Abstract: A measurement of production cross sections of the Higgs boson in proton-proton collisions is presented in the H -> tau tau decay channel. The analysis is performed using 36.1 fb(-1) of data recorded by the ATLAS experiment at the Large Hadron Collider at a center-of-mass energy of root s = 13 TeV. All combinations of leptonic (tau -> l v (v) over bar with l = e, mu) and hadronic (tau -> hadrons v) tau decays arc considered. The H -> tau tau signal over the expected background from other Standard Model processes is established with an observed (expected) significance of 4.4 (4.1) standard deviations. Combined with results obtained using data taken at 7 and 8 TeV center-of-mass energies, the observed (expected) significance amounts to 6.4 (5.4) standard deviations and constitutes an observation of H -> tau tau decays. Using the data taken at root s = 13 TeV, the total cross section in the H -> tau tau decay channel is measured to be 3.77(-0.59)(+0.60)(stat)(-0.74)(+0.87) (syst) pb, for a Higgs boson of mass 125 GeV assuming the relative contributions of its production modes as predicted by the Standard Model. Total cross sections in the H -> tau tau decay channel are determined separately for vector-boson-fusion production and gluon-gluon-fusion production to be sigma(VBF)(H -> tau tau) = 0.28 +/- 0.09 (stat)(-0.09)(+0.11) (syst) pb and sigma(ggF)(H -> tau tau) = 3.1 +/- 1.0 (stat)(-1.3)(+1.6) (syst) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.

Abstract: We perform a Bayesian statistical analysis of the constraints on the nonlinear Effective Theory given by the Higgs electroweak chiral Lagrangian. We obtain bounds on the effective coefficients entering in Higgs observables at the leading order, using all available Higgs-boson signal strengths from the LHC runs 1 and 2. Using a prior dependence study of the solutions, we discuss the results within the context of natural-sized Wilson coefficients. We further study the expected sensitivities to the different Wilson coefficients at various possible future colliders. Finally, we interpret our results in terms of some minimal composite Higgs models.

Abstract: Momentum measurements for very high momentum charged particles, such as muons from electroweak vector boson decays, are particularly susceptible to charge-dependent curvature biases that arise from misalignments of tracking detectors. Low momentum charged particles used in alignment procedures have limited sensitivity to coherent displacements of such detectors, and therefore are unable to fully constrain these misalignments to the precision necessary for studies of electroweak physics. Additional approaches are therefore required to understand and correct for these effects. In this paper the curvature biases present at the LHCb detector are studied using the pseudomass method in proton-proton collision data recorded at centre of mass energy root s = 13 TeV during 2016, 2017 and 2018. The biases are determined using Z -> mu(+)mu(-) decays in intervals defined by the data-taking period, magnet polarity and muon direction. Correcting for these biases, which are typically at the 10(-4) GeV-1 level, improves the Z -> mu(+)mu(-) mass resolution by roughly 18% and eliminates several pathological trends in the kinematic-dependence of the mean dimuon invariant mass.

Abstract: In this work we present an attempt to describe the X1(2900) found by the LHCb collaboration, in the experimental data on the invariant mass spectrum of D-K+, as a three-meson molecular state of the KpD over line system. We discuss that the interactions in all the subsystems are attractive in nature, with the pD over line interaction generating over line D1(2420) and the Kp resonating as K1(1270). We find that the system can form a three-body state but with a mass higher than that of X1(2900). We investigate the KpD system too, finding that the three-body dynamics generates an isoscalar state, which can be related to D*s1(2860), and an exotic isovector state. This latter state has a mass similar to that of the X0(2900) and X1(2900) states found by LHCb, but a very small width (similar to 7.4 +/- 0.9 MeV) and necessarily requires more than two quarks to describe its properties. We hope that our findings will encourage experimental investigations of the isovector KpD state. Finally, in the pursuit of finding a description for X1(2900), we study the K over line K*D* system where over line K*D* forms 0+, 1+, and 2+ states. We do not find a state that can be associated with X1(2900).

Abstract: Primordial black holes (PBHs) are compact objects proposed to have formed in the early Universe from the collapse of small-scale over-densities. Their existence may be detected from the observation of gravitational waves (GWs) emitted by PBH mergers, if the signals can be distinguished from those produced by the merging of astrophysical black holes. In this work, we forecast the capability of the Einstein Telescope, a proposed third-generation GW observatory, to identify and measure the abundance of a subdominant population of distant PBHs, using the difference in the redshift evolution of the merger rate of the two populations as our discriminant. We carefully model the merger rates and generate realistic mock catalogues of the luminosity distances and errors that would be obtained from GW signals observed by the Einstein Telescope. We use two independent statistical methods to analyse the mock data, finding that, with our more powerful, likelihood-based method, PBH abundances as small as fPBH approximate to 7 x 10(-6) ( fPBH approximate to 2 x 10(-6)) would be distinguishable from f(PBH) = 0 at the level of 3 sigma with a one year (ten year) observing run of the Einstein Telescope. Our mock data generation code, darksirens, is fast, easily extendable and publicly available on GitLab.