Since the first ATLAS results from LHC Run 2 were presented at this summer’s conferences (EPS-HEP 2015 and LHCP 2015) with an amount of data corresponding to an integrated luminosity of approximately 80 pb–1, the LHC has continued to ramp up in luminosity. The maximum instantaneous luminosity for 2015 was 5 × 1033 cm–2s–1, which already approaches the Run 1 record of 7 × 1033 cm–2s–1. ATLAS recorded more than 4 fb–1 in 2015, with different physics analyses using from 3.32 to 3.60 fb–1, depending on the parts of the detector required to be fully operational with good data quality.
The main goal of the early measurements presented this summer was to study in detail the performance of the detector, to characterise the main Standard Model processes at 13 TeV, and to perform the first searches for phenomena beyond the Standard Model at Run 2. These early searches focused on processes such as high-mass quantum and rotating black-hole production in dijet, multijet and lepton-jet event topologies, for which the higher centre-of-mass energy provided an immediate improvement in sensitivity beyond the reach of the Run 1 data.
The recently completed 2015 data set corresponds to more than 30 times that of this summer. With these data, the full programme of measurements and searches at Run 2 has started, and the first results were presented by the collaboration at a joint ATLAS and CMS seminar on 15 December 2015 during CERN Council week.
These new results benefitted from the first calibration of electron, muon and jet reconstruction and trigger algorithms, in situ using the data. The new insertable B layer of pixel detectors significantly improves the precision of the track measurements near the interaction region and is therefore crucial for tagging jets containing heavy quarks.
First measurements include the ZZ cross-section and single top quark, and the Wt production channels at 13 TeV. Top-quark pair production has also been investigated in measurements where the top-quark pair is produced in association with additional jets. These measurements are crucial to provide further checks of the modelling implemented in state-of-the-art generators used to simulate these processes at NLO QCD precision. These measurements can also subsequently be used to further constrain physics beyond the Standard Model that would alter these production modes.
The new data also allowed the first measurements of the Higgs boson production cross-section at 13 TeV, inclusively in the diphoton and ZZ decay channels.
With the increased centre-of-mass energy, and the availability of significantly more data than in the summer, new-particle search results were awaited with much anticipation. A large number of searches for new phenomena motivated by theories beyond the Standard Model in dijet, multijets, photon jets, diphoton, dilepton, single lepton and missing transverse energy channels were completed. Searches for vector-boson pair (VV) and Higgs and vector-boson (VH) topologies with boosted jets have also been completed. Searches for strongly produced supersymmetry (SUSY) that made use of signatures with 0 or 1 lepton, or a Z boson, jets and missing transverse energy and also topologies with B jets, have improved sensitivity from Run 1. Finally, searches for Higgs bosons from extended electroweak symmetry-breaking sectors in final states with a pair of tau leptons, and in pairs of vector bosons, have been performed.
So far, no definitive observation of new physics has been observed in the data, although two excesses have been observed. The first, with a significance of 2.2 standard deviations, was seen in the search for SUSY with gluino production with subsequent decays into a Z boson and missing energy; a 3 standard-deviation excess was observed in this channel in Run 1. The second excess was observed in the search for diphoton resonances where a peak is seen at 750 GeV with a local significance of 3.6 standard deviations, corresponding to a global significance of 2.0 standard deviations. More data will be needed to probe the nature of these excesses.
Limits on a large variety of theories beyond the Standard Model have been derived. The ATLAS experiment is completing its measurements and search programme on the data collected in 2015, and is preparing for the data to come in 2016.
• For more details on the ATLAS results presented at the seminar, see https://twiki.cern.ch/twiki/bin/view/AtlasPublic/December2015-13TeV.