Abstract: A search for the doubly charmed baryon Xi(+)(cc) is performed in the Xi(+)(c)pi(-)pi(+ )invariant-mass spectrum, where the Xi(-)(cc) baryon is reconstructed in the pK(-)pi(+) final state. The study uses proton-proton collision data collected with the LHCb detector at a centreof-mass energy of 13 TeV, corresponding to a total integrated luminosity of 5.4 fb(-1). No significant signal is observed in the invariant-mass range of 3.4-3.8 GeV/c(2). Upper limits are set on the ratio of branching fractions multiplied by the production cross-section with respect to the Xi(++)(cc) -> (Xi(+)(c) -> pK(-)pi(+)+/-)pi(+ ) decay for different Xi(+)(cc) mass and lifetime hy- potheses in the rapidity range from 2.0 to 4.5 and the transverse momentum range from 2.5 to 25 GeV/c. The results from this search are combined with a previously published search for the Xi(+)(cc) -> A(c)(+)K(-)pi(+ ) decay mode, yielding a maximum local significance of 4.0 standard deviations around the mass of 3620 MeV/c(2), including systematic uncertainties. Taking into account the look-elsewhere effect in the 3.5-3.7 GeV/c(2) mass window, the combined global significance is 2.9 standard deviations including systematic uncertainties.

Abstract: A search for the doubly heavy Xi bc0 baryon using its decay to the D(0)pK(-) 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/c(2). Upper limits are set at 95% credibility level on the ratio of the Xi bc0 production cross-section times its branching fraction to D(0)pK(-) relative to that of the Lambda b0 -> D0pK- decay. The limits are set as a function of the Xi bc0 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 x 10(-2) to 3.0 x 10(-1) for the considered Xi bc0 mass and lifetime hypotheses.

Abstract: A search for the lepton flavour violating decay tau(-) -> mu(-)mu(+)mu(-) is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of 1.0 fb(-1) of proton-proton collisions at a centre-of-mass energy of 7 TeV and 2.0 fb(-1) at 8 TeV. No evidence is found for a signal, and a limit is set at 90% confidence level on the branching fraction, B(tau(-) -> mu(-)mu(+)mu(-)) < 4.6 x 10(-8).

Abstract: A search for the lepton-flavour violating decay D-0 -> e(+/-)mu(-/+) is made with a dataset corresponding to an integrated luminosity of 3.0 fb(-1) of proton-proton collisions at centre-of-mass energies of 7 TeV and 8 TeV, collected by the LHCb experiment. Candidate D-0 mesons are selected using the decay D*(+) -> D-0 pi(+) and the D-0 -> e(+/-)mu(+) branching fraction is measured using the decay mode D-0 -> K-pi(+) as a normalization channel. No significant excess of D-0 -> e(+/-)mu(-/+) candidates over the expected background is seen, and a limit is set on the branching fraction, B(D-0 -> e(+/-)mu(-/+)) < 1.3 x10(-8), at 90% confidence level. This is an order of magnitude lower than the previous limit and it further constrains the parameter space in some leptoquark models and in supersymmetric models with R-parity violation.

Abstract: A search for the lepton-flavour violating decays B-0 -> K-*0 μe(-/+) and B-s(0)-> mu(+/-)e(-/+) is presented, using proton-proton collision data collected by the LHCb detector at the LHC, corresponding to an integrated luminosity of 9 fb(-1). No significant signals are observed and upper limits of are set at 90% (95%) confidence level. These results constitute the world's most stringent limits to date, with the limit on the decay B-s(0) -> phi mu(+/-) e(-/+) the first being set. In addition, limits are reported for scalar and left-handed lepton-flavour violating New Physics scenarios.