arXiv subjects

Nilakshi Das

Publications and source records attributed to Nilakshi Das.

12 recordsLinked to original sources

arXiv2026

Massive right-handed neutrinos in $\bar{B} \to D^* τ\bar X$ decay

We explore signatures of a massive right-handed neutrino (RHN) in angular distributions of $\bar{B} \to D^* (\to D π) τ(\to πν_τ) \bar X$ decays, where $X$ is an invisible state. We assume the new physics is described by the standard model effective field theory extended with an RHN in the MeV-GeV mass range. We calculate for the first time the full differential distributions in terms of the visible final states, including the decay of the $τ$ lepton. We evaluate the sensitivity of various distributions to the new physics operators.

hep-ph↗

arXiv2026

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

The persistent anomalies observed in $b \to s\,(\ell^+\ell^-,\,ν\barν)$ 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 $b \to s\,(μ^+μ^-,\,ν\barν)$ and $s \to d\,(μ^+μ^-,\,ν\barν)$ transitions. We perform a combined fit to $b \to s\,(μ^+μ^-,\,ν\barν)$ 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 $B \to K^{(*)}ν\barν$ decays, demonstrating their sensitivity to different new physics operators. In addition, we investigate the impact of complex Wilson coefficients on $\mathcal{CP}$ asymmetries in $B \to K^{(*)}μ^+μ^-$ decays and find that percent-level effects can arise in specific $q^2$ regions.

hep-ph↗

arXiv2026

Status of G2HDM with right handed neutrino coupling in the light of $b\to c τν$ anomalies

Recent experimental measurements of several observables in semileptonic B meson decays have pointed towards the possibility of new physics. The LHCb collaboration has reported a significant deviation, exceeding $3.2σ$, in the combined measurement of the ratio of branching ratios $R(D)-R(D^\star)$ from the predictions of the standard model. Furthermore, other observables, such as $R_{J/ψ}$, $P_τ^{D^\star}$, $F_L^{D^\star}$, and $R_{Λ_c}$ in the $b\to c\ellν$ transition, have also exhibited noticeable deviations from the standard model predictions. Motivated by these anomalies in the $b\to cτν$ transitions, we perform a log-likelihood fit incorporating new physics coming from right-handed neutrino couplings and explored the implications of a charged Higgs boson within a generic two Higgs doublet model (G2HDM). Our comprehensive analysis, focused on the $τν$ and $bτν$ final states, was performed using the High Luminosity run of the Large Hadron Collider (HL-LHC). We demonstrate that HL-LHC has the sensitivity to exclude the remaining allowed region in G2HDM model in explaining these anomalies with charged Higgs boson coupled to right-handed neutrino.

hep-ph↗

arXiv2025

Structure and dynamics of finite three-dimensional Yukawa clusters in complex plasmas : Newtonian versus Langevin Dynamics

The structure and dynamics of a harmonically confined three dimensional finite dust cluster are investigated via both Langevin Dynamics (LD) and frictionless Molecular Dynamics (fMD) simulation. The static structure of the system is analyzed through the Radial Distribution Function, Center-two-particle correlation function(C2P) and angular correlation function. The intra-shell angular correlation, Radial Distribution Function and C2P remains largely unaffected by the dynamics employed. However, the inter-shell angular correlation exhibits sharp peaks at irregular angular intervals in fMD which are not seen in LD indicating strongly correlated motion of the two spherical shells in the cluster in fMD. The single particle dynamics of the cluster is characterized by the Van - Hove self autocorrelation function and Mean Squared Displacement (MSD). Notably, the Van - Hove autocorrelation function in fMD simulations exhibits narrower width and higher peak height as compared to the LD simulations, suggesting greater particle mobility in LD. Trajectory analysis reveals a rotational motion of the particles about a common axis in fMD which disappears with progressively increasing friction coefficient. We show that the disappearance of rotational motion with the introduction of neutral friction in the dynamics is due to the much faster relaxation of the interparticle distance as well as interparticle angular separation in LD as compared to fMD.

physics.plasm-ph↗

arXiv2023

New physics analysis of $Λ_b\to (Λ^*(\to pK^-), Λ(\to pπ))(μ^{+}μ^{-},\,ν\barν)$ baryonic decays under SMEFT framework

The di-leptons and di-neutrinos observed in the final states of flavor-changing neutral b decays provide an ideal platform for probing physics beyond the standard model. Although the latest measurements of $R_{K^{(*)}}$ agree well with the standard model prediction, there exists several other observables such as $P_5^{\prime}$, $\mathcal{B}(B_s\to ϕμ^{+}μ^{-})$ and $\mathcal{B}(B_s\to μ^{+}μ^{-})$ in $b\to s \ell\ell$ transition decays that shows deviation from the standard model prediction. Similalry, very recently Belle II collaboration reported a more precise upper bound of $\mathcal{B}(B\to K^+ν\barν) < 4.1\times 10^{-5}$ by employing a new inclusive tagging approach and it also deviates from the standard model expectation. The $b\to s l^{+}l^{-}$ and $b\to sν\barν$ transition decays are related not only in the standard model but also in beyond the standard model physics due to $SU(2)_L$ gauge symmetry, and can be most effectively investigated using the standard model effective field theory formalism. Additionally, the $b\to sν\barν$ decay channels are theoretically cleaner than the corresponding $b\to s l^{+}l^{-}$ decays, as these processes do not get contributions from non-factorizable corrections and photonic penguin contributions. In this context, we study $Λ_b\to (Λ^*(\to pK^-), Λ(\to pπ))(μ^{+}μ^{-},\,ν\barν)$ baryonic decays undergoing $b\to s \ell^{+}\ell^{-}$ and $b\to sν\barν$ quark level transitions in a standard model effective field theory formalism. We give predictions of several observables pertaining to these decay channels in the standard model and in case of several new physics scenarios.

hep-ph↗

arXiv2023

Phase transition of three-dimensional finite-sized charged dust clusters in a plasma environment

The dynamics of a harmonically trapped three-dimensional Yukawa ball of charged dust particles immersed in plasma is investigated as function of external magnetic field and Coulomb coupling parameter using molecular dynamics simulation. It is shown that the harmonically trapped dust particles organize themselves into nested spherical shells. The particles start rotating in a coherent order as the magnetic field reaches a critical value corresponding to the coupling parameter of the system of dust particles. The magnetically controlled charged dust cluster of finite size undergoes a first-order phase transition from disordered to ordered phase. At sufficiently high coupling and strong magnetic field, the vibrational mode of this finite-sized charged dust cluster freezes, and the system retains only rotational motion.

physics.plasm-ph↗

arXiv2021

New physics footprints in the angular distribution of $B_s\to D_s^*(\to D_sγ,\,D_sπ)\, τ\, ν$ decays

Hints of lepton flavor universality violation observed in various flavor ratios such as $R_D$, $R_{D^*}$, $R_{J/ψ}$, $P_τ^{D^*}$ and $F_L^{D^*}$ in $B\to D^{(*)}\,\ell \, ν$ and $B_c\to J/ψ\, \ell\, ν$ charge current decays have opened new avenues to search for indirect evidences of beyond the standard model physics. Motivated by these anomalies, we perform a detailed angular analysis of $B_s\to D_s^*(\to D_sγ,\,D_sπ)\, \ell\, ν$ decays that proceed via similar $b \to c\, \ell\, ν$ quark level transition. We use the most general effective Hamiltonian for $b\to \,c\, lν$ process and give predictions of several $q^2$ and $\cosθ$ dependent observables for the $B_s\to D_s^*(\to D_sγ,\,D_sπ)\, \ell\, ν$ decays in the standard model and in the presence of various real and complex new physics couplings. The results pertaining to this decay are competent to address the anomalies in the charge current sector.

hep-ph↗

arXiv2019

Implication of $b \to cτν$ flavor anomalies on $B_s\rightarrow {D_s^*}τν$ decay observables

The experimental predictions of $R_D$, $R_{D^*}$, $R_{J/ψ}$, $P_τ^{D^*}$ and $F_L^{D^*}$ in $B$ decays mediated via $b\,\to\, c\, l\,ν$ quark level transition deviate significantly from the standard model expectations. The current world average of the ratio of branching ratios $R_D$ and $R_{D^*}$ in $B\,\to\, D^{(*)}\, l\,ν$ ($l\in e,τ$) show $1.4σ$ and $2.5σ$ deviation from the SM expectations. Similarly, the $τ$ polarization fraction $P_τ^{D^*}$ and the longitudinal polarization fraction of the $D^*$ meson $F_L^{D^*}$ in $B\,\to\, D^*\,τ\,ν$ are found to deviate from the standard model expectations at $1.6σ$ and $1.5σ$ level, respectively. In addition, the ratio of branching ratio $R_{J/ψ}$ in $B_c\,\to\,{J/ψ}\, l\,ν$ deviates from the standard model prediction at $2σ$ level. In this regard, we study the implication of $R_D$, $R_{D^*}$, $R_{J/ψ}$, $P_τ^{D^*}$, and $F_L^{D^*}$ anomalies on $B_s\,\to\, {D^*_s}\,τ\,ν$ decay observables in a model independent effective field theory formalism. We give predictions of several physical observables in the standard model and in the presence of various $1D$ and $2D$ new physics scenarios.

hep-ph↗

arXiv2017

Role of Wake Potential on Self-diffusion of Dust Particles in Dissipative System

In this paper, the structural property of complex plasma and self-diffusion coefficient of dust particles in presence of such wake potential have been investigated using Langevin dynamics simulation in the subsonic regime of ion flow. The study reveals that the self-diffusion of dust grains is strongly affected by ion flow and it changes its character in the wake potential dominant to the Debye-Hückel potential dominant regimes. The dependence of self-diffusion coefficient on parameters such as neutral pressure, dust size, ion flow velocity, Coulomb coupling parameter have been calculated for subsonic regime using Green-Kubo expression which is based on integrated velocity autocorrelation function (VACF).

physics.plasm-ph↗

arXiv2014

Self Diffusion of Yukawa system in presence of external magnetic field

Diffusion of dust particles is one of the most significant transport processes of strongly coupled dusty plasma that reflect the nature of inter particle interaction and characterize thermodynamics of the system. In this paper the effect of magnetic field on diffusion of dust particles in strongly coupled dusty plasma is investigated by using molecular dynamics simulations. Self- diffusion coefficients of Yukawa systems are obtained for a wide range of plasma parameters and magnetic field strength using Green- Kubo expression which is based on integrated velocity autocorrelation function (VACF). It is assumed that dust particles interact with each other by modified Yukawa (i.e. screened Coulomb) potential. The study gives interesting results of dust particle diffusion in magnetized plasma.

physics.plasm-ph↗

arXiv2013

Ion-Wake induced anomaly of Dust Lattice Mode in presence of external Magnetic field

We report a theoretical investigation of DL mode (Dust Lattice) in 2D Yukawa crystal in presence of asymmetric ion flow and external magnetic field perpendicular to the crystal plane. Two mutually perpendicular modes are found to be coupled due to Lorentz force. Interaction among the dust grains along vertical direction is strongly affected due to the formation of ion-wake. This causes anisotropy in interaction strength along two mutually perpendicular directions. Both hybrid modes are studied as characteristics of different ion flow speed and magnetic field strength. The study shows a fluctuation in DL mode frequency driven by the strength of the particle-wake interaction. Effect of ion flow on polarization of the hybrid wave amplitudes is discussed in details. Results show a possible mechanism of anomalous phase transition in dusty plasma.

physics.plasm-ph↗

arXiv2013

Thermodynamic properties of strongly coupled plasma in presence of external magnetic field

Thermodynamic properties of a Yukawa system consisting of dust particles in plasma are studied in presence of an external magnetic field. It is assumed that dust particles interact with each other by modified potential in presence of magnetic field. Accordingly, a modified expression for internal energy has been obtained. A molecular dynamics code is developed to calculate this internal energy for the entire system. Based on the values of internal energy given by the code Helmholtz free energy and pressure are calculated for the system. Our study shows novel kind of behaviour for internal energy in presence of magnetic field. Thermodynamic properties are affected significantly by magnetic field. The study helps to express internal energy as a function of Coulomb coupling parameter and magnetic field.

physics.plasm-ph↗