The standard model of particle physics currently provides our best description of fundamental particles and their interactions. The theory predicts that the different charged leptons, the electron, muon and tau, have identical electroweak interaction strengths. Previous measurements have shown that a wide range of particle decays are consistent with this principle of lepton universality. This article presents evidence for the breaking of lepton universality in beauty-quark decays, with a significance of 3.1 standard deviations, based on proton–proton collision data collected with the LHCb detector at CERN’s Large Hadron Collider. The measurements are of processes in which a beauty meson transforms into a strange meson with the emission of either an electron and a positron, or a muon and an antimuon. If confirmed by future measurements, this violation of lepton universality would imply physics beyond the standard model, such as a new fundamental interaction between quarks and leptons.
A search for the doubly heavy $$ {\Xi}_{bc}^0 $$
Ξ
bc
0
baryon using its decay to the D0pK– final state is performed using proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the LHCb experiment between 2016 and 2018, corresponding to an integrated luminosity of 5.4 fb−1. No significant signal is found in the invariant mass range from 6.7 to 7.2 GeV/c2. Upper limits are set at 95% credibility level on the ratio of the $$ {\Xi}_{bc}^0 $$
Ξ
bc
0
production cross-section times its branching fraction to D0pK− relative to that of the $$ {\Lambda}_b^0\to {D}^0{pK}^{-} $$
Λ
b
0
→
D
0
pK
−
decay. The limits are set as a function of the $$ {\Xi}_{bc}^0 $$
Ξ
bc
0
mass and lifetime hypotheses, in the rapidity range from 2.0 to 4.5 and in the transverse momentum region from 5 to 25 GeV/c. Upper limits range from 1.7 × 10−2 to 3.0 × 10−1 for the considered $$ {\Xi}_{bc}^0 $$
Ξ
bc
0
mass and lifetime hypotheses.
A search for the decays B 0 s → e þ e − and B 0 → e þ e − is performed using data collected with the LHCb experiment in proton-proton collisions at center-of-mass energies of 7, 8, and 13 TeV, corresponding to integrated luminosities of 1, 2, and 2 fb −1 , respectively. No signal is observed. Assuming no contribution from B 0 → e þ e − decays, an upper limit on the branching fraction BðB 0 s → e þ e − Þ < 9.4ð11.2Þ × 10 −9 is obtained at 90(95)% confidence level. If no B 0 s → e þ e − contribution is assumed, a limit of BðB 0 → e þ e − Þ < 2.5ð3.0Þ × 10 −9 is determined at 90(95)% confidence level. These upper limits are more than one order of magnitude lower than the previous values.
The first search for the rare radiative decay $$ {\Xi}_b^{-} $$
Ξ
b
−
→ Ξ−γ is performed using data collected by the LHCb experiment in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb−1. The $$ {\Xi}_b^{-} $$
Ξ
b
−
→ Ξ−J/ψ channel is used as normalization. No $$ {\Xi}_b^{-} $$
Ξ
b
−
→ Ξ−γ signal is found and an upper limit of $$ \mathcal{B} $$
B
($$ {\Xi}_b^{-} $$
Ξ
b
−
→ Ξ−γ) < 1.3 × 10−4 at 95% confidence level is obtained.
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