High Luminosity Phase Of The LHC Research Articles

Sterile neutrino searches via displaced vertices at LHCb

We explore the sensitivity of displaced vertex searches at LHCb for testing sterile neutrino extensions of the Standard Model towards explaining the observed neutrino masses. We derive estimates for the constraints on sterile neutrino parameters from a recently published displaced vertex search at LHCb based on run 1 data. They yield the currently most stringent limit on active–sterile neutrino mixing in the sterile neutrino mass range between 4.5 GeV and 10 GeV. Furthermore, we present forecasts for the sensitivities that could be obtained from the run 2 data and also for the high-luminosity phase of the LHC.

On the light massive flavor dependence of the large order asymptotic behavior and the ambiguity of the pole mass

We provide a systematic renormalization group formalism for the mass effects in the relation of the pole mass mQpole and short-distance masses such as the overline{mathrm{MS}} mass {overline{m}}_Q of a heavy quark Q, coming from virtual loop insertions of massive quarks lighter than Q. The formalism reflects the constraints from heavy quark symmetry and entails a combined matching and evolution procedure that allows to disentangle and successively integrate out the corrections coming from the lighter massive quarks and the momentum regions between them and to precisely control the large order asymptotic behavior. With the formalism we systematically sum logarithms of ratios of the lighter quark masses and mQ, relate the QCD corrections for different external heavy quarks to each other, predict the mathcal{O}left({alpha}_s^4right) virtual quark mass corrections in the pole- overline{mathrm{MS}} mass relation, calculate the pole mass differences for the top, bottom and charm quarks with a precision of around 20 MeV and analyze the decoupling of the lighter massive quark flavors at large orders. The summation of logarithms is most relevant for the top quark pole mass mtpole, where the hierarchy to the bottom and charm quarks is large. We determine the ambiguity of the pole mass for top, bottom and charm quarks in different scenarios with massive or massless bottom and charm quarks in a way consistent with heavy quark symmetry, and we find that it is 250 MeV. The ambiguity is larger than current projections for the precision of top quark mass measurements in the high-luminosity phase of the LHC.

Optimization of thin n-in-p planar pixel modules for the ATLAS upgrade at HL-LHC

The ATLAS experiment will undergo around the year 2025 a replacement of the tracker system in view of the high luminosity phase of the LHC (HL-LHC) with a new 5-layer pixel system. Thin planar pixel sensors are promising candidates to instrument the innermost region of the new pixel system, thanks to the reduced contribution to the material budget and their high charge collection efficiency after irradiation. The sensors of 50-150 μm thickness, interconnected to FE-I4 read-out chips, have been characterized with radioactive sources and beam tests. In particular active edge sensors have been investigated. The performance of two different versions of edge designs are compared: the first with a bias ring, and the second one where only a floating guard ring has been implemented. The hit efficiency at the edge has also been studied after irradiation at a fluence of 1015 neq/cm2. Highly segmented sensors will represent a challenge for the tracking in the forward region of the pixel system at HL-LHC. In order to reproduce the performance of 50x50 μm2 pixels at high pseudo-rapidity values, FE-I4 compatible planar pixel sensors have been studied before and after irradiation in beam tests at high incidence angles with respect to the short pixel direction. Results on the hit efficiency in this configuration are discussed for different sensor thicknesses.

The long journey to the Higgs boson and beyond at the LHC: Emphasis on CMS

Since 2010 there has been a rich harvest of results on standard model physics by the ATLAS and CMS experiments operating on the Large Hadron Collider. In the summer of 2012, a spectacular discovery was made by these experiments of a new, heavy particle. All the subsequently analysed data point strongly to the properties of this particle as those expected for the Higgs boson associated with the Brout–Englert–Higgs mechanism postulated to explain the spontaneous symmetry breaking in the electroweak sector, thereby explaining how elementary particles acquire mass. This article focuses on the CMS experiment, the technological challenges encountered in its construction, describing some of the physics results obtained so far, including the discovery of the Higgs boson, and searches for the widely anticipated new physics beyond the standard model, and peer into the future involving the high-luminosity phase of the LHC. This article is complementary to the one by Peter Jenni4 that focuses on the ATLAS experiment.

4D fast tracking for experiments at high luminosity LHC

The full exploitation of the physics potential of the high luminosity LHC is a big challenge that requires new instrumentation and innovative solutions. We present here a conceptual design and simulation studies of a fast timing pixel detector with embedded real-time tracking capabilities. The system is conceived to operate at 40 MHz event rate and to reconstruct tracks in real-time, using precise space and time 4D information of the hit, for fast trigger decisions. This work is part of an R&D project aimed at building an innovative tracking detector with superior time (10 ps) and position (10 μm) resolutions to be used in very harsh radiation environments, for the ultimate flavour physics experiment at the high luminosity phase of the LHC.

Test beam results of the first CMS double-sided strip module prototypes using the CBC2 read-out chip

The CMS Binary Chip (CBC) is a prototype version of the front-end read-out ASIC to be used in the silicon strip modules of the CMS outer tracking detector during the high luminosity phase of the LHC. The CBC is produced in 130nm CMOS technology and bump-bonded to the hybrid of a double layer silicon strip module, the so-called 2S-pT module. It has 254 input channels and is designed to provide on-board trigger information to the first level trigger system of CMS, with the capability of cluster-width discrimination and high-pT track identification. In November 2013 the first 2S-pT module prototypes equipped with the CBC chips were put to test at the DESY-II test beam facility. Data were collected exploiting a beam of positrons with an energy ranging from 2 to 4GeV. In this paper the test setup and the results are presented.

Thin n-in-p planar pixel modules for the ATLAS upgrade at HL-LHC

The ATLAS experiment will undergo a major upgrade of the tracker system in view of the high luminosity phase of the LHC (HL-LHC) foreseen to start around 2025. Thin planar pixel modules are promising candidates to instrument the new pixel system, thanks to the reduced contribution to the material budget and their high charge collection efficiency after irradiation. New designs of the pixel cells, with an optimized biasing structure, have been implemented in n-in-p planar pixel productions with sensor thicknesses of 270μm. Using beam tests, the gain in hit efficiency is investigated as a function of the received irradiation fluence. The outlook for future thin planar pixel sensor productions will be discussed, with a focus on thin sensors with a thickness of 100 and 150μm and a novel design with the optimized biasing structure and small pixel cells (50×50 and 25×100μm2). These dimensions are foreseen for the new ATLAS read-out chip in 65nm CMOS technology and the fine segmentation will represent a challenge for the tracking in the forward region of the pixel system at HL-LHC. To predict the performance of 50×50μm2 pixels at high η, FE-I4 compatible planar pixel sensors have been studied before and after irradiation in beam tests at high incidence angle with respect to the short pixel direction. Results on cluster shapes, charge collection- and hit efficiency will be shown.

CMS Tracker upgrade for HL-LHC: R&D plans, present status and perspectives

During the high luminosity phase of the LHC (HL-LHC), the machine is expected to deliver an instantaneous luminosity of 5×1034cm−2s−1. A total of 3000fb−1 of data is foreseen to be delivered, with the opening of new physics potential for the LHC experiments, but also new challenges from the point of view of both detector and electronics capabilities and radiation hardness. In order to maintain its physics reach, CMS will build a new Tracker, including a completely new Pixel Detector and Outer Tracker. The ongoing R&D activities on both pixel and strip sensors will be presented. The present status of the Inner and Outer Tracker projects will be illustrated, and the possible perspectives will be discussed.

Design optimization of pixel sensors using device simulations for the phase-II CMS tracker upgrade

In order to address the problems caused by the harsh radiation environment during the high luminosity phase of the LHC (HL-LHC), all silicon tracking detectors (pixels and strips) in the CMS experiment will undergo an upgrade. And so to develop radiation hard pixel sensors, simulations have been performed using the 2D TCAD device simulator, SILVACO, to obtain design parameters. The effect of various design parameters like pixel size, pixel depth, implant width, metal overhang, p-stop concentration, p-stop depth and bulk doping density on the leakage current and critical electric field are studied for both non-irradiated as well as irradiated pixel sensors. These 2D simulation results of planar pixels are useful for providing insight into the behaviour of non-irradiated and irradiated silicon pixel sensors and further work on 3D simulation is underway.

Flavour-changing top decays in the aligned two-Higgs-doublet model

We perform a complete one-loop computation of the two-body flavour-changing top decays t → ch and t → cV (V = γ, Z), within the aligned two-Higgs-doublet model. We evaluate the impact of the model parameters on the associated branching ratios, taking into account constraints from flavour data and measurements of the Higgs properties. Assuming that the 125 GeV Higgs corresponds to the lightest CP-even scalar of the CP-conserving aligned two-Higgs-doublet model, we find that the rates for such flavour-changing top decays lie below the expected sensitivity of the future high-luminosity phase of the LHC. Measurements of the Higgs signal strength in the di-photon channel are found to play an important role in limiting the size of the t → ch decay rate when the charged scalar of the model is light.

The pixel detector for the CMS phase-II upgrade

The high luminosity phase of the Large Hadron Collider (HL-LHC) requires a major pixel detector R&D effort to develop both readout chip and sensor that are capable to withstand unprecedented extremely high radiation. The target integrated luminosity of 3000 fb−1, that the HL-LHC is expected to deliver over about 10 years of operation, translates into a hadron fluence of 2×1016 1 MeV eq.n. / cm2, or equivalently 10 MGy of radiation dose in silicon, at about 3 cm from the interaction region where the first layer of the pixel detector could be located. The CMS collaboration has undertaken two baseline sensor R&D programs on thin n-on-p planar and 3D silicon sensor technologies. Together with the ATLAS collaboration it has also been established a common R&D effort for the development of the readout chip in the 65 nm CMOS technology. Status, progresses, and prospects of the CMS R&D effort are presented and discussed in this article.

Collider limits on leptophilic interactions

Leptophilic interactions can only be observed at the LHC in four-lepton final states. If these interactions are mediated by a resonance in the di-leptonic channel with renormalizable couplings, the mediator must have spin 1. We study the LHC reach for such a vector boson allowing for arbitrary couplings. We find that only couplings to muons can be probed at the LHC because lepton flavor violating couplings are constrained by rare processes, couplings to electrons by LEP and the LHC is not sensitive to final states involving taus in this case. The ILC becomes then complementary to the LHC as it will provide the best limits on $Z'$ couplings to tau leptons. A prominent example is the case of the anomaly-free $Z'$ coupling to the muon minus tau lepton number ${\rm L}_\mu - {\rm L}_\tau$. If no departure from the Standard Model is observed at the LHC, the most stringent bounds on this vector boson are provided from events with only three charged leptons plus missing energy. Masses of the order of 1 TeV can be probed at the high-luminosity phase of the LHC for $Z^\prime$ couplings of order one. Generic four-lepton operators parametrizing leptophilic interactions can be also constrained using three and four (or two at the ILC) charged-lepton samples, but the corresponding limits are marginal, if meaningful, because the resonant behavior appears to be essential for the signal to be significant.

Performance studies for the new CMS Outer Tracker module concept at HL-LHC based on measurements of charge collection properties in irradiated silicon sensors

In order to increase the discovery potential of the experiments at the Large Hadron Collider, the high-luminosity phase of the LHC (HL-LHC) is expected to deliver a total of 3000fb−1 integrated luminosity. The instantaneous luminosity will be increased by a factor of five compared to the LHC design luminosity. This results in an intensified track density and radiation level especially in the tracking systems, requiring new radiation hard silicon sensors for the CMS Outer Tracker. To cope with the increased track density and trigger rates, a new module concept based on the coincidence of hits in two closely stacked sensors is pursued for the new tracker, allowing the use of tracking and transverse momentum information already at the first trigger level. The performance of the new trigger module concept has been studied using a parametrization of the charge drift in the electric and magnetic field in the sensor. From that, the phase-space of efficient operation for this module concept and the binary readout in terms of collected charge and noise has been explored.

Timing performance of the CMS ECAL and prospects for the future

The CMS electromagnetic calorimeter (ECAL) is made of about 75000 scintillating lead tungstate crystals arranged in a barrel and two endcaps. The scintillation light is read out by avalanche photodiodes in the barrel and vacuum phototriodes in the endcaps, at which point the scintillation pulse is amplified and sampled at 40 MHz by the on-detector electronics. The fast signal from the crystal scintillation enables energy as well as time measurements from the data collected in proton-proton collisions with high energy electrons and photons. The stability of the time measurement required to maintain the energy resolution is on the order of 1 ns. The single-channel time resolution of ECAL measured at beam tests for high energy showers is better than 100 ps. The time resolution achieved with the data collected in proton-proton collisions at the LHC is presented. The time precision achieved is used in important physics measurements and also allows the study of subtle calorimetric effects, such as the time response of different crystals belonging to the same electromagnetic shower. In addition, we present prospects for the high luminosity phase of the LHC, where we expect an average of 140 concurrent interactions per bunch crossing (pile-up). It is currently being studied how precision time could be exploited for pileup mitigation and for the assignment of the collision vertex for photons. In this respect, a detailed understanding of the time performance and of the limiting factors in time resolution will be important.

Sensor R&D for the CMS outer tracker upgrade for the HL-LHC

At an instantaneous luminosity of 5 × 1034 cm−2s−1, the high-luminosity phase of the Large Hadron Collider (HL-LHC) is expected to deliver a total of 3000 fb−1 of collisions, hereby increasing the discovery potential of the LHC experiments significantly. However, the radiation environment of the tracking system will be severe, requiring new radiation hard sensors for the CMS tracker. The CMS tracker collaboration has almost completed a large material investigation and irradiation campaign to identify the silicon material and design that fulfills all requirements of a new tracking detector at HL-LHC. Focusing on the upgrade of the outer tracker region, pad diodes as well as fully functional strip sensors have been implemented on silicon wafers with different material properties and thicknesses. The samples were irradiated with a mixture of neutrons and protons corresponding to fluences as expected for various positions in the future tracker. The measurements performed on the structures include electrical sensor characterization, measurements of the collected charge and bulk defect characterization. In this paper, the performance and limitations of the different materials are presented.

Anomalous Higgs-boson coupling effects inHW+W−production at the LHC

We study the LHC associated production of a Higgs boson and a ${W}^{+}{W}^{\ensuremath{-}}$ vector-boson pair at 14 TeV, in the standard model and beyond. We consider different signatures corresponding to the cleanest $H$ and $W$ decay channels and discuss the potential of the high-luminosity phase of the LHC. In particular, we investigate the sensitivity of the $H{W}^{+}{W}^{\ensuremath{-}}$ production to possible anomalous Higgs couplings to vector bosons and fermions. Since the $b$-quark initiated partonic channel contributes significantly to this process, we find a moderate sensitivity to both the size and sign of an anomalous top-quark Yukawa coupling, because perturbative unitarity in the standard model implies a destructive interference in the $b\overline{b}$ subprocess. We show that a combination of various signatures can reach $\ensuremath{\sim}9$ standard-deviation sensitivity in the presently allowed negative region of the top-Higgs coupling, if not previously excluded.

Status of the Triple-GEM project for the upgrade of the CMS Muon System

The CMS GEM collaboration is performing a feasibility study to install triple-GEM detectors in the forward region of the muon system (1.6 < |η| < 2.4) of the CMS detector at the LHC. Such micro-pattern gas detectors are able to cope with the extreme particle rates that are expected in that region during the High Luminosity phase of the LHC. With their spatial resolution of order 100 micron GEMs would not only provide additional benefits in the CMS muon High Level Trigger, but also in the muon identification and track reconstruction, effectively combining tracking and triggering capabilities in one single device. The present status of the full project will be reviewed, highlighting all importants steps and achievements since the start of the R&D in 2009. Several small and full-size prototypes were constructed with different geometries and techniques. The baseline design of the triple-GEM detector for CMS will be described, along with the results from extensive test measurements of all prototypes both in the lab and in test beams at the CERN SPS. The proposed on- and off-detector electronics for the final system will be presented.

Planar silicon sensors for the CMS tracker upgrade

The CMS tracker collaboration has initiated a large material investigation and irradiation campaign to identify the silicon material and design that fulfills all requirements for silicon tracking detectors for the high-luminosity phase of the Large Hadron Collider (HL-LHC).A variety of silicon p-in-n and n-in-p test-sensors made from Float Zone, Deep-Diffused FZ and Magnetic Czochralski materials were manufactured by one single industrial producer, thus guaranteeing similar conditions for the production and design of the test-structures. Properties of different silicon materials and design choices have been systematically studied and compared.The samples have been irradiated with neutrons (1MeV) and protons (23MeV and 23GeV) corresponding to maximal fluences as expected for several positions of detector layers in the future tracker. Irradiations with protons of different energies have been performed to evaluate the energy dependence of the defect generation in oxygen rich material. All materials have been characterized before and after irradiations, and throughout an annealing treatment. The measurements performed on the structures include electrical sensor characterization, measurement of the collected charge injected with a beta source or laser light and bulk defect characterization. In this paper, a selection of results from the ongoing studies is presented.

Development of edgeless n-on-p planar pixel sensors for future ATLAS upgrades

The development of n-on-p “edgeless” planar pixel sensors being fabricated at FBK (Trento, Italy), aimed at the upgrade of the ATLAS Inner Detector for the High Luminosity phase of the Large Hadron Collider (HL-LHC), is reported. A characterizing feature of the devices is the reduced dead area at the edge, achieved by adopting the “active edge” technology, based on a deep etched trench, suitably doped to make an ohmic contact to the substrate. The project is presented, along with the active edge process, the sensor design for this first n-on-p production and a selection of simulation results, including the expected charge collection efficiency after radiation fluence of 1×1015neq/cm2 comparable to those expected at HL-LHC (about ten years of running, with an integrated luminosity of 3000fb−1) for the outer pixel layers. We show that, after irradiation and at a bias voltage of 500V, more than 50% of the signal should be collected in the edge region; this confirms the validity of the active edge approach.

Development and implementation of optimal filtering in a Virtex FPGA for the upgrade of the ATLAS LAr calorimeter readout

In the context of upgraded read-out systems for the Liquid-Argon Calorimeters of the ATLAS detector, modified front-end, back-end and trigger electronics are foreseen for operation in the high-luminosity phase of the LHC. Accuracy and efficiency of the energy measurement and reliability of pile-up suppression are substantial when processing the detector raw-data in real-time. Several digital filter algorithms are investigated for their performance to extract energies from incoming detector signals and for the needs of the future trigger system. The implementation of fast, resource economizing, parameter driven filter algorithms in a modern Virtex FPGA is presented.