Pile-Up Suppression in the ATLAS Level 1 Calorimeter Trigger and Searches for Higgs Boson Pair Production

The Preprocessor system of the ATLAS Level 1 Calorimeter Trigger processes analogue signals from the 7168 trigger towers in the electromagnetic and hadronic calorimeters. Almost all of the processing takes place in the Multi-Chip Module, which has been replaced by an upgraded version of the device for operation during the second long data-taking period of the Large Hadron Collider. The development and demonstration of the signal processing and noise suppression techniques incorporated into its design to reduce the sensitivity to the effects of event pile-up is presented. With these new pile-up suppression methods it is possible to maintain sufficient performance for missing transverse energy and multijet triggers at a luminosity of approximately 2 × 1034 cm−2 s −1 . A search for resonant and nonresonant Higgs boson pair production in the hh → b ¯bτ τ channel is presented for 20.3 fb−1 of √ s = 8 TeV data taken by the ATLAS experiment at the Large Hadron Collider. The results of this analysis are also combined with the corresponding results obtained in the hh → WW∗γγ, b ¯bγγ and b ¯bb¯b analyses. Evidence for their production is not observed and upper limits are set at the 95% confidence level on the production cross sections. For nonresonant hh production an upper limit of 0.69 (0.47) pb is observed (expected), which corresponds to 70 (48) times the Standard Model cross section for gg → hh. The observed (expected) upper limits for resonant hh production from the decay of a heavy Higgs boson vary between 2.1 (1.1) pb at 260 GeV and 0.011 (0.018) pb at 1000 GeV. Interpretations of these results are made in the cases of two Minimal Supersymmetric Standard Model scenarios