LHCb

Abstract:

The first evidence for the decay $B^{+}\to {\pi }^{+}{e}^{+}{e}^{-}$ is reported using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . A signal excess with a significance of $3.2\sigma$ is observed and the branching fraction is measured to be $\mathcal{B}(B^{+}\to {\pi }^{+}{e}^{+}{e}^{-})=({2.4}_{-0.8}^{+0.9}{}_{-0.2}^{+0.4})\times {10}^{-8}$ , where the first set of uncertainties is statistical and the second is systematic. The result is consistent with the Standard Model expectation.

Abstract:

The onset of nuclear effects in light-ion collisions is studied by measuring the ratio of $D^0$ meson production between NeNe and OO collisions at a center-of-mass energy per nucleon pair of 5.36 TeV recorded by the LHCb detector. The $D^0$ meson yields are measured differentially in transverse momentum ( $p_{\mathrm{T}}$ ) for $0.5<p_{\mathrm{T}}<20\mathrm{GeV}$ in the rapidity ( $y$ ) region $2.0<y<4.5$ , and are normalized to the total number of recorded inelastic nucleus-nucleus collisions in each data sample. The resulting production ratio shows evidence of variation as a function of $p_{\mathrm{T}}$ , which is inconsistent with predictions based on nuclear modification of nucleon structure alone. This measurement is consistent with the onset of quark-gluon plasma production with increasing nucleus size in high-energy nuclear collisions.

Abstract:

The first search for the lepton-flavor violating decays $B^{+}\to {\pi }^{+}{\mu }^{\pm }{e}^{\mp }$ in proton-proton collisions is presented, using data collected by the LHCb experiment between 2011 and 2018, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . No significant signal is observed and an upper limit on the branching fraction is set at $\mathcal{B}(B^{+}\to {\pi }^{+}{\mu }^{\pm }{e}^{\mp })<1.8\times {10}^{-9}$ at the 90% confidence level, 2 orders of magnitude more restrictive than the most stringent upper limit to date. This is the first constraint on lepton-flavor violating $b\to d$ quark transitions at the LHC and also sets the most stringent upper limits to date on $b\to d{\mu }^{\pm }{e}^{\mp }$ transitions. Limits on left-handed and scalar scenarios beyond the standard model are also reported.

Abstract:

This paper presents the first measurement of the forward-backward asymmetry, AFB, and the flat term, FH, that parameterise the angular distribution of the B+ → π+μ+μ− decay. The proton-proton collision dataset used in the analysis corresponds to an integrated luminosity of 9 fb−1, collected with the LHCb experiment between 2011 and 2018. The analysis is performed in two intervals of dimuon mass squared, one above and one below the region containing the J/ψ and ψ(2S) narrow charmonium resonances. The Standard Model predictions lie within the obtained 68% confidence level interval in the high-mass and within the 99% interval in the low-mass region.