Physics at 13 TeV: ATLAS - extracting the most from new LHC data
The unprecedented collision energy of LHC run 2 will bring physicists a step further into an as-yet unexplored world, where new particles should eventually leave their signature in the powerful detectors. This may well happen in the form of “missing transverse momentum” – that is, energy that is not detected directly but can be deduced by measuring the imbalance of the observed particles. Often called “missing energy” for simplicity, scientists predict (see here) that it could be the signature of many new physics processes.
“In ATLAS, we have performed many analyses, searches and measurements using the missing transverse momentum signature with the run 1 data. Reconstruction of the missing transverse momentum in ATLAS is based on calibrated jets and leptons, plus other calorimeter energy deposits,” explains David Charlton, ATLAS Spokesperson.
Not all the new processes that ATLAS plans to investigate will be studied using a missing energy signature. An example is the Brout–Englert–Higgs mechanism, which accounts for the mass difference of elementary particles and whose simplest manifestation is the Standard Model Higgs boson discovered by the LHC experiments in 2012. “Supersymmetry and string theory, which try to explain features beyond the Standard Model like dark matter and quantum gravity, suggest not one Higgs boson but five or more,” says Bill Murray, ATLAS Physics Coordinator. “We will search for evidence of these new bosons in the changed predictions they make for the behaviour of the Standard Model Higgs boson we have found. Recently, ATLAS showed that the measured properties of the Higgs boson already strongly suggest that supersymmetric Higgs bosons should weigh at least three times as much as the one found already. Precise measurements in run 2 could point to their existence. The Higgs boson could also show us dark matter by decaying into dark matter particles. We started these searches during the first run but in the next one we will have much more precise tests for such possibilities.”
Tagging the missing transverse momentum becomes increasingly difficult as the number of pile-up interactions in each bunch crossing rises. According to the current plan, run 2 will use a bunch spacing of 25 nanoseconds, which should result in a pile-up not very much higher than that of run 1. However, tougher experimental conditions with a bunch spacing of 50 ns and a higher number of protons in the bunches, which could challenge the detectors, are not completely excluded. “The information coming from the calorimeter together with very good tracking capabilities will allow us to reduce the effects of pile-up,” confirms Murray. “However, 50 ns bunch spacing would indeed be a big challenge for our detector.”
During the long shutdown of the LHC, the ATLAS collaboration has been working on improving the trigger software and performance, and introducing some new hardware trigger components. The wish to keep low trigger thresholds in run 2 results in an immense data-handling challenge, putting heavy pressure on the disks and tapes available to ATLAS through the Worldwide LHC Computing Grid. “We have been running a big programme to optimise the use of our computing resources, shrinking the size of events by dropping and condensing information, reducing the number of copies of events which need to be kept, and improving simulation flexibility and reconstruction speeds. This has taken the work of many dedicated people to implement. However, the benefits are substantial and the physics goals are large: further steps into unexplored territory,” concludes Charlton.
Check out more of our Physics at 13 TeV series in "ALICE - scratching under the surface", "CMS - scanning the unknown", "LHCb - a new data-processing strategy " and "TOTEM - a new era of collaboration with CMS". For a theory perspective on the next run, read "Life is Good at 13 TeV".
by Antonella Del Rosso