Refining new-physics searches in B→Dτν with lattice QCD.

Area of Science:

• High Energy Physics
• Particle Physics
• Quantum Chromodynamics

Background:

• The semileptonic decay B→Dτν is sensitive to new physics, particularly scalar currents.
• Experimental data from BABAR shows a discrepancy with Standard Model predictions for the R(D) ratio.

Purpose of the Study:

• To compute the R(D) ratio using unquenched lattice QCD for a precise Standard Model prediction.
• To investigate the impact of charged scalar exchange, like from a charged Higgs boson, on the R(D) ratio.

Main Methods:

• Utilized hadronic form factors computed via ab initio full QCD lattice calculations.
• Employed unquenched lattice QCD for improved accuracy in form factor determination.

Main Results:

• Calculated R(D) = 0.316(12)(7), the first Standard Model calculation from full QCD with reduced uncertainty.
• The computed R(D) value reduces the tension with experimental measurements by approximately 1σ.
• Predicted the longitudinal-polarization ratio P(L)(D) = 0.325(4)(3).

Conclusions:

• The precise lattice QCD calculation of R(D) offers a more accurate Standard Model prediction.
• The results are consistent with models involving charged scalar exchange, such as the type-II two-Higgs-doublet model.
• This work provides a crucial Standard Model benchmark for understanding B meson decays.