The start of LHC Run 2 in 2015 saw the centre-of-mass energy of proton–proton collisions increase from 8 to 13 TeV, dramatically increasing the possibility to create heavy particles predicted by many models of new physics. The ATLAS collaboration has recently released the first search results from its analysis of the full 2015 and 2016 data sets, providing the largest combined LHC data set analysed so far.
New heavy particles are likely to decay immediately inside the detector into known objects such as pairs of jets, leptons or bosons. These decay products will typically have large transverse momentum, due to the high mass of the parent particle, and this raises challenges both for the detector and the algorithms used to identify the decay products.
Utilising pairs of jets (dijets), a recent ATLAS search was able to probe the highest invariant mass of any of its searches, measuring events with energies as high as 8.1 TeV and thereby pushing up the experimentʼs sensitivity to hypothetical new resonances. Additionally, ATLAS has released the results of searches in events containing pairs of muons or electrons or single muons/electrons plus a neutrino, which extend the sensitivity to new resonance masses up to 4.5 and 5.1 TeV, respectively. Heavy particles with an affinity for coupling to the Higgs boson were also examined up to a mass of 3.7 TeV.
ATLAS has also searched for vector-like top-quark partners, which are strongly interacting particles invoked by models with new high-scale symmetries and which may be produced at the LHC. The final states sought in these analyses are a single high-transverse-momentum electron or muon, plus either several jets and a large component of missing transverse momentum or a large-radius jet consistent with a W or Z boson plus some missing transverse momentum and one b-tagged jet. The presence of vector-like top quarks is excluded for particle masses of up to 1.35 TeV, depending on the physics model chosen.
Finally, ATLAS has performed direct searches for dark matter by looking for single energetic photons plus missing transverse momentum and for a Higgs boson plus missing transverse momentum. These are potential signatures of the production and decay of a pair of weakly interacting massive particles (with the photon arising from initial-state radiation and the Higgs boson being produced in the decay of a Z’ dark-matter mediator).
The data are found to be consistent with Standard Model predictions for all of the searches conducted thus far. The second phase of Run 2 is about to begin and is scheduled to continue until the end of 2018, roughly tripling the integrated luminosity collected so far. This huge amount of data yet to be recorded will further extend the reach of these searches for new physics.