Dr Holly Pacey

Abstract:

A search is performed for dark matter particles produced in association with a resonant pair of Higgs bosons using 140 fb−1 of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the Large Hadron Collider. This signature is expected in some extensions of the Standard Model predicting the production of dark matter particles, and is interpreted in terms of a dark Higgs model containing a Z′ mediator in which the dark Higgs boson s decays into a pair of Higgs bosons. The dark Higgs boson is reconstructed through final states with at least three b-tagged jets, produced by the pair of Higgs boson decays, in events with significant missing transverse momentum consistent with the presence of dark matter. The observed data are found to be in good agreement with Standard Model predictions, constraining scenarios with dark Higgs boson masses within the range of 250 to 400 GeV and Z′ mediators up to 2.3 TeV.

Abstract:

This paper presents the reconstruction of missing transverse momentum (pTmiss) in proton–proton collisions, at a center-of-mass energy of 13 TeV. This is a challenging task involving many detector inputs, combining fully calibrated electrons, muons, photons, hadronically decaying τ-leptons, hadronic jets, and soft activity from remaining tracks. Possible double counting of momentum is avoided by applying a signal ambiguity resolution procedure which rejects detector inputs that have already been used. Several pTmiss ‘working points’ are defined with varying stringency of selections, the tightest improving the resolution at high pile-up by up to 39% compared to the loosest. The pTmiss performance is evaluated using data and Monte Carlo simulation, with an emphasis on understanding the impact of pile-up, primarily using events consistent with leptonic Z decays. The studies use 140fb-1 of data, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The results demonstrate that pTmiss reconstruction, and its associated significance, are well understood and reliably modelled by simulation. Finally, the systematic uncertainties on the soft pTmiss component are calculated. After various improvements the scale and resolution uncertainties are reduced by up to 76% and 51%, respectively, compared to the previous calculation at a lower luminosity.

Abstract:

A search for the production of three Higgs bosons (HHH) in the bb¯bb¯bb¯ final state is presented. The search uses 126 fb−1 of proton-proton collision data at s√=13 TeV collected with the ATLAS detector at the Large Hadron Collider. The analysis targets both non-resonant and resonant production of HHH. The resonant interpretations primarily consider a cascade decay topology of X→SH→HHH with masses of the new scalars X and S up to 1.5 TeV and 1 TeV, respectively. In addition to scenarios where S is off-shell, the non-resonant interpretation includes a search for standard model (SM) HHH production, with limits on the tri-linear and quartic Higgs self-coupling set. No evidence for HHH production is observed. An upper limit of 59 fb is set, at 95% confidence level, on the cross-section for Standard-Model HHH production.