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 2021, the third 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 2022. The group has also maintained its visibility within the ATLAS Collaboration through a number of important management positions.


A summary of the main analysis activities is provided below.

Higgs Boson Physics

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 ttH cross section. During Run 1 and throughout Run 2 IFAE has actively participated in the ttH(🡪bb) search, resulting in several public results. The most recent result (arXiv:2111.06712) uses the full Run-2 dataset. In addition to the inclusive cross section measurement, this result includes a differential measurement in five bins of Higgs boson transverse momentum within the Simplified Template Cross Section (STXS) framework, including a boosted selection targeting Higgs boson transverse momentum above 300 GeV (see Fig. 1)
Figure 1: Signal-strength (μ) measurements in the individual STXS pTH bins, as well as the inclusive signal strength. From arXiv:2111.06712.
Since 2018 IFAE is also involved in the ttH search with H🡪WW*/ZZ*/ττ decays, giving multilepton final states. Based on the findings from the most recent result (ATLAS-CONF-2019-04), which highlighted some issues with the modeling of the dominant ttW background, the IFAE team (N. Agaras, S. Epari, S. Kazakos, and A. Juste) is playing a leading role in first measurements of differential cross sections for ttW production, using the full Run-2 dataset. Such results will become available in 2022 and will be part of S. Kazakos’ PhD Thesis.

Searches for additional Higgs bosons

During 2021 the IFAE team (M. Bosman, Ll.M. Mir, I. Riu, and A. Salvador) completed the search for a heavy charged Higgs boson (H+) decaying to a top quark and a bottom quark. The analysis was focused on the lepton+jets final state and developed a sophisticated multivariate approach to improve the search sensitivity. Ll.M. Mir and I. Riu were corresponding editors of the paper, which was published in JHEP 06 (2021) 145. This result was shown to have complementary sensitivity to dedicated dark matter searches in the context of the 2HDM+a model (see Fig. 2). The IFAE team (N. Agaras, S. Kazakos, A. Juste, S. Epari) developed a novel search for new heavy flavor-violating neutral Higgs bosons, mediating the production of two same-sign top quarks, three top quarks, or four top quarks, and resulting in multilepton final states. The results of this search are expected during 2022.
Figure 2: Observed (solid lines) and expected (dashed lines) exclusion regions at 95% CL in the (ma, mA) plane from different searches. The assumed model parameters are given on the figure. From ATLAS-CONF-2021-036.

The IFAE team has also continued to develop a broad program of searches for light scalars that are produced in the decay of Higgs bosons or top quarks, or in association with top quarks. Such scenarios are predicted in several extensions of the Higgs sector, and are poorly tested experimentally.

In particular, the IFAE team (A. Juste, I. Riu and P. Martínez) continued its involvement in a search for h🡪aa decays, where “a” is a light pseudoscalar that decays dominantly into bb. For very light scalars, the two b-quarks are merged into a single fat jet, requiring the development of a dedicated tagging algorithm. The first result in this kinematic regime, using 36 fb-1 of Run 2 data, was published in Phys. Rev. D 102 (2020) 112006. A more sophisticated tagging algorithm has been developed for use in the full Run-2 dataset analysis, which is underway.

In addition, the IFAE team (A. Juste and N. Orlando) completed a search for a light charged Higgs boson appearing in tt events, with one of the top quarks undergoing the decay t🡪H+b, and the H+ boson decaying into a bottom quark and a charm quark (H+🡪cb). N. Orlando was the ATLAS analysis contact for this search. This search probed branching ratios as low as 0.1% for charged Higgs boson masses between 60 and 160 GeV (ATLAS-CONF-2021-037). The search found an excess above the SM prediction with a significance of 3 standard deviations at a charged Higgs boson mass of 130 GeV (see Fig. 4). The corresponding publication result will be submitted in 2022. The IFAE team has started to develop a further optimized analysis to probe this excess using the full Run 2+Run 3 dataset.

Finally, the same analysis was reoptimized to search, for the first time, for flavor-violating top-quark decays t🡪X(🡪bb)c, where “X” denotes a light pseudoscalar, a scalar, or an axion-like particle. The IFAE team (M. Bosman, A. Juste, Ll.M. Mir, N. Orlando, I. Riu, and A. Salvador) is leading this search, for which I. Riu and A. Salvador are the analysis contacts. The corresponding publication result will become available in 2022.

Figure 3: Observed and expected upper limits on the product of branching ratios B(t🡪H±b)×B(H±🡪cb) as a function of the H+ mass. From ATLAS-CONF-2021-037.

Searches for new phenomena in Jet+X

In 2021, the IFAE team (D. Bogavac, J.L. Muñoz, M. Martínez, and S. González) published the results of the mono-photon and mono-jet searches (JHEP 02 (2021) 226 and Phys. Rev. D 103 (2021) 112006, respectively), using the full Run-2 dataset. Martinez was the corresponding editor of the mono-jet publication. As illustrated in Fig. 4, the level of precision achieved in the analysis (below 2% at low ETmiss and about 4% at very high ETmiss) turns this result into a stringent test of the SM predictions leaving little room for new phenomena in this final state. The results were interpreted in a number of models. In particular, IFAE 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.

The IFAE team also continued to participate in 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. First results are expected in 2022.

Figure 4: Measured recoil transverse momentum distribution in the mono-jet final state compared to SM predictions and the predictions from different beyond-SM scenarios. From Phys. Rev. D 103 (2021) 112006.

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 2021, 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, which is the main focus of C. Moreno’s PhD Thesis (defended in 2022), 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 publication results are expected during 2022.

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 2021 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 (tt+jets). The preliminary results for this search, which is based on the full Run 2 dataset, were included in T. Van Daalen’s PhD thesis (defended in 2021), The corresponding publication is expected in 2022.

In addition, the IFAE team (A. Juste, N. Orlando, and A. Sonay) continued to participate in the search for four-top-quark (tttt, denoted 4-top) production using the full Run-2 dataset. The IFAE team 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 (referred to as the “1LOS channel”). The team has made critical contributions to the search, such as the development of a novel strategy to improve the simulation of the dominant tt+jets background by using data-based corrections, or the development of a multivariate discriminant based on a boosted-decision-tree (BDT) that significantly improved the sensitivity (see Fig. 5). The results of this search, which will be part of A. Sonay’s PhD thesis, were published in JHEP 11 (2021) 118. This publication includes the combination with a similar published SM 4-top search focused on same-charge leptons and trilepton events, to which the IFAE team also contributed. The IFAE team is also playing a leading role in a search for beyond-SM 4-top production mediated by a new, heavy scalar resonance, again in the 1LOS channel, where it has developed a new approach to improve the modeling of the tt+jets background by the simulation by applying a multidimensional correction derived using a deep Neural Network. The results of this search are expected during 2022 and will also be part of A. Sonay’s PhD thesis. N. Orlando is the coordinator of this search, as well as the corresponding editor of a similar search in multilepton final states (ATLAS-CONF-2022-008).
Figure 5: Observed and expected event yields as a function of log10(S/B), where S and B are the post-fit signal and total background yields, respectively. The bins in all fitted regions are ordered and grouped in bins of log10(S/B). The signal is shown for both the best-fit signal strength, μ=2.2, and the SM prediction, μ=1.0. From JHEP 11 (2021) 118.

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 RK() and RD(), where accurate tests of Lepton Flavor Universality can be performed. Currently, the favored beyond-SM 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. The IFAE team (S. Kazakos and A. Juste) completed the first ATLAS search for pair production of a scalar leptoquark decaying into LQ→tτ, where they led the analysis of final states with multiple leptons, including τ-leptons. This result of this search, which sets some of the most stringent direct bounds on the mass of a 3rd generation LQ dominantly decaying into tτ (see Fig. 6), was published in JHEP 06 (2021) 179, with A. Juste acting as corresponding editor. In addition, the IFAE team started working on the search for LQLQ🡪tltl, l=e,μ. Both searches will also be interpreted in the context of vector LQs, and will be included in the upcoming ATLAS LQ combination using the full Run 2 dataset. All these results will be part of S. Kazakos’ PhD Thesis.
Figure 6: Observed (solid line) and expected (dashed line) 95% CL upper limits on the branching ratio B(LQ🡪tτ) as a function of LQ mass resulting from the combination of all analysis channels. From JHEP 06 (2021) 179


In 2021, members of the IFAE group contributed strongly to the ATLAS Tile calorimeter (TileCal) maintenance, 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.

D. Bogavac took TileCal Trigger Coordination duties that included the preparation of an exhaustive list of special runs to be taken at the beginning of the LHC Run 3 to study calorimeter peculiarities in different operation conditions. Together with S. Epari, they also completed studies of the TileCal scintillator aging due to the irradiation during the whole Run 2, shown on Fig. 7. This work established the final corrections of the TileCal energy scale for the ATLAS Run 2 data reprocessing.

N. Agaras performed studies to correct for the TileCal photo-tube non-linearity vs the average number of pp interactions per crossing (μ) for the case of the passive HV dividers. The corresponding average corrections are critical for the analysis of the ATLAS luminosity measurements. In particular, these corrections were used by S. González, who had extended his analysis of the Run 2 luminosity measurement to include the TileCal A-cells equipped with passive dividers. N. Agaras also served as TileCal deputy Run Coordinator in 2021.

I. Korolkov organized and coordinated two TileCal beam-test sessions dedicated to the validation of the new TileCal readout electronics for the HL-LHC upgrade at the SpS. Spare TileCal modules were instrumented with prototype readout electronics and were exposed to different particle beams of muons, positrons and hadrons of different energies from the SPS. Several IFAE group members had participated in the tests.

Finally, following the successful pre-production, the IFAE mechanical workshop continued the production phase of the mini-drawers for the TileCal HL-LHC upgrade. A large fraction (71%) of the mini-drawers were produced, tested and prepared for the shipment to CERN, strictly in accordance to the original schedule and within the allocated budget.

Figure 7: Beam induced aging of a given TileCal cell during the LHC Run 2 measured by various TileCal calibration systems.


Since the start of the first long shutdown of the LHC in 2013 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. IFAE members have been co-leading the ATLAS-wide L1Topo commissioning group since its creation, with N. Orlando coordinating it since 2019.

In preparation for Run 3, a completely new Level-1 Calorimeter (L1Calo) system consisting of many different new electronic boards, a new L1Topo system consisting of three new electronic boards, reprogrammed Level-1 Muon electronic boards and a new Muon to Central Trigger Processor Interface (MUCTPI) system are being installed (see Fig. 8). One of the new L1Topo boards will be counting L1 trigger objects of the same type like two or more electrons or several jets for example, and issuing L1 triggers. The IFAE group took the responsibility of writing the new topological algorithms for Run 3, in particular algorithms using the charge of the muons, which were coded by P. Martínez. The new L1Calo trigger objects in Run 3 required important interface changes to the simulation of the topological algorithms, and were implemented by P. Martínez, N. Orlando and A. Sonay following the implementation of the new L1Calo simulation. In addition, C. Moreno and P. Martínez also coded the algorithms counting L1 trigger objects. N. Orlando and I. Riu co-edited the L1Topo performance paper describing the L1Topo system in Run 2 (Eur. Phys. J. C 82 (2022) 1). R. Ballester defended his final physics degree project comparing the efficiency of several L1Topo algorithms using L1 trigger objects either from Run 2 or Run 3 as inputs. A similar performance was observed for the algorithms calculating the angular distance between two tau trigger objects or the invariant mass of two electrons.

In parallel, A. Salvador continued to be involved in the tracking algorithm of the tau High Level Trigger signature. In 2019, he designed a Boosted Decision Tree (BDT) to select the best track to seed the precision tracking algorithm of the tau trigger. He took care of its optimization and adapted it to new Run 3 simulated samples. Using a sample of Z🡪ττ events, he retrained the 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.

Since April 2021, I. Riu is the co-coordinator of the ATLAS-wide tau trigger signature group. She took care of monitoring the effect of the changes in the tau trigger reconstruction code in addition to designing the tau trigger menu in preparation for the Run 3 commissioning.

Figure 8: Scheme of the Level-1 system for Run 3, including the Level-1 Calorimeter, Level-1 Muon, L1Topo and the Central Trigger Processor (CTP) systems. Parts of the Level-1 Run 2 system (named Legacy) are foreseen to be running during the Run 3 commissioning period.