LHC Run Research Articles

NLO QCD corrections to full off-shell production of toverline{t}Z including leptonic decays

Motivated by ongoing new physics searches in the top-quark sector at the Large Hadron Collider we report on the calculation of NLO QCD corrections to the Standard Model pp → toverline{t}Z + X process in the tetra-lepton decay channel. This calculation is based on the matrix elements for the {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}{tau}^{+}{tau}^{-} final state and includes all resonant and non-resonant Feynman diagrams, interferences and off-shell effects of the top quark as well as the W and Z gauge bosons. Also incorporated are photon-induced contributions. As it is customary for such studies, we show theoretical predictions for both fixed and dynamical factorisation and renormalisation scale choices and different PDF sets. Furthermore, we study the main theoretical uncertainties that are associated with neglected higher-order terms in the perturbative expansion and with the parameterisation of the PDF sets. In order to investigate the size of off-shell effects and higher-order corrections in top-quark decays, we perform a second computation for this process, which is based on the narrow-width-approximation where the top quarks, W and Z gauge bosons are kept on-shell. Results are given for the integrated and differential fiducial cross sections for the LHC Run II center-of-mass energy of 13 TeV.

Microlens-enhanced silicon photomultiplier arrays for LHCb SciFi Tracker Upgrade Ib

A large-scale Scintillating Fibre Tracker has been developed and constructed for the current LHCb Upgrade 1. The high radiation by fast neutrons reduces the light yield of the detector in the course of LHC Run 3, and is challenging the tracker’s hit detection efficiency. To mitigate radiation-induced ageing effects, the central fibre mats and photodetectors will be replaced during the next long shutdown (2026–2028). For this purpose, microlens-enhanced silicon photomultipliers with an improved photon detection efficiency are being developed. The first prototypes have been produced based on the outcome of dedicated simulation studies, and the results of the first light yield measurements are presented.

Double-charm heptaquark states composed of two charmed mesons and one nucleon

Inspired by the experimental discoveries of ${T}_{cc}$, ${\mathrm{\ensuremath{\Sigma}}}_{c}(2800)$, and ${\mathrm{\ensuremath{\Lambda}}}_{c}(2940)$ and the theoretical picture where they are $D{D}^{*}$, $DN$, and ${D}^{*}N$ molecular candidates, we investigate the double-charm heptaquark system of $D{D}^{*}N$. We employ the one-boson-exchange model to deduce the pairwise $D\text{\ensuremath{-}}{D}^{*}$, $D\text{\ensuremath{-}}N$, and ${D}^{*}\text{\ensuremath{-}}N$ potentials and then study the $D{D}^{*}N$ system with the Gaussian expansion method. We find two good hadronic molecular candidates with $I({J}^{P})=\frac{1}{2}({\frac{1}{2}}^{+})$ and $\frac{1}{2}({\frac{3}{2}}^{+})$ $D{D}^{*}N$ with only $S$-wave pairwise interactions. The conclusion remains unchanged even taking into account the $S\text{\ensuremath{-}}D$ mixing and coupled channel effects. In addition to providing the binding energies, we also calculate the root-mean-square radii of the $D{D}^{*}N$ system, which further support the molecular nature of the predicted states. They can be searched for at the upcoming LHC run 3 and run 4.

Prospects for Higgs and di-Higgs measurements at the ATLAS experiment

The large dataset of about 3000 {fb}^{-1}fb−1 that will be collected at the High Luminosity LHC (HL-LHC) will be used to measure Higgs boson processes in detail. This large dataset will also provide sensitivity to di-Higgs processes and will allow for the improvement of the constraints on the Higgs boson self coupling. Studies based on current ATLAS analyses using LHC Run 2 data have been carried out to understand the expected precision and limitations of the Higgs and di-Higgs measurements at the HL-LHC. This paper presents the ATLAS prospects for Higgs and di-Higgs results at the HL-LHC.

Proton reconstruction with the Precision Proton Spectrometer (PPS) in Run 2 and the PPS at HL-LHC

The Precision Proton Spectrometer (PPS) started operating in 2016 and has collected more than 110 fb^{-1}−1 of data over the course of the LHC Run 2, now fully available for physics analysis. This contribution covers the key features of the PPS alignment and optics calibration, which have been developed from scratch. The reconstructed proton distributions, the performance of the PPS simulation and finally the validation of the full reconstruction chain with physics data (dilepton events) are also illustrated.

Upgrade of the ALICE experiment beyond LHC Run 3

The ALICE Collaboration completed the upgrade of the detector and is now commissioning for the beginning of the data taking during LHC Run 3. In parallel, R&D activities and simulation studies are being performed to define the future of the experiment beyond LHC Run 3. Two detector upgrades are foreseen for the next long shutdown (LS3). The first is the replacement of the three layers of the inner tracking system closest to the beam with a novel vertex detector consisting of curved wafer-scale ultra-thin silicon sensors arranged in perfectly cylindrical layers to improve impact parameter resolution and significantly extend the physics capability for the study of the heavy-flavor production and the low-mass dielectrons. The second upgrade for the LS3 is the addition of a Forward Calorimeter detector at large rapidity consisting of a Si-W electromagnetic calorimeter with pad and pixel readout, that will equip the experiment with unique capabilities to measure small-x gluon distributions via prompt photon production. A proposal of a next-generation heavy-ion experiment for LHC Run 5 is also in preparation and will be discussed. The aim is to perform novel measurements of the electromagnetic and hadronic probes of the QGP, such as the production of multiply-charmed baryons, which have so far been inaccessible, both because of detector performance and luminosity. The concept of the new apparatus foresees an extensive usage of thin silicon sensors for tracking and a modern particle identification system, combining a silicon-based time of flight detector, a RICH detector, an electromagnetic calorimeter and a muon system.

The PDF4LHC21 combination of global PDF fits for the LHC Run III* *We dedicate this paper to the memory of James Stirling, Dick Roberts and Jon Pumplin, all of whom sadly died in the past few years. All three were instrumental in the development and evolution of PDF fitting, Jon for the CTEQ-TEA collaboration and James and Dick for the

A precise knowledge of the quark and gluon structure of the proton, encoded by the parton distribution functions (PDFs), is of paramount importance for the interpretation of high-energy processes at present and future lepton–hadron and hadron–hadron colliders. Motivated by recent progress in the PDF determinations carried out by the CT, MSHT, and NNPDF groups, we present an updated combination of global PDF fits: PDF4LHC21. It is based on the Monte Carlo combination of the CT18, MSHT20, and NNPDF3.1 sets followed by either its Hessian reduction or its replica compression. Extensive benchmark studies are carried out in order to disentangle the origin of the differences between the three global PDF sets. In particular, dedicated fits based on almost identical theory settings and input datasets are performed by the three groups, highlighting the role played by the respective fitting methodologies. We compare the new PDF4LHC21 combination with its predecessor, PDF4LHC15, demonstrating their good overall consistency and a modest reduction of PDF uncertainties for key LHC processes such as electroweak gauge boson production and Higgs boson production in gluon fusion. We study the phenomenological implications of PDF4LHC21 for a representative selection of inclusive, fiducial, and differential cross sections at the LHC. The PDF4LHC21 combination is made available via the LHAPDF library and provides a robust, user-friendly, and efficient method to estimate the PDF uncertainties associated to theoretical calculations for the upcoming Run III of the LHC and beyond.

Performance of the LHCb RICH detectors during LHC Run 2

The performance of the ring-imaging Cherenkov detectors at the LHCb experiment is determined during the LHC Run 2 period between 2015 and 2018. The stability of the Cherenkov angle resolution and number of detected photons with time and running conditions is measured. The particle identification performance is evaluated with data and found to satisfy the requirements of the physics programme.

The ALPs from the top: searching for long lived axion-like particles from exotic top decays

We propose a search for long lived axion-like particles (ALPs) in exotic top decays. Flavour-violating ALPs appear as low energy effective theories for various new physics scenarios such as t-channel dark sectors or Froggatt-Nielsen models. In this case the top quark may decay to an ALP and an up- or charm-quark. For masses in the few GeV range, the ALP is long lived across most of the viable parameter space, suggesting a dedicated search. We propose to search for these long lived ALPs in toverline{t} events, using one top quark as a trigger. We focus on ALPs decaying in the hadronic calorimeter, and show that the ratio of energy deposits in the electromagnetic and hadronic calorimeters as well as track vetoes can efficiently suppress Standard Model backgrounds. Our proposed search can probe exotic top branching ratios smaller than 10−4 with a conservative strategy at the upcoming LHC run, and potentially below the 10−7 level with more advanced methods. Finally we also show that measurements of single top production probe these branching ratios in the very short and very long lifetime limit at the 10−3 level.

Precise probing of the inert Higgs-doublet model at the LHC

The inert Higgs-doublet model provides a simple framework to accommodate a viable Higgs portal scalar dark matter candidate, together with other heavier scalars of mass 100 GeV or more. We study the effect of next-to-leading order (NLO) QCD corrections in this scenario in the context of the Large Hadron Collider. $\mathcal{O}({\ensuremath{\alpha}}_{s})$ corrections to the gluon-gluon-Higgs effective coupling have been taken into account in this study wherever appropriate. We find such corrections have a significant impact on various kinematic distributions and reduce scale uncertainties substantially. Fixed order NLO results are matched to the pythia8 parton shower (PS) and the di-fatjet signal associated with the missing transverse momentum is analyzed, as this channel has the ability to explore its entire parameter space during the next phase of the LHC run. A closer look at the $\mathrm{NLO}+\mathrm{PS}$ computation indicates a sizable NLO effect together with a subdued contribution from associated production of the heavy scalar compared to the pair production, thereby leading to a refined analysis strategy during the multivariate analysis of this signal.

Optimal injection voltage in the LHC

Finding the optimal rf injection voltage in the LHC is a trade-off between minimising capture and flat-bottom losses, which call for an increased voltage, and minimising rf power consumption as well as improving beam stability, both of which call for a reduced voltage. From the beam stability point of view, earlier particle-tracking simulations showed that the decrease of the injection voltage from 6MV to 4MV is beneficial. This reduction was performed in the LHC Run 2 over a three-week period in steps of 0.5MV. The impact of different voltages and injection energy errors on the evolution of beam parameters such as bunch length and beam losses are analysed in this paper. Operation at 4MV, maintained in the machine after the reduction period, is studied in more detail. The implications for the future High-Luminosity LHC operation with high-intensity beams are also discussed.

Mixing and CP violation in charm decays at LHCb

We present an overview of four recent measurements of charm decays performed at the LHCb experiment. These include the first observation of the non-zero mass difference between neutral charm-meson eigenstates, the most precise measurements of ∆YKK and ∆Yππ, and timeintegrated ACP of eight two-body charm decays. The precision of the measurements is mainly limited by statistical uncertainty, so further improvement is expected with the upcoming LHC Run 3.

The ALICE Run 3 online/offline processing

The ALICE experiment has undergone a major upgrade for LHC Run 3 and will collect data at an interaction rate 50 times larger than before. The new computing scheme for Run 3 replaces the traditionally separate online and offline frameworks by a unified one, which is called O2. Processing will happen in two phases. During data taking, a synchronous processing phase performs data compression, calibration, and quality control on the online computing farm. The output is stored on an onsite disk buffer. When there is no beam in the LHC, the same computing farm is used for the asynchronous reprocessing of the data which yields the final reconstruction output. The O2 project consists of three main parts:. The Event Processing Nodes (EPN) equipped with GPUs deliver the bulk of the computing capacity and perform the majority of the reconstruction and the calibration. The First Level Processors (FLP) receive the data via optical links from the detectors and perform local processing where it is needed, which can optionally happen in the user logic of the FPGA based readout card. Between the FLP and the EPN farms the data is distributed in the network such that the EPNs receive complete collision data for the processing. The Physics and Data Processing (PDP) group develops the software framework and the reconstruction and calibration algorithms. The current O2 setup is capable of handling in real time the peak data rate foreseen for data taking of Pb–Pb collisions at 50 kHz interaction rate.

Heavy neutrinos at future linear e+e− colliders

Neutrinos are among the most mysterious particles in nature. Their mass hierarchy and oscillations, as well as their antiparticle properties, are being intensively studied in experiments around the world. Moreover, in many models of physics beyond the Standard Model, the baryon asymmetry or the dark matter density in the Universe are explained by introducing new species of neutrinos. Among others, heavy neutrinos of Dirac or Majorana nature were proposed to solve open questions in High Energy Physics. Such neutrinos with masses above the electroweak (EW) scale could be produced at future linear e+e− colliders, like the Compact LInear Collider (CLIC) or the International Linear Collider (ILC).We studied the possibility of observing decays of heavy Dirac and Majorana neutrinos in the qqℓ final state with ILC running at 500 GeV and 1 TeV, and CLIC at 3 TeV. The analysis is based on the Whizard event generation and fast simulation of detector response with Delphes. Neutrinos with masses from 200 GeV to 3.2 TeV were considered. We estimated the limits on the production cross sections, interpreted them in terms of the neutrino-lepton coupling parameter {V}_{ell N}^2 (effectively the neutrino mixing angle) and compared them with current limits coming from the LHC running at 13 TeV, as well as the expected limits from future hadron colliders. The limits for the future lepton colliders, extending down to the coupling values of 10−7− 10−6, are stricter than any other limit estimates published so far.

Nonresonant searches for axion-like particles in vector boson scattering processes at the LHC

We propose a new search for Axion-Like Particles (ALPs), targeting Vector Boson Scattering (VBS) processes at the LHC. We consider nonresonant ALP-mediated VBS, where the ALP participates as an off-shell mediator. This process occurs whenever the ALP is too light to be produced resonantly, and it takes advantage of the derivative nature of ALP interactions with the electroweak Standard Model bosons. We study the production of ZZ, Zγ, W±γ, W±Z and W±W± pairs with large diboson invariant masses in association with two jets. Working in a gauge-invariant framework, upper limits on ALP couplings to electroweak bosons are obtained from a reinterpretation of Run 2 public CMS VBS analyses. The constraints inferred on ALP couplings to ZZ, Zγ and W±W± pairs are very competitive for ALP masses up to 100 GeV. They have the advantage of being independent of the ALP coupling to gluons and of the ALP decay width. Simple projections for LHC Run 3 and HL-LHC are also calculated, demonstrating the power of future dedicated analyses at ATLAS and CMS.

Microchannel cooling for the LHCb VELO Upgrade I

The LHCb VELO Upgrade I, currently being installed for the 2022 start of LHC Run 3, uses silicon microchannel coolers with internally circulating bi-phase CO2 for thermal control of hybrid pixel modules operating in vacuum. This is the largest scale application of this technology to date. Production of the microchannel coolers was completed in July 2019 and the assembly into cooling structures was completed in September 2021. This article describes the R&D path supporting the microchannel production and assembly and the motivation for the design choices, together with the achieved fluidic and thermal performance. The Thermal Figure of Merit of the microchannel coolers is measured on the final modules to be between 1.5 and 3.5 K cm2 W−1, depending on glue thickness. The microchannel coolers constitute 18% of the total radiation length of the VELO and less than 2% of the material seen before the second measured point on the tracks. Microchannel cooling is well suited to the VELO implementation due to the uniform mass distribution, close thermal expansion match with the module components and resistance to radiation.

Development of the ITS3: A bent-silicon vertex detector for ALICE in the LHC Run 4

In parallel with commissioning and installation of the second generation ALICE Inner Tracking System (ITS2), R&D is underway to replace the innermost tracking layers of the newly installed detector during the LHC long shutdown 3 (2026–28) with a fully cylindrical, bent-silicon tracker (ITS3) to be ready for LHC Run 4 (currently scheduled to start in 2029). By thinning down the Monolithic Active Pixel Sensors (MAPS) to 20–40μm the silicon becomes flexible; using this in conjunction with sensor stitching will allow for truly cylindrical half-barrels to be produced. In addition, the intrinsic stability of the sensors and their low power consumption (≈ 20 mW/cm2) will facilitate the removal of the supporting material for the powering, cooling and data transmission from the active area of the detector. With this design, an unprecedented low material budget of below 0.05% X0 per layer can be achieved and the innermost of three new layers can be positioned 18 mm from the interaction point.A brief motivation for the development of the ALICE ITS3, a description of the detector specifications and highlights from the R&D are discussed in this contribution.

ATLAS results on charmonium production and Bc+ meson production and decays

Recent results from the proton-proton collision data taken by the ATLAS experiment on the charmonium production and on the Bc+ meson production and decays are presented. The measurement of J/ψ meson and ψ(2S) meson differential cross sections is reported. New results on the Bc+ meson decays to J/ψ Ds(*) final states are included. Both studies are based on pp collision data collected at √s=13 TeV during the LHC Run 2, corresponding to an integrated luminosity of 139 fb-1. The measurement of the differential ratios of the Bc+and B+ mesons production cross sections at √s=8 TeV with an integrated luminosity of 20.3 fb-1is also discussed.

Strategies and performance of the CMS silicon tracker alignment during LHC Run 2

The strategies for and the performance of the CMS silicon tracking system alignment during the 2015–2018 data-taking period of the LHC are described. The alignment procedures during and after data taking are explained. Alignment scenarios are also derived for use in the simulation of the detector response. Systematic effects, related to intrinsic symmetries of the alignment task or to external constraints, are discussed and illustrated for different scenarios.

An overview of the trigger system at the CMS experiment

The trigger system of the Compact Muon Solenoid (CMS) experiment at CERN has been evolving continuously since the startup of the LHC. While the base of the current configuration will remain in use for the next LHC running period (Run 3 starting in 2022), new features and algorithms are already being developed to take care of higher data loads due to increasing LHC luminosity and pileup but also of new experimental signatures to be investigated, in particular, displaced decay vertices stemming from relatively long-lived particles created in proton-proton collisions. Beyond this period, the trigger system will undergo a major upgrade to prepare for the high-luminosity LHC (HL-LHC) operations, which will deliver a luminosity of 5–7.5 times the design value. It corresponds to 140-200 pileup events, defined as overlapping proton-proton interactions in the same or nearby bunch crossings. During HL-LHC, information from the silicon pixel and strip tracker will be available already for the Level-1 Trigger, detector granularity and pseudorapidity coverages will increase. Trigger rates will rise by a factor of about 7.5 both at Level-1 (to 750 kHz) and at the High Level Trigger (to 7.5 kHz) and the latency—the processing time available for arriving at the Level-1 trigger decision—will increase significantly from 3.8 μs to 12.5 μs, allowing for the use of more sophisticated algorithms at the Level-1 trigger.