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Soumitra Nandi

Publications and source records attributed to Soumitra Nandi.

At least 55 records · Page 3Linked to original sources
arXiv2018

New physics with the lepton flavor violating decay $τ\to 3μ$

Lepton flavour violating (LFV) processes are a smoking gun signal of new physics (NP). If the semileptonic $B$ decay anomalies are indeed due to some NP, such operators can potentially lead to LFV decays involving the second and the third generation leptons, like $τ\to 3μ$. In this paper, we explore how far the nature of NP can be unraveled at the next generation $B$-factories like Belle-II, provided the decay $τ\to 3μ$ has been observed. We use four observables with which the differentiation among NP operators may be achieved to a high confidence level. Possible presence of multiple NP operators are also analysed with the Optimal Observable technique. While the analysis can be improved even further if the final state muon polarisations are measured, we present this work as a motivational tool for the experimentalists, as well as a template for the analysis of similar processes.

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arXiv2017

Extraction of $|V_{cb}|$ from $B\to D^{(*)}\ellν_\ell$ and the Standard Model predictions of $R(D^{(*)})$

We extract $|V_{cb}|$ from the available data in the decay $B \to D^{(*)}\ellν_{\ell}$. Our analysis uses the $q^2(w)$ binned differential decay rates in different subsamples of $B\to D\ellν_\ell$ ($\ell = e, μ$), while for the decay $B\to D^*\ellν_\ell$, the unfolded binned differential decay rates of four kinematic variables including the $q^2$ bins have been used. In the CLN and BGL parameterizations of the form factors, the combined fit to all the available data along with their correlations yields $|V_{cb}| = (39.77 \pm 0.89)\times 10^{-3}$ and $(40.90 \pm 0.94)\times 10^{-3}$ respectively. In these fits, we have used the inputs from lattice and light cone sum rule (LCSR) along with the data. Using our fit results and the HQET relations (with the known corrections included) amongst the form factors, and parameterizing the unknown higher order corrections (in the ratios of HQET form factors) with a conservative estimate of the normalizing parameters, we obtain $R(D^{*}) = 0.259 \pm 0.006$ (CLN) and $R(D^*) = 0.257 \pm 0.005$ (BGL).

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arXiv2017

Looking for possible new physics in $B\to D^{(\ast)}τν_τ$ in light of recent data

We study the decays $B\to D^{(\ast)}τν_τ$ in light of the available data from BABAR, Belle and LHCb. We divide our analysis into two parts: in one part we fit the form-factors in these decays directly from the data without adding any additional new physics (NP) contributions and compare our fit results with those available from the decays $B\to D^{(\ast)}\ellν_{\ell}$. We find that the $q^2$-distributions of the form-factors associated with the pseudo-vector current, obtained from $B\to D^{(\ast)}τν_τ$ and $B\to D^{(\ast)}\ellν_{\ell}$ respectively, do not agree with each other, whereas the other form-factors are consistent with each other. In the next part of our analysis, we look for possible new effective operators of dimension 6 amongst new vector, scalar, and tensor-type that can best explain the current data in the decays $B\to D^{(\ast)}τν_τ$. We use the information-theoretic approaches, especially of `Second-order Akaike Information Criterion' (AIC$_c$) in the analysis of empirical data. Normality tests for the distribution of residuals are done after selecting the best possible scenarios, for cross-validation. We find that it is the contribution from the operator involving left or right-handed vector current that passes all the selection criteria defined for the best-fit scenario and can successfully accommodate all the available data set.

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arXiv2017

Optimal observable analysis for the decay $b \to s$ plus missing energy

The decay $b\to sν\barν$ has been a neglected sibling of $b\to s\ell^+\ell^-$ because neutrinos pass undetected and hence the process offers less number of observables. We show how the decay $b\to s~+$~invisible(s) can shed light, even with a limited number of observables, on possible new physics beyond the Standard Model and also show, quantitatively, the reach of future $B$ factories like SuperBelle to uncover such new physics. Depending on the operator structure of new physics, different channels may act as the best possible probe. We show, using the Optimal Observable technique, how almost the entire parameter space allowed till now can successfully be probed at a high luminosity $B$ factory.

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arXiv2017

Unified resolution of the $R(D)$ and $R(D^*)$ anomalies and the lepton flavor violating decay $h\toμτ$

Taking advantage of the fact that the flavor of the neutrino in semileptonic $B$ decays $B\to D^{(*)}τν$ is not known, we show how a minimal set of higher-dimensional lepton flavor violating (LFV) operators can explain the $R(D^{(*)})$ anomalies, and as a spin-off, can give rise to the LFV decay of the Higgs boson, $h\toμτ$. We also show how none but the minimal set of operators survive the present data.

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arXiv2016

Correlating new physics signals in $B \to D^{(*)} τν_τ$ with $B \to τν_τ$

Semileptonic and purely leptonic decays of B meson to $τ$, such as $B\to D^{(\ast)}τν_τ$ and $B\toτν_τ$ are studied. Recognizing that there already were some weak hints of possible deviations from the SM in the measurements of $\mathcal{B}(B\toτν_τ)$ by \Babar~and Belle and the fact that detection of the $τ$ also occurs in the measurements of $B\to D^{(\ast)}τν_τ$, we stress the importance of joint studies of these processes, whenever possible. For this purpose, as an illustration, we introduce the observable, $\mathcal{R}(D^{(*)})/\mathcal{B}(B\toτν_τ)$ where, for one thing, the unknown systematics due to $τ$ identification are expected to largely cancel. We show that all measurements of this observable are consistent with the existing data, within somewhat largish experimental errors, with the predictions of the SM. We stress that precise experimental measurement and comparison with theory of the branching ratio for $B \to τν_τ$ is extremely important for a reliable search of new physics. Furthermore, in view of the anticipated improved precision in experiments in the next few years, in addition to $\mathcal{R}(D^{(*)})$, host of other ratios analogous to $\mathcal{R}(D^{(*)})/\mathcal{B}(B \to τν_τ)$ in the SM are suggested for lattice calculations as well, so that for more stringent tests of the SM, correlations in lattice calculations can be properly taken into account to enhance precision.

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arXiv2016

Optimal-observable analysis of possible new physics in $B\to D^{(\ast)}τν_τ$

We study all possible observables in $B\to D^{(\ast)}τν_τ$ with new physics (NP), including new vector, scalar and tensor interactions, and investigate the prospects of extracting NP Wilson coefficients with optimal observables. Analysis of the full $q^2$ integrated branching fractions of $B\to D^{(\ast)}τν_τ$ show that the overall sensitivity of the observables of $B\to D τν_τ$ is more towards the scalar current, whereas the bin-by-bin analysis of $q^2$ distribution of the differential branching fraction points to regions of $q^2$ sensitive to tensor interactions. Interestingly, the observables in $B\to D^{\ast}τν_τ$ are more sensitive to tensor interactions, and bin-by-bin analysis of this mode shows the distinct regions of $q^2$ sensitive to vector or scalar interactions. In addition to that, the $τ$ polarisation asymmetry is found to be more sensitive to NP compared to the other observables, in both decay modes.

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arXiv2015

Precision determination of the CKM element Vcb

We extract the magnitude of the CKM matrix element Vcb and the most relevant parameters of the Heavy Quark Expansion from data of inclusive semileptonic B decays. Our calculation includes the recently computed O(alphas Lambda^2/mb^2) corrections and a careful estimate of the residual theoretical uncertainty. Using a recent determination of the charm quark mass, we obtain |Vcb| = (42.21+-0.78)x10-3 and mb_kin(1GeV)=(4.553+-0.020)GeV.

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arXiv2013

Probing CPT Violation in B Systems

We discuss how a possible violation of the combined symmetry CPT in the B meson system can be investigated at the LHC. We show how a tagged and an untagged analysis of the decay modes of both B(d) and B(s) mesons can lead not only to a possible detection of a CPT-violating new physics but also to an understanding of its precise nature. The implication of CPT violation to a large mixing phase in the B(s) system is also discussed.

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arXiv2013

$B_s\to D_s K$ as a Probe of CPT Violation

We discuss some possible signals of CPT violation in the $B_s$ system that may be probed at the Large Hadron Collider (LHC). We show how one can construct combinations ofobservables coming from tagged and untagged decay rates of $B_s\to D_s^\pm K^\mp$ that can unambiguously differentiate between CPT violating and CPT conserving new physics (NP) models contributing in $B_s-\bar B_s$ mixing. We choose this particular mode as an illustrative example for two reasons: (i) In the Standard Model, there is only one decay amplitude, so it is easier to untangle any new physics; (ii) $B_s$ being a neutral meson, it is possible to unambiguously identify any sign of CPT violation that occurs only in mixing but not in decay. We define an observable which is useful to extract the CPT violating parameter in $B_s$ decay, and also discuss how far the results are applicable even if CPT violation is present in both mixing and decay.

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arXiv2012

Two Higgs doublets, a 4th generation and a 125 GeV Higgs: a review

We review the possible role that multi-Higgs models may play in our understanding of the dynamics of a heavy 4th sequential generation of fermions. We describe the underlying ingredients of such models, focusing on two Higgs doublets, and discuss how they may effectively accommodate the low energy phenomenology of such new heavy fermionic degrees of freedom. We also discuss the constraints on these models from precision electroweak data as well as from flavor physics and the implications for collider searches of the Higgs particles and of the 4th generation fermions, bearing in mind the recent observation of a light Higgs with a mass of ~125 GeV.

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arXiv2012

$B_s (\bar B_s)$ -> $D^0_{CP} K {\bar K}$: Detecting and Discriminating New Physics in $B_s$-$\bar B_s$ Mixing

If the weak phase of B_s-anti B_s mixing (2 beta_s) is found to be significantly different from zero, this is a clear signal of new physics (NP). However, if such a signal is found, we would like an unambiguous determination of 2 beta_s in order to ascertain which NP models could be responsible. In addition, in the presence of NP, the width difference ΔΓ_s between the two B_s mass eigenstates can be positive or negative, and ideally this sign ambiguity should be resolved experimentally. Finally, in order to see if the NP is contributing to Γ^s_{12} in addition to M^s_{12}, the precise measurement of |Γ^s_{12}| is crucial. In this paper, we consider several different methods of measuring B_s-anti B_s mixing using two- and three-body decays with ${\bar b} \to {\bar c} u {\bar s}$ and ${\bar b} \to {\bar u} c {\bar s}$ transitions. We find that the most promising of these is a time-dependent Dalitz-plot analysis of $B_s (\bar B_s) \to D^0_{CP} K {\bar K}$. With these decays, all of the above issues can be addressed, and the measurement of the weak phase gamma is also possible. We also note that, with all three-body decays it is possible to resolve the sign ambiguity of ΔΓ_s even without determining the CP phase phi_s.

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arXiv2012

Resolving the sign ambiguity in Delta Gamma_s with B_s -> D_s K

The analysis of tagged B_s -> J/psi phi decays determines the CP phase phi_s in B_s-anti-B_s mixing with a two-fold ambiguity. The solutions differ in the sign of cos(phi_s) which equals the sign of the width difference Delta Gamma_s among the two B_s mass eigenstates. We point out that this ambiguity can be removed with the help of B_s -> D_s K decays. We compare untagged and tagged strategies and find the tagged analysis more promising. The removal of the sign ambiguity in Delta Gamma_s can be done with relatively low statistics and could therefore be a target for the early stage of B_s -> D_s K studies.

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arXiv2012

Repercussions of Flavour Symmetry Breaking on CP Violation in D-Meson Decays

We investigate to what extent the recently measured value for a non-vanishing direct CP asymmetry in D0 -> K+ K- and D0 -> pi+ pi- decays can be accommodated in the Standard Model (SM) or extensions with a constrained flavour sector, for instance from a sequential 4th generation of quarks (4G). From the comparison with D0 -> K- pi+ branching ratios, we establish large U-spin symmetry (d <-> s) breaking effects with large strong phases between different interfering amplitudes. On the basis of conservative estimates on amplitude ratios - which are supported by an analysis of the breaking of a (c <-> u) symmetry in non-leptonic B0 decays - we find that, in the SM, direct CP asymmetries in the pi+ pi- or K+ K- modes (or in their difference) of the order of several per mille are still plausible. Due to the constraints on the new CP phases in the 4G model, only moderate effects compared to the SM estimates are possible. We suggest CP studies at LHCb as well as at (Super)B-factories of several distinctive modes, such as D+ -> Kbar(*)0 pi+, phi pi+ and Ds -> K(*)0 pi+, phi pi+ (K+) etc., which should shed more light on the short- and long-distance issues underlying CP violation in non-leptonic D-meson decays.

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arXiv2011

Muon g-2 and lepton flavor violation in a two Higgs doublets model for the fourth generation

In the minimal Standard Model (SM) with four generations (the so called SM4) and in "standard" two Higgs doublets model (2HDM) setups, e.g., the type II 2HDM with four fermion generations, the contribution of the 4th family heavy leptons to the muon magnetic moment is suppressed and cannot accommodate the measured $ \sim 3 σ$ access with respect to the SM prediction. We show that in a 2HDM for the 4th generation (the 4G2HDM), which we view as a low energy effective theory for dynamical electroweak symmetry breaking, with one of the Higgs doublets coupling only to the 4th family leptons and quarks (thus effectively addressing their large masses), the loop exchanges of the heavy 4th generation neutrino can account for the measured value of the muon anomalous magnetic moment. We also discuss the sensitivity of the lepton flavor violating decays $μ\to e γ$ and $τ\to μγ$ and of the decay $B_s \to μμ$ to the new couplings which control the muon g-2 in our model.

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arXiv2011

Two Higgs doublets with 4th generation fermions - models for TeV-scale compositeness

We construct a class of two Higgs doublets models with a 4th sequential generation of fermions that may effectively accommodate the low energy characteristics and phenomenology of a dynamical electroweak symmetry breaking scenario which is triggered by the condensates of the 4th family fermions. In particular, we single out the heavy quarks by coupling the "heavier" Higgs doublet (Φ_h) which possesses a much larger VEV only to them while the "lighter" doublet (Φ_l) couples only to the light fermions. We study the constraints on these models from precision electroweak data as well as from flavor data. We also discuss some distinct new features that have direct consequences on the production and decays of the 4th family quarks and leptons in high energy colliders; in particular the conventional search strategies for t' and b' may need to be significantly revised.

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