HL-LHC Research Articles

Radiation protection challenges for the Large Hadron Collider upgrade.

In the context of the so-called Long Shutdown 3 (2026-2028), the Large Hadron Collider will be upgraded to the High-Luminosity Large Hadron Collider, allowing for approximately five more instantaneous collisions. The upgrade, maintenance and decommissioning of equipment will be mainly performed in the experimental insertions of Points 1 and 5, requiring to perform multiple interventions in high-residual radiation environment. This poses complex radiological challenges that the CERN Radiation Protection group is called to address. Radiation protection studies are performed to plan and optimise (ALARA) these future interventions using the advanced Monte Carlo techniques and tools such as FLUKA, ActiWiz, SESAME and the FCC method. This paper aims to provide an overview of the studies conducted to estimate the residual radiation field in the experimental insertions, the activation levels in terms of multiple of the Swiss clearance limits/specific activity and to provide preliminary considerations on the upgrade/decommissioning of key equipment.

TCAD Analysis of Leakage Current and Breakdown Voltage in Small Pitch 3D Pixel Sensors.

Small-pitch 3D pixel sensors have been developed to equip the innermost layers of the ATLAS and CMS tracker upgrades at the High Luminosity LHC. They feature 50 × 50 and 25 × 100 μm2 geometries and are fabricated on p-type Si-Si Direct Wafer Bonded substrates of 150 μm active thickness with a single-sided process. Due to the short inter-electrode distance, charge trapping effects are strongly mitigated, making these sensors extremely radiation hard. Results from beam test measurements of 3D pixel modules irradiated at large fluences (1016neq/cm2) indeed demonstrated high efficiency at maximum bias voltages of the order of 150 V. However, the downscaled sensor structure also lends itself to high electric fields as the bias voltage is increased, meaning that premature electrical breakdown due to impact ionization is a concern. In this study, TCAD simulations incorporating advanced surface and bulk damage models are used to investigate the leakage current and breakdown behavior of these sensors. Simulations are compared with measured characteristics of 3D diodes irradiated with neutrons at fluences up to 1.5 × 1016neq/cm2. The dependence of the breakdown voltage on geometrical parameters (e.g., the n+ column radius and the gap between the n+ column tip and the highly doped p++ handle wafer) is also discussed for optimization purposes.

Accommodating the CDF W -boson mass measurement in the beautiful mirror model

The W-boson mass measurement recently reported by the CDF II experiment exhibits a significant deviation from both the Standard Model prediction and previous measurements. There is also a long-standing deviation between the Standard Model prediction of the forward-backward asymmetry of the bottom quark ($A^{0,b}_{\rm FB}$) and its measurement at the LEP experiment. The Beautiful Mirror model, proposed to resolve the $A^{0,b}_{\rm FB}$ discrepancy, introduces vector-like quarks that modify the W-boson mass at one-loop level. In this study, we find an interesting region in the model parameter space that could potentially explain both discrepancies, which puts the new quarks in the multi-TeV region. This region is mostly consistent with current LHC bounds from direct searches and Higgs coupling measurements, but will be thoroughly probed at the High Luminosity LHC. As such, the Beautiful Mirror model as an explanation of the $m_W$ and $A^{0,b}_{\rm FB}$ discrepancies could be confirmed or falsified in the near future.

Search for leptophilic dark matter at the LHeC

The Large Hadron electron Collider (LHeC) has been designed to push the field of deep inelastic scattering to the high energy and intensity frontier using an intense electron beam with a proton beam from the High Luminosity–Large Hadron Collider. However, LHeC is also a great laboratory for new physics. In this work, we propose a search for dark matter that couples with leptons. This may yield and signals that can be potentially observed through simple missing-energy cuts that suppress the Standard Model background. Considering direct dark matter detection and LHC constraints, we show that LHeC offers a complementary probe for a weak scale dark matter fermion for masses up to 350 GeV, which reproduces the correct relic density, and has interesting implications for lepton flavor violation.

Commissioning of the LHC Injectors Upgrade fast wire scanners and first experimental results

The LHC Injectors Upgrade (LIU) program included new fast beam wire scanners (BWS) to meet the requirements of the higher brightness and intensity High Luminosity LHC (HL-LHC) beams. Seventeen LIU BWS have been installed during the LHC second long shutdown (LS2) and they are currently used in the daily operations of the LHC injectors. After giving an overview of the LIU project and the general beam instrumentation upgrades associated with it, this paper presents a short review of the mechanical design and layout of the LIU BWS, the electronics involved, and then focuses on the first results obtained during their commissioning.

Establishing the Quality Assurance programme for the strip sensor production of the ATLAS tracker upgrade including irradiation with neutrons, photons and protons to HL-LHC fluences

The successful pre-production delivery of strip sensors for the new Inner Tracker (ITk) for the upgraded ATLAS detector at the High Luminosity LHC (HL-LHC) at CERN was completed and based on their performance full production has commenced. The overall delivery period is anticipated to last 4 years to complete the approximately 22000 sensors required for the ITk. For Quality Assurance (QA), a number of test structures designed by the collaboration, along with a large area diode and miniature version of the main sensor, are produced in every wafer by the foundry Hamamatsu Photonics K.K (HPK). As well as Quality Control (QC) checks on every main sensor, samples of the QA pieces from each delivery batch are tested both before and after irradiation with results after exposure to neutrons, gammas or protons to fluences and doses corresponding to those anticipated after operation at the HL-LHC to roughly 1.5 times the ultimate integrated luminosity of 4000 fb-1. In this paper the procedures are presented and the studies carried out to establish that the seven ITk QA Strip Sensor irradiation and test sites meet all the requirements to support this very extensive programme throughout the strip sensor production phase for the ITk project.

High rate studies of the ATLAS sTGC detector and optimization of the filter circuit on the input of the front-end amplifier

The Large Hadron Collider (LHC) at CERN is expected to be upgraded to the High-Luminosity LHC (HL-LHC) by 2029 and achieve instantaneous luminosity around 5–7.5 × 10^34 cm^-2 s^-1. This represents a more than 3–4 fold increase in the instantaneous luminosity compared to what has been achieved in Run 2. The New Small Wheel (NSW) upgrade is designed to be able to operate efficiently in this high background rate environment. In this article, we summarize multiple performance studies of the small-strip Thin Gap Chamber (sTGC) at high rate using nearly final front-end electronics. We demonstrate that the efficiency versus rate distribution can be well described by an exponential decay with electronics dead-time being the primary cause of loss of efficiency at high rate. We then demonstrate several methods that can decrease the electronics dead-time and therefore minimize efficiency loss. One such method is to install either a pi-network input filter or pull-up resistor to minimize the charge input into the amplifier. We optimized the pi-network capacitance and pull-up resistor resistance using the results from our measurements. The results shown here were not only critical to finalizing the components on the front-end board, but also are critical for setting the optimal operating parameters of the sTGC detector and electronics in the ATLAS cavern.

Scrutinizing a hidden SM-like gauge model with corrections to oblique parameters

In view of the recent high precision measurement of the Standard Model W boson mass at the CDF II detector, we compute the contributions to the oblique parameters S, T and U coming from the two additional Higgs doublets (one inert and one hidden) as well as the hidden neutral dark gauge bosons and extra heavy fermions in the gauged two-Higgs-doublet model (G2HDM). While the effects from the hidden Higgs doublet and new heavy fermions are found to be minuscule, the hidden gauge sector SU(2)_H times U(1)_X with gauge coupling strength gtrsim 10^{-2} and gauge boson mass gtrsim 100 GeV can readily explain the W boson mass anomaly but is nevertheless excluded by the dilepton high-mass resonance searches at the Large Hadron Collider. On the other hand, the new global fits to the oblique parameters due to the new W boson mass measurement can give discernible impacts on the mass splitting and mixing angle for the inert Higgs doublet in G2HDM. We also study the impact on the signal strength of diphoton mode of the 125 GeV Higgs boson h rightarrow gamma gamma and the detectability of the yet to be observed process h rightarrow Z gamma at the High Luminosity Large Hadron Collider. Current constraints for the dark matter candidate W^prime including the dark matter relic density, dark matter direct detections and invisible Higgs decays are also taken into account in this study.

Analysis of interstrip capacitance of p+n MCz Si microstrip detector using TCAD simulation combining surface and bulk damage effects by mixed irradiation

Interstrip capacitance plays a crucial role on the detector electronic noise performance at the readout electrode of the irradiated detectors in the harsh radiation environment of the high luminosity Large Hadron Collider experiments. The major challenge is to optimize the design of the detector to get the low interstrip capacitance in the real ‘mixed’ irradiation environment of the experiment. A few of experimental studies have been performed within CERN RDD0 collaboration on the n-MCz Si detectors to extract the microscopic parameters for the surface and bulk damage effects especially by mixed irradiations in the detectors. The results on the microscopic parameters are fed into ATLAS TCAD commercial simulation program to compare the experimental data and simulation result. A very good agreement has been recorded between the data and simulation result. In this paper, the microscopic model parameters are used to investigate the effects of mixed irradiation up to a fluence of 8.82× 1014 neq./cm2 on the interstrip capacitance in n-MCz 〈100〉 silicon microstrip detectors. The results are discussed in detail using electric field distribution, electron concentration profile in the irradiated detectors for the different metal overhang width, and an optimized p+n-MCz Si microstrip design is proposed for the future collider experiment.

Search for mono-Higgs signals in bb¯ final states using deep neural networks

We study mono-Higgs signatures emerging in an illustrative new physics scenario involving Standard Model Higgs boson decays to bottom quark pairs using hybrid deep neural networks. We use a multilayer perceptron to analyze the kinematic observables and optimize the signal-to-background discrimination. The global color flow structure of hard jets emerging from the decay of heavy particles with different color charges is crucial to single out the mono-Higgs signature. Upon embedding the different color flow structures for signal and backgrounds into constructed images, we use a convolution neural network to analyze the latter. Specifically, the approach takes initially mono-type data as input, frittering away invaluable multisource and multiscale information. We then discuss a general architecture of hybrid deep neural networks that supports instead mixed input data. In comparison with single input deep neural networks, like multilayer perceptron or convolution neural networks, the hybrid deep neural networks provide higher capacity in feature extraction and thus in signal versus background classification performance. We provide reference results for the case of the high-luminosity Large Hadron Collider.

Constraining lepton flavor violating Higgs couplings at the HL-LHC in the vector boson fusion channel

We explore the parameter space of lepton flavor violating (LFV) neutral Higgs Yukawa couplings with the muon and tau leptons that can be probed at the high-luminosity Large Hadron Collider (HL-LHC) via the vector boson fusion (VBF) Higgs production process. Our projected sensitivities for the Standard Model Higgs ($h$) LFV branching ratio $\text{Br}(h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau})$ in the $pp\ensuremath{\rightarrow}hjj\ensuremath{\rightarrow}(h\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau})jj$ channel at the HL-LHC are contrasted with the current and future low-energy constraints from the anomalous magnetic moment and electric dipole moment of the muon, as well as with other LFV observables, such as $\ensuremath{\tau}\ensuremath{\rightarrow}3\ensuremath{\mu}$ and $\ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\gamma}$. We also study the LFV prospects of a generic beyond the Standard Model neutral Higgs boson ($H$) with a mass in the range of ${m}_{H}\ensuremath{\in}[20,800]\text{ }\text{ }\mathrm{GeV}$ and give the projected model-independent upper limits on the VBF production cross section of $Hjj$ times the branching ratio of $H\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\tau}$ at the HL-LHC. We interpret these results in the context of a two-Higgs doublet model as a case study.

Search for the electromagnetic properties of the neutrinos at the HL-LHC and the FCC-hh

The νν¯γγ couplings parametrized with the non-standard dimension-seven operators defined by the Effective Field Theory framework are investigated through the process pp→νν¯γ at the High Luminosity-LHC and the Future Circular proton-proton Collider. The effective Lagrangian of νν¯γγ couplings is implemented into FeynRules to generate a UFO module inserted into Madgraph to generate both background and signal events. These events are then passed through Pythia 8 for parton showering and Delphes to include realistic detector effects. The sensitivities on νν¯γγ couplings are obtained at 95% confidence level. We show that the analysis of the signal emerging from the process pp→νν¯γ allows to improve constraints on νν¯γγ couplings given by the LEP collaboration.

RPC background studies at CMS experiment

During Run2 the high instantaneous luminosity, up to 2.21034cm−2s−1, lead to a substantial hit rate in the Compact Muon Solenoid experiment’s muon chambers due to multiple background sources to physics processes sought for at LHC. In this article we will describe the analysis method devised to measure and identify the contributions to such background in the Resistive Plate Chambers. Thorough understanding of the background rates provides the base for the upgrade of the muon detectors for the High-Luminosity LHC.

Single vector-like quark X production in the tW channel at high energy pp colliders

Vector-like quarks (VLQs) are color-triplet spin-1/2 fermions whose left- and right-handed chiral components have the same transformation properties under the weak-isospin SU(2) gauge group. The X vector-like particle with a charge of |Q|=+5/3 exists in the (X,T) doublet or (X,T,B) triplet states. Based on a simplified model, we investigate the prospect of discovering the doublet right-handed X via the single production of X followed by X→tW at the 14 TeV High Luminosity LHC (HL-LHC), 27 TeV High Energy LHC (HE-LHC), and 100 TeV hadron-hadron Future Circular Hadron Collider (FCC-hh). We perform a detailed detector simulation, and find that the excluding and discovering capabilities on the X quark are enhanced obviously with the increase of collision energy. Assuming the system uncertainty is 30% of the number of the background events, the excluding capabilities are given as follows: (1) the HL-LHC can exclude the correlated regions of the mixing constant sin⁡θR∈[0.082,0.2] and the X quark mass mX ∈ [1000 GeV, 1680 GeV] with L=3 ab−1; (2) the HE-LHC can exclude the correlated regions of sin⁡θR∈[0.067,0.2] and mX ∈ [1000 GeV, 2230 GeV] with L=15 ab−1; (3) the FCC-hh can exclude the correlated regions of sin⁡θR∈[0.078,0.2] and mX ∈ [1000 GeV, 2560 GeV] with L=30 ab−1.

Exposing new scalars hiding behind the Higgs boson

It is possible that there is another scalar hiding behind the known 125 GeV Higgs boson. If the hidden scalar exhibits a $CP$ property different from the Higgs boson, then it can be exposed in the di-Higgs production at the high-luminosity large hadron collider and future colliders.

Sub-10 ps time tagging of electromagnetic showers with scintillating glasses and SiPMs

The high energy physics community has recently identified an e+e− Higgs factory as one of the next-generation collider experiments, following the completion of the High Luminosity LHC program at CERN. The moderate radiation levels expected at such colliders compared to hadron colliders, enable the use of less radiation tolerant but cheaper technologies for the construction of the particle detectors. This opportunity has triggered a renewed interest in the development of scintillating glasses for the instrumentation of large detector volumes such as homogeneous calorimeters. While the performance of such scintillators remains typically inferior in terms of light yield and radiation tolerance compared to that of many scintillating crystals, substantial progress has been made over the recent years. In this paper we discuss the time resolution of cerium-doped Alkali Free Fluorophosphate scintillating glasses, read-out with silicon photo-multipliers in detecting single charged tracks and at different positions along the longitudinal development of an electromagnetic shower, using respectively 150 GeV pions and 100 GeV electron beams at the CERN SPS H2 beam line. A single sensor time resolution of 14.4 ps and 5–7 ps was measured respectively in the two cases. With such a performance the present technology has the potential to address an emerging requirement of future detectors at collider experiments: measuring the time-of-flight of single charged particles as well as that of neutral particles showering inside the calorimeter and the time development of showers.

Transverse momentum resummation at N3LL+NNLO for diboson processes

Diboson processes are one of the most accessible and stringent probes of the electroweak gauge structure of the Standard Model at the LHC. They will be probed at the percent level at the high-luminosity LHC, challenging current theory predictions. We present transverse momentum resummed calculations at N3LL+NNLO for the processes ZZ, WZ, WH and ZH, compare our predictions with most recent LHC data and present predictions at 13.6 TeV including theory uncertainty estimates. For W+W− production we further present jet-veto resummed results at N3LLp+NNLO. Our calculations will be made publicly available in the upcoming MCFM release and allow future analyses to take advantage of improved predictions.

Observability of triple top quark signal at future hadron colliders

Abstract The Standard Model (SM) cross-sections for the production of single top quarks, top-quark pairs, triple top quarks, and four top quarks at three different center-of-mass energies, i.e. $\sqrt{s}=$ 7, 10, and 14 TeV, at existing particle colliders as well as at future hadron colliders, are studied. A fully kinematic analysis with the optimized preselection cuts along with invariant mass reconstruction of top quarks is performed for triple-top production processes pp→tttW, tttd, and tttb in the presence of the SM background. All three signal processes are forced to decay in fully hadronic mode. The studies are performed for the High-Luminosity Large Hadron Collider (HL−LHC) at $\sqrt{s}=$ 14 TeV and for the High-Energy LHC (HE−LHC) at $\sqrt{s}=$ 27 TeV. The signal to background ratio and signal significance of all signal and background processes are estimated for both the scenarios. It is found that the chances of signal observability at the HE−LHC are higher than those at the HL−LHC. PACS numbers: 12.60.Fr, 14.80.Fd

Long term stability studies in the presence of crab cavities and high order multipoles in the CERN super proton synchrotron and high luminosity large hadron collider

In 2018 the world’s first Crab Cavity (CC) experiments with protons were successfully performed, aiming to study the CC effect on a proton beam and evaluate the engineering challenges. Before the commencement of these experiments, the impact of the CCs and their multipoles on the long term particle stability of the SPS was studied using dynamic aperture simulations. Furthermore, in order to ensure the long term stability and luminosity performance of the LHC upgrade, the High Luminosity LHC (HL-LHC), the limits of stable motion were also examined in the presence of CC multipole errors, CC voltage errors and additional non-linearities such as the beam–beam effects. This paper presents the comprehensive dynamic aperture results for the SPS and HL-LHC for different CC, accelerator and beam configurations in view of guiding the engineering specifications of the tolerances of the crab cavity multipole errors.

Flavor changing flavon decay ɸ→tc (ɸ = HF, AF ) at the high luminosity large hadron collider

We present a study of the flavor changing decays φ → tc (φ = HF, AF ) of the CP-even and CP-odd scalar flavons at the large hadron collider and its next stage, the high-luminosity large hadron collider. The theoretical framework is an extension of the standard model that incorporates an extra complex singlet and invokes the Froggatt-Nielsen mechanism with an Abelian flavor symmetry. The projected exclusion and discovery regions in terms of the model parameters are reported. We find that AF could be detected at the LHC by considering a reasonable scenario of the model parameter space. As far as HF is concerned, we also found promising results that could be verified experimentally at the high-luminosity LHC.