arXiv subjects

Rahul Sinha

Publications and source records attributed to Rahul Sinha.

At least 19 recordsLinked to original sources

arXiv2025

Entropic active particle transport in pulsating 3D geometries

We study the transport of active Brownian particles (ABPs) in three-dimensional (3D) oscillatory geometries, which are spatially periodic. We establish a generalized Fick-Jacobs approach, which reduces a 3D system to an effective 1D system based on the assumption that a fast equilibration of particles along the transversal directions of the geometry. The transport characteristics of ABPs are computed semi-analytically and corroborated by numerical simulations. At the optimal frequency of the geometry oscillation, particles exhibit higher average velocity $\langle v \rangle$ and effective diffusion coefficient $D_{\text{eff}}$, resembling the phenomena of stochastic resonance. This effect is further enhanced by the self-propelled velocity of ABPs and the amplitude of geometry oscillations. These findings have significant implications for the development of micro- and nanofluidic devices with enhanced control over particle transport and precise manipulation of small-scale biomedical devices.

cond-mat.soft↗

arXiv2025

Implications of the evidence for direct $\mathbf{CP}$ violation in $D\to \pi^+\pi^-$ decays

The observation of $CP$ violation in the difference of $CP$ asymmetries between $D\to K^+K^-$ and $D\to \pi^+\pi^-$ has raised a debate whether the observed asymmetries can be regarded as a signal of physics beyond the standard model (SM). In this paper we obtain all the topological amplitudes and isospin amplitudes directly from measured observables for $D\to \pi\pi$. These results unambiguously imply a very large penguin contribution, having a central value $4.74$ times the magnitude of the amplitude for $D^0\to\pi^+ \pi^-$. This fitted central value differs from a reasonable SM estimate of $10\%$ with a significance greater than $3.3\sigma$. In contrast to previous studies, we present model-independent arguments based only on unitarity of re-scattering amplitudes to show that large penguins cannot arise from re-scattering alone and likely indicate physics beyond the SM. In a model-independent approach we show how a very small contribution from physics beyond the SM with a large weak phase alleviates the problem.

hep-ph↗

arXiv2023

Extracting $\gamma$ from CP violating decays of bottom baryons

The observation of CP violation in meson decays is a testament to the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing paradigm, and an integral part of the Standard Model (SM). The SM also predicts the existence of CP violation in baryon decays that is yet to be observed. A critical test of the SM requires that CP violation be measured in baryon decays as well, in order to verify that it agrees with the measurement using meson decays. In this paper, we propose a modification to the recently proposed method to measure CP violating phase $\gamma$ in $b$-baryons, using interference arising implicitly due to Bose symmetry considerations of the decay amplitudes.

hep-ph↗

arXiv2022

Measuring $\mathbf{CP}$ violating phase in beauty baryon decays

One of the outstanding problems in physics is to explain the baryon-anti-baryon asymmetry observed in nature. According to the well-known Sakharov criterion for explaining the observed asymmetry, it is essential that $CP$ violation exist. Even though $CP$ violation has been observed in meson decays and is an integral part of the standard model (SM), measurements in meson decays indicate that $CP$ violation in the SM is insufficient to explain the observed baryon-anti-baryon asymmetry. SM predicts the existence of yet to be observed $CP$ violation in baryon decays. A critical test of the SM requires that $CP$ violation be measured in baryon decays as well, in order to verify that it agrees with the measurement using meson decays. In this letter we propose a new method to measure $CP$ violating phase in $b$-baryons, using interference arising implicitly due to Bose symmetry considerations of the decaying amplitudes.

hep-ph↗

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.

hep-ph↗

arXiv2020

Beauty baryon non-leptonic decays into decuplet baryons and $CP$-asymmetries based on $SU(3)$-Flavor analysis

We consider charmless weak decays of beauty-baryons into decuplet baryons and pseudoscalar mesons in a general framework based on $SU(3)$-flavor decomposition of the decay amplitudes. The dynamical assumption independent $SU(3)$ analysis accounts for the effects of an arbitrarily broken $SU(3)$ symmetry in these decays. An alternative approach in terms of quark diagrams is also provided and compared with the $SU(3)$ decomposition in the limit of exact $SU(3)$-flavor symmetry. Furthermore, the symmetries of the effective Hamiltonian is used to relate or neglect reduced $SU(3)$ amplitudes to derive several sum rule relations between amplitudes and relations between $CP$ asymmetries in these decays and identify those that hold even if $SU(3)$ is broken.

hep-ph↗

arXiv2020

Constraining electroweak penguin graph contributions in measurements of the CKM phase alpha using $B\toππ$ and $B\toρρ$ decays

The unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix has been well established by both direct and indirect measurements without any evidence of discrepancy. The CKM weak phase $α$ is directly measured using an isospin analysis in $B\rightarrow ππ$ and $B\to ρρ$ assuming that electroweak penguin contributions are ignorable. However, electroweak penguins are sensitive to NP, hence, it is important to experimentally estimate their effects. We determine the size of both electroweak penguin and isospin amplitudes, directly from $B\rightarrow ππ$ and $B\to ρρ$ experimental data, using in addition the indirectly measured value of $α$. We find that electroweak penguin contribution are indeed small and agree with SM expectations within $1σ$. We also find that there is a mild enhancement of the $ΔI=\tfrac{1}{2}$ transition amplitude.

hep-ph↗

arXiv2019

Non-leptonic beauty baryon decays and $CP$-asymmetries based on $SU(3)$-Flavor analysis

We consider hadronic weak decays of beauty-baryons into charmless baryons and pseudoscalar mesons in a general framework based on $SU(3)$ decomposition of the decay amplitudes. The advantage of the approach lies in the ability to perform an $SU(3)$ analysis of these decays without any particular set of dynamical assumptions while accounting for the effects of an arbitrarily broken $SU(3)$ flavor symmetry. Dictated by the symmetries of the effective Hamiltonian that allow us to relate or neglect reduced $SU(3)$ amplitudes, we derive several sum rule relations between amplitudes and relations between $CP$ asymmetries in these decays and identify those that hold even if $SU(3)$ is broken.

hep-ph↗

arXiv2019

Lepton mass effects and angular observables in $Λ_b \to Λ(\to p π) \ell^+\ell^-$

The flavor changing rare decay $B\to K^{*}(\to Kπ)\ell^+\ell^-$ is one of the most studied modes due to its sensitivity to physics beyond the standard model and several discrepancies have come to light among the plethora of observables that are measured. In this paper we revisit the analogous baryonic decay mode $Λ_{b}\rightarrow Λ(\to pπ) \ell^{+}\ell^{-}$ and we present a complete set of ten angular observables that can be measured using this decay mode. Our calculations are done retaining the finite lepton mass so that the signal of lepton non-universality observed in $B\to K^{*} \ell^+\ell^-$ can be corroborated by the corresponding baryonic decay mode. We show that due to the parity violating nature of the subsequent $Λ\to pπ$ decay there exist at least one angular asymmetry that is non-vanishing in the large recoil limit unlike the case in $B\to K^{*}\ell^+\ell^-$ decay mode, making it particularly sensitive to new physics that violates lepton flavor universality.

hep-ph↗

arXiv2019

Radiative $b$-baryon decays to measure the photon and $b$-baryon polarization

The radiative decays of $b$-baryons facilitate the direct measurement of photon helicity in $b\to sγ$ transitions thus serving as an important test of physics beyond the Standard Model. In this paper we analyze the complete angular distribution of ground state $b$-baryon ($Λ_{b}^{0}$ and $Ξ_{b}^{-}$) radiative decays to multibody final states assuming an initially polarized $b$-baryon sample. Our sensitivity study suggests that the photon polarization asymmetry can be extracted to a good accuracy along with a simultaneous measurement of the initial $b$-baryon polarization. With higher yields of $b$-baryons, achievable in subsequent runs of the Large Hadron Collider (LHC), we find that the photon polarization measurement can play a pivotal role in constraining different new physics scenarios.

hep-ph↗

arXiv2019

Testing $WWγ$ vertex in radiative muon decay

Large numbers of muons will be produced at facilities developed to probe lepton flavor violating process $μ\to eγ$. We show that by constructing a suitable asymmetry, radiative muon decay $μ\to e γν_μ\barν_e$ can also be used to test the $WWγ$ vertex at such facilities. The process has two missing neutrinos in the final state and on integrating their momenta, the partial differential decay rate shows no radiation-amplitude-zero. We establish, however, that an easily separable part of the normalized differential decay rate, odd under the exchange of photon and electron energies, does have a zero in the case of standard model (SM). This "new type of zero" has hitherto not been studied in literature. A suitably constructed asymmetry using this fact, enables a sensitive probe for the $WWγ$ vertex beyond the SM. With a simplistic analysis, we find that the $C$ and $P$ conserving dimension four $WWγ$ vertex can be probed at ${\cal O}(10^{-2})$ with satisfactory significance level.

hep-ph↗

arXiv2018

$R_{K^{()}}$ and $R(D^{()})$ anomalies resolved with lepton mixing

In a recent paper arXiv:1706.08437, we have advanced a minimal resolution of some of the persistent anomalies in semileptonic $B$-decays. These include the neutral-current observables $R_K$ and $R_{K^*}$, as well as the charged-current observables $R(D)$ and $R(D^*)$. Recently, it has been observed that the semileptonic decays of the $B_c$ meson also hint at a similar type of anomaly. In this longer version, we discuss in detail why, if the anomalies are indeed there, it is a challenging task to explain the data consistently in terms of a simple and compelling new physics scenario. We find that the minimal scheme to achieve a reasonable fit involves the inclusion of just two (or, at worst, three with a possible symmetry relationship between their Wilson coefficients) new current-current operators, constructed in terms of the flavour eigenstates, augmented by a change of basis for the charged lepton fields. With only three unknown parameters, this class of models not only explain all the anomalies (including that in $B_c \to J/ψ\, \ell ν$) to a satisfactory level but also predict some interesting signatures, like $B\to Kμτ$, $B_s\to ττ$, or $B\to K$ plus missing energy, that can be observed at LHCb or Belle-II.

hep-ph↗

arXiv2018

Using time-dependent indirect $CP$ asymmetries to measure $T$ and $CPT$ violation in $B^0$-${\bar B}^0$ mixing

Quantum field theory, which is the basis for all of particle physics, requires that all processes respect $CPT$ invariance. It is therefore of paramount importance to test the validity of $CPT$ conservation. In this Letter, we show that the time-dependent, indirect $CP$ asymmetries involving $B$ decays to a $CP$ eigenstate contain enough information to measure $T$ and $CPT$ violation in $B^0$-${\bar B}^0$ mixing, in addition to the standard $CP$-violating weak phases. Entangled $B^0{\bar B}^0$ states are not required (so that this analysis can be carried out at LHCb, as well as at the $B$ factories), penguin pollution need not be neglected, and the measurements can be made even if the $B^0$-${\bar B}^0$ width difference vanishes.

hep-ph↗

arXiv2017

Prediction of the $CP$ asymmetry $C_{00}$ in $B^0 \to D^0\overline{D^0}$ decay

Of all $B \to D \overline{D}$ decays, the $B^0 \to D^0 \overline{D^0}$ decay has the smallest observed branching ratio as it takes place primarily via the suppressed $W$-exchange diagram. The $CP$ asymmetry for this mode is yet to be measured experimentally. By exploiting the relationship among the decay amplitudes of $B \to D\overline{D}$ decays (using isospin and topological amplitudes) we are able to relate the $CP$ asymmetries and branching ratios by a simple expression. This enables us to predict the $CP$ asymmetry $C_{00}$ in $B^0 \to D^0 \overline{D^0}$. While the predicted central values of $C_{00}$ are outside the physically allowed region, they are currently associated with large uncertainties owing to the large errors in the measurements of the $B^0 \to D^0 \overline{D^0}$ branching ratio ($B_{00}$), the other $CP$ asymmetries $C_{+-}$ (of $B^0 \to D^+ D^-$) and $A_{\text{CP}}$ (of $B^+ \to D^+ \overline{D^0}$). With a precise determination of $B_{00}$, $C_{+-}$ and $A_{\text{CP}}$, one can use our analytical result to predict $C_{00}$ with a reduced error and compare it with the experimental measurement when it becomes available. The correlation between $B_{00}$ and $C_{00}$ is an interesting aspect that can be probed in ongoing and future particle physics experiments such as LHCb and Belle II.

hep-ph↗

arXiv2017

Minimal unified resolution to $R_{K^{()}}$ and $R(D^{()})$ anomalies with lepton mixing

It is a challenging task to explain, in terms of a simple and compelling new physics scenario, the intriguing discrepancies between the standard model expectations and the data for the neutral-current observables $R_K$ and $R_{K^*}$, as well as the charged-current observables $R(D)$ and $R(D^*)$. We show that this can be achieved in an effective theory with only two unknown parameters. In addition, this class of models predicts some interesting signatures in the context of both $B$ decays as well as high-energy collisions.

hep-ph↗

arXiv2017

Signal of right-handed currents using $B\to K^*\ell^+\ell^-$ observables at the kinematic endpoint

The decay mode $B\to K^*\ell^+\ell^-$ is one of the most promising modes to probe physics beyond the standard model (SM), since the angular distribution of the decay products enable measurement of several constraining observables. LHCb has recently measured these observables using $3fb^{-1}$ of data as a binned function of $q^2$, the dilepton invariant mass squared. We find that LHCb data implies evidence for right-handed currents, which are absent in the SM. These conclusions are derived in the maximum $q^2$ limit and are free from hadronic corrections. Our approach differs from other approaches that probe new physics at low $q^2$ as it does not require estimates of hadronic parameters but relies instead on heavy quark symmetries that are reliable at the maximum $q^2$ kinematic endpoint.

hep-ph↗

arXiv2017

Implications from ${B\to K^*\ell^+\ell^-}$ observables using $3 \text{fb}^{-1}$ of LHCb data

The decay mode $B\to K^*\ell^+\ell^-$ results in the measurement of a large number of related observables by studying the angular distribution of the decay products and is regarded as a sensitive probe of physics beyond the standard model (SM). Recently, LHCb has measured several of these observables using $3 \text{fb}^{-1}$ data, as a binned function of $q^2$, the dilepton invariant mass squared. We show how data can be used without any approximations to extract theoretical parameters describing the decay and to obtain a relation amongst observables within the SM. We find three kinds of significant disagreement between theoretical expectations and values obtained by fits. The values of the form factors obtained from experimental data show significant discrepancies when compared with theoretical expectations in several $q^2$ bins. We emphasize that this discrepancy cannot arise due to resonances and non-factorizable contributions from charm loops. Further, a relation between form factors expected to hold at large $q^2$ is very significantly violated. Finally, the relation between observables also indicates some deviations in the forward-backward asymmetry in the same $q^2$ regions. These discrepancies are possible evidence of physics beyond the SM.

hep-ph↗

arXiv2015

Disentangling the Spin-Parity of a Resonance via the Gold-Plated Decay Mode

Searching for new resonances and finding out their properties is an essential part of any existing or future particle physics experiment. The nature of a new resonance is characterized by its spin, charge conjugation, parity, and its couplings with the existing particles of the Standard Model. If a new resonance is found in the four lepton final state produced via two intermediate $Z$ bosons, the resonance could be a new heavy scalar or a $Z'$ boson or even a higher spin particle. In such cases the step by step methodology as enunciated in this paper can be followed to determine the spin, parity and the coupling to two $Z$ bosons of the parent particles, in a fully model-independent way. In our approach we show how three uni-angular distributions and few experimentally measurable observables can conclusively tell us about the spin, parity as well as the couplings of the new resonance to two $Z$ bosons. We have performed a numerical analysis to validate our approach and showed how the uniangular observables can be used to disentangle the spin parity as well as coupling of the resonance.

hep-ph↗