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Testing Fundamental Symmetries with Lambda_b to Lambda -Vector Decay

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 Added by Olivier Leitner
 Publication date 2006
  fields
and research's language is English
 Authors O. Leitner




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Putting together kinematical and dynamical analysis, a complete study of the decay channels $Lambda_b to {Lambda} V(1^-)$, with $Lambda to p {pi}^-$ and $V (J/psi) to {ell}^+ {ell}^-$ or $V ({rho}^0) to {pi}^+ {pi}^-,$ is performed. An intensive use of the helicity formalism is involved on the kinematical side, while on the dynamical side, Heavy Quark Effective Theory (HQET) is applied for an accurate determination of the hadronic matrix elements between the baryons $Lambda_b$ and $Lambda$. Emphasis is put on the major role of the $Lambda_b$ polarization for constructing T-odd observables and the standard ${rho}^0-{omega}$ mixing has the benefit effect of amplifying the process of direct CP violation between $Lambda_b mathrm{and} {bar Lambda}_b$ decays.

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In this letter, an overview is given for interesting tests of both CP and Time Reversal symmetries with the beauty baryon $Lambda_b $. Extensive use of the helicity formalism and HQET is done for all calculations. Then, emphasis is put on sophisticated methods like analysis of resonance polarizations and particular angle distributions which can exhibit a clear signal of TR violation.
A complete study of the angular distributions of the processes, $Lambda_b to {Lambda} V(1^-)$, with $Lambda to p {pi}^-$ and $V (J/{Psi}) to {ell}^+ {ell}^-$ or $V ({rho}^0,omega) to {pi}^+ {pi}^-,$ is performed. Emphasis is put on the initial $Lambda_b$ polarization produced in the proton-proton collisions. The polarization density-matrices as well as angular distributions are derived and help to construct T-odd observables which allow us to perform tests of both Time-Reversal and CP violation.
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.
Experimental data on $ R(D^{(*)}) $, $ R(K^{(*)}) $ and $ R(J/psi) $, provided by different collaborations, show sizable deviations from the SM predictions. To describe these anomalies many new physics scenarios have been proposed. One of them is leptoquark model with introducing the vector and scalar leptoquarks coupling simultaneously to the quarks and leptons. To look for similar possible anomalies in baryonic sector, we investigate the effects of a vector leptoquark $U_3 (3,3, frac{2}{3})$ on various physical quantities related to the tree-level $ Lambda_b rightarrow Lambda_c ell ~ overline{ u}_ell$ decays ($ ell=mu, ~tau $), which proceed via $ b rightarrow c~ell ~ overline{ u}_ell$ transitions at quark level. We calculate the differential branching ratio, forward-backward asymmetry and longitudinal polarizations of lepton and $Lambda_{c}$ baryon at $ mu $ and $ tau $ lepton channels in leptoquark model and compare their behavior with respect to $ q^2 $ with the predictions of the standard model (SM). In the calculations we use the form factors calculated in full QCD as the main inputs and take into account all the errors coming from the form factors and model parameters. It is observed that ........
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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