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Journal of High Energy Physics

, 2017:125 | Cite as

A closer look at the R D and R D* anomalies

  • Debjyoti Bardhan
  • Pritibhajan Byakti
  • Diptimoy Ghosh
Open Access
Regular Article - Theoretical Physics

Abstract

The measurement of R D (R D*), the ratio of the branching fraction of \( \overline{B}\to D\tau {\overline{\nu}}_{\tau}\left(\overline{B}\to {D}^{\ast}\tau {\overline{\nu}}_{\tau}\right) \) to that of \( \overline{B}\to Dl{\overline{\nu}}_l\left(\overline{B}\to {D}^{\ast }l{\overline{\nu}}_l\right) \), shows 1.9σ (3.3σ) deviation from its Standard Model (SM) prediction. The combined deviation is at the level of 4σ according to the Heavy Flavour Averaging Group (HFAG). In this paper, we perform an effective field theory analysis (at the dimension 6 level) of these potential New Physics (NP) signals assuming SU(3)C × SU(2)L × U(1)Y gauge invariance. We first show that, in general, R D and R D* are theoretically independent observables and hence, their theoretical predictions are not correlated. We identify the operators that can explain the experimental measurements of R D and R D* individually and also together. Motivated by the recent measurement of the τ polarisation in \( \overline{B}\to {D}^{\ast}\tau {\overline{\nu}}_{\tau } \) decay, P τ (D *) by the Belle collaboration, we study the impact of a more precise measurement of P τ (D *) (and a measurement of P τ (D)) on the various possible NP explanations. Furthermore, we show that the measurement of R D* in bins of q 2, the square of the invariant mass of the lepton-neutrino system, along with the information on τ polarisation and the forward-backward asymmetry of the τ lepton, can completely distinguish the various operator structures. We also provide the full expressions of the double differential decay widths for the individual τ helicities in the presence of all the 10 dimension-6 operators that can contribute to these decays.

Keywords

Beyond Standard Model Heavy Quark Physics 

Notes

Open Access

This article is distributed under the terms of the Creative Commons Attribution License (CC-BY 4.0), which permits any use, distribution and reproduction in any medium, provided the original author(s) and source are credited.

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Copyright information

© The Author(s) 2017

Authors and Affiliations

  • Debjyoti Bardhan
    • 1
  • Pritibhajan Byakti
    • 2
  • Diptimoy Ghosh
    • 3
  1. 1.Department of Theoretical PhysicsTata Institute of Fundamental ResearchMumbaiIndia
  2. 2.Department of Theoretical PhysicsIndian Association for the Cultivation of ScienceKolkataIndia
  3. 3.Department of Particle Physics and AstrophysicsWeizmann Institute of ScienceRehovotIsrael

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