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Discriminating B→D*ℓν form factors via polarization observables and asymmetries


Fedele, Marco; Blanke, Monika; Crivellin, Andreas; Iguro, Syuhei; Nierste, Ulrich; Simula, Silvano; Vittorio, Ludovico (2023). Discriminating B→D*ℓν form factors via polarization observables and asymmetries. Physical review D, 108(5):055037.

Abstract

Form factors are a crucial theory input in order to extract |Vcb| from B→D(*)ℓν decays, to calculate the Standard Model prediction for R(D(*)) and to assess the impact of new physics. In this context, the dispersive matrix approach, a first-principle calculation of the form factors, using no experimental data but rather only lattice QCD results as input, was recently applied to B→D(*)ℓν. It predicts (within the Standard Model) a much milder tension with the R(D∗) measurements than the other form factor approaches, while at the same time giving a value of |Vcb| compatible with the inclusive value. However, this comes at the expense of creating tensions with differential B→D∗ℓν distributions (with light leptons). In this article, we explore the implications of using the dispersive matrix method form factors, in light of the recent Belle (II) measurements of the longitudinal polarization fraction of the D∗ in B→D∗ℓν with light leptons, FℓL, and the forward-backward asymmetry, AℓFB. We find that the dispersive matrix approach predicts a Standard Model value of FℓL that is in significant tension with these measurements, while mild deviations in AℓFB appear. Furthermore, FℓL is very insensitive to new physics such that the latter cannot account for the tension between dispersive matrix predictions and its measurement. While this tension can be resolved by deforming the original dispersive matrix form factor shapes within a global fit, a tension in R(D∗) reemerges. As this tension is milder than for the other form factors, it can be explained by new physics not only in the tau lepton channel but also in the light lepton modes.

Abstract

Form factors are a crucial theory input in order to extract |Vcb| from B→D(*)ℓν decays, to calculate the Standard Model prediction for R(D(*)) and to assess the impact of new physics. In this context, the dispersive matrix approach, a first-principle calculation of the form factors, using no experimental data but rather only lattice QCD results as input, was recently applied to B→D(*)ℓν. It predicts (within the Standard Model) a much milder tension with the R(D∗) measurements than the other form factor approaches, while at the same time giving a value of |Vcb| compatible with the inclusive value. However, this comes at the expense of creating tensions with differential B→D∗ℓν distributions (with light leptons). In this article, we explore the implications of using the dispersive matrix method form factors, in light of the recent Belle (II) measurements of the longitudinal polarization fraction of the D∗ in B→D∗ℓν with light leptons, FℓL, and the forward-backward asymmetry, AℓFB. We find that the dispersive matrix approach predicts a Standard Model value of FℓL that is in significant tension with these measurements, while mild deviations in AℓFB appear. Furthermore, FℓL is very insensitive to new physics such that the latter cannot account for the tension between dispersive matrix predictions and its measurement. While this tension can be resolved by deforming the original dispersive matrix form factor shapes within a global fit, a tension in R(D∗) reemerges. As this tension is milder than for the other form factors, it can be explained by new physics not only in the tau lepton channel but also in the light lepton modes.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Physics Institute
Dewey Decimal Classification:530 Physics
Scopus Subject Areas:Physical Sciences > Nuclear and High Energy Physics
Language:English
Date:29 September 2023
Deposited On:29 Dec 2023 09:58
Last Modified:28 Jun 2024 03:31
Publisher:American Physical Society
ISSN:2470-0010
OA Status:Hybrid
Publisher DOI:https://doi.org/10.1103/physrevd.108.055037
  • Content: Published Version
  • Language: English
  • Licence: Creative Commons: Attribution 4.0 International (CC BY 4.0)