LHC Run Research Articles

Ultra-Peripheral Collisions physics with ATLAS

Exclusive production of dilepton pairs, γγ → ℓℓ, ℓ = e, μ, τ, is studied using data from ultraperipheral collisions (UPC) of lead nuclei at TeV recorded by the ATLAS detector [1] at the LHC. The process of interest proceeds via photon-photon interactions in the strong electromagnetic fields of relativistic lead nuclei. In the case of the dielectron production, within experimental uncertainties the measured integrated cross section is in good agreement with the QED predictions from the Monte Carlo programs STARlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross section shows systematic differences from these predictions which are more pronounced at high absolute rapidity, |yee | and scattering angle, |cosθ*|. In the dimuon production case, the measured cross sections at large absolute rapidity |yμμ | is found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. The γγ → ττ process is observed in the UPCs with a significance exceeding 5 standard deviations. This observation can be turned into a constraint on the τ-lepton anomalous magnetic moment. Measurement of light-by-light scattering data are also performed during the LHC Run 2. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6-100 GeV.

The industrial production of Micro Pattern Gaseous Detector: experience from the ATLAS Micromegas

Resistive Micromegas is one of the detector technologies chosen by ATLAS for the Phase-1 upgrade of the Muon Spectrometer, completed in 2022 in view of the LHC Run3 start.It is the largest MPGD-based detector system ever built, covering an active area of 1280 m2, providing trigger and precise tracking capabilities to the ATLAS Muon system and able to stand a radiation background rate up to 20 kHz/cm2.The heart of the ATLAS Micromegas detectors is the anode board, which carries the resistive protection layer, the readout electrodes and the insulating spacers supporting the micro-mesh.The production of the 2048 readout boards of size up to 0.5×2.2 m2 has been assigned to high-technology PCB industries and required dedicated efforts for technology transfer, production follow-up and quality assurance and control.The paper reviews the main challenges from the design phase to the completion of the project which spanned over several years. Emphasis is also put on the thorough quality assurance and quality control protocol established, the achieved results, as well as on the logistic, supply and schedule constraints.The lessons learned from this unprecedented MPGD project are also discussed.

New techniques for jet calibration with the ATLAS detector

A determination of the jet energy scale is presented using proton–proton collision data with a centre-of-mass energy of sqrt{s}=13 TeV, corresponding to an integrated luminosity of 140 fb^{-1} collected using the ATLAS detector at the LHC. Jets are reconstructed using the ATLAS particle-flow method that combines charged-particle tracks and topo-clusters formed from energy deposits in the calorimeter cells. The anti-k_textrm{t} jet algorithm with radius parameter R=0.4 is used to define the jet. Novel jet energy scale calibration strategies developed for the LHC Run 2 are reported that lay the foundation for the jet calibration in Run 3. Jets are calibrated with a series of simulation-based corrections, including state-of-the-art techniques in jet calibration such as machine learning methods and novel in situ calibrations to achieve better performance than the baseline calibration derived using up to 81 fb^{-1} of Run 2 data. The performance of these new techniques is then examined in the in situ measurements by exploiting the transverse momentum balance between a jet and a reference object. The b-quark jet energy scale using particle flow jets is measured for the first time with around 1% precision using gamma +jet events.

Characterisation and operations of the Multianode Photomultiplier Tubes for the LHCb RICH detectors

The upgraded LHCb RICH detectors are equipped with Multianode Photomultiplier Tubes, covering a total area of approximately four square metres. In order to achieve the same excellent hadron identification performance as during LHC Runs 1 and 2 at five times the instantaneous luminosity, the photon detectors have to be sensitive to single photons with repetition rates of up to 100MHz/cm2 and to have a very low noise count rate. The main properties of the photomultipliers are presented, together with the characterisation of an unexpected source of noise observed in the Hamamatsu R11265 Multianode Photomultiplier Tubes, extending up to several microseconds after the primary signal. The quality control results, the mitigation strategies for the aforementioned noise to perform optimal single-photon counting at 40 MHz, and the first operations of the RICH system are described.

The Higgs condensate as a quantum liquid: A critical comparison with observations

The triviality of four-dimensional scalar quantum field theories poses challenging problems to the usually adopted perturbative implementation of the Higgs mechanism. In the first part of the paper, we compare the triviality scenario and the renormalized two-loop perturbation theory to precise and extensive results from nonperturbative numerical simulations of the real scalar field theory on the lattice. The proposal of triviality and spontaneous symmetry breaking turns out to be in good agreement with numerical simulations, while the renormalized perturbative approach seems to suffer significant deviations from the numerical simulation results. In the second part of the paper, we try to illustrate how the triviality of four-dimensional scalar field theory leads, nevertheless, to the spontaneous symmetry breaking in the scalar sector of the Standard Model. We show how triviality allows us to develop a physical picture of the Higgs mechanism in the Standard Model. We suggest that the Higgs condensate behaves like a relativistic quantum liquid leading to the prevision of two Higgs bosons. The light Higgs boson resembles closely the new LHC narrow resonance at 125 GeV. The heavy Higgs boson is a rather broad resonance with mass of about 730 GeV. We critically compare our proposal to the complete LHC Run 2 data collected by the ATLAS and CMS Collaborations. We do not find convincingly evidences of the heavy Higgs boson in the ATLAS datasets. On the other hand, the CMS full Run 2 data display evidences of a heavy Higgs boson in the main decay modes [Formula: see text], while in the preliminary Run 2 data there are hints of the decays [Formula: see text] in the golden channel. We also critically discuss plausible reasons for the discrepancies between the two LHC experiments.

Electroweak superpartner production at 13.6 Tev with Resummino

Due to the greater experimental precision expected from the currently ongoing LHC Run 3, equally accurate theoretical predictions are essential. We update the documentation of the Resummino package, a program dedicated to precision cross section calculations for the production of a pair of sleptons, electroweakinos, and leptons in the presence of extra gauge bosons, and for the production of an associated electroweakino-squark or electroweakino-gluino pair. We detail different additions that have been released since the initial version of the program a decade ago, and then use the code to investigate the impact of threshold resummation corrections at the next-to-next-to-leading-logarithmic accuracy. As an illustration of the code we consider the production of pairs of electroweakinos and sleptons at the LHC for centre-of-mass energies ranging up to 13.6 TeV and in simplified model scenarios. We find slightly increased total cross section values, accompanied by a significant decrease of the associated theoretical uncertainties. Furthermore, we explore the dependence of the results on the squark masses.

Global fits of simplified models for dark matter with GAMBIT

Global fits explore different parameter regions of a given model and apply constraints obtained at many energy scales. This makes it challenging to perform global fits of simplified models, which may not be valid at high energies. In this study, we derive a unitarity bound for a simplified vector dark matter model with an s-channel vector mediator and apply it to global fits of this model with GAMBIT in order to correctly interpret missing energy searches at the LHC. Two parameter space regions emerge as consistent with all experimental constraints, corresponding to different annihilation modes of the dark matter. We show that although these models are subject to strong validity constraints, they are currently most strongly constrained by measurements less sensitive to the high-energy behaviour of the theory. Understanding when these models cannot be consistently studied will become increasingly relevant as they are applied to LHC Run 3 data.

Prospects of gluino searches in multi-lepton channels in light of the ongoing LHC RUN-III

This study investigates the prospect of discovering strongly interacting gluinos in different multi-lepton channels with lepton multiplicities greater than or equal to 2 at LHC RUN-III, considering several pMSSM scenarios. The effectiveness of the multivariate analysis (MVA) method with the boosted decision tree (BDT) algorithm is explored to obtain a better significance for different models. Promising results are obtained for the 3-lepton channels, indicating that the use of MVA methods can improve the sensitivity of the search for gluinos at LHC RUN-III. The study probes the multi-lepton signatures arising from gluinos via intermediate eweakinos and sleptons at an early stage of the LHC RUN-III. The heavier eweakinos can give rise to three or four lepton signals, in which the squark hierarchy between the L and R types plays a crucial role. The study considers two sets of benchmark points that satisfy all the collider constraints obtained from the LHC RUN-II data. Moreover, these sets of benchmark points are mostly consistent with WMAP/PLANCK data and the muon (g-2) constraint. The corresponding results from the MVA technique demonstrate that, even for an integrated luminosity of 270 , the 5 σ discovery prospect of for TeV in the wino type model is promising. The study also presents various other models that may emerge at the early stage of LHC RUN-III. Wino type models in the scenario where left squarks are light and right squarks are heavy exhibit the best prospect of discovering gluinos in multi-lepton channels in the LHC RUN-III experiment. The findings of this study provide crucial insights into the potential discovery of gluinos in multi-lepton channels.

Inclusive and differential cross-sections for dilepton toverline{t} production measured in sqrt{s} = 13 TeV pp collisions with the ATLAS detector

Differential and double-differential distributions of kinematic variables of leptons from decays of top-quark pairs ( toverline{t} ) are measured using the full LHC Run 2 data sample collected with the ATLAS detector. The data were collected at a pp collision energy of sqrt{s} = 13 TeV and correspond to an integrated luminosity of 140 fb−1. The measurements use events containing an oppositely charged eμ pair and b-tagged jets. The results are compared with predictions from several Monte Carlo generators. While no prediction is found to be consistent with all distributions, a better agreement with measurements of the lepton pT distributions is obtained by reweighting the toverline{t} sample so as to reproduce the top-quark pT distribution from an NNLO calculation. The inclusive top-quark pair production cross-section is measured as well, both in a fiducial region and in the full phase-space. The total inclusive cross-section is found to beσtt¯=829±1stat±13syst±8lumi±2beampb,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{t\\overline{t}}=829\\pm 1\\ \\left(\ extrm{stat}\\right)\\pm 13\\ \\left(\ extrm{syst}\\right)\\pm 8\\ \\left(\ extrm{lumi}\\right)\\pm 2\\ \\left(\ extrm{beam}\\right)\\ \ extrm{pb}, $$\\end{document}where the uncertainties are due to statistics, systematic effects, the integrated luminosity and the beam energy. This is in excellent agreement with the theoretical expectation.

Search for high-mass Wγ and Zγ resonances using hadronic W/Z boson decays from 139 fb−1 of pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

A search for high-mass charged and neutral bosons decaying to Wγ and Zγ final states is presented in this paper. The analysis uses a data sample of sqrt{s} = 13 TeV proton-proton collisions with an integrated luminosity of 139 fb−1 collected by the ATLAS detector during LHC Run 2 operation. The sensitivity of the search is determined using models of the production and decay of spin-1 charged bosons and spin-0/2 neutral bosons. The range of resonance masses explored extends from 1.0 TeV to 6.8 TeV. At these high resonance masses, it is beneficial to target the hadronic decays of the W and Z bosons because of their large branching fractions. The decay products of the high-momentum W/Z bosons are strongly collimated and boosted-boson tagging techniques are employed to improve the sensitivity. No evidence of a signal above the Standard Model backgrounds is observed, and upper limits on the production cross-sections of these bosons times their branching fractions to Wγ and Zγ are derived for various boson production models.

The ATLAS RPC system for the LHC Run-3

Resistive Plate Chambers (RPCs) provide the barrel region of the ATLAS detector with an independent muon trigger and a two-coordinate measurement. The chambers, 3720 gas volumes in total covering a surface area of 3650 m2, are arranged in three concentric double layers and operated in a strong magnetic toroidal field.The system was originally designed to operate for 10 years with an instantaneous luminosity up to the LHC nominal value of 1034 cm−2 s−1. After a successful data taking period in Run-2, when the instantaneous luminosity more than doubled its design value, the detector has undergone an intense maintenance to ensure an efficient data taking during the just started Run-3.Several interventions have been carried out on the detector mainly concerning the gas distribution with the aim of stabilizing the system and reducing the amount of gas released into the atmosphere. The main interventions were: (a) new gas distribution racks have been added in order to increase the vertical segmentation and in view of the installation of the new chambers for the phase-2 upgrade, (b) non-return valves have been installed on the chamber outputs to avoid reverse flow with large leaks, (c) a massive repair campaign has been done for fixing the continuously developing leaks, (d) a new repair technique to fix and prevent new leaks has been tested and (e) the segmentation of the HV channels has been doubled in a third of the spectrometer to mitigate the effect of detector failures.The different aspects of the activity carried out in LS2 (Long Shutdown 2) are described, from motivation to implementation. The first measurements to assess the system performance are also presented.

Search for dark matter produced in association with a single top quark and an energetic W boson in sqrt{s}= 13 TeV pp collisions with the ATLAS detector

This paper presents a search for dark matter, chi , using events with a single top quark and an energetic W boson. The analysis is based on proton–proton collision data collected with the ATLAS experiment at sqrt{s}= 13 TeV during LHC Run 2 (2015–2018), corresponding to an integrated luminosity of 139 fb^{-1}. The search considers final states with zero or one charged lepton (electron or muon), at least one b-jet and large missing transverse momentum. In addition, a result from a previous search considering two-charged-lepton final states is included in the interpretation of the results. The data are found to be in good agreement with the Standard Model predictions and the results are interpreted in terms of 95% confidence-level exclusion limits in the context of a class of dark matter models involving an extended two-Higgs-doublet sector together with a pseudoscalar mediator particle. The search is particularly sensitive to on-shell production of the charged Higgs boson state, H^{pm }, arising from the two-Higgs-doublet mixing, and its semi-invisible decays via the mediator particle, a: H^{pm } rightarrow W^pm a (rightarrow chi chi ). Signal models with H^{pm } masses up to 1.5 TeV and a masses up to 350 GeV are excluded assuming a tan beta value of 1. For masses of a of 150 (250) GeV, tan beta values up to 2 are excluded for H^{pm } masses between 200 (400) GeV and 1.5 TeV. Signals with tan beta values between 20 and 30 are excluded for H^{pm } masses between 500 and 800 GeV.

Testing neutrino dipole portal by long-lived particle detectors at the LHC

We discuss the potential of using detectors aimed for searching long-lived particles (LLP) at the high-luminosity LHC run, to probe the neutrino dipole models. This is achieved by taking the heavy neutral leptons (HNL) of the models as candidates of the LLPs. Taking into account the dipole couplings to the weak bosons, d_{W,Z}, which control the production of the HNLs at the LHC, we discuss the reach on the electromagnetic dipole couplings, d_gamma , by searching for a single high-energy photon at LLP detectors. Four typical scenarios are considered in this paper, scenario A, B with d_{W}=0 or d_{Z}=0, and scenario C, D with d_{W,Z}gg d_gamma . We show the sensitivity on d_gamma , can be fairly different depending on the relations between the d_{W,Z} and d_gamma . And the LLP detectors can potentially extend the sensitivity on dipole couplings during the high-luminosity runs of the LHC in certain scenarios.

Ready for LHC Run III: the ATLAS New Small Wheel and the MicroMegas chambers performances

The two New Small Wheels (NSW) for the upgrade of the ATLAS Muon Spectrometer are now installed in the experiment and ready to collect data in LHC Run III, started in July 2022. The NSW is the largest Phase-1 upgrade project of ATLAS. Its challenging completion and readiness for data taking is a remarkable achievement of the Collaboration. The two wheels (10 meters in diameter) replace the first muon stations in the high-rapidity regions of ATLAS and are equipped with multiple layers of two new detector technologies: the small strips Thin Gap Chambers (sTGC) and the MicroMegas (MM). The latter, belong to the family of Micro Pattern Gaseous Detectors (MPGD), for the first time used in such a large scale in HEP experiments. Each detector technology will cover more than 1200 m2 of active area. The new system is designed to assure high tracking efficiency, reduction of fake trigger rates and precision measurement of muon tracks, also in view of the higher background environment foreseen for Hi-Lumi LHC. In this presentation, the motivation of the NSW upgrade and the steps from the commissioning to the data taking together with the first results using the Run III data will be presented, focusing on the MicroMegasperformances.

Search for pair-produced scalar and vector leptoquarks decaying into third-generation quarks and first- or second-generation leptons in pp collisions with the ATLAS detector

A search for pair-produced scalar and vector leptoquarks decaying into quarks and leptons of different generations is presented. It uses the full LHC Run 2 (2015–2018) data set of 139 fb−1 collected with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of sqrt{s} = 13 TeV. Scalar leptoquarks with charge −(1/3)e as well as scalar and vector leptoquarks with charge +(2/3)e are considered. All possible decays of the pair-produced leptoquarks into quarks of the third generation (t, b) and charged or neutral leptons of the first or second generation (e, μ, ν) with exactly one electron or muon in the final state are investigated. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section are provided for eight models as a function of the leptoquark mass and the branching ratio of the leptoquark into the charged or neutral lepton. In addition, lower limits on the leptoquark masses are derived for all models across a range of branching ratios. Two of these models have the goal of providing an explanation for the recent B-anomalies. In both models, a vector leptoquark decays into charged and neutral leptons of the second generation with a similar branching fraction. Lower limits of 1980 GeV and 1710 GeV are set on the leptoquark mass for these two models.

Energy deposition studies for the LHCb insertion region of the CERN Large Hadron Collider

The LHCb (Large Hadron Collider beauty) experiment at CERN LHC aims at achieving a significantly higher luminosity than originally planned by means of two major upgrades: the Upgrade I that took place during the Long Shutdown 2 (LS2) and the Upgrade II proposed for LS4. Such an increase in instantaneous and integrated luminosity with respect to the design values requires to reassess the radiation exposure of LHC magnets, cryogenics, and electronic equipment placed in the insertion region 8 (IR8) around LHCb. Monte Carlo simulations are a powerful tool to understand and predict the interaction between particle showers and accelerator elements, especially in case of future scenarios. For this purpose, their validation through the comparison with available measurements is a relevant step. A detailed IR8 model, including the LHCb detector, has been implemented with the fluka code. The objective of this study is to evaluate radiation levels due to proton-proton collisions and benchmark the predicted dose values against beam loss monitor measurements performed in 2018. Finally, we comment on the upcoming LHC run (Run 3, from 2022 to 2025), featuring a first luminosity jump in LHCb.11 MoreReceived 10 June 2022Revised 8 February 2023Accepted 18 April 2023DOI:https://doi.org/10.1103/PhysRevAccelBeams.26.061002Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam lossBeam techniquesParticle interactionsParticle phenomenaPhysical SystemsAccelerator subsystemsAccelerators & storage ringsHigh-energy accelerators & collidersAccelerators & BeamsParticles & Fields

Revisiting Vh(→ boverline{b} ) at the LHC and FCC-hh

Diboson production processes provide good targets for precision measurements at present and future hadron colliders. We consider Vh production, focusing on the h → boverline{b} decay channel, whose sizeable cross section makes it accessible at the LHC. We perform an improved analysis by combining the 0-, 1- and 2-lepton channels with a scale-invariant b-tagging algorithm that allows us to exploit events with either a boosted Higgs via mass-drop tagging or resolved b-jets. This strategy gives sensitivity to 4 dimension-6 SMEFT operators that modify the W and Z couplings to quarks and is competitive with the bounds obtained from global fits. The benefit of the h → boverline{b} decay channel is the fact that it is the only Vh channel accessible at the LHC Run 3 and HL-LHC, while at FCC-hh it is competitive with the effectively background-free h → γγ channel assuming ≲ 5% systematic uncertainty. Combining the boosted and resolved categories yields a 17% improvement on the most strongly bounded Wilson coefficient at the LHC Run 3 with respect to the boosted category alone (and a 7% improvement at FCC-hh). We also show that, at FCC-hh, a binning in the rapidity of the Vh system can significantly reduce correlations between some EFT operators. The bounds we obtain translate to a lower bound on the new physics scale of 5, 8, and 20 TeV at the LHC Run 3, HL-LHC, and FCC-hh respectively, assuming new-physics couplings of order unity. Finally, we assess the impact of the Vh production channel on anomalous triple gauge coupling measurements, comparing with their determination at lepton colliders.

Laser calibration of the ATLAS Tile Calorimeter during LHC Run 2

This article reports the laser calibration of the hadronic Tile Calorimeter of the ATLAS experiment in the LHC Run 2 data campaign. The upgraded Laser II calibration system is described. The system was commissioned during the first LHC Long Shutdown, exhibiting a stability better than 0.8% for the laser light monitoring. The methods employed to derive the detector calibration factors with data from the laser calibration runs are also detailed. These allowed to correct for the response fluctuations of the 9852 photomultiplier tubes of the Tile Calorimeter with a total uncertainty of 0.5% plus a luminosity-dependent sub-dominant term. Finally, we report the regular monitoring and performance studies using laser events in both standalone runs and during proton collisions. These studies include channel timing and quality inspection, and photomultiplier linearity and response dependence on anode current.

The Real-Time System for Distribution of Clock, Control and Monitoring Commands With Fixed Latency of the LHCb Experiment at CERN

The Large Hadron Collider beauty (LHCb) experiment has gone through a major upgrade for LHC Run 3 at European Organization for Nuclear Research (CERN), scheduled to start in the middle of 2022. The entire readout system of the experiment has been replaced by front-end and back-end electronics that are able to record LHC events at the full LHC bunch crossing rate of 40 MHz. To maintain synchronicity across the full system, clock and control commands are originated from a single readout supervisor (SODIN) and distributed downstream through a passive optical network (PON) infrastructure, reaching all the back-end cards of the readout system, via a two-level hierarchy and using PON splitters. A field programmable gate arrays (FPGA) firmware core, called LHCb-PON and based on the CERN timing, trigger, and control (TTC)-PON, is instantiated on SODIN and all back-end cards and provides the communication protocol while ensuring a stable distribution of control commands within the readout period of 25 ns and a clock signal with fixed and deterministic latency with a precision of a few hundreds of picoseconds. This is achieved by the firmware backbone that provides clock recovery and ensures the transmission of a control word downstream with fixed latency. In addition, the LHCb-PON also provides monitoring functionalities over the upstream link, with less strict timing requirements, connecting multiple back-end cards to the central SODIN. This is achieved by having each back-end card send data upstream through a time-division multiplexing (TDM) scheme. This article describes the implementation, measurements, and results from the usage of the system during the startup of the LHC Run 3.

Performance Evaluation of Modern Time-Series Database Technologies for the ATLAS Operational Monitoring Data Archiving Service

The Trigger and Data Acquisition system of the ATLAS [1] experiment at the Large Hadron Collider [2] at CERN is composed of a large number of distributed hardware and software components which provide the data-taking functionality of the overall system. During data-taking, huge amounts of operational data are created in order to constantly monitor the system. The Persistent Back-End for the ATLAS Information System of TDAQ (P-BEAST) is a system based on a custom-built time-series database. It archives any operational monitoring data published online, resulting in about 18 TB of highly compacted and compressed raw data per year. P-BEAST provides command line and programming interfaces for data insertion and retrieval, including integration with the Grafana platform. Since P-BEAST was developed, several promising database technologies for efficiently working with time-series data have been made available. A study to evaluate the possible use of these recent database technologies in the P-BEAST system was performed. First, the most promising technologies were selected. Then, their performance was evaluated. The evaluation strategy was based on both synthetic read and write tests, and on realistic read patterns (e.g., providing data to a set of Grafana dashboards currently used to monitor ATLAS). All the tests were executed using a subset of ATLAS operational monitoring data, archived during the LHC Run II. The details of the testing procedure and of the testing results, including a comparison with the current P-BEAST service, are presented.