LHC Run Research Articles

Studies on impurities and F-radicals production in gaseous detectors operated with Fluorinated gas mixtures at LHC Experiments

At the CERN LHC experiments several gaseous detectors are operated with gas mixtures based on Fluorinated gases. CF4 is used for wire chambers and Gas Electron Multiplier (GEM) detectors while C2H2F4 and SF6 for Resistive Plate Chambers (RPCs). Under the effects of electric field and radiation, these gases undergo radiolytic dissociation producing new molecules and radicals, which could be detrimental to detector long term operation. During LHC Run 2 several gas analyses were performed on the gas mixtures of RPCs and GEMs operated in ALICE, CMS and LHCb experiments. It was observed that several impurities and F-radicals are created inside the detectors and their concentration depends on several factors. Systematic studies on F-production in RPC and GEM detectors have been performed at CERN GIF++ with high background radiation. Correlations between impurities, gas flow, integrated charge, rate and detector currents were established. A comprehensive overview of the obtained results is presented.

Certification of a new generation of Resistive Plate Chambers for the Phase-1 BIS78 upgrade of the ATLAS Muon Spectrometer

In preparation for the coming years of LHC running at higher luminosity, two upgrade projects of the ATLAS Muon Spectrometer have been developed: the New Small Wheel project, improving the trigger in the end-cap regions, and the BIS78 project, dedicated to the transition region between the barrel and the endcaps (1< |η| <1.3). The BIS78 project will add 32 RPC triplets on the edges of the inner barrel sectors where the ATLAS toroid is present. These new generation of RPCs are characterized by thinner gas gaps (1 mm vs 2 mm of the legacy RPCs) as well as thinner resistive electrodes, together with a new high gain front-end electronics operating at lower voltage. Sixteen BIS78 stations have been already installed in the ATLAS Muon spectrometer between 2020 and 2021 and are being commissioned during this year. The status of the project, the status of the commissioning as well as the overall performances of the installed RPCs will be presented.

ALICE ITS upgrade for LHC Run 3: commissioning in the laboratory

ALICE is the CERN LHC experiment optimised for the study of the strongly interacting matter produced in heavy-ion collisions and devoted to the characterisation of the quark-gluon plasma. To achieve the physics program for LHC Run 3, a major upgrade of the experimental apparatus is ongoing. A key element of the upgrade is the substitution of the Inner Tracking System (ITS) with a completely new silicon-based detector whose features will allow the reconstruction of rare physics channels, not accessible with the previous layout. The enabling technology for such a performance boost is the adoption of custom-designed CMOS MAPS as detecting elements. The installation of the detector in the ALICE cavern is completed and few months of commissioning will allow the final integration into the central systems. In this talk, an overview of the adopted technologies as well as the status of the detector commissioning will be given.

Design and Test-Beam Results of the FoCal-H Demonstrator Prototype

The forward calorimeter (FoCal) of ALICE, planned to be operational for LHC Run 4, will cover the pseudorapidity range 3.4 ≤η≤ 5.8 allowing to probe the unexplored region of Bjorken-x down to 10−6. The hadronic section of the FoCal (FoCal-H) will be based on copper capillary tubes and scintillating fibers inside, with light read out by silicon photomultipliers (SiPM). A “proof of concept” demonstration prototype was built and tested in the H6 beamline at the CERN SPS in the beginning of October, 2021, exposing it to an unseparated charged particle beam with energy in the interval 20 GeV–80 GeV. The design of the prototype as well as the results of the energy reconstruction are presented and the validation with a GEANT4-based simulation is discussed.

The ATLAS ‘New Small Wheel’ Muon detector stations recently commissioned for LHC Run3

After nearly a decade of intense effort, the two New Small Wheels (NSW) for the upgrade of the ATLAS Muon spectrometer is now installed in the experiment and ready for final commissioning and to collect data in LHC Run3, starting March 2022. The NSW is the largest phase-1 upgrade project of ATLAS. It is 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 completely new detector technologies: the small strips Thin Gap Chambers (sTGC) and the Micromegas (MM). The latter belongs to the family of Micro Pattern Gaseous Detectors (MPGD, for the first time used in such a large scale in HEP experiment). Each detector will cover more than 1200 m2 of the active area. The new system is required to maintain the present detector’s efficiency and momentum resolution level in the expected higher background level, given the ongoing LHC luminosity upgrades. In addition, it is necessary to keep an acceptable Muon trigger rate with the same Muon momentum threshold.11Copyright 2022 CERN for the benefit of the ATLAS muon collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license.

Status of the two-Higgs-doublet model in light of the CDF mW measurement

The most recent $W$-boson mass measurement by the CDF Collaboration with a substantially reduced uncertainty indicates a significant deviation from the standard model prediction, as large as $7\ensuremath{\sigma}$ if taken literally. Then the Peskin-Takeuchi parameters of $S$ and $T$ shift to larger values, which has profound consequences in searching for physics beyond the SM. In the framework of two-Higgs-doublet models, we study the effect of the new $W$-boson mass measurement on the parameter space. Combined with other constraints including theoretical requirements, flavor-changing neutral currents in $B$ physics, the cutoff scale above 1 TeV, Higgs precision data, and direct collider search limits from the LEP, Tevatron, and LHC experiments, we find upper bounds on the masses of the heavy Higgs bosons; ${M}_{H,A,{H}^{\ifmmode\pm\else\textpm\fi{}}}\ensuremath{\lesssim}1.1\text{ }\text{ }\mathrm{TeV}$ in type I, II, X, and Y for the normal Higgs scenario; ${M}_{{H}^{\ifmmode\pm\else\textpm\fi{}}}\ensuremath{\lesssim}450\text{ }\text{ }\mathrm{GeV}$ and ${M}_{A}\ensuremath{\lesssim}420\text{ }\text{ }\mathrm{GeV}$ in type I and X for the inverted scenario where the heavier $CP$-even Higgs bosons is the observed one. Another important finding is that type II and type Y in the inverted scenario are completely excluded. Such unprecedented findings imply that the upcoming LHC run can readily close out a large portion of the, still available, parameter space.

A slice-test demonstrator for the upgrade of the CMS drift tubes at High-Luminosity LHC

Drift Tubes (DT) detectors equip the CMS muon system barrel region serving both as offline tracking and triggering devices. Existing DT chambers will operate throughout High-Luminosity LHC, but, in order to withstand event rates and integrated doses far beyond the initial design specification, an upgrade of the current readout and trigger electronics is planned. In the upgraded system, time-to-digital converters will stream hits to new back-end boards that, beyond event matching, will perform online tracking of trigger segments exploiting the ultimate DT cell resolution. During the second LHC long shutdown, prototypes of the aforementioned electronics were installed in four DT chambers with the same azimuthal acceptance, instrumenting a demonstrator of the HL-LHC DT upgrade (DT slice-test). In this report, the motivation for such an upgrade will be highlighted, and the status of the DT slice-test operation, as well as its performance measured with cosmic muons, will be presented. Plans towards future developments of the demonstrator throughout the LHC Run-3 will also be discussed.

Towards MightyPix, an HV-MAPS for the LHCb Mighty Tracker upgrade

The Mighty Tracker is a proposed upgrade to the downstream tracking system of LHCb for operations at luminosities of up to 1.5 × 1034 cm−2 s−1 starting with the LHC Run 5 data taking period. It foresees the replacement of the most central area of the current scintillating fibre tracker with High Voltage CMOS (HV-CMOS) pixel sensors. Due to the increased luminosity, occupancy will be too high for track reconstruction in the fibre tracker and the fibres would no longer withstand the radiation damage. HV-CMOS sensors have demonstrated a significant radiation tolerance and good performance making them an ideal choice of technology for the LHCb experiment. Monolithic Active Pixel Sensors (MAPS) fabricated in HV-CMOS processes provide fast charge collection via drift and allow the implementation of the readout on the same die as the sensitive volume. Due to the use of commercial processes, these sensors can be fabricated at low cost as no hybridisation with bump bonds is required. Since they are not fully depleted, the inactive volume can be reduced by thinning to achieve a total die thickness of 50 to 100 microns. A dedicated sensor called the MightyPix is developed for this programme. It is based on the HV-MAPS families MuPix and ATLASPix and tailored to the requirements of LHCb. To demonstrate the feasibility of this technology for the LHCb environment, prototypes have been irradiated. These sensors are tested in terms of efficiency, time resolution and power dissipation in temperature controlled environments.

Construction, operations, and performance of the CMS phase-1 silicon pixel detector

The CMS phase-1 pixel detector was designed to cope with an instantaneous luminosity of 2 × 1034 cm−2 s−1 and 25 ns bunch spacing with very small efficiency loss. The upgraded detector has 124 million channels that features a 4-hit coverage in the tracking volume with an additional layer in the barrel and one extra disk in both the endcaps. DC-DC converters have been used to deliver more power to the detector without the need of replacing the cable plant. A CO2 based cooling system was implemented and carbon fibre based support structures were used to reduce the material budget in the tracking volume. The data acquisition system was upgraded to cope with higher collision rates with a digital data format from the detector front-ends. The detector was installed in early 2017 and has been successfully operated since then. In this paper, the design and construction, operational experience and the performance of the phase-1 pixel detector at LHC Run 2 will be summarised.

Neutrino detection without neutrino detectors: Discovering collider neutrinos at FASER with electronic signals only

The detection of collider neutrinos will provide new insights about neutrino production, propagation, and interactions at TeV energies, the highest human-made energies ever observed. During Run 3 of the LHC, the FASER experiment is expected to detect roughly $10^4$ collider neutrinos using its emulsion-based neutrino detector FASER$\nu$. In this study, we show that, even without processing the emulsion data, low-level input provided by the electronic detector components of FASER and FASER$\nu$ will be able to establish a $5\sigma$ discovery of collider neutrinos with as little as $5~\text{fb}^{-1}$ of integrated luminosity. These results foreshadow the possible early discovery of collider neutrinos in LHC Run 3.

New Higgs decays to axion-like particles

We investigate the interactions of a light scalar with the Higgs boson and second-generation fermions, which trigger new rare decays of the Higgs boson into 4μ, 2μ2γ, 6μ and 4μ2j. We recast current LHC searches to constrain these decays and develop new collider analyses for those channels which are only poorly tested by existing studies. With the currently collected data we can probe branching ratios as small as 1.5×10−5, 8.7×10−5, 5.7×10−8 and 1.6×10−7, respectively. For the High-Luminosity LHC run, considered here to involve 3 ab−1 of integrated luminosity, these numbers go down to 1.3×10−5, 2.0×10−6, 3.0×10−9 and 5.4×10−9, respectively. We also comment on other channels that remain still unexplored.

ATLAS LAr Calorimeter Commissioning for the LHC Run 3

The Liquid Argon Calorimeters are employed by ATLAS for all electromagnetic calorimetry in the pseudo-rapidity region |η|&lt;3.2, and for hadronic and forward calorimetry in the region from |η|=1.5 to |η|=4.9. They also provide inputs to the first level of the ATLAS trigger. After a successful period of data taking during the LHC Run 2 between 2015 and 2018, the ATLAS detector entered into a long period of shutdown. In 2022, the LHC will restart and the Run 3 period should see an increase of luminosity and pile-up of up to 80 interactions per bunch crossing. To cope with these harsher conditions, a new trigger readout path has been installed during the long shutdown. This new path should significantly improve the triggering performance on electromagnetic objects. This will be achieved by increasing the granularity of the objects available at trigger level by up to a factor of ten. The installation of this new trigger readout chain also required the update of the legacy system. More than 1500 boards of the precision readout have been extracted from the ATLAS pit, refurbished and re-installed. The legacy analog trigger readout, which will remain during the LHC Run 3 as a backup of the new digital trigger system, has also been updated. For the new system, 124 new on-detector boards have been added. Those boards that are operating in a radiative environment are digitizing the calorimeter trigger signals at 40 MHz. The digital signal is sent to the off-detector system and processed online to provide the measured energy value for each unit of readout. In total up to 31 Tbps are analyzed by the processing system and more than 62Tbps are generated for downstream reconstruction. To minimize the triggering latency the processing system had to be installed underground. The limited available space imposed a very compact hardware structure. To achieve a compact system, large FPGAs with high throughput have been mounted on ATCA mezzanines cards. In total, no more than three ATCA shelves are used to process the signal from approximately 34,000 channels. Given that modern technologies have been used compared to the previous system, all the monitoring and control infrastructure is being adapted and commissioned as well. This contribution presents the challenges of the installation, the commissioning and the milestones still to be completed towards the full operation of both the legacy and the new readout paths for the LHC Run 3.

Resistive Micromegas high-rate and long-term ageing studies at the CERN Gamma Irradiation Facility

The LHC experiments are undergoing important upgrades for the High Luminosity LHC project. Experiments will be required to operate under three times higher instantaneous luminosity and higher levels of radiation when compared to LHC Run2. The integrated luminosity will increase by a factor ten. To cope with these conditions resistive Micromegas chambers were chosen by the ATLAS experiment as one of the technologies for the upgrade of the first forward muon stations. This paper describes the results of two prototype Micromegas detectors that were installed in the CERN Gamma Irradiation Facility with the aim to evaluate the detector performance under high-rate irradiation and to assess ageing effects over a 2.5 year long irradiation period and an accumulated charge in excess of 0.3 C/cm2.

Higgs boson properties and supersymmetry: Constraints and sensitivity from the LHC to ane+e−collider

The study of the Higgs boson properties offers compelling perspectives for testing the effects of physics beyond the Standard Model and has deep implications for the LHC program and future colliders. Accurate determinations of the Higgs boson properties can provide us with a distinctively precise picture of the Higgs sector, set tight bounds, and predict ranges for the values of new physics model parameters. In this paper, we discuss the constraints on supersymmetry that can be derived by a determination of the Higgs boson mass and couplings. We quantify these constraints by using scans of the 19-parameter space of the so-called phenomenological minimal supersymmetric Standard Model. The fraction of scan points that can be excluded by the Higgs measurements is studied for the coupling measurement accuracies obtained in LHC run 2 and expected for the HL-LHC program and ${e}^{+}{e}^{\ensuremath{-}}$ colliders and contrasted with those derived from missing transverse energy searches at the LHC and from dark matter experiments.

CMS Phase-1 pixel detector refurbishment during LS2 and readiness towards the LHC Run 3

The CMS Phase-1 pixel detector was extracted from the underground cavern after the end of the LHC Run 2 in 2019 and has been kept cold to protect the silicon sensors during the long shutdown period (LS2) in 2019–2021. The LHC is now preparing for the next period of data taking, Run 3, which is scheduled to start in spring 2022. The Phase-1 pixel detector was going through a series of refurbishment and repairs this year to improve the quality of the collected data and enhance the detector performance. The innermost barrel pixel layer has been replaced with new modules and features improved front-end readout chips (PROC600v4), token bit manager chips (TBM10d), and circuit boards to rectify the issues discovered in Run 2. The forward pixel detector has been equipped with new cooling inlets for safe handling and features a revised high-voltage power distribution scheme to better match the low-voltage granularity. All the DC-DC converter modules have been replaced with new modules featuring an improved FEAST2.3 ASIC, which is considerably more robust against the total ionizing dose effect and thus prevents them from breaking during operation. Overall, this article will summarize the refurbishment work of the pixel detector during LS2, and highlight the readiness towards the LHC Run 3 after installation and commissioning.

FASER’s Electromagnetic Calorimeter Test Beam Studies

FASER, or the Forward Search Experiment, is a new experiment at CERN designed to complement the LHC’s ongoing physics program, extending its discovery potential to light and weakly interacting particles that may be produced copiously at the LHC in the far-forward region. New particles targeted by FASER, such as long-lived dark photons or axion-like particles, are characterised by a signature with two oppositely charged tracks or two photons in the multi-TeV range that emanate from a common vertex inside the detector. The full detector was successfully installed in March 2021 in an LHC side tunnel 480 m downstream from the interaction point in the ATLAS detector. FASER is planned to be operational for LHC Run 3. The experiment is composed of a silicon-strip tracking-based spectrometer using three dipole magnets with a 20 cm aperture, supplemented by four scintillator stations and an electromagnetic calorimeter. The FASER electromagnetic calorimeter is constructed from four spare LHCb calorimeter modules. The modules are of the Shashlik type with interleaved scintillator and lead plates that result in 25 radiation lengths and 1% energy resolution for TeV electromagnetic showers. In 2021, a test beam campaign was carried out using one of the CERN SPS beam lines to set up the calibration of the FASER calorimeter system in preparation for physics data taking. The relative calorimeter response to electrons with energies between 10 and 300 GeV, as well as high energy muons and pions, has been measured under various high voltage settings and beam positions. The measured calorimeter resolution, energy calibration, and particle identification capabilities are presented.

Dynamic pressure evolution during the LHC operation

For the accelerator community and the vacuum scientists, the understanding of the beam interactions with a vacuum chamber is fundamental to provide solutions to mitigate pressure rises induced by electron, photon, and ion molecular desorption. Moreover, beam instabilities induced by ion and electron clouds must be investigated in order to find solutions to reduce them. This study presents in situ measurements of pressure evolutions and electrical currents performed during the LHC RUN II (2018). The proton beam circulating in the LHC vacuum chamber ionizes the residual gas producing electrons as well as positive ions. These charged particles are accelerated away from the beam and reach the vacuum chamber wall, inducing, among other phenomena, stimulated desorption and secondary electron emission. Moreover, protons emit synchrotron radiations that also induce photodesorption and photoelectron production. Experimental measurements of the electrical signals recorded on copper electrodes were compared to calculations considering both the secondary electron yield of copper and the electron energy distribution. All measurements performed with the Vacuum Pilot Sector in the LHC ring show the importance of taking into account a large variety of phenomena in order to understand the pressure evolution in the LHC. Results show that the multipacting threshold, corresponding to an increase in the electron cloud density, strongly depends on the number of protons per bunch. Finally, the ion current was measured with a biased electrode lower than $\ensuremath{-}500\text{ }\text{ }\mathrm{V}$. It was much higher than expected, pointing its origin not only from simple beam-gas ionization but also from the ionization of the residual gas by the electron cloud.

Performance and Calibration of the ATLAS Tile Calorimeter

The Tile Calorimeter (TileCal) is the central hadronic calorimeter of the ATLAS experiment at the LHC. This sampling device is made of steel plates acting as absorber and scintillating tiles as active medium. The wavelength-shifting fibers collect the light from scintillators and carry it to the photomultiplier tubes (PMTs). The analogue signals from the PMTs are amplified, shaped and digitized by sampling the signal every 25 ns and stored on detector until a trigger decision is received. The TileCal front-end electronics read out the signals produced by 9852 channels, whose dynamic range covers the interval from 30 MeV to 2 TeV. Each stage of the signal propagation from scintillation light to the signal reconstruction is monitored and calibrated. During LHC Run-2, high-momentum isolated muons and isolated hadrons have been used to study and validate the electromagnetic scale and the hadronic response, respectively. The time resolution was studied with multi-jet events. Results of performance studies that address calibration, stability, energy scale, uniformity and time resolution are presented.

Hitting two BSM particles with one lepton-jet: search for a top partner decaying to a dark photon, resulting in a lepton-jet

A maverick top partner model, decaying to a dark photon was suggested in Ref. [1]. The dark photon decays to two overlapping electrons for dark photon masses of \approx 100≈100 MeV, and results in a so-called lepton-jet. The event includes a top quark as well, which results in events with two boosted objects, one heavy and the other light. We propose a search strategy exploiting the unique signal topology. We show that for a set of kinematic selections, both in hadronic and leptonic decay channel of the SM top quark, almost all background can be eliminated, leaving enough signal events up to top partner mass of about 3.5 TeV for the search to be viable at the LHC Run 2. Further, integrated luminosity needed for the proposed search to be sensitive is presented as a function of top partner mass.

Standard model anomalies: lepton flavour non-universality, g − 2 and W-mass

We critically analyze the body of results that hints to the existence of New Physics from possible violations of lepton universality observed by the LHCb experiment in the μ/e ratios RK and {R}_{K^{ast }} to the g − 2 lepton anomalies. The analysis begins with a theoretical, in depth, study of the μ/e ratios RK and {R}_{K^{ast }} as well as the process Bs→ μ+μ−. Here we consider the impact of complex Wilson coefficients and derive constraints on their imaginary and real parts. We then move to a comprehensive comparison with experimental results. We show that, by fitting a single Wilson coefficient, the deviations from the Standard Model are at the 4.7σ level when including only the hadronic insensitive observables while it increases to 6.1σ when including also the hadronic sensitive ones. When switching on all relevant Wilson coefficients and combining both hadronic sensitive and insensitive data into the fit, the deviation from the Standard Model peaks at 7.2σ and decreases at the 4.9σ level if we assume that the central values of RK and {R}_K^{ast } are taken to be unity. We further estimate other unaccounted for SM contributions and show that their inclusion still requires New Physics to fit the data. We then introduce the g − 2 lepton anomalies as well as the most recent W-mass results. Different theoretical models are considered that can explain the discrepancies from the Standard Model. In the final part of our work we estimate the impact of the forthcoming data from LHCb (coming from LHC Run3) and Belle II, when it will have accumulated about 5 ab−1.