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. Since 2018 IFAE is 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 modelling of the dominant ttW background, the IFAE team (N. Agaras, S. Epari, S. Kazakos, and A. Juste) has played a leading role in detailed measurements of ttW production using the full Run 2 dataset, considered a pre-requisite for a precise measurement of ttH production in this channel. Such measurements (ATLAS-CONF-2023-019), performed inclusively and differentially over multiple observables (see Fig. 1), have established the required level of understanding of the process to proceed with the ttH measurement, which is currently underway. The ttW measurement was part of the S. Kazakos’ PhD Thesis (defended in 2023).
The IFAE team (N. Agaras, S. Kazakos, A. Juste, and S. Epari) developed a novel search for new heavy flavour-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 (JHEP 12 (2023) 081). An excess with a significance of up to 2.8 standard deviations was found in events with two positively charged leptons, compatible with the production of a heavy neutral Higgs boson with a sizable coupling to a top quark and an up quark (see Fig. 2). Additional data from the LHC Run 3, along with further analysis improvements, will allow drawing definite conclusions on the possible presence of a signal.
The IFAE team (A. Juste, N. Orlando, Q. Qin, and A. Sonay) has also played a leading role in a search for beyond-the-SM (BSM) 4-top production mediated by a new, heavy scalar resonance. This search, based on the full Run 2 dataset, was initially completed in multilepton final states (JHEP 07 (2023) 203) and is currently being finalized in final states with one lepton or two opposite-charge leptons.
During 2023, IFAE 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 BSM extensions of the Higgs sector, and are poorly tested experimentally.
In particular, the IFAE team (J. Jiménez, A. Juste, P. Martínez and I. Riu) continued its involvement in a search for ZH production with H🡪aa decays, where “a” is a light pseudoscalar that in turn 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 (ATL-PHYS-PUB-2022-042). The team is also leading the search for associated production of a pseudoscalar “a” and a top-antitop-quark pair (tta, a🡪bb), which is being performed for the first time in ATLAS. Both analyses, which use the full Run 2 dataset and are at a very advanced stage of completion, are part of P. Martínez’s PhD Thesis.
During 2023, the IFAE team (A. Berrocal, J. Jiménez, A. Juste, and Ll.-M. Mir) started a new 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). This search, which is part of A. Berrocal’s PhD Thesis, will use the full Run 2 and Run 3 datasets, and is a follow-up of a previous search based on the Run 2 dataset also performed by the IFAE team (A. Juste and N. Orlando). That search found an excess above the SM prediction with a significance of 3 standard deviations at a charged Higgs boson mass of 130 GeV (JHEP 09 (2023) 004). The same analysis was also reoptimized by the IFAE team (M. Bosman, A. Juste, Ll.-M. Mir, N. Orlando, I. Riu, and A. Salvador) to search, for the first time, for flavour-violating top-quark decays t🡪X(🡪bb)c, where “X” denotes a light pseudoscalar, a scalar, or an axion-like particle (JHEP 07 (2023) 199). This analysis was part of A. Salvador’s PhD Thesis (defended in 2023).
Using the full Run 2 dataset, the IFAE team (D. Bogavac, J.L. Muñoz, M. Martínez, and S. González) played a leading role in searches for monojet and monophoton final states, which are generic signatures of e.g., dark matter production or large extra spatial dimensions. The latest result on the monojet search (Phys. Rev. D 103 (2021) 112006) has been included in the ATLAS combination to set bounds on the branching ratio of the Higgs boson decaying into dark matter particles (Phys. Lett. B 842 (2023) 137963; see Fig. 3).
During 2023, IFAE team also continued to participate in searches for mono-W/Z, 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. This analysis, which uses the full Run 2 dataset, is still underway.
ne 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 electroweakinos and higgsinos are a high priority for the LHC Run 2 and beyond, and are areas of strong involvement by the IFAE team. Members of the IFAE team (A. Juste and C. Moreno) completed a search for 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, using the full Run 2 dataset (Eur. Phys. J. C 83 (2023) 561). This search yielded the most restrictive limits to date, and its preliminary results had been included in C. Moreno’s PhD Thesis (defended in 2022). On the other hand, J. Mamuzic participated in the Run 2 search for the direct production of winos and Higgsinos in events with two same-charge leptons or three leptons, interpreting the results in simplified and complete models with and without R-parity conservation (JHEP 11 (2023) 150). She also played a leading role in the statistical combination of ATLAS searches for charginos and neutralinos using various decay channels (ATLAS-CONF-2023-046; see Fig. 4), and in their interpretation in the context of the 19-parameter phenomenological minimal supersymmetric standard model (pMSSM), assuming R-parity conservation and that the lightest neutralino is the lightest supersymmetric particle (ATLAS-CONF-2023-055).
Vector-like fermions arise in many BSM scenarios, such as Composite Higgs models, models with extra spatial dimensions, supersymmetric models, and grand unified theories. A broad program of searches for vector-like quarks (VLQ) has been carried out at the LHC, with the IFAE team playing a leading role in some of the most important search channels. Most recently, the IFAE team (A. Juste and T. Van Daalen) completed 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 (JHEP 08 (2023) 153). The preliminary results of this search had been included in T. Van Daalen’s PhD Thesis (defended in 2021). The final results of this search will now be included in the upcoming ATLAS combination of single VLQ searches, whose publication is expected in 2024. Vector-like leptons (VLL) are produced in pairs via the electroweak interaction and have a much lower production cross-section than VLQs. Consequently, to date very few VLL searches have been performed at the LHC. During 2023 the IFAE team has continued to develop two ambitious searches for VLLs using the full Run 2 ATLAS dataset. Some IFAE team members (N. Agaras, S. Epari, and A. Juste) worked on a search for pair production of vector-like electrons or muons, each of which decay into a light lepton and a W, Z or Higgs boson, which is being performed for the first time in Run 2 of the LHC. This search is part of S. Epari’s PhD Thesis. Other team members (N. Agaras, S. Epari, J. Harrison, A. Juste, J. Mamuzic, G. Correa, I. Riu, A. Sonay) developed a broad search for VLLs in the context of the UV-complete 4321 model, which provides an explanation for the flavour anomalies. These VLLs decay into complex chains of third-generation fermions via an off-shell vector LQ, giving final states with tau-leptons and or light-leptons, and high jet and b-jet multiplicities. This search is part of S. Epari’s and G. Correa’s PhD Theses. Both VLL searches, which yield the most restrictive limits to date, are very advanced and their publication is expected during 2024.
Over the last few years, results from the B-factories and the LHCb experiment have shown intriguing hints of lepton flavour universality violation in rare B meson decays, known as “flavour anomalies”. Currently, the most favoured 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. Using the full Run 2 dataset, the IFAE team (A. Juste and S. Kazakos) completed the most competitive search to date for LQ pair production with each LQ decaying into a top quark and a light lepton (electron or muon), LQLQ🡪tltl (arXiv:2306.17642; see Fig. 5). This search, along with a previous search for LQLQ🡪tτtτ that was completed by the same team, were part of the S. Kazakos’ PhD Thesis (defended in 2023). Both searches will be included in the upcoming ATLAS LQ combination, whose publication is expected in 2024.
In addition, the IFAE team (G. Correa and A. Juste) is participating in a broad search for LQs in tau final states using the full Run 2 and Run 3 datasets, which will be part of G. Correa’s PhD Thesis.
The IFAE team (A. Juste and J. Harrison) continued to develop a novel model-independent search in final states with high-lepton (electron or muon) multiplicity (exactly four or at least five) based on the full Run 2 dataset. This search, which is part of J. Harrison’s PhD Thesis, employs state-of-the-art machine learning techniques for anomaly detection in a signal-agnostic way.
On the calorimeter performance side, A. Berrocal developed a method to extract optical properties of an individual scintillating tile from the calibration data taken with the Cesium (Cs) movable radioactive source (see Fig. 7). He is analysing the full Run 2 Cs calibration data and is working on understanding the ageing of the individual optical components of the TileCal under the radiation damage from the LHC collisions, with the goal of publishing these results in 2024.
In 2023, TileCal continued the 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 novel readout electronics and were exposed to different particle beams of muons, positrons and hadrons of different energies from the SPS. Several IFAE group members participated in these beam tests. The IFAE mechanical workshop successfully concluded the production phase of the mini-drawers for the TileCal HL-LHC upgrade. The whole number of mini-drawers was produced, tested and shipped to CERN in 2023, 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.
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 were installed. One of the new L1Topo boards is responsible of counting L1 trigger objects of the same type like two or more electrons or several jets for example (multiplicity algorithms), while the other two boards perform topological calculations (decision algorithms). The details of the L1Topo simulation logic are shown in Fig. 8.