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The TORCH time-of-flight detector is designed to provide a 15 ps timing resolution for charged particles, resulting in π /K particle identification up to 10 GeV/c momentum over a 10 m flight path. Cherenkov photons, produced in a quartz plate of 10 mm thickness, are focused onto an array of micro-channel plate photomultipliers (MCP-PMTs) which measure the photon arrival times and spatial positions. A half-scale (660 × 1250 × 10 mm 3 ) TORCH demonstrator module has been tested in an 8 GeV/c mixed proton-pion beam at CERN. Customised square MCP-PMTs of active area 53 × 53 mm 2 and granularity 64 × 64 pixels have been employed, which have been developed in collaboration with an industrial partner. The single-photon timing performance and photon yields have been measured as a function of beam position in the radiator, giving measurements which are consistent with expectations. The expected performance of TORCH for high luminosity running of the LHCb Upgrade II has been simulated.

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 D 0 D 0 π + 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 c c + tetraquark with a quark content of c c u ¯ d ¯ 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 π + mass distribution. To analyse the mass of the resonance and its coupling to the D * D system, a dedicated model is developed under the assumption of an isoscalar axial-vector T c c + 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 c c + state. In addition, an unexpected dependence of the production rate on track multiplicity is observed. The existence and properties of tetraquark states with two heavy quarks and two light antiquarks have been widely debated. Here, the authors use a unitarized model to study the properties of an exotic narrow state compatible with a doubly charmed tetraquark.

A bstract Measurements are reported of the central exclusive production of J/ψ and ψ (2 S ) mesons in pp collisions at a centre-of-mass energy of 13 TeV. Backgrounds are significantly reduced compared to previous measurements made at lower energies through the use of new forward shower counters. The products of the cross-sections and the branching fractions for the decays to dimuons, where both muons are within the pseudorapidity range 2 . 0 < η < 4 . 5, are measured to be σ J / ψ → μ + μ − = 435 ± 18 ± 11 ± 17 p b σ ψ 2 S → μ + μ − = 11.1 ± 1.1 ± 0.3 ± 0.4 p b . The first uncertainties are statistical, the second are systematic, and the third are due to the luminosity determination. The cross-sections are also measured differentially for meson rapidities between 2.0 and 4.5. Good agreement is observed with theoretical predictions. Photoproduction cross-sections are derived and compared to previous experiments, and a deviation from a pure power-law extrapolation of lower energy data is observed.

A bstract A measurement of the Ξ cc + + mass is performed using data collected by the LHCb experiment between 2016 and 2018 in pp collisions at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5 . 6 fb − 1 . The Ξ cc + + candidates are reconstructed via the decay modes Ξ cc + + → Λ c + K − π + π + and Ξ cc + + → Ξ c + π + . The result, 3621 . 55 ± 0 . 23 (stat) ± 0 . 30 (syst) MeV/ c 2 , is the most precise measurement of the Ξ cc + + mass to date.

A bstract A precision measurement of the B c + meson mass is performed using proton- proton collision data collected with the LHCb experiment at centre-of-mass energies of 7 , 8 and 13 TeV, corresponding to a total integrated luminosity of 9 . 0 fb − 1 . The B c + mesons are reconstructed via the decays B c + → J/ψπ + , B c + → J/ψπ + π − π + , B c + → J / ψp p ¯ π + , B c + → J / ψ D s + , B c + → J/ψ D 0 K + and B c + → B s 0 π + . Combining the results of the individual decay channels, the B c + mass is measured to be 6274 . 47 ± 0 . 27 (stat) ± 0 . 17 (syst) MeV /c 2 . This is the most precise measurement of the B c + mass to date. The difference between the B c + and B s 0 meson masses is measured to be 907 . 75 ± 0 . 37 (stat) ± 0 . 27 (syst) MeV /c .

A bstract Coherent production of J/ψ mesons is studied in ultraperipheral lead-lead collisions at a nucleon-nucleon centre-of-mass energy of 5 TeV, using a data sample collected by the LHCb experiment corresponding to an integrated luminosity of about 10 μb −1 . The J/ψ mesons are reconstructed in the dimuon final state and are required to have transverse momentum below 1 GeV. The cross-section within the rapidity range of 2 . 0 < y < 4 . 5 is measured to be 4 . 45 ± 0 . 24 ± 0 . 18 ± 0 . 58 mb, where the first uncertainty is statistical, the second systematic and the third originates from the luminosity determination. The cross-section is also measured in J/ψ rapidity intervals. The results are compared to predictions from phenomenological models.

A bstract The decays B + → J / ψπ + π − K + are studied using a data set corresponding to an integrated luminosity of 9 fb − 1 collected with the LHCb detector in proton-proton collisions between 2011 and 2018. Precise measurements of the ratios of branching fractions with the intermediate ψ 2 (3823), χ c1 (3872) and ψ(2S) states are reported. The values are B B + → ψ 2 3823 K + × B ψ 2 3823 → J / ψπ + π − B B + → χ c 1 3872 K + × B χ c 1 3872 → J / ψπ + π − = 3.56 ± 0.67 ± 0.11 × 10 − 2 , B B + → ψ 2 3823 K + × B ψ 2 3823 → J / ψπ + π − B B + → ψ 2 S K + × B ψ 2 S → J / ψπ + π − = 1.31 ± 0.25 ± 0.04 × 10 − 3 , B B + → χ c 1 3872 K + × B χ c 1 3872 → J / ψπ + π − B B + → ψ 2 S K + × B ψ 2 S → J / ψπ + π − = 3.69 ± 0.07 ± 0.06 × 10 − 2 , where the first uncertainty is statistical and the second is systematic. The decay of B + → ψ 2 (3823)K + with ψ 2 (3823) → J / ψπ + π − is observed for the first time with a significance of 5.1 standard deviations. The mass differences between the ψ 2 (3823), χ c1 (3872) and ψ(2S) states are measured to be m χ c 1 3872 − m ψ 2 3823 = 47.50 ± 0.53 ± 0.13 MeV / c 2 , m ψ 2 3823 − m ψ 2 2 S = 137.98 ± 0.53 ± 0.14 MeV / c 2 , m χ c 1 3872 − m ψ 2 2 S = 185.49 ± 0.06 ± 0.03 MeV / c 2 , resulting in the most precise determination of the χ c1 (3872) mass. The width of the ψ 2 (3823) state is found to be below 5.2 MeV at 90% confidence level. The Breit-Wigner width of the χ c1 (3872) state is measured to be Γ χ c 1 3872 BW = 0.96 − 0.18 + 0.19 ± 0.21 MeV which is inconsistent with zero by 5.5 standard deviations.

A bstract A search is performed for rare and forbidden charm decays of the form D s + → h ± ℓ + ℓ ′ ∓ , where h ± is a pion or kaon and ℓ (′)± is an electron or muon. The measurements are performed using proton-proton collision data, corresponding to an integrated luminosity of 1 . 6 fb − 1 , collected by the LHCb experiment in 2016. No evidence is observed for the 25 decay modes that are investigated and 90 % confidence level limits on the branching fractions are set between 1 . 4 × 10 − 8 and 6 . 4 × 10 − 6 . In most cases, these results represent an improvement on existing limits by one to two orders of magnitude.

A bstract The production of ϒ ( nS ) mesons ( n = 1, 2, 3) in p Pb and Pb p collisions at a centre-of-mass energy per nucleon pair s N N = 8.16 TeV is measured by the LHCb experiment, using a data sample corresponding to an integrated luminosity of 31.8 nb −1 . The ϒ ( nS ) mesons are reconstructed through their decays into two opposite-sign muons. The measurements comprise the differential production cross-sections of the ϒ (1 S ) and ϒ (2 S ) states, their forward-to-backward ratios and nuclear modification factors. The measurements are performed as a function of the transverse momentum p T and rapidity in the nucleon-nucleon centre-of-mass frame y * of the ϒ ( nS ) states, in the kinematic range p T < 25 GeV/ c and 1.5 < y * < 4.0 (−5.0 < y * < −2.5) for p Pb (Pb p ) collisions. In addition, production cross-sections for ϒ (3 S ) are measured integrated over phase space and the production ratios between all three ϒ ( nS ) states are determined. Suppression for bottomonium in proton-lead collisions is observed, which is particularly visible in the ratios. The results are compared to theoretical models.

A bstract A search is presented for the lepton flavour violating decay B + → K + μ − τ + using a sample of proton-proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of 9 fb − 1 . The τ leptons are selected inclusively, primarily via decays with a single charged particle. The four-momentum of the τ lepton is determined by using B + mesons from B s 2 ∗ 0 → B + K − decays. No significant excess is observed, and an upper limit is set on the branching fraction ℬ( B +  →  K + μ − τ + ) < 3.9 × 10 −5  at 90 %  confidence level. The obtained limit is comparable to the world-best limit.