ATLAS at the LHC

Aurelio Juste

Since 1993, the IFAE-ATLAS group has made major contributions to the construction of the ATLAS detector and its trigger system, the reconstruction software, and preparatory physics studies. Using the data collected by the ATLAS experiment during Run 1 (2010-2012) and Run 2 (2015-2018) of the Large Hadron Collider (LHC), the IFAE group has carried out a strong physics program.


During 2019, the first year of the second long shutdown of the LHC (LS2), the group has been actively involved in the analysis of the full LHC Run-2 dataset, and the maintenance and commissioning of detector elements under IFAE responsibility, in preparation for the start of the LHC Run 3 in 2021. The group has also maintained its visibility within the ATLAS Collaboration through a number of important management positions.


The year 2019 was characterized by an intense analysis effort using the full Run-2 dataset

The year 2019 was characterized by an intense analysis effort using the full Run-2 dataset, which corresponds to $139 fb^{-1}$ of proton-proton collisions at a center-of-mass energy of 13 TeV. The IFAE team continued to play a leading role in several physics research lines, including Higgs boson studies, dark-matter searches, searches for Supersymmetry, and searches for new phenomena in top-quark final states. A summary of the main analysis activities is provided below.

Higgs Boson Physics

During Run 2 the IFAE team is participating strongly in measurements of $t\bar tH$ production, and searches for additional Higgs bosons.

Probing top-Higgs interactions

Direct tests of top-Higgs interactions are extremely interesting, since the top quark is the Standard Model (SM) particle most strongly coupled to the Higgs sector. The most sensitive direct probe of the top-Higgs Yukawa coupling is provided by the measurement of the $t\bar tH$ cross section. During Run 1 and throughout Run 2 the IFAE team has actively participated in the $t\bar tH (\to b \bar b)$ search, resulting in several public results. The combination of several ATLAS searches using a partial Run-2 dataset recently led to the observation of ttH production [Phys. Lett. B 784 (2018) 173], a milestone result that represents the first step towards a precise measurement of the top-Higgs Yukawa coupling. Since 2018 the IFAE team (A. Juste and S. Kazakos) is also involved in the ttH search with $H \to WW^*/ZZ^*/\tau \bar\tau $ decays, giving multilepton final states. Using a partial Run-2 dataset, the IFAE team recently produced a preliminary result (ATLAS-CONF-2019-04) on the measurement of the $t\bar tH$ production cross section in multilepton final states (see Figure 1), with A. Juste acting as Corresponding Editor. This analysis also highlighted some issues with the modeling of the dominant $t\bar tW$ background, which would benefit from improved theoretical predictions. Motivated by these findings, the IFAE team has started participating in the measurement of differential cross sections for ttW production, using the full Run-2 dataset. Such measurement will be followed by the actual ttH analysis. Both results will be part of S. Kazakos’ PhD Thesis.

Figure 1: The observed best-fit values of the $t\bar tH$ signal strength μ (defined as the ratio of the measured cross section to the SM prediction) and their uncertainties by analysis channel and combined. The individual μ values for the channels are obtained from a simultaneous fit with the signal-strength parameter for each channel floating independently. The SM prediction corresponds to μ=1. From ATLAS-CONF-2019-04.

Searches for additional Higgs bosons

During 2019 the IFAE team (M. Bosman, J. Glatzer, Ll.M. Mir, I. Riu, and A. Salvador) continued its strong involvement in the search for a heavy charged Higgs boson $(H^+)$ decaying to a top quark and a bottom quark. IFAE is the only institute in charge of the full Run-2 analysis in the standard lepton-plus-jets final state. A sophisticated approach, which uses a parameterized neural network (see Figure 2), was developed to improve the search sensitivity. I. Riu is the ATLAS analysis contact for this search.

In addition, during 2019 the IFAE team has continued to develop a novel search for a light charged Higgs boson appearing in top-antitop-quark $(t\bar t)$ events, when one of the top quarks undergoes the decay $t \to H^+b$, followed by the decay of the $H^+$ boson into a bottom quark and a charm quark. Such scenario is predicted in several extensions of the Higgs sector, and is poorly tested experimentally. N. Orlando is the ATLAS analysis contact for this search, which capitalizes on the experience gained from a previous search also led by IFAE, this time for flavor-violating top quark decays $t \to H(\to b \bar b)c)$ (JHEP 05 (2019) 123). The results of this new search are expected during 2020.

Figure 2: Comparison of the Neural Network (NN) distribution between signal and background in one of the analysis regions for the $m_{H^+} = 400 GeV$ hypothesis.

Searches for new phenomena in Jet+X

In 2019, the IFAE team (D. Bogavac, J.L. Muñoz, M. Martínez, R. Rosten, and S. González) was a driving force in the search for a jet plus large missing transverse momentum ($E_T^{miss}$) using the full Run-2 dataset. Thanks to the increased statistics and the improvements implemented in the analysis, the SM background predictions are determined now with an ultimate total uncertainty of about 1.3% (3.6%) for E_T^{miss}$ around 400 GeV (1.2 TeV). Preliminary results indicate a good agreement between data and SM predictions and the team has focused on the interpretations in terms of different scenarios for physics beyond the SM (BSM). IFAE has led the interpretations in terms of large extra spatial dimensions, supersymmetry in compressed scenarios, and axion-like particles. In addition, IFAE took the responsibility of extracting new bounds on invisibly decaying Higgs bosons. Part of the IFAE team (D. Bogavac, and S. González) are also deeply involved in the complementary mono-photon searches, and are playing a central role in the final background determination and the interpretations related to axion-like particles. Both analyses are expected to conclude in time for the Spring conferences of 2020.
In 2019, the results of the analysis of a partial Run-2 dataset, led by IFAE and published already in 2018 (JHEP 01 (2018) 126), were a crucial ingredient of a ATLAS review on dark matter production using simplified models at the LHC (see Figure 3). Finally, in 2019 the IFAE group continued the searches for mono-V (vector boson), as a natural complement of the mono-jet analysis, accessing new interpretations related to dark matter Higgs portal models and the measurement of the invisibly decaying Higgs branching fraction. D. Bogavac is the ATLAS analysis contact for this search. In this analysis, the reconstruction of boosted vector bosons at very high transverse momentum is required, for which both QCD-inspired sub-jet quantities in large-R jets and a deep learning approach are employed in separating vector-boson decay products from gluon radiation.

Figure 3: Regions in a (mediator-mass, DM-mass) plane excluded at 95% CL by dijet, dilepton and ETmiss+X searches (led by IFAE), for leptophobic vector mediator simplified models. The exclusions are computed for a DM coupling gχ, quark coupling gq, universal to all flavors, and lepton coupling gℓ; as indicated in each case. Dashed curves labeled ’thermal relic’ correspond to combinations of DM and mediator mass values that are consistent with a DM density of ΩCh2= 0.12. From JHEP 05 (2019) 142.

Supersymmetry searches

One possible solution to the gauge hierarchy problem is provided by weak-scale supersymmetry, which extends the SM by introducing supersymmetric partners for all SM particles. Searches for gluinos, top/bottom squarks, and higgsinos are a high priority for the LHC Run 2 and beyond, and are areas of strong involvement by the IFAE team. During 2019, the IFAE team (A. Juste and C. Moreno) continued to participate in two high-profile supersymmetry searches featuring a high multiplicity of jets originating from the hadronisation of b-quarks (b-jets) and large missing transverse momentum. The first search is focused on the strong production of a pair of gluinos, with each gluino decaying into a neutralino and a tt pair or a bb pair. The second search targets the pair production of higgsinos, with each higgsino decaying into a gravitino and a Higgs boson, which in turn is required to decay into a bb pair. Both searches are based on the full Run 2 dataset and, for the first time, employ state-of-the-art machine-learning techniques, resulting in significant improvements in sensitivity compared to previous analyses. The final results are expected during 2020 and will be part of C. Moreno’s PhD Thesis.

Searches for compositeness

Models of partial compositeness represent another solution to the gauge hierarchy problem, predicting heavy vector-like quarks and new strong interactions resulting in a significant increase of the four top-quark production rate. During 2019 the IFAE team (A. Juste, N. Orlando, and T. Van Daalen) continued to lead the first ATLAS search for single production of a vector-like top quark (T), with the T-quark decaying into a top quark and a Higgs boson or a Z boson. Due to the large mass of the T-quark considered (above 1 TeV), the signal features boosted hadronically decaying SM resonances (W, Z and Higgs bosons, as well as top quarks), which are identified and used to discriminate it against the large background from top-quark pair production in association with jets $t \bar t +jets$. The results for this search, which is based on the full Run 2 dataset, are expected during Summer 2020, and will be part of T. Van Daalen’s PhD thesis. A. Juste is the ATLAS analysis contact for this search. In addition, the IFAE team (A. Juste, N. Orlando, and A. Sonay) continued to participate in the search for four-top-quark ($t \bar t t t \bar t$) production using the full Run 2 dataset. This analysis will be used to measure the SM ($t \bar t t t \bar t$) production cross-section, and to probe BSM ($t \bar t t t \bar t$) production, e.g. via a contact interaction due to an ultra-heavy new mediator particle. The IFAE team is focused on the analysis of events with exactly one lepton, or two opposite-charge leptons, and many jets, at least three of which are b-tagged. The team has made critical contributions to the search, such as the development of a novel strategy to improve the simulation of the dominant $t \bar t +jets$ background, or the development of a multivariate discriminant based on a boosted-decision-tree (BDT) that has led to a factor of two improvement in the sensitivity for this search, compared to previous analyses. The results for this search are expected during Summer 2020, and will be part of A. Sonay’s PhD thesis.

Searches for leptoquarks

Over the last few years, results from the B-factories and the LHCb experiment show intriguing deviations of ~2–3σ in the ratios $R_K^{(*)}$ and $R_D^{(*)}$, where accurate tests of Lepton Flavor Universality can be performed. Currently, the favored BSM explanation is a leptoquark (LQ) with a mass in the TeV scale, and preferentially coupled to 3rd-generation quarks and 2nd and 3rd generation leptons. During 2019 the IFAE team (S. Kazakos and A. Juste) continued to participate in the first ATLAS search for pair production of a scalar leptoquark decaying into LQ→tτ, where they are leading the analysis of final states with multiple leptons, including τ-leptons. This search, which uses the full Run 2 dataset, will be published during 2020 and will be part of S. Kazakos’ PhD Thesis.


In 2019, members of the IFAE group contributed strongly to the ATLAS Tile calorimeter (TileCal) maintenance during LS2, to the calorimeter calibration, to the measurement of the ATLAS luminosity, and to the preparation of the detector upgrades for the high-luminosity LHC (HL-LHC) operation.

IFAE engineer M. Verdaguer had designed and supervised local production of the mechanical structure that hosts TileCal crack scintillators, which are replaced during the LS2. The frames were installed in ATLAS in November 2019, as shown in Figure 4. D. Bogavac took an active role in calibrations and data quality checks of the TileCal electronics. She has started TileCal Run Coordination duties beginning of 2020.

R. Rosten developed a method to correct for the TileCal phototube non-linearity based on the measurements from slow current integrators, components developed and maintained exclusively by IFAE. While this non-linearity has to be corrected for in the TileCal energy calibration, the application of the correction to the determination of the ATLAS luminosity transfer function permitted to reduce the associated uncertainty below the 1% level on 2018 data. This is one of the most important results from the studies performed by S. González, who concluded his authorship qualification task on the analysis of the Run-2 luminosity measurement with TileCal.

Figure 4: Image of E-counter folders, which were produced at IFAE, already installed in the detector.
S. Kazakos completed his authorship qualification task on analysis of all SPS test beam runs with electrons taken during 2017-2018. His work allowed TileCal to understand to what precision the energy scale of the calorimeter response is preserved by the electronics developed for the HL-LHC upgrade. Finally, the IFAE mechanical workshop successfully produced twelve mini-drawers as a pre-production towards TileCal upgrade for the HL-LHC, strictly in time and within the budget (see Figure 5).
Figure 5: The team from the IFAE mechanical workshop involved in the pre-production of TileCal mini-drawers for the HL-LHC.


Since the first long shutdown of the LHC and during the whole Run 2, the IFAE ATLAS group has been involved in the Level-1 (L1) topological trigger system (L1Topo), consisting of two electronic boards with FPGA processors programmed to perform real-time event selection based on topological event variables. The group wrote the simulation of the topological trigger algorithms, was responsible of its evolution and provided diagnostic tools to identify sources of discrepancies or hardware malfunctioning.

In October 2018, I. Riu together with other ATLAS colleagues was granted an ATLAS Outstanding Achievement Award for her contribution and dedication to the successful commissioning of L1Topo. In 2019, N. Orlando took over the coordination of the L1Topo commissioning group. In preparation for Run 3, the IFAE group implemented the simulation of the newly proposed topological algorithms and validated their performance. C. Moreno and A. Sonay studied new ways of monitoring the algorithms behavior in a thread-safe mode, compulsory for running within the new High-Level Trigger (HLT) software in Run 3. C. Moreno and I. Riu contributed to the paper “ATLAS Data Quality Operations and Performance for 2015-2018 data-taking” (submitted to JINST). M. Bosman took several months of trigger validation shifts during 2019.

In parallel, A. Salvador got involved in the tau HLT signature, an area of expertise of IFAE. He qualified as an ATLAS author after studying how to improve the tracking part of the selection. He studied the use of a BDT to select the best track to seed the precision tracking algorithm of the tau trigger. He used a large sample of simulated $Z \to \tau \bar \tau$ events and trained a BDT to distinguish fake from truth tau tracks. Information about energies reconstructed in the various calorimeters, the transverse momentum (pT) and quality of the tracks and their distance to the calorimeter clusters were used. Figure 6 shows the tau track trigger efficiency as a function of the pT of the leading offline tau corresponding to various methods of track identification. The BDT methods show improved track trigger efficiency with respect to the method previously used.
Figure 6: Tau tracking trigger efficiency as a function of the pT of the leading offline tau, for various track trigger identification methods. The ’leading good quality’ method was used in Run 2 while the BDT methods are proposed for Run 3.