| General note |
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. Further, 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-reaching implications 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. |
