arXiv subjects

A. K. Giri

Publications and source records attributed to A. K. Giri.

At least 19 recordsLinked to original sources

arXiv2011

Study of some rare decays of B_s meson in the fourth generation model

We study some rare decays of $B_s$ meson governed by the quark level transitions $ b \to s$, in the fourth generation model popularly known as SM4. Recently it has been shown that SM4, which is a simple extension of the SM3, can successfully explain several anomalies observed in the CP violation parameters of $B$ and $B_s$ mesons. We find that in this model due to the additional contributions coming from the heavy $t'$ quark in the loop, the branching ratios and other observables in rare $B_s$ decays deviate significantly from their SM values. Some of these modes are within the reach of LHCb experiment and search for such channels are strongly argued.

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arXiv2010

Fourth generation effect on $Λ_b$ decays

The rare decays of $Λ_b$ baryon governed by the quark level transitions $ b \to s$, are investigated in the fourth quark generation model popularly known as SM4. Recently it has been shown that SM4, which is a very simple extension of the standard model, can successfully explain several anomalies observed in the CP violation parameters of $B$ and $B_s$ mesons. We find that in this model due to the additional contributions coming from the heavy $t'$ quark in the loop, the branching ratios and other observables in rare $Λ_b$ decays deviate significantly from their SM values. Some of these modes are within the reach of LHCb experiment and search for such channels are strongly argued.

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arXiv2008

Explaining B \to K pi anomaly with non-universal Z' boson

We study the effect of non-universal $Z'$ boson in the decay modes $B \to K π$. In the standard model these modes receive dominant contributions from $b \to s$ QCD penguins. Therefore, in this limit one expects $S_{π^0 K^0} \approx \sin 2 β$, $A_{π^0 K^0} \approx 0$ and $A_{π^0 K^-} \approx A_{π^+ K^-}$. The corrections due to the presence of small non-penguin contributions is found to yield $S_{π^0 K^0} > \sin 2 β$ and $ΔA_{CP}(K π) \simeq 2.5 %$. However, the measured value of $S_{π^0 K^0}$ is less than $ \sin 2 β$ and $ΔA_{CP}(K π) \simeq 15 %$. We show the model with a non-universal $Z'$ boson can successfully explain these anomalies.

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arXiv2008

$B_s^0 - \bar B_s^0$ mixing and $b \to s $ transitions in isosinglet down quark model

The recent observation of the mass difference in $B_s$ system seems to be not in complete agreement with the corresponding standard model value. We consider the model with an extra vector like down quark to explain this discrepancy and obtain the constraints on the new physics parameters. Thereafter, we show that with these new constraints this model can successfully explain other observed deviations associated with $b \to s$ transitions, namely, $B_s \to ψϕ$, $B\to K π$ and $B\to ϕK_s$.

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arXiv2008

Signature of new physics in B -> phi pi decay

We investigate the effect of an extra fourth quark generation and FCNC mediated $Z$ and $Z'$ bosons on the rare decay mode $B^- \to ϕπ^-$. In the standard model, this mode receives only $b \to d$ penguin contributions and therefore, highly suppressed with branching ratio $\sim 5 \times 10^{-9}$. This in turn makes this mode a very sensitive probe for new physics. We find that due to the above mentioned new physics contributions there is a significant enhancement in its branching ratio. Furthermore, the direct CP violation parameter which is identically zero in the SM is found to be quite significant. If this mode will be observed in the upcoming LHCb experiment, it will not only provide a clear signal of new physics but also can be used to constrain the new physics parameter space.

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arXiv2008

Probing the unparticle signal in $b \to d $ penguin processes

We investigate the effect of unparticles in the pure $ b \to d $ penguin processes $ B^0 \to K^0 \bar K^0$ and $B^{+,0} \to ϕπ^{+,0} $. Since these processes receive dominant contributions due to the {\it top} quark in the loop, direct and mixing-induced CP asymmetry parameters in these processes are expected to be vanishingly small in the standard model. We find that due to the unparticle effect sizable nonzero CP violation could be possible in these channels.

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arXiv2007

Weak phase information from the color suppressed B_d^0 -> D^{0} K^{0} modes

The decay channels $B_d^0 \to D^{*0} K^{*0}$ are investigated for extracting weak $CKM$ phase information. These channels are described by color-suppressed tree diagrams only and are free from penguin contributions. The branching ratios for these channels are found to be $\sim \cal O $ $(10^{-5} - 10^{-6})$ which can be measured at the currently running $B$ factories. The method presented here may be well-suited to determine the phase $γ$.

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arXiv2007

Determining the CKM angle gamma with B_c decays

We consider the possibility of extracting the CKM angle $γ$ with $B_c$ decays. The modes $B_c^\pm \to (D^0) D_s^\pm \to (K^{*+}K^-) D_s^\pm $ and $B_c^\pm \to (\bar D^0) D_s^\pm \to (K^{*+}K^-) D_s^\pm $ are found to be well suited for the extraction of $γ$. Since a large number of $B_c$ mesons are expected to be produced at the LHC, it would be very interesting to explore the determination of $γ$ with these modes.

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arXiv2007

Unparticle effect on B_s - \bar B_s mixing and its implications for B_s \to J/ψϕ, ϕϕdecays

We study the effect of unparticle stuff on $B_s - \bar{B}_s$ mixing and consider possible implications of it for the decaymodes $B_s \to J/ψϕ$ and $ϕϕ$. We find that due to the new contributions from the unparticles the $B_s - \bar{B}_s$ mixing phase could be observable at the LHC along with the possible sizable CP asymmetry parameters $S_{ψϕ(ϕϕ)}$ in $B_s \to J/ψϕ(ϕϕ)$ decay modes.

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arXiv2007

Possible signatures of unparticles in rare annihilation type B decays

We investigate the effect of unparticles in the pure annihilation type decays $B^- \to D_s^-ϕ$ and $ D_s^- K^{*0}$. Since these decays have only annihilation contributions their branching ratios are expected to be very small in the standard model and the direct CP asymmetry parameters to be zero. We find that due to the unparticle effect these branching ratios can be significantly enhanced from their standard model values. Furthermore, sizable nonzero direct CP violation could also be possible in these channels due to the presence of intrinsic CP conserving phase in the unparticle propagator.

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arXiv2007

Study of FCNC mediated rare B_s decays in a single universal extra dimension scenario

We study the rare semileptonic and radiative leptonic B_s decays in the universal extra dimension model. In this scenario, with a single extra dimension, there exists only one new parameter beyond those of the standard model, which is the inverse of the compactification radius R. We find that with the additional contributions due to the KK modes the branching ratios of the rare B_s decays are enhanced from their corresponding standard model values and the zero point of the forward backward asymmetries are shifted towards the left.

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arXiv2006

Probing new physics in B -> f_0(980) K decays

We study the hadronic decay modes $B^{\pm(0)} \to f_0(980) K^{\pm(0)} $, involving a scalar and a pseudoscalar meson in the final state. These decay modes are dominated by the loop induced $ b \to s \bar q q(q=s,u,d)$ penguins along with a small $ b \to u $ tree level transition (for $B^+ \to f_0 K^+$) and annihilation diagrams. Therefore, the standard model expectation of direct CP violation is negligibly small and the mixing induced CP violation parameter in the mode $B^0 \to f_0 K_S$ is expected to give the same value of $\sin(2β)$, as extracted from $B^0 \to J/ψK_S$ but with opposite sign. Using the generalized factorization approach we find the direct CP violation in the decay mode $B^+ \to f_0 K^+ $ to be of the order of few percent. We then study the effect of the R-parity violating supersymmetric model and show that the direct CP violating asymmetry in $B^+ \to f_0(980) K^+ $ could be as large as $\sim 80 % $ and the mixing induced CP asymmetry in $B^0 \to f_0 K_S$ (i.e., $-S_{f_0 K_S}$) could deviate significantly from that of $\sin(2 β)_{J/ψK_S}$.

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arXiv2006

Neutrino mass matrix in the standard parametrization with texture two zeros

We study the texture two zeros neutrino mass matrices using the standard parametrization for the neutrino mixing matrix. We find that if the origin of CP violation in the leptonic sector is not due to the Dirac-type complex phase of the mixing matrix but because of some non-standard phenomena then some of the possible texture two mass matrices, which are allowed by standard parametrization, are found to be unsuitable to accommodate the observed data in the neutrino sector. Furthermore, incorporating nonzero Dirac phase in our analysis we find that many of them do not exhibit normal hierarchy.

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arXiv2005

Study of FCNC mediated Z boson effect in the semileptonic rare baryonic decays $Λ_b \to Λl^+ l^- $

We study the effect of the FCNC mediated $Z$ boson in the rare semileptonic baryonic decays $Λ_b \to Λl^+ l^-$. We consider the model where the standard model fermion sector is extended by an extra vector like down quark, as a consequence of which it allows CP violating Z mediated flavor changing neutral current at the tree level. We find that due to this non-universal $Zbs $ coupling, branching ratios of the rare semileptonic $Λ_b$ decays are enhanced reasonably from their corresponding standard model values and the zero point of the forward backward asymmetry for $Λ_b \to Λμ^+ μ^-$ is shifted to the left.

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arXiv2004

Can there be any new physics in b -> d penguins

We analyze the possibility of observing new physics effects in the $b \to d$ penguin amplitudes. For this purpose, we consider the decay mode $B \to K^0 \bar K^0 $, which has only $b \to d$ penguin contributions. Using the QCD factorization approach, we find very tiny CP violating effects in the standard model for this process. Furthermore, we show that the minimal supersymmetric standard model with $LR$ mass insertion and R-parity violating supersymmetric model can provide substantial CP violation effects. Observation of sizable CP violation in this mode would be a clear signal of new physics.

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arXiv2004

Direct CP violation and new physics effects in the decay mode B^+ -> phi K^+

We study the direct CP violation effect in the decay mode $B^+ \to ϕK^+$. This decay mode is dominated by the loop induced $\bar b \to \bar s s \bar s$ penguin diagram with a tiny contribution from annihilation diagram. Therefore, the standard model expectation of direct CP violation is negligibly small. Using QCD factorization approach we find the CP asymmetry in the standard model to be at percent level. We consider then two scenarios beyond the standard model, the model with an extra vector like down quark (VLDQ) and the R-parity violating supersymmetric model (RPV) and show that the direct CP violating asymmetry in $B^+ \to ϕK^+ $ could be as large as ~85% (70%) in VLDQ (RPV) model.

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arXiv2004

Probing new physics in B -> phi pi decay

We analyze the rare decay mode $B \to ϕπ$, which is a pure penguin induced process, receiving dominant contribution from the electroweak penguins. Thus the standard model branching ratio is expected to be very small, which makes it as a sensitive probe of new physics. Using QCD factorization approach, we find the branching ratio in the standard model as $Br(B^- \to ϕπ^-)\simeq 5 \times 10^{-9}$. Exploring some of the beyond standard model scenarios the branching ratio is found to be $\sim {\cal O}(10^{-8})$ in the minimal supersymmetric standard model (MSSM) with mass insertion approximation and in the extended technicolor model (TC2). The existence of an extra vector-like down quark (VLDQ) model predicts it to be $\sim {\cal O}(10^{-7})$. The recent BaBar result on $B^0 \to ϕπ^0$ might be a strong indication of new physics effect present in the penguin induced process $B \to ϕπ$.

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arXiv2003

Searching for new physics in the angular distribution of B^0 -> phi K^* decay

Motivated by the possible discrepancy between the observed CP asymmetry and that of standard model expectation in the decay mode B^0 -> phi K_S, we study the corresponding vector vector decay mode B^0 -> phi K^*. In order to obtain decisive information regarding the CP violating effect, we made the angular distribution analysis of the decay products, where both the outgoing vector mesons decay into two pseudoscalars. Furthermore, we study the possible effects of new physics using the angular distribution observables.

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