Abstract: A Dalitz plot analysis of the D-s(+) -> pi(-)pi(+)pi(+) decay is presented. The analysis is based on proton-proton collision data recorded by the LHCb experiment at a centre-of-mass energy of 8TeV and corresponding to an integrated luminosity of 1.5 fb(-1). The resonant structure of the decay is obtained using a quasi-model-independent partial-wave analysis, in which the pi(+)pi(-) S-wave amplitude is parameterised as a generic complex function determined by a fit to the data. The S-wave component is found to be dominant, followed by the contribution from spin-2 resonances and a small contribution from spin-1 resonances. The latter includes the first observation of the D-s(+) -> omega(782)pi(+) channel in the D-s(+) -> pi(-)pi(+)pi(+) decay. The resonant structures of the D-s(+) -> pi(-)pi(+)pi(+) and D+ -> pi(-)pi(+)pi(+) decays are compared, providing information about the mechanisms for the hadron formation in these decays.

Abstract: The first observation of the B-s(0) -> D*+ D*- decay and the measurement of its branching ratio relative to the B-0 -> D*+ D*(-) decay are presented. The data sample used corresponds to an integrated luminosity of 9 fb(-1) of proton-proton collisions recorded by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV between 2011 and 2018. The decay is observed with more than 10 standard deviations and the time-integrated ratio of branching fractions is determined to be [GRAPHICS] . where the first uncertainty is statistical, the second systematic and the third due to the uncertainty of the fragmentation fraction ratio f(s)/f(d). The B-s(0) -> D*(+) D*(-) branching fraction is calculated to be [GRAPHICS] . where the fourth uncertainty is due to the B-0 -> D*(+) D*(-) branching fraction. These results are calculated using the average B-s(0) meson lifetime in simulation. Correction factors are reported for scenarios where either a purely heavy or a purely light B-s(0) eigenstate is considered.

Abstract: Exclusive production of dielectron pairs, gamma gamma -> e(+) e(-), is studied using L-int = 1.72 nb(-1) of data from ultraperipheral collisions of lead nuclei at root s(NN) = 5.02TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon-photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum p(T)(e) > 2.5 GeV, absolute electron pseudorapidity |eta(e)| < 2.5, dielectron invariant mass m(ee) > 5 GeV, and dielectron transverse momentum p(T)(ee) < 2 GeV. Differential cross-sections are measured as a function of mee, average peT, absolute dielectron rapidity |y(ee)|, and scattering angle in the dielectron rest frame, | cos theta* |, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be 215 +/- 1(stat.) (+23)(-20)(syst.) +/- 4(lumi.) μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs Starlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high |y(ee)| and | cos theta* | values.

Abstract: A search for forward proton scattering in association with light-by-light scattering mediated by an axion-like particle is presented, using the ATLAS Forward Proton spectrometer to detect scattered protons and the central ATLAS detector to detect pairs of outgoing photons. Proton-proton collision data recorded in 2017 at a centre-of-mass energy of root s = 13TeV were analysed, corresponding to an integrated luminosity of 14.6 fb(-1). A total of 441 candidate events were selected. A search was made for a narrow resonance in the diphoton mass distribution, corresponding to an axion-like particle (ALP) with mass in the range 150-1600 GeV. No excess is observed above a smooth background. Upper limits on the production cross section of a narrow resonance are set as a function of the mass, and are interpreted as upper limits on the ALP production coupling constant, assuming 100% decay branching ratio into a photon pair. The inferred upper limit on the coupling constant is in the range 0.04-0.09TeV-1 at 95% confidence level.

Abstract: The polarimeter vector field for multibody decays of a spin-half baryon is introduced as a generalisation of the baryon asymmetry parameters. Using a recent amplitude analysis of the Lambda(+)(c) -> pK(-)pi(+) decay performed at the LHCb experiment, we compute the distribution of the kinematic-dependent polarimeter vector for this process in the space of Mandelstam variables to express the polarised decay rate in a model-agnostic form. The obtained representation can facilitate polarisation measurements of the Lambda(+)(c) baryon and eases inclusion of the Lambda(+)(c)-> pK(-)pi(+) decay mode in hadronic amplitude analyses.