A summary of the main analysis activities is provided below.
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.
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.
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.
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 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.
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.