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Register a new userProspects for rare B decays at Belle II
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Sam Cunliffe
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Rare and flavour-changing neutral current decays of the B meson are an important probe in the search for physics beyond the Standard Model. There have recently been several anomalies in rare B decays, and lepton-universality measurements, specifically involving the $bto sell^+ell^-$ quark transition. These results tend towards a non-Standard-Model interpretation. The Belle II experiment is a next-generation b physics experiment located at SuperKEKB, an upgraded B factory $e^+e^-$ collider, in Tsukuba, Japan. The first collisions are expected in early 2018 with full physics data expected in 2019. This document describes prospects for several rare B decays at Belle II including $bto sell^+ell^-$ processes and others, such as $bto(s,,d)gamma$ and $bto s ubar u$. Areas where the Belle II program is complementary to that of the currently running LHCb experiment are highlighted.
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We report on preliminary measurements of branching fractions, charge-parity-violating asymmetries, and longitudinal polarization fractions in charmless bottom-meson decays from the Belle~II experiment. We use samples of electron-positron collisions collected in 2019 and 2020 at the $Upsilon(4S)$ resonance, corresponding to integrated luminosities of up to 62.8 ${rm fb^{-1}}$. The results are compatible with known values, indicating good understanding of early detector performance.
The flavour changing neutral current decays can be interesting probes for searching for New Physics. Angular distributions of the decay $mathrm{B}^0 to mathrm{K}^{*0} mu^ +mu^-$ are studied using a sample of proton-proton collisions at $sqrt{s} = 8~mathrm{TeV}$ collected with the CMS detector at the LHC, corresponding to an integrated luminosity of $20.5~mathrm{fb}^{-1}$. An angular analysis is performed to determine $P_1$ and $P_5$, where $P_5$ is of particular interest due to recent measurements that indicate a potential discrepancy with the standard model. Based on a sample of 1397 signal events, $P_1$ and $P_5$ angular parameters are determined as a function of the dimuon invariant mass squared. The measurements are in agreement with standard model predictions.
In this article we study the rare B-meson decay via two on-shell almost-degenerate Majorana Heavy Neutrinos, into two charged leptons and two pseudoscalar mesons ($B^{pm} to D^0 ell^{pm}_1 ell^{pm}_2 pi^{mp}$). We consider the scenario where the heavy neutrino masses are $sim 2$ GeV and the heavy-light mixing coefficients are $|B_{ell N}|^2 sim 10^{-5}$, and evaluate the possibility to measure the CP-asymmetry at Belle II. We present some realistic conditions under which the asymmetry could be detected.
With the completion of Run~I of the CERN Large Hadron Collider, particle physics has entered a new era. The production of unprecedented numbers of heavy-flavoured hadrons in high energy proton-proton collisions allows detailed studies of flavour-changing processes. The increasingly precise measurements allow to probe the Standard Model with a new level of accuracy. Rare $b$ hadron decays provide some of the most promising approaches for such tests, since there are several observables which can be cleanly interpreted from a theoretical viewpoint. In this article, the status and prospects in this field are reviewed, with a focus on precision measurements and null tests.
A search for the flavor-changing neutral-current decay $B^{+}to K^{+} ubar{ u}$ is performed at the Belle II experiment at the SuperKEKB asymmetric energy electron-positron collider. The results are based on a data sample corresponding to an integrated luminosity of $63,mbox{fb}^{-1}$ collected at the $Upsilon{(4S)}$ resonance and a sample of $9,mbox{fb}^{-1}$ collected at an energy $60mathrm{,Mekern -0.1em V}$ below the resonance. A novel measurement method is employed, which exploits topological properties of the $B^{+}to K^{+} ubar{ u}$ decay that differ from both generic bottom-meson decays and light-quark pair production. This inclusive tagging approach offers a higher signal efficiency compared to previous searches. No significant signal is observed. An upper limit on the branching fraction of $B^{+}to K^{+} ubar{ u}$ of $4.1 times 10^{-5}$ is set at the 90% confidence level.
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