ALEPH Detector Research Articles

Constraining Asymmetric Dark Matter using colliders and direct detection

We reappraise the viability of asymmetric dark matter (ADM) realized as a Dirac fermion coupling dominantly to the Standard Model fermions. Treating the interactions of such a DM particle with quarks/leptons in an effective-interactions framework, we derive updated constraints using mono-jet searches from the Large Hadron Collider (LHC) and mono-photon searches at the Large Electron-Positron (LEP) collider. We carefully model the detectors used in these experiments, which is found to have significant impact. The constraint of efficient annihilation of the symmetric part of the ADM, as well as other observational constraints are synthesized to produce a global picture. Consistent with previous work, we find that ADM with mass in the range 1–100 GeV is strongly constrained, thus ruling out its best motivated mass range. However, we find that leptophilic ADM remains allowed for ≳ 10 GeV DM, including bounds from colliders, direct detection, and stellar heating. We forecast that the Future Circular Collider for electron-positron collisions (FCC-ee) will improve sensitivity to DM-lepton interactions by almost an order of magnitude.

Compatibility and combination of world W-boson mass measurements

The compatibility of W-boson mass measurements performed by the ATLAS, LHCb, CDF, and D0 experiments is studied using a coherent framework with theory uncertainty correlations. The measurements are combined using a number of recent sets of parton distribution functions (PDF), and are further combined with the average value of measurements from the Large Electron–Positron collider. The considered PDF sets generally have a low compatibility with a suite of global rapidity-sensitive Drell–Yan measurements. The most compatible set is CT18 due to its larger uncertainties. A combination of all mW\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_W$$\\end{document} measurements yields a value of mW=80,394.6±11.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$m_W = 80{,}394.6 \\pm 11.5$$\\end{document} MeV with the CT18 set, but has a probability of compatibility of 0.5% and is therefore disfavoured. Combinations are performed removing each measurement individually, and a 91% probability of compatibility is obtained when the CDF measurement is removed. The corresponding value of the W boson mass is 80,369.2±13.3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$80{,}369.2 \\pm 13.3$$\\end{document} MeV, which differs by 3.6σ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3.6\\sigma $$\\end{document} from the CDF value determined using the same PDF set.

95 GeV diphoton and bb¯ excesses in the general next-to-minimal supersymmetric standard model

The CMS and ATLAS Collaborations recently published their results searching for light Higgs bosons, using the complete Run 2 data of the LHC. Both reported an excess in the diphoton invariant mass distribution at mγγ≃95.4 GeV with compatible signal strengths. The combined result corresponded to a local significance of 3.1σ. Besides, the mass of the diphoton signal coincided with that of the bb¯ excess observed at the Large Electron Positron. Given the remarkable theoretical advantages of the general next-to-minimal supersymmetric Standard Model, we interpret these excesses by the resonant productions of the singlet-dominated CP-even Higgs boson predicted by the theory. Using both analytic formulas and numerical results, we show that the idea can interpret the excesses by broad parameter space without contradicting current experimental restrictions, including those from the 125 GeV Higgs data, the dark matter relic abundance and direct detection experiments, and the collider searches for supersymmetry and extra Higgs bosons. Although the explanations are scarcely affected by present Higgs data and the LHC search for supersymmetry, the dark matter physics may leave footprints on them. We also survey the other signals of the light Higgs boson at the LHC. Published by the American Physical Society 2024

Measurement of two-particle correlations and flow coefficients in high multiplicity e+e− collisions using archived ALEPH data at 91-209 GeV

We present measurements of two-particle angular correlations of charged particles emitted in high-energy collisions using data collected by the ALEPH detector at LEP between 1992 to 2000. The correlation functions are measured over a wide range of pseudorapidity and azimuthal angle as a function of charged particle multiplicity. Previous measurement with LEP1 data at √S = 91 GeV shows no significant long-range correlations in either lab coordinate or thrust coordinate analyses, with associated yield distributions in agreement with predictions from the PYTHIA v6.1 event generator. The use of higher collision energy LEP2 data allows access to not only higher event multiplicity but also additional production channels beyond the e+e- → y* / Z → q¯q process. Notably, the highest multiplicity bin (Ntrk ≥ 50) suggests a tantalizing disagreement with MC and implies the potential to search for collective phenomena in small systems. This measurement is pushing the studies of long-range correlation to the smallest collision system limit and includes the first flow coefficient (vn) measurement in e+e− collisions, which uses a Fourier decomposition analysis to quantify the anisotropy in the azimuthal two-particle correlation as a function of charged particles’ transverse momentum. This work supplements our understanding of small-system references to long-range correlations observed in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Overview of high energy physics and prospective dark matter detections at HL-LHC and CEPC

This paper first takes an overview of the current development of high-energy physics, including an introduction to the fundamental particles and forces described by the standard model, physics beyond the standard model like super-symmetry and matter-antimatter asymmetry, and current high-energy physics experiments such as ATLAS and CMS at the large hadron collider. Then we focus on the prospective dark matter detections at the high-luminosity large hadron collider (HL-LHC) and circular electron-positron collider (CEPC), especially the upgrades of the current LHC and different working modes of the CEPC. Finally, the paper discusses the prospects of dark matter detection and possibly dark matter candidates.

Phenomenology of a 96 GeV Higgs boson in the 2HDM with an additional singlet

We discuss a $\ensuremath{\sim}3\ensuremath{\sigma}$ signal (local) in the light Higgs-boson search in the diphoton decay mode at $\ensuremath{\sim}96\text{ }\text{ }\mathrm{GeV}$ as reported by CMS, together with a $\ensuremath{\sim}2\ensuremath{\sigma}$ excess (local) in the $b\overline{b}$ final state at the Large Electron Positron Collider (LEP) in the same mass range. We interpret this possible signal as a Higgs boson in the 2 Higgs doublet model type II with an additional Higgs singlet, which can be either complex (2HDMS) or real (N2HDM), where the 2HDMS so far has never been analyzed as an explanation of these excesses. An emphasis of our work is the differences between and the possible distinction of the two models in this context. We find that the lightest $\mathcal{C}\mathcal{P}$-even Higgs boson of the two models can equally yield a perfect fit to both excesses simultaneously, while the second lightest state is in full agreement with the Higgs-boson measurements at 125 GeV, and the full Higgs-boson sector is in agreement with all Higgs exclusion bounds from LEP, the Tevatron and the LHC as well as other theoretical and experimental constraints. We derive bounds on the 2HDMS and N2HDM Higgs sectors from a fit to both excesses and describe how this signal can be further analyzed at future ${e}^{+}{e}^{\ensuremath{-}}$ colliders, such as the International Linear Collider (ILC). We analyze in detail the anticipated precision of the coupling measurements of the 96 GeV Higgs boson at the ILC. We find that these Higgs-boson measurements at the LHC and the ILC cannot distinguish between the two Higgs-sector realizations.

The collider landscape: which collider for establishing the SM instability?

Capabilities of future colliders are usually discussed assuming specific hypothetical new physics. We consider the opposite possibility: that no new physics is accessible, and we want to learn if the unnatural Standard Model is part of a vast landscape. We argue that a main step in this direction would be establishing the possible instability scale of the Higgs potential. This primarily needs reducing the uncertainty on the strong coupling and on the top quark mass. We show that the top quark mass can be measured well enough via a toverline{t} threshold scan with low 1033 cm−2sec−1 luminosity, that seems achievable at a ‘small’ e+e− collider in the LEP tunnel, or at a muon collider demonstrator.

Jet charge: A new tool to probe the anomalous [formula omitted] couplings at the EIC

We propose to probe the Zbb¯ interactions at the Electron-Ion Collider (EIC) by utilizing the average jet charge weighted single-spin asymmetry AebQ, which is induced by different cross sections of a left-handed and right-handed electron beam scattering off an unpolarized proton beam in the neutral current deeply-inelastic scattering processes with one observed b-tagged jet. This novel observable at the EIC is sensitive to the axial-vector component of the Zbb¯ coupling, providing similar information as the gg→Zh cross section measurement at the high-luminosity Large Hadron Collider, and is complementary to the single-spin asymmetry measurement at the EIC which is sensitive to the vector component of the Zbb¯ coupling. We show that the apparent degeneracy of the allowed Zbb¯ coupling in its vector and axial-vector components, implied by the Large Electron-Positron Collider (LEP) and the Stanford Linear Collider (SLC) precision electroweak data, can be broken by the AebQ measurement at the EIC. With a large enough integrated luminosity collected at the EIC, the measurement of AebQ could also resolve the long-standing discrepancy between bottom quark forward-backward asymmetry AFBb at the LEP and the Standard Model prediction, with a strong dependence on the b-tagging efficiency.

Jet energy spectrum and substructure in e+e− collisions at 91.2 GeV with ALEPH Archived Data

The first measurements of energy spectra and substructure of anti-kT jets in hadronic Z0 decays in e+e− collisions are presented. The archived e+e− annihilation data at a center-of-mass energy of 91.2 GeV were collected with the ALEPH detector at LEP in 1994. In addition to inclusive jet and leading dijet energy spectra, various jet substructure observables are analyzed as a function of jet energy which includes groomed and ungroomed jet mass to jet energy ratios, groomed momentum sharing, and groomed jet radius. The results are compared with perturbative QCD calculations and predictions from the Sherpa, Herwig v7.1.5, Pythia6, Pythia8 and Pyquen event generators. The jet energy spectra agree with perturbative QCD calculations which include the treatment of logarithms of the jet radius and threshold logarithms. None of the event generators give a fully satisfactory description of the data.

FCC-ee: the synthesis of a long history of e+e- circular colliders

The design of FCC-ee is relying on the accumulated experience of mathrm {e^{+}e^{-}} colliders that have been designed, constructed and operated in the past 40 years. FCC-ee will surpass the 26.7 km long Large Electron Positron collider LEP by a factor 4 in size. Like for LEP the large size is justified by the need to control the synchrotron radiation losses that for both machines reach a few percent per turn. To that end LEP had the first large super-conducting (SC) RF system with around 3.8 GV of accelerating voltage. LEP achieved for the first time very large beam-beam parameters of around 0.08, and it relied on transversely polarized beams to determine accurately the beam energy for the experiments. The DAvarPhi NE collider, together with PEP II and KEKB split the two beams into separate vacuum chambers to reach much higher Ampere-level beam currents. To overcome beam-beam lifetime and performance issues DAvarPhi NE used for the first time the Crab Waist concept for the interaction region (IR) optics. The B-factories, PEP-II and KEKB have verified the double-ring mathrm {e^{+}e^{-}} collider with multi-ampere stored currents for over 1000 bunches, small beta ^*, top-up injection, and achieved then-highest luminosity. KEKB has applied 22-mrad crossing angle at the IP with crab crossing. Both machines inherited accelerator techniques from their predecessors, PEP and TRISTAN, which was a small-scale LEP. Currently the next generation SuperKEKB collider is starting up. It has already achieved some milestones required for FCC-ee such as small beta ^* (0.8 mm) and virtual crab-waist scheme with a large Piwinski angle (>10).

Combined Effect of Beam-Beam Interaction and Beam Coupling Impedance in Future Circular Colliders

The future large scale electron-positron colliders, such as FCC-ee in Europe and CEPC in China, will rely on the crab waist collision scheme with a large Piwinski angle. Differently from the past generation colliders both luminosity and beam-beam tune shifts depend on the bunch length in such a collision scheme. In addition, for the future circular colliders with extreme beam parameters in collision several new effects become important such as beamstrahlung, coherent X-Z instability and 3D flip-flop. For all these effects the longitudinal beam dynamics plays an essential role and should be taken into account for the collider luminosity optimization. In this paper we discuss an impact of the longitudinal beam coupling impedance on the collider performance.

Search for charged-lepton-flavour violation in Z-boson decays with the ATLAS detector

Leptons with essentially the same properties apart from their mass are grouped into three families (or flavours). The number of leptons of each flavour is conserved in interactions, but this is not imposed by fundamental principles. Since the formulation of the standard model of particle physics, the observation of flavour oscillations among neutrinos has shown that lepton flavour is not conserved in neutrino weak interactions. So far, there has been no experimental evidence that this also occurs in interactions between charged leptons. Such an observation would be a sign of undiscovered particles or a yet unknown type of interaction. Here the ATLAS experiment at the Large Hadron Collider at CERN reports a constraint on lepton-flavour-violating effects in weak interactions, searching for Z-boson decays into a τ lepton and another lepton of different flavour with opposite electric charge. The branching fractions for these decays are measured to be less than 8.1 × 10−6 (eτ) and 9.5 × 10−6 (μτ) at the 95% confidence level using 139 fb−1 of proton–proton collision data at a centre-of-mass energy of sqrt{s}=13,{rm{TeV}} and 20.3 fb−1 at sqrt{s}=8,{rm{TeV}}. These results supersede the limits from the Large Electron–Positron Collider experiments conducted more than two decades ago.

Measurements of Two-Particle Correlations in e^{+}e^{-} Collisions at 91GeV with ALEPH Archived Data.

Measurements of two-particle angular correlations of charged particles emitted in hadronic Z decays are presented. The archived e^{+}e^{-} annihilation data at a center-of-mass energy of 91GeV were collected with the ALEPH detector at LEP between 1992 and 1995. The correlation functions are measured over a broad range of pseudorapidity and full azimuth as a function of charged particle multiplicity. No significant long-range correlation is observed in either the lab coordinate analysis or the thrust coordinate analysis, where the latter is sensitive to a medium expanding transverse to the color string between the outgoing qq[over ¯] pair from Z boson decays. The associated yield distributions in both analyses are in better agreement with the prediction from the pythia v6.1 event generator than from herwig v7.1.5. They provide new insights to showering and hadronization modeling. These results serve as an important reference to the observed long-range correlation in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Probing the charm Yukawa coupling at future e−p and e+e− colliders

Large Hadron Collider (LHC) has provided direct evidence of Yukawa couplings between the third generation charged fermions and the 125 GeV Higgs boson. Whether the first two generation charged fermions arise from exactly the same mechanism becomes the next interesting question. Therefore, direct measurements of charm or muon Yukawa couplings will be crucial to answering this puzzle. The charm Yukawa measurement at the LHC suffers from severe QCD background and it is extremely difficult to reach the sensitivity. In this paper, we compare the potential of probing charm Yukawa coupling at the two proposed future "Higgs Factory" experiments, the Large Hadron electron Collider (LHeC) and Circular electron positron collider (CEPC). At the LHeC, Higgs bosons will be produced via weak boson fusion and the energetic forward jet may suppress the background significantly. However, due to huge $\gamma-g$ scattering background, the potential of LHeC search is still limited. With $-80\%$ polarized electron beam of 60 GeV, the signal significance can only reach $2\sigma$ for $ \kappa_{c}\simeq 1$ with a 3 ab$^{-1}$ integrated luminosity. In comparison, measurement at the CEPC can then reach $8.0\sigma$ for $\kappa_{c}\simeq 1$ with a 2 ab$^{-1}$ of data.

Consistency of gauged two Higgs doublet model: gauge sector

We study the constraints on the new parameters in the gauge sector of gauged two Higgs doublet model using the electroweak precision test data collected from the Large Electron Positron Collider (LEP) at and off the Z-pole as well as the current Drell-Yan and high-mass dilepton resonance data from the Large Hadron Collider (LHC). Impacts on the new parameters by the projected sensitivities of various electroweak observables at the Circular Electron Positron Collider (CEPC) proposed to be built in China are also discussed. We also clarify why the Stueckelberg mass MY for the hypercharge U(1)Y is set to be zero in the model by showing that it would otherwise lead to the violation of the standard charge assignments for the elementary quarks and leptons when they couple to the massless photon.

Measurements of two-particle correlations in e+e− collisions at 91 GeV with ALEPH archived data

The first measurements of two-particle angular correlations for charged particles emitted in e+e− collisions at a center-of-mass energy of 91 GeV are presented. The archived data are collected with the ALEPH detector at LEP. The correlation functions are measured over a broad range of pseudorapidity and azimuthal angle of the charged particles. Those results are compared to predictions from the PYTHIA event generator. In contrast to the results from high charged particle multiplicity nucleon-nucleon, nucleon-nucleus and nucleus-nucleus collisions, where long-range correlations with a large pseudorapidity gap are observed, no significant enhancement of long-range correlations is observed with respect to PYTHIA predictions, which do not include additional final state interactions of the outgoing partons.

Radiological clearance of equipment from high-energy electron accelerators: The example of LEP superconducting acceleration system

The lifespan of a particle accelerator includes its final decommissioning and the disposal of potentially activated materials, one of the major challenges faced at accelerator laboratories. This paper discusses the computational and experimental techniques adopted for clearing from regulatory control the superconducting acceleration system of the former CERN Large Electron Positron collider (LEP), consisting of nearly 450 tons of valuable metals. The paper first briefly recalls the major sources of activation at high-energy electron accelerators and then discusses the general process, the computational tools employed, the methods to determine the amount of radionuclides that cannot be measured by γ-spectrometry, the selection of samples representative of the whole equipment and the outcome of the operational phase. The process ended up with the clearance of 95% of the material. The aim of this work is to provide a practical example of an approach that could be applicable to similar projects targeting radiological clearance of high-energy particle accelerator equipment.

Sharing but not caring: collider phenomenology

Based on a previous work on scenarios where the Standard Model and dark matter particles share a common asymmetry through effective operators at early time in the Universe and later on decouple from each other (not care), in this work, we study in detail the collider phenomenology of these scenarios. In particular, we use the experimental results from the Large Hadron Collider (LHC) to constrain the viable parameter space. Besides effective operators, we also constrain the parameter space of some representative ultraviolet complete models with experimental results from both the LHC and the Large Electron-Positron Collider. Specifically, we use measurements related to jets + missing transverse energy (MET), di-jets and photon + MET. In the case of ultraviolet models, depending on the assumptions on the couplings and masses of mediators, the derived constraints can become more or less stringent. We consider also the situation where one of the mediators has mass below 100 GeV, in this case we use the ultraviolet model to construct a new effective operator responsible for the sharing of the asymmetry and study its phenomenology.

ActiWiz 3 – an overview of the latest developments and their application

In 2011 the ActiWiz code was developed at CERN in order to optimize the choice of materials for accelerator equipment from a radiological point of view. Since then the code has been extended to allow for calculating complete nuclide inventories and provide evaluations with respect to radiotoxicity, inhalation doses, etc. Until now the software included only pre-defined radiation environments for CERN’s high-energy proton accelerators which were based on FLUKA Monte Carlo calculations. Eventually the decision was taken to invest into a major revamping of the code. Starting with version 3 the software is not limited anymore to pre-defined radiation fields but within a few seconds it can also treat arbitrary environments of which fluence spectra are available. This has become possible due to the use of ~100 CPU years’ worth of FLUKA Monte Carlo simulations as well as the JEFF cross-section library for neutrons < 20 MeV. Eventually the latest code version allowed for the efficient inclusion of 42 additional radiation environments of the LHC experiments as well as considerably more flexibility in view of characterizing also waste from CERN’s Large Electron Positron collider (LEP). New fully integrated analysis functionalities like automatic evaluation of difficult-to-measure nuclides, rapid assessment of the temporal evolution of quantities like radiotoxicity or dose-rates, etc. make the software a powerful tool for characterization complementary to general purpose MC codes like FLUKA. In this paper an overview of the capabilities will be given using recent examples from the domain of waste characterization as well as operational radiation protection.

Superconducting RF at CERN: Operation, Projects, and R&amp;D

The CERN SRF infrastructure was originally set up for Nb on Cu-coated cavities for large electron positron collider (LEP) and later for large hadron collider (LHC). Today, Superconducting RF has become a vital technology not only for LHC but also for high intensity and energy upgrade of isotope separator on line device (HIE-ISOLDE) and the high luminosity (HL)-LHC project. More superconducting cavities will be needed for a potential high energy (HE)-LHC or for any of the future circular collider options, which are currently under study. To meet these demands, CERN has improved its SRF infrastructure over the recent years and has started several R&D lines including improved thin film coating techniques, high-power couplers, cold-test diagnostics, etc. This paper will give an overview of CERN's SRF activities today, and the challenges of future projects.