Probing New Physics Through Bottom and Top Quark Decays.
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Date
2020-10
Researcher
Saini, Jyoti
Supervisor
Alok, Ashutosh Kumar
Journal Title
Journal ISSN
Volume Title
Publisher
Indian Institute of Technology Jodhpur
Abstract
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 1014 and is probably immeasurable at the LHC until new physics enhances its branching ratio up to the current detection level which is 104-105. 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 104 provided the couplings are complex.
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Citation
Saini, Jyoti. (2020). Probing New Physics Through Bottom and Top Quark Decays (Doctor's thesis). Indian Institute of Technology Jodhpur, Jodhpur.