HL-LHC Research Articles

Experience on Series Production of the HL-LHC Superferric High Order Corrector Magnets

INFN has developed at the LASA lab (Milano, Italy) the High Order (HO) corrector magnets for the High Luminosity-LHC (HL-LHC) project, which will equip the new interaction regions. All the HO correctors, from skew quadrupole to dodecapole, are based on a novel superferric design, never used so far in high energy colliders, which allows a relatively simple, modular, and easy way to construct a magnet. After the completion of the production of the 54 series magnets in industry, the powering of all the produced magnets has been achieved. The delivery to CERN and the assembly of the first cold mass started. Each magnet has been powered individually and most of them have also been magnetically characterized, the aggregate results are presented and discussed. The lessons learned during the production and the testing of the magnets are reported. The quality assurance and the procedures developed in during development and production in industry proved to be key points of the successful experience of the HO correctors development, together with the solid design. We also report the lessons learned during the experience of the series production of the HO correctors.

Prospects for Charged Higgs Bosons in Natural SUSY Models at the High-Luminosity LHC

We continue our examination of prospects for the discovery of heavy Higgs bosons of natural SUSY (natSUSY) models at the high luminosity LHC (HL-LHC), this time focusing on charged Higgs bosons. In natSUSY, higgsinos are expected at the few hundred GeV scale whilst electroweak gauginos inhabit the TeV scale and the heavy Higgs bosons, H, A and H± could range up tens of TeV without jeopardizing naturalness. For TeV-scale heavy SUSY Higgs bosons H, A and H±, as currently required by LHC searches, SUSY decays into gaugino plus higgsino can dominate H± decays provided these decays are kinematically accessible. The visible decay products of higgsinos are soft making them largely invisible, whilst the gauginos decay to W, Z or h plus missing transverse energy (ET). Charged Higgs bosons are dominantly produced at LHC14 via the parton subprocess, gb→H±t. In this paper, we examine the viability of observing signatures from H±→τν, H±→tb and H±→W,Z,h+ET events produced in association with a top quark at the HL-LHC over large Standard Model (SM) backgrounds from (mainly) tt¯, tt¯V and tt¯h production (where V=W,Z). We find that the greatest reach is found via the SM H±(→τν)+t channel with a subdominant contribution from the H±(→tb)+t channel. Unlike for neutral Higgs searches, the SUSY decay modes appear to be unimportant for H± searches at the HL-LHC. We delineate regions of the mA vs. tanβ plane, mostly around mA∼ 1–2 TeV, where signals from charged Higgs bosons would serve to confirm signals of a heavy, neutral Higgs boson at the 5σ level or, alternatively, to exclude heavy Higgs bosons at the 95% confidence level at the high luminosity LHC.

Discriminating between Higgs Production Mechanisms via Jet Charge at the LHC.

Discriminating between Higgs production mechanisms can play a crucial role in determining the couplings of Higgs to gauge bosons, probing the nature of electroweak symmetry breaking. We propose a novel method to distinguish the Higgs production mechanisms at the LHC by utilizing the jet charge asymmetry of the two leading forward jets in Higgs plus two jets production. This novel observable provides a way to disentangle the W fusion from the Z fusion and gluon fusion processes for the first time, due to the electric charge correlation of the two leading jets in the events. We show that the Higgs couplings to gauge bosons can be well constrained, and its conclusion does not depend on the other possible new physics effects which modify the Higgs total or partial width. We also discuss the complementary roles between the proposed jet charge asymmetry measurement and the Higgs signal strength measurements at the high luminosity LHC (HL-LHC) in determining the Higgs couplings.

Testing type II seesaw leptogenesis at the LHC* *C. H. is supported by the Sun Yat-Sen University Science Foundation, and the Fundamental Research Funds for the Central Universities, Sun Yat-sen University (23qnpy58)

Type II seesaw leptogenesis simultaneously explains the origin of neutrino masses, the baryon asymmetry of our universe, and inflation. The Large Hadron Collider (LHC) provides an opportunity to directly test type II seesaw leptogenesis by looking for the predicted triplet Higgs. In this paper, we perform an analysis of the detection prospect for the triplet Higgs at the LHC through multi-electron channels. We find that due to the contribution of the process, the sensitivity of multi-electron channels searching for doubly-charged Higgs pair production can be improved. We also investigate the signals to probe production and find that the future high luminosity LHC could probe a triplet Higgs around 1.2 TeV at the level.

Testing neutrino dipole portal by long-lived particle detectors at the LHC

We discuss the potential of using detectors aimed for searching long-lived particles (LLP) at the high-luminosity LHC run, to probe the neutrino dipole models. This is achieved by taking the heavy neutral leptons (HNL) of the models as candidates of the LLPs. Taking into account the dipole couplings to the weak bosons, d_{W,Z}, which control the production of the HNLs at the LHC, we discuss the reach on the electromagnetic dipole couplings, d_gamma , by searching for a single high-energy photon at LLP detectors. Four typical scenarios are considered in this paper, scenario A, B with d_{W}=0 or d_{Z}=0, and scenario C, D with d_{W,Z}gg d_gamma . We show the sensitivity on d_gamma , can be fairly different depending on the relations between the d_{W,Z} and d_gamma . And the LLP detectors can potentially extend the sensitivity on dipole couplings during the high-luminosity runs of the LHC in certain scenarios.

Analysis of the performance of low gain avalanche diodes for future particle detectors

Low-Gain Avalanche Diodes (LGAD) are the sensor of choice for the timing detectors of the ATLAS and CMS experiments at the High Luminosity Large Hadron Collider (HL-LHC). This paper presents the results of static and dynamic performance evaluations of LGADs manufactured by Hamamatsu Photonics K.K. (HPK) and Brookhaven National Laboratory (BNL). Timing performance was measured using β-scopes after a static characterization of the device (current-voltage and capacitance-voltage curves) and a time resolution better than 35 ps was extracted under high operational bias voltage before irradiation. This value is considered within the nominal requirements of the ATLAS project for un-irradiated sensors. Transient Current Technique (TCT) was used to observe and analyze a gain suppression mechanism, i.e. a decrease in gain correlated with increased laser intensities. TCAD simulations were carried out to interpret the gain suppression of the BNL sensors under different conditions of bias voltage and laser intensity. A good correspondence between experimental observations and TCAD simulations was found.

Heterogeneous Graph Neural Network for identifying hadronically decayed tau leptons at the High Luminosity LHC

We present a new algorithm that identifies reconstructed jets originating from hadronic decays of tau leptons against those from quarks or gluons. No tau lepton reconstruction algorithm is used. Instead, the algorithm represents jets as heterogeneous graphs with tracks and energy clusters as nodes and trains a Graph Neural Network to identify tau jets from other jets. Different attributed graph representations and different GNN architectures are explored. We propose to use differential track and energy cluster information as node features and a heterogeneous sequentially-biased encoding for the inputs to final graph-level classification.

GEM detectors for the CMS endcap muon system: status of three new detector stations

The High-Luminosity LHC (HL-LHC, or Phase-2 LHC) will deliver proton-proton collisions at 5–7.5 times the nominal LHC luminosity, with an expected number of 140–200 pp-interactions per bunch crossing (Pile-up or PU). To maintain the performance of muon triggering and reconstruction under high background, the forward part of the Muon Spectrometer of the CMS experiment will be upgraded with Gas Electron Multipliers (GEM) and improved Resistive Plate Chambers (iRPC) detectors [1,2]. A first GEM station (GE1/1) was installed during Long Shutdown 2 (LS2, 2019–2021), a 2nd station (GE2/1) of Triple-GEM detectors will be installed in winter 2023–24 and 2024–25, while a new 6-layer station (ME0) will be installed in the third Long Shutdown (LS3, 2026–2028). GE11 is considered an early Phase-2 upgrade as it will reduce the pT threshold by combining GEM and Cathode Strip Chamber (CSC) hits in the forward muon system at twice the LHC design luminosity (ℒ = 2 · 1034 cm-2 s-1, 50 PU). After a successful start of Run-3 in 2022, with almost 40 fb-1 collected, the commissioning of the GE1/1 detector is nearly complete. Most chambers are operated stably with an efficiency in excess of 95%, next being the demonstration of the combined CSC-GEM trigger in 2023. The lessons learnt with the first large-area GEM station have lead to improvements in detector and electronics design for the Phase 2 detectors GE2/1 and ME0. This proceeding will discuss the progress made since last MPGD Conference (MPGD 2019) [3], discussing the commissioning and early performance of GE1/1; the design improvements and start of construction of GE2/1; and the R&D currently ongoing for ME0.

The effect of ground motion on the LHC and HL-LHC beam orbit

The High Luminosity Large Hadron Collider (HL-LHC) will require unprecedented orbit stability at the low-beta experiments, ATLAS and CMS. Because the effect of seismic noise might become a relevant source of luminosity loss, several studies have been conducted to characterise the actual ground motion in the LHC tunnel. In preparation for the official groundbreaking of the civil engineering work for the high luminosity upgrade, that started on 15 June 2018 in parallel to LHC beam operation, seismic sensors were installed to permanently monitor the ground stability close to these experiments. Compactor work and shaft excavation were expected to induce vibrations in the accelerator magnets that would cause orbit disturbance, beam loss and potentially premature beam dumps. This paper summarises the observations made on the LHC beams and uses this data to benchmark estimates of the impact of low frequency vibrations on the closed orbit at the main interaction points and collimators. The impact of ground motion on HL-LHC operation is then estimated using the expected operational scenario and the estimated mechanical stability of the new HL-LHC triplet magnets that is based on recent measurements of mechanical magnet transfer functions.

Latest results of Longevity studies on the present CMS RPC system for HL-LHC phase

The present Compact Muon Solenoid Resistive Plate Chambers system has been worked efficiently during Run I and Run II of data taking period (Shah et al., 2020) [1]. In the coming years of operation with the High Luminosity LHC (HL-LHC), the expected rate and integrated charge are expected to be about 600 Hz/cm2 and 840 mC/cm2, respectively (including a safety factor of three). Therefore, the HL-LHC phase will be a challenge for the RPC system since the expected operating conditions are much harsher than those for which the detectors have been designed, and could introduce non-recoverable aging effects which can alter the detector properties. A longevity test has been started at the CERN Gamma Irradiation Facility to estimate the impact of HL-LHC conditions on the RPC detector performance in order to determine whether the RPC system will survive the harsher background conditions expected at HL-LHC. The latest results of the irradiation test will be presented.

Right-handed neutrino pair production via second-generation leptoquarks

No direct experimental constraints exist on Leptoquark (LQ) couplings with quarks and right-handed neutrinos (RHNs). If a LQ dominantly couples to RHNs, it can leave unique signatures at the LHC. The RHNs can be produced copiously from LQ decays as long as they are lighter than the LQs. LQ-induced RHN production has never been searched for in experiments. This channel can act as a simultaneous probe for RHNs and LQs that dominantly couple to RHNs. In this paper, we consider all possible charge-2/3 and 1/3 scalar and vector LQs that dominantly couple to second-generation quarks and RHN. We study the pair and single productions of TeV-scale LQs and their subsequent decay to sub-TeV RHNs, realised in the inverse seesaw framework. We also consider RHN pair production through a t-channel LQ exchange. The single LQ production and t-channel contributions can be significant for large LQ-RHN-quark couplings. We systematically combine events from these processes leading to a pair of RHNs plus jets to study the prospects of LQ-assisted RHN pair production. We analyse the monolepton and opposite-sign dilepton final states and estimate the discovery reach at the high-luminosity LHC.

Probing the muon (g − 2) anomaly at the LHC in final states with two muons and two taus

The longstanding muon (g−2) anomaly, as well as some hints of lepton flavor universality violation in B-meson decays, could be signaling new physics beyond the Standard Model (SM). A minimal R-parity-violating supersymmetric framework with light third-generation sfermions (dubbed as ‘RPV3’) provides a compelling solution to these flavor anomalies, while simultaneously addressing other pressing issues of the SM. We propose a new RPV3 scenario for the solution of the muon (g−2) anomaly, which leads to an interesting LHC signal of μ+μ−τ+τ− final state. We analyze the Run-2 LHC multilepton data to derive stringent constraints on the sneutrino mass and the relevant RPV coupling in this scenario. We then propose dedicated selection strategies to improve the bound even with the existing dataset. We also show that the high-luminosity LHC will completely cover the remaining muon (g−2)-preferred parameter space, thus providing a robust, independent test of the muon (g−2) anomaly.

Adiabatic quantum algorithm for multijet clustering in high energy physics

The currently predicted increase in computational demand for the upcoming High-Luminosity Large Hadron Collider (HL-LHC) event reconstruction, and in particular jet clustering, is bound to challenge present day computing resources, becoming an even more complex combinatorial problem. In this paper, we show that quantum annealing can tackle dijet event clustering by introducing a novel quantum annealing binary clustering algorithm. The benchmarked efficiency is of the order of 96%, thus yielding substantial improvements over the current quantum state-of-the-art. Additionally, we also show how to generalize the proposed objective function into a more versatile form, capable of solving the clustering problem in multijet events.

Collider constraints on electroweakinos in the presence of a light gravitino

Using the GAMBIT global fitting framework, we constrain the MSSM with an eV-scale gravitino as the lightest supersymmetric particle, and the six electroweakinos (neutralinos and charginos) as the only other light new states. We combine 15 ATLAS and 12 CMS searches at 13 TeV, along with a large collection of ATLAS and CMS measurements of Standard Model signatures. This model, which we refer to as the {{tilde{G}}}-EWMSSM, exhibits quite varied collider phenomenology due to its many permitted electroweakino production processes and decay modes. Characteristic {{tilde{G}}}-EWMSSM signal events have two or more Standard Model bosons and missing energy due to the escaping gravitinos. While much of the {{tilde{G}}}-EWMSSM parameter space is excluded, we find several viable parameter regions that predict phenomenologically rich scenarios with multiple neutralinos and charginos within the kinematic reach of the LHC during Run 3, or the High Luminosity LHC. In particular, we identify scenarios with Higgsino-dominated electroweakinos as light as 140 GeV that are consistent with our combined set of collider searches and measurements. The full set of {{tilde{G}}}-EWMSSM parameter samples and GAMBIT input files generated for this work is available via Zenodo.

System Design and Prototyping for the CMS Level-1 Trigger at the High-Luminosity LHC

The High-Luminosity Large Hadron Collider (HL-LHC) is planned to offer a very ambitious physics program, with high-precision measurement and evaluation of the Standard Model (SM), and motivate the searches for new physics. The efficient data collection and precise events reconstruction in the harsh environment of 200 proton-proton interactions are vital for achieving the success of the HL-LHC program. To realize these requirements, the CMS detector has planned to completely replace the data acquisition (DAQ) and trigger system. The CMS Level-1 trigger system will handle the enormous detector input bandwidth of 63 Tbps with the maximum output rate of 750 kHz and is desired to complete the processing within 12.5 μs. For this purpose, CMS has planned to replace the Phase-1 μTCA-based processor boards and crates with an ATCA form factor. Each ATCA board will host Xilinx large UltraScale/UltraScale+ family FPGA and support more than a hundred high-speed optical links (~28 Gbps), capable of meeting the high bandwidth and processing requirements of HL-LHC. Along with the advancement in hardware, the Level-1 trigger system will employ highly modular, flexible, and adequately sophisticated algorithms that were only possible in offline reconstruction, such as the particle-flow algorithm. The modular and flexible architecture will help address the HL-LHC dynamic physics requirements. We will discuss the details of system design, prototyping, and the algorithms employed for the Level-1 trigger system.

Charged Higgs induced 5 and 6 lepton signatures from heavy neutrinos at the LHC

We propose an anomaly free gauged U(1) extension of the SM where three right handed heavy neutrinos, being charged under the general U(1) gauge group, are introduced to explain the origin of the tiny neutrino mass through the seesaw mechanism after the general U(1) symmetry is broken. Due to the breaking of the general U(1) symmetry a neutral beyond the standard model gauge boson Z^prime acquires mass. There are two Higgs doublets in this model where one interacts with the SM fermions and the other one interacts with the right handed heavy neutrinos and charged leptons. The charged multiplet of the second Higgs can completely decay into the heavy neutrinos and charged lepton in the neutrinophilic limit of the model parameters. The charged Higgs pair production can be influenced due to presence of the Z^prime boson at the High Luminosity LHC (HL-LHC) in addition to the neutral SM gauge bosons. The pair produced charged Higgs bosons decay into SM charged leptons and heavy neutrinos. Following the leading decay modes of the heavy neutrinos into charged leptons and W boson we study the 5 and 6 lepton final states after the leptonic and hadronic decay of the W bosons considering solely muons and electrons in the final state. Combining the electron and muon final states we estimate the significance of the 5 and 6 charged lepton processes in the m_{H^pm }-m_N plane for different benchmark points of m_{Z^prime }. It is found that the 5 (6) charged lepton processes could be probed at the High Luminosity LHC (HL-LHC) with at least 5 (3) sigma significance and there are parameter regions where the significance could be larger.

Characterization of the response of IHEP-IME LGAD with shallow carbon to Gamma Irradiation

Low Gain Avalanche Detectors (LGAD) for the High-Granularity Timing Detector (HGTD) are crucial in reducing pileups in the High-Luminosity Large Hadron Collider. Numerous studies have been conducted on the bulk irradiation damage of LGADs. However, few studies have been carried out on the surface irradiation damage of LGAD sensors with shallow carbon implantation. In this paper, the IHEP-IME LGADs with shallow carbon implantation were irradiated up to 2 MGy using gamma irradiation to investigate surface damage. Important characteristic parameters, including leakage currents, breakdown voltage (BV), inter-pad resistances, and capacitances, were tested before and after irradiation. The results showed that the leakage current and BV increased after irradiation, whereas overall inter-pad resistance exhibited minimal change and remained above 109 Ω before and after irradiation. Capacitance was found to be less than 4.5 pF with a slight decrease in the gain layer depletion voltage (V gl ) after irradiation. No parameter affected by the inter-pad separation was observed before and after irradiation. All characteristic parameters meet the requirements of HGTD, and this design can be used to further optimization.

Machine learning-enhanced search for a vectorlike singlet B quark decaying to a singlet scalar or pseudoscalar

The presence of a new decay mode relaxes the current mass exclusion limits on vectorlike quarks considerably. We consider the case of a weak-singlet vectorlike $B$ quark that can decay to a singlet scalar or pseudoscalar $\Phi$. In an earlier paper [A. Bhardwaj et al., Roadmap to explore vectorlike quarks decaying to a new scalar or pseudoscalar, Phys. Rev. D, 106 (2022) 095014; arXiv:2203.13753], we mapped the possibilities to explore such setups at the LHC. We showed that it is possible for a $B$ quark to decay into $\Phi$ and the $\Phi$ to dominantly decay to a pair of gluons or $b$ quark(s) without fine-tuning the parameters. In this paper, we present a collider search strategy to look for the pair production of singlet $B$ quarks. If both $B$ quarks decay into $b\Phi$ pairs, the final state is fully hadronic: $B{B}\to(b\Phi)({b}\Phi)\to (bgg)({b}gg)/(bb{b})({b}b{b})$, which is very challenging to probe. Therefore, we consider a simpler mixed decay specific to the singlet $B$ case, $BB\to(b\Phi)(tW)$ with a lepton in the final state, to achieve the best sensitivity at the high-luminosity LHC. We use a deep neural network with a weighted cross-entropy loss to separate the signal from the huge SM background. Our analysis shows that large areas of the $M_{B}-M_{\Phi}$ parameter space are discoverable through this signature. We show how the discovery and exclusion regions scale with the branching ratio in the new decay mode. We also estimate the reach in the inclusive monolepton channel with the same network model.

Progress in Design and Testing of the DAQ and Data-Flow Control for the Phase-2 Upgrade of the CMS Experiment

The second phase of the LHC, the High-Luminosity LHC, is scheduled to start in 2029, after a shutdown during which the beam intensity and focusing will be significantly upgraded. For this HL-LHC era, also the CMS detector will receive an extensive upgrade, primarily to maintain its physics performance at increasing pileup. The Phase-2 CMS Level-1 trigger rate will increase to 750 kHz, for an estimated data rate in excess of 50 Tbit/s. The Phase-2 CMS off-detector electronics will be based on the ATCA standard, with back-end boards receiving the detector data from the on-detector front-ends via custom, radiation-tolerant, optical links. The CMS Phase-2 data acquisition design tightens the integration between trigger control and data flow, extending the synchronous regime of the DAQ system. At the core of the design is the DAQ and Timing Hub, a custom ATCA hub card forming the bridge between the different, detector-specific, control and readout electronics and the common timing, trigger, and control systems. The overall synchronisation and data flow of the experiment is handled by the Trigger and Timing Control and Distribution System. For increased flexibility during commissioning and calibration runs, the design of the Phase-2 trigger and timing distribution system breaks with the traditional distribution tree, in favour of a configurable network connecting multiple independent control units to all off-detector endpoints. In order to reduce the number of custom hardware designs required, the DAQ hardware is designed such that it can also be used to implement the Trigger and Timing Control and Distribution System.

Study of discharges in the CMS GEM GE1/1 station with LHC beam

The Large Hadron Collider (LHC) and the experiments installed there are being upgraded for the High Luminosity LHC project that will provide a greatly enlarged data sample in the search for physics beyond the Standard Model. During Long Shutdown 2, from 2018 to the first months of 2022, some of these upgrades were completed. In particular, the muon spectrometer of the CMS experiment was upgraded with installation of the GE1/1 detector station, based on Gas Electron Multiplier (GEM) technology. This station is positioned in the endcap region of the CMS muon system and covers the pseudorapidity range 1.55 < |η| < 2.18. On the 5th of July, 2022, Run 3 of the LHC began with collision energy of 13.6 TeV. The newly installed GEM detectors experienced high-voltage discharges when the beams were colliding inside CMS. To monitor these events and to understand how to safely operate the detectors, a study of these phenomena became necessary. The intensity of discharge current and the discharge rate were recorded in different detectors as a function of luminosity and time. It was observed that some chambers have a significantly higher discharge rate than the modal value of rate over the entire chamber population. The reasons responsible for this behavior are currently under investigation. In this contribution, an analysis of discharges will be presented, illustrating the dependence of discharge rate on different LHC beam configurations and the high voltage working point of the chambers.