arXiv subjects

N Rajeev

Publications and source records attributed to N Rajeev.

9 recordsLinked to original sources

arXiv2026

Neutrino mass variables in 3 active and 2 sterile neutrino scenario

The three-flavor framework of neutrino oscillations successfully explains most experimental results, but persistent anomalies at short- and long-baseline experiments hint at the existence of additional light sterile states. In particular, eV-scale sterile neutrinos are motivated by LSND and MiniBooNE results, while sub-eV sterile states with mass-squared differences at the $10^{-2}$ and $10^{-5}$~eV$^2$ scales have been proposed to address the T2K--NO$\nu$A tension and the absence of the expected upturn in the solar neutrino energy spectrum, respectively. Such sterile states are singlets under the Standard Model gauge group and mix only through their admixture with active neutrinos. In this work, we investigate the phenomenology of the $3+2$ scenario, incorporating one eV-scale sterile neutrino together with a sub-eV state, and analyze their impact on absolute-mass related observables: the sum of neutrino masses $\Sigma$ constrained by cosmology, the effective electron neutrino mass $m_\beta$ from beta decay, and the effective Majorana mass $m_{\beta\beta}$ probed in neutrinoless double beta decay. We demonstrate that the presence of two sterile states can significantly modify the allowed parameter space compared to the three-flavor and $3+1$ frameworks, with some mass-ordering schemes already disfavored by current cosmological and laboratory limits. Finally, we assess the implications of upcoming sensitivities from KATRIN, Project~8, and LEGEND-1000, highlighting the complementary role of sub-eV sterile neutrinos in probing physics beyond the minimal three-flavor paradigm.

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arXiv2025

Constraints on lepton flavor universal and non-universal New Physics in $b\, \to\, s\, \ell^+ \ell^-$ decays: a global SMEFT survey

The flavor-changing neutral current semileptonic decays of $B$ mesons provide an excellent platform for indirectly probing New Physics (NP) beyond the Standard Model (SM). Recent measurements of lepton flavor universality (LFU) ratios such as $R_K$, $R_{K^*}$, and $R_\phi$ are consistent with SM predictions, thereby reducing earlier hints of LFU violation. Nevertheless, notable tensions persist in individual branching fractions -- for instance $\mathcal{B}(B^+ \to K^+ \mu^+ \mu^-)$ and $\mathcal{B}(B^+ \to K^+ e^+ e^-)$, which deviate at the $4\sigma$--$5\sigma$ level in the low-$q^2$ region ($[1.1,6]$ GeV$^2$). Similarly, the angular observable $P'_5$ in $B \to K^{*} \mu^+ \mu^-$ shows a $3.3\sigma$ deviation, while $\mathcal{B}(B_s \to \phi \mu^+ \mu^-)$ departs from the SM by $3.6\sigma$. Although such discrepancies could in principle originate from underestimated hadronic effects, they may also indicate possible NP contributions. In this work, we perform a global fit to the latest $b \to s \ell^+ \ell^-$ data within the framework of dimension-6 SMEFT operators. Our analysis systematically incorporates hadronic uncertainties from form factors, non-factorizable corrections, and input parameters, thereby providing a balanced assessment of the NP interpretation. We allow for NP contributions not only in $b \to s \mu^+ \mu^-$ transitions but also in $b \to s e^+ e^-$ channels, thus testing both lepton flavor universal (LFU) and lepton flavor universality violating (LFUV) NP scenarios. The interplay between these possibilities yields distinctive signatures in branching fractions, angular observables, and $\Delta$-observables, offering a comprehensive view of the current flavor anomalies.

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arXiv2024

SMEFT analysis of charged lepton flavor violating $B$-meson decays

Charged lepton flavor violation (cLFV) processes, potentially important for various Beyond the Standard Model Physics scenarios are analyzed in the Standard Model Effective Field Theory (SMEFT) framework. We consider the most relevant 2 quark-2 lepton $(2q2\ell)$ operators for the leptonic and semi-leptonic LFV B-decay (LFVBD) processes $B_s\to μ^+e^- , B^+\to K^+μ^+e^-, B^0\to K^{*0}μ^+ e^-, {\rm and}~ B_s\to ϕμ^-e^+$. We analyse the interplay among the Wilson coefficients responsible for these LFVBDs and other cLFV processes like $\rm CR (μ\to e)$, $\ell_i \to \ell_j γ$, $\ell_i \to\ell_j\ell_k\ell_m$ and $Z \to \ell_i \ell_j$, to find the maximal possible LFV effects in $B$-meson decays. We probe the scale of new physics in relation to the constraints imposed by both classes of the LFV decays while considering both the present bounds and future expectations. In view of proposed experiments at LHCb-II and Belle II to study charged LFV processes, we have also provided the upper limits on the indirect constraints on such LFVBDs. For the processes where $B$ meson is decaying to $μ^{\pm}$ and $e^{\mp}$, we show that new physics can be constrained by an enhancement of 2-4 orders of magnitude on the current sensitivities of the BRs of $B^+\to K^+μ^+e^-, B^0\to K^{*0}μ^+ e^- {\rm and}~ B_s\toϕμ^{\pm}e^{\mp}$.

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arXiv2023

Consequences of $b\, \to\, s\, μ^+\, μ^-$ anomalies on $B \, \to \, K^{(*)} \,ν\, \barν$, $B_s \to\, (η,η') \, ν\,\barν$ and $B_s \, \to \, ϕ\, ν\, \barν$ decay observables

The long persistent discrepancies in $b\,\to\,s\, \ell\, \ell$ quark level transitions continue to be the ideal platform for an indirect search of new physics that lies beyond the SM. The measurements of $R_K$, $R_{K^*}$, $P_5^{\prime}$, $\mathcal{B}(B_s\, \to\, ϕ\, μ^+\,μ^-)$ and $\mathcal{B}(B_s\, \to\, μ^+\,μ^-)$ persistently deviate from the standard model expectations. Similarly, the new tests of lepton flavor universality performed using the isospin partners such as $B^0 \, \to \, K_S^{0} \, \ell\,\ell$ and $B^+ \, \to \, K^{+*} \, \ell\,\ell$ exhibit the same pattern of deviation in $R_{K_S^0}$ and $R_{K^{*+}}$ with the existing results. Motivated by these anomalies we search for the patterns of new physics in the family of flavor changing neutral decays with neutral leptons in the final state undergoing $b \, \to \,s\, ν\, \barν$ quark level transitions. There are close relations between $b\,\to\,s\, \ell\, \ell$ and $b \, \to \,s\, ν\, \barν$ transitions not only in standard model but also in various beyond the standard model scenarios. For beyond the standard model physics under the $\rm SU_L(2)$ gauge symmetry one can relate the left handed charged leptons to the neutrinos. Moreover, there are several advantages of studying $b \, \to \,s\, ν\, \barν$ transitions over $b\,\to\,s\, \ell\, \ell$ as they are free from various hadronic uncertainties beyond the form factors such as the non-factorizable corrections and photonic penguin contributions. Hence, we explore the consequences of $b\, \to\, s\, μ^+\, μ^-$ anomalies on $B \, \to \, K^{(*)} \,ν\, \barν$, $B_s \to\, (η,η') \, ν\,\barν$ and $B_s \, \to \, ϕ\, ν\, \barν$ decay observables in SMEFT platform within various 1D and 2D new physics scenarios.

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arXiv2023

Combined analysis of $B_c \to D_s^{()}\,μ^+ μ^-$ and $B_c \to D_s^{()}\, ν\barν$ decays within $Z^{\prime}$ and leptoquark new physics models

We investigate the exclusive rare semileptonic decays $ B_c \to D_{s}^{(*)}\,(\ell \ell, ν\barν)$ induced by neutral current transition $b \to s (\ell \ell,ν\barν)$ in the presence of non-universal $Z^{\prime}$, scalar and vector leptoquark new physics models. We constrain the new physics parameter space by using the latest experimental measurements of $R_{K^{(*)}}$, $P_5^{\prime}$, $\mathcal{B}(B_s \to ϕμ^+ μ^-)$ and $\mathcal{B}(B_s \to μ^+ μ^-)$. Throughout the analysis, we choose to work with the particular new physics scenario $C_9 ^{μμ}(NP)=-C_{10} ^{μμ}(NP)$ where both $Z^{\prime}$, $S_{1/3}^3$ and $U_{-2/3}^3$ leptoquarks satisfy the condition. Using these new coupling parameters we scrutinize the several physical observables such as differential branching fraction, the forward backward asymmetry, the lepton polarization asymmetry, the angular observable $P^{\prime}_5$ and the lepton flavor universal sensitive observables including the ratio of branching ratio $R_{D_s^{(*)}}$ and the few $Q$ parameters in the $B_c \to D_s^{(*)}\,μ^+ μ^-$ and $B_c \to D_s^{(*)}\, ν\barν$ decay processes.

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arXiv2021

Angular analysis of $B_s\, \to\, f_{2}'\,(1525)\,(\to K^+\,K^-)\,μ^+ \,μ^-$ decays as a probe to lepton flavor universality violation

The flavor anomalies reported in $R_K$, $R_{K^*}$, $P_{5}^{\prime}$ and $\mathcal{B}(B_s\, \to\, ϕ\, μ^+\, μ^-)$ indicate lepton flavor universality violation in $b \to s\,l^+\,l^-$ quark level transition decays. The deviation from the SM prediction reported in the underlying flavor observables currently stand at the level of $2.5σ$, $2.4σ$, $3.3σ$ and $3.7σ$, respectively. In this context, we perform an angular analysis of the four-body differential decay of $B_s\, \to\, f_{2}^{\prime}\,(1525)\,(\to K^+\,K^-)\,μ^+ \,μ^-$ in a model independent effective field theory framework. The decay mode $B_s\, \to\, f_{2}^{\prime}(1525)\,l^+\,l^-$ undergoes similar $b \to s$ neutral current quark level transition and, in principle, can provide complementary information regarding lepton flavor universality violation in $b \to s\,l^+\,l^-$ quark level transition decays. We give predictions of various physical observables such as the branching ratio, the longitudinal polarization fraction, the forward-backward asymmetry, the angular observables $P_1$, $P_2$, $P_{4}^{\prime}$, $P_{5}^{\prime}$ and also the lepton flavor sensitive observables such as the ratio of branching ratio $R_{f_{2}^{\prime}}$, $Q_{F_L}$, $Q_{A_{FB}}$, $Q_{1}$, $Q_{2}$, $Q_{4}^{\prime}$, $Q_{5}^{\prime}$ for $B_s\, \to\, f_{2}^{\prime}\,(1525)\,(\to K^+\,K^-)\,μ^+ \,μ^-$ decays in the standard model and in the presence of several 1D and 2D new physics scenarios.

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arXiv2019

Signature of lepton flavor universality violation in $B_s \to D_s τν$ semileptonic decays

Deviation from the standard model prediction is observed in many semileptonic $B$ decays mediated via $b \to c$ charged current interactions. In particular, current experimental measurements of the ratio of branching ratio $R_D$ and $R_{D^{\ast}}$ in $B \rightarrow D^{(*)}l ν$ decays disagree with standard model expectations at the level of about $4.1σ$. Moreover, recent measurement of the ratio of branching ratio $R_{J/Ψ}$ by LHCb, where $R_{J/Ψ} = \mathcal B(B_c \to J/Ψ\,τν)/\mathcal B(B_c \to J/Ψ\,μν)$, is more than $2σ$ away from the standard model prediction. In this context, we consider an effective Lagrangian in the presence of vector and scalar new physics couplings to study the implications of $R_D$ and $R_{D^{\ast}}$ anomalies in $B_s \to D_s\,τν$ decays. We give prediction of several observables such as branching ratio, ratio of branching ratio, forward backward asymmetry parameter, $τ$ polarization fraction, and the convexity parameter for the $B_s \to D_s\,τν$ decays within the standard model and within various new physics scenarios.

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arXiv2019

Probing new physics in $B_s \to (K,K^*)τν$ and $B \to πτν$ decays

Motivated by the anomalies present in $b \to u$ and $b \to c$ semileptonic decays, we study the corresponding $B_s \to (K,K^*) τν$ and $B \to πτν$ decays within an effective field theory formalism. Our analysis is based on a strict model dependent assumption, i.e., we assume that $b \to u$ and $b \to c$ transition decays exhibit similar new physics pattern. We give prediction of various observables such as the branching fraction, ratio of branching ratio, lepton side forward-backward asymmetry, longitudinal polarization fraction of the charged lepton and convexity parameter in the standard model and in the presence of vector type new physics couplings.

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arXiv2019

Implication of $R_{D^{(*)}}$ anomalies on semileptonic decays of $Σ_b$ and $Ω_b$ baryons

The flavor changing decays of heavy bottom quarks to the corresponding lighter quarks ($u$, $c$ and $s$) in various $B$-meson decays via charged current and neutral current semileptonic transitions have emerged as promising candidates to explore the physics beyond the standard model. Experimentally the lepton flavor universality violation in $b \to (c,u)\,l\,ν$ and $b \to s\,l^+ l^-$ transitions have been reported to a higher precision. The measurements of the lepton flavor violating ratios such as $R_{D^{(*)}}$, $R_{J/Ψ}$ and $R_{K^{(*)}}$ are observed to deviate from the standard model expectations at the level of $1.4σ$, $2.5σ$, $1.5σ$, $2.4σ$ and $2.2σ$ respectively. Motivated by these anomalies, we investigate the lepton flavor universality violation in $Σ_b \to Σ_c l ν$ and $Ω_b \to Ω_c l ν$ decays. We follow a model independent effective field theory formalism and study the implications of $R_{D^{(*)}}$ anomalies on $Σ_b \to Σ_c τν$ and $Ω_b \to Ω_c τν$ decay modes. We give predictions of various physical observables such as the ratio of branching ratios, total differential decay rate, forward-backward asymmetry, lepton side polarization fraction and convexity parameter within the standard model and within various new physics scenarios.

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