Angular analysis of $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ with $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ decays

The first full angular analysis of the $$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ B 0 → D ∗ − D s ∗ + decay is performed using 6 fb−1 of pp collision data collected with the LHCb experiment at a centre-of-mass energy of 13 TeV. The $$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ D s ∗ + → D s + γ and D*− → $$ {\overline{D}}^0{\pi}^{-} $$ D ¯ 0 π − vector meson decays are used with the subsequent $$ {D}_s^{+} $$ D s + → K+K−π+ and $$ {\overline{D}}^0 $$ D ¯ 0 → K+π− decays. All helicity amplitudes and phases are measured, and the longitudinal polarisation fraction is determined to be fL = 0.578 ± 0.010 ± 0.011 with world-best precision, where the first uncertainty is statistical and the second is systematic. The pattern of helicity amplitude magnitudes is found to align with expectations from quark-helicity conservation in B decays. The ratio of branching fractions [ℬ($$ {B}^0\to {D}^{\ast -}{D}_s^{\ast +} $$ B 0 → D ∗ − D s ∗ + ) × ℬ($$ {D}_s^{\ast +}\to {D}_s^{+}\gamma $$ D s ∗ + → D s + γ )]/ℬ(B0 → D*−$$ {D}_s^{+} $$ D s + ) is measured to be 2.045 ± 0.022 ± 0.071 with world-best precision. In addition, the first observation of the Cabibbo-suppressed Bs → D*−$$ {D}_s^{+} $$ D s + decay is made with a significance of seven standard deviations. The branching fraction ratio ℬ(Bs → D*−$$ {D}_s^{+} $$ D s + )/ℬ(B0 → D*−$$ {D}_s^{+} $$ D s + ) is measured to be 0.049 ± 0.006 ± 0.003 ± 0.002, where the third uncertainty is due to limited knowledge of the ratio of fragmentation fractions.

Probing QCD dynamics and the standard model with $$ {D}_{(s)}^{+}\to {P}_1^{+}{P}_2^0\gamma $$ decays

We compute 10 radiative three-body decays of charged charmed mesons $$ {D}^{+}\to {P}_1^{+}{P}_2^0\gamma $$ D + → P 1 + P 2 0 γ and $$ {D}_s\to {P}_1^{+}{P}_2^0\gamma $$ D s → P 1 + P 2 0 γ , P1,2 = π, K, in leading order QCDF, HHχPT and the soft photon approximation. We work out decay distributions and asymmetries in the standard model and with new physics in the electromagnetic dipole operators. The forward-backward asymmetry is suitable to probe the QCD frameworks, in particular the s-channel dependent weak annihilation contributions in QCDF against the markedly different resonance structure in HHχPT. These studies can be performed with Cabibbo-favored modes Ds → π+π0γ, $$ {D}^{+}\to {\pi}^{+}{\overline{K}}^0\gamma $$ D + → π + K ¯ 0 γ and $$ {D}_s\to {K}^{+}{\overline{K}}^0\gamma $$ D s → K + K ¯ 0 γ with $$ \mathcal{O}\left({10}^{-4}-{10}^{-3}\right)\hbox{-} \mathrm{level} $$ O 10 − 4 − 10 − 3 ‐ level branching ratio, which are standard model-like and induced by different hadronic dynamics. Understanding of the latter can therefore be improved in a data-driven way and sharpens the interpretation of standard model tests. Singly Cabibbo-suppressed modes such as D+ → π+π0γ, Ds → π+K0γ, Ds → K+π0γ with branching ratios within ∼ 10−5–10−4 are sensitive to new physics that can be signalled in the forward-backward asymmetry and in the CP-asymmetry of the rate, ideally in the Dalitz region but also in single differential distributions. Results complement those with neutral D0→ P1P2γ decays.

Observation of the decay $$ {\Lambda}_{\mathrm{b}}^0 $$ → χc1pπ−

The Cabibbo-suppressed decay $$ {\Lambda}_{\mathrm{b}}^0 $$ Λ b 0 → χc1pπ− is observed for the first time using data from proton-proton collisions corresponding to an integrated luminosity of 6 fb−1, collected with the LHCb detector at a centre-of-mass energy of 13 TeV. Evidence for the $$ {\Lambda}_{\mathrm{b}}^0 $$ Λ b 0 → χc2pπ− decay is also found. Using the $$ {\Lambda}_{\mathrm{b}}^0 $$ Λ b 0 → χc1pK− decay as normalisation channel, the ratios of branching fractions are measured to be$$ {\displaystyle \begin{array}{c}\frac{\mathcal{B}\left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}{\mathrm{p}\uppi}^{-}\right)}{\mathcal{B}\left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}{\mathrm{p}\mathrm{K}}^{-}\right)}=\left(6.59\pm 1.01\pm 0.22\right)\times {10}^{-2},\\ {}\frac{\mathcal{B}\left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}2}{\mathrm{p}\uppi}^{-}\right)}{\mathcal{B}\left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}{\mathrm{p}\uppi}^{-}\right)}=0.95\pm 0.30\pm 0.04\pm 0.04,\\ {}\frac{\mathcal{B}\left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}2}{\mathrm{p}\mathrm{K}}^{-}\right)}{\mathcal{B}\left({\Lambda}_{\mathrm{b}}^0\to {\upchi}_{\mathrm{c}1}{\mathrm{p}\mathrm{K}}^{-}\right)}=1.06\pm 0.05\pm 0.04\pm 0.04,\end{array}} $$ B Λ b 0 → χ c 1 pπ − B Λ b 0 → χ c 1 pK − = 6.59 ± 1.01 ± 0.22 × 10 − 2 , B Λ b 0 → χ c 2 pπ − B Λ b 0 → χ c 1 pπ − = 0.95 ± 0.30 ± 0.04 ± 0.04 , B Λ b 0 → χ c 2 pK − B Λ b 0 → χ c 1 pK − = 1.06 ± 0.05 ± 0.04 ± 0.04 , where the first uncertainty is statistical, the second is systematic and the third is due to the uncertainties in the branching fractions of χc1,2→ J/ψγ decays.

Combined theoretical study of the $$D^+ \rightarrow \pi ^+ \eta \eta $$ and $$D^+ \rightarrow \pi ^+ \pi ^0 \eta $$ reactions

We study the $$D^+ \rightarrow \pi ^+ \eta \eta $$ D + → π + η η and $$D^+ \rightarrow \pi ^+ \pi ^0 \eta $$ D + → π + π 0 η reactions, which are single Cabibbo suppressed and can proceed both through internal and external emission. The primary mechanisms at quark level are considered, followed by hadronization to produce three mesons in the $$D^+$$ D + decay, and after that the final state interaction of these mesons leads to the production of the $$a_0(980)$$ a 0 ( 980 ) resonance, seen in the $$\pi ^+ \eta $$ π + η , $$\pi ^0 \eta $$ π 0 η mass distributions. The theory has three unknown parameters to determine the shape of the distributions and the ratio between the $$D^+ \rightarrow \pi ^+ \eta \eta $$ D + → π + η η and $$D^+ \rightarrow \pi ^+ \pi ^0 \eta $$ D + → π + π 0 η rates. This ratio restricts much the sets of parameters but there is still much freedom leading to different shapes in the mass distributions. We call for a measurement of these mass distributions that will settle the reaction mechanism, while at the same time provide relevant information on the way that the $$a_0(980)$$ a 0 ( 980 ) resonance is produced in the reactions.

Radiative three-body D-meson decays in and beyond the standard model

We study radiative charm decays $$D \rightarrow P_1 P_2 \gamma $$ D → P 1 P 2 γ , $$P_{1,2}=\pi ,K$$ P 1 , 2 = π , K in QCD factorization at leading order and within heavy hadron chiral perturbation theory. Branching ratios including resonance contributions are around $$\sim 10^{-3}$$ ∼ 10 - 3 for the Cabibbo-favored modes into $$K \pi \gamma $$ K π γ and $$\sim 10^{-5}$$ ∼ 10 - 5 for the singly Cabibbo-suppressed modes into $$\pi ^+ \pi ^- \gamma , K^+ K^- \gamma $$ π + π - γ , K + K - γ , and thus in reach of the flavor factories BES III and Belle II. Dalitz plots and forward–backward asymmetries reveal significant differences between the two QCD frameworks; such observables are therefore ideally suited for a data-driven identification of relevant decay mechanisms in the standard-model dominated $$D \rightarrow K \pi \gamma $$ D → K π γ decays. This increases the potential to probe new physics with the $$D \rightarrow \pi ^+ \pi ^- \gamma $$ D → π + π - γ and $$D \rightarrow K^+ K^- \gamma $$ D → K + K - γ decays, which are sensitive to enhanced dipole operators. CP asymmetries are useful to test the SM and look for new physics in neutral $$|\Delta C|=1$$ | Δ C | = 1 transitions. Cuts in the Dalitz plot enhance the sensitivity to new physics due to the presence of both s- and t, u-channel intermediate resonances.

Role of scalar $$a_0(980)$$ in the single Cabibbo suppressed process $$D^+ \rightarrow \pi ^{+} \pi ^{0} \eta $$

Taking into account that the scalar $$a_0(980)$$ a 0 ( 980 ) can be dynamically generated from the pseudoscalar-pseudoscalar interaction within the chiral unitary approach, we have studied the single Cabibbo suppressed process $$D^+\rightarrow \pi ^+\pi ^0\eta $$ D + → π + π 0 η . We find clear peaks of $$a_0(980)^+$$ a 0 ( 980 ) + and $$a_0(980)^0$$ a 0 ( 980 ) 0 in the $$\pi ^+\eta $$ π + η and $$\pi ^0\eta $$ π 0 η invariant mass distributions, respectively. The predicted Dalitz plots of $$D^+\rightarrow \pi ^+\pi ^0\eta $$ D + → π + π 0 η also manifest the significant signals for $$a_0(980)^+$$ a 0 ( 980 ) + and $$a_0(980)^0$$ a 0 ( 980 ) 0 states. The uncertainties of the results due to the free parameters are also discussed. Our study shows that the process $$D^+\rightarrow \pi ^+\pi ^0\eta $$ D + → π + π 0 η can be used to explore the nature of the scalar $$a_0(980)$$ a 0 ( 980 ) , thus we encourage the experimental physicists to measure this reaction with more precision.

Two-body charmed baryon decays involving decuplet baryon in the quark-diagram scheme

In the quark-diagram scheme, we study the charmed baryon decays of $$\mathbf{B}_c\rightarrow \mathbf{B}^* M$$ B c → B ∗ M , where $$\mathbf{B}_c$$ B c is $$\Lambda _c^+$$ Λ c + or $$\Xi _c^{+(0)}$$ Ξ c + ( 0 ) , together with $$\mathbf{B}^*$$ B ∗ (M) the decuplet baryon (pseudoscalar meson). It is found that only two W-exchange processes are allowed to contribute to $$\mathbf{B}_c\rightarrow \mathbf{B}^* M$$ B c → B ∗ M . Particularly, we predict $${\mathcal {B}}(\Lambda _c^+ \rightarrow \Sigma ^{*0(+)} \pi ^{+(0)})=(2.8\pm 0.4)\times 10^{-3}$$ B ( Λ c + → Σ ∗ 0 ( + ) π + ( 0 ) ) = ( 2.8 ± 0.4 ) × 10 - 3 , which respects the isospin symmetry. Besides, we take into account the SU(3) flavor symmetry breaking, in order to explain the observation of $${\mathcal {B}}(\Lambda _c^+\rightarrow \Sigma ^{*+}\eta )$$ B ( Λ c + → Σ ∗ + η ) . For the decays involving $$\Delta ^{++}(uuu)$$ Δ + + ( u u u ) , we predict $${\mathcal {B}}(\Lambda _c^+\rightarrow \Delta ^{++} \pi ^-,\Xi _c^+ \rightarrow \Delta ^{++} K^-) =(7.0\pm 1.4,13.5\pm 2.7)\times 10^{-4}$$ B ( Λ c + → Δ + + π - , Ξ c + → Δ + + K - ) = ( 7.0 ± 1.4 , 13.5 ± 2.7 ) × 10 - 4 as the largest branching fractions in the singly Cabibbo-suppressed $$\Lambda _c^+,\Xi _c^+\rightarrow \mathbf{B}^*M$$ Λ c + , Ξ c + → B ∗ M decay channels, respectively, which are accessible to the LHCb, BELLEII and BESIII experiments.

Doubly Cabibbo-suppressed decays of antitriplet charmed baryons

Doubly Cabibbo-suppressed (DCS) nonleptonic weak decays of antitriplet charmed baryons are studied systematically in this work. The factorizable and nonfactorizable contributions can be classified explicitly in the topological-diagram approach and treated separately. In particular, the evaluation of nonfactorizable terms is based on the pole model in conjunction with current algebra. All three types of relevant non- perturbative parameters contributing factorizable and nonfactorizable terms are estimated in the MIT bag model. Branching fractions of all the DCS decays are predicted to be of order 10−4 ∼ 10−6. In particular, we find that the three modes $$ {\Xi}_c^{+}\to {\Sigma}^{+}{K}^0,{\Sigma}^0{K}^{+} $$ Ξ c + → Σ + K 0 , Σ 0 K + and $$ {\Xi}_c^0\to {\Sigma}^{-}{K}^{+} $$ Ξ c 0 → Σ − K + are as large as (1 ∼ 2) × 10−4, which are the most promising DCS channels to be measured. We also point out that the two DCS modes $$ {\Xi}_c^{+}\to {\Sigma}^{+}{K}^0 $$ Ξ c + → Σ + K 0 and $$ {\Xi}_c^0\to {\Sigma}^0{K}^0 $$ Ξ c 0 → Σ 0 K 0 are possible to be distinguished from $$ {\Xi}_c^{+}\to {\Sigma}^{+}{K}_S $$ Ξ c + → Σ + K S and $$ {\Xi}_c^0\to {\Sigma}^0{K}_S $$ Ξ c 0 → Σ 0 K S . The decay asymmetries for all the channels with a kaon in their final states are found to be large in magnitude and negative in sign.