Ryoutaro Watanabe
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The organizations/institutes from the scholar's latest article
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Ryoutaro Watanabe has a total of 54 co-authors. Here are the top co-authors by number of shared publications:
- Syuhei Iguro
Nagoya University
13 shared publications - Minoru Tanaka
The University of Osaka
10 shared publications - Motoi Endo
KEK Theory Center, IPNS, KEK
8 shared publications - David London
Université de Montréal
6 shared publications - Satoshi Mishima
Saitama Medical University
6 shared publications - Teppei Kitahara
Chiba University
6 shared publications
Latest Publications
- Open Access
- 2 November 2026
- Published by American Physical Society (APS) in Physical Review D
- Vol. 114 (9)
Semileptonic sum rules in heavy-to-light charm decays
We investigate semileptonic sum rules in heavy-to-light charm decays, motivated by analogous relations in b → c and b → u transitions. Focusing on the c → d ℓ ¯ ν decays, D → π ℓ ¯ ν , D → ρ ℓ ¯ ν , and Λ c → n ℓ ¯ ν , we examine a relation among their lepton-flavor universality ratios R H μ e . Although the charm sum rule is less precise than the relations in bottom-hadron decays, current low-energy and high- p T constraints on new physics restrict the actual deviation from the relation to below the percent level. The relation can therefore provide a useful consistency check of charm semileptonic measurements. As an application, we derive a prediction for the yet-unmeasured ratio R n μ e in Λ c → n ℓ ¯ ν .
- 2 November 2026
- Published by American Physical Society (APS) in Physical Review D
- Vol. 114 (9)
Open AccessSemileptonic sum rules in heavy-to-light charm decays
We investigate semileptonic sum rules in heavy-to-light charm decays, motivated by analogous relations in b → c and b → u transitions. Focusing on the c → d ℓ ¯ ν decays, D → π ℓ ¯ ν , D → ρ ℓ ¯ ν , and Λ c → n ℓ ¯ ν , we examine a relation among their lepton-flavor universality ratios R H μ e . Although the charm sum rule is less precise than the relations in bottom-hadron decays, current low-energy and high- p T constraints on new physics restrict the actual deviation from the relation to below the percent level. The relation can therefore provide a useful consistency check of charm semileptonic measurements. As an application, we derive a prediction for the yet-unmeasured ratio R n μ e in Λ c → n ℓ ¯ ν .
- Open Access
- 21 January 2026
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2026 (1) , 143
b → c semileptonic sum rule: current status and prospects
A bstract: The b → c semileptonic sum rules provide relations between the decay rates of $$B\to {D}^{(*)}\tau \overline{\nu }$$ and $${\Lambda }_{b}\to {\Lambda }_{c}\tau \overline{\nu }$$ . Starting from the heavy quark and zero-recoil limits, we revisit the derivation of the sum rule for total decay rates. We then examine deviations from the limits and investigate corrections arising from realistic hadron masses and higher-order contributions to form factors, taking account of uncertainties. We show that these corrections are negligible compared to current experimental uncertainties, indicating that the sum rule is useful for cross-checking experimental consistency and testing the validity of the Standard Model predictions. In future, precise determinations of the form factors particularly for the tensor operator will be necessary to compare the sum rule predictions with $${\Lambda }_{b}\to {\Lambda }_{c}\tau \overline{\nu }$$ data from the LHCb experiment and the Tera-Z projects.- 21 January 2026
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2026 (1) , 143
Open Accessb → c semileptonic sum rule: current status and prospects
A bstract: The b → c semileptonic sum rules provide relations between the decay rates of $$B\to {D}^{(*)}\tau \overline{\nu }$$ and $${\Lambda }_{b}\to {\Lambda }_{c}\tau \overline{\nu }$$ . Starting from the heavy quark and zero-recoil limits, we revisit the derivation of the sum rule for total decay rates. We then examine deviations from the limits and investigate corrections arising from realistic hadron masses and higher-order contributions to form factors, taking account of uncertainties. We show that these corrections are negligible compared to current experimental uncertainties, indicating that the sum rule is useful for cross-checking experimental consistency and testing the validity of the Standard Model predictions. In future, precise determinations of the form factors particularly for the tensor operator will be necessary to compare the sum rule predictions with $${\Lambda }_{b}\to {\Lambda }_{c}\tau \overline{\nu }$$ data from the LHCb experiment and the Tera-Z projects. - Open Access
- 20 January 2026
- Published by American Physical Society (APS) in Physical Review D
- Vol. 113 (1)
Constructing heavy-quark sum rule for b → c meson and baryon decays
We study heavy-hadron semileptonic decays proceeding via b → c transition, such as B → D ( * ) τ ν ¯ τ and Λ b → Λ c τ ν ¯ τ . In the heavy-quark limit, where the heavy-quark symmetry holds, we provide a fundamental framework for heavy-quark sum rules among these decays based on the spin decomposition picture. The relation holds directly for the squared amplitudes without requiring phase-space integration. We then apply this relation to reproduce the sum rule among B → D ( * ) τ ν ¯ τ and Λ b → Λ c τ ν ¯ τ . Furthermore, we derive new sum rules for Ω b → Ω c ( * ) transitions and those involving excited states, such as B → { D 0 * , D 1 * } and B → { D 1 , D 2 * } .
- 20 January 2026
- Published by American Physical Society (APS) in Physical Review D
- Vol. 113 (1)
Open AccessConstructing heavy-quark sum rule for b → c meson and baryon decays
We study heavy-hadron semileptonic decays proceeding via b → c transition, such as B → D ( * ) τ ν ¯ τ and Λ b → Λ c τ ν ¯ τ . In the heavy-quark limit, where the heavy-quark symmetry holds, we provide a fundamental framework for heavy-quark sum rules among these decays based on the spin decomposition picture. The relation holds directly for the squared amplitudes without requiring phase-space integration. We then apply this relation to reproduce the sum rule among B → D ( * ) τ ν ¯ τ and Λ b → Λ c τ ν ¯ τ . Furthermore, we derive new sum rules for Ω b → Ω c ( * ) transitions and those involving excited states, such as B → { D 0 * , D 1 * } and B → { D 1 , D 2 * } .
- Open Access
- 23 September 2025
- Published by Springer Nature in The European Physical Journal C
- Vol. 85 (9) , 1
Publisher Erratum: $b \rightarrow c$ semileptonic sum rule: extension to angular observables- 23 September 2025
- Published by Springer Nature in The European Physical Journal C
- Vol. 85 (9) , 1
Open AccessPublisher Erratum: $b \rightarrow c$ semileptonic sum rule: extension to angular observables - Open Access
- 9 September 2025
- Published by Springer Nature in The European Physical Journal C
- Vol. 85 (9) , 1-11
$b \rightarrow c$ semileptonic sum rule: extension to angular observablesLepton flavor universality is a key prediction of the Standard Model of particle physics, and any violation of it immediately indicates the existence of new physics. Given the recent discrepancy of more than $$4\sigma $$ in charged current semileptonic B meson decays and the absence of evident signals at the Large Hadron Collider, independent cross-checks become invaluable. In this context, $$b \rightarrow c$$ semileptonic sum rules based on heavy quark symmetry are interesting, since they allow us to check the consistency of experimental results. In this paper, we report newly found sum rules among angular observables of mesonic and baryonic $$b\rightarrow c l {\overline{\nu }}$$ decays holding exactly in the large mass limit of heavy quarks. Moreover, we investigate corrections to the sum rule in realistic situations and discuss phenomenological implications.- 9 September 2025
- Published by Springer Nature in The European Physical Journal C
- Vol. 85 (9) , 1-11
Open Access$b \rightarrow c$ semileptonic sum rule: extension to angular observablesLepton flavor universality is a key prediction of the Standard Model of particle physics, and any violation of it immediately indicates the existence of new physics. Given the recent discrepancy of more than $$4\sigma $$ in charged current semileptonic B meson decays and the absence of evident signals at the Large Hadron Collider, independent cross-checks become invaluable. In this context, $$b \rightarrow c$$ semileptonic sum rules based on heavy quark symmetry are interesting, since they allow us to check the consistency of experimental results. In this paper, we report newly found sum rules among angular observables of mesonic and baryonic $$b\rightarrow c l {\overline{\nu }}$$ decays holding exactly in the large mass limit of heavy quarks. Moreover, we investigate corrections to the sum rule in realistic situations and discuss phenomenological implications. - Open Access
- 15 July 2025
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2025 (7) , 1-56
On sum rules for semi-leptonic b → c and b → u decays
A bstract: The semi-leptonic b → clν processes are receiving a lot of attention, as the lepton flavor universality (LFU) violation has been hinted by the measured ratios $$ {R}_{D^{\left(\ast \right)}} $$ = Γ( B → D (∗) τν )/Γ( B → D (∗) ℓν ) for ℓ = e , μ . Recently, it has also been pointed out that the baryonic counterpart, $$ {R}_{\Lambda_c} $$ = Γ(Λ b → Λ c τν )/Γ(Λ b → Λ c ℓν ), has a strong correlation with $$ {R}_{D^{\left(\ast \right)}} $$ , referred to as the R ratio sum rule in this paper. The correlation is almost independent of the new physics (NP) implication and hence can predict $$ {R}_{\Lambda_c} $$ from the measured $$ {R}_{D^{\left(\ast \right)}} $$ . On the other hand, we have fewer measurements and/or theoretical studies of the semi-leptonic b → ulν processes, although the same arguments can be applied to the ratios R π , R ρ , and R p as above. Since these processes are measurable at the ongoing LHCb run-3 and/or Belle II experiments, precise studies on them are important as well. In this paper, we obtain the semi-analytic formulae for all the aforementioned R X ratios in the presence of NP contributions by using the available lattice QCD and/or light-cone sum rule fits to the form factors. Two novel points are highlighted: (i) We evaluate uncertainties of R X including both the Standard Model (SM) and NP terms, inherited from the form factor fits, and discuss how the uncertainties affect the R ratio sum rules. (ii) We obtain the R ratio sum rule among the semi-leptonic b → ulν processes for the first time, which provides a complementary motivation for observing these processes. In addition, based on our numerical results, we investigate how the different NP scenarios work in the b → c and b → u sectors and perform a combined study in the framework of SM effective field theory with specific flavor symmetries.- 15 July 2025
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2025 (7) , 1-56
Open AccessOn sum rules for semi-leptonic b → c and b → u decays
A bstract: The semi-leptonic b → clν processes are receiving a lot of attention, as the lepton flavor universality (LFU) violation has been hinted by the measured ratios $$ {R}_{D^{\left(\ast \right)}} $$ = Γ( B → D (∗) τν )/Γ( B → D (∗) ℓν ) for ℓ = e , μ . Recently, it has also been pointed out that the baryonic counterpart, $$ {R}_{\Lambda_c} $$ = Γ(Λ b → Λ c τν )/Γ(Λ b → Λ c ℓν ), has a strong correlation with $$ {R}_{D^{\left(\ast \right)}} $$ , referred to as the R ratio sum rule in this paper. The correlation is almost independent of the new physics (NP) implication and hence can predict $$ {R}_{\Lambda_c} $$ from the measured $$ {R}_{D^{\left(\ast \right)}} $$ . On the other hand, we have fewer measurements and/or theoretical studies of the semi-leptonic b → ulν processes, although the same arguments can be applied to the ratios R π , R ρ , and R p as above. Since these processes are measurable at the ongoing LHCb run-3 and/or Belle II experiments, precise studies on them are important as well. In this paper, we obtain the semi-analytic formulae for all the aforementioned R X ratios in the presence of NP contributions by using the available lattice QCD and/or light-cone sum rule fits to the form factors. Two novel points are highlighted: (i) We evaluate uncertainties of R X including both the Standard Model (SM) and NP terms, inherited from the form factor fits, and discuss how the uncertainties affect the R ratio sum rules. (ii) We obtain the R ratio sum rule among the semi-leptonic b → ulν processes for the first time, which provides a complementary motivation for observing these processes. In addition, based on our numerical results, we investigate how the different NP scenarios work in the b → c and b → u sectors and perform a combined study in the framework of SM effective field theory with specific flavor symmetries. - Open Access
- 14 May 2025
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2025 (5) , 1-19
Heavy quark symmetry behind b → c semileptonic sum rule
Lepton flavor universality violations in semileptonic b → c transitions have garnered attention over a decade. For $${R}_{{H}_{c}}={\text{BR}}\left({H}_{b}\to {H}_{c}\tau {\overline{\nu }}_{\tau }\right)/{\text{BR}}\left({H}_{b}\to {H}_{c}{\ell}{\overline{\nu }}_{{\ell}}\right)$$ with ℓ being e, μ, a sum rule among R D , $${R}_{{D}^{*}}$$ and R Λc was proposed to check consistency in the experimental results independently of new physics models. We revisit this relation from the perspective of the heavy quark symmetry. We derive a sum rule holding exactly in the heavy quark limit and clarify how model-dependent corrections are introduced in a realistic situation.- 14 May 2025
- Published by Springer Nature in Journal of High Energy Physics
- Vol. 2025 (5) , 1-19
Open AccessHeavy quark symmetry behind b → c semileptonic sum rule
Lepton flavor universality violations in semileptonic b → c transitions have garnered attention over a decade. For $${R}_{{H}_{c}}={\text{BR}}\left({H}_{b}\to {H}_{c}\tau {\overline{\nu }}_{\tau }\right)/{\text{BR}}\left({H}_{b}\to {H}_{c}{\ell}{\overline{\nu }}_{{\ell}}\right)$$ with ℓ being e, μ, a sum rule among R D , $${R}_{{D}^{*}}$$ and R Λc was proposed to check consistency in the experimental results independently of new physics models. We revisit this relation from the perspective of the heavy quark symmetry. We derive a sum rule holding exactly in the heavy quark limit and clarify how model-dependent corrections are introduced in a realistic situation. - Open Access
- 9 October 2024
- Published by American Physical Society (APS) in Physical Review D
- Vol. 110 (7)
Global fit to anomalies as of Spring 2024Recently, several new experimental results of the test of lepton flavor universality (LFU) in B → D ( * ) semileptonic decays were announced: the first result of R D from the LHCb run 1 data, the first results of R D and R D * from the LHCb run 2 data, and the first result of R D * from the Belle II collaboration. Including these new data, a global analysis still prefers the violation of the LFU between the tau and light leptons. A new world average of the data from the , LHCb, Belle, and Belle II collaborations is R D = 0.342 ± 0.026 and R D * = 0.287 ± 0.012 . Including this new data, we update a circumstance of the b → c τ ν ¯ measurements and their implications for new physics (NP). Incorporating recent developments for the B → D ( * ) form factors in the Standard Model, we observe a 4.4 σ deviation from the Standard Model prediction. Our updates also include model-independent NP formulas for the related observables and the global fittings of parameters for leptoquark scenarios as well as single NP operator scenarios. Furthermore, we show future potential to indirectly distinguish different NP scenarios with the use of the precise measurements of the polarization observables in B → D ( * ) τ ν ¯ at the Belle II and the high- p T flavored-tail searches at the LHC. We also discuss an impact on the LFU violation in ϒ → l + l − . Published by the American Physical Society 2024
- 9 October 2024
- Published by American Physical Society (APS) in Physical Review D
- Vol. 110 (7)
Open AccessGlobal fit to anomalies as of Spring 2024Recently, several new experimental results of the test of lepton flavor universality (LFU) in B → D ( * ) semileptonic decays were announced: the first result of R D from the LHCb run 1 data, the first results of R D and R D * from the LHCb run 2 data, and the first result of R D * from the Belle II collaboration. Including these new data, a global analysis still prefers the violation of the LFU between the tau and light leptons. A new world average of the data from the , LHCb, Belle, and Belle II collaborations is R D = 0.342 ± 0.026 and R D * = 0.287 ± 0.012 . Including this new data, we update a circumstance of the b → c τ ν ¯ measurements and their implications for new physics (NP). Incorporating recent developments for the B → D ( * ) form factors in the Standard Model, we observe a 4.4 σ deviation from the Standard Model prediction. Our updates also include model-independent NP formulas for the related observables and the global fittings of parameters for leptoquark scenarios as well as single NP operator scenarios. Furthermore, we show future potential to indirectly distinguish different NP scenarios with the use of the precise measurements of the polarization observables in B → D ( * ) τ ν ¯ at the Belle II and the high- p T flavored-tail searches at the LHC. We also discuss an impact on the LFU violation in ϒ → l + l − . Published by the American Physical Society 2024
- Open AccessSummary of the CKM 2023 Working Group on $V_{ub}$, $V_{cb}$ and semileptonic/leptonic B decays including $τ$This work summarizes recent results, both theoretical and experimental, in $B$-meson leptonic and semileptonic decays and metrology of $V_{ub}$ and $V_{cb}$, which were presented at the CKM 2023 workshop. We place these results in context and discuss future prospects in the field.Open AccessSummary of the CKM 2023 Working Group on $V_{ub}$, $V_{cb}$ and semileptonic/leptonic B decays including $τ$This work summarizes recent results, both theoretical and experimental, in $B$-meson leptonic and semileptonic decays and metrology of $V_{ub}$ and $V_{cb}$, which were presented at the CKM 2023 workshop. We place these results in context and discuss future prospects in the field.
- Open Access
- 8 March 2023
- Published by American Physical Society (APS) in Physical Review D
- Vol. 107 (5) , 055005
Impact of measurement on new physics in transitionsMeasurements of the branching ratios of and by the BABAR, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio shows a relative deficit in taus. In this paper, we critically address whether data still points towards a coherent pattern of deviations, in particular in light of the sum rule relating these decays in a model-independent way. We find that no common new physics explanation of all ratios is possible (within or , depending on the normalization to light lepton channels). While this inconsistency could be a statistical fluctuation, further measurements are required in order to converge to a coherent pattern of experimental results.- 8 March 2023
- Published by American Physical Society (APS) in Physical Review D
- Vol. 107 (5) , 055005
Open AccessImpact of measurement on new physics in transitionsMeasurements of the branching ratios of and by the BABAR, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio shows a relative deficit in taus. In this paper, we critically address whether data still points towards a coherent pattern of deviations, in particular in light of the sum rule relating these decays in a model-independent way. We find that no common new physics explanation of all ratios is possible (within or , depending on the normalization to light lepton channels). While this inconsistency could be a statistical fluctuation, further measurements are required in order to converge to a coherent pattern of experimental results.