Ricerc@Sapienza

Measurements are reported of the central exclusive production of J/ψ and ψ(2S) 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 σJ/ψ→μ+μ−=435±18±11±17pbσψ(2S)→μ+μ−=11.1±1.1±0.3±0.4pb.

The first observation of the Bs0→D̄0K+K- decay is reported, together with the most precise branching fraction measurement of the mode B0→D̄0K+K-. The results are obtained from an analysis of pp collision data corresponding to an integrated luminosity of 3.0 fb-1. The data were collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. The branching fraction of the B0→D̄0K+K- decay is measured relative to that of the decay B0→D̄0π+π- to be B(B0→D̄0K+K-)B(B0→D̄0π+π-)=(6.9±0.4±0.3)%, where the first uncertainty is statistical and the second is systematic.

The doubly charmed baryon decay Ξcc++→Ξc+π+ is observed for the first time, with a statistical significance of 5.9σ, confirming a recent observation of the baryon in the Λc+K-π+π+ final state. The data sample used corresponds to an integrated luminosity of 1.7 fb-1, collected by the LHCb experiment in pp collisions at a center-of-mass energy of 13 TeV. The Ξcc++ mass is measured to be 3620.6±1.5(stat)±0.4(syst)±0.3(Ξc+) MeV/c2 and is consistent with the previous result.

A measurement of the time-integrated CP asymmetry in D0 → KS0KS0 decays is reported. The data correspond to an integrated luminosity of about 2 fb−1 collected in 2015–2016 by the LHCb collaboration in pp collisions at a centre-of-mass energy of 13 TeV. The D0 candidate is required to originate from a D*+ → D0π+ decay, allowing the determination of the flavour of the D0 meson using the pion charge. The D0 → K+K− decay, which has a well measured CP asymmetry, is used as a calibration channel.

The cross section for prompt antiproton production in collisions of protons with an energy of 6.5 TeV incident on helium nuclei at rest is measured with the LHCb experiment from a data set corresponding to an integrated luminosity of 0.5 nb-1. The target is provided by injecting helium gas into the LHC beam line at the LHCb interaction point.

A Dalitz plot analysis of B0→ηc(1S)K+π- decays is performed using data samples of pp collisions collected with the LHCb detector at centre-of-mass energies of s=7,8 and 13TeV, corresponding to a total integrated luminosity of 4.7fb-1. A satisfactory description of the data is obtained when including a contribution representing an exotic ηc(1 S) π- resonant state. The significance of this exotic resonance is more than three standard deviations, while its mass and width are 4096±20-22+18MeV and 152±58-35+60MeV, respectively.

A search for CP violation in the Cabibbo-suppressed D 0 → K + K − π + π − decay mode is performed using an amplitude analysis. The measurement uses a sample of pp collisions recorded by the LHCb experiment during 2011 and 2012, corresponding to an integrated luminosity of 3.0 fb −1 . The D 0 mesons are reconstructed from semileptonic b-hadron decays into D 0 μ − X final states. The selected sample contains more than 160 000 signal decays, allowing the most precise amplitude modelling of this D 0 decay to date. The obtained amplitude model is used to perform the search for CP violation.

Using pp collisions corresponding to 3 fb −1 integrated luminosity, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV, the ratio of branching fractionsℬ(Λb0→ψ(2S)Λ)/ℬ(Λb0→J/ψΛ)=0.513±0.023(stat)±0.016(syst)±0.011(ℬ)is determined. The first uncertainty is statistical, the second is systematic and the third is due to the external branching fractions used.[Figure not available: see fulltext.]

The branching fractions of the doubly Cabibbo-suppressed decays D + → K − K + K + , D + → π − π + K + and D s+ → π − K + K + are measured using the decays D + → K − π + π + and D s+ → K − K + π + as normalisation channels. The measurements are performed using proton-proton collision data collected with the LHCb detector at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2.0 fb −1 .

The resonant structure of the doubly Cabibbo-suppressed decay D + →K − K + K + is studied for the first time. The measurement is based on a sample of pp-collision data, collected at a centre-of-mass energy of 8 TeV with the LHCb detector and corresponding to an integrated luminosity of 2 fb −1 . The amplitude analysis of this decay is performed with the isobar model and a phenomenological model based on an effective chiral Lagrangian.