arXiv subjects

Rusa Mandal

Publications and source records attributed to Rusa Mandal.

At least 19 recordsLinked to original sources

arXiv2026

Complex Scalar Dark Matter with a Vector-Like Quark and Lepton: Precision, Flavor, and HL-LHC

We investigate a minimal extension of the SM consisting of a $\mathbb{Z}_3$-stabilized complex scalar dark matter (CSDM) candidate, a down-type vector-like quark (VLQ), and a charged vector-like lepton (VLL). The additional vector-like fermions not only enable the CSDM to reproduce the observed relic abundance beyond the Higgs-resonance region through semi-annihilation and co-annihilation processes, but also induce correlated signatures across flavor, electroweak precision, dark matter, and collider observables. We perform a comprehensive one-loop analysis of neutral meson mixing, rare meson decays, charged lepton flavor violation, anomalous magnetic moments of charged leptons, and $Z$-pole observables. We find that neutral meson mixing and rare meson decays provide the dominant constraints on the VLQ sector, while charged lepton flavor-violating processes strongly restrict the VLL Yukawa couplings. Current direct-detection limits require Higgs-DM coupling $\lambda_{\Phi H} \lesssim5\times10^{-3}$ and the VLQ Yukawa coupling $\mathtt{y}_d \lesssim 0.05$ for TeV-scale VLQ masses, whereas present indirect-detection searches impose no additional constraints. Combining all flavor, electroweak precision, dark matter, and collider constraints, we identify viable parameter regions with CSDM masses above approximately $1.0~\rm TeV$ and VLQ masses above about $1.5~\rm TeV$. We also find that the LHC can exclude VLQs (VLLs) with masses up to approximately $1.6~(0.38)$ TeV, depending on the CSDM mass, at the $2\sigma$ confidence level. The HL-LHC can further probe an extended region of the parameter space, with discovery prospects at the $3\sigma$ level. Finally, we demonstrate the complementarity of flavor, dark matter, and collider searches in probing this framework.

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arXiv2026

Correlated $b \to s$ and $s \to d$ Rare Semileptonic Transitions in the Standard Model Effective Field Theory

The persistent anomalies observed in $b \to s\,(\ell^+\ell^-,\,\nu\bar{\nu})$ transitions continue to provide strong motivation for exploring possible extensions of the Standard Model (SM). Motivated by these discrepancies, we present a comprehensive analysis of semileptonic flavor changing neutral current processes within the Standard Model Effective Field Theory (SMEFT), encompassing both $b \to s\,(\mu^+\mu^-,\,\nu\bar{\nu})$ and $s \to d\,(\mu^+\mu^-,\,\nu\bar{\nu})$ transitions. We perform a combined fit to $b \to s\,(\mu^+\mu^-,\,\nu\bar{\nu})$ observables, allowing the relevant dimension-six Wilson coefficients to be complex. We find that the four-fermion operators involving left-handed quark and lepton doublets provide the preferred description of the current $b \to s$ data, while the electroweak operator modifying the $Z$-boson couplings also plays an important role in improving the fit. We show that flavor-universal SMEFT couplings lead to strongly enhanced rare semileptonic kaon decay branching ratios, in conflict with current experimental bounds and thus motivating the implementation of Minimal Flavor Violation. In particular, we demonstrate that flavor-symmetric frameworks based on $U(3)^5$ and $U(2)^5$ naturally restore the required CKM hierarchies and bring the predicted kaon observables into agreement with present data. We further analyze the differential distributions with respect to the dineutrino invariant mass squared $q^2$, as well as the reconstructed variable $q^2_{\mathrm{rec}}$, in $B \to K^{(*)}\nu\bar{\nu}$ decays, demonstrating their sensitivity to different new physics operators. In addition, we investigate the impact of complex Wilson coefficients on $\mathcal{CP}$ asymmetries in $B \to K^{(*)}\mu^+\mu^-$ decays and find that percent-level effects can arise in specific $q^2$ regions.

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arXiv2026

Connecting Flavor and Baryon Asymmetry via Leptogenesis in Effective Froggatt-Nielsen Theory

We investigate the hierarchical flavor structure of the Standard Model (SM) in a Froggatt-Nielsen (FN) framework, where the spontaneous breaking of a $U(1)_{\rm FN}$ symmetry by a complex flavon field generates fermion masses and mixing patterns through higher-dimensional operators. Extending the setup with three right-handed neutrinos (RHNs), light neutrino masses arise via the Type-I seesaw mechanism. Allowing complex FN coefficients enables a consistent description of the CKM and PMNS matrices while inducing CP-violating signatures in meson decays. Building on our previous work, where the lightest RHN acts as a viable dark matter (DM) candidate produced through freeze-in or freeze-out mechanisms, we investigate the origin of the baryon asymmetry of the Universe. The heavier RHNs generate a lepton asymmetry through out-of-equilibrium decays and scatterings, including both SM channels and additional flavon-induced processes in which the flavon appears as an initial-state particle. We compute the corresponding one-loop CP asymmetries and incorporate these effects in the Boltzmann equations. We show that although freeze-in and freeze-out DM production occur in two qualitatively distinct regions of the FN symmetry-breaking scale $v_\phi$, successful thermal leptogenesis can be achieved in both regimes. In the large-$v_\phi$ (freeze-in-compatible) region, the results approach the standard leptogenesis limit, while in the freeze-out-compatible region the lower value of $v_\phi$ implies lighter RHNs, requiring resonant enhancement. This tightly constrained framework, in which $v_\phi$ simultaneously controls RHN masses and the interaction strengths of the flavon and DM sectors, provides a predictive and unified description of flavor hierarchies, neutrino masses, CP violation, DM, and baryogenesis within a single effective theory.

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arXiv2025

Investigating non-local contributions in $B_{s} \to ϕ\bar{\ell} \ell$ including higher-twist effects

We analyze the impact of higher-twist three-particle $B_s$-meson light-cone distribution amplitudes (LCDAs) on the non-local form factors for the $B_s\to ϕ\bar{\ell} \ell$ transition focusing on the `charm-loop' contribution within the light-cone sum rule (LCSR) framework. To analytically continue these charm-loop contributions into the kinematically allowed region of the decay, we employ a hadronic dispersion relation that incorporates intermediate resonant states such as the $ϕ,\,J/Ψ$ and $ψ(2S)$ mesons. Here, the LCSR predictions serve as inputs, supplemented by experimental data from two-body decays $B_s \to ϕ~+$ resonance states. Our results indicate that the inclusion of twist-5 and twist-6 LCDAs enhances the non-local form factors by approximately an order of magnitude compared to previous estimates, due to partial disruption of cancellation among different twist contributions. This leads to a dilepton invariant mass-squared ($q^2$)-dependent correction to the Wilson coefficient $C_9$, which is higher than, but still consistent with the Standard Model prediction without the non-factorizable charm-loop corrections within uncertainties. Additionally, we update the local form factors to include contributions from higher-twist three-particle $B_s$-meson LCDAs. The phenomenological implications, particularly for the differential branching fraction and angular observables, are also discussed.

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arXiv2024

Probing the inverse moment of $B_s$-meson distribution amplitude via $B_s \to η_s$ form factors

We investigate the inverse moment of the $B_s$-meson light-cone distribution amplitude (LCDA), denoted as $λ_{B_s}$ and defined within the heavy quark effective theory, through the calculation of $B_s \to η_s$ form factors. The presence of the $s$-quark inside the $B_s$-meson dictates a notable departure of approximately $20\%$ in the $λ_{B_s}$ value compared to the non-strange case $λ_{B_q}$, as computed within the QCD sum rule approach, albeit with significant uncertainty. First, we compute the decay constant of the $η_s$-meson utilizing two-point sum rules while retaining finite $s$-quark mass contributions. Next, we constrain the parameter $λ_{B_s}$ by calculating $B_s \to η_s$ form factors within the light-cone sum rule approach, using $B_s$-meson LCDAs, and leveraging Lattice QCD estimates at zero momentum transfer from the HPQCD collaboration. Our findings yield $λ_{B_s}$ = 480 $\pm$ 92 MeV when expressing the $B_s$-meson LCDAs in the Exponential model, consistent with previous QCD sum rule estimate yet exhibiting a 1.5-fold improvement in uncertainty. Furthermore, we compare the form factor predictions, based on the extracted $λ_{B_s}$ value, with earlier analyses for other channels such as $B_s \to D_s$ and $B_s \to K$.

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arXiv2023

$B\to D_0^$ and $B_s\to D_{s0}^$ form factors from QCD light-cone sum rules

We present the first application of QCD light-cone sum rules (LCSRs) with $B_{(s)}$-meson distribution amplitudes to the $B_{(s)}\!\to\! D_{(s)0}^*$ form factors, where $D_{(s)0}^*$ is a charmed scalar meson. We consider two scenarios for the $D_0^*$ spectrum. In the first one, we follow the Particle Data Group and consider a single broad resonance $D_0^*(2300)$. In the second one, we assume the existence of two scalar resonances, $D_0^*(2105)$ and $D_0^*(2451)$, as follows from a recent theoretically motivated analysis of $B\to Dππ$ decays. The $B\!\to\! D_0^*$ form factors are calculated in both scenarios, also taking into account the large total width of $D_0^*(2300)$. Furthermore, we calculate the $B_s\!\to\! D_{s0}^*$ form factors, considering in this case only the one-resonance scenario with $D_{s0}(2317)$. In this LCSRs calculation, the $c$-quark mass is kept finite and the $s$-quark mass is taken into account. We also include contributions of the two- and three-particle distribution amplitudes up to twist-four. Our predictions for semileptonic $B\!\to\! D_0^*\ellν_\ell$ and $B_s\!\to\! D_{s0}^*\ellν_\ell$ branching ratios are compared with the available data and HQET-based predictions. As a byproduct, we also obtain the $D_0^*$- and $D_{s0}^*$-meson decay constants and predict the lepton flavour universality ratios $R(D_0^*)$ and $R(D_{s0}^*)$.

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arXiv2023

Constraining inverse moment of $B$-meson distribution amplitude using Lattice QCD data

We constrain the inverse moment of the $B$-meson light-cone distribution amplitude (LCDA), $λ_B$ in heavy quark effective theory, using form factor estimates from Lattice QCD collaboration. The estimation of the parameter $λ_B$ has, until now, relied solely on QCD sum rule methods and deals with significant uncertainty. In this work, we express the form factors for the $B \to K$ channel, calculated within the light-cone sum rule (LCSR) approach, in terms of the $B$-meson LCDAs. By incorporating recent Lattice results from the HPQCD collaboration for the $B \to K$ form factors at zero momentum transfer ($q^2$ = 0), we impose constraints on this parameter. Consequently, we achieve a twofold reduction in uncertainty compared to the QCD sum rule estimate, yielding $λ_B=338\pm 68$ MeV, when the $B$-meson LCDAs are expressed in the Exponential model. Additionally, we compare the form factor predictions, using the constrained $λ_B$ value, with the earlier analyses for other channels as well, such as $B\to π$ and $B \to D$.

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arXiv2023

Exploring Freeze-out and Freeze-in Dark Matter via Effective Froggatt-Nielsen Theory

Motivated by the dynamical reasons for the hierarchical structure of the Yukawa sector of the Standard Model (SM), we consider an extension of the SM with a complex scalar field, known as `flavon', based on the Froggatt-Nielsen mechanism. In an effective theory approach, the SM fermion masses and mixing patterns are generated in orders of the parameter related to the vacuum expectation value of the flavon field and the cut-off of the effective theory. By introducing right-handed neutrinos, we study the viability of the lightest right-handed neutrino as a dark matter candidate, where the same flavon field acts as a mediator between the dark and the SM sectors. We find that dark matter genesis is achieved both through freeze-out and freeze-in mechanisms encompassing the $\mathcal{O}(\text{GeV})$ -- $\mathcal{O}(\text{TeV})$ mass range of the mediator and the dark matter particle. In addition to tree-level spin-dependent cross section, the model gives rise to tree- and loop-level contributions to spin-independent scattering cross section at the direct detection experiments such as XENON and LUX-ZEPLIN which can be probed in their future upgrades. By choosing suitable Froggatt-Nielsen charges for the fermions, we also generate the mass spectrum of the SM neutrinos via the Type-I seesaw mechanism. Flavor-changing neutral current processes, such as radiative lepton decay, meson mixing, and top-quark decay remain the most constraining channels and provide testability for this minimal setup that addresses several major shortcomings of the SM.

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arXiv2022

Distinguishing signatures of scalar leptoquarks at hadron and muon colliders

While the hunt for new states beyond the standard model (SM) goes on for various well motivated theories, the leptoquarks are among the most appealing scenarios at recent times due to a series of tensions observed in $B$-meson decays. We consider $SU(2)$ singlet and triplet scalar leptoquarks separately, which contribute to charged and neutral current $B$-meson decays. Focusing on the single production of these two scalar leptoquarks, we perform a PYTHIA-based simulation considering all the dominant SM backgrounds at the current and future setups of the Large Hadron Collider (LHC). The mono-$b$-jet + $\ptmiss$ finalstate gives the strongest signal for the singlet leptoquark at the 30 TeV LHC or Future Circular Collider (FCC), with a possibility of $5σ$ signal significance with $\gtrsim 1000$ \fbi of integrated luminosity, for the chosen benchmark scenarios. The finalstate consisting of a $c$-jet and two $τ$-jets provides highest reach for the singlet leptoquark, probing an $\mathcal{O}(10^{-1})$ value of the Yukawa-type couplings for up to $3.0$ TeV leptoquark mass. For the triplet leptoquark, $1-{\rm jet}+2μ+ \ptmiss$ topology is the most optimistic signature at the LHC, probing leptoquark couplings to fermions at $\mathcal{O}(10^{-1})$ value for the leptoquark mass range up to $ 4.0$ TeV. The invariant mass edge distribution is found to be instrumental in determination of the leptoquark mass scale at the LHC. We also perform the analysis at the proposed multi-TeV muon collider, where an $\mathcal{O}(10^{-1})$ leptoquark Yukawa coupling can be probed for a $5.0$ TeV leptoquark mass.

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arXiv2022

$B\to D_1(2420)$ and $B\to D_1'(2430)$ form factors from QCD light-cone sum rules

We perform the first calculation of form factors in the semileptonic decays $B\!\to\! D_1(2420)\ellν_\ell$ and $B \to D_1^\prime (2430)\ell ν_\ell$ using QCD light-cone sum rules (LCSRs) with $B$-meson distribution amplitudes. In this calculation the $c$-quark mass is finite. Analytical expressions for two-particle contributions up to twist four are obtained. To disentangle the $D_1$ and $D_1^\prime$ contributions in the LCSRs, we suggest a novel approach that introduces a combination of two interpolating currents for these charmed mesons. To fix all the parameters in the LCSRs, we use the two-point QCD sum rules for the decay constants of $D_1$ and $D_1^\prime$ mesons augmented by a single experimental input, that is the $B \to D_1(2420)\ellν_\ell$ decay width. We provide numerical results for all $B\to D_1$ and $B\to D_1^\prime$ form factors. As a byproduct, we also obtain the $D_1$- and $D_1'$-meson decay constants and predict the lepton-flavour universality ratios $R(D_1)$ and $R(D_1')$.

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arXiv2021

Impact of $B \to K ν\bar ν$ measurements on beyond the Standard Model theories

Semileptonic flavor changing neutral current transitions with a pair of neutrinos in the final state are very accurately determined in the standard model (SM) and thus provide an accurate and sensitive probe for physics beyond the SM. Until recently, the poor tagging efficiency for the $B\to K^{(*)}ν\barν$ modes made them less advantageous as a probe of new physics (NP) compared to the charged lepton counterparts. The most recent Belle II result on $B\to K ν\barν$ uses an innovative inclusive tagging technique resulting in a higher tagging efficiency; this together with previous BaBar and Belle results indicates a possible enhancement in the branching fraction of $B^+\to K^+ ν\barν$. A reanalysis of the full Belle dataset together with upcoming Belle II dataset is expected to result in a much more precise measurement of this mode. If the branching ratio is indeed found to be enhanced with improved measurements, this would provide an unambiguous signal of NP without uncertainties due to long-distance non-factorizable effects or power corrections (in contrast to $B\to K^{(*)} \ell \ell$). We have explored the possibilities of such an enhancement as a signal of NP within several scenarios, which can also explain some of the other tensions observed in neutral as well as charged current $B$-decays. In an effective field theory approach, with the most general dimension-six Hamiltonian including light right-handed neutrinos, we explore the viability of all scalar and vector leptoquarks as well as the parameter space possible with a generic vector gauge boson $Z^\prime$ model assuming minimal new particle content. While being consistent with all data, correlations between the observed intriguing discrepancies in $B$-decays are also obtained, which will discriminate between the various NP scenarios.

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arXiv2021

Constraining flavour patterns of scalar leptoquarks in the effective field theory

We investigate the viability of extending the Standard Model with $S_1$ and $S_3$ scalar leptoquarks when the flavour structure is parametrized in terms of Froggatt-Nielsen charges. In contrast to a similar analysis with a vector leptoquark, we find essentially two solutions for the charges that fit the experimental constraints, which are dominated by the current tensions in $B$ decays. These two scenarios differ in their estimate of the anomalous magnetic moment of the muon, $(g-2)$, but they both predict sizeable contributions to $τ\toμγ$, $\bar B_s\toτ^\pmμ^\mp$ and $B^+\to K^+τ^+μ^-$ decays, whose branching ratios are close to the current experimental limits.

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arXiv2020

Inverse moment of the $B_s$-meson distribution amplitude from QCD sum rule

We derive a QCD sum rule for the inverse moment of the $B_s$-meson light-cone distribution amplitude in HQET. Within this method, the $SU(3)_{fl}$ symmetry violation is traced to the strange quark mass and to the difference between strange and nonstrange quark condensate densities. We predict the ratio of inverse moments $λ_{B_s}/λ_B= 1.19 \pm 0.14$ which can be used in various applications of these distribution amplitudes to the analyses of $B_{s}$-meson decays, provided an accurate value of $λ_B$ is available from other sources, such as the $B\to \ell ν_\ell γ$ decay.

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arXiv2020

The role of right-handed neutrinos in $b \to c τ\barν$ anomalies

Motivated by the persistent anomalies reported in the $b\to cτ\barν$ data, we perform a general model-independent analysis of these transitions, in the presence of light right-handed neutrinos. We adopt an effective field theory approach and write a low-energy effective Hamiltonian, including all possible dimension-six operators. The corresponding Wilson coefficients are determined through a numerical fit to all available experimental data. In order to work with a manageable set of free parameters, we define eleven well-motivated scenarios, characterized by the different types of new physics that could mediate these transitions, and analyse which options seem to be preferred by the current measurements. The data exhibit a clear preference for new-physics contributions, and good fits to the data are obtained in several cases. However, the current measurement of the longitudinal $D^*$ polarization in $B\to D^*τ\barν$ cannot be easily accommodated within its experimental $1σ$ range. A general analysis of the three-body $B\to D τ\barν$ and four-body $B\to D^*(\to Dπ)τ\barν$ angular distributions is also presented. The accessible angular observables are studied in order to assess their sensitivity to the different new physics scenarios. Experimental information on these distributions would help to disentangle the dynamical origin of the current anomalies.

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arXiv2020

Feeble neutrino portal dark matter at neutrino detectors

We explore the neutrino portal dark matter (DM) at its minimum of field content having two dark sector particles coupled to a right-handed neutrino. Assuming the feeble nature of their interactions with the standard model (SM) particles, we analyze the freeze-in production of the observed DM relic density characterized by three different categories depending on the major production mechanisms. The portal provides interesting signatures at the neutrino detectors like KamLAND, Super-Kamiokande and IceCube, from a very late decay of the scalar DM to the fermion DM and the SM neutrino. Such neutrino flux spectrum from the cosmic and galactic origins can produce anomalous signals at future experiments.

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arXiv2020

Angular analysis of $\bar{B}\to D_2^*(\to D π)\ell \barν$ decay and new physics

We derive the four-fold angular distribution for the semileptonic decay $\bar{B}\to D_2^*(\to D π)\,\ell \barν$ where $D_2^*$(2460) is a tensor meson. We start with the most general beyond the Standard Model (SM) dimension-six effective Hamiltonian which comprises (axial)vector, (pseudo)scalar and tensor operators for both quark and lepton currents, and it also includes the right-handed neutrinos. The decay can be described by 16 transversity amplitudes and it provides a multitude of observables which can be extracted from data. We investigate the observables in the context of the SM and the new physics scenarios which can explain the intriguing discrepancies observed in the $b \to c τ\barν$ data.

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arXiv2019

Constraints on scalar leptoquarks from lepton and kaon physics

We present a comprehensive analysis of low-energy signals of hypothetical scalar leptoquark interactions in lepton and kaon transitions. We derive the most general effective four-fermion Lagrangian induced by tree-level scalar leptoquark exchange and identify the Wilson coefficients predicted by the five possible types of scalar leptoquarks. The current constraints on the leptoquark Yukawa couplings arising from lepton and kaon processes are worked out, including also loop-induced transitions with only leptons (or quarks) as external states. In the presence of scalar leptoquark interactions, we also derive the differential distributions for flavour-changing neutral-current transitions in semileptonic kaon modes, including all known effects within the Standard Model. Their interference with the new physics contributions could play a significant role in future improvements of those constraints that are currently hampered by poorly-determined non-perturbative parameters.

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arXiv2019

Phenomenology of Higgs bosons in inverse seesaw model with Type-X two Higgs doublet at the LHC

Type-X two Higgs doublet model is known to explain the muon $g-2$ anomaly with a relatively light charged Higgs boson at large $\tanβ$. The light charged Higgs boson has been searched in the main $τν$ mode at the colliders. Invoking a scenario of inverse seesaw as the origin of neutrino masses and mixing, the charged Higgs boson can decay additionally to right-handed neutrinos which leads to interesting phenomenology. Considering generic lepton flavour violating signatures at the final states, a $5σ$ discovery can be achieved with the early data of LHC, at 14 TeV, for relatively large inverse seesaw Yukawa coupling $Y_N$. The very light pseudoscalar and charged Higgs boson mass reconstruction are performed using the new modes and the results look promising. The inverse seesaw Yukawa coupling is shown to be probed down to $Y_N \sim 0.2$ at HL LHC with 3000 fb$^{-1}$.

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