International Journal of Modern Physics A

The Standard Model predicts several rare Higgs boson decays, which have not yet been observed, but that could be enhanced in theories beyond the Standard Model. Among these are decays to light leptons, e.g. $H\to \mu \mu$. In addition, theories beyond the Standard Model may predict lepton-flavor violating decays of the Higgs boson. Results for these searches based on LHC $pp$ collision data recorded at 13 TeV by the ATLAS experiment will be presented.

The scattering of electroweak bosons tests the gauge structure of the Standard Model and is sensitive to anomalous quartic gauge couplings. In this paper, we present recent results on vector-boson scattering from the ATLAS experiment using proton–proton collisions with a center-of-mass energy of 13 TeV at the LHC. This includes the observation of $ZZ$, $WZ$, and same-sign $WW$ production via vector-boson scattering along with a measurement of $VV$ production ($V$ denotes $W$ or $Z$ boson) in semileptonic final states. The results can be used to constrain new physics that manifests as anomalous electroweak-boson self-interactions. Finally, predicted cross-sections for the electroweak scattering of two same-sign $W$ bosons in association with two jets are compared for a number of generators.

The associated production a vector boson with $J/\psi$ is a key observable for understanding of the quarkonium production mechanisms, including the separation of single and double parton scattering components. Measurements from the ATLAS detector at LHC on quarkonium production, including associated production of a $W^{\pm }+J/\psi$ and $Z+J/\psi$ are presented. A study of the hidden-charm states $X(3872)$ is also provided.

Final states containing both leptons and jets can be used to probe for physics beyond the Standard Model. Searches for new physics models with these signatures, such as heavy neutrinos or leptoquarks, for example, are performed using the ATLAS experiment at the LHC. The results of the most recent searches on 13 TeV $pp$ data will be presented.

Many theories beyond the Standard Model predict new phenomena accessible by the Lhc. Searches for new physics are performed using the Atlas experiment at the Lhc focusing on exotic signatures that are predicted in several theories, excluding supersymmetry. The results of recent searches using 13 TeV data, with the exception of those for Dark Matter signatures, and their interplay and interpretation are presented. Prospects for searches at the High Luminosity Lhc are also discussed.

A measurement of $B_s$ decay parameters using data collected by the ATLAS detector in $pp$ collisions at 13 TeV in 2015–2017 is performed. Integrated luminosity of this sample is 80.5 fb${}^{-1}$. The measurement of physical parameters are statistically combined with results obtained from Run 1 data at 7 and 8 TeV. The measured value of CP-violating phase ${\varphi }_s=-0.076\pm 0.034(\mathrm{\text{stat.}})\pm 0.019(\mathrm{\text{syst.}})$ is obtained, which has significantly better precision in comparison with Run 1 result. Precision of other physical parameters is also improved, including the decay width difference $\mathrm{\Delta }\mathrm{\Gamma }$. Further improvement is expected from the use of full Run 2 statistics, as well as from modifications in the analysis.

Electron and photon triggers are an important part of many physics analyses at the ATLAS experiment, where electron and photon final states are considered. Understanding the performance of electron and photon triggers at the High Level trigger as well as the Level-1 trigger was crucial to improve and adapt the trigger during changing run conditions of the Large Hadron Collider in Run 2 (2015–2018).

During Run 2 (2015–2018) the Large Hadron Collider has provided, at the World’s highest energy frontier, proton–proton collisions to the ATLAS experiment with high instantaneous luminosity (up to $2.1\times 10^{34}{\mathrm{\text{cm}}}^{-2}{\mathrm{\text{s}}}^{-1}$), placing stringent operational and physics requirements on the ATLAS trigger system in order to reduce the 40 MHz collision rate to a manageable event storage rate of 1 kHz, while not rejecting interesting collisions. The Level-1 trigger is the first rate-reducing step in the ATLAS trigger system with an output rate of up to 100 kHz and decision latency of less than 2.5 $\mu$s. In Run 2, an important role was played by the Level-1 Topological Processor (L1Topo). This innovative system consists of two blades designed in AdvancedTCA form factor, mounting four individual state-of-the-art processors, and providing high input bandwidth and low latency data processing. Up to 128 topological trigger algorithms can be implemented to select interesting events by applying kinematic and angular requirements on electromagnetic clusters, hadronic jets, muons and total energy reconstructed in the ATLAS apparatus. This resulted in a significantly improved background rejection and enhanced acceptance of physics signal events, despite the increasing luminosity. The L1Topo system has become more and more important for physics analyses making use of low energy objects, commonly present in the Heavy Flavor or Higgs physics events, for example. An overview of the L1Topo architecture, simulation and performance results during Run 2 is presented alongside with upgrade plans for the L1Topo system to be installed for the future Run 3 data taking period.

The upcoming luminosity upgrade of the LHC will impose new requirements for the detector installations. To perform under these conditions the Micromegas (MM) technology was selected to be adopted in the New Small Wheel (NSW) upgrade, dedicated to precision tracking. A large surface of the forward regions of the Muon Spectrometer will be equipped with 8 layers of MM modules forming a total active area of 1200 m². The NSW is planned to be installed in the forward region of $1.3<\left|\eta \right|<2.7$ of ATLAS. This new system will have to operate in a high background radiation region, while reconstructing muon tracks as well as furnishing information for the Level-1 trigger. The project requires fully efficient MM chambers with spatial resolution down to 100 $\mu$m, a rate capability up to about 15 kHz/cm² and operation in a moderate (highly inhomogeneous) magnetic field up to $B=0.3$T. The required tracking is linked to the intrinsic spatial resolution in combination with the demanding mechanical accuracy. An overview of the design, construction and QA/QC procedures followed at the Aristotle University of Thessaloniki for the Micromegas LM2 Drift panels production will be presented.

Recent results by the CMS experiment on Drell–Yan, W and multiboson events are presented, including in particular the measurement of the electroweak mixing angle, the differential distributions in Drell–Yan events, and the electroweak production of one and two vector bosons in association with two jets. No deviations from the Standard Model predictions are found and stringent bounds are set on anomalous triple and quartic gauge couplings.

The CMS Electromagnetic Calorimeter (ECAL), is a high granularity lead tungstate (PbWO₄) crystal calorimeter operating at the CERN LHC. The ECAL performance has been crucial in the discovery and subsequent characterization of the Higgs boson. The original ECAL design considerations, and the improvements to the energy reconstruction and energy calibration algorithms to cope with the LHC Run II are described. For the High-Luminosity LHC (HL-LHC) upgrades to ECAL are necessary. The crystals in the barrel region will be retained, defining the HL-LHC CMS barrel electromagnetic calorimeter ECAL. The readout electronics will be upgraded and operating at lower temperatures, to maintain the required performance of ECAL from 2027 onwards. The new readout electronics, the timing resolution and the electron and photon reconstruction efficiencies and energy resolution expected for HL-LHC are presented. The performance relevant to a number of key Higgs decay channels is reported.

The Sudbury Neutrino Observatory (SNO), whose main purpose was to study the neutrinos produced in the Sun, demonstrated that neutrinos can change flavor and, thus, they are massive particles. SNO detected and recorded neutrino and cosmic ray interactions from 1999 to 2006 and several analyses have been completed in the past year using legacy data. We present the results of the most recent ones: the measurements of neutron production in atmospheric neutrino interactions and neutron production by cosmic muons, a search for Lorentz symmetry violation in neutrino oscillations and a search for neutrino decay. A few other analyses are ongoing and we comment about their goal and status.

SNO+ is a multi-purpose experiment whose main goal is to study the nature of the neutrino mass through the observation of neutrinoless double-beta decay. Detection of this rare process would indicate that neutrinos are elementary Majorana particles, proving that lepton number is not conserved. The SNO+ detector will operate in three distinct phases with different target materials: water, pure liquid scintillator and tellurium-loaded liquid scintillator. During the water phase, the external backgrounds were confirmed to be within expectation, new limits on specific channels of invisible nucleon decay modes were set and the Boron-8 solar neutrino flux was measured and confirmed to be compatible with previous measurements. With a completed water phase, SNO+ is moving towards its main Tellurium-loaded phase. Here, we report the status of the experiment, the recent results and the potential of SNO+ for neutrinoless double-beta decay search.

The results from two currently running long-baseline neutrino experiments T2K and NO$\nu$A are presented, along with physics program for the next generation experiments — Hyper-K and DUNE. I will focus on hints and prospects of the discovery of CP violation in the leptonic sector.

We present new results of the DANSS experiment on the searches for sterile neutrinos. They are based on more than 2 million of inverse beta decay events collected at 10.7 m, 11.7 m and 12.7 m from the reactor core of the 3.1 GW Kalinin Nuclear Power Plant in Russia. This data sample is 2.4 times larger than the data sample in the previous DANSS publication. The search for the sterile neutrinos is performed using the ratio of ${\tilde{\nu }}_e$ spectra at two distances. This method is very robust against systematic uncertainties in the ${\tilde{\nu }}_e$ spectrum and the detector efficiency. We do not see any statistically significant sign for the ${\tilde{\nu }}_e$ oscillations. This allows us to exclude further a large and interesting part of the sterile neutrino parameter space. A Gaussian CL${}_s$ method was used to obtain exclusion areas. This method is more conservative than a Raster Scan method.

The knowledge of the initial flux, energy and flavor of current neutrino beams is the main limitation for a precise measurement of neutrino cross-sections. The ENUBET ERC project is studying a facility based on a narrow-band neutrino beam capable of constraining the neutrino fluxes normalization through the monitoring of the associated charged leptons in an instrumented decay tunnel. In ENUBET, the identification of large-angle positrons from $K_{e3}$ decays at single particle level can potentially reduce the ${\nu }_e$ flux uncertainty at the level of 1%. This setup would allow for an unprecedented measurement of the ${\nu }_e$ cross-section at the GeV scale. This input would be highly beneficial to reduce the budget of systematic uncertainties in the next long baseline oscillation projects. Furthermore, in narrow-band beams, the transverse position of the neutrino interaction at the detector can be exploited to determine a priori with significant precision the neutrino energy spectrum without relying on the final state reconstruction. This contribution will present the advances in the design and simulation of the hadronic beam line. Special emphasis will be given to a static focusing system of secondary mesons that can be coupled to a slow extraction proton scheme. The consequent reduction of particle rates and pile-up effects makes the determination of the ${\nu }_{\mu }$ flux through a direct monitoring of muons after the hadron dump viable, and paves the way to a time-tagged neutrino beam. Time-coincidences among the lepton at the source and the neutrino at the detector would enable an unprecedented purity and the possibility to reconstruct the neutrino kinematics at source on an event-by-event basis. We will also present the performance of positron tagger prototypes tested at CERN beamlines, a full simulation of the positron reconstruction chain and the expected physics reach of ENUBET.

The PANDA experiment at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt/Germany aims to investigate fundamental questions of hadron physics. PANDA is designed as a fixed-target experiment for an antiproton beam with a momentum range of 1.5 GeV/$c$ to 15 GeV/$c$. In order to obtain an excellent particle identification of pions and kaons, two independent DIRC detectors have been developed for two adjacent spatial regions. The Barrel DIRC covers polar angles from $22^{\circ }$–$140^{\circ }$ and performs $\pi /K$ separation with $3\sigma$ or more for momenta from 0.5 to 3.5 GeV/$c$. The novel Endcap Disc DIRC (EDD) detector will cover the forward polar angles between $5^{\circ }$ and $22^{\circ }$ and will provide a $\pi /K$ separation from 0.5 GeV/$c$ up to 4 GeV/$c$ with a separation power at least $3\sigma$. The design of the Barrel DIRC is based on the successful BaBar DIRC and the SuperB FDIRC R&D with several improvements to optimize the performance for PANDA. Both PANDA DIRC detectors use synthetic fused silica as material for radiators and light guides and lifetime-enhance Microchannel Plate PMTs (MCP-PMTs) as sensors. The Barrel DIRC uses narrow bars as a radiator, a prism-shaped expansion volume and a complex multi-layer spherical lens as focusing system. The Cherenkov radiator for the EDD is a large, 2 cm thick fused silica plate that is divided into four identical quadrants. A combination of bars and cylindrical elements with aluminum coating focus the Cherenkov light on the MCP-PMTs with segmented anode plates.

The technical design of the two DIRC detectors and the performance of prototypes, tested in a mixed hadron beam at CERN, will be discussed.

In this paper, we present two options of the toroid magnetic spectrometer dedicated to measure the energy and the polar and the azimuthal angles of the scattered from the ion’s nuclear electrons in the future electron-ion collider DERICA at JINR. These options differ by the opposite sign of the magnetic field. In one of the options, the toroid magnetic field bends electrons towards the collision line, while in the option with the inverted field a bent is done outwards from the beam axis. We show that the last case provides much larger useful fraction of a solid angle for detection of the scattered electrons. The momentum resolution of such a spectrometer is estimated.

We report on the design and the pilot phase of the $\mu$Net project that aims for the active involvement of Greek high school students in the experimental procedures of astroparticle physics and especially in cosmic ray physics. Through the anticipated educational program, the students from the geographical area of Peloponnese will construct and operate educational cosmic ray telescopes deployed at their school laboratory and/or perform distant educational activities utilizing the extensive air shower array and the relevant remotely operated experimental setups of the Hellenic Open University Physics Laboratory.