LHC Detectors Research Articles

Review of results for the NA62 gigatracker read-out prototype

The Gigatracker (GTK) is a hybrid silicon pixel detector developed for NA62, an experiment studying ultra-rare kaon decays at the CERN SPS. The main characteristics are a time-tagging resoluion of 150ps, with low material budget per station (0.5% X0) and a fluence comparable to the one expected for the inner trackers of LHC detectors in 10 years of operation. To compensate the time-walk, two read-out architectures have been designed and produced. The first architecture is based on a Constant Fraction Discriminator (CFD) followed by an on-pixel Time-to-Digital-Converter (TDC). The second architecture is based on a on-pixel group shared TDC. The GTK system developments are described: the integration steps (assembly and cooling) and the results obtained from the prototypes fabricated for the two read-out architectures.

Implementation of the type III seesaw model in FeynRules/MadGraph and prospects for discovery with early LHC data

We discuss the implementation of the "minimal" type III seesaw model, i.e. with one fermionic triplet, in FeynRules/MadGraph. This is the first step in order to realize a real study of LHC data recorded in the LHC detectors. With this goal in mind, we comment on the possibility of discovering this kind of new physics at the LHC running at 7 TeV with a luminosity of few fb^-1.

Development of micromegas muon chambers for the ATLAS upgrade

Large-area particle detectors based on the bulk-micromegas technology are an attractive choice for the upgrade of LHC detectors and/or detectors for the ILC or other experiments. In the context of the R&D for the ATLAS Muon System upgrade, we have built detectors of order 1 m2. In order to overcome the spark problem in micromegas a novel protection scheme using resistive strips above the readout electrode has been developed. This technology has undergone extensive tests with hadron beams at the CERN-SPS, X-rays in the lab, as well as in a neutron beam. In addition, four 10 × 10 cm2 micromegas chambers have been installed in the ATLAS cavern and are taking data under LHC conditions. We will discuss the underlying design of the chambers and present results on the performance of these chambers.

Fast supersymmetry phenomenology at the Large Hadron Collider using machine learning techniques

A pressing problem for supersymmetry (SUSY) phenomenologists is how to incorporate Large Hadron Collider search results into parameter fits designed to measure or constrain the SUSY parameters. Owing to the computational expense of fully simulating lots of points in a generic SUSY space to aid the calculation of the likelihoods, the limits published by experimental collaborations are frequently interpreted in slices of reduced parameter spaces. For example, both ATLAS and CMS have presented results in the Constrained Minimal Supersymmetric Model (CMSSM) by fixing two of four parameters, and generating a coarse grid in the remaining two. We demonstrate that by generating a grid in the full space of the CMSSM, one can interpolate between the output of an LHC detector simulation using machine learning techniques, thus obtaining a superfast likelihood calculator for LHC-based SUSY parameter fits. We further investigate how much training data is required to obtain usable results, finding that approximately 2000 points are required in the CMSSM to get likelihood predictions to an accuracy of a few per cent. The techniques presented here provide a general approach for adding LHC event rate data to SUSY fitting algorithms, and can easily be used to explore other candidate physics models.

A New Concept of Vertically Integrated Pattern Recognition Associative Memory

Hardware-based pattern recognition for fast triggering on particle tracks has been successfully used in high-energy physics experiments for some time. The CDF Silicon Vertex Trigger (SVT) at the Fermilab Tevatron is an excellent example. The method used there, developed in the 1990’s, is based on algorithms that use a massively parallel associative memory architecture to identify patterns efficiently at high speed. However, due to much higher occupancy and event rates at the LHC, and the fact that the LHC detectors have a much larger number of channels in their tracking detectors, there is an enormous challenge in implementing fast pattern recognition for a track trigger, requiring about three orders of magnitude more associative memory patterns than what was used in the original CDF SVT. Scaling of current technologies is unlikely to satisfy the scientific needs of the future, and investments in transformational new technologies need to be made. In this paper, we will discuss a new concept of using the emerging 3D vertical integration technology to significantly advance the state-of-the-art for fast pattern recognition within and outside HEP. A generic R&D proposal [1] based on this new concept, with a few institutions involved, has recently been submitted to DOE with the goal to design and perform the ASIC engineering necessary to realize a prototype device. The progress of this R&D project will be reported in the future. Here we will only focus on the concept of this new approach. © 2011 Elsevier BV. Selection and/or peer-review under responsibility of the organizing committee for TIPP 2011.

Results from the NA62 Gigatracker Prototype: A Low-Mass and sub-ns Time Resolution Silicon Pixel Detector

Abstract The Gigatracker (GTK) is a hybrid silicon pixel detector developed for NA62, the experiment aimed at studying ultra-rare kaon decays at the CERN SPS. Three GTK stations will provide precise momentum and angular measurements on every track of the high intensity NA62 hadron beam with a time-tagging resolution of 150 ps. Multiple scattering and hadronic interactions of beam particles in the GTK have to be minimized to keep background events at acceptable levels, hence the total material budget is fixed to 0.5% X0 per station. In addition the calculated fluence for 100 days of running is 2×10 14 1 MeV n eq /cm 2 , comparable to the one expected for the inner trackers of LHC detectors in 10 years of operation. These requirements pose challenges for the development of an efficient and low-mass cooling system, to be operated in vacuum, and on the thinning of read-out chips to 100 μm or less. The most challenging requirement is represented by the time resolution, which can be achieved by carefully compensating for the discriminator time-walk. For this purpose, two complementary read-out architectures have been designed and produced as small-scale prototypes: the first is based on the use of a Time-over-Threshold circuit followed by a TDC shared by a group of pixels, while the other uses a constant-fraction discriminator followed by an on-pixel TDC. The readout pixel ASICs are produced in 130 nm IBM CMOS technology and bump-bonded to 200 μm thick silicon sensors. The Gigatracker detector system is described with particular emphasis on recent experimental results obtained from laboratory and beam tests of prototype bump-bonded assemblies, which show a time resolution of less than 200 ps for single hits.

Microfluidic cooling for detectors and electronics

Micro-channel cooling is gaining considerable attention as an alternative technique for cooling of high energy physics detectors and front-end electronics. This technology is being evaluated for future tracking devices, where material budget limitations are a major concern. It is currently under investigation as an option for the cooling of the NA62 Gigatracker silicon pixel detector, where a micro-fabricated silicon cooling plate would stand directly in the beam. Other possible applications are also being studied in the context of LHC detectors upgrades. In this paper, the current status of this R&D at CERN is presented.

Detector Powering in the 21st Century Why stay stuck with the Good old 20th Century methods?

Future Collider Physics Detectors are envisioned with large granularity but we have a power delivery problem unless we fill a large fraction of the detector volume with copper conductors. LHC detector electronics is powered by transporting direct current over distances of 30 to 150 meters. This is how Thomas Alva Edison powered his light bulb. For example, CMS ECAL uses 50 kiloamps at 2.5 volts, supplied over a cable set with a transmission efficiency of only 30%. The transmission loss becomes waste heat in the detector that has to be removed. We have been exploring methods to transmit the DC power at higher voltage (low current), reducing to the final low voltage (high current) using DC-DC converters. These converters must operate in high magnetic fields and high radiation levels. This requires rad hard components and non-magnetic (air core) inductors.

Improvements in the Geant4 Hadronic Physics

During the last five years, development of GEANT4 hadronic physics models has been driven largely by the anticipated turn-on of the LHC detectors and the subsequent comparison of new data with the simulated detector responses.Work has concentrated on three main areas: hadronic model improvement for high energy string models, intranuclear cascade models, precompound and de-excitation models, and elastic scattering which are currently used in the simulation of LHC experiments. These development are tested against a large collection of thin target data and dedicated test beam experiments using calorimeter prototypes. Salient improvements in physics and computing performance of these models will be discussed.

Scaling HEP to Web Size with RESTful Protocols: The Frontier Example

The World-Wide-Web has scaled to an enormous size. The largest single contributor to its scalability is the HTTP protocol, particularly when used in conformity to REST (REpresentational State Transfer) principles. High Energy Physics (HEP) computing also has to scale to an enormous size, so it makes sense to base much of it on RESTful protocols. Frontier, which reads databases with an HTTP-based RESTful protocol, has successfully scaled to deliver production detector conditions data from both the CMS and ATLAS LHC detectors to hundreds of thousands of computer cores worldwide. Frontier is also able to re-use a large amount of standard software that runs the Web: on the clients, caches, and servers. I discuss the specific ways in which HTTP and REST enable high scalability for Frontier. I also briefly discuss another protocol used in HEP computing that is HTTP-based and RESTful, and another protocol that could benefit from it. My goal is to encourage HEP protocol designers to consider HTTP and REST whenever the same information is needed in many places.

What the Tevatron found?

The CDF collaboration has reported a 4.1\sigma\ excess in their lepton, missing energy, and dijets channel. This excess, which takes the form of an approximately Gaussian peak centered at a dijet invariant mass of 147 GeV, has provoked a great deal of experimental and theoretical interest. Although the D\O\ collaboration has reported that they do not observe a signal consistent with CDF, there is currently no widely accepted explanation for the discrepancy between these two experiments. A resolution of this issue is of great importance---not least because it may teach us lessons relevant for future searches at the LHC---and it will clearly require additional information. In this paper, we consider the ability of the Tevatron and LHC detectors to observe evidence associated with the CDF excess in a variety of channels. We also discuss the ability of selected kinematic distributions to distinguish between Standard Model explanations of the observed excess and various new physics scenarios.

Direct photon and photon-jet measurement capability of the ATLAS experiment at the LHC

Direct photon and photon-jet correlations are perfect tools for tomographic studies of the dense medium produced in heavy ion collisions at LHC energies. Due to their weak interactions with the medium, direct photons serve as standard candles for hard-scattering processes, providing a clean calibration of the momentum of the associated jets. The ATLAS detector has powerful opportunities to carry out these measurements. The combination of fine granularity, longitudinal segmentation and large acceptance is unique for the LHC detectors. We show how it will provide optimal capability to distinguish direct photon clusters from neutral meson clusters based on their shower profile over a wide acceptance in η − ϕ up to 200 GeV in p T . We show that combined with a photon isolation cut ATLAS would be able to measure a relatively background-free direct photon yield from 50–200 GeV along with the corresponding gamma-jet correlations in one nominal LHC Pb+Pb year. These high p T photons provide clean and statistically significant measurements of gamma-jet correlations and the fragmentation function for photon-tagged jets.

Status of the ALICE experiment and first results on heavy flavour production

The ALICE experiment is the LHC detector mainly dedicated to the study of the Quark Gluon Plasma (QGP) in Pb-Pb collisions. The detector has started the data taking less than one year ago, delivering immediately relevant results. An overview of the first physics results obtained in the first six month of running of the experiment will be summarized, giving special emphasis to heavy flavour measurements. Heavy flavours are ideal probes to explore both the formation and properties of the QGP, since they experience the full collision history and are expected to be copiously produced at LHC, much more than at any other collider. With ALICE we will measure heavy flavours down to small transverse momentum, combining hadronic and leptonic channels, both at central and forward rapidity. In particular, in the central rapidity region, it is possible to exclusively reconstruct open charm mesons and baryons via hadronic decay channels. Furthermore, the good identification of electrons allows to measure the production both of charmonium and open beauty. First results from p-p collisions at 7 TeV will be shown, including the clear signals of open and hidden charm hadrons reconstructed at ALICE. These data provide interesting insight into QCD processes in a new energy regime, are important as a baseline for the Pb-Pb program and demonstrate the potential for heavy flavour cross section measurements with the ALICE detector.

The design for test architecture in digital section of the ATLAS FE-I4 chip

This paper describes an original Design-for-Test (DfT) architecture implemented in the ATLAS FE-I4 pixel readout System-on-Chip (Soc) to accommodate the higher quality demands of future generation LHC detectors. To ensure that the highest possible number of fault-free devices is used for the detector construction, the so-called production test to detect faulty devices after the manufacturing has to be executed. For that reason, we devised a straightforward and effective DfT circuitry inside the digital part of the FE-I4 that will enable high fault coverage of potential structural faults while maintaining the performance and area penalties of the entire design negligible.

Reconstruction of % MathType!MTEF!2!1!+- % feaagaart1ev2aaatCvAUfKttLearuqr1ngBPrgarmWu51MyVXgaru % WqVvNCPvMCamXvP5wqSXMqHnxAJn0BKvguHDwzZbqegqvATv2CG4uz % 3bIuV1wyUbqee0evGueE0jxyaibaieYlf9irVeeu0dXdh9vqqj-hEe % eu0xXdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-J % b9hs0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaabau % aaaOqaaeXafv3ySLgzGmvETj2BSbacgiGaf8hXdqNbaGaadaWgaaWc % baacbaGae4xmaedabeaaaaa!417A! $$ \tilde \tau _1 $$ mass at the LHC

The cascade mass reconstruction approach was used for mass reconstruction of the lightest stau produced at the LHC in the cascade decay $\tilde{g} \to \tilde{b} b \to \tilde{\chi}_{2}^{0} b b \to \tilde{\tau}_{1} \tau b b \to \tilde{\chi}_{1}^{0} \tau \tau b b $. The stau mass was reconstructed assuming that masses of gluino, bottom squark and two lightest neutralinos were reconstructed in advance. SUSY data sample sets for the SU3 model point containing 160k events each were generated which corresponded to an integrated luminosity of about $8 \rm fb^{-1}$ at 14 TeV. These events were passed through the AcerDET detector simulator, which parametrized the response of a generic LHC detector. The mass of the $\tilde{\tau}_{1}$ was reconstructed with a precision of about 20% on average.

Performance of ALICE detector and electronics under first beam conditions

ALICE (A Large Ion Collider Experiment) is a general purpose heavy-ion detector at the CERN LHC addressing the physics of strongly interacting matter and the quark-gluon plasma in nucleus-nucleus collisions. ALICE has been recording physics data since the first proton-proton collisions at LHC as reference for the heavy-ion programme and to address physics topics for which it is complementary to the other LHC detectors. This paper aims to provide a concise description of the ALICE experiment, with entire emphasis on instrumentation and electronics related topics, as it has been operating since the beginning of the LHC 7 TeV p-p collisions programme (March 2010) up to date. The performance, stability and reliability of selected detectors and central systems are discussed. A description of the typical proton-proton data taking experience is given. Selected topics related to timing and synchronization of systems, luminosity measurements and SEU effects are finally presented.

Cosmic-ray Monte Carlo predictions for forward particle production in p-p, p-Pb, and Pb-Pb collisions at the LHC

We present and compare the predictions of various cosmic-ray Monte Carlo models for the energy (dE/deta) and particle (dN/deta) flows in p-p, p-Pb and Pb-Pb collisions at sqrt(s) = 14, 8.8, and 5.5 TeV respectively, in the range covered by forward LHC detectors like CASTOR or TOTEM (5.2<|eta|<6.6) and ZDC or LHCf (|eta|>8.1 for neutrals).

Reconstruction of stop quark mass at the LHC

The cascade mass reconstruction approach was applied to simulated production of the lightest stop quark at the LHC in the cascade decay $\tilde{g} \to \sTop \, \Top \to \tilde{\chi}_{2}^{0} \, \Top \, \Top \to \tilde{\ell}_{R} \, \ell \, \Top \, \Top \to \tilde{\chi}_{1}^{0} \, \ell \, \ell \, \Top \, \Top$ with top quarks decaying into hadrons. The stop quark mass was reconstructed assuming that the masses of gluino, slepton and of the two lightest neutralinos were reconstructed in advance. A data sample set for the SU3 model point containing 400k SUSY events was generated which corresponded to an integrated luminosity of about 20 $\rm fb^{-1}$ at 14 TeV. These events were passed through the AcerDET detector simulator, which parametrized the response of a generic LHC detector. The mass of the $\tilde{t}_{1}$ was reconstructed with a precision of about $10\%$.

Higgs boson look-alikes at the LHC

The discovery of a Higgs particle is possible in a variety of search channels at the LHC. However the true identity of any putative Higgs boson will at first remain ambiguous, until one has experimentally excluded other possible assignments of quantum numbers and couplings. We quantify to what degree one can discriminate a Standard Model Higgs boson from "look-alikes" at, or close to, the moment of discovery at the LHC. We focus on the fully-reconstructible "golden" decay mode to a pair of Z bosons and a four-lepton final state, simulating sPlot-weighted samples of signal and background events. Considering both on-shell and off-shell Z's, we show how to utilize the full decay information from the events, including the distributions and correlations of the five relevant angular variables. We demonstrate how the finite phase space acceptance of any LHC detector sculpts the decay distributions, a feature neglected in previous studies. We use likelihood ratios to discriminate a Standard Model Higgs from look-alikes with other spins or nonstandard parity, CP, or form factors. For a benchmark resonance mass of 200 GeV/c^2, we achieve a median expected discrimination significance of 3 sigma with as few as 19 events, and even better discrimination for the off-shell decays of a 145 GeV/c^2 resonance.

Geant4 models for simulation of multiple scattering

Recent progress in development of single and multiple scattering models within the Geant4 toolkit is presented. Different options available to users are discussed. The comparisons with the data are shown. The trade of precision versus CPU performance is discussed with the focus on LHC detectors simulation.