arXiv subjects

Jürgen G. Körner

Publications and source records attributed to Jürgen G. Körner.

At least 19 recordsLinked to original sources

arXiv2022

Topological tensor invariants and the current algebra approach: Analysis of 196 nonleptonic two-body decays of single and double charm baryons -- a review

We shed new light on the standard current algebra approach to the nonleptonic two-body decays of single and double heavy charm baryons. By making use of the completeness relation for the flavor wave functions of the ground state baryon {\bf 20'} representation we are able to rewrite the results of the current algebra approach in terms of the seven topological tensor invariants describing the decays. The representation of the current algebra results in terms of topological tensor invariants depends only on the initial and final state of the process. The summation over intermediate states inherent to the current algebra result is automatically taken care of in the tensor representation. In this way one arrives at a new, quick and very compact assessment of the results of the current algebra/pole model calculation. For example, one can quickly identify the decays in which the p.v.\ $S$-wave amplitude is predicted to vanish implying zero polarization asymmetries in these decays. We provide tables of the values of the seven topological tensor invariants for all Cabibbo favored and singly and doubly Cabibbo suppressed nonleptonic single and double charm baryon decays. In total we treat 196 charm baryon decays. We also discuss the charm preserving $ΔC=0$ decays of single charm baryons and the usual hyperon decays.

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arXiv2021

Analysis of the nonleptonic two-body decays of the $Λ$ hyperon

We systematically study two-body nonleptonic decays of light lambda hyperon $Λ\to p π^- (nπ^0)$ with account for both short- and long-distance effects. The short-distance effects are induced by five topologies of external and internal weak $W^\pm$ exchange, while long-distance effects are saturated by an inclusion of the so-called pole diagrams with intermediate $\frac{1}{2}^+$ and $\frac{1}{2}^-$ baryon resonances. The contributions from $\frac12^+$ resonances are calculated straightforwardly by account for nucleon and $Σ$ baryons whereas the contributions from $\frac{1}{2}^-$ resonances are calculated by using the well-known soft-pion theorem in the current-algebra approach. It allows one to express the parity-violating $S$-wave amplitude in terms of parity-conserving matrix elements. From our previous analysis of heavy baryons we know that short-distance effects induced by internal topologies are not suppressed in comparison with external $W$-exchange diagram and must be included for description of data. Here, in the case of $Λ$ decays we found that the contribution of external and internal $W$-exchange diagrams is sizably suppressed, e.g., by one order of magnitude in comparison with data, which are known with quite good accuracy. Pole diagrams play the major role to get consistency with experiment.

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arXiv2021

Form-factor-independent test of lepton universality in semileptonic heavy meson and baryon decays

In the semileptonic decays of heavy mesons and baryons the lepton-mass dependence factors out in the quadratic $\cos^2θ$ coefficient of the differential $\cosθ$ distribution. We call the corresponding normalized coefficient the convexity parameter. This observation opens the path to a test of lepton universality in semileptonic heavy meson and baryon decays that is independent of form-factor effects. By projecting out the quadratic rate coefficient, dividing out the lepton-mass-dependent factor and restricting the phase space integration to the $τ$ lepton phase space, one can define optimized partial rates which, in the Standard Model, are the same for all three $(e,μ,τ)$ modes in a given semileptonic decay process. We discuss how the identity is spoiled by New Physics effects. We discuss semileptonic heavy meson decays such as $\bar{B}^0 \to D^{(\ast)+} \ell^- \barν_\ell$ and $B_c^- \to J/ψ(η_c)\ell^- \barν_\ell$, and semileptonic heavy baryon decays such as $Λ_b \to Λ_c \ell^- \barν_\ell$ for each $\ell=e,μ,τ$.

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arXiv2020

$D^\ast$ Polarization as an Additional Constraint on New Physics in the $b\to cτ\barν_τ$ Transition

Measurements of the branching fractions of the semileptonic decays $B\to D^{(*)}τ\barν_τ$ and $B_c\to J/ψτ\barν_τ$ systematically exceed the Standard Model (SM) predictions, pointing to possible signals of new physics that can violate lepton flavor universality. The unknown origin of new physics realized in these channels can be probed using a general effective Hamiltonian constructed from four-fermion operators and the corresponding Wilson coefficients. Previously, constraints on these Wilson coefficients were obtained mainly from the experimental data for the branching fractions. Meanwhile, polarization observables were only theoretically studied. The situation has changed with more experimental data having become available, particularly those regarding the polarization of the tau and the $D^*$ meson. In this study, we discuss the implications of the new data on the overall picture. We then include them in an updated fit of the Wilson coefficients using all hadronic form factors from our covariant constituent quark model. The use of our form factors provides an analysis independent of those in the literature. Several new-physics scenarios are studied with the corresponding theoretical predictions provided, which are useful for future experimental studies.

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arXiv2019

Analysis of the semileptonic and nonleptonic two-body decays of the double heavy charm baryon states $Ξ_{cc}^{++},\,Ξ_{cc}^{+}$ and $Ω_{cc}^+$

We calculate the semileptonic and a subclass of sixteen nonleptonic two-body decays of the double charm baryon ground states $Ξ_{cc}^{++},\,Ξ_{cc}^{+}$ and $Ω_{cc}^+$ where we concentrate on the nonleptonic decay modes. We identify those nonleptonic decay channels in which the decay proceeds solely via the factorizing contribution precluding a contamination from $W$-exchange. We use the covariant confined quark model previously developed by us to calculate the various helicity amplitudes which describe the dynamics of the $1/2^+ \to 1/2^+$ and $1/2^+ \to 3/2^+$ transitions induced by the Cabibbo favored effective $(c \to s)$ and $(d \to u)$ currents. We then proceed to calculate the rates of the decays as well as polarization effects and angular decay distributions of the prominent decay chains resulting from the nonleptonic decays of the double heavy charm baryon parent states.

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arXiv2019

A new angle on an old problem: Helicity approach to neutron beta decay in the Standard Model

We elaborate on the dichotomy between the description of the semileptonic decays of heavy hadrons on the one hand and the semileptonic decays of light hadrons such as neutron $β$ decays on the other hand. For example, almost without exception the semileptonic decays of heavy baryons are described in cascade fashion as a sequence of two two-body decays $B_1 \to B_2 + W_{\rm off-shell}$ and $W_{\rm off-shell} \to \ell + ν_\ell$ whereas neutron $β$ decays are analyzed as true three-body decays $n \to p + e^- +\bar ν_e$. Within the cascade approach it is possible to define a set of seven angular observables for polarized neutron $β$ decays as well as the longitudinal, transverse and normal polarization of the decay electron. We determine the dependence of the observables on the usual vector and axial vector form factors. In order to be able to assess the importance of recoil corrections we expand the rate and the $q^2$ averages of the observables up to NLO and NNLO in the recoil parameter $δ=(M_n-M_p)/(M_n+M_p)= 0.689\cdot 10^{-3}$. Remarkably, we find that the rate and three of the four parity conserving polarization observables that we analyze are protected from NLO recoil corrections when the second class current contributions are set to zero.

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arXiv2019

Novel ideas in nonleptonic decays of double heavy baryons

The recent discovery of double charm baryon states by the LHCb Collaborarion and their high precision mass determination calls for a comprehensive analysis of the nonleptonic decays of double and single heavy baryons. Nonleptonic baryon decays play an important role in particle phenomenology since they allow to study the interplay of long and short distance dynamics of the Standard Model (SM). Further, they allow one to search for New Physics effects beyond the SM. We review recent progress in experimental and theoretical studies of the nonleptonic decays of heavy baryons with a focus on double charm baryon states and their decays. In particular, we discuss new ideas proposed by the present authors to calculate the $W$-exchange matrix elements of the nonleptonic decays of double heavy baryons. An important ingredient in our approach is the compositeness condition of Salam and Weinberg, and an effective implementation of infrared confinement both of which allow one to describe the nonperturbative structure of baryons composed of light and heavy quarks. Further we discuss an ab initio calculational method for the treatment of the so-called $W$-exchange diagrams generated by $W^{\pm}$ boson exchange between quarks. We found that the $W^{\pm}$-exchange contributions are not suppressed in comparison with the tree-level (factrorizing) diagrams and must be taken into account in the evaluation of matrix elements. Moreover, there are decay processes such as the doubly Cabibbo-suppressed decay $Ξ_c^+ \to p ϕ$ recently observed by the LHCb Collaboration which is contributed to only by one single $W$-exchange diagram.

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arXiv2019

Exclusive semileptonic decays of $D$ and $D_s$ mesons in the covariant confining quark model

Recently, the BESIII collaboration has reported numerous measurements of various $D_{(s)}$ meson semileptonic decays with significantly improved precision. Together with similar studies carried out at BABAR, Belle, and CLEO, new windows to a better understanding of weak and strong interactions in the charm sector have been opened. In light of new experimental data, we review the theoretical description and predictions for the semileptonic decays of $D_{(s)}$ to a pseudoscalar or a vector meson. This review is essentially an extended discussion of our recently published results obtained in the framework of the covariant confining quark model.

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arXiv2019

Ab initio three-loop calculation of the W-exchange contribution to nonleptonic decays of double charm baryons

We have made an ab initio three-loop quark model calculation of the $W$-exchange contribution to the nonleptonic two-body decays of the doubly charmed baryons $Ξ_{cc}^{++}$ and $Ω_{cc}^{+}$. The $W$-exchange contributions appear in addition to the factorizable tree graph contributions and are not suppressed in general. We make use of the covariant confined quark model previously developed by us to calculate the tree graph as well as the $W$-exchange contribution. We calculate helicity amplitudes and quantitatively compare the tree graph and $W$-exchange contributions. Finally, we compare the calculated decay widths with those from other theoretical approaches when they are available.

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arXiv2019

Semileptonic $D_{(s)}$-meson decays in the light of recent data

Inspired by recent improved measurements of charm semileptonic decays at BESIII, we study a large set of $D(D_s)$-meson semileptonic decays where the hadron in the final state is one of $D^0$, $ρ$, $ω$, $η^{(\prime)}$ in the case of $D^+$ decays, and $D^0$, $ϕ$, $K^0$, $K^\ast(892)^0$, $η^{(\prime)}$ in the case of $D^+_s$ decays. The required hadronic form factors are computed in the full kinematical range of momentum transfer by employing the covariant confined quark model developed by us. A detailed comparison of the form factors with those from other approaches is provided. We calculate the decay branching fractions and their ratios, which show good agreement with available experimental data. We also give predictions for the forward-backward asymmetry and the longitudinal and transverse polarizations of the charged lepton in the final state.

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arXiv2018

Nonleptonic two-body decays of single heavy baryons $Λ_Q$, $Ξ_Q$, and $Ω_Q$ $(Q=b,c)$ induced by $W$ emission in the covariant confined quark model

We have made a survey of heavy-to-heavy and heavy-to-light nonleptonic heavy baryon two-body decays and have identified those decays that proceed solely via $W$-boson emission, i.e. via the tree graph contribution. Some sample decays are $Ω_{b}^{-}\toΩ_{c}^{(*)0}ρ^{-}(π^{-}),\, Ω_{b}^{-}\toΩ^{-}J/ψ(η_{c}),\, Ξ_{b}^{0,-}\toΞ^{0,-}J/ψ(η_{c}),\, Λ_{b}\to ΛJ/ψ(η_{c}),\, Λ_{b}\to Λ_{c} D_{s}^{(\ast)},\, Ω_{c}^{0}\toΩ^{-}ρ^{+}(π^{+})$, and $Λ_c \to p ϕ$. We make use of the covariant confined quark model previously developed by us to calculate the tree graph contributions to these decays. We calculate rates, branching fractions and, for some of these decays, decay asymmetry parameters. We compare our results to experimental findings and the results of other theoretical approaches when they are available. Our main focus is on decays to final states with a lepton pair because of their clean experimental signature. For these decays we discuss two-fold polar angle decay distributions such as in the cascade decay $Ω_{b}^{-}\toΩ^{-}(\to Ξπ,ΛK^{-})+J/ψ(\to \ell^{+}\ell^{-})$. Lepton mass effects are always included in our analysis.

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arXiv2018

NLO and NNLO corrections to polarized top quark decays

We present partial results on NLO and NNLO QCD, and NLO electroweak corrections to polarized top quark decays. In parallel we derive positivity bounds for the polarized structure functions in polarized top quark decays and check them against the perturbative corrections to the structure functions.

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arXiv2018

Analyzing lepton flavor universality in the decays $Λ_b\toΛ_c^{(\ast)}(\frac12^\pm,\frac32^-) + \ell\,\barν_\ell$

Lepton flavor universality can be tested in the semileptonic decays $Λ_b\to Λ_c^{(\ast)}$ where $Λ_c^{(\ast)}$ denotes either the ground state $Λ_c(2286)$ (with $J^P=1/2^+$) or its orbital excitations $Λ_c(2595)$ (with $J^P=1/2^-$) and $Λ_c(2625)$ (with $J^P=3/2^-$). We calculate the differential decay rates as well as the branching fractions of these decays for both tauonic and muonic modes with form factors obtained from a covariant confined quark model previously developed by us. We present results for the rate ratios of the tauonic and muonic modes which provide important tests of lepton flavor universality in forthcoming experiments.

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arXiv2018

Implications of new physics in the decays $B_c \to (J/ψ,η_c)τν$

We study the semileptonic decays of the $B_c$ meson into final charmonium states within the standard model and beyond. The relevant hadronic transition form factors are calculated in the framework of the covariant confined quark model developed by us. We focus on the tau mode of these decays, which may provide some hints of new physics effects. We extend the standard model by assuming a general effective Hamiltonian describing the $b\to cτν$ transition, which consists of the full set of the four-fermion operators. We then obtain experimental constraints on the Wilson coefficients corresponding to each operator and provide predictions for the branching fractions and other polarization observables in different new physics scenarios.

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arXiv2017

Test of the multiquark structure of $a_1(1420)$ in strong two-body decays

We present an analysis of strong two-body decays of the $a_1(1420)$ with $J^{PC} = 1^{++}$ recently reported by the COMPASS Collaboration at CERN. Following the interpretation of the COMPASS Collaboration that the $a_1$ is an unusual state with a four-quark $q\bar q s \bar s$ structure we consider two possible configurations for this state --- hadronic molecular and color diquark-antidiquark structures. We find that the dominant decay mode of the $a_1$ is the decay into $K$ and $K^*$. In particular, we calculate that the four decay modes $a_1 \to VP$ with $VP = K^{*\pm}K^\mp$, $K^{*0}\bar K^0$, $\bar K^{*0}K^0$ together give a dominant contribution to the measured total width of about 150 MeV. The observational mode $a_1 \to f_0(980) + π^0$ is significantly suppressed by one order of magnitude.

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arXiv2017

Decay chain information on the newly discovered double charm baryon state $Ξ_{cc}^{++}$

We interprete the new double charm baryon state found by the LHCb Collaboration in the invariant mass distribution of the set of final state particles $(Λ_c^+\,K^-\,π^+\,π^+)$ as being at the origin of the decay chain $Ξ_{cc}^{++} \to Σ_c^{++} (\to Λ_c^+ π^+) +\bar K^{*0} (\to K^- π^+)$. The nonleptonic decay $Ξ_{cc}^{++} \to Σ_c^{++} + \bar K^{*0}$ belongs to a class of decays where the quark flavor composition is such that the decay proceeds solely via the factorizing contribution precluding a contamination from internal $W$-exchange. We use the covariant confined quark model previously developed by us to calculate the four helicity amplitudes that describe the dynamics of the transition $Ξ_{cc}^{++} \to Σ_c^{++}$ induced by the effective $(c \to u)$ current. We then proceed to calculate the rate of the decay as well as the polarization of the $Σ_c^{++}$ and $Λ_c^+$ baryons and the longitudinal/transverse composition of the $\bar K^{*0}$. We estimate the decay $Ξ_{cc}^{++} \to Σ_c^{++} \bar K^{*0}$ to have a branching rate of $B(Ξ_{cc}^{++} \to Σ_c^{++} \bar K^{*0}) \sim 10.5 \%$. As a byproduct of our investigation we have also analyzed the decay $Ξ_{cc}^{++} \to Σ_c^{++} \bar K^{0}$ for which we find a branching ratio of $B(Ξ_{cc}^{++} \to Σ_c^{++} \bar K^0) \sim 2.5 \%$.

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arXiv2017

Theoretical description of the decays $Λ_b \to Λ^{(\ast)}(\frac12^\pm,\frac32^\pm) + J/ψ$

We calculate the invariant and helicity amplitudes for the transitions $Λ_b~\to~Λ^{(\ast)}(J^P)~+~J/ψ$ where the $Λ^{(\ast)}(J^P)$ are $Λ(sud)$-type ground and excited states with $J^P$ quantum numbers $J^P=\frac12^{\pm},\frac32^{\pm}$. The calculations are performed in the framework of a covariant confined quark model previously developed by us. We find that the values of the helicity amplitudes for the $Λ^\ast(1520,\,\frac32^-)$ and the $Λ^\ast(1890,\,\frac32^+)$ are suppressed compared with those for the ground state $Λ(1116,\,\frac12^+)$ and the excited state $Λ^\ast(1405,\,\frac12^-)$. This analysis is important for the identification of the hidden charm pentaquark states $P_c^+(4380)$ and $P_c^+(4450)$ which were discovered in the decay chain $Λ_b^0~\to~P_c^+(~\to~p~J/ψ)~+~K^- $ because the cascade decay chain $Λ_b~\to~Λ^\ast(\frac32^\pm)(~\to~pK^-)~+~J/ψ$ involves the same final state.

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arXiv2017

$Z_b(10610)$ and $Z_b'(10650)$ decays in a covariant quark model

We present a calculation of the strong decays of the exotic states $Z_b(10610)$ and $Z_b'(10650)$ using a covariant quark model. We use a molecular-type four-quark current for the coupling of the $Z_b(10610)$ and $Z_b'(10650)$ to the constituent heavy and light quarks.

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