Abstract: Quantum chromodynamics, the theory of the strong force, describes interactions of coloured quarks and gluons and the formation of hadronic matter. Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. Particles with an alternative quark content are known as exotic states. Here a study is reported of an exotic narrow state in the (DD0)-D-0 pi(+) mass spectrum just below the D*+D-0 mass threshold produced in proton-proton collisions collected with the LHCb detector at the Large Hadron Collider. The state is consistent with the ground isoscalar T-cc(+), tetraquark with a quark content of cc (u) over bar(d) over bar and spin-parity quantum numbers J(P) =1(+). Study of the DD mass spectra disfavours interpretation of the resonance as the isovector state. The decay structure via intermediate off-shell D*(+) mesons is consistent with the observed D-0 pi(+) mass distribution. To analyse the mass of the resonance and its coupling to the DID system, a dedicated model is developed under the assumption of an isoscalar axial-vector T-cc(+), state decaying to the D*D channel. Using this model, resonance parameters including the pole position, scattering length, effective range and compositeness are determined to reveal important information about the nature of the T-cc(+), state. In addition, an unexpected dependence of the production rate on track multiplicity is observed.

Abstract: A data set corresponding to an integrated luminosity of 9 fb(-1) of proton-proton collisions collected by the LHCb experiment has been analysed to search for D-(s)(()*())+ ((D) over bar)(*)0 decays. The decays are fully or partially reconstructed, where one or two (8) missing neutral pions or photons from the decay of an excited charm meson are allowed. Upper limits for the branching fractions, normalised to B+ decays to final states with similar topologies, are obtained for sixteen B-c(+) decay modes. For the decay B-c(+) -> D-s(+)(D) over bar (0), an excess with a significance of 3.4 standard deviations is found.

Abstract: The production of prompt charged particles in proton-lead collisions and in proton-proton collisions at the nucleon-nucleon center-of-mass energy root s(NN) = 5 TeV is studied at LHCb as a function of pseudorapidity (eta) and transverse momentum (p(T) ) with respect to the proton beam direction. The nuclear modification factor for charged particles is determined as a function of eta between -4.8 < eta < -2.5 (backward region) and 2.0 < eta < 4.8 (forward region), and p(T) between 0.2 < p(T) < 8.0 GeV/c. The results show a suppression of charged particle production in proton-lead collisions relative to proton-proton collisions in the forward region and an enhancement in the backward region for p(T) larger than 1.5 GeV/c. This measurement constrains nuclear PDFs and saturation models at previously unexplored values of the parton momentum fraction down to 10(-6).

Abstract: An improved measurement of the decay B-s(0) -> mu(+)mu(-) and searches for the decays B-0 -> mu(+)mu(-) and B-s(0) -> mu(+)mu(-)gamma are performed at the LHCb experiment using data collected in proton-proton collisions at root s = 7, 8 and 13 TeV, corresponding to integrated luminosities of 1, 2 and 6 fb(-1), respectively. The B-s(0) -> mu(+)mu(-) branching fraction and effective lifetime are measured to be B(B-s(0) -> mu(+)mu(-)) = (3.09(-0.43-0.11)(+0.46+0.15)) x 10(-9) and tau(B-s(0) -> mu(+)mu(-)) = (2.07 +/- 0.29 +/- 0.03) ps, respectively, where the uncertain-ties include both statistical and systematic contributions. No significant signal for B-0 -> mu(+)mu(-) and B-s(0) -> mu(+)mu(-) gamma decays is found and the upper limits B(B-0 -> mu(+)mu(-)) < 2.6 x 10(-10) and B(B-s(0) -> mu(+)mu(-)gamma) 2.0 x 10(-9) at 95% confidence level are determined, where the latter is limited to the range m(mu mu) > 4.9 GeV/c(2). Additionally, the ratio between the B-0 -> mu(+)mu(-) and B-s(0) -> mu(+)mu(-) branching fractions is measured to be R mu+mu- < 0.095 at 95% confidence level. The results are in agreement with the Standard Model predictions.

Abstract: An amplitude analysis of flavor-untagged B-s(0) -> J=psi p (p) over bar decays is performed using a sample of 797 +/- 31 decays reconstructed with the LHCb detector. The data, collected in proton-proton collisions between 2011 and 2018, correspond to an integrated luminosity of 9 fb(-1). Evidence for a new structure in the J=psi p and J=psi(p) over bar systems with a mass of 4337(-4-2)(+7+2) MeV and a width of 29(-12-14)(+26+14) MeV is found, where the first uncertainty is statistical and the second systematic, with a significance in the range of 3.1 to 3.7 sigma, depending on the assigned J(P) hypothesis.