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Browsing Theses by Supervisor "Alok, Ashutosh Kumar"

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    Exploring Physic Beyond the Standard Model: Flavor and Neutrinos
    (Indian Institute of Tehcnology, Jodhpur, 06-03-2024) Alok, Ashutosh Kumar
    The Standard Model (SM) of electroweak interactions offers the most precise depiction of nature at the fundamental level. However, it falls short of being the quintessential theory of nature, as it does not account for phenomena such as matter-antimatter asymmetry of the universe, dark matter, and dark energy. This necessitates a search for physics beyond the SM. We concentrate on flavor and neutrino physics as avenues to explore potential new physics. Flavour physics, especially involving B meson decays, offers a promising avenue to uncover physics beyond the SM. Recent Large Hadron Collider (LHC) experiments at CERN have provided numerous measurements in b → sℓℓ (ℓ = e, μ) transitions, drawing attention due to their intriguing deviations from the SM predictions. The most striking discrepancy lies in the measurement of the branching ratio of Bs → ϕμ+μ− decay, which disagrees with the SM at the level of 3.5σ. The core concept involves investigating the underlying Lorentz structure of new physics, which provides a better fit to current b → sℓℓ data as compared to the SM and then looking for implications of these favored patterns in other related sectors. In order to determine the Lorentz structure of new physics, we performed a model independent global analysis of all b → sℓℓ data within the framework of effective field theory. We subsequently implemented these favored scenarios in a variety of Z′ models, both heavy (TeV scale) as well as light (MeV scale). It was observed that these models adeptly accommodate the current b → s measurements. Further, the impact of b → s measurements on rare charm decays was explored in a non-universal Z′ model where ui → uj transitions can be induced by Z′bs and Z′μμ couplings along with suitable combinations of the CKM matrix. Constraints on Z′ couplings from b → sℓℓ, ΔMs, and neutrino trident data revealed that significant enhancements beyond the SM are not viable. We then investigated the implications of CP conserving b → sℓℓ measurements on the amount of CP violation allowed in B → (K, K∗)μ+μ− decays. For statistically favoured solutions with universal complex couplings, we obtained predictions for various CP violating observables and also examined correlations between them, which provided effective discriminators for new physics solutions. We also considered the impact of b → sℓℓ (ℓ = e, μ) measurements on possible new physics in b → sτ+τ− sector under a framework where both universal and nonuniversal couplings are present. We analyzed several observables in B → K∗τ+τ− decay along with a number of lepton flavour universality violating (LFUV) observables in τ − μ sector. It was observed that the current data allows for deviations from SM spanning from 25% up to even orders of magnitude in several observables. Further, we found that the ratios Rτμ K and Rτμ K∗ diverge from SM even in the presence of only universal new physics scenarios. We also suggested methods to discriminate between these two classes of solutions. Finally, we constructed genuine LFUV observables in the τ −μ sector through an analysis of the full angular distribution of these decays. The flavor systems can also be used to investigate physics emerging from much finer length scales such as quantum decoherence. We devised a formalism using the Kraus operator method to investigate how quantum decoherence affects B meson observables. Through the analysis of purely leptonic, semileptonic, and non-leptonic decays of B mesons, we identified observables that could be influenced by decoherence. Considering that many of these observables can be measured with high precision using the abundant data collected by LHCb and Belle II, our formalism can be applied to establish constraints on the decoherence parameter through multiple decay channels. This offers an alternative set-up for such studies, which, at present, are predominantly conducted in the neutrino sector.The measurements in the flavor sector can have far reaching consequences, for e.g., specific new physics models which can accommodate the current measurements in B sector can also generate neutrino magnetic moment. This electromagnetic property of neutrino offers a unique window into physics beyond the SM. Neutrinos may possess non-zero magnetic dipole moments(μν) due to quantum effects at the loop level. This cannot only serve as clear indicators of new physics but can also have far-reaching implications in particle physics, astrophysics, and cosmology. Due to this generation of the magnetic moment, neutrinos can exhibit spin-flavor oscillations (SFO) in the presence of an external magnetic field. Also, several studies predict the existence of a primordial magnetic field (PMF) in the early Universe, extending back to the era of Big Bang nucleosynthesis (BBN) and before. The recent NANOGrav measurement can be considered as a strong indication of the presence of these PMFs. For Dirac neutrinos, we show that half of the active relic neutrinos can become sterile due to SFO well before becoming non-relativistic owing to the expansion of the Universe and also before the timeline of the formation of galaxies and hence intergalactic fields, subject to the constraints on the combined value of μνB and the cosmic magnetic field at the time of neutrino decoupling. For the upper limit of PMF allowed by the BBN,this can be true even if the experimental bounds on μν approach a few times its SM value. Furthe, we also investigate quantum coherence in neutrino SFO within the interstellar magnetic field of the Milky Way and beyond, quantified by the l1 norm and the relative entropy of coherence. We find that for flavor oscillations, coherence measures can sustain higher values over distances of several kilometers, relevant for terrestrial experiments like reactors and accelerators whereas for SFO, the coherence scale can extend to astrophysical distances.Thus, it becomes apparent that our investigations have yielded pivotal results that hold relevance within the diverse spheres of particle physics, astroparticle physics, cosmology and even Planck scale physics. This overarching theme showcases the interconnectedness and far-reachingimplications across these multifaceted sectors. The exploration of connections between pertinent measurements in particle physics, astroparticle physics, and cosmology is especially vital, given that experiments in all these domains are currently on the cusp of entering a precision era.
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    Probing New Physics Through Bottom and Top Quark Decays.
    (Indian Institute of Technology Jodhpur, 2020-10) Alok, Ashutosh Kumar
    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.
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    A Study of Quantum Mechanical Aspects in Neutrino Oscillations
    (Indian Institute of Technology Jodhpur, 2020-10) Alok, Ashutosh Kumar
    The phenomena of entanglement and the nonlocal features of quantum correlations were initially introduced to elegantly abase the opponents of quantum mechanics. However, owing to the development of the quantum information science, these quantum mechanical features have to be reassessed and to be elevated as resources that may be exploited to achieve tasks that are not possible within the realm of classical physics. Along these lines, quantum resource theories provide the framework to study and quantify these quantum effects, develop new protocols for its detection, and identify processes that optimize its use for a given application. Due to weakly interacting nature, the system of oscillating neutrinos can maintain quantum coherence over a long distance, which can be detected in long baseline experiments. Hence, neutrinos can prove to be promising candidates for various quantum information tasks. Also, the correlation measures used in our analysis can reveal important information about several open problems present in the neutrino sector, which makes this a dual study, important for both neutrino physics and quantum information theory. We study various facets of nonclassicality, quantified by spatial quantum correlations such as flavour entropy, geometric entanglement, Mermin and Svetlichny inequalities, in the system of three flavour neutrino oscillations in the context of ongoing accelerator neutrino experiments NOνA and T2K and upcoming experiment DUNE.We find that various witnesses show sensitivity to the mass-hierarchy problem and CP violation in neutrino physics. In order to test the efficiency and feasibility of neutrinos for QIP tasks, we also incorporated effects of new physics. For this study, we have chosen a measure of quantum coherence, the characteristic element of quantum mechanics which enfolds the defining features of the theory. The new physics effects are incorporated in a model-independent way within the framework of effective field theory, where higher dimensional operators (dimension-6) are added to the Standard Model (SM) Lagrangian. We show that the SM interaction provides favourable conditions for quantum information tasks for normal mass ordering, whereas new physics favours inverted ordering for such tasks. Also, we investigated the variation of coherence and mixedness encapsulated in the neutrino-system under the action of quantum decoherence. It is also interesting to see the features of quantum correlations in the effect of gravitational Zeeman-splitting, causing neutrino-antineutrino oscillations. Another interesting quantum mechanical phenomena, the geometric phase has been studied in neutrino system in the context of experiments T2K, NOνA and reactor neutrino experiments such as Daya Bay and RENO. It is seen that for neutrino experimental facilities where the geometric phase can complete one cycle, all the phase curves corresponding to different values of CP violating phase, converge to a single point, called the cluster point. There are two distinct cluster points for positive and negative signs of Δ31 (large mass-squared difference). Thus the geometric phase can contribute to our understanding of the neutrino mass hierarchy problem.