Top and Higgs processes analyses IFAE is involved in analyses of top quark and Higgs physics processes for many years. The top quark is the heaviest Standard Model particle, with a mass comparable to the electroweak symmetry scale. This raises the tantalizing possibility that the top quark may be strongly coupled to new particles beyond the SM and thus open a window to new phenomena. The Higgs boson was discovered in 2012 and processes involving it have been intensively studied since then, including potential decays to new particles.
The IFAE group has been involved in several analyses that have been published in 2025. The first LHC search for same-sign top-quark pair production giving final states with two same-sign leptons used the full Run 2 dataset and set restrictive limits on Wilson coefficients corresponding to dimension-6 operators [JHEP02 (2025) 084].
Direct tests of top-Higgs interactions are very interesting, since the top quark is the 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 Run 1 IFAE has participated in multiple searches for ttH production. In 2025, the legacy analysis of the Higgs boson production in association with top quarks in multilepton final states was performed using full Run 2 data. It included simplified template cross-section measurements, as well as the measurement of associated production with a single top. The CP structure of the top quark-Higgs boson Yukawa coupling was probed through the analysis of ttH and tH events, resulting compatible with the SM hypothesis and constraining a CP-odd top-Higgs coupling. Fig. 1 shows the exclusion contours in the plane of the signal strength of both ttH and tH processes, showing their compatibility with the SM within 2σ. [arXiv:2510.23755].
The measurement of Higgs self-couplings is critical to probe the shape of the Higgs potential and is one of the highest priority topics at the LHC. While di-Higgs production is sensitive to the trilinear self-coupling, the tri-Higgs production is sensitive to both trilinear and quartic self-couplings. These are challenging analyses and IFAE started being involved in two of the highest-profile analyses in di-Higgs production, the HH->4b and HH->2b2tau. In addition, the IFAE group started involvement and is leading the HHH->4b2tau. Publications using full Run 2 and partial Run 3 data are expected in 2026.
Related to searches for additional Higgs bosons, many beyond-the-SM (BSM) scenarios incorporate an extended Higgs sector, typically with additional neutral and at least a pair of charged scalar bosons, which may be lighter or heavier than the SM Higgs boson. The IFAE team played a leading role in a search for BSM 4-top production mediated by a new, heavy scalar resonance, ttH/A->tttt. This search, based on the full Run 2 dataset, targeted final states with one lepton or two opposite-charge leptons (1L/2LOS), and employed sophisticated neural networks (NN) to discriminate signal from background [Eur. Phys. J. C 85 (2025) 573].
IFAE continued to work on 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. IFAE played a leading role in the 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 use of a dedicated tagging algorithm. Identical scalars or a pair of scalars with a different mass were considered. Full run 2 data was used and no significant excess above the SM was observed, setting up constraints on the production and decay of the process. [Phys. Rev. D 112 (2025) 072005].
The team lead the search for associated production of a pseudoscalar “a” and a top-antitop-quark pair (tta, a->bƃ), being performed for the first time in ATLAS. It used the previously mentioned dedicated tagging algorithm, together with a dedicated mass-parameterised Neural Network. No deviation from the SM was observed and limits were extracted. HH4b and tta analyses are included in one PhD thesis defended in 2025.
During 2025, the IFAE team continued to work on the 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 uses the full Run 2 and partial Run 3 datasets and is a follow-up of a previous search by the IFAE team based on Run 2 data, which 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 new search has significantly better sensitivity and is expected to reach a conclusion regarding the previously observed excess. The publication is foreseen in 2026. At the same time, IFAE started to work on a combined H+->cb/cs analysis planned to use full Run 3 data.
Dark matter remains one of the most compelling yet elusive mysteries in high-energy physics. A wide range of cosmological and astrophysical observations provide strong evidence for its existence, suggesting it may consist of massive, weakly interacting particles. However, its fundamental nature remains unknown. Motivated by theoretical developments, attention has increasingly turned to experimental signatures that are still only loosely explored by the LHC collaborations, as outlined in the following white paper [arXiv:2504.10597]. In 2025, members of IFAE developed a search strategy targeting these models in final states featuring a top quark accompanied by dark matter and additional quarks. This analysis plans to use the full Run 2 dataset and a partial Run 3 dataset. A key prediction of these models is an enhanced production rate in final states containing same-sign quarks. To strengthen the sensitivity to such signals, the team has also contributed to the development of an algorithm to identify the charge of the jets, with a corresponding public note expected in 2026.
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, third-generation squarks, electroweakinos and higgsinos are a high priority for the LHC Run 2 and beyond, and are areas of strong involvement by the IFAE team. In 2025, the IFAE team continued to work on a high-profile search for electroweak production of supersymmetric particles in models with compressed mass spectra. The search is based on the full Run 2 and partial Run 3 datasets and targets final states with two opposite-sign leptons or three leptons and missing transverse momentum.
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. In 2025, the IFAE team was contributing in the vector-like top quark pair production search in the one lepton final state using the full Run 2 dataset. Vector-like leptons (VLL) are produced in pairs via the electroweak interaction and have a much lower production cross-section than VLQs. During 2025 the IFAE team worked on a search for pair production of vector-like leptons decaying into a third-generation SM quark and a vector leptoquark, as predicted by an ultraviolet-complete extension of the SM, referred to as the ‘4321’ model and providing an explanation for the flavour anomalies. This search targeted final states with tau leptons and b-jets in the final state, and the signal regions were optimised using neural networks parameterised in the generated mass of the vector-like lepton signal (mVLL) [Eur. Phys. J. C 85 (2025) 1335].
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. The IFAE team continued its participation in a broad search for LQs in tau final states using the full Run 2 and Run 3 datasets. It aims at covering all relevant production and decay modes. A publication is foreseen in 2026 or 2027.
The IFAE team continued to work on 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 employed state-of-the-art machine learning techniques for anomaly detection in a signal-agnostic way [EPJC 86 (2026) 247]. Model-agnostic limits were extracted for each topology, as shown in Figure 4. A model-dependent analysis was also performed to demonstrate the sensitivity of the anomaly detection technique with respect to a selection of benchmark models. The expected exclusion limits were generally competitive to the ones from dedicated searches, in some cases improving the sensitivity in corners of the available phase space.
The IFAE team started to be involved in a much broader search including events with lower lepton multiplicity using full Run 2 and partial Run 3 data.
The group continued its contribution to the study of the effect of radiation to the Tile Calorimeter and published the results of the radiation hardness studies of its optical instrumentation with Run 2 data [2025 JINST 20 P06006]. The degradation depends on the layer and the maximum degradation in barrel cells ended Run 2 with a 10% loss at most. Extrapolations to the HL-LHC operations based on the extracted degradation model were included, as shown in Figure 5. The model indicated that the less exposed cells will degrade by no more than 25% until the end of the HL-LHC phase.
At the same time, the group continued its strong contribution to the ATLAS luminosity measurement by analysing 2024 data.
Concerning the trigger activities, the group continued to maintain the validation tools of the L1 topological trigger and contributed to the validation of the topological algorithms in use during routine data taking, including multi-variate anomaly detection algorithms. IFAE members have been co-leading the ATLAS-wide L1Topo commissioning group since its creation.