Probing New Physics Through Bottom and Top Quark Decays.

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Saini, Jyoti
Alok, Ashutosh Kumar
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Indian Institute of Technology Jodhpur
There are several measurements in the decays of B mesons which show discrepancy with the predictions of the Standard Model (SM) of electro-weak interactions. Many such measurements are in the decays induced by the quark level transition b ! s l+ l􀀀 (l = e; ). In order to discriminate between various new physics solutions and pin down the type of new physics responsible for anomalies in b ! s l+ l􀀀 transition, one should look for alternative observables. The purely leptonic decay of B s meson is considered to be a golden channel to probe beyond SM Physics in b ! s l+ l􀀀 sector as it is theoretically very clean. Assuming new physics only in b ! s + 􀀀, we perform a model independent analysis of new physics in B s ! + 􀀀 decay to identify operator(s) which can provide large enhancement in the branching ratio of B s ! + 􀀀 above its SM value. We find that a significant enhancement in Br(B s ! + 􀀀) is not allowed by any of the allowed solutions. In fact, the present b ! s + 􀀀 data indicates that the future measurements of Br(B s ! + 􀀀) is expected to be suppressed in comparison to the SM. We then consider a new observable, the longitudinal polarization asymmetry of muons in B s ! + 􀀀 decay. We find that this observable is a good discriminant between the new physics solutions if it can be measured to a precision of 10%. We also investigate the potential impact of b ! c anomalies on B s ! + 􀀀 decay in a model where the new physics contributions to these two transitions are strongly correlated. We find that the branching ratio of B s ! + 􀀀 can be enhanced by three orders of magnitude. We then consider new physics only in b ! s e+ e􀀀 decay. Including all measurements in b ! se+e􀀀 sector along with lepton-universality violating ratios RK( ) , we perform a model independent analysis of new physics by considering effective operators in the form of vector/axial-vector (V/A), scalar/pseudoscalar (S/P) and tensors (T). We find that S/P operators cannot account for the anomalous measurements of RK( ) due to tight constraints coming from the upper bound on the branching ratio of Bs ! e+e􀀀. On the other hand, various V/A scenarios can alleviate the tension between RK( ) data and the SM predictions. This includes generating values for RK within 1 of its measured values in the low-q2 bin (0:045 GeV2 q2 1:1 GeV2). Further, we identify angular observables in B ! K e+e􀀀 which can discriminate between the allowed V/A solutions. Moreover, it was previously shown that various combinations of V/A and T new physics operators can also explain RK( ) measurements. We find that K longitudinal polarization fraction, FL, in B ! K e+e􀀀 decay can discriminate against pure V/A and (V/A+T) scenarios. A measurement of FL in (1 􀀀 6) GeV2 bin with an absolute uncertainty of 0.05 can either confirm or rule out any combination of V/A and T new physics scenarios by more than 2 . Finally, we study the impact of B anomalies on rare top quark decay t ! cZ. Top quarks are particularly important for hunting new physics as it is the heaviest of all the SM particles. In particular, the flavour changing neutral current top quark decay t ! cZ has immense potential to probe new physics as it is highly suppressed in the SM. The SM prediction for its branching ratios is 10􀀀14 and is probably immeasurable at the LHC until new physics enhances its branching ratio up to the current detection level which is 10􀀀4-10􀀀5. Using relevant constraints from the B and K sectors, we show that the anomalous tcZ couplings can enhance the branching ratio of t ! cZ at the level of 10􀀀4 provided the couplings are complex.
Saini, Jyoti. (2020). Probing New Physics Through Bottom and Top Quark Decays (Doctor's thesis). Indian Institute of Technology Jodhpur, Jodhpur.