The ATLAS Experiment, at CERN’s Large Hadron Collider, has access to the highest energy human-generated collisions in the world. Protons are accelerated to near the speed of light and slammed into each other at the center of the detector, recreating the conditions of the early universe soon after the big bang. These collisions provide enough energy for the production of very massive particles and rare interactions that give us insight into the fundamental building blocks of nature (fundamental particles) and the forces between them. The Demers group uses tau leptons, a 3rd generation particle with a lifetime that is only a fraction of a second, to probe these early universe conditions and to look for evidence of physics beyond what we currently understand. The group used the ATLAS detector to make the first measurement of tau polarization at a hadron collider. With this measurement, we commissioned a variable that is extremely sensitive to CP in Higgs boson decays to taus and that is used in tau identification. The group is currently involved with analyzing the dataset from Run 2 (approximately 147 fb-1 at 13 TeV center of mass energy) with data taken from 2015 - 2018.
Current ATLAS physics projects include
- A search for Higgs decaying to tau leptons, when the Higgs was produced in association with a Z or W boson. This is a small fraction of the entire production of the Higgs, but provides and nice, clean way to trigger on the events of interest given that the tau decays themselves are difficult to identify with high purity and efficiency. Yale graduate students Mariel Pettee and Emma Castiglia, postdoc Gianantonio Pezzullo, and undergraduates India Bhalla-Ladd and Claire Laffan are the lead analyzers on the VH, H->tautau team.
- A search for a new, exotic Z’ boson that decays to tau pairs. If there is a preferential coupling of new physics to the heavy and short-lived 3rd generation particles, this channel provides a fantastic probe. It also is an excellent arena for studying high momentum taus, which is important given the higher energies we expect from Run 2. Yale graduate student Andrew Leister was a lead analyzers on the Z’->tautau search in Run 1.
- A measurement of the tau polarization in Z boson decays. This measurement provides access to the weak mixing angle and provides a test-case for a measurement pioneered by the Demers group (former Yale postdoc Zofia Czyczula and student Jane Cummings) to measure tau polarization at a hadron collider, carried out in the W->tau nu decay chain. This was the first measurement of tau polarization at a hadron collider. In addition to pushing forward a tool that has the ability to probe for and eventually characterize new physics, the polarization information can be used to control Z backgrounds to Higgs events.
ATLAS Service Projects
- Demers is the co-chair of the US ATLAS Institutional Board (1/20 - 12/21) and was previously deputy chair (1/18-12/19).
- Demers is co-convener of the ATLAS Upgrade Physics group (4/18 - 10/20).
- Demers co-led, with James Frost from Cambridge (UK) and then Anyes Taffard from UC Irvine, the ATLAS Data Quality group, responsible for determining the qualtiy and usability of data coming from the ATLAS detector during the start of Run 2 (2015). Demers also wrote the tau data quality monitoring checks for the experiment, and ran the data quality for taus in Run 1 with graduate student Andrew Leister. Demers becomes Data Preparation Deputy for ATLAS in October, 2020.
- Pezzullo co-leads the tau “reconstruction and identification” working group at ATLAS.
- The Yale Group is a Trigger and Data Acquisition Institute Member. Current projects with TDAQ include the implementation of RNN in tau trigger identification (graduate student Mariel Pettee), the improvement of the tau trigger energy scale (graduate student Emma Castiglia). Former work includes the checks on reprocessing for trigger groups (Jane Cummings), with previous significant work on the tau trigger (former Yale Postdoc Cristobal Cuenca Almenar as deputy tau trigger convener.)