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The first observation of the B(s)(0)→η'η' decay is reported. The study is based on a sample of proton-proton collisions corresponding to 3.0  fb(-1) of integrated luminosity collected with the LHCb detector. The significance of the signal is 6.4 standard deviations. The branching fraction is measured to be [3.31±0.64(stat)±0.28(syst)±0.12(norm)]×10(-5), where the third uncertainty comes from the B(±)→η'K(±) branching fraction that is used as a normalization. In addition, the charge asymmetries of B(±)→η'K(±) and B(±)→ϕK(±), which are control channels, are measured to be (-0.2±1.3)% and (+1.7±1.3)%, respectively. All results are consistent with theoretical expectations.

Production of prompt D 0 mesons is studied in proton-lead and lead-proton collisions recorded at the LHCb detector at the LHC. The data sample corresponds to an integrated luminosity of 1.58±0.02 nb −1 recorded at a nucleon-nucleon centre-of-mass energy of √ s NN = 5 TeV. Measurements of the differential cross-section, the forward-backward production ratio and the nuclear modification factor are reported using D 0 candidates with transverse momenta less than 10 GeV/c and rapidities in the ranges 1.5 < y * < 4.0 and −5.0 < y * < −2.5 in the nucleon-nucleon centre-of-mass system. Keywords: Charm physics, Heavy Ion Experiments, Heavy-ion collision, Particle and resonance productionArXiv ePrint: 1707.02750Open Access, Copyright CERN, for the benefit of the LHCb Collaboration. Article funded by SCOAP 3 .https://doi.org/10.1007/JHEP10(2017)090 JHEP10(2017)090 Conclusion 17The LHCb collaboration 23 IntroductionCharm hadrons produced in hadronic and nuclear collisions are excellent probes to study nuclear matter in extreme conditions. The differential cross-sections of c-quark production in pp or pp collisions have been calculated based on perturbative quantum chromodynamics (QCD) and collinear or k T factorisation [1][2][3][4][5][6]. These phenomenological models [7] are also able to predict the differential cross-section of c-quark production including most of the commonly assumed "cold nuclear matter" (CNM) effects in nuclear collisions, where CNM effects related to the parton flux differences and other effects come into play. Since heavy quarks are produced in hard scattering (with momentum transfer squared Q 2 2m c ) typically at a short time scale, they are ideal to examine hot nuclear matter, the so-called "quark-gluon plasma" (QGP), by studying how they traverse this medium and interact with it right after their formation.These studies require a thorough understanding of the CNM effects, which can be investigated in systems where the formation of QGP is not expected. In addition, a precise quantification of CNM effects would significantly improve the understanding of charmonium and open-charm production by confirming or discarding the possibility that the suppression pattern in the production of quarkonium states, like J/ψ , at the SPS, RHIC and LHC is due to QGP formation [7].The study of CNM effects is best performed in collisions of protons with heavy nuclei like lead, where the most relevant CNM effects, such as nuclear modification of the parton densities [8,9] and in-medium energy loss [10] in initial-and final-state radiation [11,12], -1 - JHEP10(2017)090are more evident. Phenomenologically, collinear parton distributions are often used to describe the nuclear modification of the parton flux in the nucleus. The modification with respect to the free nucleon depends on the parton fractional longitudinal momentum x, Q 2 and the atomic mass number of the nucleus A [13,14]. In the low-x region, down to x ≈ 10 −5 −10 −6 , which is accessible at LHC energies at forward rapidity, a possible onset of gl...

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The LHCb Vertex Locator (VELO) is a silicon strip detector designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. During the first two years of data collection, the 84 VELO sensors have been exposed to a range of fluences up to a maximum value of approximately 45 × 10 12 1 MeV neutron equivalent (1 MeV n eq ). At the operational sensor temperature of approximately −7 • C, the average rate of sensor current increase is 18 µA per fb −1 , in excellent agreement with predictions. The silicon effective bandgap has been determined using current versus temperature scan data after irradiation, with an average value of E g = 1.16 ± 0.03 ± 0.04 eV obtained. The first observation of n + -on-n sensor type inversion at the LHC has been made, occurring at a fluence of around 15 × 10 12 of 1 MeV n eq . The only n + -on-p sensors in use at the LHC have also been studied. With an initial fluence of approximately 3 × 10 12 1 MeV n eq , a decrease in the Effective Depletion Voltage (EDV) of around 25 V is observed. Following this initial decrease, the EDV increases at a comparable rate to the type inverted n + -on-n type sensors, with rates of (1.43 ± 0.16) × 10 −12 V/ 1 MeV n eq and (1.35 ± 0.25) × 10 −12 V/ 1 MeV n eq measured for n + -on-p and n + -on-n type sensors, respectively. A reduction in the charge collection efficiency due to an unexpected effect involving the second metal layer readout lines is observed.