative couplings describing the electromagnetic decay processes of the heavy mesons H∗±→ H±γ, H∗0→ H0γ, {H_s^{ast}}^{pm } → {H}_s^{pm } γ (with H = D, B). Furthermore, we perform an exploratory comparisons of our sum rule computations of the heavy-meson magnetic couplings with the previous determinations based upon different QCD approaches and phenomenological models.

Dalitz decays of vector mesons in relativistic quark model

In the course of work, basing on the previously developed method for calculating integral representations of decay constants for pseudoscalar and vector mesons in the frame of point form of Poincare-covariant quark model, the research of ω- and ϕ-mesons decays form-factors for Dalitz decays has been carried out. It is shown that the usage of the pseudoscalar density constant with integral representations of decay constants , and leads to parameters which can be used for the study of decay form-factors. As a result of the work, the behaviour of (t) Fωπγ* (t) and Fϕηγ* (t) form-factors compared with the experimental data in the interval q < 0.5 GeV is researched.

Precision model-independent bounds from a global analysis of b→cℓν form factors

We present a model-independent global analysis of hadronic form factors for the semileptonic decays $b\rightarrow c\ell\nu$ that exploits lattice-QCD data, dispersion relations, and heavy-quark symmetries. The analysis yields predictions for the relevant form factors, within quantifiable bounds. These form factors are used to compute the semileptonic ratios $R(H_c)$ and various decay-product polarizations. In particular, we find $R(D_s^*)=0.20(3)$ and $R(J/\psi)=0.25(3)$, predictions that can be compared to results of upcoming LHCb measurements. In developing this treatment, we obtain leading-order NRQCD results for the nonzero-recoil relations between the $B_c^+ \rightarrow \{J/\psi , \eta_c \}$ form factors.

Decays of light mesons in the relativistic quark model

In the framework of the relativistic quark model based on the point form of the Poincaré-invariant quantum mechanics, the parameters were fixed using the integral representations of the lepton decay constants of pseudoscalar and vector mesons containing u-, d- and s-quarks. As a result of numerical calculations using the oscillator wave function, the basic parameters of the model are obtained using the pseudoscalar density constant and current quark masses. The analysis showed that the obtained calculation results in the framework of the model and the experimental data on the lepton decays of hadrons agree well with each other. As a result, the calculation method is generalized to the case of hadronic transitions with γ-quantum emission and a subsequent calculation of the integral representations of radiative decay constants of pseudoscalar and vector mesons. The obtained values of the anomalous magnetic moments are compared with the baryon data. As a test of the model, the authors studied the behavior of the form-factors of radiative decays of vector mesons with a subsequent comparison to the modern experimental data in the q &lt; 0.5 GeV range where the resonance effects are insignificant. As a result, self-consistent descriptions of lepton and radiative transitions were obtained within the framework of the model proposed by the authors.

W radiative decay to heavy-light mesons in HQET factorization through O(αs)

Analogous to NRQCD factorization for heavy quarkonium exclusive production, in this work we propose to employ the heavy-quark-effective-theory (HQET) factorization, which has been predominantly applied to account for exclusive $B$ decays, to study the exclusive production of the heavy-flavored mesons. We take $W\to B(D_s)+\gamma$ as a prototype process. The validity of the HQET factorization rests upon the presumed scale hierarchy: $m_W\sim m_b\gg \Lambda_{\rm QCD}$. Through an explicit analysis at next-to-leading order in $\alpha_s$ yet at leading order in $1/m_b$, we verify that the decay form factors can indeed be expressed as the convolution between perturbatively calculable hard-scattering kernel and the $B$ meson light-cone distribution amplitude (LCDA) defined in HQET. It is observed that the factorization scale dependence becomes reduced after incorporating the NLO perturbative correction. An interesting future investigation is to identify and resum large collinear logarithms of $m_W/m_b$ that arise ubiquitously in the fixed-order expressions of the hard-scattering kernel in HQET factorization.

Vector and Axial-vector form factors in radiative kaon decay and flavor SU(3) symmetry breaking

We study the vector and axial-vector form factors of radiative kaon decay within the framework of the gauged nonlocal effective chiral action from the instanton vacuum, focusing on the effects of flavor SU(3) symmetry breaking. The general tendency of the results are rather similar to those of radiative pion decays: The nonlocal contributions make the results of the vector form factor increased by about 20%, whereas they reduce those of the axial-vector form factor by almost 50%. Suppressing the prefactors consisting of the kaon mass and the pion decay constant, we scrutinize how the kaon form factors undergo changes as the mass of the strange current quark is varied. Those related to the vector and second axial-vector form factors tend to decrease monotonically as the strange quark mass increases, whereas that for the axial-vector form factor decreases. When K→eνγ decay is considered, both the results of the vector and axial-vector form factors at the zero momentum transfer are in good agreement with the experimental data. The results are also compared with those from chiral perturbation theory to p6 order.

Light-cone sum rules analysis of Xi _{QQ^{prime }q}rightarrow Lambda _{Q^{prime }} weak decays

We analyze the weak decay of doubly-heavy baryon decays into anti-triplets Lambda _Q with light-cone sum rules. To calculate the decay form factors, both bottom and charmed anti-triplets Lambda _b and Lambda _c are described by the same set of leading twist light-cone distribution amplitudes. With the obtained form factors, we perform a phenomenology study on the corresponding semi-leptonic decays. The decay widths are calculated and the branching ratios given in this work are expected to be tested by future experimental data, which will help us to understand the underlying dynamics in doubly-heavy baryon decays.

Radiative decays of the heavy tensor mesons in light cone QCD sum rules

The transition form factors of the radiative decays of the heavy tensor mesons to heavy pseudoscalar and heavy vector mesons are calculated in the framework of the light cone QCD sum rules method at the point $Q^2=0$. Using the obtained values of the transition form factors at the point $Q^2=0$ the corresponding decay widths are estimated. The results show that the radiative decays of the heavy--light tensor mesons can be measurable in the future planned experiments at LHCb.

Investigating Charged-Kaon Decays in the OKA Experiment

The OKA detector is described, and recently published results of experimental searches for a heavy neutrino and investigations of the Ke3 -decay form factors are reported. Incident-beam characteristics, the detector configuration and its major subsystems, and triggering conditions are discussed. Physical motivations for these experimental studies are provided, and the event-selection procedures are briefly described.

Relativistic description of the Ξb baryon semileptonic decays

Semileptonic decays of the $\Xi_b$ baryon are studied in the framework of the relativistic quark-diquark model based on the quasipotential approach. The weak decay form factors are calculated with the comprehensive account of all relativistic effects without employing nonrelativistic and heavy quark expansions. On this basis differential and total decay rates as well as different asymmetry parameters are calculated for the heavy-to-heavy $\Xi_b\to \Xi_c\ell\nu_\ell$ and heavy-to-light $\Xi_b\to \Lambda\ell\nu_\ell$ semileptonic decays. Predictions for the ratios of such decays involving $\tau$ lepton and muon are presented.

K+→π+νν¯ decay amplitude from lattice QCD

In Ref. [1] we have presented the results of an exploratory lattice QCD computation of the long-distance contribution to the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ decay amplitude. In the present paper we describe the details of this calculation, which includes the implementation of a number of novel techniques. The ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ decay amplitude is dominated by short-distance contributions which can be computed in perturbation theory with the only required nonperturbative input being the relatively well-known form factors of semileptonic kaon decays. The long-distance contributions, which are the target of this work, are expected to be of $O(5%)$ in the branching ratio. Our study demonstrates the feasibility of lattice QCD computations of the ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ decay amplitude, and in particular of the long-distance component. Though this calculation is performed on a small lattice ($1{6}^{3}\ifmmode\times\else\texttimes\fi{}32$) and at unphysical pion, kaon and charm quark masses, ${m}_{\ensuremath{\pi}}=420\text{ }\text{ }\mathrm{MeV}$, ${m}_{K}=563\text{ }\text{ }\mathrm{MeV}$ and ${m}_{c}^{\overline{\mathrm{MS}}}(2\text{ }\text{ }\mathrm{GeV})=863\text{ }\text{ }\mathrm{MeV}$, the techniques presented in this work can readily be applied to a future realistic calculation.

Measurement of the form factors of charged kaon semileptonic decays

A measurement of the form factors of charged kaon semileptonic decays is presented, based on 4.4 × 106K± → π0e±νe (Ke3±) and 2.3 × 106K± → π0μ±νμ (Kμ3±) decays collected in 2004 by the NA48/2 experiment. The results are obtained with improved precision as compared to earlier measurements. The combination of measurements in the Ke3± and Kμ3± modes is also presented.

Radiative leptonic decay B \u2192 \u03b3\u2113\u03bd\u2113 with subleading power corrections

We reconsider the QCD predictions for the radiative decay B → γℓνℓ with an energetic photon in the final state by taking into account the 1/Eγ, 1/mb power-suppressed hard-collinear and soft corrections from higher-twist B-meson light-cone distribution amplitudes (LCDAs). The soft contribution is estimated through a dispersion relation and light-cone QCD sum rules. The analysis of theoretical uncertainties and the dependence of the decay form factors on the leading-twist LCDA ϕ+(ω) shows that the latter dominates. The radiative leptonic decay is therefore well suited to constrain the parameters of ϕ+(ω), including the first inverse moment, 1/λB, from the expected high-statistics data of the BELLE II experiment.