[Paper Review] Impact of $Λ_b o Λ_cτν$ measurement on New Physics in $b o c \, l ν$ transitions

Marco Fedele, Monika Blanke+5 more|arXiv (Cornell University)|Nov 25, 2022

Particle physics theoretical and experimental studies4 citations

TL;DR

This paper investigates whether new physics (NP) can simultaneously explain the observed deviations in lepton flavor universality (LFU) in b → cℓν decays, focusing on the tension between the enhanced R(D*) and R(D) ratios (indicating excess taus) and the suppressed R(Λc) ratio (indicating tau deficit). Using an effective field theory (EFT) approach and a model-independent sum rule, the authors find no consistent NP explanation for all three ratios within 1.5–2σ, suggesting potential inconsistencies in current data that require further experimental scrutiny.

ABSTRACT

Measurements of the branching ratios of $B o D^{(*)}τ\barν/B o D^{(*)}\ell\barν$ and $B_c o J/ψ\, τ\barν/B_c o J/ψ\, \ell\barν$ by the BaBar, Belle and LHCb collaborations consistently point towards an abundance of taus compared to channels with light leptons. However, the ratio $Λ_b oΛ_c τ\barν/Λ_b oΛ_c \ell\barν$ 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 $2σ$ or $1.5σ$, depending on the ${\cal R}(Λ_c)$ 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.

Motivation & Objective

• To assess whether a single new physics (NP) model can explain the observed deviations in lepton flavor universality (LFU) across multiple b → cℓν decays.
• To investigate the compatibility of the R(D), R(D*), and R(Λc) ratios, which are correlated via a model-independent sum rule in the heavy quark limit.
• To test whether NP contributions to both tau and light lepton final states can reconcile the conflicting experimental data.
• To update the sum rule using the latest LHCb and Belle measurements, particularly the R(Λc) result with improved normalization.
• To evaluate whether the current data set points toward a coherent pattern of NP or if inconsistencies suggest statistical fluctuations or systematic issues.

Proposed method

• Employing an effective field theory (EFT) framework to describe NP contributions to b → cℓν transitions, including scalar, vector, and tensor operators.
• Applying a model-independent sum rule that relates R(D), R(D*), and R(Λc) in the heavy quark limit, ensuring consistency across different hadronic final states.
• Performing global fits to experimental data on R(D), R(D*), and R(Λc), including both statistical and systematic uncertainties, with a focus on the R(Λc) normalization to the SM prediction for µ decay.
• Considering UV completions such as two-Higgs doublet models (2HDM), leptoquarks, and W′ bosons to explore viable NP realizations.
• Using Bayesian analysis to constrain NP couplings and assess the compatibility of NP models with all three LFU ratios simultaneously.
• Incorporating lattice QCD form factors and CKM matrix element constraints to reduce theoretical uncertainties in the SM predictions.

Experimental results

Research questions

• RQ1 Can a single new physics model explain the observed excess of taus in B → D(*)τν and Bc → J/ψτν while also accommodating the deficit in Λb → Λcτν?
• RQ2 Is the observed tension in R(Λc) consistent with the pattern of deviations in R(D) and R(D*) within the framework of lepton flavor universality violation?
• RQ3 To what extent can NP couplings to light leptons modify the sum rule and reconcile the conflicting experimental results?
• RQ4 Does the current data set allow for a coherent NP explanation at the 1.5σ or 2σ confidence level across all three ratios?
• RQ5 How sensitive are the conclusions to the normalization of R(Λc) to the SM prediction for the µ decay mode?

Key findings

• No consistent new physics explanation can simultaneously account for the R(D), R(D*), and R(Λc) ratios within 1.5σ or 2σ confidence levels, depending on the normalization of R(Λc).
• The R(Λc) measurement shows a significant deficit in tau decays (R(Λc) = 0.242 ± 0.026 ± 0.040 ± 0.059), in contrast to the excess seen in R(D) and R(D*), indicating a potential inconsistency.
• The sum rule, which relates the three ratios in the heavy quark limit, is violated by current data, suggesting that the observed pattern may not be explained by a single NP sector.
• Including NP couplings to light leptons can modify the sum rule and improve compatibility, but no viable NP model fully reconciles all three ratios within the 2σ range.
• The tension in R(Λc) is not due to large theoretical uncertainties, as the SM prediction RSM(Λc) = 0.324 ± 0.004 has a small uncertainty due to suppression of subleading Isgur-Wise functions.
• The authors conclude that the current data may reflect a statistical fluctuation or systematic issues, and further measurements are essential to resolve the inconsistency.