High Luminosity Phase Research Articles

Effective field theory for vector-like leptons and its collider signals

We argue that in models with several high scales; e.g. in composite Higgs models or in gauge extensions of the Standard Model (SM), vector-like leptons can be likely produced in a relatively large s region of the phase space. Likewise, they can easily decay into final states not containing SM gauge bosons. This contrasts with the topology in which these new particles are being searched for at the LHC. Adopting an effective field theory approach, we show that searches for excited leptons must be used instead to test this scenario. We derive bounds on all the relevant interactions of dimension six; the most constrained ones being of about 0.05 TeV−2. We build new observables to improve current analyses and study the impact on all single-field UV completions of the SM extended with a vector-like lepton that can be captured by the effective field theory at tree level, in the current and in the high-luminosity phase of the LHC.

Development of planar pixel sensors for the CMS Inner Tracker at the High-Luminosity LHC

The LHC is planning an upgrade program which will bring the luminosity up to about 7.5×1034cm−2s−1 in 2027, with the goal of delivering an integrated luminosity of 3000 or even 4000 fb−1 by the end of 2037. This High Luminosity phase, HL-LHC, will present new challenges of higher data rates and unprecedented radiation levels for the pixel detector. A fluence of 2.3×1016neq/cm2, or equivalently 12 MGy, is expected for the inner layer of the CMS Inner Tracker (IT) for 3000 fb−1 of integrated luminosity. To maintain or even improve the performance of the present system, new technologies have to be exploited for the so-called Phase-2 upgrade. Among them is the future version of front-end chips in 65 nm CMOS technology by the CERN RD53 Collaboration, which supports small pixel sizes of 50 × 50 or 100×25μm2 and low pixel charge thresholds (≈1000 e−). Thin planar n-in-p type silicon sensors with a thickness of the active layer of 150 μm, segmented into pixel sizes of 100×25μm2 or 50×50μm2, will be used throughout most of the IT. They have been shown to allow for a good detector resolution that is much more stable with respect to radiation damage compared to the Phase-1 detector. CMS has launched several R&D submissions for the development of suitable planar silicon sensors at Hamamatsu Photonics K.K. and FBK Trento. We present results for measurements on such prototype sensors bump bonded to the RD53A prototype chip developed by the RD53 Collaboration at CERN. Different pixel cell designs are compared and evaluated in beam tests at CERN, DESY and FNAL for spatial resolution and hit efficiency at various track angles before and after irradiation. As an example, hit efficiencies of 99% at vertical incidence were reached after irradiation to 5×1015neq/cm2, which corresponds to the layer 2 lifetime fluence of the CMS IT.

Radiation resistant innovative 3D pixel sensors for the CMS upgrade at the High Luminosity LHC

Results of an extensive R&D program aiming at radiation hard, small pitch, 3D pixel sensors are reported. The CMS experiment is supporting this R&D in the scope of the Inner Tracker upgrade for the High Luminosity phase of the CERN Large Hadron Collider (HL-LHC). In the HL-LHC the Inner Tracker will have to withstand an integrated fluence up to 2.3×1016neq/cm2. A small number of 3D sensors were interconnected with the RD53A readout chip, which is the first prototype of 65 nm CMOS pixel readout chip designed for the HL-LHC pixel trackers. In this paper results obtained in beam tests before and after irradiation are reported. The irradiation of a single chip module was performed up to a maximum equivalent fluence of about 1×1016neq/cm2. The analysis of the collected data shows excellent performance: the spatial resolution in not irradiated sensors can reach about 3 to 5 μm, for inclined tracks, depending on the pixel pitch. The measured hit detection efficiencies are close to 99% measured both before and after the above mentioned irradiation fluence.

Lepton flavor violation and dilepton tails at the LHC

Starting from a general effective Lagrangian for lepton flavor violation (LFV) in quark-lepton transitions, we derive constraints on the effective coefficients from the high-mass tails of the dilepton processes pp rightarrow ell _k ell _l (with kne l). The current (projected) limits derived in this paper from LHC data with 36~mathrm {fb}^{-1} (3~mathrm {ab}^{-1}) can be applied to generic new physics scenarios, including the ones with scalar, vector and tensor effective operators. For purely left-handed operators, we explicitly compare these LHC constraints with the ones derived from flavor-physics observables, illustrating the complementarity of these different probes. While flavor physics is typically more constraining for quark-flavor violating operators, we find that LHC provides the most stringent limits on several flavor-conserving ones. Furthermore, we show that dilepton tails offer the best probes for charm-quark transitions at current luminosities and that they provide competitive limits for tauonic brightarrow d transitions at the high-luminosity LHC phase. As a by-product, we also provide general numerical expressions for several low-energy LFV processes, such as the semi-leptonic decays Krightarrow pi ell ^{pm }_k ell ^{{mp }}_l, Brightarrow pi ell ^{pm }_k ell ^{{mp }}_l and Brightarrow K^{(*)} ell ^{pm }_k ell ^{{mp }}_l.

Performance of a Front End prototype ASIC for picosecond precision time measurements with LGAD sensors

For the High-Luminosity phase of LHC, the ATLAS experiment is proposing the addition of a High Granularity Timing Detector (HGTD) in the forward region, to mitigate the effects of the increased pile-up. The chosen detection technology is Low Gain Avalanche Detector (LGAD) silicon sensors that can provide an excellent timing resolution below 50 ps. The front-end read-out ASIC must exploit the large signal derivative and small noise provided by the sensor, while keeping low power consumption. This paper presents the results on the first prototype of a front-end ASIC, named ALTIROC0, which contains the analog stages (preamplifier and discriminator) of the read-out chip. The ASIC was characterised both alone and as part of a module with a 2×2 LGAD array of 1.1×1.1 mm2 pads bump-bonded to it. The various contributions of the electronics to the time resolution were investigated in test-bench measurements with a calibration setup. Both when the ASIC is alone or with a bump-bonded sensor, the jitter of the ASIC is better than 20 ps for an injected charge of 10 fC . The time walk effect, which arises from the different preamplifier response for various injected charges, can be corrected up to 10 ps using a Time Over Threshold measurement. The combined performance of the ASIC and the LGAD sensor, which was measured during a beam test campaign in October 2018 with pions of 120 GeV energy at the CERN SPS, is around 40 ps for all measured modules. All tested modules show good efficiency and time resolution uniformity.

Performance of prototype GE1[formula omitted]1 chambers for the CMS muon spectrometer upgrade

The high-luminosity phase of the Large Hadron Collider (HL-LHC) will result in ten times higher particle background than measured during the first phase of LHC operation. In order to fully exploit the highly-demanding operating conditions during HL-LHC, the Compact Muon Solenoid (CMS) Collaboration will use Gas Electron Multiplier (GEM) detector technology. The technology will be integrated into the innermost region of the forward muon spectrometer of CMS as an additional muon station called GE1∕1. The primary purpose of this auxiliary station is to help in muon reconstruction and to control level-1 muon trigger rates in the pseudo-rapidity region 1.6≤|η|≤2.2. The new station will contain trapezoidal-shaped GEM detectors called GE1∕1 chambers. The design of these chambers is finalized, and the installation is in progress during the Long Shutdown phase two (LS-2) that started in 2019. Several full-size prototypes were built and operated successfully in various test beams at CERN. We describe performance measurements such as gain, efficiency, and time resolution of these prototype chambers, developed after years of R&D, and summarize their behavior in different gas compositions as a function of the applied voltage.

Performance study of HGCROC-v2: the front-end electronics for the CMS High Granularity Calorimeter

The High Granularity Calorimeter (HGCAL), presently being designed by the Compact Muon Solenoid collaboration (CMS) to replace the existing endcap calorimeters for the High Luminosity phase of the LHC (HL-LHC), will feature unprecedented transverse and longitudinal readout and triggering segmentation for both electromagnetic and hadronic sections. The requirements for the front-end electronics are extremely challenging, including high dynamic range (0–10 pC), low noise (0∼ 200 electrons), high-precision timing information in order to mitigate the pileup effect (25 ps binning) and low power consumption (∼ 15 mW/channel). The front-end electronics will face a harsh radiation environment which will reach 200 Mrad at the end of life. It will work at a controlled temperature of 240 K. HGCROC-v2 is the second prototype of the front-end ASIC. It has 72 channels of the full analog chain: low noise and high gain preamplifier and shapers, and a 10-bit 40 MHz SAR-ADC, which provides the charge measurement over the linear range of the preamplifier. In the saturation range of the preamplifier, a discriminator and TDC provide the charge information from TOT (Time Over Threshold) over 200 ns dynamic range using 50 ps binning. A fast discriminator and TDC provide timing information to 25 ps accuracy. Both charge and timing information are kept in a DRAM memory waiting for a Level 1-trigger decision (L1A). At a bunch crossing rate of 40 MHz, compressed charge data are sent out to participate in the generation of the L1-trigger primitives. We report on the performances of the chip in terms of signal-to-noise ratio, charge and timing, as well as results from radiation qualification with total ionizing dose (TID).

Experimental study of different silicon sensor options for the upgrade of the CMS Outer Tracker

During the high-luminosity phase of the LHC (HL-LHC), planned to start in 2027, the accelerator is expected to deliver an instantaneous peak luminosity of up to 7.5×1034 cm−2 s−1. A total integrated luminosity of 0300 or even 0400 fb−1 is foreseen to be delivered to the general purpose detectors ATLAS and CMS over a decade, thereby increasing the discovery potential of the LHC experiments significantly. The CMS detector will undergo a major upgrade for the HL-LHC, with entirely new tracking detectors consisting of an Outer Tracker and Inner Tracker. However, the new tracking system will be exposed to a significantly higher radiation than the current tracker, requiring new radiation-hard sensors. CMS initiated an extensive irradiation and measurement campaign starting in 2009 to systematically compare the properties of different silicon materials and design choices for the Outer Tracker sensors. Several test structures and sensors were designed and implemented on 18 different combinations of wafer materials, thicknesses, and production technologies. The devices were electrically characterized before and after irradiation with neutrons, and with protons of different energies, with fluences corresponding to those expected at different radii of the CMS Outer Tracker after 0300 fb−1. The tests performed include studies with β sources, lasers, and beam scans. This paper compares the performance of different options for the HL-LHC silicon sensors with a focus on silicon bulk material and thickness.

EFT triangles in the same-sign WW scattering process at the HL-LHC and HE-LHC

We investigate the Beyond Standard Model discovery potential in the framework of the effective field theory (EFT) for the same-sign WW scattering process in purely leptonic W decay modes at the High-Luminosity and High-Energy phases of the Large Hadron Collider (LHC). The goal of this paper is to examine the applicability of the EFT approach, with one dimension-8 operator varied at a time, to describe a hypothetical new physics signal in the WWWW quartic coupling. In the considered process there is no experimental handle on the WW invariant mass, and it has previously been shown that the discovery potential at 14 TeV is rather slim. In this paper we report the results calculated for a 27 TeV machine and compare them with the discovery potential obtained at 14 TeV. We find that while the respective discovery regions shift to lower values of the Wilson coefficients, the overall discovery potential of this procedure does not get significantly larger with a higher beam energy.

The electro-weak couplings of the top and bottom quarks \u2014 Global fit and future prospects

We evaluate the implications of LHC and LEP/SLC measurements for the electro-weak couplings of the top and bottom quarks. We derive global bounds on the Wilson coefficients of ten two-fermion operators in an effective field theory description. The combination of hadron collider data with Z -pole measurements is found to yield tight limits on the operator coefficients that modify the left-handed couplings of the bottom and top quark to the Z boson. We also present projections for the high-luminosity phase of the LHC and for future electron-positron colliders. The bounds on the operator coefficients are expected to improve substantially during the remaining LHC programme, by factors of 1 to 5 if systematic uncertainties are scaled as statistical ones. The operation of an e+e− collider at a center-of-mass energy above the top-quark pair production threshold is expected to further improve the bounds by one to two orders of magnitude. The combination of measurements in pp and e+e− collisions allows for a percent-level determination of the top-quark Yukawa coupling, that is robust in a global fit.

Light hidden mesons through the Z portal

Confining hidden sectors are an attractive possibility for physics beyond the Standard Model (SM). They are especially motivated by neutral naturalness theories, which reconcile the lightness of the Higgs with the strong constraints on colored top partners. We study hidden QCD with one light quark flavor, coupled to the SM via effective operators suppressed by the mass M of new electroweak-charged particles. This effective field theory is inspired by a new tripled top model of supersymmetric neutral naturalness. The hidden sector is accessed primarily via the Z and Higgs portals, which also mediate the decays of the hidden mesons back to SM particles. We find that exotic Z decays at the LHC and future Z factories provide the strongest sensitivity to this scenario, and we outline a wide array of searches. For a larger hidden confinement scale Λ ∼ O (10) GeV, the exotic Z decays dominantly produce final states with two hidden mesons. ATLAS and CMS can probe their prompt decays up to M ∼ 3 TeV at the high luminosity phase, while a TeraZ factory would extend the reach up to M ∼ 20 TeV through a combination of searches for prompt and displaced signals. For smaller Λ ∼ O (1) GeV, the Z decays to the hidden sector produce jets of hidden mesons, which are long-lived. LHCb will be a powerful probe of these emerging jets. Furthermore, the light hidden vector meson could be detected by proposed dark photon searches.

A High-Granularity Timing Detector (HGTD) for the Phase-II upgrade of the ATLAS detector

The expected increase of the particle flux at the high-luminosity phase of the LHC (HL-LHC) with instantaneous luminosities up to L = 7.5× 1034 cm−1s−1 will have a severe impact on the ATLAS detector performance. The pile-up is expected to increase on average to 200 interactions per bunch crossing. The reconstruction and trigger performance for electrons, photons as well as jets and transverse missing energy will be severely degraded in the end-cap and forward region, where the liquid Argon based electromagnetic calorimeter has coarser granularity and the inner tracker has poorer momentum resolution compared to the central region. A High Granularity Timing Detector (HGTD) is proposed in front of the liquid Argon end-cap calorimeters for pile-up mitigation and for bunch per bunch luminosity measurements. This device should cover the pseudo-rapidity range of 2.4 to about 4.0. Two Silicon sensors double-sided layers are foreseen to provide a precision timing information for minimum ionizing particle with a time resolution better than 50 pico-seconds per hit (i.e. 30 pico-seconds per track) in order to assign the particle to the correct vertex. Each readout cell has a transverse size of 1.3 mm×1.3 mm leading to a highly granular detector with about 3 millions of readout electronics channels. Low-Gain Avalanche Detector (LGAD) technology has been chosen as it provides an internal gain good enough to reach large signal over noise ratio needed for excellent time resolution. Extensive LGAD research and development (R&D) campaigns are carried out to investigate the suitability of this new technology as timing sensors for HGTD. The related readout ASIC is also being studied extensively.

RPC radiation background simulations for the high luminosity phase in the CMS experiment

The high luminosity expected from the HL-LHC will be a challenge for the CMS detector. The increased rate of particles coming from the collisions and the radioactivity induced in the detector material could cause significant damage and result in a progressive degradation of its performance. Simulation studies are very useful in these scenarios as they allow one to study the radiation environment and the impact on detector performance. Results are presented for CMS RPC stations considering the operating conditions expected at the HL-LHC.

Learning representations of irregular particle-detector geometry with distance-weighted graph networks

We explore the use of graph networks to deal with irregular-geometry detectors in the context of particle reconstruction. Thanks to their representation-learning capabilities, graph networks can exploit the full detector granularity, while natively managing the event sparsity and arbitrarily complex detector geometries. We introduce two distance-weighted graph network architectures, dubbed GarNet and GravNet layers, and apply them to a typical particle reconstruction task. The performance of the new architectures is evaluated on a data set of simulated particle interactions on a toy model of a highly granular calorimeter, loosely inspired by the endcap calorimeter to be installed in the CMS detector for the High-Luminosity LHC phase. We study the clustering of energy depositions, which is the basis for calorimetric particle reconstruction, and provide a quantitative comparison to alternative approaches. The proposed algorithms provide an interesting alternative to existing methods, offering equally performing or less resource-demanding solutions with less underlying assumptions on the detector geometry and, consequently, the possibility to generalize to other detectors.

Top quark anomalous couplings at the high-luminosity phase of the LHC

The combination of the latest and most precise measurements of several top quark properties is presented in this paper in order to establish allowed regions on anomalous contributions to the Lorentz structure of the Wtb vertex. These measurements include single top production cross-sections, W boson helicity fractions and forward–backward asymmetries, both at Tevatron and at the Large Hadron Collider (LHC), up to a center-of-mass energy of 13 TeV. The results obtained at 95% Confidence Level (CL) for the top quark anomalous couplings are compared with the limits extracted from a combination that includes the expected measurements at the future High-Luminosity run of the LHC.

First results on 3D pixel sensors interconnected to the RD53A readout chip after irradiation to 1×1016 neq cm−2

Results obtained with 3D columnar pixel sensors bump-bonded to the RD53A prototype readout chip are reported. The interconnected modules have been tested in a hadron beam before and after irradiation to a fluence of about 1×1016 neq cm−2 (1 MeV equivalent neutrons). All presented results are part of the CMS R&D activities in view of the pixel detector upgrade for the High Luminosity phase of the LHC at CERN (HL-LHC) . A preliminary analysis of the collected data shows hit detection efficiencies around 97% measured after proton irradiation.

B anomalies and dark matter: a complex connection

We study an extension of the Standard Model that addresses the hints of lepton flavour universality violation observed in Brightarrow K^{(*)} l^+l^- decays at LHCb, while providing a viable candidate for dark matter. The model incorporates two new scalar fields and a Majorana fermion that induce one-loop contributions to B meson decays. We show that agreement with observational data requires the new couplings to be complex and that the Majorana fermion can reproduce the observed dark matter relic density. This combination of cosmological and flavour constraints sets an upper limit on the dark matter and mediator masses. We have studied LHC dijet and dilepton searches, finding that they rule out large regions of parameter space by setting lower bounds on the dark matter and mediator masses. In particular, dilepton bounds are much more constraining in a future high-luminosity phase. Finally, we have computed the scattering cross section of dark matter off nuclei and compared it to the sensitivity of current and future direct detection experiments, showing that parts of the parameter space could be accessible in the future to multi-ton experiments. Future collider and direct DM searches complement each other to probe large areas of the parameter space of this model.

The CMS ECAL Phase-2 upgrade for high precision energy and timing measurements

The CMS electromagnetic calorimeter (ECAL) is a homogeneous calorimeter made of about 75,000 lead tungstate scintillating crystals. In view of the high-luminosity phase of the LHC, the ECAL electronics must be upgraded to cope with the more stringent requirements in terms of trigger latency and rate. The new electronics will transmit the data in streaming mode from the front-end electronics to the off-detector electronics, where the trigger primitives will be formed in powerful FPGAs. The front-end electronics will feature two new radiation-hard chips: a dual gain trans-impedance amplifier (TIA) and a sampling ADC with loss-less data compression. The TIA choice preserves the fast pulse shape of signals in the lead tungstate coupled to avalanche photodiodes (APD), and it is more resilient to the noise increase due to the radiation-induced APD leakage current. An important characteristic of the new design will be the capability to provide precision timing measurements, of the order of 30ps, for photons and electrons above 50GeV. The excellent time resolution will improve the overall CMS physics performance by mitigating the high pile-up effects. First characterization results of the TIA chip will be shown, and studies of energy and timing resolution performed in beam tests with the prototype electronics will be presented.

Performance of the CMS RPC upgrade using 2D fast timing readout system

We present a new generation of Resistive Plate Chambers (RPC) that are able to withstand high particle fluxes (up to 2 kHz cm−2). These chambers will be instrumented with a precise timing readout electronics and they are proposed to equip two of the four high eta stations of the CMS muon system.Two-gap RPC detectors, with each gap made of two 1.4 mm High Pressure Laminate (HPL) electrodes and separated by a gas gap of the same thickness, are proposed. The new scheme reduces the amount of the avalanche charge produced by the passage of a charged particle through the detector. This improves the RPC rate capability by reducing the needed time to absorb this charge. To keep the RPC efficiency high a sensitive, low-noise and high time resolution Front-End Electronics Board (FEB) is needed to cope with the low charge signal. An ASIC called PETIROC that has all these characteristics is proposed to read out the new chambers.A thin (0.6 mm) Printed Circuit Board (PCB), 165 cm long, equipped with pickup strips of average pitch of 0.75 cm is inserted between the two RPC gaps. The strips are read out from both ends and the arrival time difference of the two signals is used to determine the particle position along the strip (η position). The absolute time measurement will also be used to reduce the data ambiguity due to the expected high pileup at the High-Luminosity phase of the Large Hadron Collider (HL-LHC).

Mono-τ Signatures at the LHC Constrain Explanations of B-decay Anomalies.

We investigate the crossing-symmetry relation between b→cτ^{-}ν[over ¯] decay and bc[over ¯]→τ^{-}ν[over ¯] scattering to derive direct correlations of new physics in semitauonic B-meson decays and the mono-tau signature at the LHC (pp→τ_{h}X+MET). Using an exhaustive set of effective operators and heavy mediators we find that the current ATLAS and CMS data constrain scenarios addressing anomalies in B decays. Pure tensor solutions, completed by leptoquark, and right-handed solutions, completed by W_{R}^{'} or leptoquark, are challenged by our analysis. Furthermore, the sensitivity that will be achieved in the high-luminosity phase of the LHC will probe all the possible scenarios that explain the anomalies. Finally, we note that the LHC is also competitive in the b→u transitions and bounds in some cases are currently better than those from B decays.