Data Analysis at the ATLAS experiment

Overview

Since the start of data taking with the ATLAS experiment at the Large Hadron Collider (LHC), our group is contributing in various ways to the analyis of the collected data. Our main focus is the investigation of the observed Higgs boson, the electro-weak sector and the mechanism of electroweak symmetry breaking.

Precise measurements in the H--> tau tau decay mode 

Distribution of the reconstructed di-τ invariant mass (mMMCττ) for all events in the VBF_1 and signal region. The bottom panel shows the differences between the numbers of observed data events and expected background events (black points). The observed Higgs boson signal, corresponding to (σ× B)/(σ× B)SM = 0.93, is shown with a filled red histogram. Entries with values above the x-axis range are shown in the last bin of each distributions. The dashed band indicates the total uncertainty on the total predicted yields. The prediction for each sample is determined from the likelihood fit performed to measure the pp→H→ττ cross-section.

From: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2019-09/

Test of CP Invariance in Higgs boson production in association with two jets

Post-fit distributions of the event yields (divided by the bin width) as a function of the Optimal Observable in the SRs for the τ_lepτ_had  analysis channel. The values of d^˜, the signal strength μ, the normalization of background processes, and nuisance parameters for the event yield prediction are set to those which minimize the NLL. The ratios of the data to the prediction are shown in the lower panels. The size of the combined statistical, experimental and theoretical uncertainties in the predicted event yields is indicated by the hatched bands. 

From: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HIGG-2018-14/

Search for Lepton flavour violating decays of Higgs boson and other 

The discovery of the Higgs boson opened a multitude of possibilities for probing physics processes beyond the standard model (SM). One such process predicted by many beyond-SM theories is lepton flavour violation (LFV) in the decays of the Higgs boson. While lepton flavour is an accidental symmetry in the SM, LFV has already been experimentally observed in neutrino oscillations. Searches for He and H decays are conducted with leptonic -decays, leading to a final state of e+2. 

A central task of the analysis is the precise estimation of the SM backgrounds. For this a data-driven method is employed, which takes advantage of the lepton-flavour universality of the SM, causing the background processes to be symmetric under the exchange of an electron and muon. This symmetry would be broken only by the difference in the two LFV signals. In practice, however, experimental differences also need to be compensated for in order to restore the symmetry between electrons and muons. The final sensitivities are obtained from a dedicated statistical model based on the output of neural networks trained to distinguish signal and background events. 

The group is analysing the full Run 2 ATLAS data set and preparing for the Run 3 of the LHC. Additionally, the extrapolation of Run 2 sensitivities to the HL-LHC scenario are being studied.

Measurement of anomalous magnetic and electric dipole moment of the tau lepton 

The determination of the anomalous magnetic moment of the muon has shown among the largest tensions between data and predictions by the standard model (SM) of particle physics. Effects from physics beyond the standard model (BSM) could be enhanced for the tau-lepton, the heavier sibling particle of the muon.

Measurements of the anomalous magnetic moment of the tau-lepton as well as of its electric dipole moment can be performed in ultraperipheral heavy ion collisions at the LHC. The process used for this purpose is photon-induced production of di-tau pairs which was observed for the first time at the LHC in April 2022. The interaction contains two γττ-vertices which are sensitive to the electromagnetic moments of the tau-lepton. In April 2022, the ATLAS collaboration published the first constraints on the anomalous magnetic moment, reaching a sensitivity similar to the one of the world-leading measurement by the DELPHI collaboration (2004). The group participated in this analysis and plans to drive the precision of this measurement further with more data and more sophisticated techniques.

Tau trigger

L1 rates for the primary τ(+X) triggers in pp collisions recorded at √s=13 TeV in 2016. L1 rates as a function of the instantaneous luminosity. Rates are shown between 0.9 and 1.2*10³⁴ cm^-2s^-1. Rates at 1.2times10³⁴ cm^-2s^-1 are also indicated in numbers. 

From: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2017-061/

Luminosity

Fractional difference in run-integrated luminosity between the LUCID PMT-C12 algorithm, and the TILE, EMEC, FCal, track-counting and Z-counting algorithms. The LUCID luminosity has been subjected to a pile-up-dependent correction, using as a reference the track-based luminosity measured in a reference run recorded on July 16th and indicated by the arrow. Each point corresponds to an ATLAS run recorded during 25 ns bunch-train running in 2018 at √s=13 TeV; runs lasting less than about 100 minutes are not shown. The luminosity measurements by TILE, EMEC, FCal, Tracking and Z-counting have been normalized to LUCID in the reference run.

From: http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PLOTS/LUMI-2018-001/