The sensitivity of the proposed FCC-ee collider to axion-like particles (ALPs) is investigated at all planned center-of-mass energies, with focus on the case where the ALP couples primarily to electroweak gauge bosons at leading order. We study the associated production of the ALP with a photon, with the ALP decaying in turn to two photons, yielding a three-photon final state. The ALP coupling to the photon is evaluated for ALP masses of 5 to 320 GeV. In this effective model, the FCC-ee will be able to detect ALPs for couplings down to a few 10^-6GeV^-1 (10^-5GeV^-1) during the Z pole run (WW, ZH, and tt̅ threshold runs). Additionally, this final state has the potential to probe the underlying electroweak structure of ALP couplings for ALP masses below the Z-boson mass.

The decay of the Z boson into axion-like particles (ALP) at the Z-pole run of the proposed CERN FCC-ee collider is investigated. We perform a detailed study of final states with three photons or with a single photon and missing energy, yielding an evaluation of the accessible values of the ALP coupling to the photon for ALP masses between 0.1 and 85 GeV. Special attention is paid to the experimental implications of detecting the three-photon signature for the ALP mass region below 10 GeV. The FCC-ee run will be able to detect the ALP for couplings down to a few 10−3 TeV−1 over the considered mass range in an effective model where the ALP only couples to the U(1) boson of the Standard Model.

The very high luminosity run foreseen at the Z-pole for the FCC-ee will allow the detection in Z decays of new particles with very low couplings to the Standard Model. These particles can have measurable flight paths before they decay. If the timing and the position of the decay vertex can be measured with high precision, the mass of such particles can be measured by exploiting the constrained kinematics of an e^+e^- collider. The mass resolution achievable with this technique is studied through a detailed analysis in the framework of a parametrised simulation of the performance of the IDEA detector. The adopted benchmark model is the production of Heavy Neutral Leptons, which is one of the key channels for new physics discovery at the FCC-ee.

We present the results of a test beam campaign on a capillary-tube fibre-based dual-readout calorimeter, designed for precise hadronic and electromagnetic energy measurements in future collider experiments. The calorimeter prototype consists of nine modules, each composed of brass capillary tubes housing scintillating and Cherenkov optical fibres, read out using silicon photomultipliers for the central module and photomultiplier tubes for the outer modules. The performance of the detector was assessed using a positron beam with energies ranging from 10 to 120 GeV at the CERN SPS H8 beamline. The prototype is characterised in terms of the linearity and resolution of its energy response to positrons. The results confirm the performance of the capillary-tube mechanical design for large-scale dual-readout calorimetry and provide a benchmark for future detector development within the HiDRa project.

The production of a dark photon A^' at the proposed CERN FCC-ee collider is investigated. The study addresses the associated production e^+e^-^' followed by the decay A^'^+μ^-. The 95

The dual-readout calorimetric technique reconstructs the event-by-event electromagnetic fraction of the hadronic shower through the simultaneous measurement of scintillating (S) and Cherenkov (C) light produced by the shower development. The new generation of prototypes, based on Silicon Photomultipliers (SiPMs) readout, adds unprecedented granularity to the well-known high-energy resolution. A highly granular prototype (10 x 10 x 100 cm3), designed to fully contain electromagnetic showers, was recently built and qualified on beam. It consists of 9 modules, each made of 320 brass capillaries equipped with both scintillating and clear fibers. All the fibers of the central module are coupled with SiPMs, while the PMTs are used for the others. Furthermore, the new FERS-System, designed by Caen to exploit the CITIROC1A ASICs performances, is at the core of the SiPM readout.The recent test beam at DESY allowed us to qualify the readout system and define a procedure to calibrate the SiPM response from ADC to ph-e in a wide dynamic range. We measured the number of ph-e per GeV for scintillating and Cherenkov light together with the calorimetric performances in the energy range of 1-6 GeV. This work reports the system qualification and the test beam results regarding SiPM calibration.

A bstract A search for the exclusive decays of the Higgs and Z bosons to a ϕ or ρ meson and a photon is performed with a pp collision data sample corresponding to an integrated luminosity of up to 35 . 6 fb −1 collected at $$ \sqrt{s}=13 $$ s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. These decays have been suggested as a probe of the Higgs boson couplings to light quarks. No significant excess of events is observed above the background, as expected from the Standard Model. Upper limits at 95% confidence level were obtained on the branching fractions of the Higgs boson decays to ϕ γ and ρ γ of 4 . 8 × 10 −4 and 8 . 8 × 10 −4 , respectively. The corresponding 95% confidence level upper limits for the Z boson decays are 0 . 9 × 10 −6 and 25 × 10 −6 for ϕ γ and ρ γ, respectively.

A prototype of a dual-readout calorimeter using brass capillary tubes surrounding scintillating and clear plastic optical fibres was tested using beams of particles with energies between 10 and 100 GeV produced by the CERN SPS. The scope of the test was to characterise the performance of the tube-based detector response to positrons in terms of response linearity, energy resolution, and lateral granularity. After calibrating the detector and processing the output signal to correct for the energy dependency on the particle impact point, the linearity of the measurement was found to be better than 1%. The response to positron was compared to that predicted by a Geant4-based simulation, finding good agreement both in terms of energy resolution and shower profile. These results confirm the validity of the tube-based mechanical option and SiPM readout as a promising one for future developments.

A search for leptoquarks decaying into the bτ final state is performed using Run 2 proton-proton collision data from the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1 at √(s) = 13 TeV recorded by the ATLAS detector. The benchmark models considered in this search are vector leptoquarks with electric charge of 2/3e and scalar leptoquarks with an electric charge of 4/3e. No significant excess above the Standard Model prediction is observed, and 95

The total and differential Higgs boson production cross-sections are measured through a combined statistical analysis of the H → ZZ* → 4ℓ and H → γγ decay channels. The results are based on a dataset of 139 fb−1 of proton–proton collisions at a centre-of-mass energy of 13 TeV, recorded by the ATLAS detector at the Large Hadron Collider. The measured total Higgs boson production cross-section is 55.5_-3.8^+4.0 pb, consistent with the Standard Model prediction of 55.6 ± 2.5 pb. All results from the two decay channels are compatible with each other, and their combination agrees with the Standard Model predictions. A combined statistical interpretation of the measured fiducial cross-sections as a function of the Higgs boson transverse momentum is performed in order to probe the Yukawa couplings to the bottom and charm quarks. A similar interpretation is performed by including also the constraints from the measurements of Higgs boson production in association with a W or Z boson in the H → bb and cc decay channels.

This Letter reports the observation of WWW production and a measurement of its cross section using 139 fb^{-1} of proton-proton collision data recorded at a center-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Events with two same-sign leptons (electrons or muons) and at least two jets, as well as events with three charged leptons, are selected. A multivariate technique is then used to discriminate between signal and background events. Events from WWW production are observed with a significance of 8.0 standard deviations, where the expectation is 5.4 standard deviations. The inclusive WWW production cross section is measured to be 820±100 (stat)±80 (syst) fb, approximately 2.6 standard deviations from the predicted cross section of 511±18 fb calculated at next-to-leading-order QCD and leading-order electroweak accuracy.

In this White Paper for the 2021 Snowmass process, we detail the status and prospects for dual-readout calorimetry. While all calorimeters allow estimation of energy depositions in their active material, dual-readout calorimeters aim to provide additional information on the light produced in the sensitive media via, for example, wavelength and polarization, and/or a precision timing measurements, allowing an estimation of the shower-by-shower particle content. Utilizing this knowledge of the shower particle content may allow unprecedented energy resolution for hadronic particles and jets and new types of particle flow algorithms. We also discuss the impact continued development of this kind of calorimetry could have on precision on Higgs boson property measurements at future colliders.

Given the hints of lepton-flavour non-universality in B -meson decays, leptoquarks (LQs) are enjoying a renaissance. We propose novel Large Hadron Collider (LHC) searches for such hypothetical states that do not rely on strong production only, but can also receive important contributions from quark-lepton annihilation. For the cases of a resonant signal involving a bottom quark and a tau lepton ( b + τ ), a top quark and missing transverse energy ( E_T^miss ) and light-flavour jets plus E_T^miss , we develop realistic analysis strategies and provide detailed evaluations of the achievable sensitivities for the corresponding LQ signatures at future LHC runs. Our analyses allow us to derive a series of stringent constraints on the masses and couplings of third-generation singlet vector LQs, showing that at LHC Run III and the high-luminosity LHC the proposed search channels can probe interesting parts of the LQ parameter space addressing the B -physics anomalies. In view of the reach of the proposed b + τ signature, we recommend that dedicated resonance searches for this final state should be added to the exotics search canon of both ATLAS and CMS.

This letter presents a search for narrow, high-mass resonances in the Zγ final state with the Z boson decaying into a pair of electrons or muons. The s=13 TeV pp collision data were recorded by the ATLAS detector at the CERN Large Hadron Collider and have an integrated luminosity of 140 fb−1. The data are found to be in agreement with the Standard Model background expectation. Upper limits are set on the resonance production cross section times the decay branching ratio into Zγ. For spin-0 resonances produced via gluon–gluon fusion, the observed limits at 95% confidence level vary between 65.5 fb and 0.6 fb, while for spin-2 resonances produced via gluon–gluon fusion (or quark–antiquark initial states) limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb, for the mass range from 220 GeV to 3400 GeV.

Dual-readout calorimetry is now a mature and well-known technology which guarantees excellent electromagnetic and hadronic resolution in the same detector. It has recently being proposed in the framework of IDEA (Innovative Detector for Electron–Positron Accelerators) for both Future Circular Collider (FCC-ee) and Circular Electron–Positron Collider (CEPC). After being extensively tested on prototypes, the dual-readout calorimetry is now moving toward a technology design study in order to be realistically available for an experiment. In this context, a full simulation of the calorimeter has been developed and used to estimate the expected performance of the detector. At the same time, the development of a novel technique for mass production of the detector modules, at an effective cost, is ongoing. As a first step, an electromagnetic-size prototype is under construction for a testbeam data taking originally foreseen in November 2020 and now moved to spring 2021, due to the Covid-19 pandemic spread.

The first measurement of longitudinal decorrelations of harmonic flow amplitudes v_{n} for n=2-4 in Xe+Xe collisions at sqrt[s_{NN}]=5.44  TeV is obtained using 3  μb^{-1} of data with the ATLAS detector at the LHC. The decorrelation signal for v_{3} and v_{4} is found to be nearly independent of collision centrality and transverse momentum (p_{T}) requirements on final-state particles, but for v_{2} a strong centrality and p_{T} dependence is seen. When compared with the results from Pb+Pb collisions at sqrt[s_{NN}]=5.02  TeV, the longitudinal decorrelation signal in midcentral Xe+Xe collisions is found to be larger for v_{2}, but smaller for v_{3}. Current hydrodynamic models reproduce the ratios of the v_{n} measured in Xe+Xe collisions to those in Pb+Pb collisions but fail to describe the magnitudes and trends of the ratios of longitudinal flow decorrelations between Xe+Xe and Pb+Pb. The results on the system-size dependence provide new insights and an important lever arm to separate effects of the longitudinal structure of the initial state from other early and late time effects in heavy-ion collisions.

Author(s): Abe, T; Afik, Y; Albert, A; Anelli, CR; Barak, L; Bauer, M; Behr, JK; Bell, NF; Boveia, A; Brandt, O; Busoni, G; Carpenter, LM; Chen, Y-H; Doglioni, C; Elliot, A; Fujiwara, M; Genest, M-H; Gerosa, R; Gori, S; Gramling, J; Grohsjean, A; Gustavino, G; Hahn, K; Haisch, U; Henkelmann, L; Hisano, J; Huitfeldt, A; Ippolito, V; Kahlhoefer, F; Landsberg, G; Lowette, S; Maier, B; Maltoni, F; Muehlleitner, M; No, JM; Pani, P; Polesello, G; Price, DD; Robens, T; Rovelli, G; Rozen, Y; Sanderson, IW; Santos, R; Sevova, S; Sperka, D; Sung, K; Tait, TMP; Terashi, K; Ungaro, FC; Vryonidou, E; Yu, S-S; Wu, SL; Zhou, C | Abstract: Dark matter (DM) simplified models are by now commonly used by the ATLAS and CMS Collaborations to interpret searches for missing transverse energy ($E_T^mathrm{miss}$). The coherent use of these models sharpened the LHC DM search program, especially in the presentation of its results and their comparison to DM direct-detection (DD) and indirect-detection (ID) experiments. However, the community has been aware of the limitations of the DM simplified models, in particular the lack of theoretical consistency of some of them and their restricted phenomenology leading to the relevance of only a small subset of $E_T^mathrm{miss}$ signatures. This document from the LHC Dark Matter Working Group identifies an example of a next-generation DM model, called $textrm{2HDM+a}$, that provides the simplest theoretically consistent extension of the DM pseudoscalar simplified model. A~comprehensive study of the phenomenology of the $textrm{2HDM+a}$ model is presented, including a discussion of the rich and intricate pattern of mono-$X$ signatures and the relevance of other DM as well as non-DM experiments. Based on our discussions, a set of recommended scans are proposed to explore the parameter space of the $textrm{2HDM+a}$ model through LHC searches. The exclusion limits obtained from the proposed scans can be consistently compared to the constraints on the $textrm{2HDM+a}$ model that derive from DD, ID and the DM relic density.

Dark matter (DM) simplified models are by now commonly used by the ATLAS and CMS Collaborations to interpret searches for missing transverse energy (E_T^miss). The coherent use of these models sharpened the LHC DM search program, especially in the presentation of its results and their comparison to DM direct-detection (DD) and indirect-detection (ID) experiments. However, the community has been aware of the limitations of the DM simplified models, in particular the lack of theoretical consistency of some of them and their restricted phenomenology leading to the relevance of only a small subset of E_T^miss signatures. This document from the LHC Dark Matter Working Group identifies an example of a next-generation DM model, called 2HDM+a, that provides the simplest theoretically consistent extension of the DM pseudoscalar simplified model. A comprehensive study of the phenomenology of the 2HDM+a model is presented, including a discussion of the rich and intricate pattern of mono-X signatures and the relevance of other DM as well as non-DM experiments. Based on our discussions, a set of recommended scans are proposed to explore the parameter space of the 2HDM+a model through LHC searches. The exclusion limits obtained from the proposed scans can be consistently compared to the constraints on the 2HDM+a model that derive from DD, ID and the DM relic density.

The exploitation of the physical potential of future circular electron-positron colliders operating at root s = 90, 160, 240 and, optionally, 360 GeV as Z, W, H and t factories will require detectors providing superior momentum and energy resolution for lepton, jet and photon final states. The IDEA detector has been designed to match these requirements by adopting an ultra-light drift chamber, as main tracker, and a dual-readout fiber calorimeter for both hadronic and electromagnetic energy measurements. We review here some of the main results, related to calorimetry, obtained with detailed full simulation studies.

This paper presents a search for direct top squark pair production in events with missing transverse momentum plus either a pair of jets consistent with Standard Model Higgs boson decay into b-quarks or a same-flavour opposite-sign dilepton pair with an invariant mass consistent with a Z boson. The analysis is performed using the proton–proton collision data at \n$$\\sqrt{s}=13$$\n\n TeV collected with the ATLAS detector during the LHC Run-2, corresponding to an integrated luminosity of 139 fb\n$$^{-1}$$\n\n. No excess is observed in the data above the Standard Model predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark (\n$$\\tilde{t}_1$$\n\n) or the heavier top squark (\n$$\\tilde{t}_2$$\n\n), excluding at 95% confidence level \n$$\\tilde{t}_1$$\n\n and \n$$\\tilde{t}_2$$\n\n masses up to about 1220 and 875 GeV, respectively.