Measurement of the photon polarization in Λb→Λγ\Lambda_b \to \Lambda \gamma decays

The photon polarization in $b \to s \gamma$ transitions is measured for the first time in radiative b-baryon decays exploiting the unique spin structure of $\Lambda_b \to \Lambda \gamma$ decays. A data sample corresponding to an integrated luminosity of $6\;fb^{-1}$ collected by the LHCb experiment in $pp$ collisions at a center-of-mass energy of $13\;TeV$ is used. The photon polarization is measured to be $\alpha_{\gamma}= 0.82^{\,+\,0.17\,+\,0.04}_{\,-\,0.26\,-\,0.13}$, where the first uncertainty is statistical and the second systematic. This result is in agreement with the Standard Model prediction and previous measurements in b-meson decays. Charge-parity breaking effects are studied for the first time in this observable and found to be consistent with $CP$ symmetry.Comment: All figures and tables, along with machine-readable versions and any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2021-030.html (LHCb public pages

Measurement of CP -violating and mixing-induced observables in Bs0→ϕγ decays

A time-dependent analysis of the B 0 s → ϕ γ decay rate is performed to determine the C P -violating observables S ϕ γ and C ϕ γ and the mixing-induced observable A Δ ϕ γ . The measurement is based on a sample of p p collision data recorded with the LHCb detector, corresponding to an integrated luminosity of 3     fb − 1 at center-of-mass energies of 7 and 8 TeV. The measured values are S ϕ γ = 0.43 ± 0.30 ± 0.11 , C ϕ γ = 0.11 ± 0.29 ± 0.11 , and A Δ ϕ γ = − 0.67 + 0.37 − 0.41 ± 0.17 , where the first uncertainty is statistical and the second systematic. This is the first measurement of the observables S and C in radiative B 0 s decays. The results are consistent with the standard model predictions

Measurement of B+, B0 and Λb0 production in pPb collisions at √sNN=8.16 TeV

The production of B + , B 0 and Λ 0 b hadrons is studied in proton-lead collisions at a center-of-mass energy per nucleon pair of √ s NN = 8.16     TeV recorded with the LHCb detector at the LHC. The measurement uses a dataset corresponding to an integrated luminosity of 12.2 ± 0.3     nb − 1 for the case where the proton beam is projected into the LHCb detector (corresponding to measuring hadron production at positive rapidity) and 18.6 ± 0.5     nb − 1 for the lead beam projected into the LHCb detector (corresponding to measuring hadron production at negative rapidity). Double-differential cross sections are measured and used to determine forward-backward ratios and nuclear modification factors, which directly probe nuclear effects in the production of beauty hadrons. The double-differential cross sections are measured as a function of the beauty-hadron transverse momentum and rapidity in the nucleon-nucleon center-of-mass frame. Forward-to-backward cross section ratios and nuclear modification factors indicate a significant nuclear suppression at positive rapidity. The ratio of Λ 0 b over B 0 production cross sections is reported and is consistent with the corresponding measurement in p p collisions

Direct photon production at LHCb

At small Bjorken-x, the large gluon number density in the nucleon leads to gluon recombination competing with gluon splitting, which could result in saturation of the gluon PDF. This gluon saturation has yet to be conclusively observed. Direct photon production provides sensitivity to gluon densities in protons and nuclei, and the forward acceptance of LHCb detector allows for measurements of this process at low Bjorken-x, providing an ideal probe of saturation effects. Progress towards the measurement of forward direct photon production using the LHCb detector is presented

Measurement of the mass and production rate of Ξb− baryons

The first measurement of the production rate of Ξ − b baryons in p p collisions relative to that of Λ 0 b baryons is reported, using data samples collected by the LHCb experiment, and corresponding to integrated luminosities of 1, 2 and 1.6     fb − 1 at √ s = 7 , 8 and 13 TeV, respectively. In the kinematic region 2 < η < 6 and p T < 20     GeV / c , we measure f Ξ − b f Λ 0 b B ( Ξ − b → J / ψ Ξ − ) B ( Λ 0 b → J / ψ Λ ) = ( 10.8 ± 0.9 ± 0.8 ) × 10 − 2 [ √ s = 7 , 8     TeV ], f Ξ − b f Λ 0 b B ( Ξ − b → J / ψ Ξ − ) B ( Λ 0 b → J / ψ Λ ) = ( 13.1 ± 1.1 ± 1.0 ) × 10 − 2 [ √ s = 13     TeV ], where f Ξ − b and f Λ 0 b are the fragmentation fractions of b quarks into Ξ − b and Λ 0 b baryons, respectively; B represents branching fractions; and the uncertainties are due to statistical and experimental systematic sources. The values of f Ξ − b / f Λ 0 b are obtained by invoking SU(3) symmetry in the Ξ − b → J / ψ Ξ − and Λ 0 b → J / ψ Λ decays. Production asymmetries between Ξ − b and ¯ Ξ + b baryons are also reported. The mass of the Ξ − b baryon is also measured relative to that of the Λ 0 b baryon, from which it is found that m ( Ξ − b ) = 5796.70 ± 0.39 ± 0.15 ± 0.17     MeV / c 2 , where the last uncertainty is due to the precision on the known Λ 0 b mass. This result represents the most precise determination of the Ξ − b mass

Highlights from the LHCb experiment

We report recent results by the LHCb collaboration in heavy-ion collisions in collider and fixed-target mode at the LHC. A large variety of measurements show the potential of LHCb in nuclear collisions

Observation of B(s)0→J/ψpp¯ decays and precision measurements of the B(s)0 masses

The first observation of the decays B 0 ( s ) → J / ψ p ¯ p is reported, using proton-proton collision data corresponding to an integrated luminosity of 5.2     fb − 1 , collected with the LHCb detector. These decays are suppressed due to limited available phase space, as well as due to Okubo-Zweig-Iizuka or Cabibbo suppression. The measured branching fractions are B ( B 0 → J / ψ p ¯ p ) = [ 4.51 ± 0.40 ( stat ) ± 0.44 ( syst ) ] × 10 − 7 , B ( B 0 s → J / ψ p ¯ p ) = [ 3.58 ± 0.19 ( stat ) ± 0.39 ( syst ) ] × 10 − 6 . For the B 0 s meson, the result is much higher than the expected value of O ( 10 − 9 ) . The small available phase space in these decays also allows for the most precise single measurement of both the B 0 mass as 5279.74 ± 0.30 ( stat ) ± 0.10 ( syst )     MeV and the B 0 s mass as 5366.85 ± 0.19 ( stat ) ± 0.13 ( syst )     MeV

Search for CP violation in Λb0→pK− and Λb0→pπ− decays

A search for CP violation in Λb0→pK− and Λb0→pπ− decays is presented using a sample of pp collisions collected with the LHCb detector and corresponding to an integrated luminosity of 3.0fb−1. The CP -violating asymmetries are measured to be ACPpK−=−0.020±0.013±0.019 and ACPpπ−=−0.035±0.017±0.020, and their difference ACPpK−−ACPpπ−=0.014±0.022±0.010, where the first uncertainties are statistical and the second systematic. These are the most precise measurements of such asymmetries to date

Tests of lepton universality using B0→KS0ℓ+ℓ−B^0\to K^0_S \ell^+ \ell^- and B+→K∗+ℓ+ℓ−B^+\to K^{*+} \ell^+ \ell^- decays

Tests of lepton universality in $B^0\to K^0_S \ell^+ \ell^-$ and $B^+\to K^{*+} \ell^+ \ell^-$ decays where $\ell$ is either an electron or a muon are presented. The differential branching fractions of $B^0\to K^0_S e^+ e^-$ and $B^+\to K^{*+} e^+ e^-$ decays are measured in intervals of the dilepton invariant mass squared. The measurements are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of $9\, \mathrm{fb}^{-1}$. The results are consistent with the Standard Model and previous tests of lepton universality in related decay modes. The first observation of $B^0 \to K^0_S e^+ e^-$ and $B^+ \to K^{*+} e^+ e^-$ decays is reported

Searches for rare B-s(0) and B-0 decays into four muons

Searches for rare $B_s^0$ and $B^0$ decays into four muons are performed using proton-proton collision data recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 $\text{fb}^{-1}$. Direct decays and decays via light scalar and $J/\psi$ resonances are considered. No evidence for the six decays searched for is found and upper limits at the 95% confidence level on their branching fractions ranging between $1.8\times10^{-10}$ and $2.6\times10^{-9}$ are set