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Register a new userAngular distribution of polarised $Lambda_b$ baryons decaying to $Lambda ell^+ell^-$
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Rare $b to sell^+ell^-$ flavour-changing-neutral-current processes provide important tests of the Standard Model of particle physics. Angular observables in exclusive $b to sell^+ell^-$ processes can be particularly powerful as they allow hadronic uncertainties to be controlled. Amongst the exclusive processes that have been studied by experiments, the decay $Lambda_bto Lambdaell^+ell^-$ is unique in that the $Lambda_b$ baryon can be produced polarised. In this paper, we derive an expression for the angular distribution of the $Lambda_bto Lambdaell^+ell^-$ decay for the case where the $Lambda_b$ baryon is produced polarised. This extends the number of angular observables in this decay from 10 to 34. Standard Model expectations for the new observables are provided and the sensitivity of the observables is explored under a variety of new physics models. At low-hadronic recoil, four of the new observables have a new short distance dependence that is absent in the unpolarised case. The remaining observables depend on the same short distance contributions as the unpolarised observables, but with different dependence on hadronic form-factors. These relations provide possibilities for novel tests of the SM that could be carried out with the data that will become available at the LHC or a future $e^+e^-$ collider.
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The flavor changing rare decay $Bto K^{*}(to Kpi)ell^+ell^-$ 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 $Lambda_{b}rightarrow Lambda (to ppi) ell^{+}ell^{-}$ 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 $Bto K^{*} ell^+ell^-$ can be corroborated by the corresponding baryonic decay mode. We show that due to the parity violating nature of the subsequent $Lambdato ppi$ decay there exist at least one angular asymmetry that is non-vanishing in the large recoil limit unlike the case in $Bto K^{*}ell^+ell^-$ decay mode, making it particularly sensitive to new physics that violates lepton flavor universality.
We present the first lattice QCD determination of the $Lambda_b to Lambda^*(1520)$ vector, axial vector, and tensor form factors that are relevant for the rare decays $Lambda_b to Lambda^*(1520)ell^+ell^-$. The lattice calculation is performed in the $Lambda^*(1520)$ rest frame with nonzero $Lambda_b$ momenta, and is limited to the high-$q^2$ region. An interpolating field with covariant derivatives is used to obtain good overlap with the $Lambda^*(1520)$. The analysis treats the $Lambda^*(1520)$ as a stable particle, which is expected to be a reasonable approximation for this narrow resonance. A domain-wall action is used for the light and strange quarks, while the $b$ quark is implemented with an anisotropic clover action with coefficients tuned to produce the correct $B_s$ kinetic mass, rest mass, and hyperfine splitting. We use three different ensembles of lattice gauge-field configurations generated by the RBC and UKQCD collaborations, and perform extrapolations of the form factors to the continuum limit and physical pion mass. We give Standard-Model predictions for the $Lambda_b to Lambda^*(1520)ell^+ell^-$ differential branching fraction and angular observables in the high-$q^2$ region.
We present the prospects of an angular analysis of the $Lambda_b to Lambda(1520)ell^+ell^-$ decay. Using the expected yield in the current dataset collected at the LHCb experiment, as well as the foreseen ones after the LHCb upgrades, sensitivity studies are presented to determine the experimental precision on angular observables related to the lepton distribution and their potential to identify New Physics. The forward-backward lepton asymmetry at low dilepton invariant mass is particularly promising. NP scenarios favoured by the current anomalies in $bto sell^+ell^-$ decays can be distinguished from the SM case with the data collected between the Run 3 and the Upgrade 2 of the LHCb experiment.
Based on the standard model (SM) of particle physics, we study the decays $Lambda_b to Lambda ell^+ ell^-$ in light of the available inputs from lattice and the data from LHCb. We fit the form-factors of this decay mode using the available theory and experimental inputs after defining different fit scenarios and checking their consistencies. The theory inputs include the relations between the form-factors in heavy quark effective theory (HQET) and soft collinear effective theory (SCET) at the endpoints of di-lepton invariant mass squared $q^2$. Utilizing the fit results, we have predicted a few observables related to this mode. We have also predicted the observable $R_{Lambda} = Br(Lambda_b to Lambda ell_i^+ell_i^- )/Br(Lambda_b to Lambda ell_j^+ell_j^-)$ where $ell_{i}$ and $ell_j$ are charged leptons of different generations ($i e j$). At the moment, we do not observe noticeable differences in the extracted values of the observables in fully data-driven and SM like fit scenarios.
Using the responsible form factors calculated via full QCD, we analyze the $Lambda_{b}rightarrow Lambda ell^{+}ell^{-}$ transition in the standard model containing fourth generation quarks (SM4). We discuss effects of the presence of $t$ fourth family quark on related observables like branching ratio, forward-backward asymmetry, baryon polarization as well as double lepton polarization asymmetries. We also compare our results with those obtained in the SM as well as with predictions of the SM4 but using form factors calculated within heavy quark effective theory. The obtained results on branching ratio indicate that the $Lambda_{b}rightarrow Lambda ell^{+}ell^{-}$ transition is more probable in full QCD comparing to the heavy quark effective theory. It is also shown that the results on all considered observables in SM4 deviate considerably from the SM predictions when $m_{t}geq 400 GeV$.
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