A test of lepton universality, performed by measuring the ratio of the branching fractions of the B 0 → K * 0 µ + µ − and B 0 → K * 0 e + e − decays, R K * 0 , is presented. The K * 0 meson is reconstructed in the final state K + π − , which is required to have an invariant mass within 100 MeV/c 2 of the known K * (892) 0 mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3 fb −1 , collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The ratio is measured in two regions of the dilepton invariant mass squared, q 2 , to be− 0.07 (stat) ± 0.03 (syst) for 0.045 < q 2 < 1.1 GeV 2 /c 4 , 0.69 + 0.11 − 0.07 (stat) ± 0.05 (syst) for 1.1 < q 2 < 6.0 GeV 2 /c 4 .The corresponding 95.4% confidence level intervals are [0.52, 0.89] and [0.53, 0.94]. The results, which represent the most precise measurements of R K * 0 to date, are compatible with the Standard Model expectations at the level of 2.1-2.3 and 2.4-2.5 standard deviations in the two q 2 regions, respectively.

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The measured dependence of the differential decay rate upon the squared four-momentum transfer between the heavy baryons, q 2 , is compared with expectations from heavy-quark effective theory and from unquenched lattice QCD predictions. DOI: 10.1103/PhysRevD.96.112005 I. INTRODUCTIONIn the Standard Model (SM) of particle physics, quarks participate in a rich pattern of flavor-changing transitions. The relevant couplings form a complex 3 × 3 matrix, known as the Cabibbo-Kobayashi-Maskawa (CKM) matrix, characterized by just four independent parameters [1]. A vast body of measurements of individual CKM elements exists, and thus the overall consistency of the SM picture of charged current interactions is highly overconstrained. Decades of experimental work have demonstrated the impressive consistency of experimental data with the CKM paradigm [2,3]; nonetheless, the motivation to probe the CKM matrix remains strong. Effects of physics beyond the SM may be subtle; thus, more precise measurements are necessary to unveil them. Semileptonic decays of heavyflavored hadrons are commonly used to measure CKM parameters, as they involve only one hadronic current, parametrized in terms of scalar functions known as form factors. The number of form factors needed to describe a particular decay depends upon the spin of the initial-and final-state hadrons [4,5]. A precise calculation of these form factors has been elusive for many years as it is not possible in perturbative QCD. Heavy-Quark Effective Theory (HQET) provides the framework to systematically include nonperturbative corrections in computations involving hadrons containing heavy quarks. In particular, in the limit of infinite heavy-quark mass, all the form factors describing the semileptonic decay of a heavyflavored hadron are proportional to a universal function, known as the Isgur-Wise (IW) function [6]. Lattice QCD, namely the use of lattice formulations of QCD in large scale numerical simulations, has emerged in recent years as a technique with well-defined and systematically improvable uncertainties which can be applied to a wide range of processes and physical quantities [7]. Predictions from the infinite heavy-quark mass limit are useful as a check of several lattice QCD calculations [8].The decay Λ 0 b → Λ þ c μ −ν μ is described by six form factors corresponding to the vector and axial-vector components of the flavor-changing charged current [9]. In HQET, Λ is related to the squared four-momentum transfer between the heavy baryons, q 2 , by are the corresponding invariant masses. Nonperturbative corrections to the static limit can be expressed in terms of an expansion in powers of 1=m c and 1=m b , where m c and m b represent the c-and b-quark masses, respectively. It has been shown in Ref. [12] that the 1=m c term can be expressed in terms of ξ B ðwÞ and one dimensionful constant. Moreover, partial cancellations lead to small first-order corrections near w ¼ 1 [13].

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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...