arXiv subjects

Suman Kumbhakar

Publications and source records attributed to Suman Kumbhakar.

22 records · Page 2Linked to original sources

arXiv2019

New physics effects in purely leptonic $B^*_s$ decays

Recently several measurements in the neutral current sector $b\rightarrow s l^+l^-$ ($l=e$ or $μ$) as well as in the charged current sector $b \rightarrow c τ\barν$ show significant deviations from their Standard Model predictions. It has been shown that two different new physics solutions can explain all the anomalies in $b\rightarrow s l^+l^-$ sector. Both these solutions are in the form of linear combinations of the two operators $(\bar{s}γ^αP_Lb)(\barμγ_αμ)$ and $(\bar{s}γ^αP_Lb)(\barμγ_αγ_5μ)$. We show that the longitudinal polarization asymmetry of the muons in $B^*_s\rightarrow μ^+\,μ^-$ decay is a good discriminant between the two solutions if it can be measured to a precision of $10\%$, provided the new physics Wilson coefficients are real. If they are complex, the theoretical uncertainties in this asymmetry are too large to provide effective discrimination. We also investigate the potential impact of $b \rightarrow c τ\barν$ anomalies on $b \rightarrow s τ^+ τ^-$ transitions. We consider a model where the new phyics contributions to these two transitions are strongly correlated. We find that the branching ratio of $B^*_s\rightarrow τ^+\,τ^-$ can be enhanced by three orders of magnitude.

hep-ph↗

arXiv2018

New physics solutions for $R_D$ and $R_{D^*}$

Recent measurements of $R_{D^*}$ have reduced tension with the Standard Model prediction. Taking all the present data into account, we obtain the values of the Wilson coefficients of each new physics four-fermion operator of a given Lorentz structure. We find that the combined data rule out most of the solutions based on scalar/pseudoscalar operators. By studying the inter-relations between different solutions, we find that there are only four allowed solutions, which are based on operators with $(V-A)$, linear combination of $(V-A)$ and $(V+A)$, tensor and linear combination of scalar/pseudoscalar and tensor structure. We demonstrate that the need for new physics is driven by those measurement of $R_D$ and $R_{D^*}$ where the $τ$ lepton is not studied. Further, we show that new physics only in $b\rightarrow c\,μ\,\barν$ is not compatible with the full set of observables in the decays $B\rightarrow Dl\barν$ and $B\rightarrow D^*l\barν$.

hep-ph↗

arXiv2018

Resolution of $R_D$/$R_{D^*}$ puzzle

One of the exciting results in flavor physics in recent times is the $R_D$/$R_{D^*}$ puzzle. The measurements of these flavor ratios performed by the B-factory experiments, BaBar and Belle, and the LHCb experiment are about $4σ$ away from the Standard Model expectation. These measurements indicate that the mechanism of $b\rightarrow cτ\barν$ decay is not identical to that of $b\rightarrow c(μ/e)\barν$. This charge lepton universality violation is particularly intriguing because these decays occur at tree level in the Standard Model. In particular, we expect a moderately large new physics contribution to $b\rightarrow cτ\barν$. The different types of new physics amplitudes, which can explain the $R_D$/$R_{D^*}$ puzzle, have been identified previously. In this letter, we show that the polarization fractions of $τ$ and $D^*$ and the angular asymmetries $A_{FB}$ and $A_{LT}$ in $B\rightarrow D^*τ\barν$ decay have the capability to uniquely identify the Lorentz structure of the new physics. A measurement of these four observables will lead to such an identification.

hep-ph↗

arXiv2017

D* polarization as a probe to discriminate new physics in B --> D* tau nubar

The confirmation of excess in $R_{D^*}$ at the LHCb is an indication of lepton flavor non-universality. Various different new physics operators and their coupling strengths, which provide a good fit to $R_D$, $R_{D^*}$ and $q^2$ spectra, were identfied previously. In this work, we try to find angular observables in $\bar{B} \to D^* τ\barν$ which enable us to distinguish between these new physics operators. We find that the $D^*$ polarization fraction $f_L(q^2)$ is a good discriminant of scalar and tensor new physics operators. The change in $\langle f_L(q^2) \rangle$, induced by scalar and tensor operators, is about three times larger than the expected uncertainty in the upcoming Belle measurement.

hep-ph↗