ATLAS

Abstract:

The ITk is a new silicon tracker for the ATLAS experiment designed to increase detector resolution, readout capacity, and radiation hardness, in preparation for the larger number of simultaneous proton–proton interactions at the High Luminosity LHC. This paper presents the first direct measurement of the material budget of an ATLAS ITk pixel module, performed at a testbeam at the CERN Proton Synchrotron via the multiple scattering of low energy positrons within the module volume. Using a four plane telescope of thin monolithic pixel detectors from the MALTA Collaboration, scattering datasets were recorded at a beam energy of 1.2GeV. Kink angle distributions were extracted from tracks derived with and without information from the ITk pixel module, and were fit to extract the RMS scattering angle, which was converted to a fractional radiation length x/X0. The average x/X0 across the module was measured as [0.89±0.01(resolution)±0.01(subtraction)±0.08(beam momentum band)]%, which agrees within uncertainties with an estimate of 0.88% derived from material component expectations

Abstract:

A search for nonresonant Higgs boson pair production in the b¯bb¯b final state is presented. The analysis uses 126  fb−1 of pp collision data at √s=13  TeV collected with the ATLAS detector at the Large Hadron Collider, and targets both the gluon-gluon fusion and vector-boson fusion production modes. No evidence of the signal is found and the observed (expected) upper limit on the cross section for nonresonant Higgs boson pair production is determined to be 5.4 (8.1) times the Standard Model predicted cross section at 95% confidence level. Constraints are placed on modifiers to the HHH and HHVV couplings. The observed (expected) 2σ constraints on the HHH coupling modifier, κλ, are determined to be [−3.5,11.3] ([−5.4,11.4]), while the corresponding constraints for the HHVV coupling modifier, κ2V, are [−0.0,2.1] ([−0.1,2.1]). In addition, constraints on relevant coefficients are derived in the context of the Standard Model effective field theory and Higgs effective field theory, and upper limits on the HH production cross section are placed in seven Higgs effective field theory benchmark scenarios.