arXiv subjects

Soumitra Nandi

Publications and source records attributed to Soumitra Nandi.

69 records · Page 4Linked to original sources

arXiv2010

Constraining the mixing matrix for Standard Model with four generations: time dependent and semi-leptonic CP asymmetries in $B_d^0$, $B_s$ and $D^0$

Using existing experimental information from K, B and D decays as well as electroweak precision tests and oblique parameters, we provide constraints and correlations on the parameters of the 4X4 mixing matrix for the Standard Model with four generations (SM4). We emphasize that some correlations amongst the parameters have important repercussions for key observables. We work with a particular representation of this matrix which is highly suited for extracting information from B-decays. Implications of the resulting constraints for time dependent and semileptonic CP asymmetries for $D^0$, $B^0$ and for $B_s$ are also given. While we show that the semi-leptonic asymmetries may be significantly enhanced in SM4 over the SM, there are important constraints and correlations with other observables. In this context we suggest that existing data from B-factories taken on $Υ(4S)$ and $Υ(5S)$, and in the relevant continuum be used to constrain the semi-leptonic asymmetries for $B_d$, $B_s$ as well as their linear combination. Of course, the data from the Tevatron and LHCb experiments can provide non-trivial tests of SM4 as well.

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arXiv2010

Enhanced $B_s$--$\bar{B}_s$ lifetime difference and anomalous like-sign dimuon charge asymmetry from new physics in $B_s \to τ^+ τ^-$

New physics models that increase the decay rate of $B_s \to τ^+ τ^-$ contribute to the absorptive part of $B_s$--$\bar{B}_s$ mixing, and may enhance $ΔΓ_s$ all the way up to its current experimental bound. In particular, the model with a scalar leptoquark can lead to a significant violation of the expectation $ΔΓ_s \leq ΔΓ_s$ (SM). It can even allow regions in the $ΔΓ_s$-$β_s$ parameter space that are close to the best fit obtained by CDF and D\{O} through $B_s \to J/ψϕ$. In addition, it can help explain the anomalous like-sign dimuon charge asymmetry observed recently by DØ. A measurement of $BR(B_s \to τ^+ τ^-)$ is thus crucial for a better understanding of new physics involved in $B_s$--$\bar{B}_s$ mixing.

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arXiv2010

SM with four generations: Selected implications for rare B and K decays

We extend our recent work and study implications of the Standard Model with four generations (SM4) for rare B and K decays. We again take seriously the several 2-3 $σ$ anomalies seen in B, $B_s$ decays and interpret them in the context of this simple extension of the SM. SM4 is also of course of considerable interest for its potential relevance to dynamical electroweak symmetry breaking and to baryogenesis. Using experimental information from processes such as $B \to X_s γ$, $B_d$ and $B_s$ mixings, indirect CP-violation from $K_L \to ππ$ etc along with oblique corrections, we constrain the relevant parameter space of the SM4, and find $m_{t'}$ of about 400-600 GeV with a mixing angle $| V_{t'b}^*V_{t's}|$ in the range of about (0.05 to 1.4)$\times 10^{-2}$ and with an appreciable CP-odd associated phase, are favored by the current data. Given the unique role of the CP asymmetry in $B_s \to ψϕ$ due to its gold-plated nature, correlation of that with many other interesting observables, including the semileptonic asymmetry ($A_{SL}$) are studied in SM4. We also identify several processes, such as $B \to X_s ν\barν$, $K_L \to π^0 ν\bar ν$ etc, that are significantly different in SM4 from the SM. Experimentally the very distinctive process $B_s\to μ^+μ^-$ is also discussed; the branching ratio can be larger or smaller than in SM, $(3.2 \to 4.2)\times 10^{-9}$, by a factor of ${\cal{O}}(3)$.

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arXiv2010

Correlated enhancements in $D_s \to \ellν$, $(g-2)$ of muon, and lepton flavor violating $τ$ decays with two R-parity violating couplings

With just two R-parity violating couplings, $λ'_{223}$ and $λ'_{323}$, we correlate several channels, namely, $D_s \to \ell ν$ ($\ell = μ, τ$), $(g-2)_μ$, and some lepton flavor violating $τ$ decays. For $λ'_{223} = λ'_{323} \sim 0.3$ and for a common superpartner mass of 300 GeV, which explain the recently observed excesses in the above $D_s$ decay channels, we predict the following R-parity violating contributions: ${\rm Br} (τ\to μγ) \sim 4.5 \cdot 10^{-8}$, ${\rm Br} (τ\to μμμ) \sim 1.2 \cdot 10^{-8}$, ${\rm Br} (τ\to μη/η') \sim 4 \cdot 10^{-10}$, and $(g_μ-2)/2 \sim 4 \cdot 10^{-11}$. We exhibit our results through observable versus observable correlation plots.

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arXiv2010

Power suppressed effects in B-> X_s gamma at O(alpha_s)

We compute the O(alpha_s) corrections to the Wilson coefficients of the dimension five operators emerging from the Operator Product Expansion of inclusive radiative B decays. We discuss the impact of the resulting O(alpha_s Lambda_QCD^2/m_b^2) corrections on the extraction of m_b and mu_pi^2 from the moments of the photon spectrum.

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arXiv2009

The fourth family: a simple explanation for the observed pattern of anomalies in B-CP asymmetries

We show that a fourth family of quarks with $m_{t'}$ in the range of (400 - 600) GeV provides a rather simple explanation for the several indications of new physics that have been observed involving CP asymmetries of the b-quark. The built-in hierarchy of the 4$\times$4 mixing matrix is such that the $t'$ readily provides a needed {\it perturbation} ($\approx 15%$) to $\sin 2 β$ as measured in $B \to ψK_s$ and simultaneously is the dominant source of CP asymmetry in $B_s \to ψϕ$. The correlation between CP asymmetries in $B_s \toψϕ$ and $B_d\toϕK_s$ suggests $m_{t'} \approx$ (400 - 600) GeV. Such heavy masses point to the tantalizing possibility that the 4th family plays an important role in the electroweak symmetry breaking.

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arXiv2008

Two-loop neutrino masses with large R-parity violating interactions in supersymmetry

We attempt to reconcile large trilinear R-parity violating interactions in a supersymmetric (SUSY) theory with the observed pattern of neutrino masses and mixing. We show that, with a restricted number of such interaction terms with the $λ'$-type couplings in the range (0.1-1.0), it is possible to forbid one-loop contributions to the neutrino mass matrix. This is illustrated with the help of a `working example' where an econnomic choice of SUSY parameters is made, with three non-vanishing and `large' R-parity violating terms in the superpotential. The two-loop contributions in such a case can not only generate the masses in the requisite order but can also lead us to specific allowed regions of the parameter space.

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arXiv2008

A simultaneous explanation of the large phase in B_s-\bar B_s mixing and B --> ππ/ πK puzzles in R-parity violating supersymmetry

Recent data on $B$ meson mixings and decays are, in general, in accord with the standard model expectations, except showing a few hiccups: (i) a large phase in $B_s$ mixing, (ii) a significant difference ($> 3.5 σ$) between CP-asymmetries in $B^\pm \to π^0 K^\pm$ and $B_d \to π^\mp K^\pm$ channels, and (iii) a larger than expected branching ratio in $B_d \to π^0π^0$ channel. We show that selective baryon number violating Yukawa couplings in R-parity violating supersymmetry can reconcile all the measurements.

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arXiv2008

R-parity violating supersymmetry, B_s mixing, and D_s -> l nu

Recently, it was pointed out that the mixing phase in the Bs-Bsbar system is large, contrary to the expectations in the Standard Model as well as in minimal flavour violation models. The leptonic decay widths of the D_s meson are also found to be larger than expected. We show how a minimal set of four R-parity violating lambda-prime type couplings can explain both these anomalies. We also point out other phenomenological implications of such new physics.

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arXiv2007

Next-to-leading-order corrections to exclusive processes in $k_T$ factorization

We calculate next-to-leading-order (NLO) corrections to exclusive processes in $k_T$ factorization theorem, taking $πγ^*\toγ$ as an example. Partons off-shell by $k_T^2$ are considered in both the quark diagrams from full QCD and the effective diagrams for the pion wave function. The gauge dependences in the above two sets of diagrams cancel, when deriving the $k_T$-dependent hard kernel as their difference. The gauge invariance of the hard kernel is then proven to all orders by induction. The light-cone singularities in the $k_T$-dependent pion wave function are regularized by rotating the Wilson lines away from the light cone. This regularization introduces a factorization-scheme dependence into the hard kernel, which can be minimized in the standard way. Both the large double logarithms $\ln^2k_T$ and $\ln^2 x$, $x$ being a parton momentum fraction, arise from the loop correction to the virtual photon vertex, the former being absorbed into the pion wave function and organized by the $k_T$ resummation, and the latter absorbed into a jet function and organized by the threshold resummation. The NLO corrections are found to be only few-percent for $πγ^*\toγ$, if setting the factorization scale to the momentum transfer from the virtual photon.

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arXiv2007

Possibility of large lifetime differences in neutral B meson systems

We investigate new physics models that can increase the lifetime differences in the $B_q$--$\bar{B}_q$ systems ($q = d,s$) above their standard model values. If both $B_q$ as well as $\bar{B}_q$ can decay to a final state through flavour dependent new physics interactions, the so-called Grossman bound may be evaded. As examples, we consider the scalar leptoquark model and $λ''$-type R-parity violating supersymmetry. We find that models with a scalar leptoquark can enhance $ΔΓ_s/Γ_s$ all the way up to its experimental upper bound and $ΔΓ_d/Γ_d$ to as much as $\sim 2.5%$, at the same time allowing the CP violating phase $β_s$ to vary between $- 45^\circ$ and $20^\circ$. R-parity violating supersymmetry models cannot enhance the lifetime differences significantly, but can enhance the value of $β_s$ up to $\sim \pm 20^\circ$. This may bring the values of $ΔΓ_q/Γ_q$ as well as $β_s$ within the measurement capabilities of $B$ factories and LHCb. We also obtain bounds on combinations of these new physics couplings, and predict enhanced branching ratios of $B_{s/d} \to τ^+ τ^-$.

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arXiv2006

$B_s-\bar{B_s}$ mixing, B decays and R-parity violating supersymmetry

We discuss the implications of the recent measurement of the $B_s-\bar{B_s}$ oscillation frequency $ΔM_s$ on the parameter space of R-parity violating supersymmetry. For completeness, we also discuss the bounds coming from leptonic, semileptonic, and nonleptonic B decay modes, and point out some possibly interesting channels at LHC.

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arXiv2006

New Physics in b -> s bar(s) s Decay. II: Study of B -> V1 V2 Modes

Various nonleptonic decay channels mediated by the quark-level subprocess $b\to s\bar{s} s$ show hints of deviation from the Standard Model expectations. We analyse the double-vector decay $B\toϕK^\ast$ with different generic new physics structures and find the constraints on the parameter spaces of new physics. The allowed parameter spaces are compatible with, but further narrowed down from, those obtained from a similar analysis using pseudoscalar modes. We also discuss further predictions for this channel as well as for $B_s\toϕϕ$, and show how even a partial measurement of the observables may discriminate between different models of new physics.

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arXiv2005

New Physics in b --> s bar(s) s Decay

We perform a model-independent analysis of the data on branching ratios and CP asymmetries of $B\toϕK$ and $B\toη^{(')} K^{(*)}$ modes. The present data is encouraging to look for indirect evidences of physics beyond the Standard Model. We investigate the parameter spaces for different possible Lorentz structures of the new physics four-Fermi interaction. It is shown that if one takes the data at $1σ$ confidence level, only one particular Lorentz structure is allowed. Possible consequences for the $B_s$ system are also discussed.

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arXiv2004

Large electroweak penguins in B --> pi pi and B --> pi K : implication for new physics

We show that the anomalies in the $B\toππ$ decay channels, {\em viz.}, the large branching ratio of $B\toπ^0π^0$ and large direct CP asymmetry of $B\toπ^+π^-$, can both be explained, even in the framework of existing theoretical models which predict the relative strengths of the tree and the strong penguin amplitudes, if there is an abnormally large electroweak penguin contribution. We also critically examine the expectation of a similarly large electroweak penguin in the $B\toπK$ sector, and show that it can be accomodated, and may even be motivated from an SU(3) flavor symmetry between the $B\toππ$ and the $B\toπK$ amplitudes, but is not necessary to explain the data. We emphasize that if the experimental numbers on these charmless nonleptonic channels survive, they may turn out to be an indirect signal of new physics. Current data is insufficient to discriminate betwen flavor-specific and flavor-blind new physics scenarios.

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