Collider physics

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. The IFAE group is currently participating in detector operations and physics analyses during the LHC Run 3 (2022-2026).

INTRODUCTION

During 2024, the third year of the LHC Run 3, the group was actively involved in the operation of the detector elements under IFAE responsibility, while working on the completion of the planned analyses using the full Run 2 dataset, and starting new analysis projects using Run 3 data. The group also maintained its visibility within the ATLAS Collaboration through a number of important management positions.

PHYSICS ANALYSES

A summary of the main analysis activities is provided below.

Top quark and Higgs boson physics

The top quark is the heaviest Standard Model (SM) 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. If these particles are very heavy compared to the centre-of-mass energy of the LHC, then these interactions can be described via an Effective Field Theory (EFT). The IFAE team (N. Agaras, A. Juste, and T. Vázquez) performed the first LHC search for same-sign top-quark pair production giving final states with two same-sign leptons, using the full Run 2 dataset. This search was used to set the most restrictive limits on Wilson coefficients corresponding to dimension-6 operators within an EFT describing a four-fermion interaction between two light quarks (u or c) and two top quarks (arXiv:2409.14982; see Figure 1).

Figure 1: 2D likelihood scans for the ctu(1) versus cQu(1) Wilson coefficients. The observed limits at 68% and 95% CL are represented by solid and dashed lines, respectively, while the expected limits are illustrated by the inner and outer shaded regions. The best fit value is marked with a cross. The SM prediction is marked with a star. The new-physics scale is set to Λ=1 TeV. From arXiv:2409.14982.

Direct tests of top-Higgs interactions are extremely 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 targeting the H->bb and H->WW*/ZZ*/ττ decays, the latter giving multilepton final states. These results were recently interpreted, along with other Higgs-boson measurements, in the context of UV-complete extensions of the SM, in particular the two-Higgs-doublet model (2HDM) near the alignment limit and the Minimal Supersymmetric Standard Model (MSSM) for various MSSM benchmark scenarios (JHEP 11 (2024) 097). In addition, Q. Qin made major contributions to a study of the CP properties of the top-Higgs Yukawa coupling exploiting ttH and tH events and the H->bb decay mode, based on the full Run 2 dataset (Phys. Lett. B 849 (2024) 138469). Members of the IFAE team (A. Juste and T. Vázquez) are currently participating in the Run 2 legacy measurement of ttH production in multilepton final states, which will also include constraints on a CP-odd top-Higgs coupling, and whose publication is expected in 2025.

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 (A. Juste, N. Orlando, Q. Qin, and A. Sonay) 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 (arXiv:2408.17164). This analysis was combined with a previous search in multilepton final states (2LSS/ML), and interpreted in the context of a 2HDM (see Figure 2) and a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a top-antitop-quark pair.

Figure 2: Expected (black dashed line) and observed (red solid line) 95% CL limits on tanβ as a function of the mH/A mass obtained using the combination of 1L/2LOS and 2LSS/ML final states, assuming type-II 2HDM in the alignment limit. Values of tanβ below the observed limit are excluded. The yellow bands illustrate the ±1σ bands of the expected limits. The two red dashed lines correspond to the observed limits when the theory cross-sections are varied by ±1σ. From arXiv:2408.17164.

Additionally, a search for tbH±(->Wh) with h->bb with significant contribution from IFAE (Ll.-M. Mir, I. Riu, and A. Salvador) was published (arXiv:2411.03969), being the first search for this process performed at the LHC. In addition, during 2024 IFAE 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), 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 2024, the IFAE team (A. Berrocal, J. Jiménez, A. Juste, and Ll.-M. Mir) continued to develop 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 (part of A. Berrocal’s PhD Thesis) will use 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 definite conclusion regarding the previously observed excess.

Dark matter searches

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. During 2024 the IFAE team completed a search for mono-W/Z using the full Run 2 dataset (JHEP 11 (2024) 126). The results of this search were interpreted in the context of several BSM scenarios, such as 2HDMs with a pseudo-scalar mediator between the SM and the dark sector, invisible decays of the Higgs boson, or axion like particles (see Figure 3). All these results were included in an ATLAS summary paper of searches for a fermionic dark matter candidate in the context of theoretical models characterized by a mediator particle exchange in the s-channel (Eur. Phys. J. C 94 (2024) 1102).

Figure 3: Observed (solid line) and expected (dashed line) exclusion upper limits at 95% CL on the coupling cW as a function of the effective scale fa for an ALP mass of 1 MeV, with the ±1σ and ±2σ uncertainties in the expected limit (inner and outer shaded bands). The area above the line is excluded at 95% CL. From JHEP 11 (2024) 126

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, 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. J. Montejo played a leading role in a search for R-parity-violating supersymmetry in final states with high jet multiplicity (JHEP 05 (2024) 003). The search uses full Run 2 dataset and the results were interpreted in the context of R-parity-violating supersymmetry models that feature prompt gluino-pair production decaying directly to three jets each or decaying to two jets and a neutralino which subsequently decays promptly to three jets. In particular, he developed a novel technique to reconstruct the gluino mass with machine-learning methods, solving the combinatorial assignment challenge to correctly identify which jets belong to a given gluino (see Figure 4).

Figure 4: Normalised average mass spectrum comparing the shapes of the reconstructed (solid) and target (light) distributions for different masses. The reconstructed distribution is produced using the NN assignments, whereas the target distribution is built assigning jets to gluinos based on their MC generated labels. From JHEP 05 (2024) 003.

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 (Phys. Rev. Lett. 133 (2024) 031802), and in their interpretation in the context of the phenomenological minimal supersymmetric standard model (JHEP 05 (2024) 106). Currently, the IFAE team (A. Juste, P. Tricarico, and T. Vázquez) is working 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.

Searches for vector-like fermions

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. The results of this search were included in the ATLAS combination of single VLQ searches (arXiv:2408.08789). 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 2024 the IFAE team completed two ambitious searches for VLLs using the full Run 2 ATLAS dataset. Several IFAE team members (N. Agaras, S. Epari, and A. Juste) worked on a broad 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 was performed for the first time in Run 2 of the LHC. This search targeted final states with two opposite-sign leptons, three leptons, or four leptons, and employed sophisticated multivariate techniques for discrimination among different signal processes (arXiv:2411.07143). The search achieved the most restrictive direct limits to date on the mass of vector-like electrons or muons (see Figure 5). This search is part of S. Epari’s PhD Thesis, which received an ATLAS Thesis Award in 2024.

Figure 5: Combined observed and expected limits at 95% CL for the VLL muon doublet model, as a function of the mass of the VLL. The inner and outer bands around the expected limit are the ±1σ and ±2σ variations including all uncertainties, respectively. The theoretical prediction for the signal production cross-section is shown as a red solid line. Expected limits on the signal cross-section from the individual analysis channels are also shown. From arXiv:2411.07143.

The IFAE team (N. Agaras, S. Epari, J. Harrison, A. Juste, J. Mamuzic, G. Correa, I. Riu, A. Sonay) also 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. A search targeting final states with one or two hadronically decaying tau-leptons (part of G. Correa’s PhD Thesis) was completed using the full Run 2 dataset, obtaining the most restrictive direct limits to date on the mass of these particles (ATLAS-CONF-2024-008). The preliminary results of this search were presented at the ICHEP 2024 conference.

Searches for leptoquarks

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, S. Kazakos, and T. Vázquez) 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 (Eur. Phys. J. C 84 (2024) 818). This search, along with a previous search for LQLQ🡪tτtτ that was completed by the same team, were part of S. Kazakos’ PhD Thesis (defended in 2023). Both searches were included in the ATLAS combination of LQ pair production searches (Phys. Lett. B 854 (2024) 138736), illustrated in Figure 6 for a scalar LQ decaying into tμ or bν.

Figure 6: Expected and observed 95% CL exclusion limits on the LQ mass as a function of the branching ratio to a charged lepton. Limits are presented for down-type scalar LQs decaying into a third-generation quark and a muon. From Phys. Lett. B 854 (2024) 138736.

Currently, the IFAE team (G. Correa, A. Juste, and T. Vázquez) 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.

Model-agnostic searches

The IFAE team (A. Juste, J. Harrison, A. Odella, and T. Vázquez) 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 and A. Odella’s PhD Theses, employs state-of-the-art machine learning techniques for anomaly detection in a signal-agnostic way and its publication is expected in 2025.

TILECAL OPERATIONS AND UPGRADE

In 2024, members of the IFAE group contributed to the ATLAS Tile calorimeter (TileCal) Run 3 operation, to the calorimeter calibration and to the measurement of the ATLAS luminosity. Members of the group took calorimeter shifts as well as duties on data quality checks. A new PhD student, P. Tricarico, finished his ATLAS authorship qualification project based on the analysis of the TileCal dedicated laser calibrations taken in parallel to the normal data taking. The results of these studies confirmed the expected long-term aging of the TileCal phototubes obtained also with standalone laser runs. More relevantly, the results provided insights on the evolution of the phototube gains during the data taking, also of relevance to the group contribution to the ATLAS luminosity measurement.

S. Epari finished the “ATLAS calibration transfer luminosity error” analysis on the TileCal side for the 2023 Run 3 data (see Figure 7 as an illustration in the case of Run 2 data). The results of this work were reported at the Moriond 2024 conference. After the completetion of his ATLAS qualification, P. Tricarico took over from S. Epari and started contributing to the Tile luminosity measurement for the 2024 Run 3 dataset.

Figure 7: Ratio of the TileCal to track-counting luminosity measurements for runs considered to estimate the calibration transfer uncertainty in 2018. The deviation from unity amounts to the systematic error of the method.

On the calorimeter performance side, A. Berrocal finished the measurement of the optical properties of an individual scintillating tile from the calibration data taken with the Cs movable radioactive source during Run 2. He analyzed the full Run 2 Cs calibration data and concluded the measurement of the degradation of the calorimeter optical uniformity due to radiation damage to the optical components (see Figure 8). The results of his findings were included in a publication accepted by JINST (arXiv:2412.15944). A postdoc from the group, Q. Qin, took over the analysis of the aging of individual tiles with the aim to search for more subtle effects, such as a possible doze rate effect.

Figure 8: Average per layer degradation of the tile light output obtained from the Cs calibrations in Run 2 as a function of time

TITRIGGER OPERATIONS, PERFORMANCE AND UPGRADE

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. For Run 3 starting in 2022, a new L1Topo system consisting of three new electronic boards 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 group took the responsibility of writing the simulation of the L1 topological trigger algorithms, was responsible for its evolution and provided diagnostic tools to identify sources of discrepancy or hardware malfunctioning. IFAE members have been co-leading the ATLAS-wide L1Topo commissioning group since its creation. During 2024, the group maintained the validation tools and contributed to the validation of the topological algorithms in use during routine data taking. As part of her qualification task in ATLAS, H. Moyano wrote a new program to compute the event rates and trigger correlations between the various L1 topological triggers as well as their unique rates. Figure 9 shows the scheme of the program, interfacing with the trigger menu, decoding the L1 Topological simulation bits and calculating the event rates. Figure 10 shows one example of event rates correlation between various L1 multiplicity and topological triggers. At the October 2024 ATLAS Week at CERN, H. Moyano presented a poster about her work and became an ATLAS author in December 2024.

Figure 9: Scheme of the L1 Rates Calculator program.

This program is planned to be used to study and optimize the trigger menu should triggers not show additional event rates when included in the whole menu. In particular, it is going to be useful to understand the overlap between the routinely used triggers and the newly implemented anomaly detection neural network trigger planned for 2025.
In addition, J. Jiménez worked in the implementation of a data quality histogram showing discrepancies between the hardware triggers and their simulation. It should be in use during the rest of the Run 3 data taking period starting in 2025. In terms of publications, I. Riu contributed to the tau trigger section in the paper “The ATLAS Trigger System for LHC Run 3 and Trigger performance in 2022” (JINST 19 (2024) P06029). In terms of coordination roles, J. Jiménez continued to co-coordinate the Level-1 Topological algorithms group and I. Riu became the chair of the TDAQ Institute Board representatives. In addition, I. Riu was appointed as co-editor of the Run 3 tau trigger paper, planned to be published at the end of 2025 or beginning of 2026.

Figure 10: Illustrative event rates correlation matrix between several L1 Run 3 triggers.