Open Access Policy Deposits

Total Cost of Ownership and Evaluation of Google Cloud Resources for the ATLAS Experiment at the LHC

(2025)

Abstract: The ATLAS Google Project was established as part of an ongoing evaluation of the use of commercial clouds by the ATLAS Collaboration, in anticipation of the potential future adoption of such resources by WLCG grid sites to fulfil or complement their computing pledges. Seamless integration of Google cloud resources into the worldwide ATLAS distributed computing infrastructure was achieved at large scale and for an extended period of time, and hence cloud resources are shown to be an effective mechanism to provide additional, flexible computing capacity to ATLAS. For the first time a total cost of ownership analysis has been performed, to identify the dominant cost drivers and explore effective mechanisms for cost control. Network usage significantly impacts the costs of certain ATLAS workflows, underscoring the importance of implementing such mechanisms. Resource bursting has been successfully demonstrated, whilst exposing the true cost of this type of activity. A follow-up to the project is underway to investigate methods for improving the integration of cloud resources in data-intensive distributed computing environments and reducing costs related to network connectivity, which represents the primary expense when extensively utilising cloud resources.

Measurement of the top quark mass with the ATLAS detector using t t ¯ events with a high transverse momentum top quark

(2025)

The mass of the top quark is measured using top-quark-top-antiquark pair events with high transverse momentum top quarks. The dataset, collected with the ATLAS detector in proton–proton collisions at s=13 TeV delivered by the Large Hadron Collider, corresponds to an integrated luminosity of 140 fb⁻¹. The analysis targets events in the lepton-plus-jets decay channel, with an electron or muon from a semi-leptonically decaying top quark and a hadronically decaying top quark that is sufficiently energetic to be reconstructed as a single large-radius jet. The mean of the invariant mass of the reconstructed large-radius jet provides the sensitivity to the top quark mass and is simultaneously fitted with two additional observables to reduce the impact of the systematic uncertainties. The top quark mass is measured to be mt=172.95±0.53 GeV, which is the most precise ATLAS measurement from a single channel.

Observation of VVZ production at s = 13 TeV with the ATLAS detector

(2025)

A search for the production of three massive vector bosons, VVZ(V=W,Z), in proton–proton collisions at s=13 TeV is performed using data with an integrated luminosity of 140 fb−1 recorded by the ATLAS detector at the Large Hadron Collider. Events produced in the leptonic final states WWZ→ℓνℓνℓℓ (ℓ=e,μ), WZZ→ℓνℓℓℓℓ, ZZZ→ℓℓℓℓℓℓ, and the semileptonic final states WWZ→qqℓνℓℓ and WZZ→ℓνqqℓℓ, are analysed. The measured cross section for the pp→VVZ process is 660−90+93(stat.)−81+88(syst.) fb, and the observed (expected) significance is 6.4 (4.7) standard deviations, representing the observation of VVZ production. In addition, the measured cross section for the pp→WWZ process is 442±94(stat.)−52+60(syst.) fb, and the observed (expected) significance is 4.4 (3.6) standard deviations, representing evidence of WWZ production. The measured cross sections are consistent with the Standard Model predictions. Constraints on physics beyond the Standard Model are also derived in the effective field theory framework by setting limits on Wilson coefficients for dimension-8 operators describing anomalous quartic gauge boson couplings.

Development of a breathing lung phantom for proton CT imaging.

(2025)

Objective

To report on the design of a deformable lung phantom capable of imitating breathing motion with realistic tissue surrogate properties for proton imaging applications.

Approach

The phantom was manufactured via 3D printing and silicone moulding, with a customised structural design for motor-controlled breathing motion. The overall size of the phantom was rescaled to fit in the experimental proton CT (pCT) scanner prototype, featuring a 284 mm maximum size for the imaging field-of-view. Several flexible resins were evaluated in perspective of flexibility by varying ultraviolet exposure times, as increased exposure results in resin hardening at each layer. We optimised the structure to achieve ideal lung compression properties, while preserving its integrity to hold the weight of a solid tumour. Phantom material properties were characterised by segmentation of each component in X-ray CT and pCT images, to determine the CT number expressed in Hounsfield units and the relative stopping power (RSP) with respect to water.

Main results

We achieved non-homogenous compression in the lung using a grid structure with gradient thickness. The rigid ribcage was 3D printed using granite based material. The tumour motion implemented in the phantom design, as measured using template-matching in fluoroscopic X-ray imaging, revealed hysteretic motion with 10 mm peak-to-peak in the superior-inferior direction.

Significance

The developed deformable lung phantom imitated lung motion characteristics, featuring CT number and RSP values in the range comparable to human tissues. The developed breathing phantom is put forward for experimental motion studies in pCT imaging.

Erratum: Measurement of t-channel production of single top quarks and antiquarks in pp collisions at 13 TeV using the full ATLAS Run 2 data sample

(2025)

The performance of missing transverse momentum reconstruction and its significance with the ATLAS detector using 140 fb-1 of s=13 TeV pp collisions

(2025)

Abstract: This paper presents the reconstruction of missing transverse momentum ( $$p_{\text {T}}^{\text {miss}}$$ p T miss ) in proton–proton collisions, at a center-of-mass energy of 13 TeV. This is a challenging task involving many detector inputs, combining fully calibrated electrons, muons, photons, hadronically decaying $$\tau $$ τ -leptons, hadronic jets, and soft activity from remaining tracks. Possible double counting of momentum is avoided by applying a signal ambiguity resolution procedure which rejects detector inputs that have already been used. Several $$p_{\text {T}}^{\text {miss}}$$ p T miss ‘working points’ are defined with varying stringency of selections, the tightest improving the resolution at high pile-up by up to 39% compared to the loosest. The $$p_{\text {T}}^{\text {miss}}$$ p T miss performance is evaluated using data and Monte Carlo simulation, with an emphasis on understanding the impact of pile-up, primarily using events consistent with leptonic Z decays. The studies use $$140~\text {fb}^{-1}$$ 140 fb - 1 of data, collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. The results demonstrate that $$p_{\text {T}}^{\text {miss}}$$ p T miss reconstruction, and its associated significance, are well understood and reliably modelled by simulation. Finally, the systematic uncertainties on the soft $$p_{\text {T}}^{\text {miss}}$$ p T miss component are calculated. After various improvements the scale and resolution uncertainties are reduced by up to $$76\%$$ 76 % and $$51\%$$ 51 % , respectively, compared to the previous calculation at a lower luminosity.

Search for tt¯H/A→tt¯tt¯ production in proton–proton collisions at s=13 TeV with the ATLAS detector

(2025)

Abstract: A search is presented for a heavy scalar (H) or pseudo-scalar (A) predicted by the two-Higgs-doublet models, where the H/A is produced in association with a top-quark pair $$(t\bar{t}H/A),$$ ( t t ¯ H / A ) , and with the H/A decaying into a $$t\bar{t}$$ t t ¯ pair. The full LHC Run 2 proton–proton collision data collected by the ATLAS experiment is used, corresponding to an integrated luminosity of $$139~\text {fb}^{-1}.$$ 139 fb - 1 . Events are selected requiring exactly one or two opposite-charge electrons or muons. Data-driven corrections are applied to improve the modelling of the $$t\bar{t}$$ t t ¯ +jets background in the regime with high jet and b-jet multiplicities. These include a novel multi-dimensional kinematic reweighting based on a neural network trained using data and simulations. An H/A-mass parameterised graph neural network is trained to optimise the signal-to-background discrimination. In combination with the previous search performed by the ATLAS Collaboration in the multilepton final state, the observed upper limits on the $$t\bar{t}H/A \rightarrow t\bar{t}t\bar{t}$$ t t ¯ H / A → t t ¯ t t ¯ production cross-section at 95% confidence level range between 14 fb and 5.0 fb for an H/A with mass between 400 $$\text {GeV}$$ GeV and 1000 $$\text {GeV}$$ GeV , respectively. Assuming that both the H and A contribute to the $$t\bar{t}t\bar{t}$$ t t ¯ t t ¯ cross-section, $$\tan \beta $$ tan β values below 1.7 or 0.7 are excluded for a mass of 400 $$\text {GeV}$$ GeV or 1000 $$\text {GeV}$$ GeV , respectively. The results are also used to constrain a model predicting the pair production of a colour-octet scalar, with the scalar decaying into a $$t\bar{t}$$ t t ¯ pair.

Reconstruction and identification of pairs of collimated τ-leptons decaying hadronically using s=13 TeV pp collision data with the ATLAS detector

(2025)

Abstract: This paper describes an algorithm for reconstructing and identifying a highly collimated hadronically decaying $$\tau $$ τ -lepton pair with low transverse momentum. When two $$\tau $$ τ -leptons are highly collimated, their visible decay products might overlap, degrading the reconstruction performance for each of the $$\tau $$ τ -leptons. A dedicated treatment attempting to tag the $$\tau $$ τ -lepton pair as a single object is required. The reconstruction algorithm is based on a large radius jet and its associated two leading subjets, and the identification uses a boosted decision tree to discriminate between signatures from $$\tau ^+\tau ^-$$ τ + τ - systems and those arising from QCD jets. The efficiency of the identification algorithm is measured in $$Z\gamma $$ Z γ events using proton–proton collision data at $$\sqrt{s}=13$$ s = 13 TeV collected by the ATLAS experiment at the Large Hadron Collider between 2015 and 2018, corresponding to an integrated luminosity of $$139\,\text{ fb}^{-1}$$ 139 fb - 1 . The resulting data-to-simulation scale factors are close to unity with uncertainties ranging from 26 to 37%.

Erratum to: Search for tt¯H/A→tt¯tt¯ production in the multilepton final state in proton–proton collisions at s = 13 TeV with the ATLAS detector

(2025)

Search for vector-like leptons coupling to first- and second-generation Standard Model leptons in pp collisions at s = 13 TeV with the ATLAS detector

(2025)

Abstract : A search for pair production of vector-like leptons coupling to first- and second-generation Standard Model leptons is presented. The search is based on a dataset of proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb −1. Events are categorised depending on the flavour and multiplicity of leptons (electrons or muons), as well as on the scores of a deep neural network targeting particular signal topologies according to the decay modes of the vector-like leptons. In each of the signal regions, the scalar sum of the transverse momentum of the leptons and the missing transverse momentum is analysed. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No significant excess above the Standard Model background expectation is observed and limits are set at 95% confidence level on the production cross-sections of vector-like electrons and muons as a function of the vector-like lepton mass, separately for SU(2) doublet and singlet scenarios. The resulting mass lower limits are 1220 GeV (1270 GeV) and 320 GeV (400 GeV) for vector-like electrons (muons) in the doublet and singlet scenarios, respectively.

Physics Department