[Paper Review] Effects of a charged Higgs boson in $B ightarrow D^{(*)} au u$ decays

Alejandro Celis|arXiv (Cornell University)|Aug 30, 2013

Particle physics theoretical and experimental studies17 references3 citations

TL;DR

This paper investigates whether a charged Higgs boson in the Two-Higgs-Doublet Model (2HDM) can explain the observed excess in the branching ratio R(D*) from B → D*τν decays, while remaining consistent with other flavor constraints. It finds that tree-level charged Higgs contributions require a departure from family-universal Yukawa couplings, and that current data on R(D*), Br(B→τν), and D(s) leptonic decays are incompatible with the 2HDM unless new flavor structures are introduced.

ABSTRACT

In this talk I consider the current status of $B ightarrow D^{(*)} au u$ decays in the context of two-Higgs-doublet models. The BaBar collaboration has reported an excess in semileptonic $b ightarrow c au u$ transitions that has gathered recent interest as a possible signature of new physics. The sensitivity of these decays to tree-level charged Higgs boson contributions is discussed, taking into account the constraints from other semileptonic and leptonic meson decays in which the charged Higgs would enter at the same level.

Motivation & Objective

• To assess whether a charged Higgs boson in the 2HDM can explain the observed excess in R(D*) from BaBar data.
• To examine the compatibility of such a scenario with existing constraints from other semileptonic and leptonic decays involving charged Higgs at tree level.
• To determine whether the observed R(D*) anomaly can be explained without introducing tree-level flavor-changing neutral currents (FCNCs).
• To identify the required Yukawa coupling structures in the 2HDM to reconcile R(D*) with experimental data while satisfying other flavor observables.
• To explore whether specific kinematic observables in B→D(∗)τν decays can serve as clean probes of charged Higgs contributions independent of Yukawa structure.

Proposed method

• Uses the Aligned Two-Higgs-Doublet Model (A2HDM) to parametrize charged Higgs couplings to fermions via complex, flavor-non-universal alignment parameters ςf.
• Applies the effective Lagrangian L ⊃ −√2/v H+ [¯u(ςdVMdPR − ςuM†uVPL)d + ςl¯νMlPRl] + h.c. to describe tree-level charged Higgs interactions.
• Compares theoretical predictions for R(D), R(D*), Br(B→τν), and D(s) leptonic decays with experimental measurements and SM expectations.
• Performs a global fit in the complex planes of ςdς∗l/M²H± and ςuς∗l/M²H± to determine allowed regions at 95% CL.
• Identifies kinematic observables X1(q²) and XD∗₂(q²) that are insensitive to charged Higgs contributions and thus serve as clean probes of non-scalar new physics.
• Uses precision measurements from LEP and Belle/BaBar data to constrain the charged Higgs mass and Yukawa couplings.

Experimental results

Research questions

• RQ1 Can the observed R(D*) excess in B → D*τν decays be explained by a charged Higgs boson in the 2HDM?
• RQ2 What are the constraints on the charged Higgs mass and Yukawa couplings from other flavor observables, such as Br(B→τν), D(s)→τν, and π/τ decays?
• RQ3 Is it possible to explain the R(D*) anomaly in the 2HDM without introducing tree-level flavor-changing neutral currents?
• RQ4 Which kinematic observables in B→D(∗)τν decays are insensitive to charged Higgs contributions and can thus serve as clean tests of new physics?
• RQ5 What is the required structure of the Yukawa couplings to simultaneously explain R(D*) and Br(B→τν) within the 2HDM framework?

Key findings

• The BaBar measurement of R(D*) = 0.332±0.024±0.018 shows a 2.7σ excess over the SM prediction of 0.252±0.002±0.003.
• The experimental value of R(D) = 0.440±0.058±0.042 is now consistent with the SM when updated lattice QCD calculations are used, reducing the significance of the anomaly.
• The combination of R(D*), Br(B→τν), and D(s) leptonic decay rates cannot be simultaneously explained in the 2HDM without a departure from family universality in the Yukawa couplings.
• The preferred values for Re(ςuς∗l)/M²H± required to explain R(D*) are negative, while those needed to explain Br(B→τν) and D(s)→τν favor positive values, indicating a strong tension.
• Constraints from LEP on the Z→bb¯ and Z→ff¯ widths imply MH± > 39.6 GeV at 95% CL, independent of decay modes or Yukawa structure.
• Observables X1(q²) and XD∗₂(q²) are insensitive to charged Higgs contributions and can be used to test for non-scalar new physics in future q²-binned analyses.